Carlos Sotelo-Vazquez

Where Do Photogenerated Holes Go in Anatase:Rutile TiO2? A Transient Absorption Spectroscopy Study of Charge Transfer and Lifetime

Anatase:rutile TiO2 junctions are often shown to be more photocatalytically active than anatase or rutile alone, but the underlying cause of this improvement is not fully understood. Herein, we employ transient absorption spectroscopy to study hole transfer across the anatase:rutile heterojunction in films as a function of phase composition. By exploiting the different signatures in the photoinduced absorption of trapped charges in anatase and rutile, we were able to separately track the yield and lifetime of holes in anatase and rutile sites within phase composites. Photogenerated holes transfer from rutile to anatase on submicrosecond time scales. This hole transfer can significantly increase the anatase hole yield, with a 20:80 anatase:rutile composite showing a 5-fold increase in anatase holes observed from the microsecond. Hole transfer does not result in an increase in charge-carrier lifetime, where an intermediate recombination dynamic between that of pure anatase (t1/2 ≈ 0.5 ms) and rutile (t1/2 ≈ 20 ms) is found in the anatase:rutile junction (t1/2 ≈ 4 ms). Irrespective of what the formal band energy alignment may be, we demonstrate the importance of trap-state energetics for determining the direction of photogenerated charge separation across heterojunctions and how transient absorption spectroscopy, a method that can specifically track the migration of trapped charges, is a useful tool for understanding this behavior.

Photo-induced enhanced Raman spectroscopy for universal ultra-trace detection of explosives, pollutants and biomolecules

Surface-enhanced Raman spectroscopy is one of the most sensitive spectroscopic techniques available, with single-molecule detection possible on a range of noble-metal substrates. It is widely used to detect molecules that have a strong Raman response at very low concentrations. Here we present photo-induced-enhanced Raman spectroscopy, where the combination of plasmonic nanoparticles with a photo-activated substrate gives rise to large signal enhancement (an order of magnitude) for a wide range of small molecules, even those with a typically low Raman cross-section. We show that the induced chemical enhancement is due to increased electron density at the noble-metal nanoparticles, and demonstrate the universality of this system with explosives, biomolecules and organic dyes, at trace levels. Our substrates are also easy to fabricate, self-cleaning and reusable.

Interstitial boron-doped anatase TiO2 thin-films on optical fibres: atmospheric pressure-plasma enhanced chemical vapour deposition as the key for functional oxide coatings on temperature-sensitive substrates

Temperature sensitive poly(methyl methacrylate) (PMMA) optical fibres were coated with boron doped-anatase crystalline TiO2 thin films in a one-step atmospheric pressure-plasma enhanced chemical vapour deposition (AP-PECVD) process. Both the undoped and interstitial boron-doped TiO2 thin films showed photoactivity under UV irradiation, with the boron-doped thin films presenting higher photodegradation rates when compared to the undoped samples.

Single-step synthesis of doped TiO2 stratified thin-films by atmospheric-pressure chemical vapour deposition

Locally doped TiO2 thin-films were engineered by pulsed precursor delivery using atmospheric-pressure chemical vapour deposition. To our knowledge, this is the first example of stratified films deposited in this manner. The optical, structural and morphological properties of the films were investigated using absorption spectroscopy, X-ray diffraction and electron microscopy techniques. Nitrogen-doped TiO2 stratified thin-films were produced as proof that the new technique would work and that the nature and location of nitrogen species within the films could be studied by X-ray photoelectron spectroscopy. The photocatalytic performance of the films was investigated using the photodegradation of a model organic pollutant (stearic acid). The impact of a stratified configuration and the influence of the type of nitrogen species on enhanced photocatalytic activity are discussed.

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).

Dopant stability in multifunctional doped TiO2's under environmental UVA exposure

We present a UV irradiation study of three nanomaterials which have been investigated and published by peer review previously, specifically tantalum, tungsten and phosphorus doped TiO2. These nanomaterials have been previously synthesised, characterised and designed with specific applications in mind, from photo-catalysts to transparent conducting oxides (TCOs) for use in solar cells and touchscreens. We show in this work, using X-ray photoelectron spectroscopy (XPS) that under sustained levels of environmental UVA Irradiation (0.42 mW cm−2) Ta5+ and W6+ substitutional doped TiO2 exhibits little to no variation in dopant concentration and distribution as a function of irradiation time. Interestingly P5+ and P3− co-doped TiO2 experiences a pronounced and nuanced change in dopant distribution and concentration across the surface through to the bulk as a function of irradiation time. Combined with our previous work with nitrogen doped TiO2, whereby 28 days of environmental UVA irradiation causes interstitial dopant loss and the attrition of functional properties, these results demonstrate that much is still to be understood regarding dopant stability in metal oxides such as TiO2 under environmental conditions.