Hybrid amorphous MoSx-graphene protected Cu2O photocathode for better performance in H2 evolution

This paper reports on a new strategy using a combination of graphene protective layer and amorphous molybdenum sulfide (MoS_x) H₂-evolving catalyst layer to form a hybrid coating layer that enhances both photocatalytic activity and stability of the p-Cu₂O photocathode for the solar H₂ generation in water. Graphene sheet was transferred from a copper substrate onto the p-Cu₂O electrode surface by a simple solution process, resulting in the creation of p-Cu₂O/Gr. A layer of MoS_x was deposited further to generate a hybrid p-Cu₂O/Gr/MoS_x photocathode. In a pH 7 phosphate buffer electrolyte and under 1 Sun light illumination, at the applied potential of 0 V vs. reversible hydrogen electrode (RHE), the resultant p-Cu₂O/Gr and p-Cu₂O/Gr/MoS_x show 7-fold superior catalytic activity and significant enhancement of the stability compared to a pristine p-Cu₂O photocathode. The obtained results demonstrate that the p-Cu₂O/Gr and p-Cu₂O/Gr/MoS_x photocathodes are very promising for photoelectrochemical cell water splitting.     

Facile fabrication of silicon nanowires as photocathode for visible-light induced photoelectrochemical water splitting

We report the fabrication of one dimensional Silicon nanowires (Si NWs) using p-Si (100) substrate through facile two step metal assisted chemical etching (MACE) approach. The evolution of structural and optical properties of Si NWs by etching Si substrate was studied as a function of hydrogen peroxide (H₂O₂), a strong oxidation agent. The length of the NWs increased linearly with the H₂O₂ concentrations and reached maximum of 51 μm for etching of 60 min. The merits of metal free Si NWs as photocathode in the photoelectrochemical (PEC) neutral water splitting under the visible light was investigated. The performance of the photocathode highly depends on the morphology of Si nanostructure. A high density and well separated Si NWs fabricated by 0.6 M of H₂O₂ results in maximum photocurrent density of 6 mA cm^?2 with applied bias photocurrent conversion (ABPE) efficiency of 1.1% under visible light illumination.     

Plasmonic nickel nanoparticles decorated on to LaFeO3 photocathode for enhanced solar hydrogen generation

Plasmonic Ni nanoparticles were incorporated into LaFeO₃ photocathode (LFO-Ni) to excite the surface plasmon resonances (SPR) for enhanced light harvesting for enhancing the photoelectrochemical (PEC) hydrogen evolution reaction. The nanostructured LFO photocathode was prepared by spray pyrolysis method and Ni nanoparticles were incorporated on to the photocathode by spin coating technique. The LFO-Ni photocathode demonstrated strong optical absorption and higher current density where the untreated LFO film exhibited a maximum photocurrent of 0.036 mA/cm² at 0.6 V vs RHE, and when incorporating 2.84 mmol Ni nanoparticles the photocurrent density reached a maximum of 0.066 mA/cm² at 0.6 V vs RHE due to the SPR effect. This subsequently led to enhanced hydrogen production, where more than double (2.64 times) the amount of hydrogen was generated compared to the untreated LFO photocathode. Ni nanoparticles were modelled using Finite Difference Time Domain (FDTD) analysis and the results showed optimal particle size in the range of 70–100 nm for Surface Plasmon Resonance (SPR) enhancement.     

MoS2 supported on hydrogenated TiO2 heterostructure film as photocathode for photoelectrochemical hydrogen production

The substitution of noble metal platinum catalyst is one of the important research contents for sustainable development and is also the key to the practical application of photoelectrochemical (PEC) hydrogen production. In this work, we loaded the 1T-2H mixed phase MoS₂ on the hydrogenated anatase/rutile heterophase TiO₂ (A-H-RTNA) by hydrothermal method to prepare a new MoS₂/A-H-RTNA electrode material. The prepared material exhibited higher carrier density, lower PL intensity and higher conductivity than Pt/A-H-RTNA because 1T-MoS₂ has more active sites and lower charge transfer resistance than Pt. With the bias voltage of −0.4 V, the optimized 16MoS₂/A-H-RTNA as photocathode shows the largest PEC hydrogen production rate of 1840 mmol m⁻² h⁻¹, which is 2.9 and 2.2 times higher than those of A-H-RTNA (625 mmol m⁻² h⁻¹) and Pt/A-H-RTNA (848 mmol m⁻² h⁻¹), respectively. We innovatively used the prepared 16MoS₂/A-H-RTNA film as counter electrode instead of Pt electrode to construct a PEC system without any noble-metal. The result demonstrates that the noble-metal-free MoS₂ loaded on TiO₂ electrode as counter electrode has 75% PEC activity of noble metal Pt electrode. This study develops a PEC method for hydrogen evolution, which no longer depends on precious metal platinum as cathode.     

An efficient tandem photoelectrochemical cell composed of FeOOH/TiO2/BiVO4 and Cu2O for self-driven solar water splitting

An integrated solar water splitting tandem cell without external bias was designed using a FeOOH modified TiO₂/BiVO₄ photoanode as a photoanode and p-Cu₂O as a photocathode in this study. An apparent photocurrent (0.37 mA/cm² at operating voltage of +0.36 V_RHE) for the tandem cell without applied bias was measured, which is corresponding to a photoconversion efficiency of 0.46%. Besides, the photocurrent of FeOOH modified TiO₂/BiVO₄–Cu₂O is much higher than the operating point given by pure BiVO₄ and Cu₂O photocathode (∼0.07 mA/cm² at +0.42 V_RHE). Then we established a FeOOH modified TiO₂/BiVO₄–Cu₂O two-electrode system and measured the current density-voltage curves under AM 1.5G illumination. The unassisted photocurrent density is 0.12 mA/cm⁻² and the corresponding amounts of hydrogen and oxygen evolved by the tandem PEC cell without bias are 2.36 μmol/cm² and 1.09 μmol/cm² after testing for 2.5 h. The photoelectrochemical (PEC) properties of the FeOOH modified TiO₂/BiVO₄ photoanode were further studied to demonstrate the electrons transport process of solar water splitting. This aspect provides a fundamental challenge to establish an unbiased and stabilized photoelectrochemical (PEC) solar water splitting tandem cell with higher solar-to-hydrogen efficiency.     

Remarkable improvement of photoelectrochemical water splitting in pristine and black anodic TiO2 nanotubes by enhancing microstructural ordering and uniformity

Efficient photoelectrochemical (PEC) water splitting is crucial for future energy and sustainable world. We here report on the improvement of PEC activity of anodic TiO₂ nanotubes (TNTs) by enhancing tube ordering and subsequent electrochemical reduction. TNTs were prepared by two-step anodic oxidisation from an organic electrolyte containing fluoride ions. The effects of first-step anodisation time on the ordering of TNTs and subsequent electrolytic reduction were investigated on the PEC performance under simulated solar light spectrum. The photocurrent densities of TNTs anodised for 1 h, 4 h and subsequently reduced are about 25.12 μA cm⁻², 51.76 μA cm⁻² and 126.89 μA cm⁻², respectively, at 1.23 V vs RHE and their conversion efficiency of light to electrical energy achieved are about 0.016%, 0.04% and 0.08% respectively. Electrochemical impedance spectroscopy (ESI) curves revealed the improved PEC water splitting confirmed by sharper charge carrier separation and enhanced charge transfer in highly ordered pristine and black TNTs. This improvement of PEC in dopant-free TNT is at the first instance interpreted by enhancing TNT ordering and uniformity achieved by prolonging of the first-step anodisation time and its effect on the electronic band structure of TNTs. This significant effect on PEC performance of pristine TNT under visible light absorption takes place due to the induced surface defects and slower recombination rates of hole and electron. This demonstrates an efficient economic materials production appraoch for PEC hydrogen production.     

Photoelectrochemical performance of bismuth vanadate photoanode for water splitting under concentrated light irradiation

Photoelectrochemical (PEC) water splitting is a promising way to convert solar energy into hydrogen energy. It is typically carried out at room temperature (RT) and 1 sun illumination. The PEC water splitting under concentrated light is expected to be an effective route to improve PEC performance, but there are few studies on it. Herein, CoPi/Mo:BiVO₄ photoanode was selected to investigate the effect of concentrated light and the reaction temperature on its PEC performance. It was revealed that CoPi/Mo:BiVO₄ showed enhanced PEC performance under concentrated light. The photocurrent density was enhanced with increased light intensity and increased reaction temperature. At a high temperature (60 °C), the normalized photocurrent density (3.31 mA cm⁻² at 1.23 V vs. RHE) was found to be optimal at 4 suns, which was attributed to the synergistic effect of concentrating light and heating. It is proved that concentrated light can effectively improve the PEC performance, which has important guiding significance to realize the low-cost and efficient PEC water splitting.     

Sensitization of vertically grown ZnO 2D thin sheets by MoSx for efficient charge separation process towards photoelectrochemical water splitting reaction

Development of advanced materials for photoelectrochemical (PEC) water splitting has become an essential issue for efficient, green, and economical hydrogen production. In this context, vertically grown thin sheets of ZnO is developed, which can function as an efficient photoanode in PEC water splitting reaction. Further, the PEC activity of ZnO is enriched by decorating a newly developed co-catalyst, which is amorphous MoS_x through efficient charge transportation. MoS_x nanostructure is decorated on the surface of ZnO nanosheet via electrodeposition technique. MoS_x adorned ZnO shows enhanced activity towards photoanodic PEC water splitting compared to bare ZnO. ZnO@MoS_x can generate photocurrent density nearly three times higher compared to bare ZnO at an applied potential of ‘0.5998’ V vs. RHE. Sensitization of MoS_x on ZnO surface results in an enhancement in carrier density; ZnO@MoS_x shows nearly 7.4-times higher carrier density compared to bare ZnO. Maximum photoconversion efficiency, 0.934% is achieved in the case of ZnO@MoS_x. The determined band alignment of ZnO and MoS_x indicate the formation of type-II heterostructure which allow facile charge carrier separation. Efficient charge separation is also confirmed with the help of PL spectroscopy. It further restricts the electron-hole recombination in ZnO, leading to enhanced PEC activity. ZnO@MoS_x thin sheets are very stable even up to 1000 s under chopped illumination condition.     

Enhanced photoelectrocatalytic hydrogen production performance of porous MoS2/PPy/ZnO film under visible light irradiation

Photoelectrochemical (PEC) water splitting provides a “green” approach for hydrogen production. However, the design and fabrication of high-efficient catalysts are the bottleneck for PEC water splitting owing to the involved thermodynamic and kinetic challenges. Herein, we report a new strategy for constructing a porous MoS₂/PPy/ZnO thin film photocatalyst with large specific surface area and excellent conductivity to achieve photoelectrochemical water splitting under visible light irradiation. Porous PPy/ZnO was synthesized via template-assisted electrodeposition, and MoS₂ was further electrodeposited to construct porous MoS₂/PPy/ZnO thin film photocatalyst. The hydrogen evolution rate of MoS₂/PPy/ZnO exhibits about 3.5-fold increase to 40.22 μmol cm⁻² h⁻¹ under visible light irradiation. The enhancement for photoelectrochemical hydrogen production is not only ascribed to enlarged specific surface area of the porous structure, but also attributed to the synergistic effects of MoS₂ and porous PPy/ZnO, which could dramatically improve its visible light absorption capacity and enhance the separation and transfer of photogenerated charges. Thus, more abundant photogenerated electrons and holes participate in photoelectrochemical process, which significantly enhances its photoelectrochemical hydrogen production performance.     

Transition metal doped MoS2 nanosheets for electrocatalytic hydrogen evolution reaction

In the present work, the effect of transition metals (Ni, Fe, Co) doping on 2-dimensional (2D) molybdenum disulfide (MoS₂) nanosheets for electrocatalytic hydrogen evolution reaction (HER) was explored. A simple and cost-effective hydrothermal method was adopted to synthesis transition metals doped MoS₂ nanosheets. The morphological and spectroscopic studies evidence the formation of high-quality MoS₂ nanosheets with the randomly doped metal ions. Notably, the Ni–MoS₂ displayed superior HER performance with an overpotential of ?0.302 V vs. reversible hydrogen electrode (RHE) (to attain the current density of 10 mA cm^?2) as compared to the other transition metals doped MoS₂ (Co–MoS₂, Fe–MoS₂). From the Nyquist plot, superior charge transport from the electrocatalyst to the electrolyte in Ni–MoS₂ was realized and confirmed that Ni doping provides the necessary catalytic active sites for rapid hydrogen production. The stable performance was confirmed with the cyclic test and chronoamperometry measurement and envisaged that hydrothermally synthesized Ni–MoS₂ is a highly desirable cost-effective approach for electrocatalytic hydrogen generation.     

Hydrogen production by water reduction on Si photocathode coupled with Ni2P

While searching for an efficient, non-noble, earth-abundant catalyst for the hydrogen evolution reaction (HER), we synthesized hexagonal dinickel phosphide with different nanostructures using solvothermal phosphidation. Coupled atop p-type Si, this catalyst performed as a p-n heterojunction photocathode assembly and the performance varied when under different electrolyte media. Apart from changing the surface morphology, Ni₂P was crystallized with an increase in the Ni^δ+/Ni²⁺ ratio as the phosphidation temperature gradually increased. A systematic evaluation of the water splitting reaction shows that a very small amount of catalyst (>85% transmittance for the catalyst layer) exhibits a photocurrent of −10 mA cm⁻² with a positive applied potential of 0.05 V versus reversible hydrogen electrode under simulated solar irradiation of AM 1.5G. We discuss the substantial charge transfer process at the depletion layer of the electrode/catalyst and the catalyst/electrolyte interface. Mott–Shottky analysis showed a shift in the flat band potential for Ni₂P, which reveals the underlying mechanism for the role of the p-n junction for enhanced photoelectrochemical cell performance.     

Interfacing perovskite strontium molybdate to molybdenum disulfide nanoplatelts for boosting HER from water

Due to low hydrogen adsorption free energy at the edges of 2D-MoS₂ layered sheets, nanostructured MoS₂ materials recently are assigned to prospective electrocatalysts for hydrogen evolution reaction (HER) from water. However, the efficiency and stability of HER onto the MoS₂ designed on the conductive substrates are poor. To significantly increase the number of active sites and achieve a long-time working stability, the design of hybrid-type electrodes is crucial. Here, we report the synthesis of a new hybrid material composed of molybdenum disulfide and molybdenum oxides heterostructured with strontium molybdate. For this, a facile one-pot hydrothermal process was developed directly onto the TiO₂ nanotube carpet substrate. The interfacing of strontium molybdate at the electrode substrate verified by X-ray photoelectron spectroscopy and Time of flight secondary ions mass spectrometry (ToF SIMS) techniques. Considerable higher catalytic activity at the surface of this hybrid film, with the onset potential of 190 mV vs RHE and a Tafel slope of 66 mV dec^?1 attaining ~80 mA cm^?2 at 0.35 V overvoltage was ascertained. Exciting HER stability in comparison with the pure synthetic MoS₂ was verified by a prolonged potential cycling from 0.05 to ?0.35 V versus RHE potential and 45 h continuous HER processing at a constant current density.     

Functionalized 2D materials F–MoS2 and F-g-C3N4 with TiO2 as Composite Electrocatalysts for Electrochemical Hydrogen Evolution

Electrochemical hydrogen evolution is an important research field to produce renewable energy. Nanostructured two dimensional (2D) materials such as g-C₃N₄ and MoS₂ are potential electrocatalysts for hydrogen evolution reaction (HER). The incorporation of semiconducting material into 2D material enhances the hydrogen evolution. Here in, we have developed composite of acid functionalized MoS₂ and g-C₃N₄ with TiO₂ (F–MoS₂/TiO₂, F-g-C₃N₄/TiO₂). The F–MoS₂/TiO₂ composite exhibited excellent electrochemical HER activity with an overpotential of 103 mV Vs RHE at 20 mA/cm² compared to pristine F–MoS₂ of 232 mV, TiO₂ of 455 mV Vs RHE. In addition F-g-C₃N₄/TiO₂ showed high overpotential of 322 mV at 5 mA/cm² than pristine F-g-C₃N₄ and TiO₂ of 433 mV and 448 mV Vs RHE at 2.7 mA/cm² respectively.     

Bi2S3 entrenched BiVO4/WO3 multidimensional triadic photoanode for enhanced photoelectrochemical hydrogen evolution applications

The catalytic reactivity and photoactivity of WO₃ and BiVO₄ oxide semiconductors have general obstacles as electrodes in emergent photo-electrochemical (PEC) hydrogen evolution applications. The present work comprises the integration of photocatalyst with wide visible photon absorption material which is vital for hydrogen evolution in photo-electrocatalytic water splitting. Herein, the 1D WO₃ NWs have been integrated with stable water oxidation photocatalysts of BiVO₄ and Bi₂S₃ as a photoanode (Bi₂S₃/BiVO₄/WO₃) for photoelectrochemical hydrogen evolution reactions. The morphological variations in the Bi₂S₃/BiVO₄/WO₃ heterostructure manifest catalytic activity and rapid charge transfer characteristics owing to band alignment and a wide range of visible photon absorption. The optimized Bi₂S₃/BiVO₄/WO₃ multidimensional photoanode accomplishes a superior photocurrent density of 1.52 mA/cm², a seven-fold higher than pristine WO₃ photoanode counterpart (0.2 mA/cm²) at 1 V vs. RHE. A prodigious lowest onset potential of ?0.01 V vs. RHE) has been achieved which enables very high solar to hydrogen conversion. The photoelectrode with entangled morphology such as nanosheets, nanocrystals and nanorods expanded their surface to volume ratio having enhanced catalytic performance. The hybrid photoanodes have demonstrated the lowest charge transfer resistance of 360 Ohm/cm² with a 7-fold rise in hydrogen evolution performance. The resultant triadic Bi₂S₃/BiVO₄/WO₃ heterostructure appeared to be an emerging stable photo-electro catalyst for hydrogen evolution applications.     

CIS/CdS/ZnO/ZnO:Al modified photocathode for enhanced photoelectrochemical behavior under visible irradiation: Effects of pH and concentration of electrolyte solution

In this paper, the CuInS₂ films were firstly modified with CdS and CdS/ZnO/ZnO:Al/Au layers in order to improve the photoelectrochemical (PEC) water splitting efficiency. The CuInS₂ photoelectrode was synthesized by electrodeposition method as a facial and green method, on the FTO substrate. The effects of pH and concentration of Na₂S electrolyte solution on the photocurrent density of photoelectrode samples were studied. As a p-n junction photocathode, the CIS/CdS/ZnO/ZnO:Al/Au photoelectrode indicates the enhanced PEC activity. The photocurrent density of CIS/CdS/ZnO/ZnO:Al/Au photoelectrode reaches to 1.91 mA/cm², while is about 2.5 times higher than that for CuInS₂ film at pH = 8 (−0.6 V vs Ag/AgCl). The formation of a p-n junction at the CuInS₂ photoelectrode surface not only reduces the recombination of electron-hole pairs but also increases the PEC response and water splitting performance of the as-prepared CIS/CdS/ZnO/ZnO:Al/Au photoelectrode.     

1T/2H MoS2/MoO3 hybrid assembles with glycine as highly efficient and stable electrocatalyst for water splitting

Despite great efforts have been made during the past decade to improve the efficiency of hydrogen evolution reaction (HER) onto the MoS₂-based electrocatalysts via increasing the number of active sites, further improvements are crucial to avoid the detachment of 2D MoS₂ nanosheets from the substrate during the long-term water splitting under intense HER. In this study, we report on the formation of highly efficient and surprisingly stable layer composed of 2D nanoplatelets from the hybrid-type MoS₂ as a new prospective electrocatalyst for HER from acidic water solution. This layer was formed via one-pot hydrothermal synthesis in the solution containing ammonium heptamolybdate, thiourea and glycine (Gly). The products obtained were characterized by SEM, HRTEM, XRD, Raman, XPS, and potential cycling. Note that at the designed hybrid-type MoS₂/MoO₃-Gly nanoplatelets the HER rate can achieve a stable electrochemical performance for days with ~100 mA cm⁻² current density at −0.35 V potential vs RHE.     

Hierarchical mesoporous SnO2/BiVO4 photoanode decorated with Ag nanorods for efficient photoelectrochemical water splitting

Bismuth vanadate has been extensively investigated as a potential visible light photoanode for PEC water splitting. The performance of BiVO₄ is restricted by fast charge recombination and slow oxygen evolution reaction kinetic. To address these issues, hierarchical SnO₂ (HSN) mesoporous support is developed via a novel sol-electrophoretic approach, and BiVO₄ film is decorated with silver nanorods (Ag NRs). The photocurrent density of HSN/BiVO₄ photoanode is 3.98 mA/cm² at 1.23 V vs. reversible hydrogen electrode (RHE) and onset potential (V_onset) of 0.5 V vs. RHE. The PEC performance is attributed to the appropriate band alignment between SnO₂ and BiVO₄, as well as the hierarchical structure of SnO₂. Ag-HSN/BiVO₄ photoanode shows photocurrent density of 4.30 mA/cm² at 1.23 V vs. RHE and V_onset of 0.28 V vs. RHE. The enhanced photocurrent and negatively shifted V_onset can be attributed to radiative localized surface plasmon resonance decay and catalytic effect of Ag NRs, respectively.     

Photoanode of LDH catalyst decorated semiconductor heterojunction of BiVO4/CdS to enhance PEC water splitting efficiency

BiVO₄ is an ideal photoanode material for solar-driven photoelectrochemical (PEC) water splitting but it easily suffers from the recombination of photogenerated electrons and holes due to its low carrier mobility thus cause low efficiency of PEC water splitting. Herein, the BiVO₄/CdS/NiCo-LDH photoanode was prepared by combining methods of metal organic decomposition, chemical and electrodeposition. The photoanode photocurrent density reaches 2.72 mA cm⁻² at 1.23 V (vs. RHE), which is 3.6 folds of pure BiVO₄ photoanode and onset potential shifts 450 mV toward cathodic. The incident photon-to-electron conversion efficiency (IPCE) value is 2.86 folds of BiVO₄, the calculated photon–to–current efficiency (ABPE) is 1.24% at 0.62 V (vs. RHE). The obtained results are higher than that of most BiVO₄ based photoanodes published so far. The enhancement benefits from increase of visible light absorption capacity, enhancement of separation efficiency of photoexcited electron-hole and fast transfer of holes accumulated on electrode/electrolyte surface for water oxidation, which has been confirmed by calculating carrier density and carrier transport rate.     

N-doped carbon wrapped CoFe alloy nanoparticles with MoS2 nanosheets as electrocatalyst for hydrogen and oxygen evolution reactions

Developing efficient and cost-effective transition metal-based electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is crucial to generate clean and renewable hydrogen energy. The construction of hybrid catalysts with multiple active sites is an effective approach to promote catalytic performance. Herein, a molybdenum disulfide (MoS₂)-based hybrid with N-doped carbon wrapped CoFe alloy (MoS₂/CoFe@NC) was synthesized through a typical hydrothermal method. The MoS₂/CoFe@NC exhibits excellent electrocatalytic performance with overpotentials of 172 mV for HER and 337 mV for OER at 10 mA cm⁻², and long-term stability of 24-h electrolytic reaction in 1 M KOH solution. The chemical coupling between MoS₂ and CoFe@NC provides improved electronic structures and more accessible active sites. The CoFe@NC substrate accelerates the charge transfer to MoS₂ through a synergistic effect. This work demonstrates that the CoFe@NC is a promising substrate for depositing MoS₂ nanosheets (NSs) to achieve excellent catalytic performance for both HER and OER.     

Nanostructured Zn-Fe2O3 thin film modified by Fe-TiO2 for photoelectrochemical generation of hydrogen

Nanostructured semiconductor thin films of Zn-Fe₂O₃ modified with underlying layer of Fe-TiO₂ have been synthesized and studied as photoelectrode in photoelectrochemical (PEC) cell for generation of hydrogen through water splitting. The Zn-Fe₂O₃ thin film photoelectrodes were designed for best performance by tailoring thickness of the Fe-TiO₂ film. A maximum photocurrent density of 748 μA/cm² at 0.95 V/SCE and solar to hydrogen conversion efficiency of 0.47% was observed for 0.89 μm thick modified photoelectrode in 1 M NaOH as electrolyte and under 1.5 AM solar simulator. To analyse the PEC results the films were characterized for various physical and semiconducting properties using XRD, SEM, EDX and UV–Visible spectrophotometer. Zn-Fe₂O₃ thin films modified with Fe-TiO₂ exhibited improved visible light absorption. A noticeable change in surface morphology of the modified Zn-Fe₂O₃ film was observed as compared to the pristine Zn-Fe₂O₃ film. Flatband potential values calculated from Mott–Schottky curves also supported the PEC response.