Dual Functionality of Surface States in Dictating Photocurrent Generation of Au Nanocluster-Sensitized TiO2 Electrodes.

Gold nanoclusters (Au NCs), composed of only a few atoms, exhibit molecule-like behavior due to their distinct electronic structures arising from quantum confinement effects. Unlike their plasmonic nanoparticle counterparts, these nonplasmonic Au NCs possess unique properties with significant potential for photosensitizer applications. While traditional and NC-based electrodes share architectural similarities, the photoelectrochemical (PEC) behavior of the latter diverges significantly. Sensitizing TiO2 with Au NCs introduces additional surface trap states. In contrast to conventional photosensitizers, where surface states typically have a negligible impact on hole transfer, these trap states actively mediate the charge transfer process in Au NC-sensitized TiO2 electrodes. In this study, we employed impedance spectroscopy to elucidate the role of surface trap states in photocurrent generation. Our investigation revealed that these states are critical in determining PEC performance, presenting a dichotomy: they facilitate charge transfer (enhancing PEC performance) while simultaneously promoting carrier recombination (limiting efficiency). We demonstrated that the judicious control of otherwise deleterious surface trap states can significantly boost photocurrent. Our findings highlight that the dual nature of surface trap states demands a comprehensive investigation to fully understand their intricate impact on PEC performance.

Interfacial Bridging Strategy for Charge Extraction/Injection in the BiVO4/CoSn-Layered Double Hydroxide p-n Heterojunction Approach Using Graphene Quantum Dots for Enhanced Water Oxidation Kinetics.

The design of a photoanode with a bridging strategy that can enhance the charge injection and transport in a heterojunction can be an efficient approach to separate the photogenerated charge carriers and enhance the water oxidation kinetics. Aiming at such issues, herein we propose a BiVO4/GQDs/CoSn-LDH (layered double hydroxide) photoanode, which leads to the formation of a p-n heterojunction with bridged graphene quantum dots (GQDs) to accelerate the photoelectrochemical (PEC) performance. The BiVO4/GQDs/CoSn-LDH photoanode exhibits a maximum photocurrent density of 4.15 mA/cm2, which is ∼3-fold higher than for the pristine BiVO4 photoanode with an ∼250 mV cathodic shift in the onset potential. A faradaic yield of ∼91% confirms that the obtained photocurrent is mainly due to water oxidation. A mechanistic study based on the electrochemical impedance (EIS), charge separation, and charge injection efficacy measurements reveals that the introduction of GQDs between BiVO4 and CoSn-LDH provides a continuous conducting network to extract holes from the BiVO4 surface and efficiently inject into the CoSn-LDH surface for the water oxidation reaction.

Phosphorus nitride nano-dots as a versatile and metal-free support for efficient photoelectrochemical water oxidation.

Phosphorus nitride dots (PNDs) are employed as a metal-free and versatile support over a range of metal oxide-based photoanodes for efficient photoelectrochemical (PEC) water oxidation. PNDs have the ability to form various heterojunctions by virtue of their favorable band positions for enhanced charge separation leading to improved photocurrent densities.

Tuning the Electronic Structure of Monoclinic Tungsten Oxide Nanoblocks by Indium Doping for Boosted Photoelectrochemical Performance.

Photoelectrochemical (PEC) water oxidation, a desirable strategy to meet future energy demands, has several bottle-necks to resolve. One of the prominent issues is the availability of charge carriers at the surface reaction site to promote water oxidation. Of the several approaches, metal dopants to enhance the carrier density of the semiconductors, is an important one. In this work, we have studied the effect of In-doping on monoclinic WO3 nanoblocks, growing vertically over fluorine-doped tin oxide (FTO) without the aid of any seed layer. X-ray photoelectron spectroscopy (XPS) data reveals that In3+ ions are partially occupying the W6+ ions in In-doped WO3 photoanode. In3+ ions are offering better performance by adding additional charge carriers for amplifying the expression of the number of carriers. The maximum current density value of 2.18 mA/cm2 has been provided by the optimized In-doped WO3 photoanode with 3 wt% indium doping at 1.23 V vs. RHE, which is ∼3 times higher than that of undoped monoclinic WO3 photoanode. Mott-Schottky (MS) analysis reveals charge carrier density (ND ) for In-doped WO3 photoanode has been enhanced by a factor of 3. An average Faradic yield of ∼90 percent has been achieved which can serve as a model system using In3+ as a dopant for an inexpensive and attractive method for enhanced WO3 based PEC water oxidation.

Ultrasensitive NOX Detection in Simulated Exhaled Air: Enhanced Sensing via Alumina Modification of In-Situ Grown WO3 Nanoblocks.

Seedless growth of vertically aligned nanostructures, which can induce smoother transport and minimize Ohmic contact between substrate and semiconductor, can be fabricated by in situ growth utilizing modified hydrothermal methods. Such devices can be useful in designing non-invasive ultrasensitive hand-held sensors for diagnostic identification of volatile organic compounds (VOCs) in exhaled air, offering pain-free and easier detection of long-term diseases such as asthma. In the present work, WO3 nanoblocks, with a high surface area and porosity, have been grown directly over transparent conducting oxide to minimize Ohmic resistance, facilitating smoother electron transfer and enhanced current response. Further modification with porous alumina (γ-Al2 O3 ), by electrodeposition, resulted in the selective and ultrasensitive detection of NOX in simulated exhaled air. Crystal phase purity of as-fabricated pristine as well modified samples is validated with X-ray diffraction analysis. Morphological and microstructural analyses reveal the successful deposition of porous alumina over the surface of WO3 . Improved surface area and porosity is presented by porous alumina in the modified WO3 device, suggesting more active sites for the gas molecules to get adsorbed and diffuse through the pores. Oxygen vacancies, which are detrimental in the transport phenomenon in the presented sensors, have been studied using X-ray photoelectron spectroscopic (XPS) analysis. Gas sensing studies have been performed by fabricating chemiresistor devices based on bare WO3 and Al2 O3 -modified WO3 . The higher sensitivity for NOX gas in case of γ-Al2 O3 -modified WO3 based devices, as compared to bare WO3 -based devices, is attributed to the better surface area and charge transport kinetics. The presented device strategy offers crucial understanding in the design and development of non-invasive, hand-held devices for NO gas present in the human breath, with potential application in medical diagnostics.