nanocatalysts for solar water splitting and aperspective on hydrogen...

Water Splitting

Nanocatalysts for Solar Water Splitting and a Perspective onHydrogen Economy

Tobias Grewe, Mariem Meggouh, and Harun Tìysìz*[a]

Chem. Asian J. 2016, 11, 22 – 42 Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim22

Focus ReviewDOI: 10.1002/asia.201500723

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Abstract: In this review article, nanocatalysts for solar hydro-

gen production are the focus of discussion as they can con-tribute to the development of sustainable hydrogen produc-

tion in order to meet future energy demands. Achieving thistask is subject of scientific aspirations in the field of photo-and photoelectrocatalysis for solar water splitting where sys-tems of single catalysts or tandem configurations are being

investigated. In search of a suitable catalyst, a number ofcrucial parameters are laid out which need to be consideredfor material design, in particular for nanostructured materials

that provide exceptional physical and chemical properties incomparison to their bulk counterparts. Apart from synthetic

approaches for nanocatalysts, key parameters and properties

of nanostructured photocatalysts such as light absorption,charge carrier generation, charge transport, separation and

recombination, and other events that affect nanoscale cata-lysts are discussed. To provide a deeper understanding ofthese key parameters and properties, their contribution to-wards existing catalyst systems is evaluated for photo- and

photoelectrocatalytic solar hydrogen evolution. Finally, an in-sight into hydrogen production processes is given, stressingthe current development of sustainable hydrogen sources

and presenting a perspective towards a hydrogen-basedeconomy.

1. Introduction

Due to the growth of the world economy and population, theglobal demand for energy is steadily rising. To compensate for

diminishing fossil fuel resources, the development of renewa-ble and sustainable energy sources is driven forward. Amongst

all alternative energy sources, solar energy has stimulated par-

ticular interest since it is by far the most abundant renewableenergy source. The sun delivers large amounts of energy in the

form of electromagnetic radiation, in fact, the earth absorbs intotal 3.85 Õ 1024 J year¢1 of solar energy, which is about 104

greater than the world energy consumption.[1] In other words,solar energy delivers a vast surplus of energy with around

120 000 TW of light striking the surface of the earth by consid-

ering a current world energy consumption of 15 TW.[2] By cov-ering 0.16 % of the earth’s surface with solar cells that have

a 10 % solar-to-electricity efficiency, a maximum of 20 TW ofpower could be generated.[3] This corresponds to an area of

the size of France and Germany combined.[4] These numbersshow that the efficient use of a small fraction of solar energycould solve future energy concerns. The challenge is to devel-

op an efficient method for the conversion of solar energy intouseful energy in form of electricity or fuels. Nowadays, solarenergy can be effectively converted into electrical energy byusing photovoltaics, which are commercialized and the process

costs have been decreased dramatically in the past years. Themain shortcoming of this process is that electrical energy

cannot be stored for a long time. Thus, it is necessary to con-

sider the possibility of converting solar energy into chemicalenergy. This way, the energy can be stored as a dense, renewa-

ble fuel. The solar energy-to-chemical energy conversion canbe achieved through CO2 reduction into hydrocarbons or

water splitting to produce hydrogen. As illustrated in Figure 1,the ideal case will be a combination of both these paths in

order to supply a large amount of sustainable energy.

Inspired from artificial photosynthesis, producing clean H2

through photoelectrolysis of water directly with sunlight is themost promising route for the conversion of solar energy intochemical energy.[5] Overall, water splitting is an uphill reaction(DG>0) that can be used to store solar energy in the chemical

bond of hydrogen:

H2O! H2 þ 1=2 O2 DG� ¼ 237:2 kJ mol¢1 ð1Þ

Water splitting consist of two half reactions, mainly reduc-

tion and oxidation [Eq. (2) and (3)] , which can be investigated

separately. Among both reactions, water oxidation is morechallenging since it requires an extra bias (1.23 V), involving

a four-electron transfer and the formation of oxygen bonds.[6]

2 Hþ þ 2 e¢ ! H2 E� ¼ 0 V vs: RHE ð2Þ

2 H2O! O2 þ 4 Hþ þ 4 e¢ E� ¼ 1:23 V vs: RHE ð3Þ

The overall water splitting process consists of three steps:1) absorption of photons by a semiconductor catalyst and gen-

Figure 1. Conversion of solar to electrical and chemical energy and its uti-lization.

[a] T. Grewe, Dr. M. Meggouh, Dr. H. TìysìzMax-Planck-Institut fìr KohlenforschungKaiser-Wilhelm-Platz 1, 45470 Mìlheim an der Ruhr (Germany)E-mail : [email protected]

Chem. Asian J. 2016, 11, 22 – 42 www.chemasianj.org Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim23

Focus Review

http://www.chemasianj.org

eration of electron–hole pairs, 2) migration of photo-generatedelectrons/holes to the semiconductor–electrolyte interface, and

3) surface reaction of hydrogen and oxygen evolution at thesemiconductor–electrolyte interface. Powdered or slurry photo-

catalysts are still one of the most appealing platforms for solarhydrogen production on a large scale since they can function

without an electrochemical cell or external voltage bias. Anumber that is used to characterize a catalyst for solar energy

conversion is the solar-to-hydrogen conversion or efficiency

(STH). The STH efficiency can be calculated following the Gibbsfree energy of reaction:[1a, 7]

STH ¼ Output energyEnergy of incident solar light

¼ rH2 DG

PSun AGeometric

ð4Þ

with rH2as the hydrogen production rate, PSun as the energy

flux of the sunlight, and AGeometric as the area of the reactor.

2. Past, Present, and Future of Artificial WaterSplitting

The development of artificial water splitting has been the

focus of many researchers. Its history has been covered bysome review articles as well.[7, 8] In order to give a historical

view to the readers of this journal, a brief summary of the his-tory is presented in this chapter, also including recent advan-

ces in the area of tandem cells.

The idea of splitting water and using hydrogen as an energysource was mentioned already 1874 in Jules Verne’s fictional

novel The Mysterious Island. Roughly 100 years later, in 1968,the first scientific report about water splitting was reported by

Boddy (Bell Telephone Laboratories, New Jersey) where n-typesemiconductor TiO2 was used as an anode material for water

oxidation.[9] Four years later, a report by Fujishima and Honda

on the utilization of n-type TiO2 as a photoanode in a photo-electrochemical cell was published, which is often named the

starting point of artificial water splitting.[5d] Three years later,Yoneyama et al. presented the first photocathode for watersplitting, consisting of a single crystalline p-type GaP,[10] whichled to one of the first reports on photoelectrochemical tandem

cells by Bockris and co-workers in 1977 where they employedan n-SrTiO3 photoanode and n-GaP photocathode.[11] In the

same year, Nozik introduced the term ’photochemical diode’,including the Schottky-type and the p-n-type photochemicaldiode, both representing early examples of photocatalytic sys-

tems for water splitting. The Schottky-type diode consisted ofn-GaP/Pt or n-CdS/Pt and was able to evolve hydrogen from

an aqueous electrolyte solution while the p-n-type was madefrom n-TiO2/p-GaP and produced H2 and O2.[12] In order to in-

crease the light absorption efficiency of the materials/devices,

Gr�tzel presented in 1979 the first dye-sensitized solar cell, inwhich a [Ru(bipy)3]2+ complex was used as additional light ab-

sorber.[13] This report opened a new field of solar energy con-version, namely that of dye-sensitized solar cells, which result-

ed in many excellent scientific reports.[14] In 1980, the first ap-plications of semiconductor particles dispersed in solution

were reported for photocatalytic water splitting,[15] launchingan intensive investigation of semiconductor materials for solar

water splitting which is still continuing. Adopting a more pho-tosynthesis-like approach, in 2001 Domen et al. were the first

to report a Z-scheme system of rutile and anatase TiO2 forphotocatalytic overall water splitting.[16] In this system, rutilewas utilized as the oxygen evolution catalyst and anatase dec-orated with Pt as co-catalyst as the hydrogen evolution cata-lyst, which were dispersed in water with I¢/IO3

¢ as an electronmediator between the two catalysts.

The combination of photo- and electrochemistry couldcreate a photochemical cell where a thin film of semiconductoris deposited on a conductive substrate and used as photo-anode or -cathode. The design and development of solarwater splitting devices that result in hydrogen has been the

goal of many studies.[17] The integration of BiVO4 with an amor-

phous silicon solar cell in a tandem configuration has beenshown to be an effective solar water splitting device that gives

Tobias Grewe studied chemical engineeringand received his master degree from the Uni-versity of Applied Sciences in Mìnster in 2012.He is currently a Ph.D. candidate in the re-search group of Dr. Harun Tìysìz at the Max-Planck-Institut fìr Kohlenforschung. His re-search focuses on synthetic strategies fornanostructured photocatalysts and their ap-plication for photocatalytic hydrogen evolu-tion.

Mariem Meggouh received her M.Sc. inchemistry in 2008 from Radboud UniversityNijmegen and received her Ph.D. in solid-statehydrogen storage materials in 2013. Currently,she is a postdoctoral researcher at the Max-Planck-Institut fìr Kohlenforschung in the De-partment of Heterogeneous Catalysis of Prof.Ferdi Schìth. Her research interests includethe synthesis and evaluation of metal hy-drides for energy storage applications.

Harun Tìysìz received his Ph.D. in chemistryfrom the Max-Planck-Institut fìr Kohlenfor-schung in 2008. He was awarded a researchfellowship from the DFG and did his postdoc-toral work at the University of California atBerkeley. Since 2012, he is a group leader atthe Max-Planck-Institut fìr Kohlenforschung.His research interests include heterogeneouscatalysis and design of nanostructured,shape-controlled and multi-functional orderedmesoporous materials for sustainable energyapplications, mainly for water splitting, bio-mass conversion, and perovskite solar cells.


Focus Review


an STH efficiency of around 5 % under 1 sun illumination.[18] Asalternative semiconductor, organometal trihalide perovskites

have very high visible-light harvesting efficiencies and low pro-duction costs. Their combination with a BiVO4 photoanode has

been shown to be able to split water with an STH efficiency of2.5 % at neutral pH.[19] The performance of the device wasbelow the solar-to-electricity conversion efficiency of perov-skite, which was attributed to the limited photocurrent densityof BiVO4. Gr�tzel and co-workers described a tandem cellbased on NiFe-layered double hydroxide and CH3NH3PbI3 per-ovskite that reached an STH efficiency of 12.3 %. However, theinstability of the tandem cell was a major issue.[20] There isgreat room to improve the stability and performance of the

materials by modifying their morphology. Therefore, as futuredirection, more efforts in the direction of the design and fabri-

cation of nanostructured tandem cells for solar water splitting

might create stable materials and reach an STH efficiency of10 % to meet the requirements for commercialization. In addi-

tion, a steady synergy between academia and industry is alsorequired for large-scale commercial application of hydrogen.

3. The Principle of Water Splitting

Typically, two systems for solar water splitting are used,namely photo- and photoelectrochemical systems.[21] As de-

picted in Figure 2, solar water splitting can be operated ina type 1 (single catalyst, single absorber, Schottky-type) or

type 2 (two catalysts, tandem configuration, Z-scheme) reactor

set-up.[8c] The most straightforward approach is photochemicalwater splitting, which is illustrated in Figure 2 a,b. The system

is based on the utilization of a semiconductor slurry. In a typicaltype 1 photocatalytic system, the photoreactor is filled with an

aqueous dispersion of powdered semiconductor photocatalystand is then irradiated by light. The absorbed photons generate

electrons (e¢) and holes (h+) in the conduction and valence

band of the semiconductor, respectively. After the separation,these charge carriers diffuse to the catalyst surface where they

split water molecules into H2 and O2 at active surface sites. Inorder to enhance the photocatalytic activity, the semiconduc-tor can be decorated with co-catalysts (noble metals or metaloxides) that act as electron or hole sinks, thus being the active

sites for water reduction (water reduction catalyst = WRC) andoxidation (water oxidation catalyst = WOC), and thus prevent-

ing charge recombination.[22] The set-up for such a system issimple as the main components are the reactor (as container),water (as reactant), a powdered catalyst, and light (as energy

source). The bottleneck of this approach is that the band gapof the photocatalyst has to match with the potentials of the

water reduction and oxidation reactions, which limits thechoice of materials. Further, a comparison of hydrogen produc-

tion rates of catalysts from the literature is difficult as many dif-

ferent light sources and reactor geometries are used. In addi-tion, generation of H2 and O2 in the same reaction cell requires

more effort for the separation of those gases.[8c]

As illustrated in Figure 2 b, a type 2 photochemical cell, also

known as Z-scheme, consists of two catalysts that separatelypromote water oxidation and reduction. The materials are dis-

persed in an aqueous reaction solution that contains an elec-tron mediator which consumes charge carriers. The water oxi-

dation catalyst produces O2 with generated holes from the va-lence band while the electrons are consumed in the reductionof the electron mediator. The excited electrons in the water re-duction catalyst generate H2 while the holes oxidize the elec-tron mediator. This system is the artificial version of the photo-synthesis of plants, where two chlorophyll reaction centers in

photosystem II and photosystem I are responsible for the oxi-dation and reduction reaction, respectively.[23] As the watersplitting half reactions are spread out over two different cata-

lyst materials, the band gap of each semiconductor has tomatch only the potential of one of the half reactions. There-

fore, a wider range of materials can be used as catalyst materi-als compared to type 1. However, this system requires the ap-

plication of an electron mediator, and the synthetic prepara-

tion of two catalysts that can achieve overall water splitting ismore challenging compared to system 1.[8c]

The type 1 of the photoelectrochemical cell (Figure 2 c) in-volves an anode that consists of a single water oxidation cata-

lyst (working electrode) attached to a conductive substrate,usually FTO glass, and a cathode (counter electrode) that is

Figure 2. Type 1 and type 2 catalyst systems for solar water splitting separat-ed into (a, b) photocatalytic and (c, d) photoelectrocatalytic water splitting.The type 1 photocatalytic system consists of one material (a). Under illumi-nation, electron–hole pairs are generated that move to water oxidation andreduction sites at the surface of the catalyst to produce oxygen and hydro-gen, respectively. These active sites are either inherent to the catalyst mate-rial or can be constructed by the decoration of co-catalysts that function aswater reduction (WRC) or water oxidation catalysts (WOC). The type 2system consists of two catalysts and each material works as a WOC or WRC(b). They can be loaded as well with a respective co-catalyst to enhance theactivity of the material. An electron mediator (redox pair) is essential in thissystem as it allows electron/hole transfer between the two materials. Thephotoelectrochemical cell can be operated on one hand as a type 1 system,where one electrode is made of a photocatalyst material that catalyzeseither water oxidation or reduction (c). Under illumination with light, the cat-alyst, here a WOC, generates electron–hole pairs and oxidizes water, whilethe applied bias moves the electrons to the counter electrode where theyundergo water reduction. On the other hand, in a type 2 cell, both anodeand cathode consist of a WOC and WRC, respectively, and both electrodesgenerate electron–hole pairs (d).


Focus Review


typically made of Pt.[24] Underlight irradiation, the catalyst gen-

erates e¢ and h+ . Electrons aredrawn from the anode by the

applied bias and reduce watermolecules at the cathode, while

the holes oxidize water mole-cules at the anode. In this way,

hydrogen molecules can be col-

lected oxygen-free from the re-action.[24b] Typically, a lightsource is used to irradiate theworking electrode and an exter-

nal bias is applied to facilitatethe reaction. The measured cur-

rent density is used to evaluate the performance of the cata-

lyst. There are also investigations on water reduction catalystsas cathode material ; however, the number of these reports is

limited.[25] The advantages of a type 1 photoelectrochemicalcell are a simple catalyst synthesis, as there is only one photo-

catalyst, and trapping of charge carriers by the use of co-cata-lysts. On the downside, however, the cell uses a counter elec-

trode and a photocatalyst-coated electrode for oxygen evolu-

tion or hydrogen evolution. Therefore, this cell type is usedonly to evaluate oxygen evolution or hydrogen evolution cata-

lysts. Furthermore, an electrolyte as reaction solution, a lightsource, and an electrical bias are essential for the cell.

The tandem system for photoelectrochemical water splitting(Figure 2 d) is strongly based on photosynthesis as well.[21, 24a] In

this system, both anode and cathode are made out of catalyst

material. The anode and cathode consist of a water oxidationand reduction catalyst, respectively, attached to a conductive

substrate. Advantages of a type 2 photoelectrochemical cellare the effective electron–hole separation and that each cata-

lyst operates with a band gap which is suitable for eitherwater oxidation or water reduction. This allows the use of sem-

iconductors with narrow band gaps, increasing the possibilities

of material choices and allows visible-light excitation. However,the use of two catalysts complicates both the catalyst system

and set-up, and is therefore not as feasible as type 1.

4. Design of Nanocatalysts

The design and engineering of catalysts for solar water split-ting have been intensively investigated to determine impor-

tant parameters in order to produce an efficient photo-catalyst.[4, 8a, 26] As seen in Figure 3, physical and chemical key

properties for the performance of a catalyst can be groupedinto morphology and textural parameters, electronic structure,

crystal structure, and stability of the catalyst. In most cases,

these key parameters are interrelated with each other, and theoverall efficiency of the material (or power of the catalysts) is

determined by a combination of those parameters. If the per-formance of a catalyst is compared with a horse-drawn car-

riage, the overall power of the carriage will be determined byall the horses (TOF number in catalysis). The weakness of one

horse will affect the entire arrangement and overall horsepow-

er, and subsequently, the speed of the carriage.Shape, size, surface area, porosity, and dimension (1-, 2-, 3-

D) of the catalyst are dominated by the morphology and tex-tural parameters that also affect the light absorption capability.

On a flat surface, incident light is reflected to a great portion,and the number of photons that penetrate into the catalyst to

excite excitons (electron–hole pairs) is reduced. By engineering

the dimensions and morphology of a material, the incidentlight can be scattered on the surface, which can lead to reab-

sorption of the scattered light, and thus the number of pho-tons that penetrate into the catalyst will increase.[27] Further-

more, particle size and morphology also influence the possibili-ty of charge carriers to diffuse to the surface of the catalyst,[28]

as phase boundaries and defect sites represent potential traps

for electrons and holes that can lead to the recombination andthus loss of the generated excitons. The diffusion of charge

carriers through a material can be described by their diffusionlength (Ld), the diffusion coefficient of diffusivity (D), and the

carrier lifetime (t) [Eq. (5)]:[29]

Ld ¼ffiffiffiffiffiffiffiffiffiD ¡ tp ð5Þ

The equation shows that the material characteristic diffusion

length Ld is the distance that a carrier can travel through a ma-

terial in its lifetime.[29] Hence, the dimensions of the photocata-lyst have to be in the range of the charge carrier diffusion

length in order to have carriers traveling from the bulk to thesurface of the material where the catalysis takes place. The dif-

fusion length for e¢ (Le) and h+ (Le) will be disused in moredetail in section 5.1.

P-type and n-type semiconductors have an uneven numberof charge carriers due to doping of the structure. Therefore,

the charge carriers have been denoted as majority and minori-ty charge carriers, as one species exists in a higher concentra-

tion than the other one. In p-type semiconductors, the majori-

ty charge carriers are holes and the minority charge carriersare electrons. In n-type semiconductors, it is the other way

around. Due to the high concentration of majority charge carri-er, they have increased lifetimes and diffusion lengths com-

pared to the minority carriers. Thus, for optimum charge carriercollection at the semiconductor surface, the catalyst thickness

Figure 3. Key parameters for the design of a photocatalyst : the properties of the catalyst result from morphologi-cal and textural parameters, electronic structure, crystal structure, and stability.


Focus Review


has to be in the range of Le and Lh to ensure that both elec-trons and holes reach the catalyst surface, which can be ach-

ieved through a nanostructured morphology.[30] If the catalystsize exceeds the diffusion range of both charge carriers, the

concentration of charge carriers at the catalyst surface is reduc-ed.While band gap and light-harvesting efficiencies are pre-

dominantly determined by the electronic structure, charge-car-rier generation, separation, and transfer are influenced by both

electronic and crystal structures. Considering the influence on

the electronic structure, with decreasing particle size, the bandgap energy of a semiconductor is altered. Below a certain par-

ticle size, the formation of deep electronic traps near the bandedge of the valence band occur due to mild delocalized mo-

lecular orbitals on the surface, which is described by the Brus’effective-mass model.[31] Thus, it is possible to observe a de-

crease in band gap energy with decreasing particle size.[32]

With further decrease in particle size to the low nanometerrange, the size quantization effect or quantum size effect arises

due to the confinement of charge carrier movement.[33] Thisleads to an increase in band gap energy by a shift of the con-

duction band edge to more reducing and the valence bandedge to more oxidizing potentials.[34] In addition, bending of

the band structure at the semiconductor/electrolyte interface,

which results in the space charge layer, is manipulated bya nanostructured morphology. As a consequence, nanoparti-

cles can have almost flat bands, which work against the sepa-ration of electrons and holes.[35]

Catalyst properties derived from the crystal structure that in-clude charge carrier processes are related to the ordered ar-

rangement of atoms in the catalyst. A crystalline semiconduc-

tor allows a faster diffusion of charge carriers than an amor-phous material.[36] Similarly, the number of defects is related to

the crystal structure, which strongly influences the recombina-tion rate of electron–hole pairs. Important features such as

long-term stability, photo corrosion, and thermal stability,which depend on the previous parameters, are assigned to the

overall stability of the catalyst. Catalyst stability depends

strongly on the experimental conditions and semiconductormaterial. The stability of semiconductor can be increased by

the deposition of a thin protective layer (e.g. , TiO2) that allowscharge transfer but inhibits ion transfer[5b, 37] or the depositionof co-catalysts that act as reactive sites and thus avoid catalystablation.[22, 38] Although it is possible to control each parameter

separately to a certain extent during synthesis, these parame-ters are linked together and determine the overall performanceof the catalyst. Within the scope of this review, the aforemen-

tioned features of a semiconductor catalyst will be addressedin more detail.The abovementioned key properties can be con-

trolled to a certain extent by using different synthetic method-ologies to obtain nanomaterials. The preparation of nanomate-

rials can be achieved either through a top-down or a bottom-up approach; some of those common sub-methods are illus-trated in Figure 4.

Going by the name, top-down approaches start from largermaterials that get broken down into nanostructures, and

bottom-up methods start from atoms, molecules or clusters,and end up as larger particles (nanomaterials). A range of syn-

thetic methodologies have been established which fall under

one of the two categories. Top-down syntheses (Figure 4, top,from left to right) consist of ball milling of powdered sam-

ples,[39] lithography,[40] etching[41] (including anodization[42]), filmdeposition and growth,[43] hard templating (including nano-

casting[6, 44] and printing[45]) and ultrasonication breakdown,[46]

which find their application in electronics and chemical synthe-sis. In analytics and advanced syntheses, the focused ion beam

ablation[47] is an example of a top-down approach.There are certain advantages and disadvantages of top-

down and bottom-up approaches. Nanomaterials produced viathe top-down approach usually show a large number of crys-

tallographic defects on the surface due to mechanical forces

that are applied to break down the structure.[48] As the surfacearea per unit volume is very high for nanomaterials, these de-fects can play a significant role in the catalytic performance ofthese nanomaterials.[48] Particularly in photocatalysis for watersplitting, these surface defects can have a negative effect onthe catalyst activity as defects are potential carrier recombina-

tion centers. In detail, the different top-down syntheses haveboth advantages and disadvantages, which need to be consid-ered when a method is chosen for the preparation of nanoma-terials. Ball milling is one of the simplest (experimentallystraightforward) methods and inexpensive. It is useful to fabri-

cate nanostructured surfaces by mechanical attrition, but themethod does not allow for a production of controlled particle

morphologies,[49] and the minimum particle size that is possible

to obtain is about 100 nm.[50] Furthermore, ball milling can in-troduce impurities to the sample, and only 2 % of the energy

input goes into creating new surfaces.[50]

Using ultrasonication breakdown of macrostructures to yield

nanomaterials avoids the introduction of contaminants, al-though the process needs separation from the liquid phase,

Figure 4. Illustration of top-down and bottom-up approaches towards nano-materials. The top-down approaches include, from left to right, ball milling,lithography, etching, film deposition and growth, hard templating (includingnanocasting and printing), ultrasonication breakdown, and focused ionbeam ablation. In the bottom-up approaches, from left to right, self-assem-bly of molecules and clusters (e.g. , soft templating), hydrothermal and solvo-thermal synthesis, sol–gel processing, thermal decomposition (e.g. , spray py-rolysis), deposition methods (chemical vapor deposition, sputtering, atomiclayer deposition), and thermal reduction methods (e.g. , polyol synthesis) areused to produce nanostructured materials.


Focus Review


purification, and drying. Furthermore, continuous processesare difficult to establish.[49] Other abovementioned methods, in-

cluding lithography, etching, anodization, and film depositionand growth are more advanced, precise, and controllable pro-

cesses that can be used to produce fine nanostructures.Among those, vertically aligned nanowires have been demon-

strated to possess high activity as water splitting catalysts.[51]

However, these methods are not easily accessible for large-scale production. Set aside from the other top-down ap-

proaches, nanocasting is a method that is used to producea wide range of nanomaterials directed by the employed tem-plate and synthesis conditions.[52] By this versatile method,products ranging from fine nanoparticles to symmetrical nano-

architectures with ordered porosity can be obtained.[52a, 53] Onthe downside, the method is time-consuming, and it requires

synthesis and removal of the hard template.

From a catalytic point of view the bottom-up approach issynthetically more feasible and relevant, in particular for large-

scale synthesis, and nanomaterials can be easily produced inany laboratory following one of the bottom-up methods. Addi-

tionally, as the main driving force behind this process is the re-duction in Gibbs free energy, it produces nanomaterials that

are close to their equilibrium state.[48] The most common meth-

ods in this group (Figure 4, bottom, from left to right) are self-assembly of molecules and clusters[44a, 54] (e.g. , micelle forma-

tion in soft templating), hydrothermal and solvothermal syn-thesis,[55] sol–gel processing,[56] thermal decomposition (e.g. ,

spray pyrolysis[18a, 57]), deposition methods[58] (chemical vapordeposition,[59] sputtering,[60] atomic layer deposition[61]), and

thermal reduction methods (e.g. , polyol synthesis[62]). Among

these methods, hard and soft templating and thermal reduc-tion approaches are used frequently in order to prepare

a more defined structure to evaluate the influence of key pa-rameters on a catalyst individually. For instance, we could dem-

onstrate the importance of morphology, symmetry, geometry,textural parameters (surface area, pore volume), crystal struc-

ture, and composition of cobalt oxide-based materials for

water oxidation (individually) by using the soft and hard tem-plating methods.[6b, 63] The hydro- or solvothermal synthesis

allows, next to nanocasting (top-down), the preparation ofhighly crystalline materials with controlled morphology and

crystal facets. By changing the experimental conditions, themorphology of the particles including size and shape, and crys-

tallinity can be varied.[55e, f, 64] However, the batch size of eachsynthesis is limited to the size of the pressure-withstandingvessel (autoclave), which is required for the hydro- and solvo-

thermal synthesis. Other methods, such as sol–gel synthesisand flame pyrolysis allow for the preparation of amorphous

and crystalline nanoparticles and nanostructured materials ona large scale. Additionally, they offer control over the morphol-

ogy of the produced materials to a certain extent.[57b, 65] Deposi-

tion methods are used for the preparation of thin nanostruc-tured layers or the deposition of nanoparticles on a substrate.

Thus, the preparation of fine structures is possible with thesetechniques, but the preparation of nanocatalysts solely by this

method is usually inefficient, as the deposition rates are low.However, there are also large-scale applications possible by flu-

idized bed chemical vapor deposition.[66] A careful choice ofsynthetic approaches can control some physical properties of

the materials and can give a substantial insight towards theunderstanding of photo- and photoelectrocatalytic processes

and water splitting.

5. Significant Properties for Photocatalytic Pro-cesses and Water Splitting

Herein, we focus on some of the effects and selected properties,

which in our opinion are of utmost importance in order todesign efficient nanocatalysts for solar water splitting. The fol-

lowing discussion of essential parameters is inspired by earlier re-

views; for an in-depth understanding and more detailedinformation, the readers are advised to previous review

articles.[5a,8a, b,67] Due to the sequence of a photocatalytic process,the discussion is presented in the following logical order: 1) In-

teraction of the catalyst with light, 2) formation (and possible re-combination) of excitons, 3) charge carrier recombination, sepa-

ration, and diffusion, and 4) catalytic reaction on the surface.

5.1. Interaction between light and semiconductor

When the interaction between the catalyst and light is investi-

gated, the most significant parameter is light absorption inorder to drive the photocatalytic reaction. The chain of events

of a photocatalytic reaction starts with the light interaction(e.g. , light absorption), which depends on the electronic and

crystal structure of the catalyst, more precisely, the band gap(Eg) of the semiconductor between the valence band (VB) and

the conduction band (CB), morphology, dimension, particle

size, etc.[8a, b, 68]

In order to initiate a photocatalytic reaction, a semiconductor

is illuminated by light of a suitable wavelength that has thepotential to excite an electron from the VB into the CB as illus-

trated in Figure 2. One of the criteria of the photocatalytic re-action is that the energy of the light has to match the Eg of

the catalyst. From the thermodynamic standpoint, water split-

ting can be achieved by 1.23 eV under standard conditions[Eq. (2) and (3)] . However, energy loss processes are involved

in the reaction, including entropy energy losses (calculated tobe about Eg/4), kinetic overpotentials, and reaction overpoten-tials (0.2–0.4 eV) due to the four-electron transfer reaction ofthe oxygen evolution reaction.[69]

The intrinsic electronic transitions of a semiconductor arecharacterized by the absorption coefficient (a). The absorptioncoefficient also depends on the wavelength of the incidentlight. The light with shorter wavelength is absorbed closer tothe surface of the catalyst, and light with longer wavelength

can travel further into the bulk[67, 70] and can be determinedthrough transmittance and reflectance measurements [Eq. (6)]:

a ¼ 1d¡ ln ð1¢ RÞ2

T

� �ð6Þ

with T = transmittance, R = reflectance, and d = thickness of the

absorbing sample. A detailed derivation of the equation hasbeen well presented by Takanabe.[67]


Focus Review


For the synthesis of photocatalysts it is of importance thatthe particle size should not be smaller than the absorption

depth, as incident light transmits through particles of small di-mensions and light absorption is reduced.

To enhance the light-absorption and -harvesting efficiencyof the photocatalyst, the band gap of the material can be ma-

nipulated by using various strategies, such as doping with for-eign atoms to increase the number of states close to the CB or

VB,[71] partial change of the oxidation state of atoms in the

semiconductor,[72] alteration of particle size,[31, 73] or couplingwith other systems that can undergo energy or electron trans-fer and absorb light of different energy,[74] for example, plas-monic excitation.[26c, 43, 75] These band gap engineering tech-

niques are widely discussed and used to match the redox po-tentials of water reduction and oxidation and to enhance light

absorption. Furthermore, it can lead to the generation of exci-

tons through excitation with a desired wavelength, such asvisible-light excitation of semiconductors that typically respond

to UV-light excitation.[74]

Apart from the electronic structure, physical properties of

a catalyst can also strongly influence light absorption. Whenlight penetrates through the photocatalyst particles in a photo-

reactor vessel, the material partially absorbs the photons that

enter into the bulk of the particle. However, photons are alsoreflected and scattered on the surface of the catalyst. If parti-

cles are smaller than the penetration depth of the light, theybecome transparent. Reflected and scattered light can be ab-

sorbed by a second particle, which effectively increases lightabsorption. To increase the chances for absorption of scattered

light, the particle concentration in the solution as well as the

roughness of particle surface can be increased. As shown inFigure 5 a, a flat-surfaced catalyst reflects and scatters light into

one direction, whereas for a rough catalyst surface scatteringoccurs in all three directions (Figure 5 b). When small particles

with a rough surface are suspended and illuminated with light,absorption can occur from all sides of the catalyst, also from

scattered light, which strongly increases the light-absorption

and -harvesting efficiency (Figure 5 c).[8a] Furthermore, somemorphologies of the material, such as hollow micro- and nano-structures, can trap the incident light, which increases theextent of reflection, scattering, and absorption that can en-

hance the material’s light-harvesting efficiency.Light absorption can further be influenced by combining

the catalyst with an additional light-harvesting system, whichis known for dye sensitized solar cells (DSSC)[76] as well as het-erojunction structures.[26d] Another example is the utilization of

surface plasmon resonance where plasmonic metal particles

are deposited on the surface of a semiconductor catalyst.[26c, 77]

As mentioned before, this combination enables a UV-active

photocatalyst to drive catalytic reactions under visible-light ex-citation by exploiting the plasmonic effect.[75a]

5.2. Formation and recombination of excitons at semicon-ductor

After light absorption, the next step of the photocatalytic pro-

cess is exciton or charge carrier formation, which is followedby separation/transportation (discussed in the following chap-

ter) or the undesired recombination of the carriers. Due to thelight-absorption depth of a catalyst, the excitons are generated

in the bulk of the semiconductor. In order to undergo water

splitting, first the charge carriers have to diffuse to the surface.Here, electrons drive water reduction and holes drive water ox-

idation reactions. As diffusion paths to the surface bear recom-bination centers for the charge carriers, the diffusion path

length and charge carrier mobility are determining parametersfor the performances of photocatalysts.

The charge carrier recombination diminishes the mobility of

the carriers. The recombination mechanism depends on thetype of semiconductor, but in general recombination takesplace on defect sites in the structure of the bulk, on interfaces,and on the surface.[78] While in direct semiconductors the

band-to-band recombination mostly takes place, resulting ina radiative process, recombination in indirect semiconductors

occurs through defect levels that lead to thermal exchange,

which is similar to the recombination on surface and phaseboundaries.[78] The semiconductor materials need to be de-

signed for better charge separation and transportation andless recombination, which are the most crucial properties for

overall water splitting. This can be achieved to a certain extentby varying the morphology, dimensions, shape, and size of the

photocatalysts.

After an effective separation, the carriers have to diffuse tothe surface of the catalyst in order to undergo a catalytic reac-

tion. However, a long diffusion path can lead to a loss of carri-ers as the number of potential recombination sites increases

with increasing diffusion path. The diffusion path length to thecatalyst surface can be controlled by reducing the dimensions

of the catalyst particle. As shown in Figure 6, the preparationof nanometer-sized particles (Figure 6 b) or nanowire assem-blies (Figure 6 c) provide short diffusion paths of charge carri-

ers to the particle surface compared to bulk particles (Fig-ure 6 a).[51] In bulk particles, the diffusion length can limit the

activity of the material as the distance to the nearest surfacearea is long and recombination

can hinder a successful diffusionof the charge carriers. One of

the disadvantages of nanosized

catalysts is that the charge trans-port to the surface entails more

risks of undesired processes thanin bulk materials due to a higher

number of surface defects. Fur-thermore, the space charge layer

Figure 5. Reflection and scattering paths on (a) a flat catalyst surface, (b) a rough catalyst surface, and (c) dis-persed catalyst particles. The figure is partly adopted from ref. [8b]. Copyright 2013, The Royal Society of Chemis-try.


Focus Review


or depletion layer, which is typically formed at the catalyst/liquid interface, cannot effectively operate on the nanoscale.

Usually, the depletion layer is formed due to the equilibrationof the Fermi levels of the semiconductor and the electrolyte.

This leads to a band bending of the valence and conductionband of the semiconductor at the interface, thus forming an

energy barrier. This barrier supports charge carrier separationin bulk particles and suppresses recombination. The size of the

space charge layer, however, exceeds the size of ultrafine

nanoparticles. Therefore, the formation of an effective spacecharge layer or energy barrier is prevented, resulting in flat

bands for nanomaterials. In the absence of an energy barrier atthe catalyst/liquid interface, the recombination rate of charge

carriers is higher and the number of back reactions (of H2 andO2 into H2O) increases.[79]

5.3. Approaches for separation of excitons and hindrance ofcharge recombination

As mentioned before, light absorption is the most critical step

in a photocatalytic process. However, the performance ofa photocatalyst mainly depends on the charge generation and

separation as charge recombination limits the efficiency of the

photocatalyst. In order to avoid recombination, e¢ and h+

need to be separated after their generation. Strategies to ach-

ieve separation are co-catalyst decoration on the surface of thephotocatalyst and the creation of heterojunctions that alterthe electronic structure at the interface (e.g. , band bend-ing).[22, 80] This way, the photocatalytic activity towards water

splitting can be strongly enhanced.Band bending is caused by the creation of an electronic

structure at the interface which can lead to selective trappingof electrons and holes. Depositing metal particles on the sur-face of a photocatalyst forms either a barrier, the so called

Schottky barrier, or an interface called Ohmic contact, depend-ing on the work function of the co-catalyst and the Fermi level

of the semiconductor.[22, 81] For example, noble metal (e.g. , Pt)

nanoparticles form a Schottky barrier with most semiconduc-tors and extract electrons from the semiconductor to generate

H2.[82] The withdrawal of holes is usually done by metal oxidesor sulfides that donate electrons into the VB of the semicon-

ductor and function as oxidation sites.[83] This way, electrons orholes are attracted and can be trapped separately. The design

of a co-catalyst is crucial for solar water splitting and suitablecandidates are prepared and tested in electrochemical watersplitting half reactions.[6a, 25] In this manner, Nocera et al. devel-oped a cobalt-based water oxidation catalyst that formed in

situ from Co2 + and phosphate electrolyte.[84] Nanostructuredcobalt oxide water oxidation catalysts prepared by nanocasting

displayed a superior activity over bulk and nanoparticleCo3O4.[63] For the water reduction half reaction, noble Pt andRh metals are the most active co-catalysts; however, since they

are not abundant, their application on a large scale is not sus-tainable. Alternatively, cathodes consisting of an abundant ma-terial have been investigated. For example, Jaramillo et al. de-veloped molybdenum sulfide nanomaterials for the water re-duction reaction that can function as a hydrogen evolution co-catalyst.[25]

The use of semiconductor heterojunctions to ach-

ieve charge separation relies on the positions of theVB and CB of the semiconductors. As depicted in

Figure 7, in a system consisting of semiconductor Aand semiconductor B, electrons can be concentrated

in semiconductor B, if the conduction band of B islower than that of A. Efficient charge separation is re-

alized when the valence band of B is also lower than

of A, which allows holes to diffuse to A. These strat-egies are widely applied in photocatalysis to enhance

the material’s performance.[26d, 85] The morphologyand dimension of the materials can also effect the

charge separation as discussed above. For instance,the introduction of a nanowire morphology has been

shown to improve the performance of photoanode

materials.[86] The nanowire morphology (as depictedin Figure 6 c) can enhance charge collection and separation,

and provides a large surface area for catalytic centers and alsoco-catalyst loading. The length of the nanowires can increase

the light absorption while their shorter diameter allowsa faster charge transfer to the surface, which lowers the possi-

bility of charge recombination.[51]

It is generally accepted that the diameter of particles shouldbe as small as possible to create short diffusion paths, howev-er, not smaller than the light absorption depth. Otherwise, thephotons transmit through the particles without excitation of

Figure 7. Charge separation in a photocatalyst composite, semiconductor Aand B. The valence and conduction bands of B are lower than that of A, andthus electrons diffuse to B and holes diffuse to A.

Figure 6. Illustration of light penetration into semiconductor with a) bulk, b) nanoparti-cle, and c) vertically aligned nanorod morphology. Electron and hole formation, separa-tion, diffusion path length, and recombination are illustrated.


Focus Review


excitons. The mobility of charge carriers—which is one of thekey parameters for the performance of the material—depends

on the crystal structure, temperature, composition, crystallo-graphic defects, carrier lifetime, and effective mass of the carri-

er. The mobility (m) of e¢ and h+ is used to describe the diffu-sion coefficient (D) of a material through the Einstein relation

[Eq. (7)] and depends on the charge carrier life time (t) and ef-fective mass (m*) [Eq. (8)]:[87]

D ¼ kBTe

m ð7Þ

m ¼ et

m*ð8Þ

with kB = Boltzmann constant and e = elemental charge. Carrier

diffusion is influenced by internal electric fields due toa charge concentration gradient and external electric field. As

shown in Eq. (8), charge carriers with small effective mass have

a higher mobility ; therefore, electrons can travel faster thanholes. P- or n-type semiconductor catalysts can be character-

ized according to their minority charge carrier mobility to bean effective catalyst or not, as solar water splitting can only be

performed when both e¢ and h+ diffuse to the surface to un-dergo reduction and oxidation reactions, respectively. Briefly,the degree of the charge separation can be boosted to a cer-

tain extent by varying the physical and chemical properties ofphotocatalysts like morphology, textural parameters, dimen-sion, and shape in addition to creating heterojunctions in thestructure.

5.4. Consequence of interfacial and interparticle chargetransfer

Interfacial charge transfer from solid–solid or solid–liquid inter-

faces is required for photo- and photoelectrocatalytic water

splitting as charge carriers travel through the catalyst to thesurface. Nanocatalysts possess a large specific surface area that

promotes charge transfer, which makes them more effectivefor water redox reactions. Especially water oxidation, as it

needs the transfer of four electrons in a multistep reaction, isboosted by increased interfacial charge transfer.[88] However,a large surface area, for example, large catalyst/electrolyte in-terface area, also promotes the probability of back reactions.

As a consequence, the overall efficiency of nanocatalysts in-creases with a large surface area; however, reducing the junc-tion area by solid depositions (inert coatings, co-catalyst, core–shell geometry) on the catalyst surface can additionally in-crease the activity by suppressing back reactions.[89]

For photoelectrocatalytic water splitting, the preparationmethod of electrodes is crucial for charge transfer from the

catalyst into the electrode. A high interparticle charge transfer

is enabled when good contact between the dispersed catalystparticles is realized. Therefore, post-treatment of electrodes in-

clude calcination of the nanomaterials attached to the elec-trode in order to increase the contact through a low degree of

sintering and removal of a passivating oxide layer around eachparticle. Similar techniques are applied when catalysts are

grown onto the electrode and decorated with a co-catalyst.The final system is calcined to increase the contact area and

ensure interparticle charge transer across photoelectrocatalystfilms.[90] Charge transfer through a film made of partly sintered

nanoparticles is more efficient than through large-scale parti-cles, since the nanoparticle film is denser and nanoparticles

sinter at lower temperatures, thus creating a better interparti-cle contact.

Shape-controlled nanocrystals can also influence the catalyt-

ic process[91] as they result in different interfacial charge trans-fer. Due to their anisotropic behavior, tailored shapes of co-cat-alysts have shown a positive effect on solar water splitting. Ashape effect of Pt co-catalyst on the photocatalytic activity ofPt/CdS for hydrogen production was reported.[92] Cubic Pt par-ticles with an average particle size of 5.7 nm on CdS showed

a 52 % higher efficiency than that of spherical Pt particles and

25 % improvement over photodeposited Pt nanoparticles. Ashape effect of photocatalysts on the hydrogen production

rate was also observed by Li et al. where an increase in activityfrom nanorod to hollow nanosphere to solid nanosphere mor-

phology was reported.[93] For Pt/TiO2, a shape effect on thephotocatalytic hydrogen evolution was determined between

nanosphere and nanorod morphology.[94] TiO2 nanorods exhibit

an increased charge transfer between the particle surface andelectrolyte, resulting in a 1.6 times higher hydrogen produc-

tion rate than that of nanospheres. Shape control of photoca-talysts or co-catalysts influences the interfacial charge transfer

and thus can play a role in the design of a highly active cata-lyst towards solar water splitting.

6. An Overview of Specific Examples of Nano-catalysts for Solar Hydrogen Generation

The above described issues of nanocatalysts for solar water

splitting are tackled by many research groups in order to fur-ther enhance the activity of catalyst systems and to develop

novel materials and approaches. Herein, a number of examples

is given that illustrates the challenges and accomplishmentsmade using nanosized and nanostructured photo- and photo-electrocatalysts towards a clean hydrogen evolution. It needsto be kept in mind that herein only a few examples have been

chosen; for a larger gallery of photocatalysts, the reader is re-ferred to more specific review articles for water splitting.[5a, 8b, 95]

6.1. Photochemical water splitting

One of the most active powdered photocatalyst for water split-ting was reported by Kudo et al. in 2003 where it was shown

that the formation of nano-steps on the surface of the photo-catalyst strongly increases its performance.[96] Doping of

a NaTaO3 perovskite structure with up to 5 mol % of La resulted

in a material with much smaller particle size and nano-stepsurface morphology (Figure 8). After NiO deposition primarily

on the fringes of the step-shaped surface, water reduction andoxidation sites were effectively separated and in close space at

the same time. This effect was described by the authors asbeneficial for photocatalytic water splitting, as the charge carri-


Focus Review


er diffusion length is extremely short and recombination is re-duced due to separation of the reactive sites. Doping with

2 mol % La and loading of 0.2 wt % NiO as a co-catalyst

showed the highest activity with H2 and O2 production rates of19.8 and 9.7 mmol h¢1, respectively. This combination had

a maximum apparent quantum yield of 56 % at 270 nm, whichis one of the highest values for UV materials with highly stable

activity for over more than 400 h.The advantage of the modification of the surface of a photo-

catalyst was also demonstrated for TiO2 by Mao et al. to in-

crease light absorption and to tailor the electronic bandgap.[72b] Titania nanocrystals with an approximate size of 8 nm

(Figure 9 a) were prepared and treated under 20 bar H2 atmo-sphere at 200 8C for 5 days. After the H2 treatment, the initially

white-colored nanocrystals exhibited a disordered, amorphousshell (Figure 9 b) and turned black. An alteration of the elec-

tronic structure, as illustrated in Figure 9 c, was the cause of

the color change, which enabled the newly obtained, disorder-engineered black TiO2 to be photoactive under visible and in-

frared light. The material was tested under 1 sun irradiation

over 22 days and produced around 1 mmol of hydrogen overfive hours each day (Figure 9 d). In this example, the titania

nanocatalyst profits from a surface modification that allowsphotoexcitation by solar light due to the formation of an

amorphous, disordered shell with potential trapping statesthat prevent carrier recombination, as well as the benefits ofa crystalline core, and a nanoparticle dispersion with stronglight scattering.

The synthesis of nanoparticles in the confined space of

a matrix, for example, nanocasting, can be utilized to achievea catalyst with nanosized morphology and a high surface area.

Much work in this field is pursued in the research group of theauthor.[6, 44d, 54e, 63a, c, 97] Furthermore, protecting a catalyst in the

structure of a host (pore confinement) can preserve the mor-phology under post-treatment conditions. For example, nano-

structures are prone to sintering at high temperatures, which

are necessary for crystallization (under non-hydrothermal con-ditions) or anion exchange of oxides under NH3 atmosphere.[98]

Takanabe et al. used the synthesis in a confined space for thepreparation of Ta3N5

[99] and TiN nanoparticles.[100] For this pur-

pose, they prepared porous C3N4 that functions as a host, inwhich the nanoparticles can grow, and as a nitrogen donor to

obtain the desired nanoparticle metal nitrides. Using ordered

C3N4 with different pore sizes (20–80 nm) as hard template,a sample series of regularly arranged Ta3N5 nanoparticles with

different surface areas (60–36 m2 g¢1) was prepared (see Fig-ure 10 a–c as an example). Under visible-light irradiation, the

sample with the highest surface area was the most activesample, generating 8 mmol of hydrogen over 6 h with 3 wt% Pt

as co-catalyst in 10 vol % methanol aqueous solution (Fig-

ure 10 d), which is 4 times higher than the activity of conven-tional Ta3N5 macroparticles. As described by the authors, the

reason for the enhanced activity was predominantly the highsurface area and thus a high

number of active surface sitesand a low number of defects

due to a crystalline structure.[99]

On the other hand, 12 nm-sizedTiN particles that are attached tocarbon black particles (Fig-ure 10 e) were prepared and

tested as a electrocatalyst forthe oxygen reduction reaction.

They were synthesized by repli-cating a mesoporous C3N4,which was prepared via nano-

casting of 12 nm-sized silicaspheres. The fine dispersion,

nanosize, and contact of the TiNparticles to carbon black were

distinctive features why this

compound was highly active.[100]

Besides being a hard tem-

plate, the usage of the C3N4 asphotocatalysts for hydrogen pro-

duction under visible light hasbeen demonstrated as well.[101]

Figure 8. La-doped NaTaO3 particles with step-like surface structure whereNiO co-catalyst is mainly deposited on the outer edges of the steps. There-fore, water oxidation and reduction sites are effectively separated. Reprintedwith permission from [96]. Copyright 2003, American Chemical Society.

Figure 9. HR-TEM images of TiO2 nanocrystals a) before and b) after H2 treatment. The arrows indicate the shell ofthe disordered, amorphous structure. c) Schematic illustration of the density of states (DOS) of the black TiO2 andunmodified TiO2 nanocrystals and d) hydrogen production of black TiO2 in 1:1 water/methanol solution under1 sun irradiation. Reprinted with permission from [72b]. Copyright 2011, AAAS.


Focus Review


Very recently, Kang et al. made use of C3N4 due to its semicon-ducting nature and as a host for carbon nanodots (CDots) with

a diameter of 2–10 nm (Figure 10 f). They prepared a compositeof CDots and C3N4, which is able to split water with a STH effi-

ciency of 2 % over a total time of 200 days, proving an impres-

sive activity and stability of a low cost and earth-abundantphotocatalyst for solar water splitting.[102]

The limit of particle size of a photocatalyst for overall watersplitting was determined by Osterloh et al. where NiOx was

decorated on bulk and nano SrTiO3 with 30 and 6.5 nm particlesize (Figure 11).[103] The three different particle sizes were pre-pared through three different methods, namely solid–solid

state reaction, hydrothermal synthesis, and vapor diffusion sol–gel technique, respectively. All three methods gave crystallineand phase pure SrTiO3 samples. Bulk NiO-SrTiO3 showed thehighest activity, evolving hydrogen at a rate of 28 mmol g¢1 h¢1,followed by 30 nm size NiO-SrTiO3 that produced

19.4 mmol g¢1 h¢1 of hydrogen,and 6.5 nm NiO-SrTiO3 with a H2

rate of 3.0 mmol g¢1 h¢1. To inter-pret this trend, they correlated

water reduction and oxidationoverpotentials with particle size

and observed that the hydrogenreduction runs best with small

particles, whereas water oxida-

tion succeeds best with largerparticle size. In addition to

higher oxidation overpotentials,the nanometer-sized samples

show a shift of the conductionband edge to more negative en-ergies and a widening of the

band gap that reduces thenumber of absorbed photons,which is attributed to the quan-tum size effect. In conclusion,

the authors suggest that catalystparticles of SrTiO3 should be

larger than 30 nm to obtain

a better performance. These re-sults indicated that a smaller

particle size and higher surface area are not crucial require-ments for all semiconductor materials as efficient water split-

ting catalysts.A catalyst study that suggested the influence of mixed crys-

tal structures and heterojunctions on the photocatalytic activi-

ty towards hydrogen production was reported by our researchgroup.[55e, f] A series of tantalum-based oxides was obtained by

hydrothermal synthesis starting from a tantalum alkoxide pre-cursor, a sodium source, and a base. In system I, the prepara-

tion of samples containing crystalline Na2Ta2O6, highly amor-phous sodium tantalum oxide, and mixtures of both phaseswas described. The samples with amorphous phase were

found to be more active than crystalline samples, which wasattributed to the smaller particle size and higher surface areathat provide shorter charge transport lengths and more cata-lytic centers, respectively.[55e] A mixed sample that consists ofcrystalline Na2Ta2O6 and amorphous tantalum oxide showedthe best photocatalytic performance, which was ascribed to

heterojunctions between two different phases that facilitatecharge separation. In system II, a set of samples containingcrystalline Na2Ta2O6, NaTaO3, and mixtures of both phases was

prepared. The different crystal structures led to nanoparticleformation of Na2Ta2O6 and cubic microparticles of NaTaO3

(Figure 12). The nanoparticles of Na2Ta2O6 exhibit the highesthydrogen production rate (64.1 mmol h¢1 g¢1) due to a high

surface area that provides a high number of active sites and

small particle size which leads to more efficient charge carriertransfer to the particle surface. Moreover, a heterojunction

effect was observed for the composite-2 sample containingboth phases, which showed higher photocatalytic activity than

phase-pure samples with similar surface area (Figure 12 b).[55f]

The trend that was shown by the two systems is that particle

Figure 10. Field-emission SEM images of a) an ordered arrangement of 80 nm silica spheres, b) the respectiveporous C3N4 replica, and c) regularly arranged Ta3N5 nanoparticles that were templated from the porous C3N4

structure. d) Hydrogen production of all Ta3N5 samples is presented, given by the size of the silica spheres thatwere used to produce the porous C3N4 matrix. Reprinted with permission from [99]. Copyright 2011, John Wiley &Sons. e) TEM image of TiN nanoparticles on carbon black particles. Reprinted with permission from [100]. Copy-right 2010, The Royal Society of Chemistry. f) TEM image of CDots embedded in a matrix of C3N4. Reprinted withpermission from [102]. Copyright 2015, AAAS.

Figure 11. a) Illustration of nanosized SrTiO3 catalysts loaded with NiOx forwater splitting. b) The system suffers from deactivation with decreasing par-ticle size. Reprinted with permission from [103]. Copyright 2012, AmericanChemical Society.


Focus Review


size and surface area of the tantalum-based materials are more

dominant factors for photocatalytic activity than crystal struc-ture and crystallinity.

The above examples indicate the significance of the designof nanocatalysts. However, most of these synthetic methodolo-

gies are very energy- and time-consuming. Therefore, it is im-portant to use an easy handling approach to prepare nanoma-

terials. Recently, we developed a facile approach for the prepa-ration of nanostructured amorphous photocatalysts.[104] Themethod is based on direct injection of metal alkoxide precur-

sors into the reaction solution inside the photoreactor. Whenapplying the direct injection, alkoxide precursors undergo fast

hydrolysis and form metal oxo hydroxo compounds that act asphotocatalysts for hydrogen production. In this study, tantalum

and sodium tantalum oxo hydroxo materials were produced

that showed different activities depending on the surface areaand sodium content (Figure 13). Injecting Ta(V) ethoxide into

the reactor solution (water/methanol, 10 vol %) formed parti-cles with a surface area of 65 m2 g¢1 and a hydrogen produc-

tion rate of 1.7 mmol h¢1. Pre-dispersing the tantalum precur-sor in methanol before injection gave a composite with

a higher surface area of 112 m2 g¢1 that was able to evolve hy-drogen at a rate of 2.2 mmol h¢1. A sodium tantalum compo-

site was obtained when sodium ethoxide was injected at thesame time as tantalum ethoxide. This sample had a similar sur-

face area as the tantalum oxo hydroxo compound but an in-creased activity generating 3.8 mmol h¢1 of hydrogen. It is im-

portant to state that these amorphous materials had a much

better activity than their crystalline bulk counterparts in addi-tion to ordered mesoporous Ta2O5 that requires a preparation

time of almost one week.

6.2. Photoelectrochemical water splitting

The development of photoanodes for photoelectrochemical

water splitting has been the interest of many researchers. Vande Krol et al. developed a top-down method for the prepara-

tion of nanostructured monoclinic WO3 photoelectrodes bya plasma-processing technique of tungsten substrates and

thermal post-processing (Figure 14).[47a] Highest photocurrents

were measured for the nanostructured, porous WO3 disc of upto 1 mA cm¢2, which was five times higher than that of dense,

nonporous WO3 samples. Furthermore, the porous WO3 exhib-its an incident photon-to-charge carrier efficiency (IPCE) of22 % at 380 nm. Due to the top-down approach, the WO3 crys-tallites have better contact and thus avoid conductivity limita-

tions that can occur due to gaps between particles or passiva-tion layers.

Domen et al. reported the preparation of a barium-dopedtantalum nitride nanorod photoanode via a bottom-up ap-proach that yielded an ordered array of nanorods.[38a] Doping

with Ba and modifying the surface with cobalt phosphate asoxygen evolution co-catalyst greatly improved the activity for

hydrogen production via photoelectrochemical water splitting(Figure 15). The photocurrent density was 1–5.5 mA cm¢2,

which is an increase of 50–300 % compared to that of the un-

doped sample. A maximum solar energy conversion efficiencyof 1.5 % was achieved, which was three times higher than that

of reported systems on single-photon photoanodes. Overall,the system generated a total amount of 130 and

63.5 mmol cm¢2 of H2 and O2, respectively, over 100 min (Fig-ure 15 c). The enhanced activity of the nanorods was described

Figure 12. SEM images of Na2Ta2O6-2 (a), composite-2 (b), composite-3 (c),and NaTaO3-HT (d). Hydrogen production rates (e) of the sodium tantalatesprepared by a hydrothermal route and the reference material NaTaO3 pro-duced via a solid–solid state. The samples labeled as Na2Ta2O6-1 and -2 con-sist of crystalline Na2Ta2O6 nanoparticles with a size of 15 and 27 nm, respec-tively. Mixed crystalline phases of Na2Ta2O6 and NaTaO3 were determined forcomposite-1, -2, and -3 with an increasing amount of NaTaO3 and decreasingamount of Na2Ta2O6 phase. Sample NaTaO3-HT is pure NaTaO3 synthesized bya hydrothermal synthesis. Reprinted with permission from [55f] . Copyright2013, Elsevier.

Figure 13. a) Scheme of the direct injection method. Here, tantalum ethox-ide is dropped into the reaction solution. Immediately, the hydrolysis of theprecursor starts and forms a tantalum oxo hydroxo compound. Then a lampis inserted into the reactor and switched on, which initiates hydrogen gener-ation. b) Hydrogen generation rates of tantalum and sodium tantalum com-pounds prepared via direct injection and reference tantalum oxides. Reprint-ed with permission from [104] . Copyright 2015, John Wiley & Sons.


Focus Review


as a consequence of synergistic effects of enhanced interlayer

conductivity, cathodically shifted flat band potential, and in-

creased electron density in the nanorods due to Ba doping ofthe tantalum nitride structure.

A study dealing with the material design of nanocatalystsfor photochemical water splitting was reported by Yang et al.

by combining Si and TiO2 nanorods.[105] In this system, TiO2

functions as a UV absorber and Si as a visible-light absorber.

Under simulated three-sun illu-mination, the system showeda stable photocurrent of0.7 mA cm¢2, generating H2 and

O2 at rates of about 20 and10 mmol h¢1, respectively, over90 min (~13 and 7 mmol h¢1).This gives a Faradaic efficiencyof 91 % with respect to the

charge passed through the cir-cuit. As the combination ofthese electrode materials wasable to split water, they com-bined both materials with nano-rod morphology by synthesizing

nanotree heterostructures,where Si nanowires present thetrunks with branches made of

TiO2 nanowires (Figure 16). Byvariation of the TiO2 coverage,

the efficiency of the system wasoptimized and reached a total

energy conversion efficiency of

0.12 %, which is in the range of natural photosynthesis. The au-thors concluded that the high surface area of TiO2 branches,

the ohmic contact between TiO2 and Si phases, short diffusionpaths for charge carriers, and the harvest of both UV and visi-

ble light are benefits of this catalytic system.Utilizing simulated solar illumination, Smets, van de Krol and

co-workers have developed a photoelectrochemical cell based

on a BiVO4 photoanode combined with a thin-film silicon solarcell with the highest STH efficiency of 5.2 %.[18b] As shown in

Figure 17, the PEC cell with a W-doped BiVO4 photoanode is il-luminated by simulated sunlight. The transmitting light is

taken up by a Si based solar cell that generates electricity forthe photoelectrochemical cell. The design of the photoanodeis crucial in this system, as the FTO substrate has a textured

surface that acts anti-reflecting to the incident light and scat-ters photons back through the anode catalyst. Another impor-tant factor is the fabrication of the anode, which is a 250 nmthin layer of W-doped BiVO4 that is deposited on the FTO sub-

strate by spray pyrolysis with a W gradient to match thecharge carrier extraction and diffusion path length in the BiVO4

layer. The Si based solar cell was optimized to absorb the

transmitting light by the combination of amorphous and nano-crystalline silicon that creates a micromorph junction. The

5.2 % STH efficiency represent a new benchmark and is basedon the strongly enhanced light absorption of the BiVO4 photo-

anode due to the structured substrate and good charge carrierextraction.

Another progress in the field of BiVO4 photoanode-based

low-cost tandem cells for solar water splitting was reported byKamat et al. by utilizing organo–lead halide perovskites (OHP)

to prepare a solar cell.[19] As seen in Figure 18, the cell consistsof a BiVO4 photoanode (decorated with cobalt phosphate as

a co-catalyst) deposited on FTO, and a lead halide-based per-ovskite half-cell and Pt as the counter electrode.

Figure 14. a) Plasma generator used to modify the tungsten surface. b) As-made tungsten nanorods obtained byplasma treatment and c) annealed WO3 porous nanostructure. d) Photocurrent measurements of annealed WO3

and WOx nanostructure as-prepared and after a polishing finish. Reprinted with permission from [47a]. Copyright2013, American Chemical Society.

Figure 15. a, b) SEM images of vertically aligned Ba-doped tantalum nitridenanorods and c) H2 and O2 evolution of the photoelectrochemical cell at0.9 V vs. RHE (photoanode). Scale bars are 500 nm (in a) and 100 nm (in b).Reprinted with permission from [38a]. Copyright 2013, Nature PublishingGroup.


Focus Review


Incident photons with wavelengths smaller than 520 nm arebeing absorbed by the BiVO4 photoanode, which generates

charge carriers within the photoanode, where the generatedholes can oxidize water to form O2 at a co-catalyst. The trans-

mitted light with wavelengths greater than 520 nm is still suffi-ciently energetic to excite electrons into the CB of the OHP ofthe solar cell to exploit large parts of the ultraviolet and visiblespectrum. The photogenerated electrons are transferred via

a TiO2 layer from the OHP to a platinum electrode where theelectrons can reduce protons to yield hydrogen. Overall, the

system showed an STH efficiency of 2.5 % in a Na2SO4/Na2SO3

electrolyte solution. Designing a nanostructured perovskite

solar cell with enhanced light harvesting properties can be ad-

vantageous for the utilization of solar cell capabilities that willultimately boost the STH efficiency of tandem systems.

7. A Perspective of Hydrogen Economy

7.1. Introduction

Considering the growth in global primary power consumption(Figure 19) and in order to stabilize atmospheric CO2 levels,

a target of around 10 TW of carbon neutral power to be gener-ated by 2050 was set by the US Department of Energy

(DOE).[106] The utilization of naturally abundant energy and re-

sources such as wind, solar, and biomass for hydrogen andchemical production would bring us one step closer to achieve

fossil fuel independence.[107] The interest in hydrogen as anenergy carrier for a sustainable energy future has increasedsignificantly over the past decades. Apart from being the light-est and most abundant element, hydrogen (H2) produces only

water and heat when combusted with oxygen, contrary tofossil fuels which release CO2 upon combustion. Although the

energy density per mass of hydrogen is 142 MJ kg¢1, much

higher than for gasoline (44 MJ kg¢1), it has a significant lowerenergy density per volume (8 MJ L¢1 vs. 32 MJ L¢1 for gaso-

line).[108] Recently, the global hydrogen market was valued at255.3 billion cubic meters (USD 96.6 billion) in 2013 and is ex-

pected to increase to 324.8 billion cubic meters (USD 141 bil-lion) by 2020, partially due to the increasing demand for clean

transportation and the desulfurization of petroleum prod-

ucts.[109]

Apart from transportation, hydrogen has also the potential

to generate electricity and to be used in energy storage of in-termittent renewable-based technologies and in generators for

buildings.[111] However, implementation of these technologiesrequires not only major technological breakthroughs but also

Figure 16. a) Nanotree architecture showing Si trunks and TiO2 branchesdecorated with co-catalyst to generate H2 and O2 under visible-light and UV-light excitation, respectively. b) SEM image of a nanotree sample with TiO2

nanorods (green) attached to vertically aligned Si nanorods. Scale bar, 1 mm.Reprinted with permission from [105]. Copyright 2013, American ChemicalSociety.

Figure 17. Sketch of a PEC/PV device based on BiVO4 and silicon. Reprintedwith permission from [18b]. Copyright 2014, John Wiley & Sons.

Figure 18. A tandem cell consisting of a photoelectrochemical cell witha BiVO4 photoanode coupled to a lead halide perovskite-based solar cellwith a solar-to-hydrogen efficiency of 2.5 %. The figure was taken from anopen access article published under an ACS Author Choice License.[19]

Figure 19. Global energy consumption by fuel. The figure is arranged fromdata presented in the BP energy outlook 2035, summary tables, 2015.[110]


Focus Review


significant cost reductions as costs are still too high comparedto conventional fossil fuel-based technologies.[112]

Table 1 gives an overview of various hydrogen productiontechnologies and their production costs. Technological break-

throughs are especially needed in the fields of transportation,storage, and carbon capture (when hydrogen is produced

from fossil fuels), but also in the field of hydrogen generationfrom renewable sources where efficiencies are low (Table 1).[112]

This section will provide an overview of the most commonly

used hydrogen production technologies (from both fossil fuelsand renewable energy sources), hydrogen storage, hydrogendistribution, and their future prospects in a hydrogen econo-my.

7.2. Hydrogen production from fossil fuels

Currently, hydrogen is largely used for petroleum refining and

ammonia and methanol production (Figure 20), and is mainly(95 %) produced from fossil fuels via steam reforming (SMR),

partial oxidation (POX), or gasification of coal.[116]

7.2.1. Steam reforming

Steam reforming of natural gas is currently the most commonindustrial process for hydrogen production.[118] The SMR reac-

tion is highly endothermic (206 kJ mol¢1) and requires hightemperatures (700–1100 8C) and the presence of a catalyst

(nickel) to form hydrogen and carbon monoxide.[116a, 118] Addi-tional hydrogen can be produced through an exothermic(¢41 kJ mol¢1) water–gas shift reaction (WSG), performed at

lower temperatures (350 8C), which is then further purified toobtain a purity of 99.9 % or higher.[118, 119]

CO2 removal is an important step in the SMR process, how-ever, it comes with a penalty on the overall process efficien-

cy[120] just as the heat necessary for the endothermic reformingstep.[118] In order to reduce CO2 emissions from SMR, alternative

approaches such as a hybrid solar–redox process and solar

SMR have been reported.[121] Nonetheless, SMR is not the mostattractive production route to achieve a sustainable hydrogen

economy as gas reserves are finite.[118]

7.2.2. Partial oxidation and coal gasification

Partial oxidation is generally applied to heavy oil fractions andcoal, but can also be applied to methane and natural gas.[116a]

In this process, a nonstoichiometric quantity of oxygen is react-ed with the hydrocarbon fuel (e.g. , methane) to form syngas

and hydrogen. The process is exothermic, requiring tempera-tures above 1200 8C when no catalyst is used and around 800–

900 8C in the presence of a catalyst.[122] Compared to SMR, par-

tial oxidation is more flexible in terms of feedstock, and no ex-ternal heat supply is necessary due to the exothermic reaction.

The main disadvantage is that before the WGS reaction, onlytwo hydrogen molecules are produced per methane compared

to three in the case of SMR.[122]

It was reported that hydrogen produced from coal could be

delivered with costs of about $2.00–2.50 per kilogram at the

present at large scale, possibly lowered to $1.50 per kilogramin the future.[115] Gasification of coal has the potential for the

co-production of electric power and hydrogen from integratedgasification combined-cycle (IGCC) technology.[123] Unfortunate-ly, no commercial demonstrations have been reported yet.

7.2.3. Electrolysis

Water electrolysis is a mature technology and accounts forroughly 4 % of the world hydrogen production,[113] and is the

process where water molecules are split into pure hydrogenand pure oxygen using electricity in combination with an elec-

trolyzer. The two most common electrolyzers are alkaline (po-

tassium hydroxide electrolyte) and PEM (polymer membraneelectrolyte). Unfortunately, grid power is mainly used for the

electrolysis (up to 2000 kW per electrolyzer),[115] making thismethod fossil fuel-dependent and therefore not a sustainable

technology unless combined with renewable energy sources(see below).

Table 1. Hydrogen production technologies and their production costs,modified from [113] .

Technology Feedstock Efficiency Maturity H2 productioncosts ($/kg)

Steam reforming Hydrocarbons 70–85 % Commercial 0.75[b]

Partial oxidation Hydrocarbons 60–75 % Commercial 1.39[b]

Autothermal re-forming

Hydrocarbons 60–75 % Near term 1.93[b]

Plasma reform-ing

Hydrocarbons 9–85 %[a] Long term N/A

Biomass gasifica-tion

Biomass 35–50 % Commercial 1.21–2.42[b]

Aqueous phasereforming

Hydrocarbons 35–55 % Med. Term N/A

Electrolysis H2O + electricity 50–70 % Commercial 6–7[c]

Photolysis H2O + sunlight 0.5 %[a] Long term 10–30[c]

Thermochemicalwater splitting

H2O + heat 42 %(850 8C)

Long term 2.01 (S-Icycle)[b]

[a] Hydrogen purification is not included. [b] [114]. [c] [115].

Figure 20. 2010 H2 consumption market share by application, arranged fromdata presented in [117].


Focus Review


7.2.4. Nuclear energy

Hydrogen production using nuclear energy as a thermalenergy source can be achieved through thermochemical and/

or electrochemical processes.[124] Nuclear reactors most oftenuse uranium, U-235, which has finite reserves like fossil fuels

and is therefore considered as non-renewable. However, nucle-ar reactors have the advantage that they do not emit anygreenhouse gases (GHG), and they can contribute to large-

scale hydrogen production. If operated at off-peak periodsusing electrolysis, the efficient utilization of the reactor plants

would be further increased.[124] However, the potential for acci-dents, disposal of radioactive waste, and environmental and

health concerns related to mining and processing of uraniumneed to be taken into account for nuclear energy to be part of

a sustainable hydrogen economy. For example, since the 2011

Fukushima nuclear disaster in Japan, tens of thousands ofhomeowners started generating their own power from hydro-

gen fuel cells and solar panels.[125]

7.3. Hydrogen production from renewable sources

Hydrogen production from renewable sources (e.g. , solar,

wind, hydro, and biomass) provides a zero-emission alternativebut has its own challenges to overcome in order to be a com-

mercial success. High production costs, low efficiency, and theintermittent nature of the energy source (which also requires

a suitable storage system) are primarily the main issues.

7.3.1. Electrolysis from renewable sources

As mentioned before, water electrolysis can be used together

with renewable energy sources such as wind power and hy-droelectric power to become independent from fossil fuels.

The contribution towards hydrogen production from hydro-electric power is small as it is mainly used as an electricity

source.[126] Wind power has a larger potential as excess energy

can be used for hydrogen production, which in turn could becompressed for storage for later use. Hydrogen can also be im-plemented into the gas-grid as demonstrated for the first timeon an industrial scale in Germany.[127]

7.3.2. Biomass conversion

Biomass conversion is carbon neutral in its lifecycle and con-tributes to roughly 10 % of today’s world energy supply.[128]

Biomass (e.g. , wood, agricultural waste, crop residues) can beconverted into gaseous or aqueous fuels (e.g. , syngas), electric-

ity, and hydrogen via thermochemical or biological processes.From the various thermochemical processes available (combus-

tion, pyrolysis, liquefaction, and gasification), combustion and

liquefaction are least favorable for hydrogen production.[129] Inthe case of combustion, the energy efficiency is low (10–30 %),

whereas in the case of liquefaction obstacles are the operatingconditions (525–600 K in water and 5–20 MPa in the absence

of air) and the low hydrogen production.[129] Biological process-es (biophotolysis, biological water–gas shift reaction, and fer-

mentation) convert biomass through metabolic pathways ofnaturally occurring microorganisms in an oxygen-depleted en-vironment, yielding hydrogen and liquid fuels, but their practi-cal application still needs to be demonstrated.[129]

7.3.3. Solar conversion

Inspired by nature, the splitting of water into H2 and O2 usingsolar energy has been the focus of research for over

40 years.[5d] From all renewable energy sources, solar energyhas the highest potential to cover, in principle, the energydemand of a growing world economy.[130] In one hour roughly1.2 Õ 105 TW hit the earth’ surface, which is more than all

human activity can consume in one year.[131] In order to harvestthis energy, an efficient solar conversion system is required.

Solar conversion technologies can be divided into three cat-

egories : electrical, chemical, and thermal.[112, 132] Photovoltaicsconvert photon energy directly into electricity using a semicon-

ductor material to absorb the photons and release electrons,resulting in an electric current. Solar thermal technologies use

the heat which is produced from concentrated solar power

(CSP) for thermochemical reactions to produce hydrogen orfor high-temperature electrolysis for efficient water splitting.

Photosynthetic, photo- and photoelectrochemical systemsconvert solar energy into chemical energy which can be stored

in the form of chemical fuel (e.g. , hydrogen, ammonia, meth-ane). Both processes use light-sensitive materials to absorb

photons, for example, plants and algae (photosynthesis) orphotocatalysts (photo- and photoelectrochemical).

Photocatalytic water splitting has possibly the largest poten-

tial to make an important contribution towards a hydrogeneconomy. The photocatalyst, also regarded as the heart of the

chemical reaction, should not be consumed or transformedduring the water splitting reaction and have energy levels ap-

propriate to initiate the water splitting reaction. Continuous ef-forts are made to increase the photocatalytic activity in order

to reach a solar-to-fuel efficiency of 5–10 % needed for eco-

nomic viability.[126, 130]

7.4. Infrastructure and distribution

The infrastructure and distribution of hydrogen plays an im-portant role for its effective utilization. Figure 21 shows a car-toon of how hydrogen can play a significant role in a sustaina-

ble energy future and how a hydrogen economy could exist.Apart from hydrogen production expenses, costs associatedwith the infrastructure for transportation, distribution, and stor-age of the generated hydrogen also play a crucial role in itswidespread use.

Compressed or liquefied hydrogen are both in commercial

use but come with a large energy penalty for compression orliquefaction and also with high costs due to safe tankdesign.[112, 115] The infrastructure depends largely on how the

hydrogen is produced (centralized or decentralized).[112] Cen-tralized hydrogen production would mean large-scale plants

and therefore has the economic advantage of scale, for exam-ple, in the case of steam reforming and coal gasification. The


Focus Review


infrastructure involves transportation and distribution over

large distances, which would result in additional costs. Usingan existing gas distribution network could lower these costs if

a small amount of hydrogen is mixed together with natural

gas for household applications. Having a decentralized hydro-gen production plant would result in lower distribution costs

as hydrogen is produced close to the center of demand. Usual-ly, these are smaller plants and therefore require less invest-

ment capital, although carbon capture would be more expen-sive than in centralized plants.[112] Location, demand, feedstock,

costs, and technological developments will probable deter-

mine which type of plant would be more suitable for each oc-casion.

8. Conclusion

Solar water splitting that yields hydrogen is the most promis-ing strategy to convert solar energy into fuels. Although there

is a large number of photocatalysts that can split water, a mate-rial that fulfills all the requirements has not yet been discov-

ered. Thus, novel approaches and synthetic strategies need tobe developed to discover more effective materials/devices for

solar water splitting. Nanoscale materials have exceptionalphysical and chemical properties in comparison to their bulk

counterparts, and those properties can play a significant rolefor the design and engineering of more effective photocata-lysts. These attributes include morphology (counting dimen-

sion, geometry, symmetry, shape and size of the particles), tex-tural parameters (including surface area, porosity, and pore

volume), and electronic and crystal structure. Among theseproperties, the major benefits of nanoscale materials are their

high specific surface area and short charge carrier diffusion

paths. By decreasing the particle size, which results in a largersurface area and an increased number of active sites, the light-

harvesting efficiency of semiconductors can be enhanced. Nextto the enhanced light absorption in nanoparticle suspensions,

a small particle size also allows for a fast diffusion of chargecarriers from the bulk to the surface as the distances are on

the nanometer scale. However, a high surface area also pos-sesses a higher number of defect sites which creates more ex-

citon recombination sites and too fine particles do not exhibitany space charge layer and therefore charge carrier separation

becomes more challenging. Furthermore, the activity and sta-bility of nanocatalyst systems are commonly enhanced

through the deposition of co-catalysts. In the described exam-ples of nanostructured systems various catalyst materials were

investigated, and the common observation was that a high

surface area and small particle size are the determining factorsthat increase the catalytic hydrogen production rate.

Further research in this field has to lead to more feasiblesynthesis strategies similar to the described direct injection

method in order to produce nanostructured or nanoscale cata-lysts with sufficient hydrogen production rates that can lead to

the development of large-scale hydrogen-producing solar

plants. Although the concept of using hydrogen as an energycarrier for industrial and domestic use dates back to the 1800s,

it has recently gained much attention in the light of global cli-mate change and security of energy supply. Safety concerns re-garding its handling and storage seem to be a major concern,despite the fact that it is no more hazardous than other con-

ventional fuels. However, with consumers becoming moreaware of the finite supply of fossil fuels, renewable technolo-gies are sought after. To promote a hydrogen economy further,

incentives from the government pertaining to vehicles, localauthorities, and research grants remain crucial.

Acknowledgements

This work was supported by MAXNET Energy consortium of

Max-Planck-Society, and the Cluster of Excellence RESOLV (EXC1069) funded by the Deutsche Forschungsgemeinschaft (DFG)

and Fonds der Chemischen Industrie (FCI).

Keywords: hydrogen · hydrogen economy · nanocatalysts ·solar energy · water splitting

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Manuscript received: July 13, 2015

Revised: August 20, 2015

Accepted Article published: September 3, 2015

Final Article published: September 28, 2015


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