Molybdenum doping effects for bismuth vanadate photocatalysts on electrochemical performances using the solution process

Metal-doping is widely used to enhance charge generation and reduce carrier recombination of BiVO₄ for catalyzing water oxidation. In this work, different Mo-doping levels are applied to synthesize Mo-doped BiVO₄ (MBVO) on conductive glasses. Molybdenum plays multiple roles of dopant, structure-confined mediator, and sources for forming additional semiconductor. The MBVO with lower Mo-doping levels presents bi-layered structure with nanowire overlayer and nanorod underlayer, which can develop facile one-dimensional charge-transfer paths and efficient heterojunction. With preferable design of nanowire and nanorod layers, the high light absorption, small charge-transfer resistance and high carrier density are attained for MBVO with 1% Mo-doping, which shows the highest photocurrent density of 2.5 mA/cm² at 1.23 V_RHE, smallest onset potential of 0.22 V_RHE and highest maximum photoconversion efficiency of 2.20%. This work carefully illustrates the function of molybdenum and firstly constructs the unique bi-layered structure for MBVO to display outstanding photoelectrochemical performance for catalyzing water oxidation.     

Enhancement of photoconversion efficiency and light harvesting ability of TiO2 nanotube-arrays with Cu2ZnSnS4

We report an underpotential deposition (UPD) route of Cu₂ZnSnS₄/TiO₂ nanotube arrays (TiO₂-NTAs) in which the Kesterite (Cu₂ZnSnS₄) was employed as a sensitizer to enhance the photoconversion efficiency of the TiO₂-NTAs. Cu₂ZnSnS₄ was simultaneously coated on TiO₂-NTAs by depositing its constituent metals from the precursor ions via electrochemical atomic layer deposition (EC-ALD) and subsequent annealing. The detailed synthesis process, the surface morphology, crystalline structure, photoelectrochemical properties and hydrogen production rate of the as-prepared Cu₂ZnSnS₄/TiO₂-NTAs were discussed. Thickened TiO₂ nanotubes were observed, suggesting that the Cu₂ZnSnS₄ coating was about 5 ± 0.5 nm. The results showed that the light harvesting of TiO₂-NTAs has an obvious improvement after sensitizing them with Cu₂ZnSnS₄. In comparison with pure TiO₂-NTAs, a two-fold increment in photoconversion efficiency was achieved using the composite of Cu₂ZnSnS₄/TiO₂-NTAs. The novel photoanode of CZTS/TiO₂ NTAs achieved a maximum hydrogen generation rate of 49 ml h^?1 cm^?2.     

Appropriate strategies for determining the photoconversion efficiency of water photoelectrolysis cells: A review with examples using titania nanotube array photoanodes

Proper determination of light to chemical energy conversion efficiency of a photoelectrochemical cell is critical in evaluating its performance. Since the demonstration of photocatalytic water splitting using semiconductor electrodes, many strategies have been suggested and employed for the determination of photoconversion efficiency. We review these approaches as well as factors limiting ideal case efficiencies. Cell efficiency values are found to vary considerably depending upon the errors involved in the basic assumptions and measurement procedures. With researchers using different expressions for efficiency calculation, the values can be inconsistent and a direct comparison meaningless; we demonstrate this with the help of photocurrent data obtained from a photoelectrolysis cell employing titania nanotube array photoanodes. We find, and demonstrate, that realistic solar photoconversion efficiencies can be estimated with the help of incident photon to electron conversion efficiency (IPCE) values and solar irradiance data using the expression: ratio of the net power output to the power supplied by the incident light, where the net power output is the difference between the maximum electrical power available from the hydrogen produced and the power supplied by an external source. The power from the external source is determined by taking product of the photocurrent and the potential difference between the working and counter electrodes.     

Visible light driven photosplitting of water using one dimensional Mg doped ZnO nanorod arrays

In the present work, we have introduced Mg doped ZnO nanorods based photoanodes for photoelectrochemical water splitting applications. Vertically aligned Mg doped ZnO nanorods were fabricated by sol-gel and hydrothermal technique. The as-prepared nanorod samples exhibited hexagonal wurtzite structure as confirmed from XRD measurements. We achieved a photocurrent density of 0.35 mA/cm² at 1.5 V vs. Ag/AgCl for 10% Mg doped ZnO photoanode which is 9 times higher than that of undoped ZnO nanorods (0.03 mA/cm²). Incorporation of Mg resulted in faster charge transport and longer life time of electrons with reduced recombination rate. Mg dopant tuned the optical band gap of ZnO and increased the carrier concentration boosting the PEC performance of the photoanodes. Since seawater is one of the most abundant natural resource on earth, we further carried out seawater splitting of 10MgZ under visible light illumination which indicated its high photostability in natural seawater for 5 h of continuous illumination.     

Dye sensitized solar cells using freestanding TiO2 nanotube arrays on FTO substrate as photoanode

The front-side illuminated dye-sensitized solar cells with TiO₂ nanotube arrays/FTO based photoanode were fabricated by a simple detachment and transfer method. The morphology and crystalline phase of the TiO₂ nanotubes were studied by scanning electron microscopy and X-ray diffraction. A photoconversion efficiency of 1.78% was obtained using TiO₂-nanotube/FTO glass as a photoanode. It was found that the average tubes diameter increased with anodizing voltage, and higher photoconversion efficiency was obtained for nanotubes synthesized at the higher voltage.     

Photoresponse of n-TiO2 thin film and nanowire electrodes

Thin film and nanowire electrodes of n-type titanium oxide (n-TiO₂) were fabricated and their photoresponses towards water-splitting reaction were studied. A more than twofold increase in maximum photoconversion efficiency was observed when a single-layer thin film of n-TiO₂ was replaced by nanowires. Highest photoconversion efficiency was observed at an applied potential of 0.61 V vs. E_aoc where the electrode potential at open circuit, E_aoc was found to be −1.0 V SCE⁻¹ at illumination intensity of 40 mW cm⁻² in 5.0 M KOH solution. The maximum photoconversion efficiency was approximately of another twofold increase for water splitting at nanowire electrodes when methanol was used as a sacrifacial hole scavenger (depolarizing agent) in the electrolyte. Also, this maximum photoconversion efficiency was found at a lower applied potential of 0.41 V vs. E_aoc in presence of methanol. The band-gap energy of the n-TiO₂ films and nanowires annealed at 750°C was found to be 2.98 eV, which indicates their rutile structure.     

Photogalvanic behaviour of [Cr2O2S2(1-Pipdtc)2(H2O)2] in aqueous DMF

The photogalvanic behaviour of [Cr₂O₂S₂(1-Pipdtc)₂(H₂O)₂] was performed in a Honda Cell using 100% DMF and different percentages of water-DMF systems. The maximum potential of 200 mV with 18 μA of current was generated in DMF. The experiments were performed at different pH conditions and added dyes. The system was found to generate a maximum potential of 86, 67 and 36 mV at pH∼4, 7 and 8, respectively, at 65 °C. Malachite green and methylene blue (10⁻⁴ M) produced maximum potential of 100 and 82 mV, respectively. The system was reversible when the irradiated solution was aerated immediately. When the solution was kept in dark for a long time (12 h) and then aerated, the solution was found to be irreversible. The Cr(V) is photoreduced to Cr(IV) with the light irradiation and the unstable Cr(IV) reverts to Cr(V) by aerial oxidation.     

An efficient approach to control the morphology and the adhesion properties of anodized TiO2 nanotube arrays for improved photoconversion efficiency

We investigate an efficient approach to control the surface morphology and the geometrical parameter of TiO₂ nanotubes, while improving its adhesion to underlying titanium substrate, which is the key to successful fabrication of electro-optical devices. By controlling the water content and using the previously used electrolyte, nanotube arrays with surface morphology, geometrical parameters and adhesion properties relevant to required applications can be achieved. On the basis of these results, a mechanism of chemical dissolution rate is discussed in detail, in particular, with respect to the variation of nanotube length, presence of surface aggregation on the top surface, and the adhesion of nanotubes to underlying titanium substrate. It is found that the enhanced chemical dissolution rate can help to dissolve off the surface aggregation parts and reduce the internal stress at the barrier layer–metal interface. As a result this adhesion characteristic between the nanotubes and the underlying metal layer can be significantly improved. The contrastive photoelectrochemical experiments suggested that the photoconversion efficiency greatly depends on the geometric roughness factor of nanotubes and its adhesion to underlying metal layer. It shows that the 3.0% water content in previously used electrolytes has a combinative advantage to meet the optimal condition of photoconversion efficiency.     

Photovoltaic properties of PSi impregnated with eumelanin

A bulk heterojunction of porous silicon and eumelanin, where the columnar pores of porous silicon are filled with eumelanin, is proposed as a new organic-inorganic hybrid material for photovoltaic applications. The addition of eumelanin, whose absorption in the near infrared region is significantly higher than porous silicon, should greatly enhance the light absorption capabilities of the empty porous silicon matrix, which are very low in the low energy side of the visible spectral range (from about 600 nm downwards). The experimental results show that indeed the photocarrier collection efficiency at longer wavelengths in eumelanin-impregnated samples is clearly higher with respect to empty porous silicon matrices.