Photoelectrochemical properties of rhodamine-C18 sensitized p-CuSCN photoelectrochemical cell (PEC)

Surface states in p-type Cu^I thiocyanate (CuSCN) were detected from I−V characteristics, diffuse reflectance spectra, and photocurrent action spectra. The p-CuSCN films are sensitized by rhodamine with octadecyl-alkyl chain, and the sensitized photocurrent is observed with the visible light illumination. In spite of the surface states in p-CuSCN, the maximum photocurrent quantum efficiency (gf_max) at λ = 560 nm, in 1 × 10⁻⁴ M KI + I₂ solution, pH = 6, reached 8.6%, where the surface dye concentration of photocathode Cu/p-CuSCN/Dye was 1.1 × 10¹⁴ molecules cm⁻². Photocathodes were biased at −0.25 V versus AgCl/Ag to give a zero dark current. From the variation of φ values with the reduction potential of electron acceptors, the cathodic sensitization mechanism presented is further confirmed.     

Branched multiphase TiO2 with enhanced photoelectrochemical water splitting activity

An efficient hierarchical structure, nano-branch containing anatase TiO₂ nanofibers and rutile nanorods, was prepared via the combination of the electrospinning and hydrothermal processes. This novel configuration of TiO₂ multiphase possessed higher surface area, roughness, and fill factors compared with each single phase component prepared in the same condition, which significantly enhanced its light absorption. Our experimental results showed that within the interface of multiphase TiO₂, the heterojunction promoted the charge separation and improved the charge transfer rate, leading to higher efficiency for photoelectrochemical water splitting. The photocurrent density of the nano-branched TiO₂ electrode could reach 0.95 mA/cm², which was almost twice as large as that of the pristine TiO₂ nanorod. Our work provides a simple and feasible routine to synthesize complex TiO₂ nanoarchitectures, which lays a foundation for improving energy storage and conversion efficiency of TiO₂-based photoelectrodes.     

Single-step fabrication of phase-controllable nanocrystalline TiO2 films for enhanced photoelectrochemical water splitting and dye-sensitized solar cells

We introduce a single-step procedure for growing a phase-controllable bilayer-structured TiO₂ film directly onto transparent conductive oxide glass by precipitation from hydrolysis of TiCl₄ in acid solution containing sulfate ions. The obtained bilayer-structured film with anatase nanoparticles in the inner layer which provide high surface area, and an outer layer of larger rutile particles for incident light scattering. In both the water splitting and the dye-sensitized solar cells under AM 1.5 simulated solar light, the bilayer-structured film outperformed the single layer-structured films with either anatase or rutile TiO₂ alone by at least 50%.     

Ruthenium dye-sensitized SnO2/TiO2 coupled solar cells

The photoelectrochemical behaviors of RuL₂(NCS)₂ dye-sensitized SnO₂/TiO₂ coupled solar cell was studied and compared with TiO₂ single system. The coupled system shows higher incident photon-to-current conversion efficiency (IPCE) value than the single system. A maximum IPCE value in the coupled system with 3.5 μm-thick SnO₂ and 7 μm-thick TiO₂ attained 82.4% at 530 nm wavelength. The higher IPCE value in the coupled system is attributed to the charge separation by fast electron transfer process from the excited RuL₂(NCS)₂ dye to TiO₂ to SnO₂ in the system with different energy level.     

Photocurrent enhancement of interlocked LB film dye layers in a p-CuSCN sensitised photoelectrochemical cell

Dye sensitised photoelectrochemical (PEC) cells based on Cu/p-CuSCN/LB films have been studied with mixed Langmuir Blodgett (LB) films as the dye layer. The effects of mixed layers were investigated in detail by observing the changes of optical absorption and photocurrent in a PEC cell configuration. Enhancements in both optical absorption and photocurrent were found when a mixture of octadecyl methylviolet–C₁₈ (M–C₁₈) and dioctadecyl rhodamine (C₁₈–R–C₁₈) were deposited using the LB technique on p-CuSCN wide band gap semiconductor. The maximum photocurrent quantum efficiency of the PEC cell reached ≈36% in KI (10⁻² M)+I₂ (10⁻⁴ M) electrolyte solution when mixed LB films were used as the dye layer. Photocurrent enhancement is believed to be the enhancement of light absorption of the dye layers due to the interlocking of M–C₁₈ between the double C₁₈ chains of rhodamine.     

Layered WO3/TiO2 nanostructures with enhanced photocurrent densities

Layered WO₃/TiO₂ nanostructures, fabricated by magnetron sputtering, demonstrate significantly enhanced photocurrent densities compared to individual TiO₂ and WO₃ layers. First, a large quantity of compositions having different microstructures and thicknesses were fabricated by a combinatorial approach: diverse WO₃ microstructures were obtained by adjusting sputtering pressures and depositing the films in form of wedges; later layers of TiO₂ nanocolumns were fabricated thereon by the oblique angle deposition. The obtained photocurrent densities of individual WO₃ and TiO₂ films show thickness and microstructure dependence. Among individual WO₃ layers, porous films exhibit increased photocurrent densities as compared to the dense layer. TiO₂ nanocolumns show length-dependent characteristics, where the photocurrent increases with increasing film thickness. However, by combining a WO₃-wedge type layer with a layer of TiO₂ nanocolumns, PEC properties strikingly improve, by about two orders of magnitude as compared to individual WO₃ layers. The highest photocurrent that is measured in the combinatorial library of porous WO₃/TiO₂ films is as high as 0.11 mA/cm². Efficient charge-separation and charge carrier transfer processes increase the photoconversion efficiency for such films.     

Photoelectrochemical properties of ruthenium dye-sensitized nanocrystalline SnO2:TiO2 solar cells

The photoelectrochemical properties of RuL₂(NCS)₂ dye-sensitized nanocrystalline SnO₂:TiO₂ coupled and composite solar cells are reported. The coupled (bilayer) system shows higher incident photon-to-current conversion efficiency (IPCE) value than the composite (mixture) system. A maximum IPCE value attained 82.4% at 530 nm wavelength in the coupled system with 3.5 μm-thick SnO₂ and 7 μm-thick TiO₂. The higher IPCE value in the coupled system is attributed to the promotion of the charge separation by fast electron transfer process in the SnO₂/TiO₂/RuL₂(NCS)₂ system with different energy levels, different conduction band edge energy positions.     

NiO decorated Mo:BiVO4 photoanode with enhanced visible-light photoelectrochemical activity

Photoelectrochemical water splitting has attracted increasing attention recently in the perspective of clean and sustainable energy economy. Herein, we reported the synthesis of NiO functionalized Mo doped BiVO₄ (denoted as NiO/Mo:BiVO₄) nanobelts and their enhanced photoelectrochemical activity for efficient water oxidation. The prepared NiO/Mo:BiVO₄ p–n junction structure showed much higher water splitting activity than the pristine BiVO₄ and Mo:BiVO₄. Such obvious enhancement are due to the increased donor density by doping with Mo and fast separation of the photoexcited electron–hole pairs by the novel p–n junction composite structure. Under light irradiation, photoexcited holes in the conduction band (CB) of Mo:BiVO₄ will conveniently transfer to the p-type NiO with the effect of the inner electric field. Meanwhile, the holes would oxidize the water into oxygen and the electrons transfer to the counter electrode (Pt electrode) to produce hydrogen. This novel p–n junction structure could open up new opportunities to develop high-performance photoanode for water splitting.     

H2 evolution from a photoelectrochemical cell with n-Cu2O photoelectrode under visible light irradiation

H₂ evolution was observed for the first time from a photoelectrochemical cell using an n-type Cu₂O photoelectrode under visible light irradiation. Three-electrode configuration was used in the photoelectrochemical cell to observe H₂ evolution. AgCl/Ag calomel electrode and a platinum plate were used as the reference and counter electrodes, respectively. Fe²⁺/Fe³⁺ redox couple was used as the electrolyte. H₂ evolution was visible on the platinum electrode in the photoelectrochemical cell.     

Analysis of photocurrent in the i-layer of GaAs-based n–i–p solar cell

A numerical investigation of the intrinsic layer effect on the improvement of GaAs n–i–p solar cell performances is presented. Solution of Poisson's equation together with continuity equations for electrons and holes allows the determination of carrier's density, electric field and recombination profiles within the i-layer. The analysis examines the effect of i-layer thickness on the electric field, recombination rate and collection efficiency. It is found that increasing the i-layer thickness increases the absorption while it reduces the electric field and increases the recombination rate. The three competing parameters have to be monitored simultaneously so as to obtain an optimal thickness. To achieve this, the variation of the total photocurrent is used as indicator. The photocurrent shows a sharp increase in the domain of very thin i-layers (<0.5 μm) then a saturation is reached for thicker layers (>1 μm), the simulation is performed for thicknesses up to 2 μm.     

Stability of the SnO2/MgO dye-sensitized photoelectrochemical solar cell

Dye-sensitized solar cells made of TiO₂ are extensively studied as a cheap alternative to conventional photovoltaic cells. The other familiar stable oxide material of similar band gap suitable for dye sensitization is SnO₂. Although cells based only of SnO₂ are prone to severe recombination losses, the cells made of SnO₂/MgO films where the SnO₂ crystallite is surface covered with an ultra-thin shell of MgO, deliver reasonably high efficiencies. It is found that SnO₂/MgO cells resist dye and electrolyte degradation better than TiO₂ cells. Furthermore, the ultra-thin barrier of MgO on SnO₂ remains intact during prolonged usage or storage of the cell.     

Chemical bath deposition synthesis of TiO2/Cu2O core/shell nanowire arrays with enhanced photoelectrochemical water splitting for H2 evolution and photostability

In this study, we have developed a facile chemical bath deposition (CBD) method to grow p-type Cu₂O nanoparticles on n-type TiO₂ nanowire arrays (TiO₂ NWAs) to fabricate TiO₂/Cu₂O core/shell heterojunction nanowire arrays (TiO₂/Cu₂O core/shell NWAs). When used as photoelectrode, the fabricated TiO₂/Cu₂O core/shell NWAs show improved photoelectrochemical (PEC) water splitting activity to pure TiO₂ NWAs. The effects of the CBD cycle times on the PEC activities have been studied. The TiO₂/Cu₂O core/shell heterojunction nanowire array photoelectrode prepared by cycling 5 times in the CBD process achieves the highest photocurrent of 2.5 mA cm^?2, which is 2.5 times higher than that of pure TiO₂ NWAs. In addition, the H₂ generation rate of this photoelectrode reaches to 32 μmol h^?1 cm^?2, 1.7 times higher than that of pure TiO₂ NWAs. Furthermore, the TiO₂/Cu₂O core/shell heterojunction nanowire array photoelectrode shows excellent photostability and achieves a stable photocurrent of over 2.3 mA cm^?2 during long light illumination time of 5 h. The enhanced photocatalytic activity of TiO₂/Cu₂O core/shell heterojunction nanowire array photoelectrode is attributed to the synergistic actions of TiO₂ and Cu₂O for improving visible light harvesting, and efficient transfer and separation of photogenerated electrons and holes.     

A novel photoelectrochemical cell with self-organized TiO2 nanotubes as photoanodes for hydrogen generation

A photoelectrochemical (PEC) cell with an innovative design for hydrogen generation via photoelectrocatalytic water splitting is proposed and investigated. It consisted of a TiO₂ nanotube photoanode, a Pt/C cathode and a commercial asbestos diaphragm. The PEC could generate hydrogen under ultraviolet (UV) light-excitation with applied bias in KOH solution. The Ti mesh was used as the substrate to synthesize the self-organized TiO₂ nanotubular array layers. The effect of the morphology of the nanotubular array layers on the photovoltaic performances was investigated. When TiO₂ photocatalyst was irradiated with UV-excitation, it prompted the water splitting under applied bias (0.6 V vs. Normal Hydrogen Electrode, NHE.). Photocurrent generation of 0.58 mA/cm² under UV-light irradiation showed good performance on hydrogen production.     

Enhancement of photoelectrochemical activity of Fe2O3 nanowires decorated with carbon quantum dots

Owing to its up-conversion photoluminescence, photo-induced electron transfer property, and excellent conductivity, carbon quantum dots (CQDs) have been established as effective sensitizers in combination with Fe₂O₃ nanowires for enhancing the catalytic activity of photoelectrochemical water oxidation. In comparison to pristine Fe₂O₃ nanowires, Fe₂O₃ nanowires decorated with CQDs demonstrate 27 orders of magnitude increase in photocurrent density at 0.23 V vs. Ag/AgCl. The mechanism of enhanced photoelectrochemical activity of CQDs/Fe₂O₃ composite was also investigated. Thereby, it is confirmed that the enhanced optical absorption, accelerated interfacial charge carrier transfer and effective separation of photogenerated electron-hole pairs induced by CQDs decoration account for the enhancement of CQDs/Fe₂O₃ nanowire arrays in photoelectrochemical application.     

Fabrication of n-Cu2O electrodes with higher energy conversion efficiency in a photoelectrochemical cell

We observed an n-type photoresponse in cuprous oxide films, which were prepared by a simple method of immersing copper plates in a HCl solution of 3 pH at 40°C temperature, when they were used in a PEC cell. This photoresponse was much higher than the previously published values for n-Cu₂O electrodes which were prepared by other methods. The photocurrent obtained was in the order of 0.3 mA/cm² when the cell was illuminated with light intensity of 50 mW/cm². (The semiconductor electrode was biased to get a zero dark current.) The power conversion efficiency of the cell was 0.01%. The maximum quantum efficiency obtained was 15%. It is hoped that these values could be improved with a better understanding of the photoelectrochemical properties of the cell.     

Unique photoelectrochemical behavior of TiO2 nanorods wrapped with novel titanium Oxy-Nitride (TiOxNy) nanoparticles

In this work, we developed novel titanium oxynitride (TiO_xN_y) nanoparticles with diameter of 25 ± 2 nm and crystalline size of ～15 nm on hydrothermally grown one-dimensional (1D) TiO₂ nanorod (TNR) arrays. Herein, the TiO_xN_y nanoparticles were synthesized by facile nitridation using TiO₂ powder at 100% NH₃ gas atmosphere. Titanium oxynitride composed of potentially energetic metal-nitrogen bonds (TiN), compared to the weaker TiO bond, becomes chemically stable in the alkaline environment, and is considered as a suitable material for photoelectrochemical (PEC) system. The PEC performance of TiO_xN_y decorated TNR (abbreviated as TiO_xN_y @TNR) films was evaluated in 0.1 M KOH solution under solar illumination condition, and achieved the potentially high photocurrent density (J) of 2.1 mA/cm² at 1.23 V versus reversible hydrogen electrode (RHE) (abbreviated as V_RHE) in the TiO_xN_y@TNR arrays, in comparison with the poor photoresponse (0.7 mA/cm² at 1.23 V_RHE) of the pristine TNR arrays. A nearly three-fold enhancement was attained in the TiO_xN_y decorated TNR arrays, attributed to the high visible light absorption and fast carrier separation, due to the hybridization with the visible active TiO_xN_y nanoparticles in the cascading band alignment between the TiO_xN_y and TNR materials. Furthermore, the introduction of TiO_xN_y layer on the TNR surface quite reduces the interfacial resistance in the solid-liquid interface region, and further, the TiO_xN_y layer contributes to the passivation of the surface states (e.g., defect, trap sites etc.) where the charge recombination reaction frequently happens, leading to the improvement of PEC performance.     

Ag grid induced photocurrent enhancement in WO3 photoanodes and their scale-up performance toward photoelectrochemical H2 generation

The hydrogen generation from photoelectrochemical (PEC) water splitting under visible light was investigated using large area tungsten oxide (WO₃) photoanodes. The photoanodes for PEC hydrogen generation were prepared by screen printing WO₃ films having typical active areas of 0.36, 4.8 and 130 cm² onto the conducting fluorine-doped tin oxide (FTO) substrates with and without embedded inter-connected Ag grid lines. TiO₂ based dye-sensitized solar cell was also fabricated to provide the required external bias to the photoanodes for water splitting. The structural and morphological properties of the WO₃ films were studied before scaling up the area of photoanodes. The screen printed WO₃ film sintered at 500 °C for 30 min crystallized in a monoclinic crystal structure, which is the most useful phase for water splitting. Such WO₃ film revealed nanocrystalline and porous morphology with grain size of ∼70-90 nm. WO₃ photoanode coated on Ag grid embedded FTO substrate exhibited almost two-fold degree of photocurrent density enhancement than that on bare FTO substrate under 1 SUN illumination in 0.5 M H₂SO₄ electrolyte. With such enhancement, the calculated solar-to-hydrogen conversion efficiencies under 1 SUN were 3.24% and ∼2% at 1.23 V for small (0.36 cm²) and large (4.8 cm²) area WO₃ photoanodes, respectively. The rate of hydrogen generation for large area photoanode (130.56 cm²) was 3 mL/min.     

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.     

Modelling and development of photoelectrochemical reactor for H2 production

Photoelectrolysis of aqueous solutions, using one or more semiconducting electrodes in a photoelectrochemical reactor, is a potentially attractive process for hydrogen production because of its prospectively high energy efficiency, simplicity and potentially low cost. The design requirements and preliminary results of modelling a photoelectrochemical (PEC) reactor are described. Potential and current density distributions, due to ohmic potential losses in thin (non-photo) anodes on poorly conducting fluoride-doped tin oxide coated glass substrates, were modelled. The predicted current densities decayed rapidly from the terminals at the edges, towards the centre of a 0.1 × 0.1 m² anode, so limiting scale-up with such substrates. Spatial distributions of dissolved oxygen concentrations were also modelled, aiming to define operating conditions that would avoid forming bubbles, which reflect light specularly decreasing photon absorption efficiencies of photoelectrodes. The implications for the future optimization of the reactor are discussed.     

Influence of chelating agents on the growth and photoelectrochemical responses of chemical bath-synthesized AgIn5S8 film electrodes

An aqueous method for the deposition of silver-indium-sulfide ternary semiconductor film electrodes is presented. Various deposition parameters, such as reaction temperature and molar ratios of different chelating agents, were changed in order to grow uniform and adherent films on indium-tin-oxide glass substrates. With a reaction temperature higher than 65 °C, a film composed of AgIn₅S₈ was grown on the substrates in our experimental conditions. The direct and indirect energy band gaps of samples prepared in this study varied from 1.70 to 1.97 eV and 1.61 to 1.72 eV, respectively. The maximum photocurrent density of samples in this study reached 3.0 mA/cm² at an external potential of +1.0 V vs. a Pt electrode under illumination using a 300 W Xe lamp system with the light intensity set at 100 mW/cm².