Modification of titania films by chemical vapour deposition for enhanced photocatalysis

Titanium dioxide (TiO2) is the leading material for self-cleaning applications due to its intrinsic properties, such as chemical inertness, mechanical robustness, high photocatalytic activity and durability to extend photocatalytic cycling. However, its relatively wide bandgap limits its outdoor applications. There has been a strenuous effort to try and improve the photocatalytic efficiency of TiO2, in particular by modifying its electronic structure to enhance its function under solar illumination. The most commonly studied approaches for achieving this have been to incorporate anionic and/or cationic species into the TiO2 structure and the design of TiO2-based heterojunction systems. The addition of nitrogen, phosphorus and sulfur species into the matrix of TiO2 was investigated. Films were grown using atmospheric-pressure chemical vapour deposition (APCVD). The nitrogen-doped system has been investigated most prominently to enhance and extend the photocatalytic response of TiO2 materials into the visible region of the electromagnetic spectrum. Nitrogen can either replace oxygen sites (Ns, substitutional doping) or sit within the TiO2 structure (Ni, interstitial doping) and form N-O groups with lattice oxygen. Interestingly, these NOx groups, as well as NHx surface species present similar binding energies, ca. 400 eV, hindering the identification of the nitrogen species and their role in the photocatalytic response of the material. Various synthesis conditions were experimented using different nitrogen precursors (tert-butylamine, benzylamine and ammonia), which were used to establish a correlation between surface and bulk nitrogen species and the photocatalytic behaviour of the N-TiO2 films. A loss of the Ni environment (as observed by X-ray photoelectron spectroscopy), as well as a decrease in photoactivity over time was observed, suggesting a direct participation of the nitrogen species in photocatalytic iv processes. In addition to traditional CVD methods, a pulse precursor approach was used for the first time, to the best of our knowledge, to synthesise stratified N-doped TiO2 thin films, by adding nitrogen into specific regions of the N-TiO2 film. Physical and functional comparison of stratified and non-stratified N-TiO2 films with similar structural and morphological features allowed us to evaluate the benefits of this synthetic approach, which not only resulted in an increase in the photocatalytic efficiency of the stratified N-TiO2 films but also did not affect the overall crystallinity of the films. The addition of phosphorus and sulfur was investigated as the most promising alternative to the use of nitrogen doping, as both could be added to the lattice of TiO2 either as cations or anions. Through functional testing, it was found that both dopant species were beneficial from a photocatalytic point of view. Interestingly, the use of APCVD techniques to deposit P-TiO2 thin films resulted in the addition for the first time, to the best of our knowledge, of P3- species, as well as P5+, to the TiO2 structure with the relative proportion being determined by the synthesis conditions. Through Hall effect probe, photocatalytic testing and transient absorption spectroscopy (TAS) analyses, it was found that the incorporation of P3- species was detrimental from an electrical conduction and photocatalytic point of view; however, the presence of solely P5+ species resulted in P-TiO2 films with enhanced self-cleaning and TCO properties. These results provide important insights on the influence of dopant nature and its location within a semiconductor’s structure. Heterojunction semiconductor materials are used in a wide range of applications including catalysis, electronic devices, sensors and solar-to-chemical energy conversion. These materials benefit from effective electron transfer processes, electron tunnelling, surface passivation and other synergistic effects to enhance their performance beyond the individual components. By using CVD methods, two different v heterojunction systems, rutile/ anatase TiO2 and WO3/TiO2, were grown. The interposition of an amorphous TiO2-based interlayer allowed direct vapour deposition of anatase on a rutile substrate, which is otherwise hindered by templating. The subsequent crystallisation of the amorphous interlayer after annealing, allowed us to investigate the impact of an efficient interface between the two rutile-anatase phases in the photodegradation of an organic model pollutant, stearic acid. Clear evidence on the synergy between the two polymorphs and more importantly, on the charge flow across the interface, which is against much conventional understanding, was evaluated through the photoreduction of silver particles. This charge flow involves electron transfer from rutile to anatase. Likewise, a conformal coating of WO3 nanorods with TiO2 was performed using APCVD techniques. The resulting WO3/TiO2 heterojunction films showed an electron transfer phenomenon, where electrons moved from WO3 into TiO2, against widely reported observation. State-of-the-art hybrid density functional theory (DFT) and hard X-ray photoelectron spectroscopy (HAXPES) were employed to elucidate the electronic interaction at the heterojunction of the WO3 and TiO2 crystalline phases. This vectorial charge separation reduces electron-hole recombination and most likely extends the lifetime and relative population of photogenerated charges. These results provide important insights on the influence of vectorial charge separation in heterojunctions. These phenomena had a dramatic impact on the photocatalytic efficiency of the heterojunction films, which are among the very highest ever reported by a thin film

Iron-Intercalated Zirconium Diselenide Thin Films from the Low-Pressure Chemical Vapor Deposition of [Fe(η⁵-C₅H₄Se)₂Zr(η⁵-C₅H₅)₂]₂

Transition metal chalcogenide thin films of the type FexZrSe2 have applications in electronic devices, but their use is limited by current synthetic techniques. Here, we demonstrate the synthesis and characterization of Fe-intercalated ZrSe2 thin films on quartz substrates using the low-pressure chemical vapor deposition of the single-source precursor [Fe(η5-C5H4Se)2Zr(η5-C5H5)2]2. Powder X-ray diffraction of the film scraping and subsequent Rietveld refinement of the data showed the successful synthesis of the Fe0.14ZrSe2 phase, along with secondary phases of FeSe and ZrO2. Upon intercalation, a small optical band gap enhancement (Eg(direct)opt = 1.72 eV) is detected in comparison with that of the host material

Critical influence of surface nitrogen species on the activity of N-doped TiO thin-films during photodegradation of stearic acid under UV light irradiation

Atmospheric-pressure chemical vapour deposition (APCVD) was used to produce a series of nitrogen-doped titania (N-TiO) thin-films using tert-butylamine as the nitrogen source. The films were deposited as the anatase phase on glass and quartz substrates and characterised using X-ray diffraction, optical and vibrational spectroscopy and electron microscopy. The nature and location of the nitrogen species present on the surface and bulk of the films was studied by X-ray photoelectron spectroscopy. Thorough comparison amongst films with similar structural and morphological features allowed the role of nitrogen species to be evaluated during photo-oxidation of a model organic pollutant (stearic acid). Sequential photocatalytic experiments revealed a drastic decrease in the UV activity of the films which were correlated with changes involving surface nitrogen groups. The existence of concomitant nitrogen species with similar binding energies (ca. 400eV) but different chemical nature is proposed, as well as the direct participation of at least one of these species in the oxidation reaction. A similar mechanism for the visible light activity of N-TiO materials is also suggested. © 2014

Stoichiometrically driven disorder and local diffusion in NMC cathodes

Major structural differences in lithium nickel manganese cobalt oxides (NMC) prepared under identical conditions have been uncovered using neutron powder diffraction. Sample NMC-622 was obtained as a single R[3 with combining macron]m crystal structure with little defects, whereas NMC-811 showed significant Li deficiency and NMC-433 formed three distinct phases; ordered R[3 with combining macron]m, disordered R[3 with combining macron]m and a C2/m phase. Local diffusion behaviour was also studied by muon spin relaxation (μSR). It was observed that single phase R[3 with combining macron]m NMC-622 showed a higher lithium diffusion coefficient (4.4 × 10−11 cm2 s−1) compared to lithium deficient NMC-811 (2.9 × 10−11 cm2 s−1), or the highly disordered NMC-433 (3.4 × 10−11 cm2 s−1). Furthermore, activation energies for the Li diffusion process were estimated to be 58 meV, 61 meV and 28 meV for NMC-811, NMC-622 and NMC-433, respectively

Multifunctional P-Doped TiO2 Films: A New Approach to Self-Cleaning, Transparent Conducting Oxide Materials

Multifunctional P-doped TiO2 thin films were synthesized by atmospheric pressure chemical vapor deposition (APCVD). This is the first example of P-doped TiO2 films with both P5+ and P3– states, with the relative proportion being determined by synthesis conditions. This technique to control the oxidation state of the impurities presents a new approach to achieve films with both self-cleaning and TCO properties. The origin of electrical conductivity in these materials was correlated to the incorporation of P5+ species, as suggested by Hall Effect probe measurements. The photocatalytic performance of the films was investigated using the model organic pollutant, stearic acid, with films containing predominately P3– states found to be vastly inferior photocatalysts compared to undoped TiO2 films. Transient absorption spectroscopy studies also showed that charge carrier concentrations increased by several orders of magnitude in films containing P5+ species only, whereas photogenerated carrier lifetimes—and thus photocatalytic activity—were severely reduced upon incorporation of P3– species. The results presented here provide important insights on the influence of dopant nature and location within a semiconductor structure. These new P-doped TiO2 films are a breakthrough in the development of multifunctional advanced materials with tuned properties for a wide range of applications

Multiple diffusion pathways in LixNi0.77Co0.14Al0.09O2 (NCA) Li-ion battery cathodes

Experimental evidence for the presence of two computationally theorised diffusion pathways, namely the oxygen dumbbell hop (ODH) and tetrahedral site hop (TSH), has been given for the first time by muon spin relaxation (µSR) on sub-stoichiometric LixNi0.77Co0.14Al0.09O2. µSR has proven to be a powerful tool that is able to discriminate between diffusion pathways that occur on different timescales on a local level, where bulk electrochemical techniques cannot. Whereas the estimated values of DLi at lithium concentrations of 0.87 and 0.71 were found to be on the order of 10-11 by electrochemical impedance spectroscopy, contributions to diffusion from ODH and TSH were determined to be on the order of 10-11 and 10-10 cm2 s-1, and a factor of four decrease in Ea for both samples, in excellent agreement with theoretical calculations on related compounds. Rietveld refinement of both X-ray and neutron diffraction data was also used to interrogate the local structure of the materials where no contribution from Li+/Ni2+ cation mixing was observed

Charge Transport Phenomena in Heterojunction Photocatalysts: The WO₃/TiO₂ System as an Archetypical Model

Recent studies have demonstrated the high efficiency through which nanostructured core–shell WO3/TiO2 (WT) heterojunctions can photocatalytically degrade model organic pollutants (stearic acid, QE ≈ 18% @ λ = 365 nm), and as such, has varied potential environmental and antimicrobial applications. The key motivation herein is to connect theoretical calculations of charge transport phenomena, with experimental measures of charge carrier behavior using transient absorption spectroscopy (TAS), to develop a fundamental understanding of how such WT heterojunctions achieve high photocatalytic efficiency (in comparison to standalone WO3 and TiO2 photocatalysts). This work reveals an order of magnitude enhancement in electron and hole recombination lifetimes, respectively located in the TiO2 and WO3 sides, when an optimally designed WT heterojunction photocatalyst operates under UV excitation. This observation is further supported by our computationally captured details of conduction band and valence band processes, identified as (i) dominant electron transfer from WO3 to TiO2 via the diffusion of excess electrons; and (ii) dominant hole transfer from TiO2 to WO3 via thermionic emission over the valence band edge. Simultaneously, our combined theoretical and experimental study offers a time-resolved understanding of what occurs on the micro- to milliseconds (μs–ms) time scale in this archetypical photocatalytic heterojunction. At the microsecond time scale, a portion of the accumulated holes in WO3 contribute to the depopulation of W5+ polaronic states, whereas the remaining accumulated holes in WO3 are separated from adjacent electrons in TiO2 up to 3 ms after photoexcitation. The presence of these exceptionally long-lived photogenerated carriers, dynamically separated by the WT heterojunction, is the origin of the superior photocatalytic efficiency displayed by this system (in the degradation of stearic acid). Consequently, our combined computational and experimental approach delivers a robust understanding of the direction of charge separation along with critical time-resolved insights into the evolution of charge transport phenomena in this model heterojunction photocatalyst

Charge transport phenomena in heterojunction photocatalysts: the WO3/TiO2 system as an archetypical model.

Recent studies have demonstrated the high efficiency through which nanostructured core-shell WO3/TiO2 (WT) heterojunctions can photocatalytically degrade model organic pollutants (stearic acid, QE ≈ 18% @ λ = 365 nm), and as such, has varied potential environmental and antimicrobial applications. The key motivation herein is to connect theoretical calculations of charge transport phenomena, with experimental measures of charge carrier behavior using transient absorption spectroscopy (TAS), to develop a fundamental understanding of how such WT heterojunctions achieve high photocatalytic efficiency (in comparison to standalone WO3 and TiO2 photocatalysts). This work reveals an order of magnitude enhancement in electron and hole recombination lifetimes, respectively located in the TiO2 and WO3 sides, when an optimally designed WT heterojunction photocatalyst operates under UV excitation. This observation is further supported by our computationally captured details of conduction band and valence band processes, identified as (i) dominant electron transfer from WO3 to TiO2 via the diffusion of excess electrons; and (ii) dominant hole transfer from TiO2 to WO3 via thermionic emission over the valence band edge. Simultaneously, our combined theoretical and experimental study offers a time-resolved understanding of what occurs on the micro- to milliseconds (μs-ms) time scale in this archetypical photocatalytic heterojunction. At the microsecond time scale, a portion of the accumulated holes in WO3 contribute to the depopulation of W5+ polaronic states, whereas the remaining accumulated holes in WO3 are separated from adjacent electrons in TiO2 up to 3 ms after photoexcitation. The presence of these exceptionally long-lived photogenerated carriers, dynamically separated by the WT heterojunction, is the origin of the superior photocatalytic efficiency displayed by this system (in the degradation of stearic acid). Consequently, our combined computational and experimental approach delivers a robust understanding of the direction of charge separation along with critical time-resolved insights into the evolution of charge transport phenomena in this model heterojunction photocatalyst

Correlation of Optical Properties, Electronic Structure, and Photocatalytic Activity in Nanostructured Tungsten Oxide

Tungsten trioxide nanorod arrays are deposited using aerosol assisted chemical vapor deposition. The electronic structure, defect chemistry, optical bandgap, and photocatalytic activity are found to vary progressively with nanorod length. Nanorods less than 1 μm in length show a widening of the optical bandgap (up to 3.1 eV), more disorder states within the bandgap, an absence of reduced tungsten cation states, and increased photocatalytic activity for destruction of a test organic pollutant (stearic acid) compared to nanorods of 2 μm length or greater which possessed bandgaps close to the bulk value for tungsten oxide (2.6-2.8 eV), the presence of reduced tungsten states (W4+), and lower photocatalytic activity. The results indicate that for maximum photocatalytic performance in organic pollutant degradation, tungsten oxide should be engineered such that the bandgap is widened relative to bulk WO3 to a value above 3 eV; although less photons are expected be absorbed, increases in the overpotential for oxidation reactions appear to more than offset this loss. It is also desirable to ensure the material remains defect free, or the defect concentration minimized, to minimize carrier recombination

Accessing new 2D semiconductors with optical band gap: synthesis of iron-intercalated titanium diselenide thin films via LPCVD

Fe-doped TiSe2 thin-films were synthesized via low pressure chemical vapor deposition (LPCVD) of a single source precursor: [Fe(η⁵-C₅H₄Se)₂Ti(η⁵-C₅H₅)₂]₂ (1). Samples were heated at 1000 °C for 1–18 h and cooled to room temperature following two different protocols, which promoted the formation of different phases. The resulting films were analyzed by grazing incidence X-ray diffraction (GIXRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM) and UV/vis spectroscopy. An investigation of the Fe doping limit from a parallel pyrolysis study of FeₓTiSe₂ powders produced in situ during LPCVD depositions has shown an increase in the Fe–TiSe₂–Fe layer width with Fe at% increase. Powders were analyzed using powder X-ray diffraction (PXRD) involving Rietveld refinement and XPS. UV/vis measurements of the semiconducting thin films show a shift in band gap with iron doping from 0.1 eV (TiSe₂) to 1.46 eV (Fe₀.₄₆TiSe₂)