Role of graphene in photocatalytic water splitting for hydrogen production

C H A P T E R

5 Role of graphene in photocatalytic water splitting for hydrogen production Mohamed I. Fadlalla, Sundaram Ganesh Babu Department of Chemical Engineering, University of Cape Town, Rondebosch, Cape Town, South Africa

1. Introduction Renewable energy sources are currently a critical area for researchers, due to the increase in the energy demand for different usage (e.g., manufacturing and transportation) and the major environmental impact of nonrenewable energy sources. There are different types of renewable energy, such as wind, solar, and hydrogen, where the latter has a high burning energy, thus being a promising energy source candidate [1]. Although hydrogen is considered as a green energy source, it is produced mainly via fossil fuel (̴ 95%), the balance of 5% is generated by water electrolysis [1e3]. Thus, clean and environmental processes from hydrogen production are from water. Photochemistry and photocatalysis are processes for hydrogen generation from water, where in the former the reactants are in the ground state initially and in the latter the reactants are in the excited state. The two processes also differ in other process aspects and advantages and disadvantages. To start with the photocatalysis process, advantages include the use of cheap, readily available visible light in form of sunlight. Furthermore, the photocatalysis process has no environmental impact; on the contrary, the process provides an environmental and energy balanced process. Photocatalysis process consist of three steps: (1) atoms or groups in the catalyst are excited via light and result in formation of radical photocatalyst, (2) The electron transfer between the substrate and the catalyst results in formation of radical anion or cation, and lastly (3) the reaction takes place on the metal centers dispersed on the catalyst surface [4]. On the other hand, the photochemical process faces major disadvantages, namely, the fact that organic compounds only absorb high energy light, thus, expensive and high-energy ultraviolent lamp (photoreactor).

Graphene-Based Nanotechnologies for Energy and Environmental Applications https://doi.org/10.1016/B978-0-12-815811-1.00005-3

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5. Role of graphene in photocatalytic water splitting for hydrogen production

Furthermore, the need for a photoreactor caused industrial challenges for industrial-scale hydrogen production via photochemical process. In 2004, Novoselov and coworkers reported the discovery of carbon-based material, in which the carbon atoms are in hexagonal geometry and organized in a two-dimensional monolayer sheet [5]. The properties of graphene can be tailored or controlled via the synthesis method and the chemical state of graphene; furthermore, the graphene surface can be modified by adding functionalized groups [4]. The key properties of graphene are high electronic property, mechanical strength, and efficient thermal stability and surface area [6,7]. These properties are critical in the catalysis field in general; therefore, (un)doped graphene is used in different areas in the catalysis field [6e10], such as hydrogen production [11,12] metal/oxide-free graphene catalysis [13], and oxygen reduction reaction [4,14]. In this chapter, the focus is to combine two critical topics in the catalysis sphere, in the form of hydrogen production via water splitting during photocatalysis and the use of graphene-based catalyst, as well as the type of semiconductors and number of metals (e.g., mono-semiconductor and bi-semiconductor), and use of cocatalyst such as noble metals (e.g., Pt) and transition metals (e.g., Ni). The mechanism and sacrificial agent role will also be discussed in this chapter in relation to presence or lack of cocatalyst as well as graphene loading or lack of altering the electron transport route and charge separation.

2. Role of graphene in water splitting Owing to the high surface of area of graphene, it is an excellent support material for the semiconducting photocatalyst materials. Furthermore, the defective cites present in graphene-based materials act as a nucleation center for the semiconducting metal oxide nanoparticles. Similarly, the graphene-based material, specifically reduced graphene oxide (rGO) with negatively charged surface, exhibits an attractive material for the metal oxides to adsorb. Due to this attractive force, the metal oxide nanoparticle becomes immobile, which prevents the agglomeration of the metal oxide semiconducting nanoparticles that improve the durability of the photocatalyst and also enhance the reusability.

2.1 Graphene based mono-semiconducting catalyst TiO2 is extensively used in the hydrogen production from water splitting via photocatalysis due to it low cost, nontoxicity, and high stability [15]. Zhang et al. [16] investigated the photocatalytic activity (using 500 W Xe lamp and infrared part by water filters) of TiO2/graphene sheets (GS) in comparison to P25. The results obtained (see Fig. 5.1) shows the synergistic effect of TiO2/GS, as demonstrated by increase H2 evolution from 4.5 mmol he1 to 5.4 mmol h
1, over TiO2/0.8 wt% GS and TiO2/2.0 wt% GS, respectively. The increase in H2 production with increase in GS content is believed to be due to the high GS electron conductivity. However, a further increase in GS content to 5 wt% shows the H2 evolution decrease to 3.9 mmol h
1, and H2 evolution increase to 4.7 mmol h
1 over 10 wt% GS. The authors suggested this due to changes in the TiO2 average size with increase in GS content as shown by the XRD results. The authors concluded the improved activity is due to GS

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FIGURE 5.1 The effect of graphene oxide (GO) content in TiO2/GO composite on H2 evolution as function of time. GO content of 0.8, 2, 5, 10, and 0 wt% donated as a, b, c, d, and e, respectively. Reproduced with permission from Zhang X, Sun Y, Cui X, Jiang Z. A green and facile synthesis of TiO2/graphene nanocomposites and their photocatalytic activity for hydrogen evolution. Int J Hydrogen Energy 2012;37:811e815.

suppressed electronehole recombination [17]. A different report by Zhang et al. [18] showed the influence of GS content and calcination environment. The photocatalytic activity was carried out with 500 Xe lamp as light source. Fig. 5.2 shows the effect of GS content and calcination atmosphere, the results showed the increase in GS content (up to 5 wt% GS) increased H2 evolution and this is due to GS high electron conductivity and limiting electronehole recombination, in which a further increase in GS content leads to a decrease in activity by

FIGURE 5.2 The effect of heat treatment environment (i.e., air or nitrogen) of the photocatalytic production of H2 over TiO2/graphene sheet (GS) with varied GS content. Reproduced with permission from Zhang X-Y, Li H-P, Cui X-L, Lin Y. Graphene/TiO2 nanocomposites: synthesis, characterization and application in hydrogen evolution from water photocatalytic splitting. J Mater Chem 2010;20:2801e2806.

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5. Role of graphene in photocatalytic water splitting for hydrogen production

H2 evolution rate (µmol h-1)

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FIGURE 5.3 The effect of P25/rGO (mass ratio1/0.2) synthesis method on photocatalytic production of H2 and H2 production over P25/CNT with different CNT loading. CNT, carbon nanotube; rGO, reduced graphene oxide. Reproduced with permission from Fan W, Lai Q, Zhang Q, Wang Y. Nanocomposites of TiO2 and reduced graphene oxide as efficient photocatalysts for hydrogen evolution. J Phys Chem C 2011;115:10694e10701.

generating electronehole recombination centers. Catalyst thermal treatment under nitrogen atmosphere leads to an increase of the catalyst activity as shown in Fig. 5.2. This is due to oxygen vacancies generated under thermal treatment conditions [19], where these vacancies play the role of electron traps and therefore, the photocatalytic activity increased. Fan et al. [20] synthesized P25 TiO2 and rGO by hydrazine reduction, UV-assisted photocatalytic reduction, and hydrothermal method in order to study the effect of composite synthesis method on the H2 evolution in water splitting, using methanol as a sacrifice agent. The result obtained (Fig. 5.3) shows the most active composite prepared via the hydrothermal method. The Raman spectroscopy results showed this method yields rGO with lower defects in comparison to the other methods (i.e., hydrazine reduction and UV-assisted photocatalytic reduction); these results are in agreement with the report by Zhou et al. [21]. Furthermore, the optimum TiO2/rGO ratio was determined as 1/0.2, and methanol is the most efficient sacrifice agent in comparison with ethanol and iso-propanol. Li et al. [22] reported a template-free self-assembly method for the synthesis of mesoporous anatase nanosphere/graphene composite for the hydrogen production via photocatalytic water splitting reaction, with methanol as a scavenger. The results obtained shows the superior H2 evolution of TiO2/graphene composite versus TiO2. This improved activity is attributed to the high conductivity of graphene, in which the photoexcited electron from the TiO2 conduction band is transferred to the graphene, since the Fermi level in graphene is lower than conduction band on TiO2 (Fig. 5.4) [23].

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FIGURE 5.4 Schematic energy-level diagram for the GS/TiO2 dye-sensitized solar cells (DSSCs). Reproduced with permission from Tang Y-B, Lee C-S, Xu J, Liu Z-T, Chen Z-H, He Z, Cao Y-L, Yuan G, Song H, Chen L, Luo L, Cheng H-M, Zhang W-J, Bello I, Lee S-T. Incorporation of graphenes in nanostructured TiO2 films via molecular grafting for dye-sensitized solar cell application. ACS Nano 2010;4:3482e3488.

The superiority of TiO2/rGO in hydrogen production was further demonstrated by Shen et al. [24]. The composite was synthesized through a modified one-step hydrothermal method and test in photocatalytic water splitting with UV light (150 mW cm
2) and Na2S þ Na2SO3 as scavengers. Fig. 5.5 showed the improved H2 evolution with composite in comparison with the pure TiO2 as function of exposure time. The improved activity of the composite is a result of electron transfer through the interface from TiO2 to graphene; this transfer is facilitated by the difference in the energy levels between the TiO2 and graphene. This transfer limits or prevents the electronehole charge recombination [25,26]. Cheng et al. [27] prepared P25 TiO2/graphene composite with different graphene content by solvothermal method and investigated the composites in photocatalytic hydrogen production from water splitting. The light source and scavenger used in the study were Xe lamp and methanol, respectively. Graphene content in the composite played a key role in terms of H2 evolution (Fig. 5.6), where increase of graphene content increased the amount of H2

FIGURE 5.5 H2 production via photocatalytic water splitting reaction as function of catalyst content and irradiation time. Reproduced with permission from Shen J, Yan B, Shi M, Ma H, Li N, Ye M. One step hydrothermal synthesis of TiO2-reduced graphene oxide sheets. J Mater Chem 2011;21:3415e3421.

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FIGURE 5.6 H2 production as function of GR content in the composite with 0.5 wt% Pt and irradiation time. Reproduced with permission from TiO2egraphene nanocomposites for photocatalytic hydrogen production from splitting water. Int J Hydrogen Energy 2012;37:2224e2230.

produced, up to 0.5% graphene content. Further increase in graphene content led to a decrease in the catalytic activity. The increase in H2 production is attributed to graphene limiting or preventing an electronehole recombination. This is achieved by transferring the photoexcited electron from the conduction band of P25eTiO2 to the graphene via percolation mechanism [28]. Moreover, graphene consists of a two-dimensional p-conjugative structure; this structure makes graphene a competitive acceptor material [29]. Xiang et al. [30] used a microwaveehydrothermal method for the treatment of graphene oxide (GO), while TiO2 with (001) facets exposed were synthesized by the hydrothermal method to yield a graphene/TiO2 nanosheet composite. The activity of the composite was investigated in the photocatalytic activity for production of H2 from water splitting reaction, using methanol and UV light irradiation as sacrificial agent and light source, respectively. The influence of the graphene content in the composite on the photocatalytic activity is shown in Fig. 5.7. The H2 production increased with the increase in graphene up to 1 wt% (41 times higher than TiO2), where a further increase in the graphene content led to a decrease in H2 production. The authors explain the improved activity by the limitation of electronehole recombination, which is because of electron transfer from the conduction band of TiO2 to graphene due to close proximity. This was confirmed by comparing the optimum composite with TiO2/Degussa (P1.0) (Fig. 5.7). The decrease in the photocatalytic activity with graphene content (greater than 1 wt%) is due to the graphene loading shielding TiO2 from the light via light scattering and opacity [31e34]. Furthermore, the authors proposed a photocatalytic H2 production over the TiO2/graphene composite mechanism as illustrated in Fig. 5.8. Ye et al. [35] studied the water splitting photocatalytic H2 production over CdSegraphene and CdSecarbon nanotubes, using Na2S and Na2SO3 and Xe lamp (200 W) as sacrificial agent and light source, respectively. The photocatalytic water splitting increased significantly with the incorporation of the carbon structure (i.e., GO or carbon nanotubes) into CdS, as indicated by the increase in H2 production (Fig. 5.9 and Fig. 5.10). The mass ratio of CdS:GO or carbon nanotube was also investigated, and the optimum CdS:GO determined is 1:0.01, while 1:0.05 ratio of CdS:carbon nanotube was the ideal mass ratio. The comparison by the CdS:graphene

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FIGURE 5.7 Hydrogen production as function of graphene content in graphene/TiO2 catalyst. G0, G0.2, G0.5, G1, G2.0, G5.0 represents 0, 0.2, 0.5, 1.0, 2.0, 5.0 graphene wt%. P1.0 is 1.0 wt% graphene/Degussa P25 composite. Reproduced with permission from Xiang Q, Yu J, Jaroniec M. Enhanced photocatalytic H2-production activity of graphenemodified titania nanosheets. Nanoscale 2011;3:3670e3678.

and CdS:carbon nanotube showed superior H2 evolution over CdS:graphene than over CdS:carbon nanotube nanocomposite (Fig. 5.11). The improved H2 evolution over the nanocomposite is attributed to the separation of generated electron and hole. Moreover, the higher activity of CdS:graphene is explained by the size of the interface area between CdS and graphene or the stronger interaction between the two components of the nanocomposite, which increases electron transfer and limitation in the electronehole recombination, thus, increasing the H2 evolution. Furthermore, the authors compared the nanocomposites with CdS:TiO2, which was reported to increase the CdS photocatalytic activity [36], and the results obtained showed the higher efficiency of the nanocomposite and the importance of the synthesis method to obtain high nanocomposite activity. Lv et al. [37] studied both CdS or TiO2/graphene composite prepared by a one-pot solution method, for the production of H2 from water splitting reaction using methanol as a sacrificial agent. The results obtained are in agreement with other studies. The inclusion of graphene in the composite increased the H2 production by altering the electron transfer route to the graphene, hence suppressing charge recombination taking place (Fig. 5.12). However, there is an ideal graphene content (Fig. 5.13) to obtain higher electron transfer activity without limiting light scattering due to the opacity of graphene. Peng et al. [38] prepared GOeCdS nanocomposite via facile precipitation method to study the influence of GO as a cocatalyst and support in photocatalytic hydrogen production, utilizing Na2S/Na2SO3 as sacrificial agent and Xe lamp (300 W) as light source. The authors demonstrated the effect of GO content in the nanocomposite (Fig. 5.14), following a volcanic plot type. The highest hydrogen evolution was achieved with the 5 wt% GO; this enhancement in activity is attributed to graphene introducing charge separation by acting as an electron acceptor [37,39e41], thus limiting electronehole recombination. However, a further increase in graphene content leads to a decrease in hydrogen evolution; this is due to graphene acting as a shield, limiting CdS and light interaction. These results are in an agreement with the Lv et al. [37] findings. It is important to point to the low hydrogen production over

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FIGURE 5.8 (A) Illustration of charge movement and separation over the graphene/TiO2 composite, (B) proposed photocatalytic mechanism for H2 production via photocatalysis over graphene/TiO2 composite. Reproduced with permission from Xiang Q, Yu J, Jaroniec M. Enhanced photocatalytic H2-production activity of graphenemodified titania nanosheets. Nanoscale 2011;3:3670e3678.

GO, which is due to GO antibonding p* orbital having higher energy than the energy need for hydrogen production [42]. The authors proposed the mechanism illustrated in Fig. 5.15 based on the results obtained. The conduction band in CdS is responsible for the production of photoexcited electron, which is then transferred rapidly via the GO sheet to carbon atoms where Hþ ions adsorbed and H2 formation takes place. This is possible due to the difference in energy levels of GO, CdS, and water. GO energy level is higher than water reduction potential [37,43] and lower than CB of CdS. The role of the sacrificial agent is to provide electrons to the remaining holes in CdS VB. Wang et al. [44] used a one-pot hydrothermal process for the synthesis of graphene@TiO2 nanocomposite with controlled crystal facets for H2 production via water splitting using methanol and Xe arc lamp (300 W) as sacrificial agent and light source, respectively. Fig. 5.16 shows the enhanced H2 production over as prepare graphene@TiO2 in comparison

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FIGURE 5.9 Effect of graphene content in CdSeGR on the amount of H2 generated during photocatalytic water splitting. Reproduced with permission from Ye A, Fan W, Zhang Q, Deng W, Wang Y. CdSegraphene and CdSeCNT nanocomposites as visible-light photocatalysts for hydrogen evolution and organic dye degradation. Catal Sci Technol 2012;2:969e978.

to graphene@P25. This improved H2 production is attributed to the exposure of highly active TiO2 facets and/or well and homogenous dispersion of TiO2 on the surface of the GS, leading to an increase in the number of active sites to carry out the photocatalytic water splitting reaction.

FIGURE 5.10 Effect of carbon nanotube content in CdSeCNT on the amount of H2 generated during photocatalytic water splitting. Reproduced with permission from Ye A, Fan W, Zhang Q, Deng W, Wang Y. CdSegraphene and CdSeCNT nanocomposites as visible-light photocatalysts for hydrogen evolution and organic dye degradation. Catal Sci Technol 2012;2:969e978.

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FIGURE 5.11 Amount of H2 generated via photocatalytic water splitting reaction over different catalyst. Reproduced with permission from Ye A, Fan W, Zhang Q, Deng W, Wang Y. CdSegraphene and CdSeCNT nanocomposites as visible-light photocatalysts for hydrogen evolution and organic dye degradation. Catal Sci Technol 2012;2:969e978.

FIGURE 5.12 Illustration of charge transfer and separation during irradiation in the CdsemG composite. Reproduced with permission from Lv X-J, Fu W-F, Chang H-X, Zhang H, Cheng J-S, Zhang G-J, Song Y, Hu C-Y, Li J-H, Hydrogen evolution from water using semiconductor nanoparticle/graphene composite photocatalysts without noble metals. J Mater Chem 22 (2012) 1539e1546.

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FIGURE 5.13 Effect of modified graphene (mG) content in CdSemG composite in H2 production via photocatalytic water splitting reaction. Reproduced with permission from Lv X-J, Fu W-F, Chang H-X, Zhang H, Cheng J-S, Zhang G-J, Song Y, Hu C-Y, Li J-H, Hydrogen evolution from water using semiconductor nanoparticle/graphene composite photocatalysts without noble metals. J Mater Chem 22 (2012) 1539e1546.

FIGURE 5.14 The rate of H2 evolution as function of graphene oxide (GO) content in GOeCdS composite. Reproduced with permission from Peng T, Li K, Zeng P, Zhang Q, Zhang X. Enhanced photocatalytic hydrogen production over graphene oxideecadmium sulfide nanocomposite under visible light irradiation. J Phys Chem C 2012;116:22720e22726.

2.2 Graphene-based bi-semiconducting catalyst Xiang et al. [45] used a two-step hydrothermal method for the preparation of nanocrystal growth of TiO2 on MoS2/graphene composite. The composite was tested for hydrogen evolution via photocatalytic water splitting reaction using ethanol and xenon arc lamp irradiation as scavenger agent and light source, respectively. Furthermore, the effect of MoS2 and GO content was investigated to obtain the highest hydrogen evolution. Fig. 5.17 shows the

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FIGURE 5.15 Charge separation and transfer pathway and proposed H2 production mechanism during photocatalytic water splitting over GOeCdS composite. Reproduced with permission from Peng T, Li K, Zeng P, Zhang Q, Zhang X. Enhanced photocatalytic hydrogen production over graphene oxideecadmium sulfide nanocomposite under visible light irradiation. J Phys Chem C 2012;116:22720e22726.

photocatalytic activity of the different composites, in which the highest activity was obtained over TiO2/95MoS25GO, where a further increase in GO content led to a decrease in activity. It is important to note, both MoS2 and GO acted as efficient cocatalysts as indicated by the increase in hydrogen production, in comparison to pure TiO2. This is attributed to charge

FIGURE 5.16 H2 production via photocatalytic water splitting reaction over graphene@P25 (A) and graphene@TiO2 (B) nanocomposites. Reproduced with permission from Wang Z, Huang B, Dai Y, Liu Y, Zhang X, Qin X, Wang J, Zheng Z, Cheng H. Crystal facets controlled synthesis of graphene@TiO2 nanocomposites by a one-pot hydrothermal process. Cryst Eng Comm 2012;14:1687e1692.

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FIGURE 5.17 Effect of MoS2 and graphene content in TiO2/MG catalyst in the photocatalytic production of H2. Reproduced with permission from Xiang Q, Yu J, Jaroniec M. Synergetic effect of MoS2 and graphene as cocatalysts for enhanced photocatalytic H2 production activity of TiO2 nanoparticles. J Am Chem Soc 2012;134:6575e6578.

separation of electronehole recombination introduced by MoS2 and GO. The stability study of the TiO2/95MoS25GO showed no significant decrease in catalyst activity after four cycles. The study proposed a mechanism (Fig. 5.18) to illustrate the activity of TiO2/95MoS25GO, in which the VB electrons of TiO2 are excited by UV illumination to CB band, leading to hole formation in the VB. The CB electron is transferred to the GO sheet since the redox potential of graphene is slightly lower than the CB of TiO2. MoS2 acts as electron acceptor and H2 production site [46e48], since MoS2 (nanoscale) is reported to be an active catalyst for H2 evolution due to the quantum confinement effect [49e51]. The synergistic effect of MoS2 and graphene resulted in an increase in the photocatalytic active sites, limitation of charge recombination, and enhancement of charge transfer. Min et al. [52] studied a limited layer of MoS2 confined on rGO as a photocatalyst for H2 production, utilizing triethanolamine coupled with Eosin Y (EY) as sacrificial agent and Xe lamp (300 W) as light source. Fig. 5.19 shows the H2 evolution over the different catalysts. The low amount of H2 evolution over RGO is attributed to the larger p-p conjected structure, likely due to the high electron affinity and lack of favorable surface sites for H2 production [53,54]. Physical mixture of MoS2þRGO showed lower H2 evolution than synthesized MoS2/RGO; these results indicated the importance of the interfacial contacts between the two materials (i.e., MoS2 and RGO) since the improved activity of MoS2/RGO is due to the electron transfer between the two materials resulting in charge separation. Lv et al. [55] studied the used of Cuegraphene as a cocatalyst on TiO2 for H2 production through photocatalytic water splitting, utilizing methanol and Xe lamp (500W) as sacrificial agent and light source, respectively. The authors studied the effect of graphene content on H2 production, and the results obtained (Fig. 5.20) showed the optimal graphene content (highest H2 production) is 2 wt%. This improvement is attributed to the graphene electron acceptability and transfer efficiency, leading to limitation in electronehole recombination (i.e., charge separation). Further increase in graphene content resulted in decrease in photocatalytic activity due to graphene shielding and scattering the light [56]. Furthermore, the effect of Cu content was investigated using the optimum nanocomposite catalyst (i.e., GP2.0) in H2

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FIGURE 5.18 Proposed mechanism for electron transfer in TiO2/MG composite during photocatalytic H2 production. Reproduced with permission from Xiang Q, Yu J, Jaroniec M. Synergetic effect of MoS2 and graphene as cocatalysts for enhanced photocatalytic H2 production activity of TiO2 nanoparticles. J Am Chem Soc 2012;134:6575e6578.

FIGURE 5.19 H2 production over pure RGO, MoS2, mixed MoS2þRGO, and MoS2/RGO composite. Reproduced with permission from Min S, Lu G. Sites for high efficient photocatalytic hydrogen evolution on a limited-layered MoS2 cocatalyst confined on graphene sheetsethe role of graphene. J Phys Chem C 2012;116:25415e25424.

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FIGURE 5.20 Effect of graphene content in P25-G composite for photocatalytic H2 production. Reproduced with permission from Lv X-J, Zhou S-X, Zhang C, Chang H-X, Chen Y, Fu W-F. Synergetic effect of Cu and graphene as cocatalyst on TiO2 for enhanced photocatalytic hydrogen evolution from solar water splitting. J Mater Chem 2012;22:18542e18549.

production. Fig. 5.21 shows the influence of Cu content from 0.5 to 8.0 wt% in H2 production, where the H2 evolution increased from 2 to 10.2 mmol over GP2.0 (no Cu) and GP 2.0 þ 1.5% Cu, respectively. This improvement is due to increase in interfacial charge transfer via increase in the trapping sites [57]. Khan et al. [58] studied the effect of Al2O3 and ZnO in CdS/GO in the photocatalytic water spitting reaction for H2 production, using Na2SO3 and Na2S as sacrificial agents, and Phoenix

FIGURE 5.21 Influence of Cu loading on P25-2.0 G for photocatalytic H2 production. Reproduced with permission from Lv X-J, Zhou S-X, Zhang C, Chang H-X, Chen Y, Fu W-F. Synergetic effect of Cu and graphene as cocatalyst on TiO2 for enhanced photocatalytic hydrogen evolution from solar water splitting. J Mater Chem 2012;22:18542e18549.

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FIGURE 5.22 Influence of Al2O3 and ZnO in Cds/GO composite on amount of H2 evolution and composite surface area. Reproduced with permission from Khan Z, Chetia TR, Vardhaman AK, Barpuzary D, Sastri CV, Qureshi M. Visible light assisted photocatalytic hydrogen generation and organic dye degradation by CdSemetal oxide hybrids in presence of graphene oxide. RSC Adv 2012;2:12122e12128.

tungsten halogen lamp as irradiation source. The presence of oxide (i.e., Al2O3 and ZnO) and graphene in the nanocomposite increased the hydrogen production via water splitting (Fig. 5.22). This increase is due to the enhanced surface area and surface area to volume ratio in CdS/Al2O3/GO and CdS/ZnO/GO, respectively. Furthermore, the charge separation is more efficient over CdS/ZnO/GO, which results in higher hydrogen evolution (see Fig. 5.22). The authors combined these results with the results reported in the literature [59e61] to propose a mechanism for H2 production (Fig. 5.23) over CdS/ZnO/GO. The electrons excited from the VB of CdS to the CB of the same; however, the CB of CdS and ZnO are in close proximity [60,62,63]. Therefore, the reductive electrons transferred from CB of CdS to ZnO CB prevent charge formation. Furthermore, the electrons will move from ZnO to GO due to the high charge carrier mobility of GO.

2.3 Noble-metal doped graphene-based photocatalyst Cheng et al. [27] studied the H2 evolution in the absence of sacrificial agent, with the 0.5 wt% Pt (Fig. 5.24), using Xe lamb (300 w). The results obtained indicate the advantage of graphene presence in the composite, by increasing the amount of H2 evolution (see Fig. 5.24). These are very encouraging results for H2 production in the absence of sacrificial agent and from pure water. Another approach was taken by Kim et al. [64] to investigate the effect of GS size and position of the graphene. Two composites were prepared, which are reduced nanosized GOcoated TiO2 nanoparticles (rNGOTs) and TiO2 nanoparticles loaded on larger graphene sheet (r-LGOT). The photocatalytic testing was carried out using methanol and UV light (l > 320 nm) as sacrificial agent and irradiation source, respectively, and 0.05 wt% of Pt as

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FIGURE 5.23 Proposed charge transfer and separation pathway over CdS/ZnO/GO composite for photocatalytic H2 production. Reproduced with permission from Khan Z, Chetia TR, Vardhaman AK, Barpuzary D, Sastri CV, Qureshi M. Visible light assisted photocatalytic hydrogen generation and organic dye degradation by CdSemetal oxide hybrids in presence of graphene oxide. RSC Adv 2012;2:12122e12128.

cocatalyst. The photocatalytic results (Fig. 5.25) showed higher H2 production over the core/ shell structure (Pt/r-NGOT) than over TiO2 on reduced graphene sheet (i.e., Pt/r-LGOT); the author attributes the improved in photocatalytic activity over Pt/r-NGOT due to efficient contact between GO and TiO2 in the core shell structure, allowing for transfer of electrons from the conduction band in TiO2 to GO, unlike in the case of Pt/r-LGOT, where limited contact between GO and TiO2 is available for electron transfer. Furthermore, Pt synergistic effect

FIGURE 5.24 Photocatalytic H2 production over P25 and P25e0.5% GR with 0.5 wt% Pt, without the presence of sacrificial agent. Reproduced with permission from Cheng P, Yang Z, Wang H, Cheng W, Chen M, Shangguan W, Ding G. TiO2egraphene nanocomposites for photocatalytic hydrogen production from splitting water. Int J Hydrogen Energy 2012;37:2224e2230.

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FIGURE 5.25

Effect of composite structure on photocatalytic production of hydrogen. Reproduced with permission from Kim H.-I, Moon G-H, Monllor-Satoca D, Park Y, Choi W. Solar photoconversion using graphene/TiO2 composites: nanographene shell on TiO2 core versus TiO2 nanoparticles on graphene sheet. J Phys Chem C 2012;116:1535e1543.

is more prominent over r-NGOT than r-LGOT, due to the core/shell structure facilitating intimate contact for ease of electron transfer. Li et al. demonstrated the decoration of graphene nanosheets with clusters of CdS, and the use of the composite for hydrogen generation, using Pt, visible light, and lactic acid as cocatalyst, irradiation source, and sacrificial agent, respectively [65]. The influence of graphene content was investigated, and the results (Fig. 5.26) show the graphene content goes through maxima at 1.0 wt% of graphene, corresponding to 1.12 mmol h
1 hydrogen production rate. The improved activity in the presence of graphene is attributed to two reasons. The high electron transfer ability of graphene makes it an acceptor of CdS cluster-generated electron; this

FIGURE 5.26 Effect of the graphene content in the visible light catalytic hydrogen production with CdS cluster 0.5 wt% Pt as cocatalyst and 10 vol% lactic acid as sacrifice agent. Sample GCx (x ¼ wt% content of graphene). Reproduced with permission from Li Q, Guo B, Yu J, Ran J, Zhang B, Yan H, Gong JR. Highly efficient visible-light-driven photocatalytic hydrogen production of CdS-Cluster-Decorated graphene nanosheets. J Am Chem Soc 2011;133:10878e10884.

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99

FIGURE 5.27 Photocatalytic H2 production as function of GO loading and Pt as cocatalyst. Reproduced with permission from Zeng P, Zhang Q, Zhang X, Peng T. Graphite oxideeTiO2 nanocomposite and its efficient visible-light-driven photocatalytic hydrogen production. J Alloy Comp 2012;516:85e90.

process prevents/limits charge recombination. Furthermore, graphene enhances the surface area of the catalyst, thus, more active sites are available to carry out the reaction. It is important to note, further increase in the graphene content leads to drop in the activity, due to the optic property of graphene which hinders the irradiation from CdS cluster. Pt cocatalyst role is to prevent the reverse reaction (producing water) by limiting overpotential generated during the water splitting reaction. Zeng et al. [66] compared H2 production via photocatalytic water splitting reaction as a function of graphene content and Pt presence (Fig. 5.27). The increase in GO loading went through a maximum at 2 wt% of GO on TiO2, and a further increase in GO loading resulted in a decrease in terms of H2 production. GO improved photocatalytic activity due to its electronic structure [20,67,68] and electron transfer efficiency. Moreover, further increase in the H2 production was achieved with inclusion of Pt as a cocatalyst. The results obtained in the study led the authors to propose a mechanism (Fig. 5.28) for the photocatalytic H2 production via water splitting. Electrons are generated at the antibonding p* and injected into the conducting band of TiO2 through dep interaction. Thereafter, GO transfers the electrons to the active site where H2 is produced through water or protons reduction. This proposed mechanism agrees with the reported mechanism over multiwall carbon nanotubes/TiO2 catalysts [69,70]. Mou et al. [71] used RuO2/TiSi2/graphene for H2 generation from water through photocatalytic water splitting reaction. Two-component (i.e., RuO and TiSi2) and three-component catalysts (i.e., RuO, TiSi2 and graphene) in the photocatalytic reaction use GY-10 xenon lamp (150 W) as light. The results obtained shown in Fig. 5.29 indicates the highest activity in terms of H2 evolution was obtained over RuO2-1/TiSi2/RGO-1. RuO2 acts as an electron trap [72] on the surface of TiSi2; however, further increase of RuO2 loading led to charge recombination centers. Moreover, similar behavior was observed with the RGO loading, where 1% RGO loading acts as charge carrier and at higher loading acts as light shield from TiSi2 leading to drop in H2 evolution.

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5. Role of graphene in photocatalytic water splitting for hydrogen production

FIGURE 5.28 Proposed mechanism for H2 production via photocatalytic water splitting from GOeTiO2 nanocomposites. Reproduced with permission from Zeng P, Zhang Q, Zhang X, Peng T. Graphite oxideeTiO2 nanocomposite and its efficient visible-light-driven photocatalytic hydrogen production. J Alloy Comp 2012;516:85e90.

FIGURE 5.29 Effect of RuO2 and RGO content in the composite on H2 production via photocatalysis water splitting. Reproduced with permission from Mou Z, Yin S, Zhu M, Du Y, Wang X, Yang P, Zheng J, Lu C. Phys Chem Chem Phys 2013;15:2793e2799.

Tran et al. [73] studied the hydrogen generation over a composite of Cu2O and rGO. The authors attempted to overcome the disadvantages associated with Cu2O as a photocatalyst, such as the oxidation and reduction of Cu2O into CuO and metallic Cu, respectively, falls within the bandgap (i.e., photocorrosion phenomenon). The photostability of Cu2O is improved with combining Cu2O with n-type semiconductor (e.g., ZnO and TiO2) for efficient transport of photogenerated electron for Cu2O to the semiconductor [74e76]. Therefore, this study made use of rGO due to it efficient transportation capacity. The authors reported high H2 generation and composite (i.e., Pt Cu2O-rGO) photostability, and this is due to the high

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specific surface area of rGO. Furthermore, the synthesis method is key to form stable, welldispersed, nano-size Cu2O on rGO, which is key to obtain Cu2OerGO junctions with high electron transport efficiency. This was evident by the low activity and stability of the physically mixed Cu2O and rGO. It is important to note the importance of rGO loading, since low loading results in Cu2O agglomeration and high loading of rGO results in light absorption competition.

2.4 Transition-metal doped graphene-based photocatalyst Babu et al. studied hydrogen generation over Cu2O, TiO2, and/or rGO containing composites; the results obtained (Fig. 5.30) shows the highest hydrogen production was obtained over Cu2OeTiO2/rGO, utilizing glycerol as the sacrificial agent [77]. The significant improvement of hydrogen production over Cu2OeTiO2/rGO is attributed to rGO electron transfer property, which provides a layer at the heterojunction of the three composite components (i.e., Cu2O, TiO2, and rGO) as indicated by photoelectrochemical analysis, thus preventing electronehole recombination as demonstrated by photoluminescence study. The authors also reported the effect of rGO loading which goes through a maximum; therefore, the rGO loading is key since high loading results in irradiation shielding and low electron transfer capability at low rGO loading. Agegnehu et al. [78] studied nanoparticles (i.e., 2 and 3 nm) of Ni and NiO on GO sheets, for photocatalytic water splitting for H2 production, using methanol as sacrificial agent and Hg lamp (400 W) as irradiation source. Fig. 5.31 shows the highest amount of H2 produced over Ni/GO followed by NiO/GO and the lost amount over GO. This trend is explained

FIGURE 5.30 Water splitting over different Cu2O-, TiO2-, and rGO-based catalysts as function of time. Reproduced with permission from Babu SG, Vinoth R, Praveen Kumar D, Shankar MV, Chou H-L, Vinodgopal K, Neppolian B, Influence of electron storing, transferring and shuttling assets of reduced graphene oxide at the interfacial copper doped TiO2 pen heterojunction for increased hydrogen production. Nanoscale 2015;7:7849e7857.

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5. Role of graphene in photocatalytic water splitting for hydrogen production

FIGURE 5.31 H2 evolution over GO, NiO/GO, and Ni/GO via photocatalytic water splitting reaction. Reproduced with permission from Agegnehu AK, Pan C-J, Rick J, Lee J-F, Su W-N, Hwang B-J. Enhanced hydrogen generation by cocatalytic Ni and NiO nanoparticles loaded on graphene oxide sheets. J Mater Chem 2012;22:13849e13854.

by the effectiveness of Ni metal in charge separation between the excited electron and the hole [79], while NiO a less effective charge separation or electron trapping on the surface [80].

2.5 Nonmetal doped graphene-based photocatalyst Xiang et al. prepared a composite of graphene/C3N4 and studied the water splitting reaction activity over the prepared composite [81]. The authors studied the graphene content effect and the results showed 1 wt% graphene content is the optimum loading and results in 451 mmol h
1 hydrogen production, while 147 mmol h
1 was reported over C3N4 attributing to the material moderate energy gap and electronic structure. The composite mechanism for H2 production is highlighted in Fig. 5.32, where the visible light excited electrons from the VB (i.e., N2p orbital) to the CB (i.e., C2p orbital); this movement creates a hole in the VB,

FIGURE 5.32 Proposed mechanism for photocatalytic H2 production in water splitting over graphene/g-C3N4. Reproduced with permission from Xiang Q, Yu J, Jaroniec M. Preparation and enhanced visible-light photocatalytic H2-production activity of graphene/C3N4 composites. J Phys Chem C 2011;115:7355e7363.

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therefore limited number of electrons gets to the Pt particle to reduce H2O to H2, due to electronehole recombination via Schottky barrier [82,83]. Methanol (sacrificial agent) will react with the hole in g-C3N4. The presence of graphene in the composite increases the number of electrons transported to Pt particles (via percolation mechanism) [26], thus increasing H2 production. This is due to the electronic conductivity of graphene which limits charge recombination. The reaction equation for the mechanistic step in shown in Eqs. (5.1)e(5.4). Graphene=g
C3 N4 /graphene ðe
Þ=g
C3 N4 ðhþ Þ

(5.1)

Grapheneðe
Þ þ Pt/graphene þ Ptðe
Þ

(5.2)

Ptðe
Þ þ 2Hþ /Pt þ H2

(5.3)

g
C3 N4 ðhþ Þ þ CH3 OH þ 6OH
/g
C3 N4 þ CO2 þ 5H2 O

(5.4)

Mukherji et al. synthesized nitrogen doped Sr2Ta2O7 and utilized the composite (with 0.5 wt% Pt and graphene) for H2 production via photocatalytic water splitting reaction. Doping of Sr2Ta2O7 with N2 resulted in an increase in H2 production, since N2 decreases the energy gap and forms intermediate steps as a result of N2p and O2p orbitals [84]. Fig. 5.33A shows clearly the stability and increase in H2 production with increase in time, while Fig. 5.33B shows the influence of graphene content of catalyst activity, where 5 wt% of graphene showed the highest H2 production of 293 mM h
1; the improved activity is due to charge transfer role played by graphene, since it has high charge carrier mobility.

3. Conclusion and future direction Graphene is a two-dimensional rising star material which is currently being used for a wide range of application such as in catalysis, photocatalysis, energy storage, energy transport, and environmental remediation. The interesting physiochemical properties of graphene such as high surface area, thermal stability, and high carrier mobility make it more attractive for the photocatalysts. The role of graphene and graphene-based material in preventing the recombination of electronehole pair recombination in mono- and binary semiconducting photocatalyst is well discussed. Besides, graphene also acts as an electron transport layer for the noble metal and transition metal doped semiconducting photocatalytic materials. From the discussion given in this chapter it is very clear that graphene and graphenebased materials are effective supports for semiconducting photocatalyst, especially for the water splitting to produce hydrogen fuel. Although the outcome of graphene-based photocatalyst in water splitting is vital, the production cost of graphene slightly slows down the commercialization process. However, the worldwide recent research imposes to decrease the production cost and improve the quality of graphene-based materials which in turn moves positively toward commercialization.

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FIGURE 5.33 Effect of graphene on Sr2Ta2O7-xNx on H2 production in water splitting photocatalysis as a function of (A) reaction time and (B) graphene content. Reproduced with permission from Mukherji A, Seger B, Lu GQ, Wang L. Nitrogen doped Sr2Ta2O7 coupled with graphene sheets as photocatalysts for increased photocatalytic hydrogen production. ACS Nano 2011;5:3483e3492.

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