Nanomaterial-Based Photocatalytic Hydrogen Production

3 NANOMATERIAL-BASED PHOTOCATALYTIC HYDROGEN PRODUCTION Fang Deng, Jian-Ping Zou, Li-Na Zhao, Gang Zhou, Xu-Biao Luo, Sheng-Lian Luo Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycle, Nanchang Hangkong University, Nanchang, P. R. China

CHAPTER OUTLINE 3.1 The Demand for Nanomaterial-Based Photocatalytic Hydrogen Production 60 3.2 The Principles of Photocatalytic Hydrogen Generation 60 3.2.1 The Mechanisam of Photocatalytic Hydrogen Generation 60 3.2.2 Evaluation of Photocatalytic Activity for Hydrogen Evolution 62 3.2.3 The Photocatalytic H2-Generation System 63 3.3 Material Development for Photocatalytic Hydrogen Production 64 3.4 Performance Enhancement Strategies 66 3.4.1 Doping With Metal and Nonmetal Elements 66 3.4.2 Dye-Sensitization 68 3.4.3 Sensitization With Noble Metal Particles 69 3.4.4 Fabrication of Heterojunction With Narrow Bandgap Semiconductors 70 3.4.5 Modification of Phocatalysts With Cocatalysts 72 3.4.6 Formation of Solid Solutions 74 3.4.7 Surface Modification With Graphene and Other Carbon Material 74 3.4.8 Increasing Photocatalytically Active Area 75 3.5 Photocatalytic Reactors for Hydrogen Production From Water Splitting 76 3.5.1 Batch-Type Photoreactor 76 3.5.2 Continuous Annular Photoreactors 77 3.5.3 Photocatalytic Membrane Reactors 77 3.6 Conclusions and Perspectives 78 References 79

Nanomaterials for the Removal of Pollutants and Resource Reutilization. https://doi.org/10.1016/B978-0-12-814837-2.00003-2 Copyright # 2019 Elsevier Inc. All rights reserved.

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3.1

The Demand for Nanomaterial-Based Photocatalytic Hydrogen Production

Fossil energies are important sources for human society. Especially in recent years, all the world has realized the importance of energy resources to the development of each country. The destructive exploitation of fossil energy accelerated the depletion of these nonrenewable resources. Meanwhile, the emissions of greenhouse gases and other toxic pollutants associated with fossil energy are environmentally unacceptable. Therefore there is an urgent need for the development of clean and sustainable sources of energy. Solar energy is considered to be one of the most promising alternative energy sources because of its cleanness and richness, but it is difficult to capture, transform, and store. Thus more researchers are trying to convert solar energy into a more available renewable energy source. Hydrogen is considered to be the most ideal alternative clean energy because of its high calorific value, zero pollution, and storablility, and it is widely considered to be the future clean energy carrier in many applications, such as environmentally friendly vehicles, domestic heating, and stationary power generation. If we can transform the scattered, inexhaustible solar energy into the highly concentrated and clean hydrogen energy by photochemical, electrochemical, and photoelectrochemical methods, energy shortage can be solved. Among the limited methods for solar energy conversion and utilization, solar water splitting has been considered as the most attractive way to produce hydrogen due to its several advantages over other mehods [1, 2]: (1) this technology is based on photon (or solar) energy and water, which is a clean, perpetual, and renewable source of energy; (2) it is an environmentally friendly technology without harmful by-products and pollutants; and (3) the photochemical conversion of solar energy into hydrogen can effectively deal with the intermittent character and seasonal variation of the solar influx.

3.2 3.2.1

The Principles of Photocatalytic Hydrogen Generation The Mechanisam of Photocatalytic Hydrogen Generation

Honda and Fujishima discovered photoassisted electrochemical water splitting to H2 and O2 in 1972 [3], and since then many approaches and photocatalysts have been developed to drive

Chapter 3 NANOMATERIAL-BASED PHOTOCATALYTIC HYDROGEN PRODUCTION

Fig. 3.1 Photocatalytic hydrogen generation upon suitable semiconductor.

catalytic H2 production under solar irradiation. The photocatalytic hydrogen generation using a suitable semiconductor is shown in Fig. 3.1. The photocatalytic H2 generation process from water splitting includes two chemical half reactions. One is the proton reduction half reaction and the other is water oxidation half reaction (Eqs. 3.1–3.4). 2γ ! 2eCB + 2ehv + Photon induced e =h + generation

(3.1)

+

2H2 O + 4h ! O2 + 4H + Water oxidation half reaction EOX O ¼ 1:23eV (3.2) 4H + + 4e ! H2 Proton reduction half reaction ERedO ¼ 0eV (3.3) 2H2 O + 2γ ! 2H2 + O2 Overall water splitting

(3.4)

It can be seen from Fig. 3.1 and the above equations that photocatalytic water splitting is a delicate balancing process that can drive the photocatalytic half reactions. In this system, light energy is converted into chemical energy, and electrons will transfer from the valence band (VB) to the conduction band (CB), generating photoproduced electron-hole pairs. The photo-generated electron-hole pairs can drive the photocatalytic half reactions. The redox ability of a semiconductor relies on the highest energy state of the VB and the lowest energy state of the CB. The CB level must be more negative than the hydrogen production level (EH2/ H2O) to catalyze reduction of water to hydrogen, and the VB must

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be more positive than water oxidation level (EO2/H2O) to oxidize water in the relatively unfavorable four-hole process [4]. The minimum energy required to drive the reaction referred to that of two moles of impinging photons is equal to 1.23 eV. The efficiency of photocatalytic hydrogen generation from water splitting is relatively low due to the fast recombination of the photogenerated electrone-hole pairs, the inability to use visible light of many photocatalysts, and the possibility of a reverse reaction that involves the rapid hydrogen and oxygen recombination.

3.2.2

Evaluation of Photocatalytic Activity for Hydrogen Evolution

Various criteria have been utilized to estimate the photocatalytic activity of a photocatalyst for hydrogen evolution. Generally, for photocatalytic water splitting and hydrogen evolution reaction (HER), the time-based H2 evolution rate (μmol h1 g1catalyst) is an important index to estimate the photocatalytic activity of a photocatalyst. In order to compare the efficacy of the photocatalyst, it is essential to utilize standard conditions for hydrogen production to estimate the performance of photocatalysts; for example, similar reactor vessels, light sources, filters, and temperatures. Other parameters, such as quantum yield (QY), apparent quantum yield (AQY), incident photon-to-current efficiency (IPCE), and absorbed photon-to-current efficiency (APCE) are utilized to estimate the extent of solar energy utilized for water splitting or HER by normalization with respect to light intensity and area. The time-based H2 evolution rate can be calculated according to Eq. (3.5) Q¼

A Vm  M  T

(3.5)

where Q is the amount of actual hydrogens (mmol g1 h1), A is the actual peak area of hydrogen (L), and Vm is the standard molar volume (22.4 L mol1), M is the mass of the catalyst ( g), and t is time (h). The overall quantum yield for O2 and H2 formation is shown in Eqs. (3.6), (3.7), respectively. 

Overall quantum yield ð%Þ ¼  ð2  Number evolved H2 moleculesÞ  100ðforH2 evolutionÞ Number of incident photons (3.6)

Chapter 3 NANOMATERIAL-BASED PHOTOCATALYTIC HYDROGEN PRODUCTION



Overall quantum yield ð%Þ ¼

 ð4  Number evolvedO2 moleculesÞ  100ðforO2 evolutionÞ Number of incident photons (3.7)

3.2.3

The Photocatalytic H2-Generation System

To investigate the reaction mechanism of H2 production, researchers generally develop the solar H2-generation system in virtue of water-oxidation half-reaction using an applicable sacrificial reductant [5–9]. The photocatalytic H2-generation system has drawn great attention since Lehn and Sauvage. In this system, [Ru(bpy)3]2+ (bpy ¼ 2,20-bipyridine), [Rh(bpy)3]3+, colloidal Pt, and triethanolamine (TEOA) act as chromophore, electron relay, catalyst, and sacrificial reductant, respectively [10, 11]. Fig. 3.2 shows the key components of these photocatalytic H2-generating systems. The three-component solar H2-generation system (Fig. 3.2A) attracted increasing attention in the late 1970s and 1980s [10]. In the three-component solar H2-generation system, an electron relay functions as an electron mediator, which can receive electrons from the excited chromophore and transfer them to the catalyst. Compared with the three-component system, hydrogen can be evolved by the two-component photocatalytic system (Fig. 3.2B) in the absence of an electron relay species [12–16]. Electrons can be transferred from the excited chromophore to the catalyst for HER, thereby simplifying the H2-evolution system. However, in these three- and two-component systems it is difficult to control the influencing factors of the electron-transfer

Fig. 3.2 Three different photocatalytic H2 production systems: (A) three-component photocatalytic system; (B) two-component system; and (C) single-component system. Sacrificial reagent (SR), sensitizer (S), electron relay (ER), and catalyst (Cat).

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process. To control the electron-transfer process, supramolecular photocatalysts, where the light-harvesting unit is coupled with a catalytic center, have been designed in recent years (Fig. 3.2C). In the supramolecular photocatalyst it is possible to control intramolecular electron transfer from the photoactivated unit to the catalytic center, leading to more efficient charge transfer for solar H2 generation.

3.3

Material Development for Photocatalytic Hydrogen Production

According to properties of elements across the periodic table, a large number of semiconductors have been found to be capable of photocatalytic water splitting for hydrogen production. The required properties of a photocatalyst for viable use on a large scale include: providing a high quantum yield; being inexpense, earth abundant, nontoxic, recyclable, photostable, and corrosion resistant; and having a long lifetime. TiO2 has been considered as one of the most superior photocatalysts for water splitting due to its advantages, such as being inexpensive, abundant, stable, noncorrosive, and environmentally friendly. However, TiO2 can only utilize UV light for hydrogen production due to its wide bandgap of 3.2 eV, the VB holes of TiO2 can quickly recombine with CB electrons and deliver energy in the form of heat or unproductive photons, which restricts practical application in solar photocatalytic hydrogen production. The utilization of solar energy is strongly dependent on the spectral range in which photocatalyst is active. Up to 400 nm, the solar conversion efficiency is only 2%. When the active spectral range moves to 600 and 800 nm, the solar conversion efficiency increases to 16% and 32%. Other oxides, such as WO3, Cu2O, BiVO4, and Fe2O3, also were used for photocatalytic hydrogen production, but their photocatalytic performance for hydrogen production is still limited by low light harvesting, high charge recombination rates, and poor charge transport. The CB edge position of BiVO4 is unfavorable for HER. Although the theoretical solar-to-fuel conversion efficiency of Cu2O is 18%, it is susceptible to self-reduction in aqueous solution. Thus it is crucial to develop novel photocatalysts to utilize the visible light regime and near infrared region. The sulfides and nitrides of d0 or d10 transition metal cations have been developed for photocatalytic hydrogen production. Metal sulfides with more negative VB have been considered as attractive candidates for visible-light-driven photocatalytic hydrogen production due to a narrower bandgap than metal oxides. Metal sulfides such as CdS and ZnS, and their solid

Chapter 3 NANOMATERIAL-BASED PHOTOCATALYTIC HYDROGEN PRODUCTION

solutions, have attracted much attention. CdS has a narrow bandgap of 2.4 eV and suitable band positions for visible-light photocatalytic water splitting. However, CdS is not stable due to the oxidation of S2 in CdS by photogenerated holes and the elution of Cd2+ into the solution. Such photocorrosion is also a common phenomenon in other metal sulfides. In spite of photocorrosion, CdS is a good visible-light-driven photocatalyst for H2 production in the presence of a hole scavenger (S2 or SO3 2 ). ZnS is another excellent photocatalyst with a wide bandgap of 3.6 eV for H2 production, which is only active in UV light region. Earth-abundant and metal-free catalysts like g-C3N4, graphene, graphene oxide, carbon nanotubes, and carbon quantum dots (CDQs) also have been investigated extensively due to potential cost reduction during up scaling. Recently a metal-free material of graphitic carbon nitride (g-C3N4) has become popular for use in water splitting. Different from traditional organic semiconductor, g-C3N4 shows a particular stability, such as heat endurance and chemical resistance. Wang et al. were the pioneers in utilizing g-C3N4 to split water. g-C3N4 has since been applied in photochemistry and photocatalytic hydrogen generation from water splitting, relying on its extensive visible light absorption and proper VB position [5, 17, 18]. However, g-C3N4 still suffers from many shortcomings, such as small surface area, low conductivity, and high recombination rate. Metal-organic frameworks (MOFs) are novel photocatalysts for photocatalytic hydrogen production. MOFs consist of inorganic metal ions and organic moieties, which act as connecting centers and as linkers, respectively. MOFs have received wide attention due to their high surface areas, crystalline open structures, tunable pore size, and good functionality. Because of the modification by functional groups, MOFs exhibit obvious chemical diversity, and have potential application in the fields of catalysis, energy storage, gas sorption, and membrane. Recently MOFs have been widely utilized as photocatalysts for H2 production from water splitting, but have been dismissed as important alternatives to semiconductor and metal complexes. Many efforts have been made to design visible-light-responsive MOFs for enhanced hydrogen production from water splitting. In hydrogen generation from water splitting, MOFs function as photo- or catalytic active sites instead of support materials. Under light illumination, the linker of the MOFs can capture light, electrons were excited from HOMO to LUMO state of organic linker, then the photogenerated electrons further transferred to nodes of MOFs for activation of the metal nodes through a linker-to-cluster charge-transfer mechanism (LCCT), subsequently the photogenerated electrons react with proton to produce hydrogen over the inorganic cluster of MOFs (Fig. 3.3).

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Fig. 3.3 Schematic illustration of photocatalytic water splitting over semiconducting MOFs; (Pathway 1) light harvesting by an organic linker; (Pathway 2) the electron transportation pathway; (Pathway 3) reaction of proton with electron to produce hydrogen over the inorganic cluster of MOFs; (Pathway 4) quenching of h+ by a donor. (Inset) Basic components of MOFs: inorganic cluster and organic linker of MOFs, these components can be varied and modified to tailor the physical and chemical properties of the resulting MOFs [19].

3.4

Performance Enhancement Strategies

Recently considerable efforts have been made to deal with the above problems as well as to promote the solar photocatalytic hydrogen evolution of photocatalysts. Common strategies include doping metal ion and anion, loading precious metal and dye sensitization, heterojunction formation, nanostructure design, surface plasmonic enhancement, defect control, and co-catalysts utilization. Proper methods for synthesis, surface modifications, and chemical treatments specific to the composition of photocatalysts are also advantageous to enhance photocatalytic activity. Some performance enhancement strategies will be discussed in detail in the following paragraphs.

3.4.1

Doping With Metal and Nonmetal Elements

Structure and chemical composition have important influence on the photocatalytic activity of photocatalysts. In the case of TiO2, titanium and oxygen can be replaced by metal and nonmetal doping. Therefore the incorporation of a secondary active cation and/or anion into the lattice of TiO2 can extend the visible-light response as well as suppress the recombination of electron/hole pairs.

Chapter 3 NANOMATERIAL-BASED PHOTOCATALYTIC HYDROGEN PRODUCTION

Nonmetals such as N, C, B, S, F, and P can be incorporated into the lattice of TiO2 by hydrothermal methods, plasma ion implantation, or sputtering in a special atmosphere, thermal treatment in a gas atmosphere (N2, CO, Ar, etc.) and Ti alloy anodization, etc. Although ion implantation and sputtering in a special atmosphere are effective doping methods, they require harsh experimental conditions, high energy accelerators, and limited doping depth. Thermal treatment in an N2, H2, and Ar atmosphere is considered as a facile doping technique and is widely utilized. Carbon and nitrogen doping are the most studied among all of the nonmetal elements. C and N doping can decrease the bandgap and produce a subbandgap above the VB of TiO2, which extends the visible-light response. N doping is a highly effective method for narrowing the bandgap by mixing p states with O 2p states. S doping can narrow the bandgap, but it is difficult to be incorporated into the TiO2 crystal due to its large ionic radius. Additionally, the energy excitation pathway can be lowered by intrinsic defects of TiO2, such as oxygen vacancies and reduced Ti species after calcination. Transition metal cations such as Fe, V, Cu, Co, and Mn can be incorporated in the lattice of TiO2 and have been confirmed to extend the visible light absorption range, inhibit the recombination of photo-generated electron/hole pairs, and enhance the photoelectric performance. Fe is one common transition metal for doping TiO2 due to its strong oxidizing ability. Hu et al. prepared Cu-doped TiO2 films by a facile magnetron sputtering method under different atmospheres. The Cu-doped TiO2 fabricated under an oxygen atmosphere exhibited high H2 evolution rates to the order of 2.80 mmol cm2 h1, which is 55 times higher than that of pure TiO2 [20]. Liu et al. prepared C3N4 xSx via putting the as-prepared g-C3N4 samples in a gaseous H2S atmosphere [21]. The sulfur-doped g-C3N4 shows good photocatalytic performance for H2 generation from water splitting under visible light, and the highest H2 evolution rate reaches 12.16 mmol h1, nearly six times higher than that of the pure g-C3N4 due to the active sites of sulfur species. Our group developed novel Zn-doped SrTiO3 and BaTiO3 photocatalysts for photocatalytic hydrogen production from water with no co-catalysts loading [22]. SrTiO3 and BaTiO3 were substitutionally doped with Zn(II) ions at A-sites (Sr or Ba) for the first time, and Sr2/3Zn1/3TiO3 and Ba5/6Zn1/6TiO3 with perovskite structures were successfully prepared by sol-gel method. It is worthwhile to note that Sr2/3Zn1/3TiO3 and Ba5/6Zn1/6TiO3 without co-catalysts loading exhibited improved photocatalytic activity for hydrogen production from H2O splitting with a small

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quantity of ethanol (3%) as a sacrificial electron reagent in comparison with the pure SrTiO3 and BaTiO3. This indicated that Zn (II) ions doping was an effective method to enhance photocatalytic efficiency of H2 production for the titanates catalysts.

3.4.2

Dye-Sensitization

Organic dye sensitization has been considered as an efficient approach for developing visible-light-responsive photocatalysts for H2 production because their HOMO and LUMO states are tunable by anchoring different ligands, they are low-cost, and they offer great variety. Up to now, many organic dyes such as erythrosine B(ErB), eosin Y (EY), rhodamine B (RhB), and rose bengal (RB) have been utilized to sensitize catalysts for H2 production. Fig. 3.4 shows the schematic description of the water photosplitting process in the presence of dye sensitizer. MOFs are limited in the application of photocatalytic H2 production due to low activity, poor visible-light utilization, and instability. However, it is found that there are inherent shortcomings with organic dyes, such as low stability resulting from selfdegradation and dissolution, fast carrier recombination, and deactivation. Immobilizing the dye onto the porous semiconductor materials is an effective way to suppress self-degradation and inhibit carrier recombination. The unique organic properties and large surface area of MOFs encourage the physical and chemical adsorption of organic dye via Van Der Waals interaction and strong-stacking between the benzene rings of both MOFs and dyes, which can promote the transfer of electrons from dye to

Fig. 3.4 Schematic illustration of the water photo-splitting process in the presence of dye sensitizer.

Chapter 3 NANOMATERIAL-BASED PHOTOCATALYTIC HYDROGEN PRODUCTION

co-catalyst. Moreover, the metal clusters of MOFs can accept the photo-excited electron from dye and function as active sites for the photon-reduction reaction. Dye-sensitized MOFs for photocatalytic H2 production are usually composed of organic dye, a sacrificial reagent, and metal nanoparticle-supported MOFs, which serve as photosensitizer, electron donor, and co-catalyst, respectively. Under light irradiation, dye molecules harvest light and electrons of dye are excited from HOMO state to LUMO state. The photo-excited electrons transfer to the metal nanoparticle and inject into the LUMO state of MOFs simultaneously, then the electrons in the LUMO state migrate to the surface of metal nanoparticles and are finally involved in the reduction of water. The oxidized dye molecules also come return to their original state by receiving electrons from the sacrificial reagents. However, back electron transfer, fluorescence decay of excited dye molecules and possible recombination pathways from LUMO state’s electrons to HOMO state of dye molecule accompanied the photon reduction process. In order to prevent the back electron transfer, it is necessary to increase the contacting interface between the dye and the co-catalyst and reduce the size of metal nanoparticles. Erythrosin (ErB) was used to sensitize water-stable UiO-66 for H2 production, and the highest H2 evolution rate with 0.46 mmol h1 g1 was obtained when 30 mg ErB and Pt was used under visible-light illumination [23]. Rhodamine B (RhB) dye was used to sensitize amine-functionalized chromium-based MIL-101(Cr) with imbedded Pt nanoparticles [24], and the photocatalytic activities of the catalysts were evaluated by photocatalytic H2 production in the presence of TEOA as a sacrificial agent. The pristine NH2MIL-101(Cr) is inactive for H2 evolution, and RhB sensitized NH2-MIL-101(Cr) shows low photocatalytic activity for H2 production. When Pt nanoparticles were imbedded into NH2MIL-101(Cr), H2 evolution rate increased tremendously. When the Pt loading amount was 1.5 wt%, Pt/NH2-MIL-101 exhibited the highest H2 evolution rate with 0.57 mmol h1 g1 due to the synergistic effects of Pt, MOF, and RhB dye solution in enhancing light harvest, protecting the photosensitizer, reducing the selfquenching of RhB, and promoting the separation and transportation of photoexcited electrons.

3.4.3

Sensitization With Noble Metal Particles

TiO2 can be active only in the UV region, thus it is important to sensitize TiO2 with metallic nanoparticles by utilizing the localized surface plasmon resonance (LSPR) effect to improve the visible-light harvesting capacity. The primary noble metal

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nanoparticles include Au, Ag, Pt, Pd, and their alloys, due to their absorption in the visible region [25–28]. Under visible-light irradiation, the electrons in the CB can be “excited” by plasmon to form highly energetic “hot electrons” through the LSPR effect, which functions as an electron sink. Then the “hot electrons” transfer to the CB of TiO2 in direct contact thereby producing a metalsemiconductor Schottky junction. Meanwhile, a strong local electronic field can be formed due to the LSPR effect to enhance the energy of trapped electrons, which facilitate their transfer and reaction with electron acceptors. Thus the photo-generated electrons and holes are efficiently separated [29–31]. Decorating TiO2 nanomaterials with noble metal nanoparticles can be achieved by electrodeposition, UV reduction, electrospinning plasma sputtering, or hydrothermal methods. Zhu et al. have synthesized hollow Pt/TiO2 spheres using a simple sol-gel method [24]. In comparison with pure hollow TiO2 sphere, Pt/TiO2 hollow spheres show higher photocatalytic water splitting activity under visible light as a result of improved donor density and reduced recombination of photogenerated electron-hole pairs.

3.4.4

Fabrication of Heterojunction With Narrow Bandgap Semiconductors

Construction of a heterojunction with other p-type or n-type semiconductors is another effective strategy to enhance photocatalytic activity for hydrogen production. When the heterojunction is fabricated, the photo-generated electrons are driven to the n-type semiconductor side, and holes transfer in the opposite direction due to the resulting local electric field, leading to improved separation of photon-generated electron and hole pairs, and enhanced photocatalytic activity (Fig. 3.5). The separation process of photon-generated electron and hole pairs in the heterojunction is similar to dye sensitization. The difference between heterojunction and dye sensitization is that electrons are injected from one semiconductor to another semiconductor in the heterojunction, rather than from excited dye to semiconductor. Successful fabrication of heterojunctions for visible-light photocatalytic hydrogen production from water splitting requires the following conditions: (1) semiconductors should be free of photocorrosion, (2) the semiconductor with small bandgap must be excited by visible light, and (3) electron injection should be efficient and fast. CdS is a narrow bandgap (2.4 eV) semiconductor and it is extensively utilized to modify TiO2 by fabricating heterojunctions. Zhao

Chapter 3 NANOMATERIAL-BASED PHOTOCATALYTIC HYDROGEN PRODUCTION

Fig. 3.5 Schematic illustration for charge transfer process in heterojunction semiconductors.

modified meso-porous anatase TiO2 with CdS nanoparticles (TiO2/CdS) by a chemical bath deposition method and found that TiO2/CdS photocatalyst showed a broad visible-light absorption region and high photocatalytic activity and stability for hydrogen evolution [32]. The p-type semiconductor Cu2O is a common semiconductor that couples with TiO2 for photocatalytic hydrogen evolution from water splitting. Tamiolakis et al. synthesized the mesoporous structures of Cu2O and TiO2 to form a heterojunction and found that the Cu2O-TiO2 heterojunction showed high H2 evolution rates under visible-light irradiation, which was attributed to the fact that the formation of the heterojunction was beneficial to the large surface area and efficient charge separation [33]. Our group also designed and synthesized the CdS/Ba1 xSrxTiO3 heterojunction with excellent photocatalytic H2 evolution activity and long-term durability from an aqueous Na2S/Na2SO3 solution with no loading of noble metals as co-catalysts [34]. Under simulated solar illumination, electrons in VB are excited to CB of CdS and Ba0.4Sr0.6TiO3, and meanwhile the same number of holes were left in the VB of both CdS and Ba0.4Sr0.6TiO3. Electrons from the CB of Ba0.4Sr0.6TiO3 were then injected into the CB of CdS and the photogenerated holes in the VB of CdS flowed into the VB of Ba0.4Sr0.6TiO3 through the Schottky barrier, leading to efficient

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separation of photogenerated electrons and holes. The photogenerated electrons on the surface of CdS nanoparticles have enough time to reduce H+ to H2 while the holes can be consumed by Na2S/ Na2SO3 sacrificial reagents. Meanwhile, the photocorrosion of CdS can be greatly alleviated due to the fast moving photogenerated holes of CdS to Ba0.4Sr0.6TiO3, leading to long-term stability of CdS/Ba1 xSrxTiO3 heterojunction. More recently our group developed CdS/Ba1 xZnxTiO3 heterostructured photocatalysts for highly efficient and stable hydrogen evolution from water splitting. In this system, CdS increases the visible-light absorption as a photosensitizer, as well as suppresses the photogenerated electron-hole recombination due to the formation of heterojunctions with Ba1 xZnxTiO3. The CdS/Ba1 xZnxTiO3 showed excellent photocatalytic activity and stability with a high H2 production rate of 1473 μmol h1 g1 under simulated solar light irradiation for 480 h without loading any noble metal as cocatalyst or any reagents for regeneration [35]. Ye et al. synthesized a direct Z-scheme CoTiO3/g-C3N4 (CT-U) photocatalytic system by using a facile in situ growth method for H2 evolution from water splitting. The 0.15% CT-U sample showed the highest photocatalytic activity for H2 production and possesses excellent photostability after four consecutive photocatalytic runs, which was attributed to the synergistic effect of CoTiO3 and g-C3N4 associated with the formation of heterojunctions on the solid-solid contact interface [36]. Aiming at enhancing the photocatalytic activities of MOFs, Yuan et al. prepared a novel quasipolymeric metal-organic framework UiO-66/g-C3N4 heterojunction through a thermal annealing process [37]. The prepared g-C3N4/UiO-66 hybrid photocatalyst presented a much higher hydrogen evolution rate compared with the pure g-C3N4 and UiO-66 under visible-light irradiation, which is attributed to the efficient interfacial charge transfer from photoexcited g-C3N4 to UiO-66.

3.4.5

Modification of Phocatalysts With Cocatalysts

Novel co-catalysts such as noble metals and other catalysts can match with phocatalysts and enhance the photocatalytic hydrogen production activity by promoting the separation efficiency of photogenerated charge carriers. When CdS was loaded with only 0.2 wt% of MoS2, the rate of H2 evolution was enhanced by >36 times, which is even higher than CdS photocatalysts loaded with Pt, Ru, Rh, Pd, and Au [38]. Sun et al. prepared CdS nanoparticle supported on MIL101 by hydrothermal treatment, then Pt nanoparticles were

Chapter 3 NANOMATERIAL-BASED PHOTOCATALYTIC HYDROGEN PRODUCTION

photo-deposited on CdS/MIL-101(Cr) as the co-catalyst [39]. The photocatalytic performance of Pt/CdS/MIL-101(Cr) was evaluated by H2 production from water splitting. MOFs can effectively inhibit aggregation of CdS, and are beneficial to the dispersion of Pt nanoparticles, but they also provide more active sites for proton reduction. 10 wt% of CdS loaded Pt/MIL-101(Cr) exhibited the highest photocatalytic activity among pure CdS, CdS/MCM-41, and CdS/MOF-5, and different amounts of CdS-loaded MOFs, which was an almost 20 times higher photocatalytic activity than the MOF itself. Additionally, CdS/MIL-101(Cr) is considerably stable during the photocatalytic reaction. Exploiting noble-metal-free cocatalysts has attracted huge interest for photocatalytic hydrogen production from water splitting using solar energy. MoS2 is an efficient cocatalyst and is a low-cost alternative to Pt in photocatalysis for hydrogen evolution. MoS2 shows a three stacked atom layer structure (S-Mo-S) and the active S atoms on exposed edges attract much attention in terms of hydrogen evolution. Ye et al. prepared MoS2 with layer number ranging from 1 to 112, and fabricated layer-structured MoS2/CdS for HER, and found that the HER activity of layerstructured MoS2/CdS is dependent on the number of MoS2 layers. The hydrogen production activity of MoS2/CdS increases with decreasing MoS2 layer numbers and MoS2/CdS with single-layer MoS2 exhibits the highest H2 production rate, which can be attributed to the fact that the decrease of MoS2 layer numbers can increase active S atoms and promote the separation of charge carriers, and the conduction band potential of a single-layer MoS2 is negative than H+/H2 potential [40]. Wu et al. decorated UiO-66/CdS hybrids with noble-metal-free MoS2 co-catalysts (MoS2/UiO-66-CdS) for efficient photocatalytic H2 production. In these UiO-66/CdS hybrids, MOFs act as ideal support and MoS2 functions as a cocatalyst. The MoS2/UiO-66/ CdS sample with 50 wt% UiO-66 and 1.5 wt%MoS2 shows an unusual H2 production rate of 650 μmol h1, which is almost 60 times higher than that of pure CdS and also two times higher than that of Pt/UiO-66/CdS under the same photocatalytic conditions. Moreover, MoS2/UiO-66-CdS exhibits higher photostability than that of pure CdS. The enhanced photocatalytic activity and stability of MoS2/U6-CdS can be ascribed to the well-dispersion of MoS2 and CdS, efficient separation of photogenerated charge carries, and increased electron transfer pathways and active sites. MOFs also can function as a co-catalyst. Yuan et al. [41] prepared CdS/UiO-66 hybrids via in situ growth of CdS nanoparticles on UiO-66 metal-organic framework octahedrons. In these hybrids MOFs function as support matrices and co-catalysts for

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CdS semiconductors. No H2 was produced in the presence of UiO66 MOF under visible-light irradiation. The photocatalytic activity of CdS/UiO-66 hybrids for H2 production is dependent on CdS content, and the photocatalytic H2 production rate over 16 wt% CdS supported UiO-66 was 235 mol h1 in the presence of L-ascorbic acid and Pt nanoparticlea as co-catalyst, which is much higher than that of bare CdS and UiO-66. The enhanced photocatalytic H2 evolution of CdS/UiO-66 is due to the efficient interfacial charge transfer pathway from CdS to UiO-66 then to Pt nanoparticles. Small-sized Ni with exposed (111) facet was utilized as a co-catalyst for fabricating an efficient MOFs [42]. MOF-5 has been selected as support, Ni nanoparticles were well dispersed on MOF-5 (Ni@MOF-5), Eosin Y (EY) and TEOA was used as a photosensitizer and electron donor in the photocatalytic H2 production. The optimal hydrogen evolution rate of Ni@MOF-5 with 30.22 mmol h1 g1 was obtained, which was attributed to the large capacity of MOFs for absorbing EY, the high loading amount of Ni nanoparticles, the semiconducting properties of MOFs, strong interaction between MOFs and dye, and the low overpotential of Ni@MOF-5.

3.4.6

Formation of Solid Solutions

The formation of solid solutions of ZnS and narrow bandgap semiconductors is a good way to extend the light absorption of ZnS to the visible-light region and enhance visible-light utilization. (AgIn)xZn2(1 x)S2 solid solutions between ZnS and AgInS2 with a narrow bandgap exhibited high visible-light photocatalytic H2 evolution activities from aqueous S2 and SO3 2 solutions as sacrificial reagents. By increasing the ratio of AgInS2 to ZnS, the absorption of the (AgIn)xZn2(1 x)S2 solid solutions shifted to longer wavelengths, and the photophysical and photocatalytic properties of (AgIn)xZn2(1 x)S2 solid solutions were dependent upon the composition due to a change of band position. The crystal structures of ZnS are similar to those of CdS, leading to the easy formation of ZnS-CdS solid solution. ZnS-CdS solid solutions are excellent visible-light-responsive photocatalysts for H2 production [43–46].

3.4.7

Surface Modification With Graphene and Other Carbon Material

Recently surface modification of photocatalysts has been carried out with carbon materials like graphene, CQDs, carbon nanodots, fullerenes (C60), and carbon nanotubes (CNTs). CNTs for

Chapter 3 NANOMATERIAL-BASED PHOTOCATALYTIC HYDROGEN PRODUCTION

increasing hydrogen production in UV, UV-Vis, and visible-light region has attracted much attention due to the extended visible-light absorption range as well as improved charge transfer. Graphene shows superior charge carrier mobility (200,000 cm2 V1 s1), large specific surface area (2630 m2 g1), excellent electrical and thermal conductivity (5000 W•m1•K1), and good physical and chemical stability, and it can be easily obtained from bulk graphite by mechanical cleavage, thermal exfoliation, and chemical methods [47–51]. Zhang et al. reported the synthesis of graphene/TiO2 nanocomposites and the enhanced photocatalytic H2 evolution from water splitting, and found that the photocatalytic activity of graphene/TiO2 nanocomposites depends on the rGO content and calcination process [52]. Li et al. synthesized S and N co-doped graphene quantum dot/ TiO2 (S,N-GQD/TiO2) composite photocatalysts for enhancing photocatalytic H2 evolution, and S,N-GQD/TiO2 exhibited much higher photocatalytic activity than that of pure TiO2, which is due to the enhanced visible-light absorption and effective separation and migration of photogenerated electrons and holes [53]. Fan et al. studied the effect of different reduction approaches on the hydrogen evolution efficiency of TiO2/RGO nanocomposites systematically, and TiO2/RGO composite prepared by the hydrothermal method exhibited the highest hydrogen evolution performance under UV-Vis light irradiation [54]. Hao et al. anchored TiO2 with graphene quantum dots via in situ photoassisted method and found that TiO2 with graphene quantum dots showed enhanced photocatalytic H2 evolution activity in methanol aqueous solution [55], which is due to the function of graphene quantum dots as efficient electron reservoirs and excellent photosensitizers for TiO2.

3.4.8

Increasing Photocatalytically Active Area

The photocatalytically active area of photocatalysts is an important factor that directly influences hydrogen production rates, thus much attention was paid to how to increase the photocatalytically active area. There are two common and effective methods to increase the specific surface. One method is to construct coarse surfaces with uniform nanoparticles and the other one is incorporating another material (nanobelts, nanoparticles, nanorods, etc.) on the surface of the primary photocatalysts. One-dimensional CdS nanoro and nanowires with high photocatalytic activity for H2 production can be prepared by solvothermal method and hydrothermal method. Mesoporous CdS

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nanoparticles can be successfully synthesized by ultrasonicmediated precipitation, two-step aqueous route, and other methods.

3.5

Photocatalytic Reactors for Hydrogen Production From Water Splitting

Slurry-based photocatalytic reactor is usually utilized for photochemical water splitting. In this system, the photocatalysts will be mixed with water to form a homogeneous or heterogeneous slurry suspension, which will absorb photons from natural solar light or artificial light source, and then react at the solid-liquid-gas interfaces. Theoretically, a photoreactor can absorb incident light effectively with minimal photonic loss, which is beneficial for photocatalytic reaction. Moreover, technical challenges and economic cost are important factors to be considered for fabricating an efficient reactor from the point of practical application. A number of photoreactors were focused on photocatalytic watertreatment application, but only a few photoreactors focused on H2 production from water splitting. However, water-splitting photoreactors are different from water-treatment photoreactors, good sealing is needed in photocatalytic water-splitting photoreactors in order to protect the reactant from air and/or avoid hydrogen loss. In the design and construction of photoreactors, the geometric set-up of photoreactor, irradiation source (natural or artificial), light source position (immersed or external), catalyst (slurry or immobilized on a support), etc., all should be considered. Here we will introduce some common semiconductor particles-based photoreactors.

3.5.1

Batch-Type Photoreactor

Currently the batch-type reactor is the primary type of photoreactor. Fig. 3.6 shows a typical batch-type reactor for photocatalytic H2 production. This type of batch-type reactor is composed of a reaction tank, light source, quartz window, cooling water system, evacuation system, sample/product collecting device, and gas detection instrument. The reaction tank is made of stainless steel, Pyrex or quartz, etc. The water jacket surrounding the tank can allow cooling water to run around the tank in order to keep the temperature of the reaction process within a certain range. Light sources will irradiate upon the quartz window. The slurry suspension will react in the reaction tank under continuous magnetic stirring.

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77

Simulated solar radiation Thermocouples Hydrogen outlet Ar purging inlet Cooling water outlet

Quartz window

Photolyte and photocatalyst Cooling water inlet

Stirrer and hot plate

3.5.2

Continuous Annular Photoreactors

Continuous annular photoreactors consist of an annularshaped reactor with a lamp located in its center. The reactor is placed in a nitrogen atmosphere to separate the system from the surrounding air. Ar gas is bubbled continuously through the reaction slurry to keep the suspension in a good mixed state. Part of the generated H2 gas is introduced to a gas valve and analyzed by gas chromatography (GC). The advantage of continuous annular photoreactors is that the increased liquid-gas interfacial area, which facilitates desorption of H2 from the catalyst surface. The shortcoming of this reactor is that more photons can be accepted by the interior of the annular reactor than the exterior.

3.5.3

Photocatalytic Membrane Reactors

Photocatalytic membrane reactors (PMRs) are efficacious in sustainable hydrogen production by inhibiting the backward reaction between H2 and O2, and pure hydrogen without further purification can be obtained in a single step. In the design of

Fig. 3.6 A typical batch-type photoreactor built from stainless steel vessel and quartz window on the top, water jacket is placed around the vessel with cooling water to keep the temperature constant [56].

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PMRs, photocatalysts and metal loading, the mechanical resistance, permeability, and morphology of the membrane are important factors to be considered and need to be optimized to obtain excellent performance of the catalytic membrane system in hydrogen generation. There are several different types of PMRs for hydrogen generation, such as H-type photocatalytic reactor, membrane electrode photocatalyst-assembly, membrane twin reactor, and polymer membrane electrode assembly (MEA).

3.6

Conclusions and Perspectives

Nanomaterials for photocatalytic hydrogen production from splitting water have attracted much attention. The demand and main principles for nanomaterial-based photocatalytic hydrogen production have been introduced in this chapter. The development of photocatalyst materials, common performance enhancement strategies, and photocatalytic reactors for photocatalytic hydrogen production were also summarized. Although significant progress has been made in the design and fabrication of nano-materials for efficient photocatalytic hydrogen in recent years, there are still many intrinsic defects, such as relatively low efficiency for hydrogen production, solar conversion efficiency, recyclability, reusability, and stability, all of which need to be overcome for industrial application to be realistic. In future research and development on the photocatalytic hydrogen production from water splitting, some breakthroughs are expected in the following aspects: (1) Enhancement of the visible-light response of the photocatalyst: we try to design and synthesize novel visible-light-active semiconductor photocatalysts by bandgap energy engineering and cost-effective methods to realize maximum absorbance and effective harvesting of low-energy photons. (2) Development of organic pollutants or industrial wastes as sacrificial reagents: For an effective photocatalytic hydrogen generation system, sacrificial reagents or electron donors need to be added to the reaction system for improvement of the overall activity. If sacrificial reagents such as methanol or ethanol are more expensive than the commercial value of the hydrogen generation, their application in the photocatalytic hydrogen production from water splitting is meaningless. If organic pollutants or industrial wastes can be used as sacrificial reagents, the photocatalytic hydrogen generation and pollutants degradation can be achieved simultaneously, which transform the waste into useful material and reduces the economic cost.

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(3) Design of low-cost and highly effective photoreactor: The design of a low-cost and highly efficient photoreactor has an important effect on the hydrogen generation rate from photocatalytic splitting water. An ideal photoreactor should be capable of absorbing the incident radiations and promote photocatalytic reactions with minimum photonic losses. Moreover, photoreactors can separate H2 and O2 generation effectively to avoid the risk of explosion and reduce the cost of hydrogen purification. Through sustained efforts, we believe that the photocatalytic hydrogen generation will be an important path to obtain clean energy in the further.

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