Andreas Kafizas

Superhydrophobic Photocatalytic Surfaces through Direct Incorporation of Titania Nanoparticles into a Polymer Matrix by Aerosol Assisted Chemical Vapor Deposition

A new class of superhydrophobic photocatalytic surfaces that are self-cleaning through light-induced photodegradation and the Lotus effect are presented. The films are formed in a single-step aerosol-assisted chemical vapor deposition (AACVD) process. The films are durable and show no degradation on continuous exposure to UV-C radiation.

Ultrafast Charge Carrier Recombination and Trapping in Hematite Photoanodes under Applied Bias

Transient absorption spectroscopy on subpicosecond to second time scales is used to investigate photogenerated charge carrier recombination in Si-doped nanostructured hematite (α-Fe2O3) photoanodes as a function of applied bias. For unbiased hematite, this recombination exhibits a 50% decay time of ∼6 ps, ∼103 times faster than that of TiO2 under comparable conditions. Anodic bias significantly retards hematite recombination dynamics, and causes the appearance of electron trapping on ps−μs time scales. These ultrafast recombination dynamics, their retardation by applied bias, and the associated electron trapping are discussed in terms of their implications for efficient water oxidation.

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.

Aerosol assisted chemical vapour deposition of hydrophobic TiO2–SnO2 composite film with novel microstructure and enhanced photocatalytic activity

Aerosol assisted chemical vapour deposition (AACVD) was used to synthesise a TiO2–SnO2 composite film onto a glass substrate. For comparison a TiO2 film and a SnO2 film were also prepared. All films were characterised by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and wavelength dispersive X-ray spectroscopy (WDX). XPS and WDX of the composite film revealed a TiO2 rich film with a high level of SnO2 segregation at the surface. Highly structured pyramid-like features gave rise to hydrophobic films with static water contact angles of 134°. Photocatalytic activities were determined by monitoring the degradation of intelligent ink (containing Resazurin redox dye) via UV-visible spectroscopy. Under UVA irradiation, the TiO2 film only began to degrade the dye after being irradiated in excess of 100 minutes, whereas the composite TiO2–SnO2 film required only 6 minutes of irradiation before degradation was observed. The formal quantum efficiency (FQE) for the TiO2–SnO2 composite was determined to be 1.01 × 10−2 molecules per incident photon and the formal quantum yield (FQY) was 1.17 × 10−2 molecules per absorbed photon. This is an order of magnitude superior to Pilkington Activ™ self-cleaning glass a commercial self-cleaning TiO2 coating on glass. This improved photocatalytic activity is attributed to the presence of electron scavenging SnO2 sites that increase charge separation and the increased surface area due to the highly structured morphology.

Simple method for the rapid simultaneous screening of photocatalytic activity over multiple positions of self-cleaning films.

An intelligent ink, previously shown to be capable of rapidly assessing photocatalytic activity, was simply applied via a felt-pen onto a commercially available piece of Activ self-cleaning glass. The ink, comprising of redox dye resazurin and the sacrificial electron donor glycerol within an aqueous hydroxy ethyl cellulose (HEC) polymer media, was photocatalytically degraded in a two-step process. The key initial stage was the photo-reductive conversion of resazurin to resorufin, whereby a colour change from blue to pink occurred. The latter stage was the subsequent photo-reduction of the resorufin, where a slower change from pink to colourless was seen. Red and green components of red-green-blue colour extracted from flat-bed scanner digital images of resazurin ink coated photocatalytic films at intervals during the photocatalysis reaction were inversely proportional to the changes seen via UV-visible absorption spectroscopy and indicative of reaction kinetics. A 3 x 3 grid of intelligent ink was drawn onto a piece of Activ and a glass blank. The photocatalysis reaction was monitored solely by flat-bed digital scanning. Red-green-blue values of respective positions on the grid were extracted using a custom-built program entitled RGB Extractor. The program was capable of extracting a number of 5 x 5 pixel averages of red-green-blue components simultaneously. Allocation of merely three coordinates allowed for the automatic generation of a grid, with scroll-bars controlling the number of positions to be extracted on the grid formed. No significant change in red and green components for any position on the glass blank was observed; however, the Activ film displayed a homogenous photo-reduction of the dye, reaching maxima in red and minima in green components in 23 +/- 3 and 14 +/- 2 min, respectively. A compositionally graded N-doped titania film synthesised in house via a combinatorial APCVD reaction was also photocatalytically tested by this method where 247 positions on a 13 x 19 grid were simultaneously analysed. The dramatic variation in photocatalysis observed was rapidly quantified for all positions (2-3 hours) allowing for correlations to be made between thicknesses and N : Ti% compositions attained from Swanepoel and WDX analysis, respectively. N incorporation within this system was found to be detrimental to film activity for the photocatalysis reaction of intelligent ink under 365 nm light.

Combinatorial Atmospheric Pressure Chemical Vapor Deposition (cAPCVD): A Route to Functional Property Optimization

We demonstrate how combinatorial atmospheric pressure chemical vapor deposition (cAPCVD) can be used as a synthetic tool for rapidly optimizing the functional properties of thin-films, by analyzing the self-cleaning properties of tungsten doped anatase as an example. By introducing reagents at separate points inside the reactor, a tungsten/titanium compositional gradient was formed and a diverse range of film growth conditions were obtained. By partially mixing the metal sources, a combinatorial film with a compositional profile that varied primarily in the lateral plane was synthesized. A combinatorial thin-film of anatase TiO(2) doped with an array of tungsten levels as a solid solution ranging from 0.38-13.8 W/Ti atom % was formed on a single glass substrate. The compositional-functional relationships were understood through comprehensively analyzing combinatorial phase space, with 200 positions investigated by high-throughput methods in this study. Physical and functional properties, and their compositional dependencies, were intercorrelated. It was found that increases in photocatalytic activity and conductivity were most highly dependent on film crystallinity within the 0.38-13.8 atom % W/Ti doping regime. However, enhancements in photoinduced surface wetting were primarily dependent on increases in preferred growth in the (211) crystal plane.

Does a Photocatalytic Synergy in an Anatase–Rutile TiO2 Composite Thin‐Film Exist?

It has often been suggested that anatase-rutile mixtures/composites synergistically enhance photocatalysis. However, in the case of dense thin-films containing an intimate mix of both anatase and rutile phases, such an effect has not been observed. In synthesising combinatorial films with graded film thickness and phase, and applying established photocatalytic mapping methods, we were able to assess how dense thin-films of intimately mixed anatase-rutile mixtures affect photocatalytic performance. We found that no photocatalytic synergy between anatase-rutile composites (29≤rutile %≤83) within such dense thin-film systems exists. In fact, an increased presence of rutile caused the photocatalytic activity to fall. This was explained by the unfavourable energetics in the multiple electron transfers required between several neighbouring rutile and anatase sites for the photo-generated electron to reach the materials surface; encouraging the trapping of electrons within the bulk and increasing the likelihood of charge recombination. The decrease in photocatalytic activity was found to vary linearly with rutile component.

The relationship between photocatalytic activity and photochromic state of nanoparticulate silver surface loaded titanium dioxide thin-films

Anatase titania thin-films were prepared by a modified spray-pyrolysis method. Glass substrates were coated at room temperature with an aerosol-spray of a titania sol-gel solution and then annealed at 500 °C to form rough, transparent, crystalline thin-films of anatase TiO(2). Silver nanoparticles were deposited on the surface of these films by a photo-assisted deposition method; films were dip-coated in methanolic solutions of silver nitrate salt and then photo-irradiated for 5 h with UVC light. The AgNO(3) concentration was adjusted to create an array of films with varying silver loadings. The films displayed photochromism; changing colour to orange-brown in UV-light to colourless under white light. The rates of photochromic change, when subjected to four different lighting conditions (UVC, UVA, white light and dark), were analysed by UV-visible spectroscopy. By assessing the photocatalytic activity to these light sources it was found that the initial photochromic state of the material had a profound effect on the films photocatalytic ability. This effect was more pronounced in the more concentrated silver loaded films; where significant enhancements in photoactivity occurred when reactions were initiated from the photo-reduced state. The mode of improved photocatalysis was attributed to the photo-generated electron trapping by silver nanoparticles, which stabilised photo-generated holes and drove photo-oxidation processes. We believe this is the first study in which the relationship between the photochromic state of a thin-film and its subsequent photocatalytic activity is reported.

Combinatorial Atmospheric Pressure Chemical Vapor Deposition of Graded TiO2–VO2 Mixed-Phase Composites and Their Dual Functional Property as Self-Cleaning and Photochromic Window Coatings

A combinatorial film with a phase gradient from V:TiO₂ (V: Ti ≥ 0.08), through a range of TiO₂-VO₂ composites, to a vanadium-rich composite (V: Ti = 1.81) was grown by combinatorial atmospheric pressure chemical vapor deposition (cAPCVD). The film was grown from the reaction of TiCl₄, VCl₄, ethyl acetate (EtAc), and H₂O at 550 °C on glass. The gradient in gas mixtures across the reactor induced compositional film growth, producing a single film with numerous phases and compositions at different positions. Seventeen unique positions distributed evenly along a central horizontal strip were investigated. The physical properties were characterized by wavelength dispersive X-ray (WDX) analysis, X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and UV-visible spectroscopy. The functional properties examined included the degree of photoinduced hydrophilicity (PIH), UVC-photocatalysis, and thermochromism. Superhydrophilic contact angles could be achieved at all positions, even within a highly VO₂-rich composite (V: Ti = 1.81). A maximum level of UVC photocatalysis was observed at a position bordering the solubility limit of V:TiO₂ (V: Ti ≈ 0.21) and fragmentation into a mixed-phase composite. Within the mixed-phase TiO₂: VO₂ composition region (V: Ti = 1.09 to 1.81) a decrease in the semiconductor-to-metal transition temperature of VO₂ from 68 to 51 °C was observed.

The extended time evolution size decrease of gold nanoparticles formed by the Turkevich method

Gold nanoparticle (Au NP) solutions were synthesised by the Turkevich reduction method and stored in either the light or dark. All solutions were monitored daily using UV-visible absorption spectroscopy and displayed surface plasmon resonance (SPR), typical of Au nanoparticle colloids. An increase in SPR intensity, a narrowing of the SPR peak as well as a gradual shift towards lower wavenumbers over time indicated a decrease in average nanoparticle diameter and a more mono-dispersed particle size. After two weeks no further changes were observable by UV-visible absorption spectroscopy. A series of high resolution transmission electron micrographs (HRTEM) taken over the evolution period confirmed that the plasmon resonance shifts correlated to a decrease in nanoparticle size. A systematic size decrease in nanoparticle size was also observed for solutions even after centrifugation to remove the excess un-reacted citrate and auric acid. This indicated that the size evolution was independent of further excess reactant chemistries and charge stabilities. The gold nanoparticle evolution followed an inverse Ostwald type growth, by which the size of the NPs decreases, in effect a digestive ripening. The aging process provides a reliable route to fairly mono-dispersed gold nanoparticles of ca. 11.5-12.5 nm in size via the Turkevich method.