Ujwal Kumar Thakur

Anodically formed TiO ₂ nanotube arrays (TNTAs) constitute an optoelectronic platform that is being studied for use as a photoanode in photoelectrocatalytic cells, as an electron transport layer (ETL) in solar cells and photodetectors, and as an active layer for chemiresistive and microwave sensors. For optimal transport of charge carriers in these one-dimensional polycrystalline ordered structures, it is desirable to introduce a preferential texture with the grains constituting the nanotube walls aligned along the transport direction. Through x-ray diffraction analysis, we demonstrate that choosing the right water content in the anodization electrolyte and the use of a post-anodization zinc ion treatment can introduce a preferential texture in sub-micron length transparent TNTAs formed on non-native substrates. The incorporation of 1.5 atom% of Zn in TiO ₂ nanotubes prior to annealing, was found to consistently result in the strongest preferential orientation along the [001] direction. [001] oriented TNTAs exhibited a responsivity of 523 A W ⁻¹ at a bias of 2 V for 365 nm photons, which is among the highest reported performance values for ultraviolet photodetection using titania nanotubes. Furthermore, the textured nanotubes without a Zn ²⁺ treatment showed a significantly enhanced performance in halide perovskite solar cells that used TNTAs as the ETL.

As the search for efficient catalysts for CO ₂ photoreduction continues, nanostructured perovskite oxides have emerged as a class of high-performance photocatalytic materials. The perovskite oxide candidates for CO ₂ photoreduction are primarily nanostructured forms of titanates, niobates, tantalates and cobaltates. These materials form the focus of this review article because they are much sought-after due to their nontoxic nature, adequate chemical stability, and tunable crystal structures, bandgaps and surface energies. As compared to conventional semiconductors and nanomaterial catalysts, nanostructured perovskite oxides also exhibit an extended optical-absorption edge, longer charge carrier lifetimes, and favorable band-alignment with respect to reduction potential of activated CO ₂ and reduction products of the same. While CO ₂ reduction product yields of several hundred μ mol ⁻¹ h ⁻¹ are observed with many types of perovskite oxide nanomaterials in stand-alone forms, yield of such quantities are not common with semiconductor nanomaterials of other types. In this review, we present current state-of-the-art synthesis methods to form perovskite oxide nanomaterials, and procedures to engineer their bandgaps. This review also presents a comprehensive summary and discussion on crystal structures, defect distribution, morphologies and electronic properties of the perovskite oxides, and correlation of these properties to CO ₂ photoreduction performance. This review offers researchers key insights for developing advanced perovskite oxides in order to further improve the yields of CO ₂ reduction products.

Anodically formed, vertically oriented, self-organized cylindrical TiO ₂ nanotube arrays composed of the anatase phase undergo an interesting morphological and phase transition upon flame annealing to square-shaped nanotubes composed of both anatase and rutile phases. This is the first report on heterojunctions consisting of metal nanoparticles (NPs) deposited on square-shaped TiO ₂ nanotube arrays (STNAs) with mixed rutile and anatase phase content. A simple photochemical deposition process was used to form Cu, CuPt, and Pt NPs on the STNAs, and an enhancement in the visible light photoelectrochemical water splitting performance for the NP-decorated STNAs was observed over the bare STNAs. Under narrow band illumination by visible photons at 410 nm and 505 nm, Cu NP-decorated STNAs performed the best, producing photocurrents 80% higher and 50 times higher than bare STNAs, respectively. Probing the energy level structure at the NP–STNA interface using ultraviolet photoelectron spectroscopy revealed Schottky barrier formation in the NP-decorated STNAs, which assists in separating the photogenerated charge carriers, as also confirmed by longer charge carrier lifetimes in NP-decorated STNAs. While all the NP-decorated STNAs showed enhanced visible light absorption compared to the bare STNAs, only the Cu NPs exhibited a clear plasmonic behavior with an extinction cross section that peaked at 550 nm.

Thin films consisting of copper oxide nanowires (NWs) containing a mixture of copper oxide phases were formed by electrochemical anodization in de-ionized water (DI) and methanol based electrolytes. Well-defined arrays of vertical nanopores were obtained via anodization in ethylene glycol (EG)-based electrolytes. Optical absorption spectra revealed the electronic bandgap of the NWs formed in DI and methanol to be 2.50 eV, and that of nanopores formed in EG electrolytes to be 1.94 eV. The nanostructures were found to exhibit p-type conduction through Mott-Schottky analysis. The as-formed nanostructures exhibited two photon photoluminescence with lifetimes ranging between 200 and 1100 ps. They also displayed strong optical limiting in the form of a 25% decrease in optical transmission at 800 nm at a relatively low fluence of 0.1 J cm-2 and an absolute transmittance <50% over the entire range of laser fluence values used in this study. The optical transmittance decreased from 40% at an incident Ti-sapphire laser fluence of 20 mJ cm-2 to ∼9% when the fluence was increased to 700 mJ cm-2. Furthermore, the damage threshold of 0.7 J cm-2 compares favorably with advanced optical limiting materials such as gold-graphene composites and single-walled carbon nanotube composites even without further optimization of nanoscale morphology to maximize scattering. Therefore the nanostructures of mixed copper oxide films studied in this report are particularly promising as optical limiters for protecting low damage threshold (<1 J cm-2) photonic devices and are an important addition to the narrow selection of optical limiting materials that are currently available.

This is the first report of a 17.6% champion efficiency solar cell architecture comprising monocrystalline TiO ₂ nanorods (TNRs) coupled with perovskite, and formed using facile solution processing without non-routine surface conditioning. Vertically oriented TNR ensembles are desirable as electron transporting layers (ETLs) in halide perovskite solar cells (HPSCs) because of potential advantages such as vectorial electron percolation pathways to balance the longer hole diffusion lengths in certain halide perovskite semiconductors, ease of incorporating nanophotonic enhancements, and optimization between a high contact surface area for charge transfer (good) versus high interfacial recombination (bad). These advantages arise from the tunable morphology of hydrothermally grown rutile TNRs, which is a strong function of the growth conditions. Fluorescence lifetime imaging microscopy of the HPSCs demonstrated a stronger quenching of the perovskite PL when using TNRs as compared to mesoporous/compact TiO ₂ thin films. Due to increased interfacial contact area between the ETL and perovskite with easier pore filling, charge separation efficiency is dramatically enhanced. Additionally, solid-state impedance spectroscopy results strongly suggested the suppression of interfacial charge recombination between TNRs and perovskite layer, compared to other ETLs. The optimal ETL morphology in this study was found to consist of an array of TNRs ∼300 nm in length and ∼40 nm in width. This work highlights the potential of TNR ETLs to achieve high performance solution-processed HPSCs.