K.
Pramoda
*a and
C. N. R.
Rao
*b
aCentre for Nano and Material Sciences, Jain (Deemed-to-be University), Jain Global Campus, Kanakapura, Bangalore, Karnataka 562112, India
bNew Chemistry Unit, School of Advanced Materials and International Centre for Material Science, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur P. O., Bengaluru, 560064, India. E-mail: cnrrao@jncasr.ac.in
First published on 10th July 2023
The utilization of hydrogen (H2) as a renewable substitute for fossil fuel can mitigate issues related to energy shortage and associated global warming. The generation of H2via water splitting by photo and electrocatalytic means is of significant importance. The employment of a semiconductor catalyst reduces the high energy barrier (237 kJ mol−1) associated with a water-splitting reaction to yield H2 and O2. Sustainable H2 production demands the utilization of noble-metal-free, efficient, and stable catalysts over a wide pH range. In recent times, layered transition metal thio(seleno) phosphates (MPX3, X = S, Se) are reported to be highly efficient for H2 evolution reaction owing to their earth-abundance, rich active sites, wide spanned band gap of 1.2 to 3.5 eV, and high chemical stability. In this perspective, advances in 2D monometallic and bimetallic MPX3 compounds are reviewed comprehensively from the viewpoint of water splitting, especially the hydrogen evolution reaction (HER). This study includes the composition, structural engineering, heterostructure, and hierarchically structural design in enhancing the HER activity of MPX3. Computational results providing insights into the intrinsic photo and electrocatalytic HER activity of 2D MPX3 are presented. Finally, the challenges and opportunities in further improving MPX3 activity towards HER and associated catalysis reactions are discussed.
Akin to TMDCs, 2D transition metal phosphides (TMPs) are reported to show bifunctional properties as HER and oxygen evolution reaction (OER) catalysts and also display long-term stability in a wide pH range.41–44 Theoretically, it is predicted that the phosphorous atom of TMPs acts as an active site for intermediate hydrogen adsorption and desorption during water splitting. Jia et al.45 compared the Gibbs free energy change (ΔGH*) of hydrogen adsorbed, which is a main intermediate state in HER, on to the different active sites MoP, MoS2, and Mo2C catalysts by density functional theory (DFT) calculations (Fig. 1a and b). According to the Sabatier principle, too strong or too weak adsorption is not favorable for the water reduction reaction. The ΔGH* values for Mo-terminated MoP, MoS2, and Mo2C were calculated as 0.16, 0.19, and 0.24 eV, respectively. These results indicate that the P atom in MoP plays the equivalent role as S of MoS2 and C in Mo2C. Further, a subtle smaller ΔGH* value for MoP as compared to MoS2 and Mo2C, indicates that MoP possesses higher catalytic activity. In another study, Miguel et al.46 using DFT calculations demonstrated that the ternary pyrite-type cobalt phosphosulfide can be an efficient catalyst for both photocatalytic and electrocatalytic HER since the cobalt octahedra contain more-electron donating P2− sites compared to S2−, owing to thermoneutral H+ adsorption at the active sites. The cubic crystal structure of CoPS can be envisaged as Co3+ octahedra surrounded by dumbbells with a uniform arrangement of P2− and S2− ligands (Fig. 1c and d). Further, DFT calculations revealed that ΔGH* on the (100) surface of CoPS at the Co site is more favorable compared to the CoS2 constituent. As shown in Fig. 1f–i, after spontaneous adsorption of H+ at open P sites, adjacent Co3+ atoms are reduced to Co2+, which brings ΔGH* value for cobalt near the thermoneutral value. Subsequent H+ adsorption at Co2+ sites induces the oxidation of Co2+ to Co3+ as shown in Fig. 1f–i. Such synergistic interaction is not realized in the case of CoS2 since Co3+ sites are energetically more favorable for H+ adsorption than open S2− ligands. In addition, Miguel et al.46 showed that CoPS nanostructures display electrocatalytic HER activity comparable to the Pt/C catalyst with a current density of 10 mA cm−2 at an overpotential as low as 48 mV and also shows exceptional stability. The Pt/C-like HER activity of CoPS inspired other researchers to study MPX3 systems containing Mn, Fe, Ni, Cd, etc. metals. But, some of the 2D family MPX3 systems such as MnPS3, FePS3, and NiPS3 showed marginal electrocatalytic H2 evolution activity due to low intrinsic electronic conductivity.47 Coupling of MPX3 with conducting graphene47 and doping of Co into MnPS3 and NiPS3 structures was reported to improve HER activity.48,49 Doping modifies the valence band and conduction edges and improves the electron transport rate. Further, MoS2 in combination with MPX3 facilitates hydrogen spillover during HER from MoS2 edge sites to TMPCs.50,51
Fig. 1 (a) Schematic depiction of the layered structure of 2D molybdenum phosphide (MoP), molybdenum disulfide (MoS2), and molybdenum carbide (Mo2C). (b) Gibbs free energy diagram of the H2 evolution for MoP, MoS2, and Mo2C at different sites. Reprinted with permission.45 Copyright 2017, American Chemical Society. (c and d) Crystal structures of CoS2 and CoPS systems. (e) Free energy diagrams of the H2 evolution for CoS2 and CoPS at different sites. Structural demonstration of hydrogen adsorption (H*) at the Co site on (100) plane of CoS2 (f), at the Co site (g), P site (h), and at the P site after H* at the Co site (i) on (100) plane of CoPS. Reprinted with permission. Copyright 2015, Springer nature.46 |
As discussed in prior sections, water splitting is thermodynamically energy-intensive as it demands a relatively large free energy of 237 kJ mol−1 to produce one mole of H2 and O2 each.15,19 To overcome this large energy barrier, researchers are utilizing semiconductor photocatalysts for water splitting, which can mimic the natural photosynthesis process involving the following two half-reactions.52,53
Water reduction: 2H++2e− → H2, ΔEo = −0.41 V |
Water oxidation: 2H2O → O2+4H+ + 4e−, ΔEo = +0.82 V |
The high Gibbs free energy barrier for water redox reactions can be reduced by employing semiconductors as photocatalysts and, therefore, the H2 evolution rate can be improved. The standard reduction potentials for H+/H2 reduction and oxidation potential for H2O/O2 are estimated to be −0.41 and +0.82 V (vs. NHE), respectively. To achieve overall water splitting by utilizing a semiconductor, the conduction band minimum (CBM) energy of the semiconductor should be higher than the H+/H2 reduction potential and its valence band maximum (VBM) energy must be lower than the H2O/O2 oxidation potential.54–56 The water splitting utilizing semiconductor photocatalyst comprises three essential steps: (1) absorption of solar energy by the semiconductor; (2) generation and mobilization of electron–hole pair from bulk to the active sites of the semiconductor; (3) water reduction (H+/H2) and oxidation (H2O/O2) reactions by photogenerated electron and hole pair at the conduction band (CB) and valence band (VB) of the semiconductor, respectively (Fig. 2). As shown in Fig. 2a and b, water splitting using semiconductors could be a one-step or two-step approach based on whether a single component is utilized or a combination of two or more semiconductor systems. The one-step route involves the photoexcitation of the semiconductor and water reduction and oxidation reaction by the electron and holes at the CB and VB of the semiconductor. While the two-step method includes concomitant photoexcitation of two semiconductors, hydrogen (H+/H2) and oxygen (H2O/O2) evolution occurs individually at different semiconductors similar to the usual Z-scheme photosynthesis.
In 1972, Fujishima et al.57 primarily described the use of TiO2 electrodes as photocatalysts for water-splitting reactions, since then, numerous semiconductors such as ZnO, Sb2O3, WO3, and TiO2 have been utilized as photocalaysts.58,59 However, the utilization of semiconductor photocatalyst requires the use of noble-metal cocatalysts in combination to realize higher water splitting efficiency.60,61 Hence, researchers are relentlessly exploring non-noble metal catalysts for water splitting to make the process cost-effective. In this regard, 2D materials such as metal nitrides, carbides, and phosphides have been explored as proficient catalysts for water reduction reactions. As discussed earlier, photon absorption, electron–hole pair separation and water redox reactions at the catalyst active sites are three crucial stages in photocatalytic water splitting. 2D material band gap can be modified by varying the number of stacked sheets, which fulfills the first condition. Secondly, due to the atomic thickness of 2D material, photogenerated charge-carriers have to cover lesser distances to reach the surface energetic sites. Further, the high specific surface area of 2D materials provides abundant active sites and higher accessibility for H+ adsorption. Lately, Ida et al.62 demonstrated how solar flux density and charge carrier lifetime in 0D nanocrystals and 2D crystal affects the water-splitting process (Fig. 3). A nanocrystal requires four excited electrons to ensure complete water splitting. But, photogenerated charge carriers possess a very short average lifetime of 1 μs before recombination. Therefore, to harvest four electrons, the nanocrystal should absorb four photons within a shorter period of 1 μs time. The solar flux density in the Earth's atmosphere is estimated to be 2000 μmol s−1 m−2. Thus, 4 ms is essential for photons to get adsorbed by the 0D nanocrystal and it is tough to afford such high solar flux to achieve water splitting using 0D nanocrystal. 2D materials, on the other hand, could minimize recombination rate by providing shorter pathways for charge-carriers to reach the active sites, thereby low solar energy flux is paramount in the case of 2D materials to achieve water splitting.
Fig. 3 Schematic illustration of the photocatalytic water redox reaction under solar light illumination for (a) 0D crystal and (b) 2D system. Replicated with the agreement.62 Copyright 2014, American Chemical Society. |
2D transition metal thio(seleno) phosphates (MPX3, X = S, Se) nanosheets containing Mn, Fe, Co, Ni, Cu, etc. metals have been investigated for photocatalytic HER due to their tunable composition, electronic structure, and wide band gap ranging from 1.3 to 3.5 eV.63–65 The electronic structure and properties of the MPX3 systems are investigated by optical absorption, photoluminescence, and DFT simulations. Zhang et al.66 calculated the band structure of MPS3 (M = Fe, Mn, Ni, Cd, Zn) and MPSe3 (M = Fe, Mn) systems using the Heyd–Scuseria–Ernzerhof (HSE06) functional, which is presented in Fig. 4a–f. This study specifies that the band gap of single-layer MPX3 varies from 1.90 to 3.44 eV, which surpasses the thermodynamic potential of 1.23 eV essential for water splitting. In addition, Yang et al.67 calculated the dynamic stability of MnPX3 (X = S or Se) nanosheets with the finite displacement method, and the corresponding phonon dispersion curve is shown in Fig. 4h and g. Interestingly, no imaginary phonon modes are observed in the phonon spectra, confirming the dynamic stability and they can be isolated as free-standing nanosheets. As displayed in the right panel of Fig. 4h and g, phonon bands below 8 THz are assigned to Mn–S/Se interactions, while the high frequency region above 8 THz corresponds to P2X6 moiety vibrations. Secondly, the radius of Se is greater than that of S, hence, the bond length of Mn–Se is larger than that of Mn–S and phonon bands of MnSe3 are shifted towards low frequencies as compared to the MnPS3 system.
Fig. 4 Band structure near the Fermi level for (a) FePS3, (b) MnPS3, (c) NiPS3, (d) CdPS3, (e) ZnPS3, and (f) FePSe3 single-layers. Reprinted with permission.66 Copyright 2018, Wiley publication. Phonon dispersion and the corresponding density of states for (g) MnPS3 and (h) MnPSe3 nanosheets. Reprinted with permission.67 Copyright 2020, The Royal Society of Chemistry. |
Apart from the band gap, the valence band and conduction band edges must straddle with H+/H2 reduction and H2O/O2 oxidation potentials. Liu et al.68,69 using HSE06 functional and partial density of states, calculated the orientation of CBM and VBM edges relative to water redox potential (Fig. 5a). Further, Zhang et al.66 by taking into consideration of stability of monolayered MPX3 systems and variation in their redox potentials with changes in pH, calculated the band edges based on HSE06 functional studies (Fig. 5b). Among the studied materials, FePSe3 displayed the narrowest band gap, which was further confirmed by optical absorption spectra as well. However, at pH = 7, FePSe3 displayed a CBM edge lower than water reduction potential and hence it is not suitable for H+/H2 reduction. Therefore, a particular MPS3 system can be chosen for either HER or OER based on the alignment of VBM and CBM edges relative to water redox potential. In addition, MnPS3, FePS3, and NiPS3 displayed strong optical absorption in the visible range, which indicates their high capability to harvest visible light (Fig. 5c). These simulations have been experimentally verified by Du et al.70 as well.
Fig. 5 (a) Band edge potential of mono-layered MPX3 compounds. The energy scale is represented with respect to the normal hydrogen electrode (right y-axis) and vacuum level (left y-axis) in eV for reference. The redox potential of water is incorporated for assessment. Reproduced with permission.68 Copyright 2001, Macmillan Publishers Ltd. (b) Location of VBM and CBM edge potentials of monolayer MPS3 and MPSe3 calculated using HSE06 functional. The redox potential of water at pH = 0 and pH = 7 is incorporated for comparison. (c) The optical absorption coefficient for MPS3 compounds. The region between the red and purple lines indicates the visible range. Reprinted with permission.66 Copyright 2018, Wiley publication. (d) Schematic shows that the exfoliated ZnPS3 shows higher photocatalytic HER activity as compared to the bulk due to more exposed edge sites.112 |
Strong visible light absorption characteristics and high carrier mobility (625.9 cm2 V−1 s −1 for MnPSe3) specify the MPX3 system's potential capability for H2 generation under solar light.65,66,70 In addition, members of the MPX3 family possess the aforementioned properties along with abundant phosphorous active sites for HER in mono or few-layer form; thus, it is realistic to assume that they can be good candidates for many catalysis reactions as non-precious metal catalysts. 2D CdPS3 showed high HER performance of 10880 μmol h−1 g−1 while MnPS3 and FePS3 displayed 21.2 and 141.9 μmol h−1 g−1, respectively, under solar light.65,71,72 One drawback of 2D TMPCs is their weak oxidizing ability of the photogenerated holes because of the misaligned valence band maximum (VBM) edge for H2O/O2 oxidation potential and hence shows low stability because of photocorrosion.64,73 Combination of 2D MPX3 with other Cs4W11O35, CdS, TiO2, and g-C3N4 nanostructures is reported to be beneficial for water splitting as latter systems possess strong hole oxidizing ability.
In this perspective, we provide an overview of various 2D TMPCs utilized for photocatalytic and electrocatalytic HER applications. The chemical vapor transport (CVT) method for bulk MPX3 preparation is mentioned briefly and micromechanical exfoliation and liquid-phase exfoliation approaches to prepare 2D MPX3 from bulk crystal are discussed. The DFT approach demonstrating the significance of proper alignment of VBM and conduction band minimum (CBM) edges of 2D MPX3 concerning water redox potential towards HER is discussed at length. To improve the catalytic activity of MPX3, phase engineering, band structure engineering, and electronic state modulation by doping strategies have been employed (Fig. 6). Generation of the heterojunction between 2D MPX3 with other Cs4W11O35, CdS, TiO2, and g-C3N4 nanostructures is reported to improve hole oxidizing ability. Further, photogenerated electrons are readily available for water reduction due to an in-built electric field gradient and therefore heterostructures are likely to display a higher HER rate and good stability. Besides, ferroelectric CuInP2S6 displays good HER characteristics and its activity can be further enhanced by coupling with other photocatalytically HER-active materials such as g-C3N4 and ZnIn2S4. The permanent polarization electric field and large exciton binding energy characteristics of ferroelectric CuInP2S6 induce spatial separation of the photogenerated carriers and reduction potential of electrons, respectively, thereby showing good H2 evolution. Notably, the CuInP2S6/g-C3N4 heterostructure shows extraordinary stability under harsh photochemical conditions, suggesting its practical applicability.
Fig. 6 Schematic highlights of 2D MPX3 compounds and their nanocomposites for photocatalytic and electrocatalytic HER applications. |
Fig. 8 (a) Optical microscopy, (b) AFM, and (c) conductive AFM image of few-layer MnPS3 obtained by the scotch-tape exfoliation method. Adapted with permission.89 Copyright 2016, AIP Publishing. (d) Schematic representation of the CVT method adapted for the preparation of MPX3 crystals and (e) photographs of the bulk NiPS3, FePS3, MnPS3, and FePSe3 prepared by the CVT method. TEM images of the exfoliated (f) Ag0.5In0.5PS3 and (g) Ag0.5In0.5PSe3 nanosheets.93 |
Rao and coworkers93 extended the chemical vapor transport (CVT) method to prepare monometallic (MnPS3, FePS3, NiPS3, ZnPS3, CdPS3, FePSe3, and CdPSe3) and bimetallic Ag0.5In0.5PS3 and Ag0.5In0.5PSe3) MPX3 systems (Fig. 8d and e). The synthesis involved heating the stoichiometric ratios of the constituent elements sealed in a quartz ampoule to ∼700 °C under an inert atmosphere. The pure bulk MPX3 compounds were formed at the other end of the quartz ampoule. The presence of sulfur and phosphor elements enhanced the vapor transport rate during material growth and well-developed crystals are obtained. The obtained bulk crystals were exfoliated in a water and ethanol mixture under sonication conditions. Fig. 8f and g shows the TEM images of the exfoliated Ag0.5In0.5PS3 and Ag0.5In0.5PSe3 systems, which revealed a few-layer nature. In literature, the majority of MPX3 at large-scale are prepared using the CVT method and this strategy is even applicable for doping other transition (Ni, Fe or Co) elements into MnPS3 (ref. 94) as well as for the synthesis of ferroelectric CuInP2S6 (ref. 95) and CuBiP2S6 materials.
Fig. 9 (a) Structure and the corresponding free energy diagram of hydrogen evolution at equilibrium for NiPS3, Ni0.9Fe0.1PS3, and FePS3. Reprinted with permission.96 Copyright 2019, American Chemical Society. (b) The Volcano curve of exchange current density for various transition metal anchored CuPS3. Reprinted with permission.97 Copyright 2022, Springer. (c and d) Cyclic voltammograms recorded in the ferri/ferrocyanide redox probe and the Nyquist plots of various 2D TMPCs. Reprinted with permission.98 Copyright 2017, American Chemical Society. |
During electrolytic water splitting, the catalyst material is coated on the substrate (glassy carbon and indium tin oxide) and utilized as the anode. As depicted in Fig. 10a, the water-splitting electrolyzer consists of three main constituents, the anode, cathode, and electrolyte. Irrespective of the electrolytic media (acidic or basic), the water splitting reaction involves a change in free energy of 237 kJ mol−1 at 1 atm and 25 °C, corresponding to a theoretical cell voltage of 1.23 V to initiate water splitting.99 However, experimentally a voltage higher than 1.23 V is required due to mass, electrolyte, and transport resistances and also because of the sluggish kinetics of HER and OER reactions.100 The overpotential can be reduced by coating the catalyst at the anode and cathode surfaces, which decreases the energy barrier. Water splitting via HER involves the reduction of a H+ ion or H2O, based on the pH of the electrolytic medium.100,101
2H+ + 2e− → H2 (acidic) |
2H2O + 2e− → 2OH− + H2 (basic) |
Fig. 10 Schematic of the (a) water-splitting electrolyzer and (b) possible reaction routes for electrocatalytic HER in the acidic medium. Reproduced with permission.122 Copyright 2019, Elsevier. (c and d) Linear sweep voltammogram (LSV) curves obtained in 0.5 M H2SO4 and (e) Tafel plots for various bulk MPX3. Reproduced with permission.123 Copyright 2017, American Chemical Society. (f) Free energy diagram of the H2 evolution for NiPS3 and LSV curves of bulk NiPS3, exfoliated NiPS3, and NiPS3/RGO composites. Reproduced with permission.106 Copyright 2016, Wiley publication. |
Under acidic conditions, HER follows three possible reaction pathways, namely Volmer, Tafel, or Heyrovsky pathways. The Volmer step involves the reaction of electrons with H3O+ to yield catalytically adsorbed H+ ion (Had) (2H+ + e− → Hads). Subsequently, H2 evolution occurs either by following the Tafel (2Hads → H2) or Heyrovsky (Hads + H++e− → H2) pathway or both (Fig. 10b).41–43 Irrespective of the paths by which H2 evolution happens, Had is always associated with the water reduction reaction. The Tafel plots could help in obtaining the reaction kinetics and mechanistic pathways for any catalyst, which shows water splitting via HER. The slope corresponds to the log(current density) versus overpotential plot, in the cathodic potential (η) range, which gives the Tafel slope value. The Tafel slope value signifies the amount of additional potential essential to increase a current density by 10 mA cm−2. For an ideal catalyst Pt, the Tafel slope values of 30, 40, and 120 mV dec−1 indicate that the Volmer, Heyrovsky, and Tafel steps are the rate-determining steps.102,103 The charge carrier mobility and the number of active sites also play an important role in describing the intrinsic HER activity for MPX3. For instance, MnPSe3 shows high charge carrier mobility (electron and hole mobilities of 625.9 and 34.7 cm2 V−1 S−1, respectively), and the massive divergence in the carrier mobility values suggests that efficient charge-separation for H2 and O2 evolution reactions.66 Further, exfoliated layers show high water redox activity compared to the bulk due to increasing in-plane conductivity and more exposed edge sites.
2D MPX3-based electrocatalysts display H2 evolution activity starting from very low to high, some of them even showing activity comparable to benchmark Pt/C catalysts. Experimentally, the electrocatalytic HER performance is accessed by the onset potential with respect to a standard hydrogen electrode, Tafel slope, and charge-transfer resistance concerning the benchmark Pt/C catalyst. The lower the value of these factors, the superior the catalytic activity. We have compared these parameters of 2D MPX3-based catalysts in Table 1, which suggest that the MPX3 containing Ni, Co, and Fe atoms, bimetallic TMPCs and their composite with other 2D systems display superior activity. Generally, it has been described that bimetallic phosphochalcogenides show better activity than monometallic systems due to synergistic interactions. Some of the widely examined 2D MPX3 and their heterostructures are discussed in the following section.
Catalysts | Electrode substrate | Overpotential at 10 mA cm−2vs. RHE (mV) | Tafel slope (mV dec−1) | Electrolyte | Reference |
---|---|---|---|---|---|
a GCE – glassy carbon electrode. | |||||
Exfoliated MPX3 nanosheets | |||||
MnPS3 | GCEa | 835 ± 68 | — | 0.5 M H2SO4 | 124 |
MnPS3 | GCE | 632 | 272 | 0.5 M H2SO4 | 94 |
MnPS3 | GCE | 1090 ± 71 | — | 0.1 M KOH | 124 |
MnPSe3 | GCE | 640 ± 87 | — | 0.5 M H2SO4 | 124 |
MnPSe3 | GCE | 992 ± 56 | — | 0.1 M KOH | 124 |
FePS3 | GCE | 211 ± 3 | 42 | 0.5 M H2SO4 | 47 |
FePS3 | GCE | 337 ± 4 | — | 1M KOH | 47 |
FePS3 | GCE | 673 ± 4 | 3.5 wt% NaCl | 47 | |
FePS3 | GCE | 530 | 56 | 0.5 M H2SO4 | 98 |
CoPS3 | Graphite | 222 ± 2 | 71 ± 5 | 0.5 M H2SO4 | 125 |
CoPS3 | GCE | 590 | 84 | 0.5 M H2SO4 | 98 |
NiPS3 | GCE | 297 | 69 | 0.5 M H2SO4 | 106 |
NiPS3 | GCE | 398 | 159 | 1M KOH | 106 |
NiPS3 | GCE | 816 | 54 | 3.5 wt% NaCl | 106 |
NiPS3 | GCE | 205 | 74 | 0.5 M H2SO4 | 126 |
MPX3 nanosheet composite with reduced graphene oxide (RGO) | |||||
FePS3/RGO | GCE | 108 ± 2 | 54 | 0.5 M H2SO4 | 47 |
FePS3/RGO | GCE | 467 ± 3 | — | 1M KOH | 47 |
FePS3/RGO | GCE | 192 ± 2 | — | 3.5 wt% NaCl | 47 |
NiPS3/RGO | GCE | 178 | 55 | 0.5 M H2SO4 | 106 |
NiPS3/RGO | GCE | 281 | 48 | 1M KOH | 106 |
NiPS3/RGO | GCE | 543 | 94 | 3.5 wt% NaCl | 106 |
Transition element doped MPX3 nanosheets | |||||
Ni0.95Fe0.05PS3 | GCE | 130 | 114 | 1M KOH | 96 |
Ni0.9Fe0.1PS3 | GCE | 72 | 73 | 1M KOH | 96 |
Ni0.85Fe0.15PS3 | GCE | 152 | 187 | 1M KOH | 96 |
Ni0.95Mn0.05PS3 | GCE | — | 135 | 1M KOH | 94 |
Ni0.90Mn0.10PS3 | GCE | — | 142 | 1M KOH | 94 |
Ni0.85Mn0.15PS3 | GCE | — | 143 | 1M KOH | 94 |
Ni0.97Co0.03PS3 | GCE | 112 | 103 | 1M KOH | 49 |
Ni0.95Co0.05PS3 | GCE | 71 | 77 | 1M KOH | 49 |
Ni0.93Co0.07PS3 | GCE | 105 | 110 | 1M KOH | 49 |
Ni0.91Co0.09PS3 | GCE | 145 | 113 | 1M KOH | 49 |
Co0.6(VMnNiZn)0.4PS3 | GCE | 66 | 65 | 1M KOH | 107 |
MnFePS3 | Carbon paper | 102 | 49 | 1M KOH | 127 |
NiCoFePS3 | Ni foam | 231 | 86 | 1M KOH | 128 |
Single-atom anchored MPX3 nanosheets | |||||
NiFePS3 | GCE | 356 | 170 | 1M KOH | 109 |
CoFePS3 | GCE | 490 | 132 | 1M KOH | 109 |
PdFePS3 | GCE | 471 | 128 | 1M KOH | 109 |
2D MPX3–based heterostructures | |||||
FePS3/MoS2 | Ni foam | 175 | 127 | 0.5 M H2SO4 | 50 |
FePS3/MoS2 | Ni foam | 168 | 107 | 1 M KOH | 50 |
NiPS3/MoS2 | GCE | 112 | 64 | 1 M KOH | 51 |
NiPS3/Ni2P | GCE | 85 | 82 | 1 M KOH | 110 |
NiPS3/Ni foam | Ni foam | 74 | 86 | 1 M KOH | 129 |
CoNiPS3/N-doped carbon | GCE | 140 | 60 | 1 M KOH | 130 |
In recent work, Martinez et al.98 studied the electrocatalytic HER performance of a series of bulk MPX3 (M = Mn, Fe, Co, Ni, Zn, Cd, Ga, and Sn) in acid electrolytes. Fig. 10c and d show the HER polarization curves for these MPS3 compounds and the resulting onset potential values at 10 mA cm−2. Among these MPX3 compounds, only NiPS3 and CoPS3 showed lower overpotential, −590 and −530 mV (vs. RHE), respectively, than bare GCE (−890 mV vs. RHE), indicating that these systems can be beneficial for electrocatalysis.69,98 The high H2 evolution activity of NiPS3 and COPS3 is ascribed to preferential orientation of these systems in the (001) crystal plane, as these planes are reported to be highly HER active in the case of Ni2P catalyst.33,104 Further, the MPS3 system containing Ni and Co metals is estimated to display ΔGH* value near to thermoneutral potential, thereby display superior HER activity.49,92,105 In another study, Sampath and coworkers106 have shown that HER activity of bulk NiPS3 can be further improved by exfoliating bulk crystals into few-layers by liquid phase exfoliation. These few-layer NiPS3 showed stable H2 evolution over a long period in a wide pH range of 1 to 14, including in 3.5 wt% NaCl solution, which is close to seawater composition. Fig. 10f shows the iR-corrected LSV polarization curves for bulk and exfoliated NiPS3, which show the onset potential of −450 and −159 mV, respectively, in 0.5 M H2SO4, indicating a higher activity in the case of the latter. Further, bulk NiPS3 exhibited a Tafel slope value of −119 mV dec−1, while a few-layer sample revealed a value of −69 mV dec−1. Further, exfoliated NiPS3 showed an overpotential of 297, 398 and −816 mV in acidic (pH = 1), basic (pH = 14) and in neutral 3.5 wt% NaCl electrolyte, respectively, at 10 mA cm−2. These studies indicate that the kinetics of HER is more facile in the case of exfoliated samples due to an enhanced charge-transfer rate. The authors also showed by DFT calculation that the P atom is the most favorable site for hydrogen adsorption by evaluating the ΔGH* value (Fig. 10f).
Sampath and coworkers47 reported HER activity of the exfoliated FePS3 system as well. The reported overpotentials at 10 mA cm−2 for exfoliated FePS3 in pH = 1, 14, and in neutral 3.5 wt% NaCl were −211, −337, and −637 mV respectively. According to Sampath and coworkers, exfoliated NiPS3 showed higher overpotential at 10 mA cm−2 and hence lower activity than FePS3. On the contrary, Martinez et al.98 reported that bulk NiPS3 showed lower overpotential at 10 mA m−2 as compared to FePS3. Therefore, the physicochemical properties of these materials need to be investigated in detail to provide further insights into the activity and mechanism of H2 evolution. In addition, coupling of exfoliated NiPS3/FePS3 with conductive reduced graphene oxide (RGO) substrate further improved HER activity as reported by Sampath and coworkers.47,106 The onset potential values obtained for exfoliated NiPS3 and FePS3 were −159 mV and −95 mV, while NiPS3/RGO and FePS3/RGO show −62 and −50 mV, respectively. The charge-transfer resistance (Rct) value obtained from AC impedance measurements indicates that the Rct value for NiPS3 and FePS3 was lowered on coupling with graphene. This implied that the conductive RGO support improved the HER activity of the MPX3 system due to enhanced charge-transfer characteristics. Also, NiPS3/RGO and FePS3/RGO nanocomposites showed long-term stability, suggesting these materials can be highly efficient electrocatalysts for HER in a wide pH range.
As indicated earlier, 2D MPS3 showed marginal electrocatalytic performance due to poor electronic conductivity; thus conductive graphene is coupled to improve the HER performance. Even though, NiPS3/RGO and FePS3/RGO composite required overpotentials of −178 and −192 mV, respectively, to reach 10 mA cm−2 in a basic electrolyte. In general, the catalyst electrocatalytic activity can be improved either by increasing the active site through catalyst loading or exposing the edge site via nanostructuring, or enhancing the intrinsic activity of the individual site. With this in mind, Li et al.49 doped Co into NiPS3 and with Ni0.95Co0.05PS3 nanosheets achieved a current density of 10 mA cm−2 at an overpotential of as low as −48 mV (vs. RHE) with a Tafe slope of 77 mV dec−1 in 1.0 M KOH electrolyte (Fig. 11a). The synthetic procedure involved initially obtaining the bulk NiCoPS3 by heating the stoichiometric ratio of individual elements in a sealed quartz ampoule. Later, the bulk sample was ultrasonicated in N,N-dimethyl formamide solvent to obtain 2D nanosheets. The authors also prepared Ni1−xCoxPS3 containing various cobalt amounts (x = 0.03, 0.05, and 0.07) to know the effect of Co doping content on HER activity. The Rct values of 18.6, 12.1, 23.4, and 41.7 Ω were obtained for Ni1−xCoxPS3 with cobalt doping amounts of x = 0.03, 0.05, 0.07, respectively, while bare NiPS3 showed 82.3 Ω (Fig. 11b). The reduced Rct value in the case of Ni1−xCoxPS3 implied that Co doping improved intrinsic HER kinetics of NiPS3 through a higher charge-transfer rate. However, an increase or decrease of Co amount other than x = 0.05, more or fewer declines HER performance. Later, theoretical studies have shown that H affinity at the P site can be improved by Co doping along with electrical conductivity.48 In another study, Rakov et al.94 showed that Ni-doped (lower oxophilicity) MnPS3 displayed higher HER activity than the bare sample. With Ni doping, the ability of binding of the surface intermediate into the catalyst surface decreased, which in turn assisted the desorption steps such as the Tafel or Heyrovsky steps. In addition, doping of multi-metal atoms such as V, Mn, Ni, and Zn into CoPS3 increased the density of active sites due to the formation of high entropy alloys.107 The generated S edge sites and basal plane P sites improved H+ ion adsorption, whereas Mn ion boosted water dissociation during the Heyrovsky step. Recently, Megha et al.108 using DFT calculations showed that Sc, Y, and Mo elemental doping into the MnPSe3 lattice reduced the ΔGH value of pristine MnPSe3 to 1.41 eV, to 0.24, 0.18, and 0.23 eV, respectively. Further, the density of state (DOS) plots suggested that DOS at the Fermi energy was infinite for 25, 12.5, and 6.25% Sc elemental doping concentrations in MnPSe3, indicating activation of the inert basal plane of MnPSe3 towards HER (Fig. 11c and d). Apart from metal atom doping, non-metal carbon doping can transform semiconducting FePS3 into metallic form and show enhanced HER activity. The dopant with fewer electrons than the substitutional atom can improve electronic conductivity and also greatly activate surface sites for hydrogen adsorption. Further, C, N-codoping in FePS3 pushed ΔGH (−0.188 eV) value closer to zero and showed HER activity comparable to that of Pt/C with a small overpotential of 53.2 mV to achieve a current density of 10 mA cm−2.105 However, studies on non-metal doping in 2D MPX3 are very limited and these systems need to be investigated in detail both by computational and experimental studies.
Fig. 11 (a) LSV polarization curves and (b) Nyquist plots for Ni1−xCoxPS3 containing various cobalt amounts (x = 0.03, 0.05, 0.07).49 (c and d) Density of states plots for 25 and 6.25% Sc element doped MnPSe3. Reproduced with permission.108 Copyright 2023, Elsevier. (e) Schematic illustration of anchoring single-atom catalysts (Pt, Co, and Ni) over 2D FePS3. (f) Gibbs free energy profile of FePS3 and Ni–FePS3. (g and h) LSV polarization curves for HER and the corresponding Tafel plots, (i) polarization curves for OER for FePS3, Ni–FePS3, Co–FePS3, and Pd–FePS3 catalysts. Reproduced with permission.109 Copyright 2022, Wiley publication. |
Recently, Tang et al.109 showed that anchoring of single-atom (SA) catalysts such as Ni, Co, and Pd over 2D FePS3 can tune the surface electronic structure and reinforces water adsorption and dissociation capacity for both OER and HER. DFT calculations showed that Ni SA anchored FePS3 facilitated electron aggregation from Fe to Ni–S and enhanced electron-transfer rate at Ni and S sites (Fig. 11e and f). In addition, the electron-rich Ni site acted as an active site to diminish hybridization between O and Ni in Ni–FePS3, thereby, facilitating conversion from O* to OOH* group and reducing the barrier for OER. Ni–FePS3 delivered an outstanding performance as a bifunctional catalyst for water splitting with an overpotential of 356 and 287 mV at a current density of 10 mA cm−2 for HER and OER, respectively (Fig. 11g–i). This study indicates that engineering surface active sites at the atomic scale are an effective way to modulate the catalytic activity of 2D MPX3 systems.
MPX3 nanostructures with a large number of active phosphorus edges are considered to be efficient water-splitting catalysts by both theoretical and experimental studies. The generation of heterojunctions by coupling two or more different materials can incorporate the advantages of distinct materials into one hybrid. Huang et al.50 reported the 2D/2D FePS3/MoS2 heterojunction, in which interfacial coupling between FePS3 and MoS2 causes significant enhancement in H2 evolution performance. FePS3/MoS2 requires low overpotentials of only 175 and 168 mV to reach 10 mA cm−2 current density under acidic and basic conditions, respectively (Fig. 12a and b). Further, FePS3/MoS2 showed lower Tafel slope values of 107 and 125 mV dec−1 at pH = 1 and pH = 14, respectively, with little decay in potential even after 1000 cycles (Fig. 12c). The in situ grown MoS2 can provide more exposed active sites, which facilitate diffusion of active species and release of the formed H2 bubble. Secondly, the intercalated MoS2 layers can stop restacking of FePS3 layers, enhancing conductivity and exposing active sites of FePS3. Since the electron transfer resistance is higher in adjacent layers than along the MPX3 layer, reduction in the stacking of the number of FePS3 layers improves electronic conductivity and carrier flow to MoS2.50 In another study, Liu et al.51 synthesized NiPS3/MoS2 composite and demonstrated that internal polarization field (IPF) facilities hydrogen spillover (HSo) during HER from the MoS2 edge site to NiPS3. Secondly, IPF boosts the hydroxyl ion diffusion pathway, thereby enhancing the rate of oxygen evolution reaction (Fig. 12d). The NiPS3/MoS2 composite requires a low overpotential of 112 and 296 mV to reach 10 mA cm−2 current density for H2 and O2 evolution reactions, respectively. Further, the NiPS3/MoS2 composite delivered overall water splitting with a cell voltage of 1.64 V at 10 mA cm−2 and exhibited long-term stability up to 100 h (Fig. 12e). The chronopotentiometry study conducted for 50 h indicates that NiPS3/MoS2 does not lose any catalytic activity, implying the practical advantages of the catalyst in terms of stability (Fig. 12f). Besides, Liang et al.110 grew nickel phosphide nanostructures on NiPS3, which showed a lower bias voltage of 1.65 V for overall water splitting, even surpassing that of the commercial Pt/C//IrO2 electrocatalyst (Fig. 12g–i). Generation of the MPX3 heterostructure with MoS2 would be beneficial as a bifunctional catalyst since these systems decrease the energy barrier for hydrogen adsorption and also improve sluggish OER kinetics due to built-in electric field gradient.115
Fig. 12 (a) Schematic of the in situ CVT method adapted for the growth of FePS3/MoS2 heterostructure. (b) Polarization curves for FePS3/MoS2 heterostructure. (c) Electrochemical durability of FePS3/MoS2 in 0.5 M H2SO4. Reproduced with permission.50 Copyright 2020, Elsevier. (d and e) Polarization curve of NiPS3/MoS2 as both anode and cathode catalyst for overall water splitting driven by a solar cell (∼1.65 V). (f) Chronopotentiometry study for NiPS3/MoS2 carried for 50 h. Reproduced with permission.51 Copyright 2012, Wiley publication. (g) Gibbs free energy of hydrogen adsorption estimated for NiPS3/Ni2P composite, NiPS3 (110), Ni2P (110) and Ni2P (110) planes. (h) Charge density distribution at the junction of NiPS3/Ni2P composite. (i) Polarization curve of the NiPS3/Ni2P heterocatalyst for overall water splitting compared with the benchmark Pt/C//IrO2 electrocatalyst. Reproduced with permission.110 Copyright 2019, American Chemical Society. |
Fig. 13 AFM images of MnPS3 (a), MnPSe3 (b), Tauc plot (c), HER data (d), Mott–Schottky plots (e), and schematic illustration of alignment of VBM and CBM edge potentials with respect to water reduction potential derived from Mott–Schottky plots (f) for MnPS3 and MnPSe3. Reprinted with permission.65 Copyright 2018, Wiley publication. Two-dimensional NiPS3 HER data under a xenon lamp and in solar simulated light (g), ultraviolet photoelectron spectroscopy data (h), and schematic illustration of alignment of VBM and CBM edge potentials of NiPS3 with respect to water reduction potential derived from Mott–Schottky plots (i). Reproduced with permission. Copyright 2017, Elsevier.113 |
Layered metal thiophosphates containing Fe, Ni, Sn, and Zn have also been reported as efficient catalysts for photocatalytic hydrogen evolution reactions. For instance, Sendeku et al.111 synthesized Sn2P2S6 nanosheets with Pc monoclinic phase by chemical vapor conversion process employing SnS2, P, and S precursors (Fig. 14a and b) and further utilized for H2 generation from pure water under solar light irradiation. To reveal the light-harvesting ability of Sn2P2S6, the UV-vis diffuse reflectance spectrum was recorded, as shown in Fig. 14c. The absorption edge of Sn2P2S6 was up to 520 nm with a weak absorption in the 520–600 nm region. Meanwhile, the corresponding Tauc plot shown in the inset of Fig. 14c shows the estimated band gap (Eg) value of 2.25 eV. Further, Mott–Schottky plots gave a positive slope revealing the n-type feature along with a flat band potential of −0.48 V (Fig. 14d). Secondly, ultraviolet photoelectron spectroscopy (UPS) studies were also carried out to obtain the band alignment of Sn2P2S6 nanosheets. From the UPS spectrum, the VB edge of Sn2P2S6 was calculated to be at −5.68 versus the vacuum level, as shown in Fig. 14e. The CB level was calculated from the equation ECB = EVB − Eg, which was estimated to be at −3.43 eV versus the vacuum level. On the basis of the previous results, a schematic band diagram of Sn2P2S6 is described in Fig. 14e, which implied that the VB and CB edges were suitably oriented to employ a water redox reaction. Fig. 14f shows the time course hydrogen evolution curve of Sn2P2S6 under simulated solar light (AM 1.5G) without any sacrificial agent. The H2 evolution rate increases linearly over time with a rate of 202.06 μmol h−1 g−1, which is ∼10 times higher than previously discussed MnPS3 nanosheets. In another study, Zhao et al.112 prepared exfoliated ZnPS3 by a liquid-phase exfoliation from its bulk crystal and realized a high HER activity of 640 μmol h−1 g−1 under a xenon lamp using Na2S and Na2SO3 as sacrificial agents (Fig. 14g–i). While bulk ZnPS3 showed relatively low activity of 640 μmol h−1 g−1 under similar conditions. The enhanced activity in the case of exfoliated ZnPS3 is attributed to a high specific area and more exposed edge sites.
Fig. 14 (a and b) Schematic of the synthesis of Sn2P2S6 nanosheets using a tubular furnace. UV-vis diffuse reflectance spectrum (c), Mott–Schottky plots (d), energy band diagram (e), and HER data (f) for Sn2P2S6 nanosheets. Reprinted with permission.111 Copyright 2021, The American Chemical Society. UV-vis extinction spectra (g), UPS spectra (h), and time course HER curves (i) for bulk and exfoliated ZnPS3 catalyst. Reprinted with permission.112 Copyright 2022, Elsevier. |
In 2017, Wang et al.113 prepared NiPS3 nanosheets by a modified CVD method and studied their HER activity under xenon light without any cocatalyst or sacrificial agent. As shown in Fig. 7g, NiPS3 showed the HER activity of 26.42 μmol h−1 g−1 in neutral or pure water until the light irradiation was turned off. Further, the addition of the hole scavenger Na2S/Na2SO3 to the photosystem enhanced the H2 yield to 74.67 μmol h−1 g−1. While the NiPS3 catalyst under simulated solar light showed a lower HER activity of 6.46 μmol h−1 g−1 in the sacrificial agent-free water. The band structure of NiPS3 was studied by using ultraviolet photoelectron spectroscopy and electrochemical impedance measurements indicated that the VB of NiPS3 was not straddling with water oxidation potential and hence was not energetically favorable to offer holes for oxygen evolution (Fig. 7h and i). Therefore, some of the single-component MPX3 systems utilized for photochemical HER without the cocatalyst or sacrificial agent did not evolve oxygen due to misalignment of the VB relative to the water oxidation potential. Thus, the MPX3 catalyst also possesses the common disadvantage of the single-catalyst photosystem, wherein photoactivity decreases with time due to photocorrosion. The photogenerated electrons at CB are consumed by H+ to generate H2 while holes accumulate at the VB due to misalignment. These highly reactive holes further react with the semiconductor itself, which results in decomposition and reduced activity of the catalyst. Even with the presence of a sacrificial agent, the activity of ZnPS3 and NiPS3 nanosheets was reduced by more than half of the initial value only after 24 h of the hydrogen evolution test, which suggests deprivation in structural stability under photochemical conditions. Therefore, the activity and stability obtained with a single-component MPX3 system are still far from reality, and photocorrosion of these materials under photocatalytic conditions impedes their applicability as photocatalysts.
In an effort to increase the H2 evolution yield, Barua et al.93 examined the photocatalytic HER activity of a range of MPX3 systems (MnPS3, FePS3, NiPS3, ZnPS3, CdPS3, FePSe3, CdPSe3, Ag0.5In0.5PS3, and Ag0.5In0.5PSe3) using eosin Y as a photosensitizer and triethanolamine (TEOA) as a sacrificial agent. This study indicated that the combination of photosensitizer and sacrificial agent somewhat assisted in extending the structural stability and long-term H2 evolution activity. Experimentally, band gaps of all synthesized MPX3 systems were obtained using absorption spectra and the corresponding Taucs plots revealed band gaps varied in the range of 1.29–3.3 eV, suggesting that the band gap and band edges of MPX3 were thermodynamically suitable for the water reduction reaction. Fig. 15a shows the schematic of the probable mechanism of HER by MPX3 compounds in the presence of Eosin Y and TEOA. Initially, the dye molecule absorbs light to yield a photoexcited dye (EY*), which then transforms to a triplet excited state (EY3*) via an intersystem crossing. The intermediate EY3* then takes an electron from TEOA via reductive quenching forming EY− species. The electrons from EY− are then transported to the CBM of the MPX3 where H+/H2 reduction takes place.97 The HER activity of monometallic TMPCs varies in the order NiPS3 > FePS3 > CdPS3 ∼ MnPS3 ∼ ZnPS3 where NiPS3 displays the highest HER rate of 2600 μmol h−1 g−1 (Fig. 15b and c). Fig. 15d presents the variation in the H2 evolution rate of the monometallic MPX3 with a change in P–P bond distance. The enhancement in the P–P bond distance leads to a reduction in the population of electrons at the P center, which is the active site, and therefore overall H2 evolution rate decreases. Under similar conditions, bimetallic Ag0.5In0.5PS3 and Ag0.5In0.5PSe3 showed appreciable hydrogen yields of 1900 and 500 μmol h−1 g−1, respectively (Fig. 15c). The catalytic stability is the additional significant factor for an HER catalyst. Under photochemical conditions, the NiPS3 system showed steady H2 evolution up to 5 cycles (25 h) without any decrement in H2 evolution, suggesting long-term stability of the photocatalyst (Fig. 15e).
Fig. 15 (a) Schematic illustration of the mechanism of HER by exfoliated TMPCs nanosheets under a xenon lamp using eosin Y photosensitizer and TEOA sacrificial agent. (b) The yield of hydrogen evolved using monometallic (MnPS3, FePS3, NiPS3, ZnPS3, CdPS3) phosphosulfides. (c) Comparison of HER activity of monometallic (MnPS3, FePS3, NiPS3, ZnPS3, CdPS3 and bimetallic (Ag0.5In0.5PS3, Ag0.5In0.5PSe3) phosphosulfides. (d) Variation of H2 evolution rate of monometallic phosphosulfides with p–p bond distance. (e) Cyclic stability curves for NiPS3 studied up to 5 cycles for 25 h.93 |
2D MPX3 compounds are coupled with other semiconductors possessing good oxidation capability of holes to enhance H2 evolution yield and avoid photocorrosion. Theoretically, Mi and coworkers114 investigated the electronic structure of the MnPSe3 heterostructure with MoS2 using DFT calculations, where different stacking patterns of mono-layer MnPSe3 with MoS2 were investigated, as shown in Fig. 16a and b. In some stacking patterns, spin splitting at VBM of MnPSe3 is evident due to the hybridization of the d orbital of Mn, which enhances electron mobility. Secondly, MnPSe3/MoS2 forms a type II heterostructure where the top of the VB is majorly contributed by MnPSe3 while the bottom of CB is from MoS2. Normally, the type I heterojunction possesses a symmetrical offset of potential barriers, where direct exciton transition occurs at the heterointerface. While in the case of type II heterojunction holes and electrons are accumulated on different sides of the heterointerface, leading to indirect exciton transition. In type II heterojunction, the spatial separation of photoexcited electrons and holes as they are localized in different sides of heterojunction, reduces the recombination rate and enhances the photocatalytic efficiency. In the case of MnPSe3/CrSiTe3 heterojunction type I heterostructure is formed due to a similar crystal structure and low lattice mismatch. However, under tensile strain the band alignment changes from type I to type II due to the transition from an indirect bandgap to a direct bandgap. The above studies demonstrated the possibility of the modulation of the electronic structure of MPX3 by forming heterojunctions, implying the potential applicability of heterojunctions for photocatalysis applications.
Fig. 16 (a) Structure and side view of the charge density difference of the MnPSe3/MoS2 nanocomposite with various stacking orders V1–V5. (b) Total and partial DOS of V1, V2, and V4 arrangements. The Fermi level is represented by the vertical shadow line and set to zero. Reprinted with permission.114 Copyright 2017, Springer Nature. |
Experimentally, Chen et al.73 anchored 0D Cs4W11O35 nanoparticles onto 2D MnPS3 and reported higher HER yield in the case of MnPS3–Cs4W11O35 as compared to bare MnPS3. 2D MnPS3 is reported to be a non-toxic direct band gap semiconductor with appropriate VBM and CBM band edge potentials for photocatalytic water redox reaction. However, higher electron–hole recombination rates and their inability to oxidize water to generate oxygen limit their photo applicability. Bare MnPS3 was reported to show a low H2 production yield of 21.2 μmol h−1 g−1 under solar light illumination. To prepare MnPS3–Cs4W11O35 composites, initially, MnPS3 sheets are exfoliated in NMP solvent and then tetrabutylammonium hydroxide-modified Cs4W11O35 nanoparticles are anchored onto the MnPS3 sheets by electrostatic interaction. The homogeneous distribution of Cs4W11O35 nanoparticles on the MnPS3 sheets via strong interfacial interaction of the S–W–O bond is evident in TEM images (Fig. 17a). The photocatalytic HER performance of the MnPS3–Cs4W11O35 hybrid was examined under solar light with the sacrificial Na2S/Na2SO3 solution. Fig. 17b depicts the photocatalytic H2 evolution rate of bare MnPS3, MnPS3–Cs4W11O35 hybrid composites with Cs4W11O35 mass percentages of 5.4, 10.9, and 16.5%. The H2 evolution rates for MnPS3–Cs4W11O35 with 5.4, 10.9 and 16.5% of Cs4W11O35 were 38, 99, and 58 μmol h−1 g−1, respectively, while bare MnPS3 showed HER activity of 21.2 μmol h−1 g−1only. With the addition of Cs4W11O35 nanoparticles, the photocatalytic HER activity increased first and then decreased. The maximum H2 evolution rate of 99.6 μmol h−1 g−1 was achieved with the MnPS3–Cs4W11O35 composite with a Cs4W11O35 mass percentage of 10.9%. In order to investigate the long-term stability of the MnPS3–Cs4W11O35 composite, photocatalytic HER experiments were performed for four cycles, and during each cycle, the quartz reactor is evacuated prior to the start of the new experimental cycle. Cyclic stability curves shown in Fig. 17d indicate that HER activity for the second cycle is significantly higher than the first one and there is stable H2 evolution after the second cycle. The stable H2 evolution rate after the second cycle to the fourth cycle signifies the long-term photostability of the MnPS3–Cs4W11O35 composite. Further, TEM images of the MnPS3–Cs4W11O35 composite after photocatalytic HER studies, shown in Fig. 17e, reveal that Cs4W11O35 nanoparticles are coupled with MnPS3 sheets even after 24 h reaction. The PXRD pattern of MnPS3–Cs4W11O35 recorded after HER studies also showed that the catalyst was stable (Fig. 17f). Based on UV-vis, photoluminescence, valence band XPS spectra, and electrochemical impedance spectra studies, Chen et al.60,73 proposed the formation of the Z-scheme heterojunction, which effectively suppresses the charge-hole recombination rate, charge-transfer ability, and hole oxidation ability (Fig. 17g). The internal polarization field generated due to the heterojunction formation generates more positive charges on MnPS3 and more negative charges on Cs4W11O35. Upon light irradiation, electrons and holes are produced in VB and CB of both MnPS3 and Cs4W11O35 components of the composite. Further, the internal electric field suppresses the electron transfer from CB of Cs4W11O35 to CB MnPS3 and facilitates the electron migration from CB of Cs4W11O35 to VB of MnPS3, which results in the formation of the Z-scheme heterojunction. Hence, photogenerated electrons left in the CB of MnPS3 are utilized for water reduction whereas holes in the VB of Cs4W11O35 are utilized for the oxidation of Na2S/Na2SO3.
Fig. 17 (a) TEM image of the MnPS3–Cs4W11O35 heterojunction. (b and c) Time course hydrogen production data and photocurrent responses for MnPS3–Cs4W11O35 catalyst with 0, 5.4, 10.9, and 16.5% of Cs4W11O35, and these composites designated as MnPS3, 5.4MPSCWO, 10.9MPSCWO, and 16.5MPSCWO, respectively. (d) Cyclic stability curves for HER of MnPS3–Cs4W11O35 catalyst recorded for 4 cycles and 24 h. (e) TEM image of MnPS3–Cs4W11O35 after photocatalytic HER studies. (f) Raman spectra of MnPS3–Cs4W11O35 before and after photocatalytic HER studies and (g) schematic of the reaction mechanism of photocatalytic HER on Z-scheme MnPS3–Cs4W11O35 heterojunction. Adapted with permission.73 Copyright 2021, Elsevier. (h) Time course HER curves for NiPS3/CdS composite. (i) Cyclic stability curves for HER of MnPS3–Cs4W11O35 catalyst recorded for 4 h and (j) Schematic representation of generated type-I (straddling type) heterostructure between NiPS3 and CdS components. (k) XPS spectra of NiPS3/CdS composite with light on and off respectively. Adapted with permission.115 Copyright 2022, Springer Nature. |
As indicated earlier, in the case of 2D NiPS3, due to rapid charge recombination rate and photo-corrosion issues because of misalignment of the VBM concerning water oxidation potential, their reported HER activity is marginal. To overcome the weak oxidizing ability of NiPS3, Ran et al.115 coupled 2D NiPS3 with 0D CdS photocatalyst, which can supply oxidative photogenerated holes rather than being utilized. Cadmium sulfide (CdS) was reported to show a narrow band gap of 2.4 eV at room temperature, which accounted for its proficient visible light absorption characteristics. Secondly, CdS possess suitable VB and CB positions for water redox reactions, which make it an excellent photocatalyst for water redox reactions. Ran et al. examined the photocatalytic water splitting performance of NiPS3/CdS under visible-light illumination (λ > 400 nm) using TEOA as a sacrificial agent. Under light illumination, NiPS3/CdS showed an extremely high H2 evolution rate of 13600 μmol h−1 g−1 while bare NiPS3 showed negligible activity under similar conditions (Fig. 17h). The stability of NiPS3/CdS for photocatalytic HER was examined for 9 h, with a cycle every 3 h. As shown in Fig. 17i, the H2 evolution rate at cycle 3 (6687 μmol h−1 g−1) accounted for 49.17% of that in the first cycle (13600 μmol h−1 g−1). This result implied the appreciable stability of the NiPS3/CdS catalyst. In addition, in situ XPS measurements were conducted to reveal the dissociation and migration pathways of photogenerated electrons and holes on the surface of the NiPS3/CdS catalyst (Fig. 17k). During the in situ XPS measurement, a light-emitting diode was utilized as the light source to excite the catalyst. As presented in Fig. 17k, under light irradiation Ni 2p, P 2p and S 2p signals shifted towards higher binding energy as compared to dark conditions, signifying that more photogenerated holes than electrons migrate to the surface of both NiPS3 and CdPS3 components. This study suggests that during photocatalysis, holes migrate to the surface and are captured by the sacrificial agent, TEOA, leaving the electrons for HER. To explain the origin of such high HER activity of the NiPS3/CdS catalyst, the authors proposed a formation of type-I (straddling type) heterostructure between NiPS3 and CdS based on photoluminescence, Mott–Schottky plots, ultrafast transient absorption spectroscopy, steady-state and transient-state surface photovoltage (SPV) spectroscopy studies (Fig. 17j). Upon light irradiation, electrons and holes are photogenerated in both NiPS3 and CdS systems. Due to strong interfacial electronic coupling between NiPS3 and CdS, the electrons and holes in CB and VB of CdS migrate to NiPS3. However, most of the holes generated in CdS are consumed by sacrificial hole quencher TEOA before migrating to NiPS3, resulting in the oxidation of TEOA. While a low fraction of photogenerated holes are transported from VB of CdS to VB of NiPS3. On the other hand, much more photogenerated electrons in the CB of NiPS3 are utilized for the water reduction reaction. The significantly high HER activity of NiPS3/CdS can be attributed to the decreased recombination rate of photogenerated electron–hole pairs and abundant active sites on NiPS3 for hydrogen evolution.
Xia et al.116 reported the formation of S-scheme heterojunction between TiO2 and 2D FePS3 by self-assembling TiO2 nanoparticles on FePS3 nanosheets. In this study, FePS3/TiO2 nanocomposites are obtained by adding TiO2 (P25, Degussa AG) nanoparticles to the exfoliated FePS3 suspension under grinding conditions at room temperature. FePS3/TiO2 showed a higher hydrogen evolution rate of 99.5 μmol h−1 g−1 using ethanol solvent as a sacrificial electron donor under UV light illumination (Fig. 18a). While bare 2D FePS3 showed negligible H2 evolution under similar conditions. The Mott–Schottky plot for the FePS3/TiO2 composite indicated that both positive and negative slopes were obtained in distinct regions of the composite, implying the successful formation of the p–n heterojunction between 2D FePS3 and TiO2 (Fig. 18b). Further, in situ atomic force microscopy (AFM) combined with Kelvin probe force microscopy (AFM-KPFM) was utilized to investigate the electron transport pathway of the FePS3/TiO2 composite under light irradiation. As shown in Fig. 18d, aggregation of the TiO2 nanoparticles on the surface of FePS3 was evident in the AFM image of the FePS3/TiO2 composite. Fig. 18e and f show the corresponding KPFM images of FePS3/TiO2 in the dark and with 365 nm UV light irradiation, respectively. Accordingly, the surface potential plots of FePS3/TiO2 in dark and UV light irradiation are displayed in Fig. 18g. The surface potential across the line is enhanced under light illumination as compared to dark conditions. Particularly, the surface potential at A1 is elevated by 124 mV. This result indicated that the photogenerated holes accumulate at the surface of FePS3 and TiO2 components under light illumination. Upon light excitation, photoexcited electrons and holes are produced in both FePS3 and TiO2 components. Then, photogenerated electrons are retained in the CB of the FePS3 and holes preserved in VB of TiO2 to carry water reduction and oxidation, respectively, attributed to a strong in-built electric field gradient between the individual components. Meanwhile, photogenerated electrons in the VB of TiO2 recombine with the holes in the CB of FePS3. Due to the generation of the S-scheme heterojunction, the reduction ability of photogenerated electrons in FePS3 and the oxidation ability of holes in TiO2 are preserved (Fig. 18c). Secondly, a reduced recombination rate between the photogenerated electrons and holes was also simultaneously achieved. The photogenerated holes in VB of TiO2 were utilized for the oxidation of ethanol to produce the oxidized products.
Fig. 18 (a) Photocatalytic HER activity of FePS3/TiO2 with various amounts of FePS3. (b) Mott–Schottky plots for FePS3/TiO2 acquired in 0.5 M Na2SO4 at 3000 MHz. (c) Schematic of the reaction mechanism of photocatalytic HER on S-scheme FePS3/TiO2 components. (d and e) Atomic force microscopy (AFM) and Kelvin probe force microscopy (AFM-KPFM) image of FePS3/TiO2 in dark conditions. (f) KPFM image with UV light irradiation. (g) The corresponding surface potential plot of FePS3/TiO2 in dark and UV light irradiation. Reprinted with permission.116 Copyright 2018, Wiley publication. |
In recent years, 2D ferroelectric materials such as CuInP2S6, CuBiP2S6, and AgInP2Se6 are receiving increased attention as HER photocatalysts. The permanent polarization electric field in these materials serves as a driving force for the spatial separation of charges, thereby showing good HER activity.117–119 Secondly, large exciton binding energy suppresses the reduction potential of the photogenerated electrons, prompting electrons readily available for the water reduction reaction. Among these MPX3 compounds, CuInP2S6 experimentally showed good HER characteristics due to intermediate band gap and room temperature ferroelectricity.119 Lin et al.120 studied the photocatalytic HER performance of CuInP2S6, prepared by the solid-state reaction under visible light using a TEOA sacrificial electron donor. Bare CuInP2S6 nanosheets showed moderate HER activity of 18.3 μmol h−1 g−1 due to the high charge-transfer ability of the ferroelectric material. Further, the formation of 2D/2D heterojunction between CuInP2S6 and graphitic carbon nitride (g-C3N4) substantially accelerated the charge-transfer rate, due to which CuInP2S6/g-C3N4 displayed 2.5 times higher (45.1 μmol h−1 g−1) activity than that of bare CuInP2S6 (Fig. 19a–d). Transient photocurrent response studies indicated that the composite showed a higher photocurrent density of 2.92 μA cm−2, which was 1.62 and 2.25 times higher than that of individual CuInP2S6 and g-C3N4, respectively. This result implies the rapid charge separation and migration in the composite through high-speed microchannels. Further, photoluminescence studies implied a much lower emission intensity for the CuInP2S6/g-C3N4 composite compared to g-C3N4, indicating a much reduced electron–hole recombination rate in the composite. Based on the above results, the generation of type-II heterojunction between CuInP2S6 and g-C3N4 components is projected, as depicted in Fig. 19e. Upon light illumination, owing to the intrinsic dipole moment, CuInP2S6 can generate ferroelectric polarization fields with the deposition of negative and positive at opposite directions of the surface, which induces spatial charge separation. These photogenerated electrons at the CuInP2S6/g-C3N4 interface are utilized for water reduction to generate hydrogen, while holes are consumed by the sacrificial hole scavenger TEOA. Further, due to the in-built electric field gradient holes in the VB of g-C3N4 are transferred to the VB of CuInP2S6 by nanochannels, which further reduced the electron–hole recombination rate. Further, the cyclic stability curve shown in Fig. 19c implies that CuInP2S6/g-C3N4 still keeps high activity even after 5 cycles (15 h), suggesting their promising potential application in solar-energy conversion. In another study, Ren et al.121 synthesized a CuInP2S6/ZnIn2S4 heterostructure by a surfactant-assisted hydrothermal method and utilized it for photocatalytic H2 generation. As shown in Fig. 19f, bare ZnIn2S4 showed a quite inferior H2 evolution rate of 5.88 μmol h−1 g−1, while CuInP2S6/ZnIn2S4 displayed an enhanced activity of 76.2 μmol h−1 g−1. The band gap energy values for CuInP2S6 and ZnIn2S4 estimated using a UV-vis diffuse reflectance study were 2.37 and 2.02 eV, respectively (Fig. 19g). Owing to the matched band gap energy value between CuInP2S6 and ZnIn2S4, a type I heterostructure was effectively generated, which led to the efficient separation of the photogenerated electrons and holes (Fig. 19h). On visible-light irradiation, photogenerated electrons in the CB of CuInP2S6 migrated to the CB of ZnIn2S4 at the interface and were utilized for H+ reduction to yield H2. While photogenerated holes in the VB of CuInP2S6 migrated to the VB of ZnIn2S4 and were finally captured by the sacrificial agent TEOA. Markedly, due to the generation of the heterostructure, abundant charge-migration pathways were created, which reduced the charge recombination rate and improved the HER yield.
Fig. 19 (a) Schematic of the photogenerated charge carriers in 2D CuInP2S6. (b) Time course H2 evolution data for g-C3N4(CN), CuInP2S6 (CIPS) and CuInP2S6/g-C3N4 (CN/CIPS). (c) Cyclic stability curve of HER for the CuInP2S6/g-C3N4 (CN/CIPS) heterojunction. (d) Temperature-dependent remnant polarization of CuInP2S6 inset: polarization-electric field (P-E) hysteresis loop obtained using piezoresponse force microscopy and (e) schematic representation of generated type-II heterostructure between CuInP2S6 and g-C3N4 components. Adapted with permission.120 Copyright 2020, Elsevier. (f) Time course H2 evolution data for ZnIn2S4 (ZIS) and CuInP2S6/ZnIn2S4 (CIPS/ZIS). (g) UV-vis diffuse reflectance spectrum of ZnIn2S4 (ZIS), CuInP2S6 (CIPS), and CuInP2S6/ZnIn2S4 (CIPS/ZIS) and (h) schematic illustration of generated type-I heterostructure at CuInP2S6/ZnIn2S4 interface. Adapted with permission.121 Copyright 2023, Elsevier. (i) Band edge potentials of CuInP2S6/Mn2P2S6 and CuInP2S6/Zn2P2Se6 ferroelectric heterostructures obtained using DFT+U method. The HER and OER potentials are shown for comparison. +P and −P designate CuInP2S6 in two oppositely polarized states.119 |
Theoretically, Huang et al.119 projected that the generation of ferroelectric hetero-interfaces such as CuInP2S6/Mn2P2S6 and CuInP2S6/Zn2P2Se6 would be beneficial for photocatalytic water splitting due to strong visible light absorption and type-II band alignment, which can facilitate rapid charge separation (Fig. 19i). To date, there are very limited experimental studies on ferroelectric hetero-interfaces containing 2D MPX3 compounds for photocatalysis and these systems need to be investigated in more detail. In Table 2, we have listed the photocatalytic HER activity of various 2D metal thiophosphates and their heterostructures reported in the literature along with the reaction conditions. It should be noted that, while comparing the HER activity between two individual systems, the strength of the light source (simulated solar light or xenon lamp, 300 W or 400 W), photosensitizer added and type of sacrificial agent (TEOA and Na2S/Na2SO3) need to be taken into consideration since these factors can affect the activity significantly. Secondly, different synthetic approaches generate MPX3 layers of various thicknesses, which further affect the HER activity as accessibility for catalytic edge (active) sites decreases with an increase in the layer thickness.
Photocatalyst system | Synthesis method, layer thickness | Light source | Sacrificial agent | HER activity (μmol h−1 g−1) | Ref. |
---|---|---|---|---|---|
a CVD – chemical vapor deposition. b AM 1.5G – simulated solar light. c SS – solid state method. d LPE – liquid phase exfoliation. | |||||
MnPS3 | CVDa, 6 nm | AM 1.5Gb | — | 3.1 | 55 |
MnPS3 | CVD, 6 nm | AM 1.5G | Na2S/Na2SO3 | 21.2 | 55 |
MnPSe3 | CVD, 28 nm | AM 1.5G | — | 6.5 | 55 |
MnPSe3 | CVD, 28 nm | AM 1.5G | Na2S/Na2SO3 | 43.5 | 55 |
FePS3 | CVD, 20 nm | 300 W Xe lamp | FeSO4 | 46.6 | 131 |
FePS3 | CVD, 20 nm | 300 W Xe lamp | Triethanolamine | 141.9 | 131 |
FePS3 | CVD, 20 nm | 300 W Xe lamp | Na2S/Na2SO3 | 402.4 | 131 |
FePS3 | SS methodc | 300 W Xe lamp | Triethanolamine | 166.2 | 132 |
FePS3 | LPEd,10 nm | 300 W Xe lamp | Triethanolamine | 290.0 | 133 |
Porous FePS3 | SS 7 nm | 300 W Xe lamp | Triethanolamine | 305.6 | 132 |
NiPS3 | CVD, 3.5 nm | AM 1.5G | — | 26.42 | 113 |
NiPS3 | CVD, 3.5 nm | AM 1.5G | Na2S/Na2SO3 | 74.67 | 113 |
CuPS3 | Hot injection | 150W simulator | Na2S/Na2SO3 | 2085 | 63 |
ZnPS3 | LPE | 300 W Xe lamp | Na2S/Na2SO3 | 640.0 | 112 |
CdPS3 | CVD | AM 1.5G | Na2S/Na2SO3 | 786.6 | 131 |
CdPS3 | LPE, 3 nm | 300 W Xe lamp | Ethanol | 10880.0 | 71 |
N-doped CdPS3 | LPE | 300 W Xe lamp | DL-lactic acid | 6280.0 | 134 |
InPS3 | CVD | AM 1.5G | Na2S/Na2SO3 | 379.1 | 131 |
Sn2P2S6 | CVD | AM 1.5G | Na2S/Na2SO3 | 202.06 | 135 |
ZnIn2S4 | SS method | Visible light (λ ≥ 420 nm) | — | 5.88 | 121 |
2D MPX3 HER activity reported with photosensitizer eosin Y (EY) | |||||
EY/MnPS3 | LPE | 400 W Xe lamp | Triethanolamine | 200.0 | 93 |
EY/FePS3 | LPE | 400 W Xe lamp | Triethanolamine | 600.0 | 93 |
EY/NiPS3 | LPE, 4.9 nm | 400 W Xe lamp | Triethanolamine | 2600.0 | 93 |
EY/ZnPS3 | LPE | 400 W Xe lamp | Triethanolamine | 100.0 | 93 |
EY/CdPS3 | LPE | 400 W Xe lamp | Triethanolamine | 300.0 | 93 |
EY/MnPSe3 | LPE | 400 W Xe lamp | Triethanolamine | 200.0 | 93 |
EY/FePSe3 | LPE, 6.2 nm | 400 W Xe lamp | Triethanolamine | 1700.0 | 93 |
EY/CdPSe3 | LPE | 400 W Xe lamp | Triethanolamine | 200.0 | 93 |
EY/Ag0.5In0.5PS3 | LPE, 2.1 nm | 400 W Xe lamp | Triethanolamine | 1900.0 | 93 |
EY/Ag0.5In0.5PS3 | LPE, 3.1 nm | 400 W Xe lamp | Triethanolamine | 500.0 | 93 |
2D MPX3 heterojunctions | |||||
MnPS3/Cs4W11O35 | — | 300 W Xe lamp | Na2S/Na2SO3 | 99.0 | 73 |
NiPS3/CdS | — | 300 W Xe lamp | Triethanolamine | 13600.0 | 64 |
FePS3/TiO2 | — | 350 W Xe lamp | Ethanol | 99.5 | 116 |
MnPS3/carbon dot | — | 300 W Xe lamp | — | 339.63 | 136 |
CuInP2S6 | — | 300 W Xe lamp | Triethanolamine | 18.3 | 120 |
CuInP2S6/g-C3N4 | — | 300 W Xe lamp | Triethanolamine | 45.1 | 120 |
CuInP2S6/ZnIn2S4 | SS method | Visible light (λ ≥ 420 nm) | — | 76.2 | 121 |
Among the reported various 2D MPX3 compounds, CdPS3 showed a significantly high H2 evolution rate of 10880 μmol h−1 g−1, while CuPS3 and ZnPS3 displayed 2085 and 640 μmol h−1 g−1, respectively (Table 1). However, in a single-component MPX3 system, the photogenerated holes are confined to the semiconductor and not available for water oxidation, hence resulting in photocorrosion of the semiconductor. The combination of sacrificial hole scavengers such as Na2S/Na2SO3 or triethanolamine with 2D MPX3 alleviates photocorrsion, and increases the activity to some extent. Generally, the formation of hetero-interfaces between 2D MPX3 compounds and material with good oxidation capability for holes such as Cs4W11O35 and CdS enhances the HER yield and photostability of 2D MnPS3. The combination of NiPS3 and FePS3 with CdS and TiO2 appears to be advantageous for photocatalytic HER. Mainly, the NiPS3/CdS heterostructure displayed a significantly high H2 evolution rate of 13600 μmol h−1 g−1. The generation of heterojunction effectively suppresses the electron–hole recombination rate and improves the charge-transfer and hole oxidation ability due to the internal polarization field. In Table 3, we have compared the photocatalytic HER activity of 2D MPX3 nanocomposites with other 2D materials (C3N4, metal sulfides, phosphides, and carbides) and their nanocomposites reported in the literature. The HER activity obtained with NiPS3/CdS surpasses the highest reported for TMDCs-based nanocomposites, indicating that 2D MPX3-based composites could be beneficial for large-scale hydrogen production with exceptional stability.
2D nanomaterials | |||||||
---|---|---|---|---|---|---|---|
Photocatalyst system | Light source | Sacrificial agent | HER activity (μmol h−1 g−1) | Cyclic stability | References | ||
No. of cycles | Total time of cycle (h) | HER activity after cyclic stability as compared with 1st cycle | |||||
a TEOA – triethanolamine. b NR – not reported. c 2D phosphochalcogenides. d EY – eosin Y dye. | |||||||
C3N4 | 300 W Xe lamp | TEOAa | 169.5 | 4 | 20 | Negligible change | 137 |
MoS2 | 300 W Xe lamp | TEOA | 339.5 | — | — | NRb | 138 |
ZnInS4 | 300 W Xe lamp | Na2S/Na2SO3 | 3890.0 | 3 | 9 | Negligible change | 139 |
CdS | 300 W Xe lamp | MeOH | 370.0 | — | — | NR | 140 |
FeSe2 | 300 W Xe lamp | Na2S/Na2SO3 | 955.3 | — | — | NR | 141 |
Fe–BiOCl | 300 W Xe lamp | Na2SO4 | 3540.0 | — | — | NR | 142 |
Phosphorene | 300 W Xe lamp | Na2S/Na2SO3 | 512.0 | 2 | 20 | Negligible change | 143 |
CuPS3c | 150W simulator | Na2S/Na2SO3 | 2085.0 | 3 | 24 | Negligible change | 63 |
EY/NiPS3c,d | 400 W Xe lamp | TEOA | 2600.0 | 5 | 25 | Negligible change | 93 |
CdPS3c | 300 W Xe lamp | Ethanol | 10880.0 | — | — | NR | 71 |
2D nanomaterial-based heterostructures | |||||||
MoS2/CdS | 300 W Xe lamp | Lactic acid | 4060.0 | 3 | 15 | Negligible change | 144 |
MoS2/TiO2 | 300 W Xe lamp | TEOA | 10046.0 | 3 | 12 | Negligible change | 145 |
MoS2/C3N4 | 300 W Xe lamp | TEOA | 1155.0 | 3 | 12 | 1098 μmol h−1 g−1 | 146 |
MoS2/C3N4 | AM 1.5G | TEOA | 8300.0 | 6 | 16 | Negligible change | 147 |
Ti3C2/CdS | 530 W Xenon lamp | MeOH | 1730.0 | 3 | 18 | Negligible change | 140 |
Cu3P/ZnInS4 | 300 W Xe lamp | TEOA | 5461.0 | 6 | 18 | Negligible change | 148 |
WS2/CdS | 400 W Xe lamp | Lactic acid | 13132.0 | 4 | 12 | Negligible change | 149 |
FeSe2/C3N4 | 300 W Xe lamp | Na2S/Na2SO3 | 1655.6 | 2 | 12 | Negligible change | 141 |
Ti3C2/TiO2 | 300 W Xe lamp | MeOH | 4672.0 | 5 | 25 | Negligible change | 150 |
C3N4/phosphorene | 300 W Xe lamp | TEOA | 5850.0 | 3 | 12 | Negligible change | 151 |
MnPS3/Cs4W11O35c | 300 W Xe lamp | Na2S/Na2SO3 | 99.0 | 4 | 24 | Negligible change | 73 |
NiPS3/CdSc | 300 W Xe lamp | TEOA | 13600.0 | 4 | 4 | 6800 μmol h−1 g−1 | 64 |
In this perspective, we discussed the recent development of non-noble metal catalysts for water-splitting reactions with a focus on 2D MPX3 compounds by emphasizing novel strategies developed for activating MPX3 for photocatalytic and electrocatalytic HER. Theoretically, Gibbs free energy of hydrogen adsorption is considered to be a good descriptor in evaluating HER performance, and material with an approximate ΔGH value of zero is considered to be an ideal catalyst. DFT calculations revealed that pyrite type CoPS shows thermoneutral H+ adsorption, and also experimentally substantiated that CoPS possess Pt/C-like electrocatalytic H2 evolution activity. The exfoliation of bulk MPX3 into thin sheets enhances the HER activity due to the increased in-plane conductivity and more exposed edge sites. The electronic conductivity and HER activity of MPS3 sheets can be further improved by coupling a conducting matrix such as graphene. Doping of Co atoms into FePS3 and NiPS3 systems also enhances HER kinetics due to the electronic state modulation with doping. Coupling of TMPCs with other 2D systems such as MoS2 facilitates hydrogen spillover during H2 evolution from the MoS2 edge site to MPX3. These MPX3/MoS2 composites also showed good OER characteristics, indicating that these composites can be useful for overall water-splitting applications.
Strong visible light absorption characteristics, high carrier mobility, and thermoneutral H+ adsorption of 2D MPX3 make them suitable catalysts for photocatalytic water splitting. However, single-component MPX3 as a photocatalyst showed marginal HER activity and low stability due to misaligned VBM with respect to water oxidation potential. The development of MPX3 for photocatalytic HER is presently concentrating on enhancing the light absorption efficiency, the alleviation of photocorrosion, industrial scale H2 production, and improving stability. The addition of sacrificial hole scavengers such as Na2S/Na2SO3 or triethanolamine alleviates photocorrsion, and increases the activity and stability to some extent. Nevertheless, band gap engineering is required to enhance light absorption and reduce the photogenerated electron–hole recombination rate.
By coupling MnPS3 with a Cs4W11O35 semiconductor possessing good oxidation capability of holes, the HER activity of individual components can be enhanced. The generation of Z-scheme heterojunction between MnPS3 and Cs4W11O35 effectively suppresses the electron–hole recombination rate and improves charge-transfer and hole oxidation ability due to the internal polarization field. Further, a combination of NiPS3 and FePS3 with CdS and TiO2, respectively appears to be advantageous for photocatalytic HER. Particularly, the NiPS3/CdS heterojunction showed superior HER activity of 13600 μmol h−1 g−1, which is comparable to that of some of the highest reported transition metal-based nanocomposites reported in the literature.
In spite of the overwhelming progress in photocatalysis/electrocatalysis of 2D MPX3, there is still a lot of scope to enhance the performance and stability towards commercialization, some points are listed below.
After potential developments of proficient H2 evolution catalysts, recent research has demonstrated that 2D MPX3 catalysts can be highly efficient for photocatalytic and electrocatalytic water splitting. Solar water-splitting reactors need to be established to sustain large-scale H2 production at a low cost. In this context, photoelectrochemical cells (PECs) have the benefit of exploiting both solar and electrical energy and concurrently separating evolved H2 from O2. Band gap tunability and good electronic conductivity make 2D MPX3 a potential candidate for HER with high solar-to-hydrogen conversion efficiency. Also, strategies for safe H2 storage and transport need to be developed.
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