Based Heterostructured Photocatalysts for Solar Water Splitting

Copyright © 2014 by American Scientific Publishers All rights reserved. Printed in the United States of America

Energy and Environment Focus Vol. 3, pp. 339–353, 2014 (www.aspbs.com/efocus)

BiVO4-Based Heterostructured Photocatalysts for Solar Water Splitting: A Review Jin Hyun Kim1 and Jae Sung Lee2, ∗ 1

School of Environmental Science and Engineering and Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), San 31, Hyoja-Dong, Pohang, 790-784 Republic of Korea 2 School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan, 689-798 Republic of Korea

ABSTRACT

CONTENTS 1. Photoelectrochemical Water Splitting for Solar Hydrogen Production . . . . . . . . . . . . . . . . . . . . . 2. BiVO4 as a Photoanode for Photoelectrochemical Cell 3. Synthesis of BIVO4 and Fabrication of Heterostructures Film . . . . . . . . . . . . . . . . . . . . . 4. BIVO4 Heterostructures . . . . . . . . . . . . . . . . . . . 4.1. Heterojunction . . . . . . . . . . . . . . . . . . . . . . 4.2. Charge Transfer Mediator . . . . . . . . . . . . . . . 4.3. Co-Catalyst Loading . . . . . . . . . . . . . . . . . . 5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . References and Notes . . . . . . . . . . . . . . . . . . . . .

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Author to whom correspondence should be addressed. Email: [email protected] Received: 9 February 2014 Accepted: 2 August 2014

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1. PHOTOELECTROCHEMICAL WATER SPLITTING FOR SOLAR HYDROGEN PRODUCTION Solar energy is the most promising renewable energy that could counter the dwindling supply of fossil fuels and relentless emission of carbon dioxide responsible for global warming.1–5 The most favorable feature of solar energy is its enormous supply corresponding to 10,000 times world annual energy consumption. But it is highly diffused and has to be harvested in a concentrated form and converted to a convenient form for practical utilization. Currently, it is converted to heat in solar thermal technologies or to electricity in photovoltaic. Photovoltaic is one of the fastest growing industries today and electricity is a clean and convenient form of energy. Yet, it has the critical drawback of difficult storage. Hence we need to develop technology to produce clean and CO2 -neutral

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Hydrogen is a clean and storable energy carrier of the future when it is produced from a renewable energy source via a CO2 -neutal process. Solar water splitting is such a renewable and sustainable energy production method utilizing sun light and water, the most abundant resources on earth. Among conceived solar hydrogen production methods, the photoelectrochemical (PEC) cell is most promising, in which the semiconductor photoelectrodes have to satisfy a number of requirements; a proper band gap energy for strong visible light absorption, band edge positions enabling oxidation or reduction of water, chemical and electrochemical stability in water under illumination, fast transport of the photo-generated electrons and holes in the semiconductor, low over-potentials for the electrode reactions, and low cost. Monoclinic BiVO4 satisfies many of these material requirements; a moderate band gap (∼ 2.4 eV) that allows 9.2% of theoretical solar-to-hydrogen (STH) efficiency, fine hole conductivity, good stability in neutral electrolytes, low price, and environmentally benign Delivered by Publishing Technology to: ? characteristics. But it also has critical drawbacksOn: of poor electron transfer and sluggish water oxidation kinetics. IP: 93.91.26.29 Fri, 14 Aug 2015 08:30:48 Formation of heterostructures is aCopyright: simple butAmerican effectiveScientific strategy Publishers to improve the performance of BiVO4 -based photocatalysts in light harvesting, long term stability and solar-to-chemical energy conversion efficiency in solar water splitting. This article reviews three types of heterostructures to modify BiVO4 including formation of heterojunctions with another semiconductor, addition of a charge transfer mediator, and loading a co-catalyst, which lead to great performance improvement in PEC water splitting. In all cases, improved performance comes from suppressed electron–hole recombination by facilitated charge separation in various interfaces along the charge transfer pathways. Only a limited number of selected examples are discussed that could provide good illustration of the underlying concepts and recent developments. KEYWORDS: BiVO4 Heterostructures, Heterojunction, Charge Mediators, Co-Catalyst, Charge Separation.

BiVO4 -Based Heterostructured Photocatalysts for Solar Water Splitting: A Review

Kim and Lee

CB and VB positions straddle water reduction (0.0 VRHE ) and oxidation (1.23 VRHE ) potentials. But much larger Eg is needed to overcome over potential of the reactions. In addition, the rate of hydrogen production is much smaller than the rate of light absorption because of dominant electron–hole recombination, which is the main energy loss process. The photoelectrochemical (PEC) water splitting cell employs two electrodes to produce O2 at anode and H2 at cathode. Figure 1 illustrates schematic configurations of PEC cells. The most basic configuration is composed − of an n-type semiconductor as photoanode and a metal+ h+ Light absorption (1) SC + h≥ Eg → ecb vb lic cathode separated by an aqueous electrolyte. Upon − Electrons in CB (ecb ) with an energy level above 0.0 eV light absorption by the semiconductor film, photoelectrons diffuse to the surface and reduce water to generate hydroand holes are generated in conduction and valance bands, gen usually on a hydrogen evolution co-catalyst. Holes in respectively. Because of the band bending formed at the VB (h+ vb ) with an energy level below 1.23 eV can also difn-semiconductor/electrolyte interface, holes move to the fuse to the surface and oxidize water to generate oxygen electrolyte to oxidize water by the reaction 3. Electrons are sometimes on an oxygen evolution co-catalyst. These procollected by the transparent conducting glass (F:SnO2 or cesses complete overall water splitting reaction that proFTO glass here), withdrawn to external circuit, pump up duces hydrogen from water with solar energy. by a bias potential (Vapp ), and come to the metallic cath− ode to reduce water for H2 evolution. The bias voltage is → 2H2 Water reduction (2) 4H+ +4ecb usually supplied by a photovoltaic device that is located + 2H2 O+4h+ (3) vb → O2 +4H Water oxidation behind the photoanode and utilizes the transmitted photons Overall reaction: 2H2 O → 2HDelivered +O G power generation 2 2 by Publishingfor Technology to: ? in a tandem configuration. IP: 93.91.26.29 On: Fri, 14 Aug 2015 08:30:48 There could be a wide variety of variations from this = 238 kJ mol−1 (4) Scientific Publishers Copyright: American basic PEC cell as shown in Figures 1(b)–(d). When only the photoanode is considered, the objective of the device is to produce the largest photocurrent (JPEC ). This could Theoretically, a semiconductor can split water if its Eg is greater than 1.23 eV (water dissociation energy) and its be achieved by maximizing light absorption (Jabs ) by using

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solar fuels and hydrogen is the most promising fuel when it is produced from the most abundant resources on earth, i.e., sinlight and water.1–11 To produce solar hydrogen from water, photocatalytic or photoelectrochemical water splitting.1–13 is employed with sun light as the energy source of the reaction and a solid semiconductor (SC) of small band gap energy (Eg ) as the photocatalyst or photoelectrode. Thus, the semiconductor can absorb light to generate electron/hole pairs when electrons are excited from valence band (VB) to conduction band (CB).

Jin Hyun Kim achieved his undergraduate degree at The University of Seoul (Korea) in 2012. He is now in Jae Sung Lee’s group in Ulsan National Institute of Science and Technology for his Ph.D. course related to environmental science. He is working for development of photocatalytic and photoelectrochemical solar energy conversion.

Jae Sung Lee achieved his B.S. degree from Seoul National University in 1975, M.S. degree from Korea Advanced Institute of Science and Technology in 1977, Korea. He received his Ph.D. degree under Professor Michel Boudart’s supervision for chemical engineering and catalysis from Stanford University in 1984. After postdoctoral carrier from Stanford University, he became assistant (86) and full (96) professor in Pohang University of Science and Technology, department of chemical engineering, Korea. He and his group recently moved to Ulsan National Institute of Science and Technology in 2013. He has been working for catalysis for environmental friendly applications, including traditional, electro and photocatalysis. Regarding photocatalysis, his research is related to development of materials possessing potential usage for solar energy conversion, using heterojunction, surface modification, doping and solid solution technologies. 340

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Delivered by Publishing Technology to: ? IP: 93.91.26.29 On: Fri, 14 Aug 2015 08:30:48 Copyright: American Publishers a semiconductor of proper band gap and minimizing loss Scientific Among hundreds of semiconductors known for photocatalytic activity, there is no single material that satisfies all of charges due to bulk recombination (Jbr ) and surface these requirements. Hence the materials development is recombination (Jsr ) by proper modification of the material. still the key issue in this research field. Instead of external bias voltage, the potential of cathParticulate form of monoclinic sheelite BiVO4 has been ode could be raised by using p-type semiconductor phoused as an oxygen evolution photocatalyst in the presence tocathode (b),14 a photovoltaic (PV) device behind the of Ag+ as the electron scavenger17 and in a Z-scheme photoanode in a tandem configuration (c),15 or double 16 together with a hydrogen evolution photocatalyst.4 More photoanodes with a redox mediator (d). The second recently, however, it has received great attention as a phoabsorbers in Figures 1(b)–(d) utilize photons transmittoanode for PEC water splitting because it satisfies many ted through the front absorbers, hence the first photoanof the above requirements.1 2 18 19 As an n-type semiode films should have an excellent transmittance. The conductor with a direct band gap of 2.4 eV, it absorbs second absorber should also have a smaller band gap ample visible light and is stable in neutral electrolyte, than the material for the front photoanode, so it extends nontoxic and relatively cheap.8 14 20 Compared to other the range of light harvesting to longer wavelength solar common oxygen evolution photocatalysts like WO3 and photons. Fe2 O3 , BiVO4 has a relatively high CB edge (0.02 VRHE ) and, as a consequence, requires less bias potential to 2. BiVO4 AS A PHOTOANODE FOR raise the potential of photoelectrons above water reducPHOTOELECTROCHEMICAL CELL tion potential (0.0 VRHE ). But it has its own shortcomings A photoanode material for PEC cell should satisfy a numof poor electron conductivity and sluggish water oxidaber requirements for efficient water splitting performance; tion kinetics. To make BiVO4 an efficient photoanode of (i) n-type semiconductor with a band gap energy of PEC cell, these undesired properties have to be improved 10% solar energy spectrum, by a variety of modifications including formation of het(ii) the top of the valence band more positive than water erostructures with other functional materials like another oxidation potential for oxygen generation (1.23 VRHE ), semiconductor, charge mediator, or co-catalyst. The for(iii) long-term durability in aqueous electrolytes, mation of BiVO4 -based heterostructures is the subject of (iv) good charge transport properties, this review. (v) high crystalline quality with minimal defects, and BiVO4 has three crystal systems of tetragonal scheelite, monoclinic scheelite and tetragonal zircon, and (vi) low cost material made of earth-abundant elements. Energy Environ. Focus, 3, 339–353, 2014

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Fig. 1. Schematic configurations of PEC cells: (a) Basic photoanode-bias cell, (b) photoanode (n) – photocathode (p) cell, (c) photoanode (n) – PV tandem cell, and (d) double photoanodes with a redox mediator.


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Fig. 2. (a) Crystal structure of monoclinic sheelite BiVO4 .24 (b) Photographs of as prepared BiVO4 with coprecipitation, paste blending and the metal-organic decomposition (MOD). Reproduced with permission from [24], A. Walsh, SPIE Newsroom 1 (2009). © 2009, SPIE.

monoclinic scheelite is known to be the most photoactive with only O 2p orbital-driven VB.23 26 This characteristic 1 2 14 21 22 phase. Phase transition from tetragonal to monis owing to distortion of metal polyhedron (BiO8 ) in monoclinic sheelite occurs irreversibly at 670–770 K.1 2 14 As oclinic sheelite phase compared to other phases.19 27 The shown in Figure 2(a), the scheelite structure contains V tetragonal BiVO4 does not have this hybridization and thus ion coordinated by four O atoms in a tetrahedral site and a larger band gap of 2.9 eV with VB mostly composed Bi ion is coordinated by eight O atoms from eight differof O 2p. This hybridization also gives BiVO4 good charge ent VO4 tetrahedral units.1 2 19 23 The monoclinic crystal mobility,28 which is a desired attribute for high photocatsystem has a space group of I2/b with a = 51935Å, b = alytic activity. Another unique advantage of this electronic 50898Å, c = 116972Å, and = 903871. structure is related Delivered Technology to: ?to direct band gap formation via unocThe electronic structure of BiVO4 was studiedby inPublishing detail IP: 93.91.26.29 On: Fri, 14 Aug 2015 08:30:48 cupied V 3d and Bi 6s+O 2p hybridized band. This allows 19 25 by density functional theory (DFT) calculations. and Scientific Publishers Copyright: American facile light absorption and thus a thin semiconductor film confirmed by many spectroscopic techniques. As shown in is enough to absorb fully the incoming light. The band gap Figure 3, its CB is composed of mainly V 3d states with energy of 2.4 eV corresponds to 516 nm wavelength of small contributions from O 2p and Bi 6p. For VB, Bi 6s light, which gives theoretical solar-to-hydrogen (STH) effioccupies upper part of VB while O 2p takes lower part, ciency of about 9.2%. As an n-type photocatalyst, a good and light absorption occurs from Bi 6s + O 2p hybridized hole conductivity of BiVO4 is highly favorable as hole VB. Hybridization of Bi 6s + O 2p is responsible for relatively smaller band gap of BiVO4 than usual binary oxides transfer is critical for both photocatalytic and photoanodic

Fig. 3. (a) Ion-projected electronic density of states (DOS) of BiVO4 (the highest occupied state is set to 0 eV).25 (b) Schematic band structure of BiVO4 .17 23 Reproduced with permission from [25], J. Yang et al., Chem. Eur. J. 19, 1320 (2013). © 2013, Wiley.

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are shown in Figure 2(b). The sample should be highly transparent to fabricate a tandem cell with a photovoltaic device, and should be highly porous to allow the facile access of electrolyte.

4. BIVO4 HETEROSTRUCTURES

Heterostructure formation is widely utilized to improve the properties of a semiconductor by combining with other functional materials.5 12 It has a large scope of materials as well as applications including photocatalysis, photovoltaics, light emitting devices and optoelectronics.46–51 For BiVO4 , three kinds of heterostructuring techniques are commonly employed by combining with other semiconductors (heterojunction), charge mediators, and oxygen evolution co-catalysts. Table I lists typical examples and representative performances. To demonstrate general effects of heterostructuring, Figure 4 shows a scheme and performance evolution of Co–Pi/W:BiVO4 /WO3 that we studied. As mentioned, doping increases carrier density and mobility to improve the charge transport properties of BiVO4 . BiVO4 and WO3 forms a heterojunction so that electrons can trickle down from CB of BiVO4 to that of WO3 by the CB energy level offset to improve charge separation. Oxygen evolution cocatalysts like by Publishingcobalt Technology to:(Co–Pi) ? 3. SYNTHESIS OF BIVO4 ANDDelivered FABRICATION phosphate improves hole transfer at elecIP: 93.91.26.29 On: Fri, 14 Aug 2015 08:30:48 trolyte/photocatalyst interface, increasing surface charge OF HETEROSTRUCTURESCopyright: FILM American Scientific Publishers separation. Thus the observed photocurrents at 1.23 VRHE Before the heterostructure formation, effective synthesis under AM 1.5 G solar light improve with successive of pristine BiVO4 is critically important to obtain high modifications: bare BiVO4 (0.2 mA/cm2 ) < doped BiVO4 efficiency after modification. Among various methods, the (0.7 mA/cm2 ) < heterojunction with WO3 (1.6 mA/cm2 ) < metal organic decomposition (MOD) method is most pop6 8 15 19 29 32 35 36 Co–Pi cocatalyst (3.7 mA/cm2 ). Loading of the co-catalyst The MOD synthesis is ular and effective. also brings a cathodic shift of 0.3 V in the current onset conducted with an organic solution (acetic acid/acetyl ace3+ potential. Details of these heterostructuring techniques are tonate) containing Bi source (bismuth (III) nitrate) and 5+ discussed below. V source (vanadyl acetylacetonate, vanadium (V) tri-ipropoxy oxide). The solution is deposited on a transpar4.1. Heterojunction ent conducting glass with spin coating, spray pyrolysis or drop casting. Thermal treatment (600–770 K) decomCombination of two semiconductors (n/n or p/n) with poses organometallic Bi + V species to form crystalline proper band positions can make cascade electron transBiVO4 without formation of discrete phases of Bi2 O3 and fer from CB of upper potential to CB of lower potential. V2 O5 .37 The MOD method is also suitable for the fabriSuccessful heterojunction formation of BiVO4 has been cation of heterostructures and modification of BiVO4 with reported with WO3 , SnO2 , Fe2 O3 , CuWO4 , and CdS and doping. WO3 /BiVO4 has been most common. WO3 (Eg = 26– Recently, other effective methods have been also 2.8 eV) is one of the most active metal oxide photocatalyst reported for improved BiVO4 preparation. Electrodepowith CB at 0.42 VRHE and VB at 3.12 VRHE .9 52 54–57 With 38 39 sition (ED). or thermal conversion. involves BiOI such band configurations presented at Figure 5(a), elecas precursor. By making amorphous phase of Bi + trons transfer from BiVO4 to WO3 , whereas holes cannot. V by ED, thermal treatment and post basic treatThis prevents electron/hole recombination in BiVO4 . Since ment produce nanoporous BiVO4 . Also reported are WO3 has better mobility and longer diffusion length than coprecipitation.4 14 40 , paste blending (PB),41 chemical BiVO4 , electrons collected in WO3 can be more efficiently bath deposition (CBD),42 hydrothermal43 sputtering,44 converted to photocurrents with much reduced recombinachemical vapor deposition (CVD).45 and simple precursor tion compared to the case when the electrons are locked in drop casting (water base with PEG polymer) for synthesis BiVO4 . As BiVO4 has a smaller band gap and wider pH of BiVO4 and fabrication of heterostrucures. The images stability, BiVO4 /WO3 heterojunction can absorb larger porof the typical samples prepared by some of these methods tion of solar light and has better neutral stability relative Energy Environ. Focus, 3, 339–353, 2014

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water oxidation.1 2 6 15 29 From time-resolved microwave conductivity (TRMC) analysis, the hole diffusion length of BiVO4 was calculated to be 70 nm, which is relatively longer than that of Fe2 O3 (5 nm) but shorter than that of WO3 (150–500 nm).28 30 31 In spite of the advantages of BiVO4 as a photoanode material, it also has critical weaknesses, i.e., low electron conductivity and poor water oxidation kinetics. In addition, surface of BiVO4 tends to be damaged by V 5+ ion leaching, which can be temporary deterred by forming Bi-rich oxide layer.1 2 32 33 Even though BiVO4 has far larger electrolyte tolerance (pH 3 ∼ 14).1 2 32 33 than WO3 (pH ∼ 1) and Fe2 O3 (pH ∼ 14), higher stability is still required for practical applications. These shortcomings could be overcome by a number of strategies, including morphology control, doping, and formation of heterostructures with other semiconductors, charge mediator, and oxygen evolution co-catalysts. This review focuses only on heterostructure formation, although morphology control (including 1D or 2D nanostructuring) and impurity doping are equally important particularly to improve the charge transport characteristics of BiVO4 . For these topics, readers are referred to excellent accounts published already.1 2 23 26 34


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Table I. Heterostrucuring techniques for BiVO4 . Techniques Heterojunction

Charge mediator

Electrocatalyst

Reported substance WO3 SnO2 Fe2 O3 CuWO4 Rh:SrTiO3 (z scheme) CdS RGO PRGO CNT, MWCNT SiO2 TiO2 Co–Pi Co–Bi, Ni–Bi FeOOH Co3 O4 (CoOx ), RhO2 , IrOx , RuOx , MnOx , IrOx Pt, Au, Ag+

Typical figure of merit [reference]

Functionality

BiVO4 /SnO2 /WO3 (3.04 mA/cm2 at 1.23 VRHE )52

Charge separation

BiVO4 /PRGO/SrTiO3 /Ru (1.05% AQY)53

Electron mediator

Co–Pi/gradient W:BiVO4 (3.6 mA/cm2 at 1.23 VRHE , ∼ 5% STH with tandem cell)15

Surface charge separation and improving OER kinetics.

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Notes: AQY = Apparent quantum yield. Pi = phosphate ion.

to bare WO3 . The improved charge transfer characteristics (1-D) nanorods or nanowires makes the BiVO4 /WO3 of BiVO4 /WO3 heterojunction was confirmed by electroheterojunction more effective. The particular geometry chemical impedance spectroscopy (EIS) that showed that reduces the distance that holes have to travel in radial resistance of the heterojunction is reduced almost to that direction to reach the electrolyte. On the other hand, elecof WO3 .58 have to to: flow Delivered by Publishingtrons Technology ? along the axial direction making a vectorial flow. This configuration has been proven to be An interesting feature of BiVO4 /WO is the partial dopIP: 93.91.26.29 On: Fri, 14 Aug 2015 08:30:48 3 Copyright: Publishers It American has been Scientific ing effect of W into BiVO4 at the interface. highly effective for various photocatalysts (TiO2 , ZnO, shown that precursor state of BiVO4 can intake a sigFe2 O3 , Ta3 N5 ).1 2 26 59–61 and BiVO4 /WO3 .42 62 Recent nificant amount of W from interfacing WO3 or H2 WO4 report of W:BiVO4 /WO3 core–shell structure showed one even in the solid form,21 although intermetallic oxide like of the highest PEC performance reported for BiVO4 /WO3 (3.1 mA/cm2 at 1.23 VRHE ).62 The estimated bulk charge Bi2 WO6 is not formed. It is well known that W doping into separation coefficient of 77% for W:BiVO4 /WO3 was as BiVO4 lattice increases major carrier density and PEC performance of BiVO4 .19 21 28 But the presence of this effect good as that of WO3 nanowires. There has been also depends on the method of heterojunction fabrication and efforts to improve the properties of BiVO4 itself by introducing dopants,19 21 28 35 58 63–65 pore-forming surfactant,35 the effect is much smaller than intentional W doping. and combustion additive65 during the synthesis. Thus Nanostructured WO3 was found to be more effective. BiVO4 /WO3 prepared with both Triton-X, NH4 NO3 As shown in Figure 5(b), WO3 prepared in 1-dimentional

Fig. 4. (a) A scheme of heterojunction configuration with a co-catalyst (co-catalyst/BiVO4 /WO3 ) and (b) I –V curve of Co–Pi/W:BiVO4 /WO3 photoanode (light source - AM 1.5 G, 0.1 M KPi (pH 7), scan rate 20 mV/s, front illumination).

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showed 3.41 mA/cm2 at 1.23 VRHE in 0.1 M carbonate (2.3 eV) has smaller band gap than BiVO4 (2.4 eV), its more light absorption did not give better conversion showsolution.65 ing the same IPCE wavelength threshold. This might be Another successful heterojunction is BiVO4 /SnO2 as owing to the indirect band gap property of CuWO4 . And shown in Figure 5(c). SnO2 has a large band gap back side illumination showed smaller photocurrent than of 3.5 eV and potentials of CB (0.27 VRHE ) and VB the front side, suggesting that CuWO4 mostly acted as a (3.77 VRHE ) are favorable for cascade electron transfer 52 hole mirror like WO3 , SnO2 and Fe2 O3 . Heterojunction from BiVO4 . Another important property of SnO2 is a 69 of BiVO passivation effect of FTO glass. Thus, a large population 4 /CdS was prepared by spray pyrolysis method. Unlike previous cases of oxides, CdS has a higher CB of interfacial defects, the potential electron trap states of potential than BiVO4 . But authors claimed that electron FTO, can be passivated by a thin SnO2 layer, improv6 15 28 29 66 formed at BiVO4 could move toward VB of CdS to recoming charge transfer at BiVO4 /FTO interface. bine with holes generated in CdS. This process can supAlso very positive VB of SnO2 prevents backward hole 52 67 press the self-oxidation of CdS by its own holes, while transfer through SnO2 layer forming a ‘hole mirror.’ its electrons can be utilized for reduction reactions. Thus, As CB potential of BiVO4 , SnO2 and WO3 is aligned BiVO4 layer can protect CdS from meeting directly aquein cascade (0.02/0.27/0.41 VRHE ), ternary composite of ous electrolyte and also increase PEC activity in Na2 SO3 BiVO4 /SnO2 /WO3 could be prepared as an effective hetelectrolyte. 52 erojunction as shown in Figure 5(d). Recently, Xie et al. reported TiO2 (5 wt%)/BiVO4 susA thin SnO2 layer deposited by spray pyrolysis showed pension that produced hydrogen from methanol decoman excellent bulk charge separation property and several position under visible light.70 The result is unexpected BiVO4 /SnO2 heterojunctions like Co–Pi/BiVO4 /SnO2 , Co– because only BiVO4 can absorb the visible light, but Pi/1%W:BiVO4 /SnO2 and Co–Pi/gradient W:BiVO4 /SnO2 cannot produce hydrogen because its CB lies below the 6 15 28 29 exhibited the state-of-the-art PEC performance. water reduction potential (0.0 VRHE ). By transient state surThus, Saito et al., reported BiVO4 /SnO2 /WO3 photoanface photovoltage spectroscopy analysis, they found that ode showing a photocurrent of 3.04 mA/cm2 at 1.23 VRHE /BiVO TiO 2 4 showed longer carrier lifetime than BiVO4 in 2.5 M KHCO3 solution as an electrolyte.52 Sayama because of improved charge separation by the heterojuncby Publishing Technology to: ? et al. compared PEC performance in Delivered Na2 SO4 , NaH 2 PO4 , tion formation. In addition, steady state surface photoOn: − Fri, 14 Aug 2015 08:30:48 NaBO2 , NaHCO3 electrolytes and IP: the 93.91.26.29 presence of HCO 3 voltage spectroscopy Copyright: American Publishers indicated the electron transfer from in carbonate electrolyte can activate surface reaction and Scientific to CB of TiO2 . Thus, the high energy (‘hot’) phoBiVO 4 prevent backward reaction (O2 + 2H+ → H2 O), resulting toelectrons formed in CB of BiVO4 transfer to CB of TiO2 in improved PEC performance and stability.36 67 As coninstead of relaxation to the bottom of its own CB. This firmation of SnO2 as hole mirror, CVD deposition of unusual charge transfer would be possible because of the BiVO4 (200 nm) on 1%Mo:BiVO4 (20 nm)/FTO and SnO2 long carrier life time induced by the heterojunction forma(40 nm)/FTO showed similar effect of hole mirror, where tion. Now the photoelectrons in CB of TiO2 can produce both thin Mo:BiVO4 and SnO2 layers cover interfacial trap H2 because of its CB band position negative enough for states on FTO and promote charge transfer.45 water reduction. BiVO4 was combined with Fe2 O3 nanostructure array (NA) with reduced graphene oxide (RGO) as an elec4.2. Charge Transfer Mediator tron mediator68 (to be discussed in the next section). As semiconductor photocatalyst is less conductive than Thus Fe2 O3 −NA/RGO/BiV1−x Mox O4 was sequentially conductors, combination with conductive materials could prepared (Fig. 6) by hydrothermal, photoreduction and improve its conductivity problem. Because of high conMOD. The formed core/shell composite structure exhibited ductivity from conjugated carbon networks, carbon mateincreased photocurrent of 2.4-fold from that of Fe2 O3 -NA rials like graphene, fullerene, carbon nanotube (CNT) and and 1.7-fold from that of Fe2 O3 -NA/RGO. Note that their derivatives have been frequently used as an electron BiVO4 or BiV1−x Mox O4 (or doped Mo:BiVO4 ) deposited mediator and even as a photocatalyst itself.53 71–74 In paron Ti substrate has much smaller photocurrents. Here, ticular, BiVO4 is known for poor electron conductivity and Mo:BiVO4 layer is the main light absorber and RGO mediCNT and graphene derivatives were reported as effective ates electron transfer from Mo:BiVO4 to Fe2 O3 -NA. Also, conductive charge mediator. An example is already proFe2 O3 -NA showed much better effect than planar Fe2 O3 , vided in Figure 6, where RGO is the electron mediator reflecting the advantage of 1-D nanostructures for the elecbetween poorly conducting BiVO4 and Fe2 O3 to connect tron transfer.68 them electronically and make the heterojunction working. BiVO4 was deposited by spray pyrolysis on elecFigure 7 shows scheme, morphology and visual observatrodeposited CuWO4 and the BiVO4 /CuWO4 heterojunction of BiVO4 /RGO composite.53 71 74 This heterostructurtion showed significantly improved PEC performance, ing induced 10-fold increase in photocurrent generation ∼ 0.8 mA/cm2 in 1 M Na2 SO4 and ∼ 1.7 mA/cm2 in and greatly reduced transient photocurrent decay indicat1 M NaHCO3 at 0.6 V Ag/AgCl.63 Even though CuWO4 ing reduced charge recombination. Although the obtained


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Fig. 5. Schemes of BiVO4 -based heterojunction: (a) BiVO4 /WO3 ,9 (b) BiVO4 /WO3 1D nanostructure,42 (c) BiVO4 /SnO2 (hole mirror),2954 and (d) BiVO4 /SnO2 /WO3 dual hole mirror.52

absolute photocurrent was rather low owing to unoptisolution of pH 3), particles were connected to those in mized materials synthetic procedure, the report demonthe next layers leading to favorable electron transfer, i.e., strated potential of carbon-based materials as an effective BiVO4 → PRGO → Rh:SrTiO3 /Ru → H+ for hydrogen electron transfer mediator. A good contact between BiVO4 evolution reaction. and the carbon mediator was essential for high efficiency. A unique heterostructure of electron mediator/BiVO4 For example, BiVO4 synthesized in situ with MWCNT was reported by Pilli et al.64 Their SiO2 /BiVO4 (1:2 showed much higher photocurrent than a simple physical mole ratio) prepared by a surfactant-assisted MOD was mixture of BiVO4 and MWCNT although both of the modhighly transparent compared to normal BiVO4 . The hetDelivered by to Publishing Technology to: ? ifications showed increased photocurrent compared bare erostructure containing amorphous SiO2 showed two-fold IP: 93.91.26.29 On: Fri, 14higher Aug 2015 08:30:48 BiVO4 .74 photocurrent and this tendency was maintained with Copyright: American Scientific Publishers Kudo and coworkers reported BiVO4 /electron and without cobalt phosphate (Co–Pi) as a co-catalyst. mediator/Rh:SrTiO3/Ru, so called the Z-scheme sysAlthough the role of SiO2 was not clearly established, tem for powder-based overall water splitting.4 23 40 53 it was suggested that the presence of SiO2 on the semiAs electron mediators, both liquid (IO3− /I− , Fe3+ /Fe2+ , conductor surface favored the charge separation of the [Co(bpy)3]3+ /2+ and [Co(phen)3]3+ /2+ ) redox couples and photo-generated electron/hole pairs by facilitating the tunsolid (partially reduced graphene oxide, PRGO) elecneling of electrons and holes through grain boundaries and tron mediators were used. They showed that interaction thereby improves the PEC characteristics. The work proamong the components of BiVO4 /PRGO/Rh:SrTiO3 /Ru vides an example of another kind of charge transfer mediwas affected by pH. The apparent quantum yield was ator by insulating SiO2 . the highest (1.03%) when these components were ‘visually’ attached to each other. In acidic media (H2 SO4 4.3. Co-Catalyst Loading As a photocatalyst for PEC applications, BiVO4 has two shortcomings; poor water oxidation kinetics and inefficient charge transfer.39 Loading an oxygen evolution co-catalyst on the semiconductor surface can improve the problems by providing sites for hole collection and its injection to water for the oxygen evolution reaction (OER). These functions can improve not only photocatalytic performance but also the stability (durability) of material.2 8 38 75–77 By collecting holes, proton generation via water oxidation occurs on the co-catalyst instead of semiconductor surface and local pH drop on the semiconductor can be avoided. This would suppress the photo dissolution of Fig. 6. I –V curves: (a) BiVO4 /SnO2 /WO3 , (b) BiVO4 /WO3 , (c) bare BiVO4 . By funneling the holes to the main water oxidation BiVO4 and (d) bare WO3 in 0.1 M KHCO3 aqueous solution. reaction, photooxidation of the semiconductor itself could (e) BiVO4 /SnO2 /WO3 in 0.1 M Na2 SO4 aqueous solution. Insertion is also be avoided.26 There are reports that both PEC percross sectional SEM image of BiVO4 /SnO2 /WO3 .52 Reproduced with formance and stability of nitrides (Ta3 N5 ) and oxinitrides permission from [52], R. Saito et al., Chem. Commun. 48, 3833 (TaON, BaTaO2 N, SrNbO2 N) are remarkably improved by (2012). © 2012, The Royal Society of Chemistry. 346


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Delivered by Publishing Technology to: ? loading Co–Pi, IrOx , RhOx , CoOx IP: and93.91.26.29 CoOx /RhOx as co-Fri, 14 other Fe2 O3 .12 86–88 ZnO, WO3 ,89 Si/ITO,90 On: Augphotocatalysts; 2015 08:30:48 78 91 Ta N , C N , and TiO2 .92 93 It is currently the most catalysts.11 75 78–83 Copyright: American Scientific Publishers 3 5 3 4 active heterogeneous oxygen evolution co-catalyst and Effects of the co-catalyst on BiVO4 are illustrated in is composed of earth-abundant elements. Deposition of Figure 8 for Co–Pi as an example.84 85 The main role of Co–Pi has been conducted in various ways; photodeco-catalyst is to lower activation energy of water oxidation position (PD), photo assisted electrodeposition (PED) by providing more efficient pathway of holes, instead of and electrodeposition (ED),6 8 15 35 85 94 which oxidize the one through semiconductor itself. Correlations among electro- or photo-chemically a Co2+ precursor (cobalt(II) different parameters are summarized as follows:84 nitrate, cobalt (II) chloride) in a phosphate anion (PO3− 4 ) − Photocatalysis Eg ≥ E + Ea medium. Resulting Co–Pi has various valences for Co (2+ /3+ ) in amorphous phase. Different methods give dif− Photoelectrocatalysis Eg + EV ≥ E + EAOP ference in morphology, thickness, color and PEC perArrhenius equation:k = A exp−EAOP /RT formance, but in common is the significantly increased PEC performance.6 8 15 On BiVO4 , a thin Co–Pi layer (5– where E − = chemical energy at the standard potential for 30 nm) is most effective but the effect depends on surwater oxidation (1.23 V); Ea = activation energy for phoface status of BiVO4 .6 8 15 29 35 85 Also the catalyst works tocatalytic and photoelectrocatalytic water oxidation; Eg = best in a phosphate buffer solution giving higher photocurthe band gap (2.45 eV) of BiVO4 ; EAOP = the activation rents and stability than usual electrolyte like Na2 NO3 and overpotential; EV = the externally applied electric energy Na2 SO4 . at the onset potential. As overpotential is required to oxiQuantitative effects of co-catalysts were demonstrated dize water in addition to the standard potential, the coby Gamelin and coworkers.8 Loading Co–Pi on W:BiVO4 catalyst can lower this EAOP , reduce Ev requirement for brought a cathodic onset potential shift of ∼ 0.4 V, the reaction and offer faster photocatalytic water oxidaphotocurrent increment and faradaic OER efficiency of tion. Figure 8(b) shows how the co-catalyst lowers EAOP ∼ 100%. The analysis is based on the fact that PEC oxiin case of Co–Pi. Specific charge transfer of Co–Pi/BiVO4 is dation of easily oxidizable species like H2 O2 or SO2− 3 involves charge/discharge of Co(2+ /3+ /4+ ) species in Co– so facile that all the holes arriving at the photocatalyst Pi, resulting in hole transfer to water molecule and oxygen surface are used to oxidize those species without any evolution.85 loss by surface charge recombination. Hence, comparison Among many co-catalyst, Co–Pi has been most popof photooxidation currents of water and H2 O2 gives the surface charge separation efficiency. These two currents ular and widely effective for BiVO4 as well as many Energy Environ. Focus, 3, 339–353, 2014

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Fig. 7. (a) Schematic for the energy band structure of the Mo:BiVO4 /RGO/Fe2 O3 – NA heterojunction (denoted as Fe2 O3 -NA/RGO/BiV1−x Mox O4 ) and proposed mechanism of PEC water splitting, (b) synthetic route to the heterojunction.68 Reprinted with permission from [68], Y. Hou et al., Nano Lett. 12, 6464 (2012). © 2012, American Chemical Society.


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Fig. 8. (a) Schematic diagram of BiVO4 /RGO deposited on FTO (a) SEM images (b), (c) and photographs (d) of BiVO4 and BiVO4 /RGO. (Scale bars correspond to 600 nm).71 Reprinted with permission from [71], Y. H. Ng et al., J. Phys. Chem. Lett. 1, 2607 (2010). © 2010, American Chemical Society.

became almost the same on Co–Pi/W:BiVO4 , indicating steady oxygen evolution efficiency. It was also loaded that surface charge separation efficiency was nearly 100% onto BiVO4 prepared by ED,38 96 thermal conversion.39 because Co–Pi suppressed the surface recombination proand dip coating.7 Thus, FeOOH loaded on ED BiVO4 cess almost completely making water photooxidation as showed the best results with an onset potential of 0.2 VRHE efficient as H2 O2 oxidation. and only a very small difference in photocurrents between Similar to Co–Pi, nickel borate (Ni–Bi) prepared by sulfite and water oxidation, indicating that it suppressed both ED and PD showed significant PEC performance surface recombination effectively. Interestingly, Co–Pi did increment.20 The oxidation state of Ni varied (2+ /3+ /4+ ) not work as well, suggesting that effects of co-catalyst during the water oxidation reaction. Optimum thickness are dependent on surface state of BiVO4 derived from of Ni–Bi was ca. 40 nm and obtained by ED for 10 sec different preparation methods. Furthermore, Mo:BiVO4 and PD for 30 min. Deposition of Ni–Bi also decreased prepared by electrodepsoition or dip coating was comcharge transfer impedance of BiVO4 photoanodes. It was bined with known electrocatalysts (CoPi, Ag+ , Pt, RuOx . concluded that Ni–Bi works just like Co–Pi and other Delivered by PublishingMnO Technology to:09?Fe01 Ox , NiFe2 O4 and carbonate solux , NiOx , Ni IP: 93.91.26.29 Fri, 14tions) Aug but 2015 08:30:48 co-catalysts to reduce the overpotential needed forOn: OER most of them showed negligible effect compared 7 Copyright: American Scientific Publishers and to transfer holes generated in BiVO to FeOOH. 4 into the reaction rapidly and efficiently. Co–Bi was found to have simiMany common metal oxides are known as electrocatlar effects as a co-coatalyst.95 For both Ni–Bi and Co–Bi, alysts for water electrolysis and are also effective as copresence of borate anion (BO3− catalysts for PEC water oxidation on BiVO4 . These include 3 ) was very important to obtain high photocurrents and stability.20 95 It was also CoOx (including Co3 O4 ), MnOx , IrOx , RuOx and RhOx . noted that borate has a proton capturing property as a Thus, Co3 O4 /BiVO4 prepared with Co3 O4 and BiVO4 buffer solution.36 67 powders showed increased PEC performance.41 Co3 O4 Recently, FeOOH was reported to have a performance prepared by precipitation had a p-type character with Eg comparable with Co–Pi.2 38 39 96 Thus, the co-catalyst preof 2.0 eV. From IPCE, however, it was found that photons pared by PED on BiVO4 showed both exceptional stability absorbed by Co3 O4 did not contribute to water oxidaand moderate photocurrent under low bias (0.5 VRHE ) with tion because its VB position (+ 0.54 VNHE at pH 7) was

Fig. 9. Scheme of water oxidation mechanism by Co–Pi. (a) Activation energy shift.84 (b) Water oxidation mechanism of Co–Pi/BiVO4 .85 Reprinted with permission from [84], D. Wang et al., J. Phys. Chem. C 116, 5082 (2012). © 2011, American Chemical Society, and Reproduced with permission from [85], T. H. Jeon et al., Phys. Chem. Chem. Phys. 13, 21392 (2011). © 2011, PCCP Owner Societies.

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Table II.


PEC performance of BiVO4 -based heterostructures.

Material

Conditions (Light source, Electrolyte)

Preparation method (co-catalyst/BiVO4 /others)

Performance

References [9]

PADD/PADD

0.5 M Na2 SO4

1.5 mA/cm (1.23 VRHE ) IPCE 38%at420 nm (1.23 VRHE )

CBD/Solvothermal

0.5 M Na2 SO4

∼ 1.6 mA/cm2 (1.0 V vs. Pt counter electrode)

[42]

BiVO4 /WO3

MOD/spin coat

50 W tungsten lamp ( ≥ 420 nm) 0.5 M Na2 SO4

0.09 mA/cm2 (1.0 V Ag/AgCl)

[57]

BiVO4 /SnO2

SP/SP

0.1 M KPi (pH 7)

IPCE 30% at 420 nm (1.63 VRHE )

[66]

BiVO4 /WO3 BiVO4 /WO3 (nano rod)

BiVO4 /SnO2

2

2


0.02 mA/cm (1.0 V Ag/AgCl)

[54]

ED/MOD/spin coat



[56]

5%Mo:BiVO4 /WO3

Hydrothermal/drop cast

0.5 M Na2 SO4

1.7 mA/cm2 (1.23 VRHE )

[100]

BiVO4 /SnO2 /WO3

MOD/spin coat/spin coat

0.1 M, 2.5 M KHCO3

2.52 mA/cm2 (0.1 M KHCO3 ) 3.04 mA/cm2 (2.5 M KHCO3 ) (1.23 VRHE ) Maximum EQE = 0.90%

[52]

Auto combustion/spin coat

0.1 M KHCO3

3.41 mA/cm2 (0.1 M KHCO3 ) (1.23 VRHE )

[65]

CVD/drop cast/sputter

0.5 M Na2 SO3 + KPi buffer (pH 7)

0.95 mA/cm2 (1.23 VRHE )

[45]

Au/BiVO4 /WO3

BiVO4 /WO3 BiVO4 /Mo:BiVO4 /SnO2 BiVO4 /WO3

Delivered Sputter/sputter

by0.5Publishing to: ? 0.7 mA/cm2 M Na2 SO4 + Technology 0.1 M IP: 93.91.26.29 On: Fri, 14 KPi buffer (pH Aug 7). 2015 08:30:48 Copyright: American Scientific Publishers 2

(1.23 VRHE )

[44]

MOD/ flame vapor deposition

0.5 M KPi (pH 8)

3.1 mA/cm (1.23 VRHE )

[62]

Precursor spin coating/spray pyrolysis

0.5 M Na2 SO3

3.4 mA/cm2 (0.4 Vsce ) (BiVO4 : 0.03 mA/cm2 )

[69]

Spray pyrolysis/ED

1.0 M NaHCO3

2.18 mA/cm2 (1.0 VAgAgCl )

[63]

MOD/photoreduction/ hydrothermal

0.01 M Na2 SO4

∼197 mA/cm2 (1.0 VAg/AgCl ) Max. EQE =∼ 053%

[68]

Precipitation/ photoreduction

300 W Xe ( ≥ 420 nm) 0.1 M Na2 SO4

IPCE 3.5% at 420 nm (0.75 VAg/AgCl ) (bare : 0.1%)

[71]

2%W, 6%Mo:BiVO4 + RGO Drop casting/Hummer’s method (power form)

120 mW/cm2 0.2 M KPi + 0.1 M Na2 SO4 + 0.1 M Na2 SO3 (pH 7)

∼13 mA/cm2 (0.6 VNHE )

[101]

BiVO4 + MWCNT

Co-precipitation

300 W Xe ( ≥ 420 nm) 0.5 M Na2 SO4

0.14 mA/cm2 (bare 0.03 mA/cm2 )

[74]

Thermal treatment/ hydrothermal

500W Xe 1 M NaOH (PEC) 300 W Xe (200–400 nm reflection filter) (HER)

0.18 mA/cm2 (0.2 V Ag/AgCl) HER:2.2 mol/h (0.1 g, 1%wt Pt cocatalyst, 20%vol methanol)

[70]

MOD/MOD

0.1 M KPi (pH 8)


[102]

Precipitation/ photoreduction/SSR

300 W Xe ( ≥ 420 nm) (pH 3)

AQY = 1.05% at 420 nm (activity 3 fold higher than wo PRGO)

[53]

BiVO4 /Rh:SrTiO3 /Ru

Precipitation/SSR

300 W Xe (420 nm ≤ ≤ 800 nm) (pH 7)

AQY = 1.6% at 420 nm

[4]

RhO2 /3%Mo:BiVO4

MOD/wet impregnation

AM 1.5 G Sea water or 0.1 M K2 SO4

2.7 mA/cm2 (1.23 VRHE ) IPCE 49% at =∼ 420 nm (1.0 VRHE )

[97]

7%W:BiVO4 /WO3 (nano wire) BiVO4 /CdS BiVO4 /CuWO4 3%Mo:BiVO4 /RGO/ Fe2 O3 (nano array) BiVO4 + RGO

5% TiO2 /BiVO4

TiO2 /BiVO4 BiVO4 /PRGO/Rh:SrTiO3 /Ru


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MOD/spin coat


Kim and Lee

Table II. Continued. Preparation method (co-catalyst/ BiVO4 /others)

Conditions (Light source, Electrolyte)

Wet impregnation/EPD

MnOx /BiVO4

Performance

References

300 W Xe ( ≥ 420 nm) 0.1 M KPi

Co–Pi (0.11), CoOx (0.09), IrOx (0.07), RuOx (0.02), MnOx (0.01) (mA/cm2 , 0.5 VSCE ) (bare: 0.02 mA/cm2 )

[84]

PD (-110- facet selective)

300 W Xe ( ≥ 420 nm) 0.5 M Na2 SO4

0.05 mA/cm2 (0.55 VSCE ) (bare 0.02 mA/cm2 )

[99]

CoOx /BiVO4

ALD/MOD

AM 1.5G KOH (pH 13)

1.49 mA/cm2 (1.23 VRHE )

[33]

Co3 O4 /BiVO4

Co-precipitation/PB(film)

1000 W Xe (IPCE) 0.5 M Na2 SO4

IPCE 6.5% at 420 nm (1.6 VRHE )

[41]

Pt/Mo,W:BiVO4 (Bi:V:Mo:W = 4.6:4.6:0.6:0.2)

PD/simultaneous Evaporation and annealing

120 mW/cm2 0.2 M KPi + 0.1 M Na2 SO4 (pH 7)

IPCE 37% at 420 nm (1.1 VRHE )

[100]

PD/drop casting

Xe (100 mW/cm2 ). (≥420 nm) 0.2 M NaPi (pH 6.8)

IPCE ∼ 50% at 420 nm (1.23 VRHE ) (Pt, best result)

[98]

PED/MOD

0.5 M Na2 SO4 + KPi (pH 7)

1.2 mA/cm2 (1.23 VRHE )

[64]

ED or PD/MOD

AM 1.5G ( ≥400 nm) 0.1 M KPi

∼ 0.5 mA/cm2 (PD, 0.5 VSCE )

[85]

PED/MOD

0.5 M Na2 SO4 + KPi

1.0 mA/cm2 (1.0 VAg/AgCl )

[35]

(1.23 VRHE ) IPCE 33% at 330 nm (1.23 VRHE )

[8]

Material Co–Pi,CoOx ,IrOx , RuOx ,MnOx /BiVO4

Pt/W:BiVO4 (Bi:V:W = 4.5:5:0.5)

REVIEW

Co–Pi/SiO2 /BiVO4 Co–Pi/BiVO4 Co–Pi/2%Mo:BiVO4 Co–Pi/7%W:BiVO4

Delivered by Publishing Technology to: ? (pH 7) IP: 93.91.26.29 On: Fri, 14 Aug 2015 08:30:48 2 PED/MOD 0.1 Scientific M KPi (pH 8) Copyright: American Publishers 1.5 mA/cm

Co–Pi/BiVO4 /SnO2

ED/spray pyrolysis/spray pyrolysis

0.1 M KPi

1.8 mA/cm2 (1.23 VRHE )

[29]

Co–Pi/1%W:BiVO4 /SnO2


0.1 M KPi

2.3 mA/cm2 (1.23 VRHE )

[6]

Co–Pi/gradient 1%W:BiVO4 /SnO2


0.1 M KPi

3.6 mA/cm2 (1.23 VRHE ) STH =∼5% (tandem, 2-jn a-Si cell)

[15]

ED/ED

300 W Xe ( ≥ 420 nm) 0.2 M NaBi buffer (pH 9)

∼ 2.0 mA/cm2 (0.4 VSCE )

[95]

PED/MOD/drop cast

0.1 M KPi (pH 7)

2.4 mA/cm2 (1.23 VRHE )

[10]

PED/spray pyrolysis/ spray pyrolysis

0.5 M Na2 SO4 + KPi (pH 7)

2.5 mA/cm (1.23 VRHE )

[58]

PED/Hydrothermal

300 W Xe 0.1 M KPi

1.25 mA/cm2 (1.23 VRHE )

[94]

PD or ED/MOD

0.1 M KBi (pH 9.2)

∼ 1.1 mA/cm2 (PD, 1.23 VRHE ) ∼ 100% faradaic OER eff.

[20]

FeOOH/BiVO4

PED/thermal conversion

0.1 M KPi

1.17 mA/cm2 (0.55 VRHE )

[39]

FeOOH/BiVO4

PED/ED

0.1 M KPi (pH 7)

∼ 2.2 mA/cm2 (1.23 VRHE )

[96]

PED/Dip coating

1 M Na2 HPO4 buffer (pH 7)

2.77 mA/cm2 (1.23 VRHE )

[38]

PED/Electrodeposition

0.1 M KPi

∼ 3.3 mA/cm2 (1.23 VRHE )

[7]

Ion treatment/MOD

300 W Xe( ≥ 420 nm) 0.5 M NaSO4

IPCE 44% at 420 nm (1.6 VRHE )

[32]

Co–Bi/BiVO4 Co–Pi/Mo:BiVO4 /WO3 Co–Pi/BiVO4 /WO3 (1D) Co–Pi/BiVO4 (heteroepitaxial) Ni–Bi/BiVO4

FeOOH/1.8%Mo:BiVO4 FeOOH/3%Mo:BiVO4 +

Ag ion treated BiVO4

2

Notes: Light source = AM 1.5G (100 mW/cm2 ) unless otherwise specified. KPi = potassium phosphate buffer. NaPi = sodium phosphate buffer. KBi = potassium borate buffer. Potential was denoted if different from 1.23 VRHE ED = electrodeposition. PED = photoassisted electrodeposition. PD = photodeposition. MOD = metal-organic decomposition. PADD = polymer-assisted decomposition deposition. PB = paste blending. CVD = chemical vapor deposition. ALD = atomic layer deposition. SP = spray pyrolysis.

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not positive enough for the oxidation. Instead, its loading produce solar hydrogen. It has many desired properties as reduced the difference between oxidations of water and an efficient photoanode including a small band gap energy hole scavengers (methanol, KI, KSCN, KBr), an indication (2.4 eV), good hole conductivity and acceptable stabilof co-catalyst effect. ity under various photochemical reaction conditions. Yet, When CoOx was deposited by atomic layer depoBiVO4 also has critical shortcomings as well including sition (ALD), the effect was greater.33 Apparently, poor charge transfer property and sluggish water oxidation ALD gives more homogenous deposition than wet kinetics. To overcome such difficulties, various successimpregnation or particle loading. It was reported that RhO2 ful modifications have been reported. Among them, for(0.3 wt%)/Mo:BiVO4 can operate in sea water with a mation of heterostructures is an effective approach, which significant PEC performance and stability.97 Deteriorating includes formation of heterojunctions between semiconoxygen evolution was observed because of precipitation ductors, addition of charge transfer mediator, and loading of Mg2+ and Ca2+ salts on Pt cathode. The results show co-catalyst. the possibility of sea water splitting with BiVO4 photoanHeterojunction with another photocatalyst semiconducode modified with a co-catalyst, but the performance and tor induces cascade charge transfer at interface according stability have to be greatly improved. to the energy level of involved bands leading to electron Various co-catalysts were compared including Pt, Co– and hole movements between the two semiconductors in Pi, IrOx , CoOx deposited on CVD-prepared BiVO4 .98 In opposite directions and consequently improved charge sepPEC water oxidation, Pt deposited by photo-reduction aration efficiency. With BiVO4 , WO3 and SnO2 are best showed the highest photocurrent and IrOx was the lowest known hole mirrors which block holes from recombining (Pt > Co–Pi > CoOx > IrOx ). But in electrochemical water with electrons. Conductive carbons such as graphene and oxidation, IrOx was the best and Pt was the worst. This CNT are often employed as charge transport mediators indicates importance of co-catalyst/photocatalyst interface to enhance the charge transfer through grain boundaries for photogenerated hole transfer in PEC OER, whereas of poorly conducting semiconductors. Co-catalysts loaded potential induced charge transfer is important for electroon the semiconductors can improve both surface charge chemical OER. The co-catalysts were also compared in transfer and stability of semiconductors. The most popususpension photocatalysis and PEC water oxidation with larTechnology Co–Pi loaded Delivered by Publishing to:on ? BiVO4 showed greatly increased PEC RuOFri, co-catalyst/BiVO4 .84 Among Co–Pi, IP: CoO 93.91.26.29 14 Aug 2015 08:30:48 x , IrOx , On: x performance, stability and good oxygen evolution faradaic Publishers and MnOx , the first two catalysts Copyright: showed theAmerican highest Scientific efficiency. These heterostructuring techniques could be PEC water oxidation activity, but for photocatalytic OER, applied singly or with another heterostructuring techRuOx performed best. Thus, the hole transfer mechanism nique or other modification techniques not discussed is slightly different between two water oxidation systems. here. Pt is unique co-catalyst active for both hydrogen evolution Although these modification techniques and improved and oxygen evolution reactions.16 21 98 performance have elevated the status of BiVO4 to one of Usually these metal oxides co-catalysts are randomly the most promising oxide photoanodes today, there are still dispersed on BiVO4 surface. But if BiVO4 crystals are many remaining challenges for practical application. For synthesized exposing preferentially (110) and (010) facets, charge separation, near complete surface charge separation holes and electrons are separated and directed to these was accomplished by highly efficient and generic oxygen respective facets. When co-catalysts were loaded by phoevolution co-catalysts like Co–Pi. Yet, bulk charge sepatodeposition (PD), oxide co-catalysts MOx (M : Mn, Pb) ration efficiency still remains less than 50% and should were deposited selectively on (110) face by hole transfer be greatly improved. The solar light absorption of BiVO4 and metallic species M (Ag, Au, Pt) on (010) face by elecis limited by its band gap energy with theoretical solartron transfer.99 By this selective deposition of oxidizing to-hydrogen (STH) efficiency of 9.2%. The STH should and reducing co-catalysts at different faces, PD process be greater than 10% for practical application. Heteroshowed far better PEC performance than wet impregnajunction formation with a semiconductor of smaller band tion process. In addition, MnOx at (110)/BiVO4/Pt at (010) gap energy could expand the solar light utilization to showed the best photocatalytic water oxidation activity longer wavelength photons. Other modification techniques compared to randomly deposited MOx and M species by are also in order for better solar light harvesting and physical separation of HER and OER sites. management. Table II summarizes performance of BiVO4 -based heterostructures as photoanodes for PEC water oxidation Acknowledgments: This work was supported reported in the recent literature. by grants from the BK + Program and Basic Science Research Program (No. 2012-017247) and the 5. SUMMARY Korea Center for Artificial Photosynthesis (KCAP) funded by the National Research Foundation of Korea Monoclinic BiVO4 is a promising photoanode material for (No. 2012M1A2A2671779). photocatalytic and photoelectrochemical water splitting to


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References and Notes

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