Najia Mahdi

Enhanced CH4 yield by photocatalytic CO2 reduction using TiO2 nanotube arrays grafted with Au, Ru, and ZnPd nanoparticles

Metal nanoparticle (NP) co-catalysts on metal oxide semiconductor supports are attracting attention as photocatalysts for a variety of chemical reactions. Related efforts seek to make and use Pt-free catalysts. In this regard, we report here enhanced CH4 formation rates of 25 and 60 μmol·g–1·h–1 by photocatalytic CO2 reduction using hitherto unused ZnPd NPs as well as Au and Ru NPs. The NPs are formed by colloidal synthesis and grafted onto short n-type anatase TiO2 nanotube arrays (TNAs), grown anodically on transparent glass substrates. The interfacial electric fields in the NP-grafted TiO2 nanotubes were probed by ultraviolet photoelectron spectroscopy (UPS). Au NP-grafted TiO2 nanotubes (Au-TNAs) showed no band bending, but a depletion region was detected in Ru NP-grafted TNAs (Ru-TNAs) and an accumulation layer was observed in ZnPd NP-grafted TNAs (ZnPd-TNAs). Temperature programmed desorption (TPD) experiments showed significantly greater CO2 adsorption on NP-grafted TNAs. TNAs with grafted NPs exhibit broader and more intense UV–visible absorption bands than bare TNAs. We found that CO2 photoreduction by nanoparticle-grafted TNAs was driven not only by ultraviolet photons with energies greater than the TiO2 band gap, but also by blue photons close to and below the anatase band edge. The enhanced rate of CO2 reduction is attributed to superior use of blue photons in the solar spectrum, excellent reactant adsorption, efficient charge transfer to adsorbates, and low recombination losses.

Heterojunctions of mixed phase TiO2 nanotubes with Cu, CuPt, and Pt nanoparticles: interfacial band alignment and visible light photoelectrochemical activity

Anodically formed, vertically oriented, self-organized cylindrical TiO2 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 TiO2 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.

Core–shell titanium dioxide–titanium nitride nanotube arrays with near-infrared plasmon resonances

Titanium nitride (TiN) is a ceramic with high electrical conductivity which in nanoparticle form, exhibits localized surface plasmon resonances (LSPRs) in the visible region of the solar spectrum. The ceramic nature of TiN coupled with its dielectric loss factor being comparable to that of gold, render it attractive for CMOS polarizers, refractory plasmonics, surface-enhanced Raman scattering and a whole host of sensing applications. We report core-shell TiO2-TiN nanotube arrays exhibiting LSPR peaks in the range 775-830 nm achieved by a simple, solution-based, low cost, large area-compatible fabrication route that does not involve laser-writing or lithography. Self-organized, highly ordered TiO2 nanotube arrays were grown by electrochemical anodization of Ti thin films on fluorine-doped tin oxide-coated glass substrates and then conformally coated with a thin layer of TiN using atomic layer deposition. The effects of varying the TiN layer thickness and thermal annealing on the LSPR profiles were also investigated. Modeling the TiO2-TiN core-shell nanotube structure using two different approaches, one employing effective medium approximations coupled with Fresnel coefficients, resulted in calculated optical spectra that closely matched the experimentally measured spectra. Modeling provided the insight that the observed near-infrared resonance was not collective in nature, and was mainly attributable to the longitudinal resonance of annular nanotube-like TiN particles redshifted due to the presence of the higher permittivity TiO2 matrix. The resulting TiO2-TiN core-shell nanotube structures also function as visible light responsive photocatalysts, as evidenced by their photoelectrochemical water-splitting performance under light emitting diode illumination using 400, 430 and 500 nm photons.

Ultraviolet sensing using a TiO2 nanotube integrated high resolution planar microwave resonator device

This paper presents a unique integrated UV light sensing concept and introduces a device with a detection limit of 1.96 nW cm-2. The combination of a high quality factor, a microwave planar resonator (Q ∼ 50 000) with a semiconducting nanomaterial enables a revolutionary potential paradigm for photodetection of low light intensities and small form factors. The presenting device employs a high-resolution microwave microstrip resonator as the signal transducer to convert the variant dielectric properties (permittivity and conductivity) of the nanotube membrane into electrical signals such as the resonant frequency, quality factor and resonant amplitude. The microwave resonator has an active feedback loop to improve the initial quality factor of the resonator from 200 to 50 000 and leads to boosting of the sensing resolution by orders of magnitude. Anatase TiO2 nanotubes are assembled on the surface of the microwave resonator. Upon exposure to UV light, electron-hole pair generation, trapping and recombination in the nanotubes are exploited as a unique signature to quantify the UV light intensity. The change of dielectric properties of the nanotube membrane is monitored using the underlying active microwave resonator. The proposed concept enables the detection and monitoring of UV light at high resolution, with very small exposure power and integrated form factors.

All-solid-state formation of titania nanotube arrays and their application in photoelectrochemical water splitting

The present work demonstrates for the first time the facile fabrication of TiO2 nanotube arrays (TNTAs) by a fluoride-free solid-state anodization process using LiClO4 containing solid polymeric electrolyte. The resulting nanotubes were tested for photoelectrochemical water splitting. The elimination of liquid electrolytes in electrochemical anodization constitutes a paradigm shift for the formation of nanoporous and nanotubular metal oxides. Our results open a new area of research that uses the distinctive properties of solid polymer electrolytes to achieve targeted doping and nano-morphologies. Characterization of the grown TNTAs indicated solid state anodized TNTAs to consist purely of the anatase phase of titania. The solid-state anodization process provides several advantages over conventional liquid electrolytes such as easy handling and processing, better charge transport, environmentally benign chemicals and methodology. Photoelectrochemical water splitting experiments were performed which confirmed the viability of TNTAs grown by the new solid-state process for photocatalytic applications.

Distinguishing between Deep Trapping Transients of Electrons and Holes in TiO2 Nanotube Arrays Using Planar Microwave Resonator Sensor

A large signal direct current (DC) bias and a small signal microwave bias were simultaneously applied to TiO2 nanotube membranes mounted on a planar microwave resonator. The DC bias modulated the electron concentration in the TiO2 nanotubes and was varied between 0 and 120 V in this study. Transients immediately following the application and removal of DC bias were measured by monitoring the S-parameters of the resonator as a function of time. The DC bias stimulated Poole-Frenkel-type trap-mediated electrical injection of excess carriers into TiO2 nanotubes, which resulted in a near-constant resonant frequency but a pronounced decrease in the microwave amplitude due to free electron absorption. When ultraviolet illumination and DC bias were both present and then stepwise removed, the resonant frequency shifted due to trapping-mediated change in the dielectric constant of the nanotube membranes. Characteristic lifetimes of 60-80, 300-800, and ∼3000 s were present regardless of whether light or bias was applied and were also observed in the presence of a hole scavenger, which we attributed to oxygen adsorption and deep electron traps, whereas another characteristic lifetime >8000 s was only present when illumination was applied, and is attributed to the presence of hole traps.

Microwave resonator sensor integrated with nanostructured semiconductor membranes for photodetection and carrier lifetime measurement

Free-standing TiO2 nanotube array membranes are mounted in the active coupling gap of a planar ring-type microwave resonator with f0 of 5.5 GHz and Q of 220. When illuminated by ultraviolet radiation, the S-parameters of the resonator vary as a function of time due to changes in the dielectric constant and electrical conductivity of the membranes, which in turn occur due to the Alling of trap states and photoexcitation of charge carriers respectively. Monitoring the amplitude and frequency response of the resonator enables photodetection and carrier lifetime measurements in a simple, compact and low-cost configuration. TiO2 nanotube membranes decorated with gold nanoparticles are found to have a much faster temporal microwave response upon illumination compared to bare, undecorated membranes, which we attribute to fast and enduring charge separation at the Au-TiO2 heterojunction.