Stability and Performance of Sulfide-, Nitride-, and Phosphide-Based

Perspective Cite This: J. Phys. Chem. Lett. 2017, 8, 5228-5238

pubs.acs.org/JPCL

Stability and Performance of Sulfide‑, Nitride‑, and Phosphide-Based Electrodes for Photocatalytic Solar Water Splitting Jinzhan Su, Yankuan Wei, and Lionel Vayssieres* International Research Center for Renewable Energy (IRCRE), State Key Laboratory of Multiphase Flow in Power Engineering, School of Energy & Power Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China ABSTRACT: With the past decade of worldwide sustained efforts on artificial photosynthesis for photocatalytic solar water splitting and clean hydrogen generation by dedicated researchers and engineers from different disciplines, substantial progress has been achieved in raising its overall efficiency along with finding new photocatalysts. Various materials, systems, devices, and better fundamental understandings of the interplay between interfacial chemistry, electronic structure, and photogenerated charge dynamics involved have been developed. Nevertheless, the overall photocatalytic performance is yet to achieve its maximum theoretical limit. Moreover, the stability of well-known semiconductors (as well as novel ones) remains the biggest challenge that scientists are facing to develop durable industrial-scale devices for large-scale water oxidation and overall solar water splitting. In this Perspective, we summarize the major achievements and the different approaches carried out to improve the stability and performance of photoelectrodes based on sulfide, nitride, and phosphide semiconductors.

T

ransitioning from a coal/hydrocarbon-based energy source to renewable and sustainable clean energy sources has become a top priority in our societies due to the severe effects of pollution endangering public health and the environment as well as to gain energetic independence. A hydrogen-based energy source seems ideal (i.e., zero carbon emission, only water!), and consumer hydrogen fuel cellpowered cars as well as public transportation are becoming available in our societies. However, most of the hydrogen produced nowadays still comes from nonrenewable sources, made by steam reforming of natural gas/hydrocarbons (i.e., syngas and water gas shift reaction), which produce CO and CO2; yet, the natural and cleanest way to sustainably produce hydrogen is by splitting water (e.g., photosynthesis). As a result, a great increase in research during the past decade, with dedicated studies on material design, surface and electronic structure engineering, heteronanostructuring, and fundamental theoretical work, has been conducted to search and identify ideal materials and systems for artificial photosynthesis.1−3 As one of the promising approaches to realize efficient artificial photosynthesis, photoelectrochemical water splitting is yet very challenging. Different from photovoltaic, the photoelectrochemical process requires the photogenerated charge carriers not only to be created, transported, and collected efficiently but also to perform (electro)chemical reactions at the semiconductor/electrolyte interface, as shown in Figure 1. The charge carrier collection and delivery to an electrical circuit of a solar cell is relatively efficient due to its sole electric process. However, for a photoelectrochemical cell, more steps and more complicated processes are involved, including thermodynamics and kinetics of solid/liquid/gas interfaces, reactant adsorption, product desorption, ion/molecule diffusion, solution flow/ © 2017 American Chemical Society

Figure 1. Operational mechanisms of a photoelectrochemical (left) and a photovoltaic (right) cell. CB represents the bottom of the conduction band, VB represents the top of valence band, Voc represents the open-circuit voltage, and Ef represents the Fermi level.

Received: March 30, 2017 Accepted: October 3, 2017 Published: October 3, 2017 5228

DOI: 10.1021/acs.jpclett.7b00772 J. Phys. Chem. Lett. 2017, 8, 5228−5238

The Journal of Physical Chemistry Letters

Perspective

convection and production separation/collection, etc.4 In addition, the photoelectrodes cannot be sealed (i.e., to prevent degradation) like photovoltaic cells can (e.g., dye-sensitized solar cell) as the photoelectrodes must be in direct and constant contact with the aqueous electrolyte to perform. From the material point of view, precise nanoscale manipulation to create (hetero)structures that have unique or enhanced properties was put forward as a promising approach to search for breakthrough in PEC performance of the already-known materials and systems. It is indeed typically assumed that the semiconductor−electrolyte junction will be a key feature of the cells. Therefore, in addition to specific materials properties, the surface electrochemical process is another influential factor that is undergoing intense research. In this process, charge carriers and ions exchange electrons and holes through the semiconductor/electrolyte interface. The very difference of phase and electronic environment makes the electrons going through the interface a much more difficult issue. Furthermore, the stability, which is a more profound issue that directly affects the use and implementation (or lack thereof) of photoelectrochemical systems in our society compared to photovoltaic systems, is determined by the semiconductor/electrolyte interface and has been recognized as the critical issue for several decades.5 Indeed, the most stable system rather than the most efficient one will most likely be the successful one.

whereas N3− and S2− tend to shift the valence band edge to more positive potentials.9 The band structure results in narrow bandgaps and more efficient light absorption. The photoexcitation from the VB to the conduction band in semiconductors such as sulfides, nitrides, and phosphides is therefore a bond-breaking transformation that promotes an electron from a bonding orbital to an antibonding one, as shown in Figure 2. This bond breaking behavior under illumination increases the susceptibility to surface corrosion.

Figure 2. Photoexcitation of a GaN semiconductor. Left: The VB is mainly composed of N 2p states, which determine the position of the VB, while the bottom of the conduction band is mainly determined by Ga 4s states.10,11 The Ga dangling-bond surface state SGa and the N dangling-bond surface-resonance state SN derived energy bands are highlighted in red and green. Light absorption will induce electron excitation from SN to SGa. Right: Electron and hole excitation in GaN resulted in a bond-breaking charge transfer from surface N to Ga.

Precise nanoscale manipulation to create (hetero)structures that have unique or enhanced properties was put forward as a promising approach to search for breakthrough in PEC performance of the already-known materials and systems.

From the chemical analysis point of view, corrosion is thermodynamically favorable compared to proton reduction or water oxidation for unstable materials in an aqueous environment. When looking into the semiconductor in terms of energy levels, two important parameters, which are thermodynamic reduction/oxidation potentials (or anodic/cathodic decomposition potentials, Edecomp,re/Edecomp,ox), determine the stability of the semiconductors in aqueous solutions. The Edecomp,re and E decomp,ox can be derived from the Pourbaix diagram experimentally,12 by which Gerischer has calculated the corrosion potentials of several binary semiconductors.13 To check whether a semiconductor is inherently stable or not, a simple comparison of Edecomp,re and Edecomp,ox with the conduction band minimum (CBM) and valence band maximum (VBM) can be used. As shown in Figure 3, if Edecomp,re is higher than the CBM and Edecomp,ox is lower than the VBM, the semiconductor is stable; otherwise, the semiconductor will suffer from corrosion by an electron in the CBM transfer to Edecomp,re or a hole in the VBM transfer to Edecomp,ox to drive self-reduction or self-oxidation. Because the photoelectrode is in direct contact with the aqueous electrolyte, the photogenerated electrons/holes in the CBM/VBM can oxidize/reduce water rather than itself if Edecomp,re/Edecomp,ox is higher/lower than the water reduction potential of H+/H2 and water oxidation potential of O2/H2O. With newly developed first-principle calculations and experimental electrochemistry data of Edecomp,re/Edecomp,ox of a series of metal oxides, II−VI and III−V related semiconductors can be estimated to facilitate future suitable candidates for photocatalytic applications.14 However, almost all nonoxide semiconductors are unstable when operating as photoanodes for oxygen evolution reactions.

From the 4 decades of research on photoelectrochemical water splitting, TiO2 is regarded as one of the best candidate, yet its wide bandgap limited its application to the UV range only. Doping was used to extend its light absorption. However, no significant improvement of performance was achieved by doping due to the electron−hole recombination role of the introduced impurity levels.6 Stable metal oxides with a narrow bandgap then became the hot spot of this research field. Hematite7 is a promising material with an optimal band gap, but the low charge mobility limits its performance for water oxidation. Even though significant efforts7 have been made by doping, nanostructural engineering, and surface co-catalyst loading, its current performance (4.32 mA/cm2) is yet far from its maximum theoretical limit (12.7 mA/cm2).8 Recently, some non-oxide-efficient semiconductors, typically metal sulfide, nitride, oxynitride, phosphide, and silicon, have been subjected to intensive research. These materials showed outstanding photocurrents but suffered from strong instability in aqueous solutions under illumination (i.e., photocorrosion). The origin of the stability difference for sulfides and nitrides is the anion in these compounds. Stability with respect to self-oxidation is correlated with the valence band-edge energy, which determined the bandgap to a certain extent. Because O is more electronegative than N and S, stronger covalent M−O bonds showed a valence band edge at more negative potentials, 5229



Perspective

an electric bias or connection to a sacrificial metal anode; however, electric energy is actively consumed and chemical reactions may be changed with this additional bias. More promising and practical approaches may rely on engineering the catalyst side. The currently developed approaches could be differentiated into three different approaches (Figure 4).

Figure 3. Diagram of energy level alignment for Edecomp,re and Edecomp,ox, the water reduction potential of H+/H2 and the water oxidation potential of O2/H2O, and the CBM and VBM for stable and unstable semiconductors.

Photoexcitation from the valence band to the conduction band in semiconductors such as sulfides, nitrides, and phosphides is a bond-breaking transformation that promotes an electron from a bonding orbital to an antibonding one. This bond breaking behavior under illumination increases the susceptibility to surface corrosion.

Figure 4. Corrosion of a semiconductor in photocatalytic conditions (GaN as an example) (a) and three approaches used for its protection: (b) protective layer; (c) co-catalyst; (d) surface engineering.

The first approach is the physical protection of the unstable surface. This method uses a dense layer of a stable material to protect the unstable material from direct contact with the electrolyte. The protective layer should allow as much light as possible to pass through and efficiently transfer the charge from the semiconductor to the surface catalytic sites. The earliest attempts were to stabilize efficient solar water splitting with a tandem photovoltaic device.18 New approaches with different materials are emerging nowadays to overcome this challenge. Metal thin films of Pt19 or Ni20 were deposited onto n-type Si photoanodes, which showed improved stability under oxidative conditions. Transparent conductive oxide (TCO) coatings such as fluorine-doped tin oxide (FTO) and tin-doped indium oxide (ITO) were also used but are not stable under alkaline or acidic conditions.21 P-type TCO such as NiCo2O422 was used as a protective layer, which produce selective hole conduction to the surface under illumination. A thin layer of amorphous Ta2O5 was also used because of its hole conductivity and thermal and chemical stability.23 The interfacial energetic alignment is important for the transport of charge carriers to the surface. For example, the VB of the protective layer must be energetically aligned with (or lie higher than that of) the light absorber so that VB hole transport can proceed without a barrier.24 If the VB of the protective layer is lower than that of the absorber, the holes can be transported by tunneling, but the thickness of the oxide layer needs to be carefully controlled within a few nm for an efficient tunneling transfer. The atomic layer deposition (ALD) technique provides a conformal protection layer without pin holes to physically separate the unstable surface from the electrolyte. This layer can be a highly conductive layer or an unconducive layer with thickness thin enough to ensure facile charge carrier exchange by tunneling. This technique has boosted the research on ultrathin oxide layers such as NiOx,25 TiO2,26 or MnOx27 as the protective layer, which are stable in acidic or alkaline

To realize efficient solar water splitting systems, the first approach of making electrochemically stable semiconductors more efficient using various strategies as described above could be an everlasting task. Before novel efficient semiconductors that are inherently stable under water splitting conditions are discovered, the simple approach of making efficient semiconductors more stable under water splitting conditions could be a more effective one. Early research efforts have been devoted to adjusting solution compositions and additives that stabilize the semiconductor/ electrolyte interface. The pH of the aqueous electrolytes is investigated because the proton and hydroxyl ions actively participate in the corrosion reactions and have significant influences on the stability.15 For an electrochemical cell, if the ion transport is not efficient enough, a significant potential gradient will be developed under high current densities.16 Extreme acid or alkaline solutions are capable of reducing the impending pH gradient in the electrolyte and to ensure ion transport with minimal resistance losses in overall solar water splitting devices.17 Changing the electrolyte composition usually alters the surface reaction path and may avoid corrosive reduction or oxidization reactions. For instance, sacrificial reagents are often introduced to evaluate the PEC activity of sulfides for water splitting. Indeed, when the photocatalytic reaction is carried out in an aqueous solution including a reducing reagent, in other words, electron donors or hole scavengers, such as alcohol and a sulfide ion, photogenerated holes irreversibly oxidize the reducing reagent instead of water itself.9 Another protection technique is cathodic protection, which adjusts the electrochemical potential of the interface by 5230



Perspective

Reviews focusing on the use of protective layers to stabilize unstable semiconductor photoelectrodes for water splitting have been reported for instance by Lewis34 and McIntyre.35 The second approach is using a co-catalyst to accelerate the desired reaction. This structure accelerates the water oxidation/ reduction reactions and in turn attenuates surface charge recombination and the unwanted self-oxidation and reduction of the semiconductor itself (corrosion). This co-catalyst can also be deposited as a full covering layer, which also served as a protective layer. The newly developed HER and OER cocatalysts have been reviewed elsewhere.2,36 As the co-catalysts were usually not deposited in a conformal fashion, it is difficult to entirely cover the porous semiconductor with effective cocatalyst, which leads to failure through photocorrosion.37 For the co-catalyst or electrocatalyst, amorphous ones (e.g., TiOx or MoSx) are usually better than crystalline ones as defect sites provide higher activity for HER or OER. The third approach is the surface (band structure or space charge region) engineering. In this approach, the surface is modified to form a built-in electric field layer to drive the unwanted charge carrier (the electron or hole depends on the material) away from its surface. It was proposed and demonstrated by Kibria et al. that one has to identify the electronic and atomic structure of the photocatalyst surfaces down to the atomic level to overcome the poor stability of a Ga(In)N-based semiconductor photocatalyst.38 Indeed, GaN surfaces can be easily oxidized (to Ga2O3) in air or in aqueous electrolytes. The efficiency and stability can be improved by tuning the atomic and electronic structure by (1) engineered optimum surface band bending with controlled p-type doping for efficient carrier extraction, (2) nitrogen-polar c-plane (000− 1) and associated reverse polarization for enhanced carrier separation, and (3) reconstructed nitrogen-rich m-plane (10− 10) surfaces as a passivation layer against photocorrosion. The surface band bending can be modified by tuning the surface Fermi level through controlled Mg-dopant incorporation.39 It was also found that not only the band-edge energy of a semiconductor but also the width of the depletion region and the band-edge energy profile affected the photocurrent density from the evaluation.40 For these efficient semiconductors, the corresponding protection approaches are different, which largely depends on their physical and chemical properties. In this Perspective, we are focusing on metal sulfides, nitrides, oxynitrides, and phosphides as silicon has been well reviewed in the literature41 and is thus not included. Metal Sulf ides. Metal sulfides are known as promising semiconductors for PEC applications for their suitable bandgaps and favorable band-edge positions. Almost all sulfide photocatalysts consist of metal cations with a d10 configuration, as shown in Table 1. Their conduction bands are composed of d and sp orbitals, while their VBs consist of S 3p orbitals, which

aqueous solution. Although not as conductive as TCOs, their ultrathin thickness makes the holes conducting via a tunneling mechanism.28 When the protective layer is too thick (2−5 nm for the InP/TiO2 junction as an example) for electrons to tunnel through the barrier that arises from the conduction band mismatch in the junction, amorphous TiO2 can be employed because electrons can transport through defect states.29 For ideal protection, the protective layer must be robust, with full coverage and without a pinhole, yet long-term stability of this photoanode was limited by the somewhat irregular coverage of the protective layer. A thicker layer that gives better protection usually reduces the charge transfer across itself or possesses poor optical transmissivity. Therefore, the trade-off between the protection and conductivity limits its efficiency. The possible breakthrough then falls into the improvement of the conducting and catalytic properties. It was demonstrated in TiO2/GaP30 that the protective layer not only prevented the electrode from photocorrosion but also increased the opencircuit voltage, which was attributed to an overall increase in the buried built-in potential and the asymmetric potential drop distribution across the junction.

Before novel efficient semiconductors that are inherently stable under water splitting conditions are discovered, the simple approach of making efficient semiconductors more stable under water splitting conditions could be a more effective one. Another interesting protective layer that acts as a holestorage layer was reported by Can Li et al.31 They used ferrihydrite to protect the unstable Ta3N5 photoanode against photocorrosion. It was found that photogenerated holes can be transported from the Ta3N5 to the ferrihydrite layer and stored in the form of positively charged states, waiting for discharge to drive water oxidation. A hole-storage bilayer, Ni(OH)x/MoO3, is able to efficiently extract and store holes from Ta3N5, resulting in more than 24 h of sustained water oxidation.32 Recently, they further improved the hole-storage layer system by using Ni(OH)x/ferrihydrite as the new hole-storage layer to mediate interfacial charge transfer from Ta3N5 to coupled molecular catalysts (Co cubane and Ir complexes) for water oxidation.33 With an additional TiOx blocking layer that spatially separated electrons and oxidizing equivalents in water oxidation, a record photocurrent of 12.1 mA/cm2 at 1.23 V vs RHE was obtained on a Ta3N5 photoanode, which is nearly its theoretical photocurrent limit (12.9 mA/cm2) under sunlight.

Table 1. Composition and Electronic Configuration of Metal Sulfides Reported with Promising Photocatalytic Activitya period\group

IB

II B

III A

IV A

VA

4 5 6

Cu 3d104s1 Ag 4d105s1

Zn 3d104s2 Cd 4d105s2

Ga 3d104s24p1 In 4d105s25p1

Ge 3d104s24p1 Sn 4d105s25p2 Pb 5d106s26p2

Bi 5d106s26p3

a The d10 configuration of the metals makes the conduction bands of the sulfides composed of d and sp orbitals, resulting in conduction band positions negative enough to reduce H2O to H2. However, their VBs consist of S 3p orbitals, which are much more negative than O 2p orbitals, thus maintaining low bandgaps for more efficient light absorption.42

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Perspective

Table 2. Summary of Water Splitting Performance and Stability of Most Common II−VI Semiconductors semiconductor CdSe/ZnOnanowire arrays CdSe/ZnO/TiO2

condition 0.2 M Na2S (pH = 13), 1 sun AM1.5G

CdS/CdSe

0.24 M Na2S + 0.35 M Na2SO3 1 sun AM1.5G 0.25 M Na2S + 0.35 M Na2SO3 1 sun AM1.5G 0.5 M Na2SO3 1 sun AM1.5G

CdSe/TiO2

0.5 M Na2SO3 1 sun AM1.5G

ZnO/ZnSe/CdSe/ CuxS WO3/Bi2S3

0.5 M Na2S + 0.5 M Na2SO3 1 sun AM1.5G 0.1 M Na2SO4 1 sun AM1.5G

Bi2S3/RGO-TiO2

0.25 M Na2S + 0.35 M Na2SO3 1 sun AM1.5G 0.25 M Na2S + 0.35 M Na2SO3 AM1.5G (λ > 400 nm) 0.1 M Na2SO4 (pH = 9) AM 1.5 G 1 M KOH 100 mW/cm2 Xe light

ZnO/CdS/CdSe

Bi2S3/BiVO4 Pt/In2S3/CuInS2 TiO2/ln2S3/AgInS2 (Cu2Sn)0.45Zn1.65S3/ CdS/Pt

0.1 M Na2SO4 (pH = 3.0)

performance 14.9 mA/cm2 (0.8 V RHE) 15.7 mA/cm2 (0 V Ag/ AgCl) 17.5 mA/cm2 (0.4 V RHE) 5.3 mA/cm2 (0.7 V Ag/ AgCl) 4.22 mA/cm2 (0.57 V RHE) 12.0 mA/cm (0.2 V Ag/ AgCl) 1.33 mA/cm2 (1.2 V Ag/ AgCl) 2.7 mA/cm2 (1.23 V RHE) 7.81 mA/cm2 (0.97 V RHE) 15 mA/cm2 (0 V RHE) 22.13 mA/cm2 (0.56 V Ag/AgCl) 1.00 mA/cm2 (0 V RHE)

efficiency

decay rate

ref

IPCE = 34.9% at 475 nm

14% in 1 h

50

IPCE = 60% at 305−750 nm (0 V Ag/AgCl) STH = 2.45% (0.2 V RHE)

0% in 0.11 h

51

15% in 1.66 h

52

ABPE = 1.6% (0.2 V Ag/AgCl)

6% in 1 h

53

ABPE = 2.8% (0.366 V RHE)

5.2% in 1 h

54

IPCE = 89.5% at 500 nm (0.2 V Ag/AgCl) IPCE = 60% at 375 nm (1.0 V Ag/ AgCl.) ABPE = 1.2% (0.47 V RHE)

15% in 0.5 h

55

69% in 1 h

56

34% in 1 h

57

ABPE = 3.9% (0.6 V RHE)

60% in 2 h

58

ABPE = 1.97% (0.28 V RHE) STH = 14.83% (0.56 V Ag/AgCl)

0% in 2.33 h 0% after 20 I−V scans 0% in 2 h

59 48

PCE = 27.4% at 450 nm (0 V RHE)

60,61

the VB of CdS to the corresponding coupled semiconductors by a type-II staggered band alignment. Reduced graphene oxide sheets were also used to wrap CdS particles as a chainmail to facilitate the charge separation and suppress the recombination.63 Heterojunctions between MoS 2 and CdS were also constructed to promote the interfacial charge transfer process and suppressed the charge recombination due to their well matching VB. The photocorrosion was inhibited by promoting the holes on the VB of CdS to transfer to that of In2S3, preventing the accumulation of holes on the VB of CdS.64 Surface modification by co-catalyst loading is also an efficient strategy to prevent the photocorrosion of the CdS photocatalyst via the rapid consumption of photogenerated holes. For example, IrOX·nH2O, a well-known oxygen-evolution catalyst, as a hole scavenger, was introduced to CdSe/CdS/TiO2 thin films to solve the low stability of the quantum dot sensitizers. The modified thin and well-spread IrOX·nH2O nanoparticles can effectively scavenge the holes, resulting in suppression of the hole-induced anodic corrosion of the CdSe/CdS quantum dots.52 Low-cost, nontoxic, and earth-abundant amorphous Ti(IV) and Ni(II) co-catalysts deposited on the CdS surface to enhance its photoinduced stability and photocatalytic activity were also reported.65 The photogenerated holes on the VB of CdS would first transfer to the Ti(IV) hole co-catalyst and then be rapidly consumed by the Na2SO3−Na2S scavenger, causing less oxidation of surface lattice S2− ions; as a result, the photoinduced stability of the CdS photocatalyst was greatly improved. The Ni(II) electron co-catalyst can also rapidly capture the photogenerated electrons and then function as the reduction active site to promote the interfacial H2− evolution reaction. A Au embedded CdS system was reported with improved efficiency and stability.66 Even though the electrons generated from plasmons in Au nanoparticles are blocked from transferring to CdS by the nonconducting organic stabilizer molecules between Au and CdS, the plasmon resonance energy field could still positively influence the excitation of CdS,

are much more negative than O 2p orbitals, thus resulting in a low bandgap yet with conduction band positions negative enough to reduce H2O to H2.42 Bare metal sulfides are reported with high efficiencies. Metal sulfide nanostructures directly coated on TCO reach photocurrents ranging from 5.18 to 8 mA/cm2.43,44 By forming heterojunctions of CdS/ZnS core/ shell nanorods, the photocurrent can be improved to 14.0 mA/ cm2 at 0.0 V vs SCE.45 However, coating sulfide on a conductive structure, such as ZnO/CdS/Au, can reach a high photocurrent of 21.53 mA/cm2 at 1.2 V vs Ag/AgCl, respectively.46 Ternary sulfides are reported with a good performance. For example, CuInS2 on hierarchical TiO2 shows a photocurrent up to 19.07 mA/cm2.47 In2S3/AgInS2 coated on branched TiO2 nanorods gives a photocurrent density up to 22.13 mA/cm2.48 However, metal sulfides usually show strong photocorrosion during photocatalytic reactions under visible-light irradiation. The main cause can be ascribed to the excess photogenerated holes accumulating on the VB surface and subsequently inducing rapid oxidation of the surface lattice S2− ions to form S0. Another oxidation product is soluble sulfates during and after photocatalytic reactions.49 The surface structure also showed a significant effect on the stability of spherical-shaped CdS nanoparticles for instance. According to an electromagnetic field simulation, the V-shaped defect changed the electric field distribution of the outmost shell and thus the charge distribution, resulting in electrons gathering at the corners and holes transferring into the V-shaped regions.49 Photodegradation thus accelerates in the V-shaped areas. For the application of metal sulfide in photocatalytic water splitting, photocorrosion is therefore one of the major obstacles for its practical use, especially with a high efficiency. To improve its stability (prevent its photocorrosion), several strategies were developed, as shown in Table 2. The most frequently reported strategy is to develop composite photocatalysts coupled with other materials such as CdS−C3N4.62 The principal mechanism for stability enhancement can be attributed to the rapid transfer of photogenerated holes from 5232



Perspective

Table 3. Summary of Performances and Stabilities for Sulfides, (Oxy)nitrides, and Phosphides passivation layer/ cocatalyst

semiconductor CdS

CdIn2S4

CdS

ZnS

CZTS

CdS/ALD-TiO2/Pt

Cu2BaSnS4

CdS/ZnO/TiO2

Cu(InGa)Se2

CdS/ZnO/Pt

Ta3N5

Co(OH)x

Ta3N5

Co3O4/ferrihydrite

Ta3N5

CoPi

Ta3N5

Co cubane/Ir complex/ Ni(OH)x/ferrihydrite/ TiOx

porous Ta3N5

efficiency

decay rate

ref

0.25 M Na2S/0.35 M Na2SO3 5.5 mA/cm2 at 1 V vs RHE solution 1 sun AM1.5G 7.8 mA/cm2 at 0 V vs 0.25 M Na2S/0.35 M Na2SO3 1 sun AM1.5G SCE 0.5 M Na2SO4/0.25 M Na2HPO4 13 mA cm−2 at −0.2 V vs RHE (pH = 6.85) 1 sun AM1.5G

conditions

performance

0.45% STH at 0.49 V vs RHE

20% in 10 h

89

IPCE = 35.2% at 300 nm −0.5 V vs SCE 49% IPCE at 0 V vs RHE at 500 nm

0% in 0.28 h

90 69

7.8 mA/cm2 at −0.1 V vs RHE

60% IPCE at 0 V vs RHE at 580 nm

25% in 1h and 70% in 5h 0% in 0.42 h

32.5 mA/cm2 at −0.7 V vs Ag/AgCl, 6.3 mA/cm2 at 1.23 V vs RHE 5.2 mA/cm2 at 1.23 V vs RHE 6.1 mA/cm2 at 1.23 V vs RHE 12.1 mA/cm2 at 1.23 V vs RHE

45% IPCE at−0.48 V vs Ag/ AgCl at 400 nm 65% IPCE at 1.23 V vs RHE at 400 nm 96% FE

6% in 6 h

31

60% SE at 1.3 V vs RHE

20% in 0.5 h

37

90−100% IPCE 400−550 nm range

0% in 1.25 h

33

0% in 1 h

93

420 nm) 0.45 wt % 30.2 μmol h−1 H2 PtOx/WO3 and NaI 1 mM

resulting in an enhanced formation rate and lifetime of e−/h+ pairs in CdS. ALD has also been proposed as an effective way to suppress the corrosion of sulfides. One cycle of ALD TiO2 or Al2O3 was proposed to passivate CdS powder to balance the carrier transportation and corrosion suppression.49 Moreover, with the passivation induced by one cycle of ALD, the catalyst lifetime was prolonged by up to 14 times compared to that of the asprepared CdS. An effective p-type hole mediator CuxS was also used in ZnO/ZnSe/CdSe/CuxS core−shell nanowire arrays, which displayed a high photocurrent of 12.0 mA/cm2 under

0.25% IPCE at 350−450 nm at −1 V vs Ag/AgCl 14% ECE

88 39

97 0% in 4 h

29

6.4% PCCE 15.8% PCCE at 0.65 V vs RHE 0.14% ABPE at 1.0 V vs RHE

50% in 1 h 0% in 6 h

87 98

20% IPCE at 400−460 nm at 1.23 V vs RHE 0.7% half-cell STH

75% in 1 h

100

20% in 6 h

101

0.1% AQE at 420 nm

0% in 15 h

102

99

AM1.5G illumination.55 The CuxS not only passivates the defects and surface states of the semiconductors but also forms a p−n junction with CdSe to promote the hole transport and better kinetics at the semiconductor/electrolyte interface. The photocorrosion of ZnS was reduced by N doping, which raised the VBM and thus reduced the oxidative capacity of holes (h+) in the VB.67 This N treatment makes the ZnS show outstanding stable capability for photocatalytic hydrogen evolution from water under simulated sunlight irradiation for 12 h. Binary, ternary, and even quaternary chalcogenides are promising materials that possess a high absorption coefficient, 5233



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low toxicity, and suitable bandgaps.68 Cu2ZnSnS4 (CZTS) and CuBaSnS4 (CBTS) are two emerging competitive earthabundant candidates for photovoltaic and PEC applications, and both possess a high optical absorption coefficient due to their VB containing Cu d10 states (Table 3). ALD was used to coat the TiO2 layer on CZTS and the obtained conformal thin layer show much better stability compared to the spin-coated TiO2 layer due to a more complete coverage.69 CdS/ZnO/ TiO2 overlayers were also coated on the CBTS surface to improve the photocurrent and device stability. With a bandgap of 2.05 eV and high absorption coefficient (>104 cm−1), the protected CBTS showed a saturated cathodic photocurrent of 7.8 mA/cm2 at −0.10 V versus RHE in a neutral electrolyte solution.70,71 (Oxy)nitrides and Phosphides. (Oxy)nitrides used as photocatalysts containing a d0 transition metal, such as Ti4+, Nb5+, and Ta5+, have reasonable absorption in the visible light region.72 Nitrogen is less electronegative and more polarizable than oxygen and thus shows a higher bonding covalency with metal. The introduction of N makes the VB of oxynitrides consist of a hybridization of N 2p and O 2p orbitals, where the N 2p contribution is larger than that of O 2p. The higher VB edge than that of O 2p results in a lower bandgap than the oxides.73 These materials are therefore recognized as promising candidates for high-performance photoanodes. Bare Ta3N5 nanorods obtained by nitridation of Ta2O5 nanorod arrays grown in situ on Ta substrates reached a photocurrent of 3.8 mA/cm2 at 1.23 V vs RHE under 1 sun AM1.5G simulated sunlight.74 An indium gallium nitride (InxGa1−xN) alloy semiconductor with a tuned bandgap showed a wide range of visible light absorption. With a small diameter and nearly defectless structure, ultrathin InGaN nanowires without a co-catalyst showed a photocurrent density of ∼32 mA/cm2 at 1.0 V against a Pt counter electrode under AM1.5G 1 sun illumination.75 However, the long-term stability of the nitrides is a challenging issue that might limit their use. Tantalum (oxy)nitrides show attractive bandgaps ranging from 2.5 to 1.9 eV, which correspond to theoretical STH efficiencies varying from 9.3 to 20.9%.76 However, the N 2p orbitals in the VB makes this material prone to oxidative decomposition to molecular nitrogen (2N3− + 6h+ → N2) and Ta2O5.77 Furthermore, the formed Ta2O5 layer hinders hole transport to the electrolyte, thus reducing its performance The instability of n- or p-GaN photoelectrodes originated from its surfaces; these can be easily oxidized (to Ga2O3) in air and in aqueous electrolytes, together with the presence of extensive dislocations and defects.78−80 Different behaviors were observed for InxGa1−xN at different pH values. At a pH of 7.4,79 for example, the photocurrent quenches completely after only ∼5 min due to Ga−O being the most predominant bond on the surface after surface oxidation, while at a pH of 11.3, the photocurrent increases with time and reaches a steady state after ∼100 min, which was ascribed to the formation of In−O bonds during the partial dissolution of the InGaN layer. Surface states cannot only act as recombination centers but also lead to Fermi level pinning, which in turn increases overpotentials and decreases photovoltages.81 For example, surface band pinning of the exposed GaN is detrimental to the efficient transfer of electrons to H+ ions in photoelectrolysis. This problem can be overcome by Rh loading, which forms a Schottky contact with GaN.82

However, in other reports, no surface Fermi level pinning was observed in the surfaces of III-nitride nanowires. Due to strong ionic bonds,83 the surface states of III-nitride semiconductors were bunched and largely positioned near the band edges, where they do not serve as nonradiative recombination centers. Therefore, some surface states of GaN may not contribute to surface oxidization and thereby lead to long-term stability against corrosion.84 Surface states of GaN nanowires can also be passivated with thiol compounds, resulting in an improvement of the IPCE from 8.1 to 18.3% along with an enhanced photocurrent density of −31 mA/cm2 at −0.2 V versus RHE. As the surface states were removed, the stability was also improved with the photocurrent retaining 80% of its initial value over 55 h at pH = 0.85 The surface passivation with a protection layer or co-catalyst is the most commonly used approach, and some of the most commonly reported photoelectrodes are listed in Table 3. When loaded with a co-catalyst, improved water splitting performance was attributed to the fast transport of photogenerated holes in the VB from GaN to NiO and efficient water reduction at the NiO/electrolyte interface.78 Metal phosphides typically possess high carrier mobility and low surface recombination velocity and thus are promising candidates for photoelectrochemical applications. Indium phosphide (InP) has a relatively narrow bandgap (1.35−1.42 eV) that can take advantage of a large portion of the incident solar spectrum and has been shown to exhibit exceptionally high photocurrent of 18 mA/cm2 in a PEC cell.86 Fast electron transfer and less surface charging current can be achieved by decorating InP nanowires with MoS3, resulting in higher stability.87 With a co-catalyst as a protective layer, the performance of a GaInP2 electrode exhibited no loss in photocurrent, onset potential, fill factor, and saturated photocurrent density after 60 h of operation but still suffered slow corrosion of the MoS2 layer, which led to exposure of the GaInP2 layer. This small-scale failure was considered to be the result of pinholes in the MoS2 layer.88 Recently, perovskite oxynitrides,103 which consist of AB(O,N)3 structure (A = Ca, Sr, Ba, and La, B = Ti, Ta, and Nb) and bandgaps that can be tuned by employing different combinations of elements on the A and B sites have drawn some interest. For example LaTiO2N and BaTaO2N have absorption edges up to 600 nm and exhibit theoretical solar-tohydrogen (STH) conversion efficiencies of over 15%.104 CoOx and C3N4 were used to extract holes and electrons from the BaTaON to avoid photocorrosion.105 Oxynitrides such as Ta3N5, TaON, and LaTiO2N still suffer from high defect densities. The most stable nitride reported could possibly be the graphitic carbon nitride (g-C3N4) as it can maintain a high rate of hydrogen and oxygen production with robust stability in 200 runs of recycling use over 200 days.106 As the most stable structural pattern of an allotrope of carbon nitride, it is stable at high temperature (600 °C in air) and in acid, base, and organic solvents. As a stable earth-abundant visible light photocatalyst, g-C3N4 has been reviewed elsewhere.107,108 Another type of oxynitride with a unique composition and structure that is a solid solution of GaN and ZnO with a wurtzite-type structure was also reported. This material achieves overall water splitting into H2 and O2 under visible light irradiation and received wide investigation.109 Many promising sulfide-, nitride-, and phosphide-based systems have emerged as very good candidates for efficient 5234



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photocatalytic solar water splitting systems along with new approaches to ensure their stability. With continued efforts on this truly critical stability issue, more and more promising systems will be identified. Among the sulfide-, nitride- and phosphide-based semiconductors, all of these materials could achieve photocurrents up to 30 mA/cm2 under 1 sun illumination. However, what hinders them moving forward to industrial application is the stability and fabrication cost. Among them, the sulfides could have the cheapest fabrication approach, for example, an aqueous solution-based one, but they may be most prone to photocorrosion in aqueous conditions. Nitrides are probably the most robust ones, but their fabrication approaches for high-quality electrodes usually required expensive fabrication equipment. Providing the durability issue of efficient, low-cost, earth-abundant, and nontoxic materials is fully understood and solved; artificial photosynthesis for solar water splitting will “see the light of day”. As stated by Landsmann et al.,23 a general strategy can be applied: identification and analysis of loss mechanisms in photoelectrodes and selection and deposition of specifically designed materials to address these problems. To assist these efforts on our knowledge-driven design of photocatalysts, new approaches, characterization techniques, and benchmark measurements are needed to gain better insights into the origin of the photocorrosion. For instance, the transient photocurrents were observed in many unstable materials that arise from photogenerated carriers that accumulate at the semiconductor/electrolyte interface in which slow reaction kinetics or surface state traps were involved.110 Moreover, operando/in situ measurements at synchrotron radiation facilities have emerged as very powerful tools to investigate the loss mechanisms and interface behavior during the actual photoelectrochemical reactions. For instance, downward band bending at the TiO2/electrolyte interface was probed by X-ray photoelectron spectroscopy.111 It was confirmed by a binding energy decrease of 250 meV when probing deeper into the particle. Analysis of the electrolyte solutions used in the stability test by inductively coupled plasma (ICP) spectroscopy can be used to test the material detachment from the electrode surface, which was caused by mechanical erosion due to oxygen evolution and photocorrosion. In conclusion, long-term stability, rather than highest efficiency, is defining the future of photocatalytic solar water splitting systems, and if one believes the old Chinese saying that “Whoever started the trouble should end it”, the critical stability problem must be solved by chemistry at the atomic scale!38

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ACKNOWLEDGMENTS

Financial support from the National Natural Science Foundation of China (No. 51323011, 51236007) is gratefully acknowledged.

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ABBREVIATIONS ABPE, applied bias photon-to-current efficiency; AQE, apparent quantum efficiency; IPCE, incident photon-toelectron conversion efficiency;; IQE, internal quantum efficiency; FE, Faradaic efficiency; SE, separation efficiency; STH, solar-to-hydrogen; PCCE, photocatahode conversion efficiency; ECE, energy conversion efficiency; np, nanoparticle; RHE, reversible hydrogen electrode; SCE, standard calomel electrode

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REFERENCES

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Corresponding Author

*E-mail: [email protected]. ORCID

Lionel Vayssieres: 0000-0001-5085-5806 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. The authors contributed equally. Notes

The authors declare no competing financial interest. 5235



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