The effect of silver nanoparticles/graphene-coupled TiO2 beads photocatalyst on the

photoconversion efficiency of photoelectrochemical hydrogen production

Chun-Ren Ke #, Jyun-Sheng Guo, Yen-Hsun Su, Jyh-Ming Ting *

Department of Materials Science and Engineering, National Cheng Kung University, Tainan

70101, Taiwan

* Corresponding author. Tel.: +886 6 2757575, ext. 62949

E-mail address: jting@mail.ncku.edu.tw (J.M. Ting)

# Present address: (C.R. Ke)

School of Physics and Astronomy and the Photon Science Institute, University of Manchester,

Manchester M13 9PL, United Kingdom

Abstract

In this work, a novel configuration of the photoelectrochemical hydrogen production device is

demonstrated. It is based on TiO2 beads as the primary photoanode material with the addition

of a heterostructure of silver nanoparticles/graphene. The heterostructure not only caters to a

great improvement in light harvesting efficiency (LHE) but also enhances the charge

collection efficiency. For LHE, the optimized cell based on TiO2 beads/Ag/graphene shows a

47% gain as compared to the cell having a photoanode of commercial P25 TiO2 powders. For

the charge collection efficiency, there is a pronounced improvement of an impressive value of

856%. The reason for the improvement in light absorption is attributed to either the light

scattering of TiO2 beads or the surface plasmonic resonance on the Ag

nanoparticles/graphene. The photoconversion efficiency (PCE) of the resulting cells is also

presented and discussed. The PCE of the TiO2 beads/Ag/graphene cell is approximately 2.5

times than that of pure P25 cell.

Keywords:

Graphene; surface plasmonic resonance; TiO2 beads; hydrogen production; silver

nanoparticles; water splitting

1. Introduction

One of the most promising ways for pure energy generation is hydrogen production [1].

The production of hydrogen can be traced back to the year of 1972 when Honda and

Fujishima reported the generation of H2 from a cell containing a TiO2 electrode exposed to

ultraviolet (UV) light irradiation [2]. Following this, researchers have made enormous efforts

in this field [2-19]. Currently, hydrogen production can be achieved by photosensitized water

splitting through photochemical or photoelectrochemical (PEC) reaction. Owing to the fact

that in a PEC system an external circuit is connected, allowing the generated photoelectrons to

be transferred to the counter electrode via the external connection, the charge separation

efficiency is generally better than that of the photochemical type. Therefore separation of

product gases is not required in a PEC cell. In the context of the photocatalyst material

selection for a PEC cell, there are a wide range of UV active semiconducting materials that

have been investigated or developed. Based on the electronic configuration, these UV active

photocatalysts can be classified into four groups: (1) d0 metal (Ti4+, Zr4+, Nb5+, Ta5+, W6+, and

Mo6+) oxides, (2) d10 metal (In3+, Ga3+, Ge4+, Sn4+, and Sb5+) oxides, (3) f0 metal (Ce4+) oxides,

and (4) a small group of non-oxides. Among them, TiO2 has been widely accepted as an

efficient photocatalyst, which is low-cost, non-toxic, and photostable [20-22]. However, the

problem of this material is that it is only active under the UV light, which merely occupies

approximately 4% of the total solar spectrum. In order to utilize solar energy more

effectively, a photocatalyst needs to absorb a wider range of the solar spectrum by expanding

the absorption to the visible light (around 43% of the solar spectrum). There are many

approaches to achieve this goal, of which modifying the band structure and developing

composite materials are pertinent examples. One of the most effective ways to develop

visible light active photocatalysts is to create impurity levels in the forbidden band through

metal ion doping. This enables wide band gap photocatalysts to be active in the visible light

region, and such approach has been known for a long time. Over the past decades, there have

been numerous reports on the modification of wide band gap photocatalysts, including doped

TiO2, SrTiO3, La2Ti2O7, and ZnS. As early as 1982, Borgarello et al. found that Cr5+-doped

TiO2 can constantly produce hydrogen and oxygen under visible light (400-550 nm)

irradiation [23]. Until now, a lot of different metals, such as V, Ni, Cr, Mo, Fe, Sn, and Mn,

have been doped into TiO2 to improve the visible light absorption and photocatalytic

activities. Furthermore, non-metal ion doping is another approach to modify UV light active

photocatalysts. This approach has been widely used to narrow the band gap and improve the

visible light photocatalytic activity. Unlike metal ion dopants, non-metal ion dopants are less

likely to form donor levels in the forbidden band but instead shift the valence band edge

upward. Although visible light active photocatalysts with proper band structures are thus

developed, the efficiency enhancement seems to reach a limit [24].

On the other hand, the issue of photogenerated charge separation is another vital factor

strongly affecting the efficiency of photocatalytic water splitting process. In order to increase

the use of the photogenerated charges and obtain high photocatalytic water splitting activities,

these charges must be effectively separated by transferring the positive and negative charges

to separate sites on the surfaces of the photocatalysts, thus restricting the backward reaction of

hydrogen and oxygen to form water. There are some methods to reach this goal through co-

catalyst loading, semiconductor combinations, and modification of crystal structure and

morphology. For co-catalyst loading, taking Pt as an example, adding this kind of noble metal

can effectively separate the electron-hole pair since the Fermi energy level of noble metal is

always lower than that of semiconductor photocatalysts. For semiconductor combinations, a

well-known example is the use of CdS [25, 26]. Under visible light irradiation, the

photogenerated electrons in the CdS particles rapidly transfer to TiO2 particles, but

photogenerated holes still remain in CdS. By utilizing this method, effective electron-hole

separation and charge recombination prevention can be obtained to improve the photocatalytic

activity. Although this material is an appealing visible light photocatalyst for hydrogen

production, it is unstable in the system, leading to a serious self-oxidation induced by the

photogenerated holes in the valence band. For the modification of crystal structure and

morphology, it has been found that the anatase TiO2 is more active than the rutile TiO2 [27,

28]. This is because the photogenerated electrons trapped in oxygen vacancies of anatase

TiO2 can be easily quenched by the added Pt particles, whereas those in the intrinsic defects of

rutile TiO2 are hardly influenced by the existence of Pt [29]. Decreasing the particle size also

yields a higher efficiency in photocatalysis [30, 31]. Lee et al. reported that smaller NaTaO3

particles with a higher surface area lead to a higher photocatalytic activity in the overall water

splitting owing to the increased probability of surface reactions for gas generation (either O2

or H2) rather than recombination in the bulk [32].

The methods or the materials mentioned above only partially solve the problems such

that the improvement of hydrogen production is limited. For this, graphene provides an

opportunity to break this limit. Graphene discovered in 2004 has been shown to possess

various superior properties, namely, fast room-temperature mobility of charge carriers

(200,000 cm2V-1s-1), exceptional conductivity (106 Scm-1) similar to that of silver, large

theoretical specific surface area (2,630 m2g-1), and excellent optical transmittance (around

97.7%) [33]. With these unique properties, graphene finds itself a great potential to be used in

PEC electrode for hydrogen production for decreasing the recombination rate of

photogenerated electrons-holes pairs due to its fantastic conductivity. It transfers the electrons

in an ultrafast speed, leading to a rapid decrease in the carrier recombination rate, which is

expected to further increase the performance of hydrogen production. The positive

contribution of graphene addition in TiO2 photoanode has been demonstrated in, for example,

dye-sensitized solar cell [34] and perovskite solar cell [35]. Furthermore, graphene also

exhibits advantageous surface plasmonic resonance (SPR) effect as described below.

SPR has been observed between graphene and a semiconductor. This means that, in the

presence of graphene, a semiconductor photoanode can absorb more light, resulting in

increased photocurrent. SPR also occurs in the interface between nanoparticle-Ag and

graphene interface under illuminations of 262 and 422 nm light [36]. Furthermore, loading

metal nanoparticles (such as Au and Ag) on to, for examples, TiO2 or KNbO3, allows the

absorption of visible light also via SPR effect [37-40]. Therefore, the possibility of

combining three materials, for example, Ag, graphene, TiO2, for even enhanced light

absorption through SPR can be expected. However, there is no or little study that takes the

advantage and applies to water splitting cell. The possible positive effects of these two highly

conductive materials (Ag and graphene) on the performance of photoanodes are seldom

addressed. As a result, we have investigated the effects of Ag and graphene additions into

TiO2 photoanode in PEC. Not only the light - harvesting efficiency (LHE) for characterizing

SPR but also charge collection performance for evaluating TiO2/G/Ag bridging are discussed.

The improvement of photoconversion efficiency (PCE) of PECs by using this heterostructures

is also demonstrated. Also, we have used homemade TiO2 beads that give better performance

than the commercial P25 TiO2 particles.

2. Experimental

The active materials used in this study include commercial P25 TiO2 powders (Degussa),

homemade TiO2 beads, home-made graphene oxide (GO), and silver nanoparticles. The beads

were synthesized using a two-step process [41, 42]. In short, a sol-gel method was first used

to form amorphous TiOx particles, followed by a microwave-assisted hydrothermal process

for transforming the particles to crystalline anatase TiO2 beads. The GO was prepared using a

modified Hummer’s method as described elsewhere [43]. The silver nanoparticles were

obtained using a precipitation process where a solution consisting of AgNO3, NaBH4, and

polyvinyl pyrrolidone were used. To prepare the photoanode, the paste was first made by

mixing P25 powders or beads with desired amounts of GO and/or silver nanoparticles. A

mixture of de-ionized (DI) water and t-butanol was used as the solvent for P25-containg

paste. For the paste having beads, anhydrous ethanol was used as the solvent with the

addition of HCl (37%) for forming a homogeneous paste. The concentration of silver

nanoparticles in the resulting paste was 0.001 M. Subsequently, the paste was spin-coated

onto indium tin oxide (ITO)-coated glass substrate. Following 10 min drying in ambient air,

the coating was heated at 450 oC for 4 hr to form photoanodes, which were all controlled to be

around 10 μm. In the PEC water splitting cell, Pt-coated glass was used as the counter

electrode, while a mixture of methanol and NaHCO3 (1 M) was used as the electrolyte. The

methanol served as a sacrificing agent and facilitate the wetting of photoanodes [33]. The

photoanodes and the resulting cells share the same designations, which are, for example, the

photoanode made from the paste containing bead (B), 2 weight % graphene oxide (G), and

silver nanoparticles (A) is designated as BG2%A. If P25 was used, the designation is

PG2%A.

The resulting photoanodes and cells were subjected to various characterizations. The

photoanode morphology was examined using scanning electron microscopy (SEM, JEOL

6701F). UV-vis spectrophotometry (UV-vis, PerkinElmer LAMBDA 950) was used to

determine the light harvesting efficiency (LHE) of the photoanode. The LHE (or so-called

absorption) was calculated via the equation of LHE (%) = 1 – T (%) = 1 – 10 -A, where T is

transmission and A is absorbance obtained from the UV-vis measurement under transmission

mode. X-ray photoelectron spectrometer (XPS, VersaProbe PHI 5000) was used for

analyzing the surface chemistry of the photoanode. Incident photon-to-electron conversion

efficiency (IPCE, IQE 200) or the so-called external quantum efficiency (EQE) of the cells

was also determined. Photochemical hydrogen production was performed in a gas-closed

circulation system with a solar simulator (100 mWcm-2, AM1.5G; Newport 91160A) at room

temperature and the PCE was thus obtained.

3. Results and discussion

The mesoporous TiO2 beads have diameters ranging from 300 to 400 nm and are

composed of numerous nanoparticles (~20 nm in diameter), which are slightly smaller than

the commercial P25 powders (~25 nm in diameter), as shown in Figures S1A in the

Supplementary Materials for the P25 and S1B for the mesopourous TiO2 beads photoanodes,

similar to our previous research [41]. The bead sizes allow light scattering to occur as shown

later. The beads and P25 powders were respectively mixed with GO for making photoanodes

with or without Ag silver particles. The characteristics of the used RGO were examined using

Raman spectroscopy and transmission electron microscopy (TEM) in our previous work [43].

We have found that the Raman peaks were broadened, indicating the multilayer feature of the

obtained GO [44, 45], and the number of the graphene layers in the RGO was 12 based on the

TEM analysis. After the 450 oC heat treatment for making into photoanode, the GO became

reduced GO (RGO), as shown in Fig. 1 for Sample BG2%. The XPS C1s spectrum of BG2%

before the 450 oC heat treatment is shown in Fig. 1A, while that after the heat treatment is

shown in Fig. 1B. After deconvolution using XPS PEAK 41 software, the various bonding

obtained and their concentrations are summarized in Table 1. After the deconvolution, four

distinct peaks were found in the samples before and after heat treatment of 450 oC. The peak

positions are almost identical before and after the heat treatment. For the sample without heat

treatment, the four characteristic peaks are C=C (graphitic sp2 network, 284.6 eV), C-H

(localized hydrocarbon, 285.5 eV), C-O (286.3 eV), and COOH (288.8 eV) [34, 46]. Among

these, C-O shows the highest concentration (near half, 47.3%) and COOH also has a high

percentage of 11.5%, both representing the characteristic of GO. However, after the heat

treatment, there is a significant reduction in the C-O concentration from 47.3% to 15.1%.

This reduction reaction also results in the increase in the C-H ratio, from 1.0% to a

pronounced percentage of 35.7%. Surprisingly, C=O (287.4 eV) has a concentration 6.7%

that cannot be ignored, possibly stemming from the COOH. In addition the concentration of

C=C slightly rises from 40.2% to 42.5%, which is not obvious due to the insufficient heat

treatment temperature. The transformation during heat treatment generally matches previous

study regarding the XPS difference between GO and reduced GO (RGO) [46].

In order to evaluate the light utilization of the photoanodes, the light-harvesting

efficiency (LHE) was determined using the relevant UV-vis measurement data and is shown

in Fig. 2, where Fig. 2A and Fig. 2B represent the P25 and bead group, respectively.

Although the LHE decreases when the wavelength exceeds 400 nm, differences can be found

due to the use of beads and addition of RGO or RGO/silver nanoparticles. For comparison,

the integrated LHE values normalized to that of the P25 sample (LHEnorm) are shown in Table

2. In the P25 group, the LHEnorm of PG1% (0.92) is lower than that of pristine P25 (1.00).

Due to the fact that graphene is more optical transparent than TiO2, the LHEnorm is therefore

lower in PG1%. Should this optical effect continue, the LHEnorm would continue to decrease

as the amount of GO doubles (PG2%). However, the LHEnorm of PG2% (0.97) is better than

that of PG1% (0.92). This suggests that there is another phenomenon happening. We believe

that this is due to the fact that the addition of a sufficient amount graphene, for example, 2%,

generates SPR [47], which therefore enhances the light absorption. For the same reason, the

LHEnorm of PG2%A is higher than that of PG1%A. Furthermore, the LHEnorm of PG1%A

(0.93) and PG2%A (1.00) are respectively higher than that of the counterpart samples PG1%

and PG2% without the addition of Ag. It was further observed that the LHE improvement

due to the addition of graphene or Ag in the P25 group occurs primarily between 400 nm and

470 nm, as shown in Fig. 2C, centering near 430 nm. It has been reported that combining

graphene and silver nanoparticles, more significant resonance at a typical wavelength of 422

nm can be generated [36], and the advantage gained from the Ag nanoparticles without the

addition of graphene has also been demonstrated in ZnO nanocomposites [48]. For the bead

group, the addition of RGO leads to the same effect as in the case of the P25 group. However,

the LHEnorm of the bead group is significantly better than that of the P25 group. As shown in

Fig. 2D, more improvement of LHE occurs from 700 nm and beyond. It has been reported

that the TiO2 beads induce a strong light scattering in long wavelength region [49]. The

scattering effect leads to an increased light travelling distance, allowing the light to travel

among bead, graphene, and silver nanoparticles for a longer time. As this happens, a very

important consequence is that the aforementioned SPR would have an increased probability to

occur. As a result, the LHE of BG2%A is nearly 50% higher than that of pristine P25.

While LHE is used to evaluate photon utilization, it is equally important to investigate

the amount of the generated photoelectrons. Fig. 3 shows the external quantum efficiency

(EQE) data. The EQE values of the bead group are higher than that of the P25 group. Also,

the differences in the EQE values are more significant in the bead group. This appears to

echo the differences in the LHE values (Fig. 2). As seen in Fig. 3B, there is a broad peak in

the vicinity of 850 nm, which corresponds to the increase in the LHE in Fig. 2D. This is

attributed to the scattering effect of the beads, occurring when the light wavelength is a

multiple of the diameters of the beads. To examine the EQE data in details, the normalized

integral EQE values, namely EQEnorm, are also summarized in Table 2. In general, the EQEnorm

of the bead group are higher than that of the P25 group. For instance, the EQEnorm of the bead

is approximately 2.5 times better than that of P25, which is attributed to LHE enhancement as

mentioned above and also the charge collection improvement due to the use of the beads [50].

The addition of GO (1%) improves the EQEnorm, due to enhanced electron transport ability of

the RGO. However, as shown in Table 2, too much RGO (PG2%A) may lead to a lower

EQEnorm as compared to PG1%A. It might be due to the aggregation of RGO, which is often

observed [34], diminishing the RGO bridging effect among TiO2 particles or blocking the

photocatalyst active sites on the TiO2 and the silver nanoparticles from the light [51, 52].

With the addition of Ag, the EQEnorm apparently increases. As shown in Table 2, the

enhancement in the EQE is more than that in the LHE. For the EQE, it is attributed, in part,

to the much better electrical conductivity of both the RGO and the silver. These materials

provide a faster electron transport routes among the TiO2 nanoparticles and between the

substrates and the photocatalysts. On the other hand, SPR occurs as shown above. However,

the SPR is much more apparent in the bead groups. This is due to the fact that the light

scattering inside the bead-containing photocatalysts results in much more light utilization.

This allows more light to pump the SPR to be observed. This also gives a high EQE,

contributing to the much more enhancement in the EQE.

To evaluate the performance of hydrogen production, the photocurrent density Jp and

PCE have been determined using the following equation [53], PCE = Jp*(Vrev-|Vapp|)/Io*100%,

where Vrev is the standard potential for the water splitting reaction (1.23 V), Vapp is the applied

voltage which is self-determined, and Io is the intensity of the incident light (100 mWcm-2).

According to this equation, a higher Jp and/or a lower |Vapp| could result in a better PCE. In

normal circumstances, a larger applied bias leads to a higher Jp, but the relationship between

Vapp and Jp depends on the photoanode material characteristics [53]. This implies that the PCE

can be optimized under a certain applied bias and the current density, so that the PCEs shown

herein are all maximized through tuning the applied voltages. The relevant data are

summarized in Table 3. Both PCE and Jp are important factors to evaluate the efficiency of

hydrogen production. For this, we define a value, CF, to be equal to EQE/LHE and the

normalized CF, i.e., CFnorm, is used to represent the charge collection efficiency. Overall, the

variations of Jp from sample to sample follow that of CFnorm, as shown in Fig. 4. The

correlation coefficient between Jp and CFnorm was found to be 0.51 (medium). These two

parameters both represent a capacity for generated photoelectrons collected by photoanode

and subsequently transferred via external circuit. It is noted that the Jp was measured under

different applied voltages while CFnorm was determined from IPCE that was obtained without

any applied voltage. In general, for both the P25 and bead groups, the existence of RGO

improves the Jp (at an average of ~83% gain) and the addition of silver nanoparticles further

boost the photocurrent (at an average of ~147% gain). In addition, the average Jp of the P25

group, 0.510 mAcm-2, is only slightly lower (~3.4%) than that of the bead group, 0.528

mAcm-2. This is due to the higher average absolute value of the applied voltage (0.806 V for

the P25 group versus 0.785 V for the bead group). It means that the gap between these two

groups should be larger under the same applied voltage, indicating the advantage of using the

beads. Also, it further implies that the equilibrium Fermi level is lower for P25 group so that

the higher Vapp is needed for reaching the optimized PCE, matching to the result that the a

lower LHE generates less photoelectrons. Although it is well known that the rate of hydrogen

generation is proportional to Jp, it should be noted that Jp can be manipulated by varying the

applied bias as mentioned above, and a larger supplied voltage represents a higher cost of

electricity. Therefore, PCE is more worthy of discussion, despite of the fact that many reports

failed to do so. The overall variations of PCE also correlate well with that of the normalized

integrated CFnorm, as shown in Fig. 4. The correlation coefficient between PCE and CFnorm was

found to be 0.87, a very good correlation, owing to the fact that the Eq. 1 includes the

contribution from applied voltage. The figures of PCE show similar correlations with either

the EQE or photocurrent, demonstrating again the effectiveness of the addition of RGO (an

average of 50% gain) and RGO/silver nanoparticles (an average of 100% gain) through the

aforementioned charge collection enhancement as well as the LHE improvement.

Furthermore, the amount of RGO only slightly affects the PCEs except that of BG1%A and

BG2%A. The latter possesses a higher Jp at a lower applied voltage, suggesting not only more

photoelectrons are generated but also are collected, which can be evaluated by the LHE and

CFnorm. Especially for the CFnorm, the obvious enhancement (~3 times) makes major

contribution to the final PCE, indicating again that the usefulness of the CFnorm. Furthermore,

it is seen again that the use of beads enhances not only the LHE but even also the CF norm due

to its specific pore structure which well accommodates the RGO and silver nanoparticles.

Therefore, BG2%A and BG1%A have the best and the second best PCE, 0.38% and 0.28%,

respectively. The PCE of BG2%A is approximately 2.5 times higher than that of P25,

indicating a pertinent strategy for improving efficiency through the combination of bead,

RGO, and silver. However, it was surprisingly found that the PCE of bead-based cell is worse

than that of the P25-based cell. It is speculated that the surface of the beads is relatively

unsuitable for oxygen generation as compared to P25. This can be attributed to the relatively

low oxygen vacancy concentration (more negative) on the bead surface. The negative oxygen

ions in water tend to attach onto P25 rather than TiO2 beads due to electron negativity. As a

result, the oxygen reaction is suppressed and the PCE reduces. Nevertheless, the existence of

highly conductive RGO and/or silver nanoparticles helps photoelectrons move toward the

external circuit and not to be recombined with the accumulated holes.

4. Conclusions

For the first time, the utilization of mesoporous TiO2 beads and Ag/graphene in PEC

cells has been demonstrated. The performance of photoanodes and/or cells has been

discussed in terms of LHE, EQE, CFnorm or charge collection ability, and most importantly,

PCE. The significant improvement of 47% in LHE was obtained in a cell using TiO 2

beads/Ag/graphene structure (BG2%A) as compared to that uses pristine P25. Furthermore,

there is a pronounced enhancement in charge collection ability, which was embodied by

introducing a factor CFnorm, reaching an impressive value of 856% as compared with the same

P25 cell. The improvement for LHE is attributed to either the light scattering of TiO2 beads or

the surface plasmonic resonance on the Ag nanoparticles/graphene, which was shown from

UV-vis measurement. Moreover, we have also demonstrated the PCE of the water splitting

devices via a reasonable equation. Our best cell (BG2%A) shows approximately 2.5 times

PCE as compared to pure P25 cell. This suggests further opportunity for future TiO2-based

hydrogen production cells by employing a light scattering structure like beads as well as a

functional heterostructures of Ag/graphene.

Acknowledgements

This research was supported by the Ministry of Science and Technology in Taiwan under

Grant No. ： MOST 104-2221-E-006-026-MY3, and the Headquarters of University

Advancement at National Cheng Kung University, which is sponsored by the Ministry of

Education, Taiwan.

References

[1] Gao M-R, Xu Y-F, Jiang J and Yu S-H 2013 Nanostructured metal chalcogenides:

synthesis, modification, and applications in energy conversion and storage devices

Chemical Society Reviews 42 2986-3017

[2] Fujishima A and Honda K 1972 Photolysis-decomposition of water at the surface of an

irradiated semiconductor nature 238 37-8

[3] Walter M G, Warren E L, McKone J R, Boettcher S W, Mi Q, Santori E A and Lewis

N S 2010 Solar water splitting cells Chemical reviews 110 6446-73

[4] Bard A J 1979 Photoelectrochemistry and heterogeneous photo-catalysis at

semiconductors Journal of Photochemistry 10 59-75

[5] Kudo A and Miseki Y 2009 Heterogeneous photocatalyst materials for water splitting

Chemical Society Reviews 38 253-78

[6] Chen X, Shen S, Guo L and Mao S S 2010 Semiconductor-based photocatalytic

hydrogen generation Chemical reviews 110 6503-70

[7] Maeda K and Domen K 2010 Photocatalytic water splitting: recent progress and future

challenges The Journal of Physical Chemistry Letters 1 2655-61

[8] Fox M A and Dulay M T 1993 Heterogeneous photocatalysis Chemical reviews 93

341-57

[9] Fujishima A, Zhang X and Tryk D A 2007 Heterogeneous photocatalysis: from water

photolysis to applications in environmental cleanup International journal of hydrogen

energy 32 2664-72

[10] Kamat P V 2007 Meeting the clean energy demand: nanostructure architectures for

solar energy conversion The Journal of Physical Chemistry C 111 2834-60

[11] Esswein A J and Nocera D G 2007 Hydrogen production by molecular photocatalysis

Chemical reviews 107 4022-47

[12] Tong H, Ouyang S, Bi Y, Umezawa N, Oshikiri M and Ye J 2012 Nano‐photocatalytic

Materials: Possibilities and Challenges Advanced Materials 24 229-51

[13] Chen X and Mao S S 2007 Titanium dioxide nanomaterials: synthesis, properties,

modifications, and applications Chemical reviews 107 2891-959

[14] Linsebigler A L, Lu G and Yates Jr J T 1995 Photocatalysis on TiO2 surfaces:

principles, mechanisms, and selected results Chemical reviews 95 735-58

[15] Osterloh F E 2007 Inorganic materials as catalysts for photochemical splitting of water

Chemistry of Materials 20 35-54

[16] Li C-J, Xu G-R, Zhang B and Gong J R 2012 High selectivity in visible-light-driven

partial photocatalytic oxidation of benzyl alcohol into benzaldehyde over single-

crystalline rutile TiO< sub> 2</sub> nanorods Applied Catalysis B: Environmental

115 201-8

[17] Li C-J, Wang J-N, Wang B, Gong J R and Lin Z 2012 A novel magnetically separable

TiO< sub> 2</sub>/CoFe< sub> 2</sub> O< sub> 4</sub> nanofiber with high

photocatalytic activity under UV–vis light Materials Research Bulletin 47 333-7

[18] Li C-J, Wang J-N, Wang B, Gong J R and Lin Z 2012 Direct formation of reusable

TiO2/CoFe2O4 heterogeneous photocatalytic fibers via two-spinneret electrospinning

Journal of nanoscience and nanotechnology 12 2496-502

[19] Zhang K, Liu Q, Wang H, Zhang R, Wu C and Gong J R 2013 TiO2 Single Crystal

with Four‐Truncated‐Bipyramid Morphology as an Efficient Photocatalyst for

Hydrogen Production Small 9 2452-9

[20] Jang J S, Kim H G and Lee J S 2012 Heterojunction semiconductors: a strategy to

develop efficient photocatalytic materials for visible light water splitting Catalysis

Today 185 270-7

[21] Kubacka A, Fernandez-Garcia M and Colon G 2011 Advanced nanoarchitectures for

solar photocatalytic applications Chemical reviews 112 1555-614

[22] Tran P D, Wong L H, Barber J and Loo J S 2012 Recent advances in hybrid

photocatalysts for solar fuel production Energy & Environmental Science 5 5902-18

[23] Borgarello E, Kiwi J, Graetzel M, Pelizzetti E and Visca M 1982 Visible light induced

water cleavage in colloidal solutions of chromium-doped titanium dioxide particles

Journal of the American Chemical Society 104 2996-3002

[24] Kudo A 2007 Recent progress in the development of visible light-driven powdered

photocatalysts for water splitting International journal of hydrogen energy 32 2673-8

[25] Peng T, Dai K, Yi H, Ke D, Cai P and Zan L 2008 Photosensitization of different

ruthenium (II) complex dyes on TiO< sub> 2</sub> for photocatalytic H< sub>

2</sub> evolution under visible-light Chemical Physics Letters 460 216-9

[26] Jang J S, Li W, Oh S H and Lee J S 2006 Fabrication of CdS/TiO< sub> 2</sub>

nano-bulk composite photocatalysts for hydrogen production from aqueous H< sub>

2</sub> S solution under visible light Chemical Physics Letters 425 278-82

[27] Spanhel L, Weller H and Henglein A 1987 Photochemistry of semiconductor colloids.

22. Electron ejection from illuminated cadmium sulfide into attached titanium and

zinc oxide particles Journal of the American Chemical Society 109 6632-5

[28] Bickley R I, Gonzalez-Carreno T, Lees J S, Palmisano L and Tilley R J 1991 A

structural investigation of titanium dioxide photocatalysts Journal of Solid State

Chemistry 92 178-90

[29] Shi J, Chen J, Feng Z, Chen T, Lian Y, Wang X and Li C 2007 Photoluminescence

characteristics of TiO2 and their relationship to the photoassisted reaction of

water/methanol mixture The Journal of Physical Chemistry C 111 693-9

[30] Ikeda S, Tanaka A, Shinohara K, Hara M, Kondo J N, Maruya K-i and Domen K 1997

Effect of the particle size for photocatalytic decomposition of water on Ni-loaded K<

sub> 4</sub> Nb< sub> 6</sub> O< sub> 17</sub> Microporous materials 9 253-8

[31] Hidalgo M, Aguilar M, Maicu M, Navío J A and Colón G 2007 Hydrothermal

preparation of highly photoactive TiO< sub> 2</sub> nanoparticles Catalysis Today

129 50-8

[32] Lee Y, Watanabe T, Takata T, Hara M, Yoshimura M and Domen K 2007

Hydrothermal synthesis of fine NaTaO3 powder as a highly efficient photocatalyst for

overall water splitting Bulletin of the Chemical Society of Japan 80 423-8

[33] Geim A K and Novoselov K S 2007 The rise of graphene Nature Materials 6 183-91

[34] Wu T T and Ting J M 2014 Bridging TiO2 nanoparticles using graphene for use in

dye‐sensitized solar cells International Journal of Energy Research 38 1438-45

[35] Wang J T-W, Ball J M, Barea E M, Abate A, Alexander-Webber J A, Huang J, Saliba

M, Mora-Sero I n, Bisquert J and Snaith H J 2013 Low-temperature processed

electron collection layers of Graphene/TiO2 nanocomposites in thin film perovskite

solar cells Nano Letters 14 724-30

[36] Li J and Liu C y 2010 Ag/graphene heterostructures: synthesis, characterization and

optical properties European Journal of Inorganic Chemistry 2010 1244-8

[37] Zhang H, Chen G and Bahnemann D W 2009 Photoelectrocatalytic materials for

environmental applications Journal of Materials Chemistry 19 5089-121

[38] Tada H, Kiyonaga T and Naya S-i 2009 Rational design and applications of highly

efficient reaction systems photocatalyzed by noble metal nanoparticle-loaded titanium

(IV) dioxide Chemical Society Reviews 38 1849-58

[39] Lan J, Zhou X, Liu G, Yu J, Zhang J, Zhi L and Nie G 2011 Enhancing photocatalytic

activity of one-dimensional KNbO 3 nanowires by Au nanoparticles under ultraviolet

and visible-light Nanoscale 3 5161-7

[40] Naya S-i, Teranishi M, Isobe T and Tada H 2010 Light wavelength-switchable

photocatalytic reaction by gold nanoparticle-loaded titanium (IV) dioxide Chemical

Communications 46 815-7

[41] Hsu C-W, Chen P and Ting J-M 2013 Microwave-Assisted Hydrothermal Synthesis of

TiO2 Mesoporous Beads Having C and/or N Doping for Use in High Efficiency All-

Plastic Flexible Dye-Sensitized Solar Cells Journal of The Electrochemical Society

160 H160-H5

[42] Ke C R, Chen L C and Ting J M 2012 Photoanodes Consisting of Mesoporous Anatase

TiO2 Beads with Various Sizes for High-Efficiency Flexible Dye-Sensitized Solar

Journal of Physical Chemistry C 116 2600-7

[43] Wu T-T and Ting J-M 2013 Preparation and characteristics of graphene oxide and its

thin films Surface and Coatings Technology 231 487-91

[44] Zhang D, Pu X, Ding G, Shao X, Gao Y, Liu J, Gao M and Li Y 2013 Two-phase

hydrothermal synthesis of TiO 2–graphene hybrids with improved photocatalytic

activity Journal of Alloys and Compounds 572 199-204

[45] Gao Y, Pu X, Zhang D, Ding G, Shao X and Ma J 2012 Combustion synthesis of

graphene oxide–TiO 2 hybrid materials for photodegradation of methyl orange Carbon

50 4093-101

[46] Paredes J, Villar-Rodil S, Solis-Fernandez P, Martinez-Alonso A and Tascon J 2009

Atomic force and scanning tunneling microscopy imaging of graphene nanosheets

derived from graphite oxide Langmuir 25 5957-68

[47] Papasimakis N, Luo Z, Shen Z X, De Angelis F, Di Fabrizio E, Nikolaenko A E and

Zheludev N I 2010 Graphene in a photonic metamaterial Optics express 18 8353-9

[48] Zhang D, Pu X, Li H, Yu Y M, Shim J J, Cai P, Kim S I and Seo H J 2015 Microwave-

assisted combustion synthesis of Ag/ZnO nanocomposites and their photocatalytic

activities under ultraviolet and visible-light irradiation Materials Research Bulletin 61

321-5

[49] Chen D H, Huang F Z, Cheng Y B and Caruso R A 2009 Mesoporous Anatase TiO2

Beads with High Surface Areas and Controllable Pore Sizes: A Superior Candidate for

High-Performance Dye-Sensitized Solar Cells Advanced Materials 21 2206-+

[50] Ke C R and Ting J M 2012 Anatase TiO2 beads having ultra-fast electron diffusion

rates for use in low temperature flexible dye-sensitized solar cells Journal of Power

Sources 208 316-21

[51] Li Q, Guo B, Yu J, Ran J, Zhang B, Yan H and Gong J R 2011 Highly efficient visible-

light-driven photocatalytic hydrogen production of CdS-cluster-decorated graphene

nanosheets Journal of the American Chemical Society 133 10878-84

[52] Xiang Q, Yu J and Jaroniec M 2011 Preparation and enhanced visible-light

photocatalytic H2-production activity of graphene/C3N4 composites The Journal of

Physical Chemistry C 115 7355-63

[53] Khan S U, Al-Shahry M and Ingler W B 2002 Efficient photochemical water splitting

by a chemically modified n-TiO2 Science 297 2243-5

Table 1 XPS data obtained from Fig. 1

ParameterHeat

treatmentC=C C-H C-O C=O COOH

Integrated ratio Before 40.2% 1.0% 47.3% 0.0% 11.5%

Integrated ratio After 42.5% 35.7% 15.1% 6.7% 0.0%

Table 2 The data from UV-vis and IPCE measurement for various hydrogen production samples

Sample P25PG1

%PG2% PG1%A PG2%A Bead BG1% BG2% BG1%A BG2%A

Normalized integrated

LHEnorm (a.u.)1.00 0.92 0.97 0.93 1.00 1.42 1.22 1.39 1.45 1.47

Normalized integrated

EQEnorm (a.u.)1.00 3.06 - 4.85 3.73 2.45 4.30 - 4.80 12.60

Normalized collection factor

CFnorm (a.u.)1.00 3.31 - 5.19 3.71 1.72 3.51 - 3.31 8.56

Table 3 The data from photo conversion efficiency (PCE) measurement for various hydrogen production samples

Sample P25 PG1% PG2% PG1%A PG2%A Bead BG1% BG2%BG1%

ABG2%A

Vapp

(V)-0.710 -0.716 -0.710 -0.963 -0.931 -0.748 -0.939 -0.961 -0.679 -0.598

Jp

(mAcm-2)0.28 0.34 0.36 0.87 0.70 0.26 0.88 0.40 0.50 0.60

PCE

(%)0.15% 0.17% 0.19% 0.23% 0.21% 0.13% 0.26% 0.22% 0.28% 0.38%

(A)

(B)

Fig. 1. XPS C1s spectra of BG2% photoanode (A) before (B) after the 450 oC heat treatment.

(A)

(B)

(C)

(D)

Fig. 2. LHE of (A) P25 and (B) Bead groups. Enlarged views showing the LHE of (C) P25

group between 400 and 470 nm, and (D) Bead group between 700 and 1100 nm.

(A)

(B)

Fig. 3 The values of EQE obtained from IPCE measurements for the (A) P25 and (B) bead

groups.

Fig. 4 The variations of CF, Jp, and PCE.