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ReviewJournal of

Nanoscience and NanotechnologyVol. 19, 307–331, 2019

www.aspbs.com/jnn

Shape Tailored TiO2 Nanostructures and

Their Hybrids for Advanced Energy and

Environmental Applications: A Review

Kamal Kumar Paul1 and P. K. Giri1�2�∗1Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, India

2Centre for Nanotechnology, Indian Institute of Technology Guwahati, Guwahati 781039, India

Shape tailored TiO2 nanostructures with various dimensionality (zero to three dimension) haveunique physicochemical and functional properties that facilitates its efficient energy and environ-ment applications, e.g., solar light driven photocatalytic hydrogen generation and decontaminationof organic/inorganic toxic pollutants, CO2 reduction into the hydrocarbon fuels, solar cells, superca-pacitors and lithium-ion batteries etc. However, the wide band gap nature and the fast recombinationof the photogenerated charge carriers in TiO2 usually limit its overall performance under solar lightillumination. In this review, we present a state of the art on the fabrication techniques of shapetailored TiO2 nanostructures and the strategies employed to make the system catalytically moreefficient. Though shape tailored TiO2 nanostructures with large specific surface area and highlyenergetic (001) facet exposed TiO2 nanostructures (2D and 3D) can enhance the photocatalyticefficiency to a reasonable extent, further surface engineering is needed for the modification of theelectronic band arrangement, visible light sensitization and efficient charge separation. Herein, TiO2

heterostructures (HSs) with metal/non-metal doping, surface fluorination, plasmonic noble metalnanoparticles (NPs) and coupling with the narrow band gap suitable semiconductor (type-II) arediscussed in details covering from zero dimensional to three dimensional heterostructures. Thesynthesis strategies, charge transfer mechanism and their participation in the photocatalysis areelaborated. Though one dimensional TiO2 HSs have been widely studied, we present the recentdevelopment of critical surface engineering strategies of two and three dimensional systems, whichgive rise to the excellent properties including the enlargement of surface area, light absorption capa-bility and efficient separation of electrons/holes resulting in the superior performance in advancedapplications. Based on recent breakthroughs in the field, future directions and outlook of the fieldare presented at the end.

Keywords: Shape Tailored TiO2 Nanostructures, 0D TiO2, 1D TiO2, 2D TiO2, 3D TiO2,Plasmonic Heterostructure, Type-II Heterostructure, Visible Light Photocatalysis,Hydrogen Production.

CONTENTS1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307

2. Crystal Structures of Various Phases of TiO2 . . . . . . . . . . . . . . 309

3. Synthesis of Shape Tailored TiO2 Nanostructures . . . . . . . . . . . 310

3.1. Synthesis of 0D TiO2 Nanostructure . . . . . . . . . . . . . . . . . 310

3.2. Synthesis of 1D TiO2 Nanostructures . . . . . . . . . . . . . . . . 310

3.3. Synthesis of 2D TiO2 Nanostructures . . . . . . . . . . . . . . . . 315

3.4. Synthesis of 3D TiO2 Nanostructures by Hydrothermal/

Solvothermal Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317

4. Surface Engineering of TiO2 Nanostructure and Its

Application in Catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320

∗Author to whom correspondence should be addressed.

4.1. 0D TiO2 Quantum Dots and Its HSs . . . . . . . . . . . . . . . . . 320

4.2. 1D TiO2 HSs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321

4.3. 2D TiO2 HSs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324

4.4. 3D TiO2 HSs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326

5. Conclusion and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328

Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328

References and Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328

1. INTRODUCTION

The rapid industrialization, continuous growth of global

economy and population in the past few decades have

J. Nanosci. Nanotechnol. 2019, Vol. 19, No. 1 1533-4880/2019/19/307/025 doi:10.1166/jnn.2019.15778 307

Shape Tailored TiO2 Nanostructures and Their Hybrids for Advanced Energy and Environmental Applications Paul and Giri

steadily raised the awareness of potential global crisis of

energy because of our rapacious consumption of fossil

fuels at an extremely fast rate. Thus, the global warming

and climate change becomes the inevitable consequence

of various pollution in the environment resulting from the

extensive use of fossil fuels.1–3 For the sustainable devel-

opment of our society, the advancement of technologies

towards the renewable green energy systems as well as the

environmental decontamination becomes the most urgent

task. Parallel to the ongoing various renewable energy

projects, such as supercapacitors, solar cells and lithium

ion (Li-ion) batteries,4–6 optical photon induced catalysis

or photocatalysis by a suitable semiconductor has attracted

great attention and research fascination due to its diverse

potential application in the field of energy and environ-

ment involving the decomposition of organic and inorganic

pollutants,7 CO2 reduction into hydrocarbon fuels8�9 and

photocatalytic water splitting for hydrogen production.10

After the remarkable discovery of photocatalytic water

splitting in TiO2 electrode under the ultraviolet (UV) irra-

diation by Fujishima and Honda,10 notable progress has

been made in designing the efficient semiconductor pho-

tocatalysts. Thus, the semiconductor TiO2 has been paid

great attention, established to be most advantageous and

extensively used for the various catalytic and energy appli-

cations such as, photocatalytic degradation of industrial

pollutants,11�12 CO2 reduction,13�14 water splitting for H2

evolution,15�16 solar cells,17�18 supercapacitors,19�20 lithium

ion batteries21�22 and biomedical devices.23�24 TiO2 satisfies

Kamal Kumar Paul graduated (Bachelor of Science (with Honors in Physics)) from the

University of Kalyani in 2011 and obtained his Masters of Science degree in Physics from

Indian Institute of Technology Delhi in 2013. Then he joined the Department of Physics,

Indian Institute of Technology Guwahati for his Ph.D. in 2013 and his research areas of

interest are TiO2 and 2D materials (e.g., MoS2, g-C3N4, WS2 etc.) hybrids for energy and

environment applications.

P. K. Giri obtained his Ph.D. in Physics from Indian Institute of Technology Kanpur in

1998 and then moved to Italy for postdoctoral research with ICTP TRIL fellow-ship. Dur-

ing 1999–2001, he worked in Indira Gandhi Centre for Atomic Research, Kalpakkam as

a Scientist. Later he moved to Indian Institute of Technology Guwahati as an Assistant

Professor of Physics. Presently he is a full Professor of Physics and Center for Nanotech-

nology at the same institute. For his outstanding research contributions, he received several

national/international awards/fellowships including ICTP TRIL fellowship (1998), DAE

Young Scientist Award (2000), DAAD Exchange visit Fellowship (2010), JSPS Invitation

Fellowship for log-term research in Japan (2012) etc. He has published 120 Research

articles including 5 review articles in high profile international journals and holds one

Patent to his credit. He is an Executive Council Member of Electron Microscopy Society

of India and a life member of Materials Research Society of India. His research areas of interests are semiconductor

nanostructures, optoelectronics, 2D Materials based hybrid nanostructures for energy and environmental applications,

nano biosensors, etc.

the prerequisite to be an efficient photocatalyst keeping

the redox potential of highly reactive oxygenated species

(hydroxyl and superoxide radicals, hydrogen peroxide etc.)

in forbidden energy gap. Initially the investigations were

concentrated mainly on the TiO2 nanoparticles (NPs) and

an excellent performance was observed only under the

UV light irradiation. This is because of the large band

gap of TiO2 (∼3.2 eV for anatase and brookite, ∼3.0 eV

for rutile) and thus the threshold of excitation wave-

length falls in the UV region of the solar spectrum.25

Since, only ∼5% of the incident radiation on the earth’s

surface lies in the UV zone and the rest 95% in the

visible (∼43%) and infrared zone (∼52%), the perfor-

mance of TiO2 nanostructures as a natural solar light

photocatalyst is usually not satisfactory and it needs fur-

ther investigation for lowering the activation energy of

TiO2.26 Another unavoidable difficulties with the TiO2 NPs

is the high recycling cost after each reaction due to its

small size and high dispersion in the aqueous solution.

As the photocatalysis is fully a surface property of a

semiconductor, enhancement in the specific surface area

may lead to the higher efficiency by increasing the cat-

alytically active sites.27 Thus, shape tailored TiO2 nano-

structures with various dimensionality such as (i) zero

dimensional (0D) quantum dots (QDs), (ii) one dimen-

sional (1D) nanorods (NRs), nanowires (NWs), nanobelts

(NBs), nanotubes (NTs), nanoribbons (NRbs), (iii) two

dimensional (2D) nanosheets (NSs) and three dimensional

(3D) superstructures, nanoflowers (NFs), nanospheres etc.

308 J. Nanosci. Nanotechnol. 19, 307–331, 2019

Paul and Giri Shape Tailored TiO2 Nanostructures and Their Hybrids for Advanced Energy and Environmental Applications

have been studied extensively in the last decade resulting

in the significant advancement in the field of photocataly-

sis and energy applications.

In bare homogeneous TiO2 nanostructure, the recom-

bination process of photogenerated excitons is observed

to be random and thus recombination is highly probable

before reaching the surface to initiate redox reactions caus-

ing low efficiency of photocatalysis. Additionally, TiO2

nanostructure can utilize only ∼5% of the incident solar

light28 and shows slow charge carrier transfer, making it

inefficient in the industrial scale.

To overcome these limitations, modification of physico-

chemical and optical properties of the shape tailored titania

and its heterostructures (HSs) is indispensable. Therefore,

enhancement in the efficiency of the photocatalytic pro-

cess by tuning the electronic band structure and light har-

vesting range of TiO2 to the visible and NIR region has

been achieved through various strategies, such as cou-

pling with a narrow band gap semiconductor (type-II

HSs),29�30 doping with metal and non-metal ions,31–36 co-

doping with multiple foreign elements,37 depositing noble

metal NPs38–40 to form plasmonic HSs, surface sensitiza-

tion with organic dyes,41�42 surface fluorination,43�44 etc.

In the last decade, an appreciable development has been

made on the fabrication of shape tailored nanoscale TiO2

HSs with various dimensionality. In this review, we first

provide a summary of the crystal structures of TiO2 in

distinct phases followed by the detailed methodologies

for the fabrication of shape tailored TiO2 nanostructures,

including 0D, 1D, 2D and 3D nanostructures. Next, we

focus on the tuning of electronic structure, the range of

optical absorption and charge separation efficiency in the

TiO2 HSs fabricated by depositing plasmonic noble metal

NPs, coupling with various appropriate narrow band gap

semiconductors, doping etc. Finally, we look into their

influence on the photocatalytic hydrogen evolution and

environmental decontamination properties.

2. CRYSTAL STRUCTURES OF VARIOUSPHASES OF TiO2

Titania (TiO2� generally exists in three distinct poly-

morphs (phases): rutile, anatase, and brookite, where

anatase and rutile phases have tetragonal crystal struc-

ture with space groups I41/amd and P42/mnm, respec-

tively and brookite has orthorhombic crystal structure with

space group Pbca.45 In each polymorph, titanium cations

are found to be six-fold coordinated to oxygen anions,

which forms distorted TiO6 octahedra. The crystal struc-

ture of TiO2 differs with phase due to the distinct spatial

arrangement of the TiO6 octahedra building blocks (see

Fig. 2). While anatase has corner sharing TiO6 octahedra,

in rutile phase edge sharing is observed and the brookite

shows the both. Among the different crystal phases, rutile

is considered to be the most thermodynamically stable

bulk phase. At the nanoscale, anatase and brookite are

Figure 1. A schematic illustration of various steps of photocatalysis by

TiO2: (I) formation of free excitons, (II) photoinduced charge recombi-

nation, (III) photoinduced charge transfer to the oxygen or hydroxyl ion

(IV, V) oxidation and reduction caused by valence band hole and conduc-

tion band electron, respectively. Reproduced with permission from [28],

Y. Wang, et al., Nanoscale 5, 8326 (2013). © 2013, RSC.

generally regarded as more stable due to their low sur-

face energy, although there exists some debate on this

issue.46�47 Additionally, there exists another exotic crys-

tal phase of pure TiO2 (space group C2/m), known as

TiO2 (B) (Fig. 2(d)). This phase is generally produced by

solution based process, like hydrothermal method. It has

a very open framework structure despite having the iden-

tical chemical formula. Thus, it possesses a lower density

and a larger specific capacity than other phases, which are

extremely advantageous in the applications of photocatal-

ysis, solar cells and energy storage, such as lithium-ion

batteries.48–51 The detailed growth studies have revealed

that the crystal phases are determined by the experimen-

tal conditions and also post growth treatments (synthesis

method, pH of the medium, annealing duration and anneal-

ing temperature). Zhang et al.52 have thermodynamically

analyzed the limiting crystal size to be stable under a cer-

tain crystal phase. Anatase can have the size up to 11 nm,

for brookite 11–35 nm and for the bulk phase, rutile, it

should be more than 35 nm. Yang et al.53 obtained dif-

ferent crystal phases (rutile, anatase, and brookite) by a

hydrothermal method using different pH values of perox-

ide titanic acid solution. With pH value lower than 8, only

rutile phase was found. When the pH was varied in the

range 8–10, brookite and rutile phases were obtained in the

mixed phases. However, with the pH value higher than 10,

only anatase phase was obtained. Santara et al.54 discussed

the effect of annealing temperature and environment on

the crystal phases and surface feature of the TiO2 nano-

structures. Prior to the annealing, no peak was observed

in the XRD pattern of the TiO2 nanoribbon. The growth

of TiO2 (B) phase from hydrogen titanate was observed at

annealing temperature 500 �C. When the temperature was

increases to 700 �C, the B-phase started converting into

anatase and a mixture of these two was obtained. When

annealed at 900 �C, the TiO2 nanoribbons were found in

anatase and rutile mixed phase, indicating a partial trans-

formation of anatase into rutile phase. Beyond 900 �C only

J. Nanosci. Nanotechnol. 19, 307–331, 2019 309


Figure 2. A schematic representation of various TiO2 polymorphs: (a) Rutile; (b) anatase; (c) brookite; and (d) TiO2 (B). Each purple sphere represents

Ti atom, and the blue octahedra represent TiO6 blocks. Oxygen atoms at the corner of the octahedra are omitted for clarity. Reproduced with permission

from [45], Y. Zhang, et al., RSC Adv. 5, 79479 (2015). © 2015, RSC.

phase detected was rutile in bulk phase. Tay et al.55 synthe-

sized a mixed phase anatase/brookite TiO2 nanostructure

by a simple hydrothermal method, which shows enhanced

hydrogen production activity compared to the highly crys-

talline pure brookite and P25.

3. SYNTHESIS OF SHAPE TAILOREDTiO2 NANOSTRUCTURES

Distinct morphological feature of semiconductor nano-

structures acts as extremely determining factor for the

growth of high performance photocatalysts for hydrogen

evolution as well as environmental cleaning, Li-ion batter-

ies, solar cells, etc. Thus, shape tailored various surface

morphology of TiO2 including zero dimension (quantum

dots), one dimension (nanorods, nanowires, nanoribbons,

etc.), two dimension (nanosheets, nanoplates, etc.) and

three dimension (flowers, hierarchical architectures) are

being extensively investigated to address the challenges of

energy crises, environmental issues, etc. of the modern era.

3.1. Synthesis of 0D TiO2 Nanostructure0D TiO2 quantum dots (QDs) based nanostructures are

comparatively least studied system among various dimen-

sion nano-heterostructures (1D, 2D and 3D) for the energy

and environmental applications. To grow the shape tai-

lored nanostructures, hydrothermal method is one of the

most widely used methods for its versatility with con-

trollability, repeatability, relatively lower cost and indus-

try based large scale production. Recently, few approaches

have been explored to prepare the 0D TiO2 QDs based

various HSs after the breakthrough work by Pan et al.56

on the hydrothermal synthesis of (001) facet dominated

TiO2 nanosheet with self-decorated TiO2 QDs. Their long

time hydrothermal strategy and subsequent defect cur-

ing by the hydrothermal washing are the key factors to

obtain defect-free nanosheets decorated with TiO2 QDs

with sizes 1.5–3 nm, as shown in Figure 3. The band

gap enlargement of TiO2 due to the formation of QDs

and its HSs with TiO2 nanosheet increases photoactivity

significantly which is highly promising for photocatalytic

performances.

Xu et al.57 designed TiO2 QDs on Cu2O octadecahedra

(Cu2O–O) single crystals with (110) and (100) exposed

facets synthesized by a sol gel method. Figures 4(a)–(d)

shows the details of the structural and morphological prop-

erties of the TiO2 QDs and Cu2O HSs. The Figure 4(e)

demonstrates the synthesis pathways of the Cu2O octadec-

ahedra, TiO2 QDs and their HSs. Special technique of

coupling the TiO2 QDs with the Cu2O facets to form

numerous stable p–n heterojunctions is used here for the

extremely efficient charge migration through the interface

of the heterostructures.

3.2. Synthesis of 1D TiO2 Nanostructures3.2.1. Sol–Gel and Template Assisted MethodOne of the key synthesis processes of wet chemistry

method is the sol–gel method with low temperature and

easy control in surface morphology is generally used to

synthesize 1D TiO2 nanostructures. In general, the TiO2

sol–gel is prepared by mixing titanium isopropoxide or

titanium butoxide in acetic acid followed by a heating to

high temperature with vigorous magnetic stirring which

allows hydrolysis and condensation reactions. Thus shape

tailored TiO2 nanostructures are obtained. Joo et al.58

have prepared 3.4 nm (diameter)× 38 nm (length) sized

TiO2 nanocrystals from sol–gel reaction between tita-

nium(IV) isopropoxide and oleic acid at 270 �C. They

have successfully controlled the length of the as-grown

NRs between 28–39 nm by adding 1-hexadecylamine to

the reaction mixture as a co-surfactant. The template-

assisted method is also extensively used to fabricate the 1D

TiO2 nanostructures. An anodic aluminum oxide (AAO)

nanoporous membrane consisting of parallel straight array

of nanopores with well controllable dimension (diameter

and length), is usually considered as a template. Finally,

the template is removed by chemical etching and 1D TiO2

nanostructures remain on the substrate. Sander et al.59 have

fabricated dense, thermally stable and well-aligned TiO2

nanotubes by sequential atomic layer deposition (ALD)

process with TiCl4 as the precursor material using tem-

plate assisted method. A combination of sol–gel and

template assisted method produces better control over



Figure 3. HRTEM images of [001] faceted TiO2 nanosheet with TiO2 QDs decoration after the hydrothermal reaction of 72 hrs (a–d) and 168 hrs

(f–i). (b, g) the corresponding lattice d-spacing and the inset of (c, d) exhibit the size distribution of TiO2 QDs. HRTEM images of [001] faceted TiO2

nanosheet with TiO2 QDs decoration. (e, j) XRD and Raman spectra of TiO2 nanosheets, respectively. Reproduced with permission from [56], L. Pan,

et al., ChemComm. 49, 6593 (2013). © 2013, RSC.

the growth of 1D TiO2 nanostructures and high perfor-

mance in the real life applications. For example, Qiu,60

and Lin et al.61 synthesized 1D TiO2 nanorods, nano-

tube arrays and nanowires, respectively by a combined

template-assisted sol–gel method as shown in Figure 5.

3.2.2. Hydrothermal MethodHydrothermal method is the most used chemical method

for the fabrication of shape tailored TiO2 nanostructures

including various 1D architectures. Usually the reaction is

conducted in a stainless steel chamber with a Teflon liner

under high temperature with autogenous pressure.

It is probably the simplest method for tuning the sur-

face morphology at the lowest production cost. Kasuga

Figure 4. (a) The XRD patterns of TiO2 QDs, Cu2O octadecahedra and HSs. (b–d) their TEM images. (e) Schematic pathways of synthesis of

TiO2 QDs, Cu2O octadecahedra and HSs. Reproduced with permission from [57], X. Xu, et al., ACS Appl. Mater. Interfaces 8, 91 (2016). © 2016,

ACS, (a–e).

et al.62�63 discovered a template free new route to fabricate

TiO2 nanotubes with diameter ∼8 nm and length ∼100 nm

considering amorphous TiO2 as a precursor material treat-

ing at high temperatures with concentrated NaOH sol-

vent. Afterwards, many research articles have reported

on the growth of TiO2 nanotubes,64�65 nanowires,66�67

nanorods,47�68�69 nanobelts,70�71 nanosheets72–75 by the

hydrothermal method. Depending upon the solvent used,

the synthesis procedure can generally be divided into

two methods: first, acid hydrothermal method where the

reactants are titanium salts and concentrated hydrochlo-

ric acid (HCl) and second, alkali hydrothermal method

where the reactants are TiO2 nanoparticles and concen-

trated sodium hydroxide (NaOH) solution. These two



Figure 5. (a) TEM image of as-synthesized TiO2 nanocrystals. (b) HRTEM image of TiO2 nanorods. (c–d) Top and side view of titania nanotube array

on Si substrate with outer diameter 40 nm and length 1.5 �m. TEM image of TiO2 nanotubes oriented in cross section (e) and a single nanotube (f).

(g–h) FESEM images of well aligned TiO2 and ZnO nanotube arrays. (i–j) XRD and SEM image of TiO2 nanowires (NWs), respectively. Reproduced

with permission from [58], J. Joo, et al., J. Phys. Chem. B 109, 15297 (2005). © 2005, ACS, (a–b). Reproduced with permission from [59], M. S.

Sander, et al., Adv. Mater. 16, 2052 (2004). © 2004, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, (c–f). Reproduced with permission from [60],

Q. Jijun, et al., Nanotechnology 17, 4695 (2006). © 2006, IOP Science, (g–h). Reproduced with permission from [61], Y. Lin, et al., J. Phys. Condens.Matter. 15, 2917. © 2003, IOP Science, (i–j).

methods have different reaction mechanism which pro-

duces different 1D morphological features of TiO2. Dur-

ing alkali-hydrothermal process, layered sodium titanate

(Na2Ti4O9� is produced in an intermediate reaction stage.

As the Na+ ions are residing between the edge-shared

TiO6 octahedral layers, it can be easily replaced by pro-

tons (H+ ion) in a dilute acidic medium. Thus, hydro-

gen titanate (H2Ti5O11 ·H2O) is formed and the interlayer

distance becomes enlarged because of the larger interca-

lated size effect of H2O molecules causing weaker static

interaction between neighboring TiO6 octahedral sheets.

Thus, hydrogen titanate can easily be exfoliated to form

nanosheets which in turn curl up from the edges to form

TiO2 nanotubes to minimize the high surface tension.76�77

Additional hydrothermal conditions like type of precursor,

its size, concentration of NaOH solvent, temperature and

reaction duration may affect the structure and morphology

of the nanostructures.

Morgan et al.78 showed the effect of concentration of

NaOH and the reaction temperature on the formation of

Figure 6. Morphological shape evolution of P25 (TiO2� to (a) nanotube

and (b) nanoribbon after hydrothermal treatment at 160 �C and 220 �C,respectively. Reproduced with permission from [78], D. L. Morgan, et al.,

Chem. Mater. 20, 3800 (2008). © 2008, ACS.

various nanostructure from P25 (TiO2� through alkaline

hydrothermal treatment (see Fig. 6). Similar results have

been reported by Tanaka79 and Peng et al.,80 an inves-

tigation of the effect of concentration of NaOH, tem-

perature and reaction time duration on the nanostructure

formation.

For further modification and improvement in the nano-

structures, several approaches like microwave assisted

heating process, ultrasonication and magnetic rotation of

the reactants during the hydrothermal treatment were

adopted. Stirring based hydrothermal method has been

proved to be an extremely beneficial for the growth of

much longer nanotube (up to tens of micrometers) (see

Fig. 7). The optimized nanotubes with very high aspect

ratio exhibit ultrafast rechargeable Li-ion battery materials

with high stability.81�82

3.2.3. Solvothermal MethodSimilar to the hydrothermal process, the solvothermal

method is also a common growth process for various

TiO2 nanostructures. The only difference between the

hydrothermal and the solvothermal method is the use

of organic solvent (ethanol, ethylene glycol, n-hexane,etc.) in solvothermal method, while only water is used

as reaction solution in hydrothermal method.83–87 Wang,88

Santara et al.53�89 have grown nanowires, nanotubes

and nanoribbons by a solvothermal method consider-

ing ethanol, glycerol and ethylene glycol as solvents,

respectively, see Figures 8(a), (b), (e)–(h). The as-grown

nanowires and nanotubes are suitable for the high perfor-

mance Li-ion battery materials, whereas the nanoribbons

with modified effective band gap and optical absorption

window are promising for the photocatalysis under solar

light. Nitrogen and fluorine co-doped TiO2 nanobelts have

also been reported for high performance photocatalysis,

see Figure 8(c).88 Selection of appropriate solvent is the



Figure 7. (a) FESEM image of TiO2 nanotubes obtained at 500 rpm. (b) TEM image of (a), the arrow shows nanotubular structure. (c–f) FESEM

images of TiO2 nanotubes obtained from the hydrothermal reaction with temperature maintained at 130 �C for 24 hrs at stirring rates of 0, 300, 400,

500 and 1000 rpm, respectively. (g) XRD patterns of TiO2 nanotubes at various stirring speed. Reproduced with permission from [81], Y. Tang, et al.,

Angew. Chem. Int. Ed. 53, 13488 (2014). © 2014, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

key factor for the growth and various surface modifications

of the nanostructures.

Despite its versatility and advantages of the hydrother-

mal and solvothermal method over the other growth

processes, it has still some disadvantages. First, the nano-

structures grown by this method are not always uniform in

Figure 8. SEM micrographs of different TiO2 nanostructures synthesized by the solvothermal method: (a) Nanowires, (b) nanotubes, (c) nanobelts.

Reproduced with permission from [88], Q. Wang, et al., Inorg. Chem. 45, 6944 (2006). © 2006, ACS (a, b). Reproduced with permission from [90],

Z. He, et al., ACS Appl. Mater. Interfaces 4, 6816 (2012). © 2012, ACS, (c). (d–f) TEM and FESEM images, respectively of TiO2 (B) nanoribbons

synthesized at 180 �C for 16 hrs and 500 �C post growth treatment. The inset of (e) shows the EDX spectrum. Reproduced with permission from [54],

B. Santara, et al., Nanoscale 5, 5476 (2013). © 2013, RSC. (g) High magnification TEM image of TiO2 nanoribbons calcined at 700 �C showing

nanobricks and nanopits on its surface, and the inset shows the corresponding SAED pattern. (h) FESEM images of TiO2 nanoribbons calcined at

various temperatures: 500 �C, 700 �C and 900 �C, respectively. Reproduced with permission from [89], B. Santara, et al., J. Phys. Chem. C 117, 23402

(2013). © 2013, ACS.

size and shape. Second, the as-grown 1D nanostructures

are generally too small for the better performance in the

real life applications. Third, the reaction rate is very slow

and it takes long time to grow desired materials. Fourth,

the growth of monolayer 2D materials (MoS2, MoSe2,

WS2, etc.) is not well controllable by this method.



3.2.4. Electrochemical Anodization MethodIn 1984, Assefpour-Dezfuly et al.91 reported the growth of

porous, well aligned 1D TiO2 nanotube arrays by two step

process: etching in alkaline peroxide and then anodization

in chromic acid. Presently, anodization process is com-

monly used for the synthesis of 1D TiO2 nanostructured

materials on Ti foil substrate. The TiO2 nanotubes synthe-

sized by this method are very promising for the photoan-

ode material due to its high performance, stability and easy

synthesis procedure. Afterwards, Zwilling et al.92 made

a breakthrough by growing nanoporous titania nanotube

in fluoride containing electrolytes on Ti foil. Since then,

a lot of attention has been paid to optimize the controlled

growth of the 1D TiO2 nanostructures.93–95

It is now well understood that the nature of electrolyte,

it’s pH, applied voltage, reaction duration and temperature

of the reaction solution determine the morphology and the

dimension of the TiO2 nanostructures. The as-grown TiO2

nanotubes of various generations are shown in Figure 9.

Even though, a remarkable progress on the optimized

Figure 9. SEM images of the TiO2 nanotubes categorized under (a) 3rd generation in ethylene glycol/fluoride electrolytes, (b) shows a inclined view

of (a) and (c) is the TEM image of a TiO2 nanotube corresponding to (a), (d, e) top and cross section view, respectively of 5th generation with multi

step anodization in ethylene glycol/fluoride electrolytes, (f, g) top and cross section view, respectively of 5th generation with two-step anodization

process with ethylene glycol and fluoride electrolytes. Reproduced with permission from [93], M. Paulose, et al., J. Phys. Chem. C 111, 14992 (2007).© 2007, ACS, (a–c). Reproduced with permission from [94], M. Ye, et al., J. Am. Chem. Soc. 134, 15720 (2012). © 2012, ACS, (d–e). Reproduced

with permission from [95], M.-Z. Ge, et al., J. Mater. Chem. A 3, 3491 (2015). © 2015, RSC, (f–g).

growth of 1D TiO2 nanostructure has been achieved, devel-

opment is still going on for the further improvement of

the length of the nanotubes in F− based organic-inorganic

neutral electrolytes.

3.2.5. Electrospinning MethodIn 2003, Li et al.96 synthesized TiO2 nanofibers having

diameter ∼55 nm by a electrospinning method for the first

time. In this process, a polymer solution is injected from

a needle of a syringe like chamber under the influence of

strong electric field. Due to the continuous gathering elec-

tric charges, the polymer solution starts stretching to form

ultralong and hollow nanofibers. More and more attention

are being paid to this method due to the easy formation

of single-phase and highly crystalline TiO2 nanofibers.

Figure 10(a) shows a schematic of the electrospinning pro-

cess and the (b) is the FESEM images of TiO2 nano-

fibers grown by it. Zhu et al.98 reported the growth of

porous-shaped anatase TiO2, cluster-shaped anatase TiO2,

hierarchical-shaped rutile TiO2 and nano-necklace rutile



Figure 10. (a) Schematic of an electrospinning setup. (b) SEM image of TiO2/PVP nanofibers grown from an ethanol solution. (c) Change in the

surface morphology of the nanofibers after thermal treatment. (d–g) FESEM images of TiO2 nanofibers calcined at 500, 600, 700, 800 �C, respectively.Reproduced with permission from [97], Y. Yang, et al., Nanoscale 5, 10267 (2013). © 2013, RSC. (a, c–g). Reproduced with permission from [96],

D. Li and Y. Xia, Nano Lett. 3, 555 (2003). © 2003, ACS (b).

TiO2 by the electrospinning method followed by the

annealing at temperatures 500, 600, 700 and 800 �C,respectively. It is observed that the diameter of the nano-

fibers shrink from 200 nm to <100 nm due the effect of

annealing at high temperatures though the axial continuity

is unaltered. Ag, CuO, ZnO, carbon, etc. can be introduced

into the nanofibers to modify the surface features for the

improvement in many application like photocatalysis.97–99

3.3. Synthesis of 2D TiO2 Nanostructures2D faceted TiO2 nanosheets with various surface energies

are at the focus of intense research in the field of energy

and environmental remediation. The key factor on tun-

ing the exposed crystallographic facets of TiO2 as well

as its facet percentage without disturbing its stability is

the nature of solvents used, capping and shape controlling

agents and impurities introduced into the crystals.100–102

The capping agents used to be adsorbed selectively and

interact differently with the different crystalline facets.

This strategy is used to design nanocrystals with various

dominant reactive facets.103 The capping agents play the

important role to reduce the surface free energy of the

facets which in turn guide the nanocrystal growth along

the specific facets. Additionally, the solvent used for the

reaction is also very significant and it determines the facet

orientation of the crystal during the growth process. This

is due to the fact that the atomic arrangement near the

surface and the surface affinity of the solvent vary con-

siderably with orientation of the facet and can influence

the growth rates in a favorable direction.1�100 Ong et al.104

succinctly explained the role of solvent and impurities

or additive in controlling the shape and facet orientation.

Several fabrication techniques have been developed for

the growth of 2D TiO2 nanocrystals dominated with (001)

facets. Among all, most explored and easy method is the

surface fluorination to stabilize the highly reactive (001)

facets.105

3.3.1. Hydrothermal MethodRecently, researches are focused on the development of

2D anatase single crystals with large percentage of reac-

tive (001) facets due to its extremely high reactivity

using a simple facile hydrothermal method.106–110 Vari-

ous techniques are used to calculate the exposed facets

like, FESEM, TEM and XRD simply by assuming the

whole surfaces are occupied by the (101) and (001)

facets.69�104�111 Yang et al.75 reported for the first time

the hydrothermal synthesis of anatase TiO2 single crys-

tals with 47% exposed (001) facets considering TiF4 as

a precursor material and hydrofluoric acid (HF) as a

shape controlling agent. Each single truncated bipyramid

shaped anatase TiO2 nanocrystal is composed of two flat

square surfaces corresponding to (001) facets while the

four isosceles trapezoidal surfaces are ascribed to the (101)

facets. The average interfacial angle between these two

facets is estimated to be ∼68.3� which is consistent with

the theoretical value.112 Following this work, synthesis

of (001) facet dominated 2D TiO2 nanosheets have been

extensively studied with various surface modification for

photocatalytic applications. Wen et al.113 synthesized large

area highly reactive and purely anatase TiO2 nanosheets

fully dominated by (001) and (100) facets with rela-

tive amount of 98.7% and 1.3%, respectively which are

generally thermodynamically unfavourable due to their



Figure 11. (a) SEM image of (001) faceted TiO2 single crystal. Inset shows the SAED pattern. (b, c) FESEM images of TiO2 nanosheets synthesized

at 210 �C for 24 hrs. Inset of (c) shows the corresponding TEM image. (d–h) TEM images of various TiO2 nanosheets efficient for the catalytic

application. Reproduced with permission from [105], H. G. Yang, et al., Nature 453, 638 (2008). © 2008, Nature Publishing. Group (a). Reproduced

with permission from [113], C. Z. Wen, et al., ChemComm. 47, 4400 (2011). © 2011, RSC, (b, c). Reproduced with permission from [114], J. Yu,

et al., Nanoscale 2, 2144 (2010). © 2010, RSC, (d). Reproduced with permission from [106], X. Han, et al., J. Am. Chem. Soc. 131, 3152 (2009).© 2009, ACS, (e). Reproduced with permission from [117], F. Tian, et al., J. Phys. Chem. C 116, 7515 (2012). © 2012, ACS, (f–g). Reproduced with

permission from [116], B. Yan, et al., RSC Adv. 6, 6133 (2016). © 2016, RSC (h).

high surface energy. The as-grown TiO2 nanosheets are

very large in the length ∼4.1 �m and (001) facet domi-

nated surfaces are showing appreciably high photocatalytic

activity (see Fig. 11). Yu114 and Wang et al.115 synthe-

sized (001) facet dominated anatase TiO2 nanosheets by

the hydrothermal method, which are highly beneficial for

the photoelectric conversion efficiency in dye-sensitized

solar cells as compared to the P25. Han,106 Yan,116 Tian117

and Liu et al.118 also synthesized (001) faceted TiO2

nanosheets which behave as good photocatalysts. Wu

and co-workers reported a facile non-aqueous synthesis

Figure 12. TEM images of 2D TiO2 NCs synthesized using precursor TiF4 (a, c), a mixed precursor of TiF4 and TiCl4 (b, d). (a, b) and (c, d) are

the images of TiO2 synthesized in the presence of oleylamine and 1octadecanol, respectively. TEM images of TiO2 having (e) the rhombic shape

(f) truncated rhombic shape, (i) spherical shape, (g) dog-bone-shape, (h) truncated and elongated rhombic shape, (j) elongated spherical shape and

(k) dumbbell shape. Insets show high-magnification images of the corresponding images. Reproduced with permission from [111], T. R. Gordon, et al.,

J. Am. Chem. Soc. 134, 6751 (2012). © 2012, ACS, (a–d). Reproduced with permission from [120], C.-T. Dinh, et al., ACS Nano 3, 3737 (2009).© 2009, ACS, (e–k).

route for the preparation of rhombic-shaped anatase TiO2

nanocrystals with exposed (001) for the first time.119

Following this work, Gordon et al.111 synthesized TiO2

NCs having size 10–100 nm by a controlled non-aqueous

seeded growth considering the precursors TiF4 and TiCl4where the TiF4 enables the exposure of (001) facets (see

Fig. 12). Dinh et al.120 demonstrated the TiO2 nanocrystal

growth with various shapes: rhombic, truncated rhom-

bic, spherical, dog-bone, truncated and elongated rhombic,

etc. by using a modified hydrothermal route employing

both as capping agents (see Fig. 12). In the water vapor



Figure 13. (A, B) Schematic representation along with the FESEM micrographs of (001) facet dominated anatase and rutile TiO2 nanosheets by CVD

method with H2 flow. Reproduced with permission from [122], W.-J. Lee and Y.-M. Sung, Cryst. Growth Des. 12, 5792 (2012). © 2012, ACS.

environment a simple variation of oleic acid and oley-

lamine molar ratio, precursor or the reaction temperature

make extremely fine tuning over the shape evolution of the

TiO2 nanocrystals, similar to the works discussed in the

Ref. [121].

3.3.2. Chemical Vapor DepositionMethod

Though the chemical based growth mechanism of TiO2 is

now firmly established, the synthesis of pure TiO2 nano-

structure by physical method is not yet well explored. Lee

et al.122 designed high density (001) facet dominated TiO2

nanosheets both in anatase and rutile phases on silicon and

silicon coated substrates via a chemical vapor deposition

method, as shown in Figure 13. They have shown that the

H2 autoignition induced below its autoignition tempera-

ture (536 �C) could prevent the phase transformation from

anatase to rutile.

Figure 14. (a–f) FESEM micrographs of (001) facet dominated anatase TiO2 nanocubic structures. Reproduced with permission from [75], H. G.

Yang, et al., J. Am. Chem. Soc. 131, 4078 (2009). © 2009, ACS (a). Reproduced with permission from [124], C. Z. Wen, et al., ChemComm. 47, 6138(2011). © 2011, RSC, (b–c). Reproduced with permission from [126], L. Wu, et al., CrystEngComm. 15, 3252 (2013). © 2013, ACS, (d). Reproduced

with permission from [43], Z. Lai, et al., J. Mater. Chem. 22, 23906 (2012). © 2012, RSC, (e, f).

3.4. Synthesis of 3D TiO2 Nanostructures byHydrothermal/Solvothermal Method

3.4.1. (001) Faceted Nanocubic StructuresNumerous techniques have been explored for the fabrica-

tion of 3D TiO2 architectures with different shapes. Notable

progress has been reported for the preparation of (001)

facet dominated 3D nanostructures by hydrothermal and

modified hydrothermal methods. As the average surface

formation energy is maximum for (001) facet (0.90 J m−2�which is almost double than that of the (101) facet

(0.44 J m−2� these facets are highly catalytically active and

are very promising for the future generation of energy and

environmental cleaning.72 3D TiO2 single crystal architec-

tures have been synthesized by various precursor materials

like TiF4, TiCl4, Tetrabutyl titanate, titanium isopropoxide,

TiOSO4, etc.32�43�123 Yang and co-workers demonstrated

a novel solvothermal route for the shape tailored synthe-

sis of 3D single crystals of anatase TiO2 with dominating



Figure 15. (a, b) FESEM and TEM images of nearly 100% (001) facet dominated anatase TiO2 nanosphere, respectively. (c) FESEM image of (001)

faceted TiO2 nanoflowers. (d) FESEM image of etched (001) faceted TiO2 nanoflowers with size 1.5–2 �m. Reproduced with permission from [21],

J. S. Chen, et al., J. Am. Chem. Soc. 132, 6124 (2010). © 2010, ACS, (a, b). Reproduced with permission from [72], M. Liu, et al., Nanoscale 2, 1115

(2010). © 2010, RSC, (c). Reproduced with permission from [73], Q. Xiang, et al., ChemComm. 47, 4532 (2011). © 2011, RSC, (d).

Figure 16. (a, b) FESEM images of 3D microsphere in purely anatase phase. (c) FESEM image of electrosprayed TiO2 spheres and (d) shows

its magnified view clearly showing the core–shell structure. Reproduced with permission from [131], S. Liu, et al., J. Am. Chem. Soc. 132, 11914(2010). © 2010, ACS, (a). Reproduced with permission from [132], J. Yu, et al., CrystEngComm. 12, 872 (2010). © 2010, RSC, (b). Reproduced with

permission from [133], B. Liu, et al., Langmuir 27, 8500 (2011). © 2011, ACS, (c, d).



percentage of reactive facets prepared by considering a

water-2-propanol as solvent (see Fig. 14).75 Wen et al.124

demonstrated the solvothermal synthesis of single crys-

talline TiOF2 nanocube and its transformation to anatase

TiO2 for the first time by the calcination under various

atmospheres such as pure argon, moist argon and pure

hydrogen sulfide (H2S). After this breakthrough work, sim-

ilar reports have been published on the solvothermal syn-

thesis of 3D TiO2 single crystals with cubic morphology

for the efficient photocatalysis as well as the energy appli-

cations (see Fig. 14).43�125�126

3.4.2. (001) Faceted Nanoflower StructuresChen et al.21 reported the growth of 3D hierarchical

nanoporous spheres of lateral size a few hundred nm

composed of ultrathin anatase TiO2 nanosheets (thickness

∼3 nm) for the first time with nearly 100% exposed (001)

facets, as shown in Figure 15. The nanostructures with

high specific surface area (∼170 m2 g−1� is reported to be

extremely beneficial for the reversible and ultrafast Li-ion

battery application. Besides, Liu et al.72 reported a novel

growth process via a simple hydrothermal route of 3D

anatase TiO2 nanoflowers composed by truncated tetrago-

nal pyramidal TiO2 nanocrystals. Ti powder was used as

precursor material and the hydrofluoric acid (HF) as the

shape controlling agent. The size of the nanoflowers are

reported to be 300–700 nm while the truncated tetrago-

nal pyramidal TiO2 nanocrystals have size 50–150 nm.

Following this work, several reports have been published

with tuned (001) facet percentage with various precursor

material. Ma et al.127 considered the shape controlling

agents as disodium ethylene diamine tetraacetate (EDTA)

along with the HF solution while TiF4 as the precur-

sor material and they observed much larger nanoflowers

with length up to 1.2 �m. Numerous studies have been

performed thereafter to enhance the photocatalytic perfor-

mance, dye sensitized solar cell and Li-ion battery applica-

tion simply by controlling the (001) facet percentage.128–129

3.4.3. 3D MicrospheresLiu et al.131 demonstrated a fluoride mediated self-

transformation based fabrication of hollow TiO2 micro-

spheres of diameter 1–2 �m, which is composed of anatase

polyhedral, as shown in Figure 16. Yu and co-workers132

also have shown a novel one-step hydrothermal strat-

egy for the growth of surface-fluorinated TiO2 hollow

microspheres and tabular-shaped anatase single crystals

(001) facets exposed considering ammonium bi-fluoride

(NH4HF2� as a shape controlling agent. Recently, Liu

et al.133 showed a new facile method consisting of elec-

trospray and hydrothermal treatment to grow mesoporous

core–shell TiO2 microspheres composed of tiny anatase

TiO2 nanocrystals and dominated by high-energy facets.

It is proposed that the amorphous TiO2 network formed

by the electrospray plays a crucial role for the forma-

tion of mesoporous TiO2 spheres. The core–shell structure

with very specific surface area as well as exposed high

energy facets makes the system a very promising candi-

date for application in photocatalytic hydrogen evolution

and DSSCs.

3.4.4. 3D Hierarchical MicrostructuresFabrication of 3D hierarchical flower-like nanostructure

has attracted enormous attention due to their large den-

sity of active sites, high surface area and greater pore vol-

ume, as these properties are extremely beneficial for the

adsorption and catalytic application. A glycerol mediated

solvothermal technique followed by calcinations under

various preselected conditions has been demonstrated by

Tian et al.134 (see Fig. 17). The 3D morphology evolution

by dissolution and recrystallization is fully determined by

the reaction species, temperature, and duration of nucle-

ation. Further, H2O2 was also employed as a structure-

tuning agent to grow purely anatase TiO2 microflowers

with (001) facets exposed. As HCl has a tendency to

assist the growth in the rutile phase, an interplay between

the HCl and H2O2 finally determines the crystal struc-

ture, phase, morphology, and facet orientation. The phase

evolution of TiO2 has been observed to vary gradually

Figure 17. (a, b) FESEM images of 3D microstructures at different

magnifications obtained from 24 h solvothermal reaction. (c, d) FESEM

and TEM images of TiO2 microstructures, respectively using 1.0 M H2O2

and 1.6 M HCl. (e, f) FESEM images at different magnifications of (001)

facet exposed 3D TiO2 superstructures synthesized without any kind of

additives or capping agents. Reproduced with permission from [134],

G. Tian, et al., CrystEngComm. 13, 2994 (2011). © 2011, RSC, (a, b).

Reproduced with permission from [135], T. Li, et al., Ind. Eng. Chem.Res. 52, 6704 (2013). © 2013, ACS, (c, d). Reproduced with permission

from [123], G. Li, et al., CrystEngComm. 16, 10547 (2014). © 2014,

RSC, (e, f).



from pure rutile to bi-crystalline phases of anatase and

rutile and finally to pure anatase with increasing H2O2

concentration in the range 0–0.7 M.135 Recently, titanium

glycerolate precursor has been used by Kim et al.136 as

both the TiO2 precursor as well as the sacrificial tem-

plates which discards the requirement of additional sur-

factants or ionic additives. Similar growth mechanism i.e.,

nucleation-aggregation-recrystallization of 3D hierarchical

anatase superstructures with high percentage of exposed

(001) facets has been successfully demonstrated via a sim-

ple one-pot solvothermal strategy.123

4. SURFACE ENGINEERING OF TiO2

NANOSTRUCTURE AND ITSAPPLICATION IN CATALYSIS

In the last few decades, various photocatalysts have been

explored, such as TiO2, ZnO, WO3, Bi2WO6, ZnWO4,

etc. Among them, TiO2 nanostructures have proved its

extremely high potential as a photocatalyst and a posi-

tive sign can be observed in its commercialization as a

photocatalyst. The coupling of different semiconductors

results in improved photocatalytic activity. TiO2 shows

appreciably good photocatalytic activity irrespective of the

nature of the medium of catalytic reaction: in aqueous

pollutants, air pollutants or the CO2 reduction from the

hydrocarbon fuels. Energy and environmental problems

gradually become more serious with the fast development

of technology and economy. Specially, huge amount of

waste water and heavily toxic gases from the various fac-

tories are destroying the diversity of water life by con-

taminating it and introduces non-curable disease in the

human life. Thus, an urgent as well as effective solution

for these problems was needed to maintain the growth

of economy and sustainable development of technology.

TiO2 has been established to be an excellent photocat-

alyst and it can be commercialized due to its abundant

availability in the nature, non-toxicity, strong oxidizing as

well as reducing power and long-term stability towards

the photo and chemical corrosion. The limitations of the

bare TiO2 nanostructures is its UV illumination sensitivity

(wide band gap, anatase: 3.2 eV, rutile: 3.0 eV) and weak

separation efficiency of the photogenerated charge carriers

leading to the efficient recombination which results in low

photocatalytic activity under the solar irradiation.137 After

the photo-excitation, electrons at the conduction band may

attack the oxygen molecules to produce superoxide radi-

cals, and the valence band holes can react with water to

generate hydroxyl radicals simultaneously which in turn

degrades the pollutants, see Figure 1, showing a pos-

sible simple reaction mechanism of the photocatalysis.

In comparison to the photocatalysis, photoelectrocataly-

sis has been shown to have higher efficiency of catalysis

i.e., the degradation pollutants as well as hydrogen gener-

ation. In this case, a low bias voltage applied to the TiO2

electrode can transfer the photocarriers efficiently through

the external circuit inhibiting their fast recombination.

Thus, plenty of active species (hydroxyl, superoxide radi-

cals) present on the TiO2 surface accelerates the catalytic

efficiency.

Being a wide band gap semiconductor, the optical

absorption of TiO2 fully falls in the UV region (∼380 nm).

Besides, the excited state average life time of the photo-

generated electrons and holes is too short to participate

in the redox reaction for bare TiO2 nanoparticles. How-

ever shape tailored 1D, 2D and 3D TiO2 nanostructures

exhibit superior charge transport due to the high specific

surface area, exposed high energy facets, low axial resis-

tivity (for 1D structures), etc. Though the shape tailored

TiO2 architectures are showing better results in degrada-

tion as well as hydrogen evolution it is still necessary

to modify the electronic arrangement in the nanostructure

by incorporating several techniques to make it industry

level efficient. Several modifications have been explored

to prevent recombination of electrons/holes and to achieve

higher photocatalytic activity such as: (1) doping with

metal and non-metal elements, (2) loading noble metal

nanoparticles to drive surface plasmon resonance (SPR)

and to create hot electrons and (3) fabrication of staggered

type (type-II, in general) heterostructure with another

semiconductor for synergic absorption and efficient inter-

facial charge separation for better utilization of solar

energy.

4.1. 0D TiO2 Quantum Dots and Its HSsThe TiO2 QDs can have even larger band gap than its

bulk counterpart making it disadvantageous for the uti-

lization of solar light. As the separation of the photo-

catalyst from the pollute solution after photocatalysis is

an important and inevitable part, use of only TiO2 QDs

for photocatalysis is not recommended. But, a proper het-

erostructure with different phase or microstructure can be

beneficial to create longer life time of charge carriers

through the interfacial charge transfer, making the system

promising. Pan et al.56 demonstrated a TiO2 QD HSs with

2D (001) faceted TiO2 nanosheet. The band position of

narrow band gap 2D TiO2 and large band gap 0D QDs

are suitable enough for the efficient transfer of photogen-

erated electrons from the 2D to 0D surface and vice versa

for the holes. Thus, a uniformly coated TiO2 nanosheet

with TiO2 QDs (synthesized after 168 hrs of hydrothermal

treatment) shows superior degradation efficiency under UV

light, see Figures 18(a)–(c). Besides, a highly stable and

recyclable p–n heterojunction photocatalyst composed of

Cu2O octadecahedra and TiO2 QDs has been studied by

Xu and co-workers.57 Incorporation of TiO2 QDs enhances

the visible light sensitization and the charge transfer from

the p-type Cu2O to n-type TiO2 is thermodynamically

favorable leading to high efficiency (97% in 60 min under

visible light), see the Figures 18(d)–(g).



Figure 18. Photocatalytic degradation rate constants in unit surface area of as-grown samples and P-25 (a), scheme of interfacial charge transfer

between TiO2 QDs and nanosheets (b) and for clean nanosheets (c). (d) UV-visible absorption spectra of TiO2-QDs, Cu2O octadecahedra and their HSs.

The same for P25 has been recorded for comparison, degradation curves of methyl orange (MO) in presence of various catalysts with irradiation (visible)

time (t) (e), stability of Cu2O/TiO2 QDs as photocatalysts (f) and interfacial charge transfer mechanism for enhanced photocatalytic performance (g).

Reproduced with permission from [56], L. Pan, et al., ChemComm. 49, 6593 (2013). © 2013, RSC, (a–c). Reproduced with permission from [57],

X. Xu, et al., ACS Appl. Mater. Interfaces 8, 91 (2016). © 2016, ACS, (d–g).

4.2. 1D TiO2 HSs4.2.1. Doping: Metal and Non-Metal ElementsA controlled incorporation of a suitable secondary element

into the TiO2 lattice may introduce additional energy levels

in between the forbidden region of TiO2 nanostructured

materials and thus can sensitize the enhanced visible light

absorption and suppressed recombination of the photogen-

erated electrons and holes. The first report on the suc-

cessful doping of nitrogen into TiO2 by sputtering method

in a nitrogen rich environment showed extended optical

absorption band from UV to visible region, resulting to the

superior visible light photodegradation of methyl orange

(MO).31 Following this work, several reports have been

published on the doping with various non-metal ions such

as, sulfur (S)139 and fluorine (F)142 fabricated by various

methods including hydrothermal, sputtering and thermal

treatment method (see Fig. 19). Introduction of doping ele-

ment causes the narrowing of the effective band gap of the

TiO2 which facilitates the interfacial transfer of photogen-

erated carries and thus, dramatically enhanced photocat-

alytic activity under visible light irradiation.

Besides the individual doping, the existing literature

is mainly concentrated on the co-doping with N and

F, or N and S (see Fig. 19).90�143 These results mainly

demonstrate the incorporation of additional energy band,

enhanced visible light absorption, lower recombination

rates, making the TiO2 nanostructures suitable to be com-

mercially used. sp2-hybridized single layer graphene has

been widely used for the surface modification of TiO2

nanostructures due to its unique mechanical, electrical, and

thermal properties.138�139�151 Recently, Ge et al.144 showed

improved photocatalytic dye degradation and hydrogen

evolution by a reduced graphene oxide/TiO2 nanotube

composite fabricated by electrodeposition followed by car-

bonation technique.95

Doping with the ions of transition metal such as Fe,

V and Cu has also been explored for the improvement

of the photoelectron conversion efficiency under the solar

light illumination suppressing the recombination of pho-

togenerated charge carriers and stability.145–147 Though the

doping is beneficial for the improvement of charge separa-

tion and photocatalysis, it is observed that the performance

of TiO2 is fully dependent on the doping amount, energy

state, electron configuration of the doped element and also

on the percentage of doping. Excessively high amount of

doping may act as the recombination center rather than the

trap centre, resulting in low photocatalytic activity.

4.2.2. Plasmonic 1D TiO2 HSs with Noble MetalsThe most promising approach for the enhancement of vis-

ible light absorption, high charge separation at the inter-

face and hence the photocatalytic activity is to decorate

the TiO2 nanostructures with noble metal nanoparticles

such as Au, Ag, Pd, Pt, Cu, etc.148–153 In general, Fermi

level position of Au, Ag, Pt, etc. noble metals lies just

below the conduction band of TiO2. Thus, the metal NPs

can act as trap centers for the photogenerated electrons

from valence band to conduction band upon visible light

illumination.154 Besides, a properly designed TiO2 HS with

noble metals involves a coupling interaction between the

plasmonic oscillations and the electromagnetic radiation

incident on the nanostructures with the same frequency.

Due to this strong coupling, excitation of extremely intense

and highly localized surface plasmon resonance (LSPR) is

observed. These plasmonic nanostructures with TiO2 may

be advantageous for the efficient light trapping compo-

nent integrated in photocatalytic devices. The plasmonic



Figure 19. (a, b) Morphological features of N−F co-doped TiO2 nanobelts with different magnifications. (c) Pore diameter distribution and inset

shows the BET surface area analysis of bare TiO2 nanobelt (represented as “B”) and N-F co-doped nanobelts (represented as “A”). (d) UV-visible

absorption spectra of nanobelts and corresponding Tauc plot. (e) Degradation of dye solution with irradiation time in presence of various catalysts.

Reproduced with permission from [90], Z. He, et al., ACS Appl. Mater. Interfaces 4, 6816 (2012). © 2012, ACS, (a–e).

nanostructures drive the LSPR excitation following the

light absorption and eventually the plasmons may decay

non-radiatively in a femtosecond time scale, giving its

energy to the electrons in the conduction band of TiO2.

Thus, highly energetic electrons (1–4 eV for the Ag or

Au NPs) are produced in thermal non-equilibrium with

the atoms of the TiO2 known as “hot electrons” (see

Fig. 20). After the migration of these electrons from the

plasmonic material to the TiO2 in contact forms a metal-

semiconductor Schottky junction, which may accelerates

the rate of photocatalysis.68�155�156

Among the various existing strategies of fabrica-

tion of plasmonic noble metal NPs (photoreduction

method, plasma sputtering, electrodeposition, electrospin-

ning and hydrothermal method) on TiO2 nanostructures,

the hydrothermal method has been established to be

the most easy and versatile method in controlling the

shape, size and dispersion over the TiO2 template. Ye

et al.94 sensitized TiO2 nanotube arrays with Pd QDs

(of size ∼3.3 nm) by a facile hydrothermal method (see

Fig. 21), which showed a superior photoelectrocatalytic

water splitting under UV irradiation. Lin et al.157 reported

the deposition of Pt NPs of size 1.0–3.0 nm titanate nano-

tubes by a wet impregnation method. Nam et al.158 dec-

orated Ag and Au NPs separately with size 5–10 nm on

the TiO2 nanofibers via a one-step electrospinning process

which enables the specific capacity to increase up to 20%

in Li-ion battery application (see Fig. 21). Recently, our

group has reported on the ultrahigh photodegradation effi-

ciency of Ag NPs (size ∼17 nm) decorated anatase TiO2

NRs HSs synthesized by a solvothermal method followed

by a UV light induced photoreduction procedure. Almost

Figure 20. A schematic of generation of hot electrons, their transfer

and creation of Schottky junction at the interface of both systems.



Figure 21. (a, b) SEM and TEM images of Pd/TiO2 nanotube HSs. (c) Amount H2 evolved in photocatalytic water splitting. (d) TEM image of Au

NRs encapsulated by TiO2 layers (e) extinction spectra of Au NRs display two SPR bands at 516 and 655 nm, corresponding to their transverse and

longitudinal SPR modes, respectively. (f) Shows the normalized degradation curves with irradiation time. (g) TEM image of Ag/TiO2 NRs with 1.5:1

weight ratio and (h) sequential decay kinetics of HSs. (i–k) TEM images of TiO2 nanofibers decorated with Au, Pt and both, respectively. Inset shows

the HRTEM lattice fringe patterns. (l) Shows improved H2 evolution by the bi-metal decoration. (m) Schematic diagram showing the LSPR absorption,

generation of hot electrons and their transfer to accelerate the photocatalytic reaction. Reproduced with permission from [94], M. Ye, et al., J. Am.Chem. Soc. 134, 15720 (2012). © 2012, ACS, (a–c). Reproduced with permission from [160], N. Zhou, et al., Nanoscale 5, 4236 (2013). © 2013, RSC,

(d–f). Reproduced with permission from [68], K. K. Paul and P. K. Giri, J. Phys. Chem. C 121, 20016 (2017). © 2017, ACS, (g, h, m). Reproduced

with permission from [161], Z. Zhang, et al., J. Phys. Chem. C 117, 25939 (2013). © 2013, ACS, (i–l).

100% of degradation efficiency with two sequential rate

constants is observed, which was explained on the basis

of plasmonic effect induced enhanced visible light absorp-

tion, hot electrons generated charge transfer through the

interface of the HSs and N-de-ethylation process of RhB.68

Yang et al.159 reported 100% photodegradation of RhB by

Ag nanocrystal (size 3.8 nm) decorated TiO2 nanotubes

under visible light. Zhou et al.160 and Zhang et al.161 have

incorporated Au encapsulation and Au/Pt NPs, respec-

tively with the TiO2 nanostructures and superior photocat-

alytic activity has been achieved.

Wang et al.37 showed a hydrothermally grown TiO2

NWs with a diameter of 100–200 nm anchored with 1 At%

of Pt NPs with the average size ∼5 nm exhibit very high

photodegradation of MB under UV light. Thus, plasmonic

metal NPs modified TiO2 nanostructures can effectively

enhance visible light absorption, suppress the recombina-

tion of photogenerated electron/hole pairs, and facilitate

their transfer through in the interfaces improving the pho-

tocatalytic hydrogen production activity, solar cells, super-

capacitors and lithium batteries.159�161–163

4.2.3. TiO2 Type-II HSsThe discovery of semiconductor heterostructure by Herbert

Kroemer and Zhores I. Alferov opened up a wide

research window in the field of high speed optoelectronics,

photocatalysis and energy storage applications. The band

position and their alignment at near the interface solely

determine the catalytic behavior of a semiconductor het-

erojunction. Among the various semiconductor hetero-

junctions, a type-II (staggered type) band arrangement

of TiO2 with a suitable semiconductor develops a band

bending at the heterojunction resulting to a built-in elec-

tric field, which actually drives the photogenerated elec-

trons and holes in two opposite directions (see Fig. 22).

The formation of heterojunction by coupling an n-type

Figure 22. Schematic band alignment of staggered type semiconductor

heterojunction showing charge separation at the interface.



Figure 23. (a, b) SEM and TEM images of TiO2(B)/anatase core–shell nanowires, (c) Photocatalytic performance in decomposition of dye. (d) TEM

micrograph of Ag2O/TiO2(B) NRs, (e) UV-visible absorption spectra of HSSs along with the bare TiO2, (f) enhanced degradation efficiency of organic

dye MO under visible light. (g) FESEM image of few layer MoS2 coated TiO2 nanobelts (h, i) corresponding photocatalytic activity in dye degradation

and H2 evolution, respectively. (j, k) FESEM images of Copper(II) Phthalocyanine decorated TiO2 nanofibers, (l) corresponding UV-visible absorption,

and (m) degradation efficiency under visible light. (n) A schematic diagram explaining the charge separation and their participation in the redox reaction

to initiate the decomposition process. Reproduced with permission from [165], B. Liu, et al., ACS Appl. Mater. Interfaces 3, 4444 (2011). © 2011,

ACS, (a–c). Reproduced with permission from [48], K. K. Paul, et al., Nanotechnology 27, 315703 (2016). © 2016, IOP Science, (d-f,n). Reproduced

with permission from [167], W. Zhou, et al., Small 9, 140 (2013). © 2013, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim (g–i). Reproduced with

permission from [172], M. Zhang, et al., ACS Appl. Mater. Interfaces 3, 369 (2011). © 2011, ACS, (j–m).

semiconductor (say, TiO2� with a p-type semiconductor

(say, Cu2O, Ag2O)47�163�164 develops an electric field from

n-type to p-type semiconductor, which increases the car-

rier separation efficiency at the interface.

Though numerous fabrication techniques have been

developed for the synthesis of semiconductor HSs

including physical (sputtering, physical vapor deposition)

and chemical (hydrothermal, sol gel, sonochemical, elec-

trodeposition, microwave and chemical vapor deposition)

approaches, hydrothermal technique is extremely popular

due to its low cost, versatility against the shape tuning

and multipurpose advantages over the other processes. Liu

et al.165 synthesized a type-II heterojunction composed of

TiO2 (B)/anatase core–shell nanowire (see Fig. 23). As

the conduction as well as the valence band of anatase lies

above the respective bands of TiO2 (B), an efficient charge

transfer may happen at the junction, which is described

to be responsible for the enhanced photocatalytic activity

towards the decomposition of organic dye, methyl orange

(MO). Zhou and co-workers demonstrated the high photo-

catalytic activity by anatase TiO2 nanobelt designed with

Ag2O NPs in the UV irradiation.166 Following this work

several reports have been published focusing on the mech-

anism of photocatalytic degradation of dye with various

ionic nature.47�164 Zhou et al.167 reported a novel synthesis

route of TiO2 nanobelt wrapped with few layer MoS2 for

the first time via the hydrothermal method (see Fig. 23).

Rough surface of TiO2 nanobelt was used as the template

for the growth of few layer MoS2 film. The 50 wt% of

MoS2 loaded on TiO2 showed maximum photocatalytic

hydrogen production activity at rate of 1.6 mmol h−1g−1.

Following this work, several unique approaches have been

reported for the further improvement with MoS2 layered

structure.168�169 Wang et al.170 incorporated wide band gap

SnO2 nanocrystals on the TiO2 nanofibers via electrospin-

ning method followed by a facile hydrothermal method.

Cadmium sulfide (CdS) is also a commonly used can-

didate to form a type-II HSs. It has the band gap of

∼2.4 eV which is able to absorb visible light up to 520 nm.

Shao et al.171 developed a novel route to deposit CdS NPs

uniformly on the TiO2 nanotubes via a constant current

electrochemical deposition process. Compared to the bare

TiO2 nanotube arrays the CdS/TiO2 HSs showed around

11-fold enhancement in photoelectrochemical activity.

Similar reports with semiconductors CdS QDs,173 CdSe,174

CdS/CdSe,175 C3N4,176 Sn3O4,

177 RuO2,178 Bi2MoO6,

179

Copper(II) phthalocyanine,172 ZnO,180 ZnIn2S1814 have also

explored the development of type-II HSs towards the com-

mercialization of the photocatalysts.

4.3. 2D TiO2 HSsNoble metal NPs loaded highly energetic (001) faceted

2D TiO2 HSs have also been explored extensively.



As discussed earlier, the (001) facets have high aver-

age surface energy resulting to the highly active sites

for the photocatalytic application towards environmental

remediation as well as energy applications. Thus, sev-

eral strategies have been developed to increase the surface

area of exposed (001) facets. The plasmonic noble metal

NPs (Ag, Au, Pd, Pt, etc.) anchored with the 2D TiO2

nanosheets can influence the spectral response, generation

of photoinduced carriers, their separation and participation

in the redox reactions which actually control the kinet-

ics of photocatalysis, as discussed for the 1D TiO2 HSs

case (Section 4.2.2). Till date, various fabrication tech-

niques have been developed to prepare plasmonic noble

metal decorated 2D TiO2 nanostructures such as, impreg-

nation, UV light induced photodeposition, co-precipitation

and sol–gel methods.182–184 As, the photocatalytic activity

in a plasmonic semiconductor nanocomposites is strongly

influenced by the size, shape and the spatial distribution of

the NPs, it is important to develop an easily controllable

methodology to decorate uniform and well-dispersed metal

NPs on the TiO2 nanosheets. Wang et al.185 proposed the

growth of plasmonic coupling photocatalyst, Au/TiO2/Au

nanostructure directly on a Ti foil. The enhancement in the

near-field amplitude of LSPR in the vicinity of metal NPs

boosts the kinetics of photocatalysis of the neighboring

semiconductor. Here, authors have optimized the thickness

(∼5 nm) of the middle layer TiO2 nanosheets to be satis-

fied the condition of required for the coupling effect. Thus,

this system advantageous as a “plasmonic coupling pho-

tocatalyst” compared to the bare TiO2 nanosheet. Besides

the Au NPs, Pt NPs loaded TiO2 nanosheets also reported

by several groups for the enhanced H2 production by

the photocatalytic water splitting. Yu et al.157 reported

Figure 24. (a, b) TEM micrographs of TiO2 nanosheets before and after Pt NPs deposition, respectively, (c) UV-visible absorption spectra of HSs

along with the bare TiO2, (d) enhanced photocatalytic H2 evolution by the HSs. (e, f) TEM micrographs of 2D TiO2 nanosheets prepared by C3N4

template method, before and after Pt loading, respectively, (g) UV-visible absorption spectra of HSs along with the bare TiO2, (h) degradation of C2H4

by the various catalysts. Reproduced with permission from [148], J. Yu, et al., J. Phys. Chem. C 114, 13118 (2010). © 2010, ACS, (a–d). Reproduced

with permission from [187], X. Pan, et al., ACS Appl. Mater. Interfaces 8, 10104 (2016). © 2016, ACS, (e–h).

that the 2 wt% of Pt NPs decorated (001) facet exposed

anatase TiO2 nanosheets exhibit highest H2 evolution of

333.5 �mol h−1. Following this work, Qi et al.186 fabri-

cated CdS sensitized Pt/TiO2 nanosheet HSs with exposed

(001) facets via a hydrothermal treatment followed by a

photochemical reduction (see Fig. 24). It is observed that

the H2 evolved under the visible and UV irradiation are

265 and 590 �mol h−1, respectively. Pan et al.187 have

synthesized TiO2 nanosheets by C3N4 template method

and introduced Pt NPs into it for the higher photocat-

alytic dye degradation. Jiang and co-workers developed a

hybrid nanostructure of TiO2 nanosheets designed with Au

nanocubes and Au nanocages for the sensitization of broad

solar spectrum from visible to NIR region and plasmon-

mediated photocatalytic hydrogen production.188 Addition-

ally Pd nanocubes increase the catalytically active sites

enormously to enhance the hydrogen production rate.

Doping with metal and non-metal elements in (001)

facet exposed TiO2 nanosheet can improve the optical

absorption simply by modifying the band arrangement of

the nanostructure and shifting the threshold of the optical

response value from UV to the visible region. Introduction

of a new element may substitute the lattice oxygen and

form interstitial species in the vicinity of near surface

regions. Thus, visible light sensitization can effectively

be incorporated by narrowing the band gap due to the

formation of mid band gap energy levels of doping ele-

ment. Doping of TiO2 (001) facets with non-metal ele-

ments such as nitrogen was demonstrated by Xiang et al.189

for the first time (see Fig. 25). They observed superior vis-

ible light photocatalytic H2-production activity by the self-

doped TiO2 nanosheets (exposed facet 67%) with nitrogen.

To put the step forward, it was expected that employment



Figure 25. (a) TEM image of N-doped (001) faceted TiO2 sheet, (b) UV-visible absorption spectra along with the P25, inset shows the digital

photographs of the samples. (c) HRTEM image of N-S co-doped TiO2 nanosheet, (d) UV-visible absorption spectra of as grown and doped TiO2. The

absorption band in the visible region is increasing with the doping percentage. Band structure plots (e, g) and total density of states (DOS) (f, h)

for pure TiO2 and N-S co-doped TiO2, respectively, (i) shows the corresponding photocatalytic dye degradation of RhB. Reproduced with permission

from [189], Q. Xiang, et al., ChemComm. 47, 6906 (2011). © 2011, RSC, (a–b). Reproduced with permission from [143], Q. Xiang, et al., Phys.Chem. Chem. Phys. 13, 4853 (2011). © 2011, RSC, (c–i).

of other non-metal element into the (001) faceted TiO2

may also boost the optical absorption. S-doping in the

TiO2 lattice may introduce additional states in between

the wide spaced optical band gap of TiO2 facilitating the

enhanced visible light sensitivity and thus the solar light

driven photocatalysis.

Rather than the single element doping as discussed

above, doping by multiple elements in 2D TiO2 nanosheets

may give synergistic effect resulting to the enhanced pho-

tocatalytic performance.190 Xiang et al.150 reported the

hydrothermal preparation of N and S co-doped (001) facet

exposed TiO2 nanosheets considering thiourea as the dop-

ing precursor. Compared to the bulk TiO2, the N-S co-

doped TiO2 nanosheet exhibits the appearance of a new

absorption shoulder at ∼390–500 nm due to the introduc-

tion of localized states near the valence band edge and

the improved absorption in the range of 550–750 nm as

a result of the combined effect of N and S co-doping,

further confirmed by the DFT calculation. An exception-

ally advantageous hybrid structure by directly growing 2D

anatase TiO2 nanosheets with (001) exposed facets on the

graphene support is useful for the enhanced photocatalytic

hydrogen evolution, fast lithium storage, etc.191–195

4.4. 3D TiO2 HSsAmong the various shaped TiO2 nanostructures, the 3D

superstructures and its composites are least studied system

in the field of energy and environmental applications.

In spite of having several advantageous properties such as

high specific surface area, large number of catalytically

active sites, easy charge transfer through the interfaces, the

3D nanostructure is not well explored may be because of

the tedious synthesis process. But, in 2009 Wu et al.196

reported the growth of truncated wedge-shaped TiO2 nano-

structures as shell over the Au NPs as core by a simple

and flexible hydrothermal route (see Fig. 26). To stabilize

the structure, the immediate layer on the Au NPs is of

(101) faceted TiO2 which minimizes the average surface

energy of the whole system. Then, epitaxially segmented

orientation growth of (001) faceted TiO2 was observed on

the TiO2 layer. The F− ions hydrolyzed TiF4 precursor

acts as the shape controlling agent to form well defined

wedge-like TiO2 shells and also contributes to the exposed

(001) facets. The superior photocatalytic activity of the

Au@TiO2 core–shell HSs was investigated by decom-

position of acetaldehyde in gaseous phase. Wu et al.197

has developed an exceptional strategy for the bottom up

synthesis of AuNR/TiO2 nano-dumbbell like structures

with spatially separated Au and TiO2 regions, by wet-

chemistry technique (see Fig. 26). The as prepared Au

NRs are exposed in their side wall which acts as the

optical window for the incident light as well as it offers

a fast lane transfer of the photogenerated hot electrons

to the TiO2 side, making the AuNR/TiO2 HSs extremely

high efficient for the H2 generation under visible/NIR

light. Separation of photocatalysts after the reaction still

remains a tedious job for the researchers. Shen and

co-workers198 developed a new magnetically separable

plasmonic 3D sandwich-structured Fe3O4/SiO2/Au/TiO2

photocatalyst, which shows easy magnetic separability,

higher absorption in the visible region and thus the

enhanced photocatalytic activity towards the decompo-

sition of organic compounds under the illumination of

simulated sunlight (see Fig. 26). Following these works,

similar reports with other plasmonic NPs like Ag and

graphene have also discussed on the enhancement strategy

of photocatalysis.199–202

The formation of type-II HS composed of 3D TiO2

microstructure grown by hydrothermal method and an

another semiconductor (Ag2O NPs) having suitable band

position was discussed by Sarkar et al.164 Uniform



Figure 26. (a, b) TEM images of the as-grown core–shell Au@TiO2 HSs, (c) schematic diagram showing the proposed pathways of formation of

core-shell Au@TiO2 HSs with truncated wedge-shaped morphology, (d) UV-visible absorption spectra along with the P25, (e) comparative diagram of

photocatalytic decomposition of acetaldehyde in the gas phase. (f) Schematic illustration of the synthesis process of the sandwiched magnetic TiO2

HSs, (g) the detailed illustration of the sandwich-structure, (h) typical SEM image of the composite, (i) TEM image of a single magnetic TiO2 HS,

the enlarged sections show the truncated facets of TiO2. (j, k) Enhanced UV-visible absorption and so as the photocatalysis by the magnetic TiO2

HSs, as shown respectively. (l) TEM image and (m) STEM elemental mapping of AuNR/TiO2 HSs, (n, o) comparison of H2 evolution rate and

degradation of methylene blue by various catalysts under visible illumination, (p) structure and mechanism of enhanced photocatalysis under visible

light by a AuNR/TiO2 dumbbell over a core/shell AuNR@TiO2. Reproduced with permission from [196], X.-F. Wu, et al., Langmuir 25, 6438 (2009).© 2009, ACS, (a–e). Reproduced with permission from [198], J. Shen, et al., J. Mater. Chem. 22, 13341 (2012). © 2012, RSC, (f–k). Reproduced with

permission from [197], B. Wu, et al., J. Am. Chem. Soc. 138, 1114 (2016). © 2016, ACS, (l–p).

decoration of p-type Ag2O NPs on the n-type TiO2 forms

numerous number of type-II p–n heterojunction which

promotes an efficient charge separation due to built-in

electric field at the junction as shown in Figure 27.

Figure 27. (a) FESEM image of Ag2O/TiO2 HSs where (b) shows the enlarged view, (c) TEM image of the above mentioned HSs, (d) exhibits the

enhanced UV light induced dye degradation. (e) FESEM micrograph of TiO2/CdS porous hollow microspheres, (f) the corresponding TEM image,

(h) UV-visible absorption spectra of CdS, TiO2, and TiO2/CdS HSs. (i) shows the average rate of hydrogen evolution and TiO2/CdS porous hollow

microspheres HS gives the maximum rate, labelled as ‘C’. Reproduced with permission from [164], D. Sarkar, et al., ACS Appl. Mater. Interfaces5, 331 (2013). © 2013, ACS, (a–d). Reproduced with permission from [203], Y. Huang, et al., Dalton Trans. 45, 1160 (2016). © 2016, RSC, (e–i).

Thus, an enhanced UV light induced photocatalytic

activity was demonstrated by the optimized HSs with

apparent first-order rate constant of 0.138 min−1. Differ-

ent staggered type HSs of 3D TiO2 demonstrating superior



photocatalytic activity was reported with CdS,203 Cu2O,204

and discussed in details in Refs. [28, 55, 205].

5. CONCLUSION AND OUTLOOK

Since 1972, intense research efforts have been devoted

to the fabrication, properties and surface engineering

of TiO2 nanostructures and their influence in various

applications has given rise to a rich database for the

future developments. The latest breakthrough works on

the synthesis and modifications of shape controlled TiO2

and its composites certainly brought fascinating as well

as advantageous properties for the enhanced photocat-

alytic performance. This review article provided a state

of the art on the advancement of strategies in the

novel fabrication of shape controlled TiO2 with various

dimensionalities followed by their applications in envi-

ronment and energy issues. A proper understanding of

the surface chemistry in controlling the morphology of

TiO2 and its HSs with distinct facets exposed is neces-

sary for the designing of high quality and atmosphere

stable devices. Most of the frequently used fabrica-

tion techniques from 0D to 3D nanostructures includ-

ing hydrothermal/solvothermal method, electrochemical

anodization method, sol–gel method, template-assisted

method, electrospinning method and chemical vapor depo-

sition method have been critically reviewed. Among them,

the hydrothermal route is found to be the most popular

for the fabrication of shape controlled different dimen-

sional TiO2 and its nanoheterostructures. This is because

of the easy control over the size, shape and for the low

cost. Enhancement of the specific surface area as well

as the catalytically active sites, surface modification with

the metal/non-metal doping, formation of type-II and plas-

monic HSs can accelerate the kinetics of photocataly-

sis, solar cells, supercapacitors and lithium-ion battery

performances.

Even though the shape tailored TiO2 based photocat-

alysts have evolved to be most promising material and

capable enough to solve the future needs for diverse appli-

cations, several issues such as efficiency, stability and the

secondary contamination by the photocatalysts limits its

commercial applications and adequate market presence.

The extremely beneficial sulfide based semiconductors like

MoS2, CdS etc. allow superior solar energy harvesting

when coupled to the TiO2. But the aqueous soluble sul-

fur easily gets detached from the system and contaminates

the solution meant to be decontaminate. Besides, the plas-

monic HSs with noble metal NPs are not well stable in

the aqueous medium as most of them tend to be oxidized,

severely reducing the efficiency as a photocatalysts. Addi-

tionally, only 1D TiO2 HSs are observed to be investi-

gated enormously even though the other dimensional (001)

faceted 2D TiO2 nanosheet and 3D superstructures nano-

structures are also expected to be promising and superior

than the 1D structures in some extent (larger specific sur-

face area, active catalytic sites and easy recycling process).

We believe that the low cost TiO2 HSs with high atmo-

sphere stability will be realized in future, which will reach

the needs of the society in the field of energy and environ-

mental applications.

Acknowledgment: We acknowledge the financial sup-

port from MEITY (Grant No. 5(9)/2012-NANO (VOL-II))

for carrying out part of this work.

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Received: 30 September 2017. Accepted: 9 October 2017.


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