Hui Ling Tan

BiVO4 {010} and {110} Relative Exposure Extent: Governing Factor of Surface Charge Population and Photocatalytic Activity

The {010} and {110} crystal facets of monoclinic bismuth vanadate (m-BiVO4) has been demonstrated to be the active reduction and oxidation sites, respectively. Here, we show using dual-faceted m-BiVO4 with distinctly different dominant exposed facets, one which is {010}-dominant and the other {110}-dominant, contrary to prediction, the former m-BiVO4 exhibits superior photooxidation activities. The population of photogenerated electrons and holes on the surface are revealed to be proportional to the respective surface areas of {010} and {110} exposed on m-BiVO4, as evidenced by steady-state photoluminescence (PL) measurements in the presence of charge scavengers. The better photoactivity of {010}-dominant m-BiVO4 is attributed to prompt electron transfer facilitated by the presence of more photogenerated electrons on the larger {010} surface. Additionally, the greater extent of electron trapping in {110}-dominant m-BiVO4 also deteriorates its photoactivity by inducing electron-hole pair recombination.

Alternative strategies in improving the photocatalytic and photoelectrochemical activities of visible light-driven BiVO4: a review

The research interest on bismuth vanadate (BiVO4) has heightened over the past decade due to its proven high activity for water oxidation and organic degradations under visible light. Although metal doping and water-oxidation cocatalyst loading have been widely demonstrated to be useful to overcome the poor electron transport and slow water oxidation kinetics of BiVO4, the efficiency of this material is still greatly limited by poor charge separation. Various efforts directed at modifying the surface and bulk properties to improve the performance of BiVO4-based materials have therefore been developed, including crystal facet engineering, coupling with graphitic carbon material, annealing treatment, and nanoscaling. This review aims to provide insights into the most recent progress in these strategies in regard to their influences on the charge separation, transport, and transfer aspects of BiVO4, all of which are crucial to govern photochemical conversion efficiency. Understanding of these charge kinetics in relation to the properties of BiVO4 is of fundamental importance for rational design of BiVO4 with optimum structures, which may serve as a general guideline for the fabrication of metal oxide photocatalysts.

Scaffolding an ultrathin CdS layer on a ZnO nanorod array using pulsed electrodeposition for improved photocharge transport under visible light illumination

An ultrathin layer of CdS (∼8 nm) was successfully coated on an array of vertically aligned ZnO nanorods using pulsed electrodeposition. The pulse was essential during the deposition of CdS to ensure equilibrium between diffusion and nucleation of CdS precursors. These ZnO nanorods functioned as a large contact base for the deposition of CdS. This enlarged interface between CdS and ZnO together with its close intimacy facilitated efficient charge transfer from the excited CdS to ZnO upon visible illumination. Owing to the high electron mobility of ZnO, it shuttled the electrons efficiently for enhanced photocurrent generation. Compared to the bare CdS film, the CdS–ZnO photoelectrode yielded a doubled anodic visible photocurrent density of 6 mA cm−2 at 0 V vs. Ag/AgCl. Photoluminescence spectroscopy and photoelectrochemical characterization showed that the charge recombination within CdS was suppressed and the appropriate band alignment favored the electron transfer.

Interfacing BiVO4 with Reduced Graphene Oxide for Enhanced Photoactivity: A Tale of Facet Dependence of Electron Shuttling

Efficient interfacial charge transfer is essential in graphene-based semiconductors to realize their superior photoactivity. However, little is known about the factors (for example, semiconductor morphology) governing the charge interaction. Here, it is demonstrated that the electron transfer efficacy in reduced graphene oxide-bismuth oxide (RGO/BiVO4 ) composite is improved as the relative exposure extent of {010}/{110} facets on BiVO4 increases, indicated by the greater extent of photocurrent enhancement. The dependence of charge transfer ability on the exposure degree of {010} relative to {110} is revealed to arise due to the difference in electronic structures of the graphene/BiVO4 {010} and graphene/BiVO4 {110} interfaces, as evidenced by the density functional theory calculations. The former interface is found to be metallic with higher binding energy and smaller Schottky barrier than that of the latter semiconducting interface. The facet-dependent charge interaction elucidated in this study provides new aspect for design of graphene-based semiconductor photocatalyst useful in manifold applications.

Exploring the Different Roles of Particle Size in Photoelectrochemical and Photocatalytic Water Oxidation on BiVO4.

Water oxidation on visible-light-active bismuth vanadate (BiVO4) has commonly been demonstrated to be viable in powder suspension (PS) and particulate photoelectrochemical (PEC) systems. Here, we demonstrate that particle size reduction, which is known to be efficacious in promoting charge carrier extraction and boosting surface active sites, has an opposite effect on BiVO4s photoactivity in the two systems. With three BiVO4 samples of distinctive particle sizes, smaller BiVO4 particle size is shown to be beneficial for enhancing PEC photocurrent generation, but deleterious for photocatalytic O2 evolution on suspended BiVO4. Such contrary effect of particle size in the PEC and PS systems is revealed to be due to the different governing factors of the systems: charge transport in the former and charge separation in the latter. Smaller particle size was found to enrich the interparticle and the particle/FTO substrate contacts which improve charge transport and charge collection efficiency in BiVO4 particulate electrode. On the contrary, larger particle size is necessary for improved photocatalytic O2 evolution because of increased crystallinity and greater band bending, which are essential for charge separation.

Polyurethane sponge facilitating highly dispersed TiO2 nanoparticles on reduced graphene oxide sheets for enhanced photoelectro-oxidation of ethanol

Three-dimensional (3D) porous polyurethane sponge serves as the sacrificial scaffold for the hydrothermally-synthesized anatase TiO2-reduced graphene oxide (RGO) composites. The internal channel of the 3D polyurethane structure provides larger exposed TiO2 seeding and crystal growth area on the pre-adsorbed graphene oxide during the synthesis. The uniform pore size within the polyurethane also prevents severe aggregation of TiO2 nanoparticles. As a result, TiO2 nanoparticles are finely dispersed on the RGO sheets. In addition to the general advantages of introducing graphitic carbon to TiO2 (i.e. hydrophobic carbon for preferential organic adsorption and excellent electron transport of RGO), the presence of polyurethane sponge during the synthesis offers larger contact area between TiO2 and RGO which is facilitated by the excellent dispersion of TiO2. Besides, formation of Ti–O–C species is found at the interface and it extends the light absorption of the composite into visible light region. Combining the localized organic adsorption adjacent to TiO2, efficient charge transfer from large contact area of TiO2–RGO and the extended light response of the composite, this material demonstrates enhanced photoelectrochemical oxidation of ethanol.

Photo-driven synthesis of polymer-coated platinized ZnO nanoparticles with enhanced photoelectrochemical charge transportation

Incorporation of both metal nanoparticles and a polymer layer is proposed to benefit charge transport in a semiconductor in the absence of electron and hole scavengers. Therefore in this work, zinc oxide is deposited with platinum metal and a layer of polymeric species via a photocatalytic route: platinum cation photoreduction and phenol photooxidative polymerization which utilize photogenerated electrons and holes of ZnO, respectively. The photoelectrochemical performances of these photocatalysts follow the order of Poly/Pt/ZnO > Pt/ZnO > bare ZnO, revealing the effectiveness of both the platinum and polymer in enhancing charge separation within ZnO. Although platinum is known to be an excellent electron sink for promoting electron transfer, the performance of Pt/ZnO was limited by the transport of holes to the ZnO surface for oxidation reactions. Thus, the polymer layer plays an important role in facilitating hole transport, resulting in the enhanced performance of Poly/Pt/ZnO.

A dual-electrolyte system for photoelectrochemical hydrogen generation using CuInS2-In2O3-TiO2 nanotube array thin film

The utilization of Na2S/Na2SO3 mixture as the electrolyte solution to stabilize sulfide anode in a photoelectrochemical cell for hydrogen evolution generally compromises the current-to-hydrogen efficiency (η-current) of the system. Here, the employment of a dual-electrolyte system, that is, Na2S/Na2SO3 mixture and pH-neutral Na2SO4 as the respective electrolyte solutions in the anode and cathode chambers of a water splitting cell is demonstrated to suppress the photocorrosion of CuInS2-In2O3-TiO2 nanotube (CIS-In2O3-TNT) heterostructure, while simultaneously boosts the η-current. Although n-type CIS and In2O3 nanoparticles can be easily formed on TNT array via facile pulse-assisted electrodeposition method, conformal deposition of the nanoparticles homogeneously on the nanotubes wall with preservation of the TNT hollow structure is shown to be essential for achieving efficient charge generation and separation within the heterostructure. In comparison to Na2S/Na2SO3 solution as the sole electrolyte in both the anode and cathode chambers, introduction of dual electrolyte is shown to not only enhance the photostability of the CIS-In2O3-TNT anode, but also lead to near-unity η-current as opposed to the merely 20% η-current of the single-electrolyte system.摘要光电化学电池产氢过程中利用 Na2S/Na2SO3混合物作为电解质溶液稳定硫化物阳极通常会牺牲电流产氢效率(ηcurrent). 本文采用Na2S/Na2SO3和pH中性的Na2SO4分别作为光解水电池的阳极和阴极电解液可有效抑制CuInS2-In2O3-TiO2 (CIS-In2O3-TNT)纳米管杂化结构的光腐蚀, 同时提高ηcurrent. 通过脉冲辅助电沉积法可将n型CIS和In2O3纳米粒子沉积在TNT阵列表面, 在保留TNT原有中空结构的前提下将纳米粒子均匀沉积在纳米管上对于在杂化结构中获得高效电荷聚集和分离非常必要. 与Na2S/Na2SO3单电解液电池相比双电解液的引入不仅提高了CIS-In2O3-TNT阳极的光稳定性, 而且ηcurrent接近于1并远高于单电解液电池(20%).