CuInS2/SiNWs/Si composite material for application as potential photoelectrode for photoelectrochemical hydrogen generation

This work aims at developing a new composite material based on nanosized semiconducting CuInS₂ (CIS) particles combined with silicon nanowires grown on a silicon substrate (SiNWs/Si) for photoelectrochemical (PEC)-splitting of water. The CIS particles were prepared via a colloidal method using N-methylimidazole (NMI) as the solvent and an annealing treatment. The SiNWs were obtained by chemical etching of silicon (100) substrates assisted by a metal. The CIS/SiNWs/Si composite material was obtained by deposition of an aliquot of a suspension of CIS particles onto the SiNWs/Si substrate, using spin coating followed by a drying step. The XRD pattern demonstrated that CuInS₂ grows in the tetragonal/chalcopyrite phase, while SiNWs/Si presents a cubic structure. The SEM images show semi-spherical particles (～10 nm) distributed on the surface of silicon nanowires (～10 μm). The EIS measurements reveal n-type conductivity for CIS, SiNWs/Si and CIS/SiNWs/Si materials, which could favour the oxidation reaction of water molecules.     

A subtle review on the challenges of photocatalytic fuel cell for sustainable power production

The revolution in the arena of functional materials for the development of well advanced engineered photocatalyst can efficiently harness photon energy from a wide spectrum of electromagnetic radiation. These next-generation smart materials would be a spectacular approach in designing devices such as photovoltaic cells, photoelectrochemical cells, and photocatalytic fuel cells. Photocatalytic oxidation of water or wastewater for concurrent production of hydrogen and electric current has turned out as a principal concept for the construction of modern photocatalytic fuel cells (PFCs). Such PFCs mimics reverse photosynthesis process where electrical energy is generated from organic pollutants. In recent years many reviews on focusing the design, fabrication, and theoretical efficiency of the PFCs have been published. Hence the present review is aimed to unveil the wall-to-wall information starting from fundamentals spanning to working principles, structural configuration, electrochemical degradation of pollutants and photoelectrochemical properties, electron transport, thermodynamic behavior and columbic efficiency of studied PFCs.     

Improved photon management in a photoelectrochemical cell with Nd-modified TiO2 thin film photoanode

Photon management involving particularly an up-conversion process is proposed as a relatively novel strategy for improving the efficiency of hydrogen generation in photoelectrochemical cells (PEC) with wide-band gap photoanodes. Optically active photoanode has been constructed by electrodeposition of titanium dioxide nanopowders containing Nd³⁺ ions, synthesized via a sol-gel method, onto ITO/TiO₂(thin film) substrates. Thin films of TiO₂ have been deposited by means of RF magnetron sputtering in an ultra-high-vacuum system. X-ray diffraction, scanning electron microscopy, UV-VIS-NIR spectrophotometry, and photoluminescence have been applied to assess the properties of photoanodes. In experiments involving photon-assisted water splitting, an external up-converter containing Yb³⁺/Er³⁺ rare-earth ions has been used. Photocurrent as a function of voltage (V_B) under illumination with white light is relatively high (280 μA at V_B = 0 V) for pure TiO₂ thin films and it is not affected by the electrodeposition of TiO₂:Nd³⁺ powders. NIR-driven up-conversion with laser excitation at λ = 980 nm has been found responsible for a 13-fold increase in photocurrent at V_B = 0 V in the modified PEC configuration.     

Study of photoactivity of tungsten trioxide (WO3) for water splitting

The paper presents the properties of WO₃ films considering the possibility to build a photoelectrochemical cell (PECC) for hydrogen production. The photocurrent response of the PECC containing WO₃/TCO as photoanode and Pt as cathode was analysed. The morphology, crystalline structure and electrical aspects were investigated. Tungsten trioxide (WO₃) thin film with porous morphology and high crystallinity was obtained using the spray pyrolysis deposition technique.     

Effects of the incorporation of silver and gold nanoparticles on the photoanodic properties of rose bengal sensitized TiO2 film electrodes prepared by sol-gel method

Rose bengal-deposited TiO₂ film electrodes bearing dispersed Ag or Au nanoparticles were prepared by the sol-gel method. The dye-induced visible region photoresponse of the electrodes decreased with increasing Ag content up to a mole ratio of Ag/TiO₂ = 0.0207, while the UV photoresponse increased. On the other hand, the dye-induced visible region photoresponse decreased to a less extent by incorporation of a larger amount of Au particles of Au/TiO₂ = 0.06, along with decreased UV photoresponse. The effects of the metal particles on the dye sensitization of the electrodes were discussed in terms of band edge fluctuation induced by the surface metal particles, Schottky barriers at TiO₂/metal interfaces, and surface plasma resonance.     

Characterization of structured α-Fe2O3 photoanodes prepared via electrodeposition and thermal oxidation of iron

In this paper we explore the feasibility of using electrodeposition as a low-cost, versatile and easily upscalable technique for preparing α-Fe₂O₃ (hematite) photoanodes for water splitting applications. The photoelectrodes are prepared on transparent conducting glass substrates by electrodeposition of Fe, using a non-aqueous precursor solution at room temperature, followed by thermal oxidation in air. Variations in deposition parameters yield films with diverse morphologies. The effects of the different morphologies on the structural, optical, and photoelectrochemical properties are investigated by photocurrent measurements under AM1.5 illumination. The photocurrent could be improved by growing the first part of the Fe film at low current densities, yielding a dense underlayer, followed by the deposition of a more structured, porous film at high current densities. X-ray diffraction reveals that high deposition currents result in smaller crystallites and a (110) preferred orientation. This orientation is favorable when using hematite as a photoanode, since the conductivity in the [110] direction is known to be up to four orders of magnitude higher than in directions perpendicular to this.     

A hematite photoelectrode grown on porous and conductive SnO2 ceramics for solar-driven water splitting

Photoelectrochemical water splitting using solar energy is a highly promising technology to produce hydrogen as an environmentally friendly and renewable fuel with high-energy density. This approach requires the development of appropriate photoelectrode materials and substrates, which are low-cost and applicable for the fabrication of large area electrodes. In this work, hematite photoelectrodes are grown by aerosol assisted chemical vapour deposition (AA-CVD) onto highly-conductive and bulk porous SnO₂ (Sb-doped) ceramic substrates. For such photoelectrodes, the photocurrent density of 2.8 mA cm^-2 is achieved in aqueous 0.1 M NaOH under blue LED illumination (λ = 455 nm; 198 mW cm^-2) at 1.23 V vs. RHE (reversible hydrogen electrode). This relatively good photoelectrochemical performance of the photoelectrode is achieved despite the simple fabrication process. Good performance is suggested to be related to the three-dimensional morphology of the porous ceramic substrate resulting in excellent light-driven charge carrier harvesting. The porosity of the ceramic substrate allows growth of the photoactive layer (SnO₂-grains covered by hematite) to a depth of some micrometers, whereas the thickness of Fe₂O₃-coating on individual grains is only about 100–150 nm. This architecture of the photoactive layer assures a good light absorption and it creates favourable conditions for charge separation and transport.     

Efficient and economical dye-sensitized solar cells based on graphene/TiO2 nanocomposite as a photoanode and graphene as a Pt-free catalyst for counter electrode

Dye-sensitized solar cells have been fabricated by employing graphene/TiO₂ nanocomposites as photoanodes and graphene as a counter electrode. The mixing technique is used to prepare graphene/TiO₂ nanocomposites. The dispersion of graphene in TiO₂ is affirmed by transmission electron microscopy analysis. X-ray photoelectron spectroscopy is carried out to confirm the interstitial incorporation of carbon atoms in the TiO₂ matrix through O TiC and TiOC surface states. The electrochemical activity and stability of graphene as a catalyst for counter electrode are investigated by cyclic voltammetry and chronoamperometry measurements. Solar cells fabricated are characterized by photocurrent–voltage characteristic, Incident photon-to-current efficiency, and electrochemical impedance spectroscopy analyses. The solar cell assembled with 0.08%GR-TiO₂/N3/GR shows power conversion efficiency of 7.70%. This efficiency is superior to that of TiO₂/N3/Pt based solar cell (7.28%). The improvement in efficiency can be attributed to a fast electron transport, improved light harvesting efficiency, and enhanced electron collection at photoanodes.     

Physical and photoelectrochemical characterizations of hematite α-Fe2O3: Application to photocatalytic oxygen evolution

The physical properties and photoelectrochemical characterization of α-Fe₂O₃, synthesized by co-precipitation, have been investigated in regard to solar energy conversion. The optical gap is found to be 1.94 eV and the transition is indirectly allowed. The chemical analysis reveals an oxygen deficiency and the oxide exhibits n-type conductivity, confirmed by a negative thermopower. The plot log σ vs 1/T shows linearity in the range (400-670 K) with the donor levels at 0.14 eV below the conduction band and a break at ∼590 K, attributed to the ionization of the donors. The conduction occurs by small polaron hopping through mixed valences Fe^2+/3+ with an electron mobility μ_400 K of 10⁻³ V cm² s⁻¹. α-Fe₂O₃ exhibits long term chemical stability in neutral solution and has been characterized photoelectrochemically to assess its activity as bias-free O₂-photoanode. The flat band potential V_fb (−0.45V_SCE) and the electron density N_D (1.63 × 10¹⁸ cm⁻³) were determined, respectively, by extrapolating the linear part to C⁻² = 0 and the slope of the Mott Schottky plot. At pH 6.5, the valence band (+1.35V_SCE) is suitably positioned with respect to the O₂/H₂O level (+0.62 V) and α-Fe₂O₃ has been evaluated for the chemical energy storage through the photocatalytic reaction: (, ΔG = 213.36 kJ mol⁻¹). The best photoactivity occurs in solution (0.025 M, pH 8) with an oxygen rate evolution of 7.8 cm³ (g catalyst)⁻¹ h⁻¹.     

Efficient photoelectrochemical water splitting of CaBi6O10 decorated with Cu2O and NiOOH for improved photogenerated carriers

Broadening the light absorption and accelerating the separation of photogenerated electron-hole pairs is of crucial importance for strongly enhancing the photoelectrochemical (PEC) water splitting performances of photoelectrode. In this paper, a novel CaBi₆O₁₀/Cu₂O/NiOOH photoanode for photoelectrochemical water splitting is prepared, where, the NiOOH acts as water oxidation catalyst to accelerate water oxidation taking place in the interfaces between electrode and electrolyte, Cu₂O is chosen to extend the absorption range of the light absorber, enhancing an efficient separation and transfer of the electron-hole pairs. This triple CaBi₆O₁₀/Cu₂O/NiOOH photoanode negatively shifts the onset potential and exhibits an improved photocurrent density 1.89 mA·cm^?2 at 1.23 V vs RHE, which is 1.4 and 4.8 times higher compared to CaBi₆O₁₀/Cu₂O and CaBi₆O₁₀, respectively. More importantly, the CaBi₆O₁₀/Cu₂O/NiOOH electrode shows excellent photoelectrochemical stability in comparison with CaBi₆O₁₀/Cu₂O after 2 h irradiation. The amazing photoelectrochemical performance is due to the broader light absorption spectrum, the improved photogenerated carriers separation, transfer and consumption. The research results demonstrate a promising ternary semiconductor structure, which can improve photoelectrochemical performance effectively. Moreover, these results also imply that the CaBi₆O₁₀/Cu₂O/NiOOH heterojunction structure has a great potential application for photoelectrochemical water splitting systems.