Crystal Facet Engineering of TiO2 Nanostructures for Enhancing Photoelectrochemical Water Splitting with BiVO4 Nanodots

Highlights Two types of BiVO4/TiO2 heterostructure photoanodes comprising TiO2 nanorods (NRs) and TiO2 nanoflowers (NFs) with different (001) and (110) crystal facets, respectively, were designed. The higher photoactivity of BiVO4/TiO2 NFs than BiVO4/TiO2 NRs was attributed to the improvement of charge separation by the TiO2 NFs. The formation of type II band alignment between BiVO4 nanodots and TiO2 NFs expedited electron transport and reduced charge recombination.Abstract Although bismuth vanadate (BiVO4) has been promising as photoanode material for photoelectrochemical water splitting, its charge recombination issue by short charge diffusion length has led to various studies about heterostructure photoanodes. As a hole blocking layer of BiVO4, titanium dioxide (TiO2) has been considered unsuitable because of its relatively positive valence band edge and low electrical conductivity. Herein, a crystal facet engineering of TiO2 nanostructures is proposed to control band structures for the hole blocking layer of BiVO4 nanodots. We design two types of TiO2 nanostructures, which are nanorods (NRs) and nanoflowers (NFs) with different (001) and (110) crystal facets, respectively, and fabricate BiVO4/TiO2 heterostructure photoanodes. The BiVO4/TiO2 NFs showed 4.8 times higher photocurrent density than the BiVO4/TiO2 NRs. Transient decay time analysis and time-resolved photoluminescence reveal the enhancement is attributed to the reduced charge recombination, which is originated from the formation of type II band alignment between BiVO4 nanodots and TiO2 NFs. This work provides not only new insights into the interplay between crystal facets and band structures but also important steps for the design of highly efficient photoelectrodes

Composite of bentonite/CoFe2O4/hydroxyapatite for adsorption of Pb (II)

In this contribution, a composite from bentonite (B), CoFe _2 O _4 (CF), and hydroxyapatite (HAP) was developed by chemical synthesis route for adsorption of a lead ion, Pb (II) from wastewater. Initially, a composite of B/CF was synthesized by varying the weight ratio of CF, i.e., (1– x )B/( x )CF ( x  = 0.05, 0.15, 0.25, 0.50), followed by ternary composite synthesis, which was formulated from the sample of (0.85B/0.15CF) and different weight ratios of HAP, i.e., (1– y )[(0.85B/0.15CF)]/ y HAP where y  = 0.25, 0.35 and 0.45 weight ratios of HAP. The sample of 0.85B/0.15CF was found to be optimal in its adsorption capacity of about 20 mg g ^−1 from the binary composite samples, while among the ternary composites, a sample with a composition of 0.65[(0.85B/0.15CF)]/0.35HAP revealed an optimum adsorption capacity of about 36 mg g ^−1 , which was then selected for further studies. The adsorption kinetics of Pb (II) by the optimum 0.65[(0.85B/0.15CF)]/0.35HAP sample was studied at different contact times from 30–120 min, where the equilibrium was reached at around 90 min of contact time and the kinetic behavior adopted Pseudo-second order adsorption mechanism. The initial concentration of Pb (II) was also varied from 50–200 mg l ^−1 to study the adsorption isotherm, which resulted that adsorption capacity of 0.65[(0.85B/0.15CF)]/0.35HAP towards Pb (II) was increased to about 66 mg g ^−1 and the adsorption isotherm data best fitted with Langmuir adsorption isotherm model. Therefore, the result of this study pinpoints that the present composite material is a potential candidate for the adsorption of Pb (II) ion

A wafer-scale antireflective protection layer of solution-processed TiO2 nanorods for high performance silicon-based water splitting photocathodes

Sustainable and efficient conversion of solar energy to transportable green energy and storable fuels, hydrogen, represents a solution to the energy crisis and reduces the consumption of fossil fuels, which are mainly responsible for the rise in global temperature. Solar water splitting using semiconductors, such as silicon, is promising to satisfy the global energy demand by producing hydrogen molecules. However, the solar to hydrogen conversion efficiency of a silicon photoelectrode is suppressed by overpotential, high reflectance and/or instability in liquid electrolytes. Herein, we report the synthesis of multifunctional solution-processed TiO2 nanorods on a 4-inch p-silicon wafer with controllable heights and diameters for highly efficient water splitting photocathodes. The solution-processed passivation layer of TiO2 nanorods reduces the overpotential of the silicon photocathode due to its catalytic properties. The TiO2 NRs also dramatically improves the light absorption of silicon due to the antireflective ability of the nanorods. The reflectance of silicon is decreased from 37.5% to 1.4% and enhances the saturated photocurrent density. The Pt-decorated (1-2.5 nm diameter) TiO2 nanorods/p-Si photocathodes show a short circuit current density of up to 40 mA cm(-2), an open circuit voltage similar to 440 mV and incident photon to current conversion efficiency of >90% using 0.5 M H2SO4 electrolyte with simulated 1 sun irradiation. The heterostructure photocathodes are stable for more than 52 h without noticeable degradation and an ideal regenerative cell efficiency of 2.5% is achieved.1118sciescopu

Toward High-Performance Hematite Nanotube Photoanodes: Charge-Transfer Engineering at Heterointerfaces

Vertically ordered hematite nanotubes are considered to be promising photoactive materials for high-performance water-splitting photoanodes. However, the synthesis of hematite nanotubes directly on conducting substrates such as fluorine-doped tin oxide (FTO)/glass is difficult to be achieved because of the poor adhesion between hematite nanotubes and FTO/glass. Here, we report the synthesis of hematite nanotubes directly on FTO/glass substrate and high-performance photoelectrochemical properties of the nanotubes with NiFe cocatalysts. The hematite nanotubes are synthesized by a simple electrochemical anodization method. The adhesion of the hematite nanotubes to the FTO/glass substrate is drastically improved by dipping them in nonpolar cyclohexane prior to postannealing. Bare hematite nanotubes show a photocurrent density of 1.3 mA/cm² at 1.23 V vs a reversible hydrogen electrode, while hematite nanotubes with electrodeposited NiFe cocatalysts exhibit 2.1 mA/cm² at 1.23 V which is the highest photocurrent density reported for hematite nanotubes-based photoanodes for solar water splitting. Our work provides an efficient platform to obtain high-performance water-splitting photoanodes utilizing earth-abundant hematite and noble-metal-free cocatalysts