Ivan P. Parkin

Band alignment of rutile and anatase TiO2

The most widely used oxide for photocatalytic applications owing to its low cost and high activity is TiO₂. The discovery of the photolysis of water on the surface of TiO₂ in 1972 launched four decades of intensive research into the underlying chemical and physical processes involved. Despite much collected evidence, a thoroughly convincing explanation of why mixed-phase samples of anatase and rutile outperform the individual polymorphs has remained elusive. One long-standing controversy is the energetic alignment of the band edges of the rutile and anatase polymorphs of TiO₂ (ref. ). We demonstrate, through a combination of state-of-the-art materials simulation techniques and X-ray photoemission experiments, that a type-II, staggered, band alignment of ~ 0.4 eV exists between anatase and rutile with anatase possessing the higher electron affinity, or work function. Our results help to explain the robust separation of photoexcited charge carriers between the two phases and highlight a route to improved photocatalysts.

Mechanochemistry: opportunities for new and cleaner synthesis

The aim of this critical review is to provide a broad but digestible overview of mechanochemical synthesis, i.e. reactions conducted by grinding solid reactants together with no or minimal solvent. Although mechanochemistry has historically been a sideline approach to synthesis it may soon move into the mainstream because it is increasingly apparent that it can be practical, and even advantageous, and because of the opportunities it provides for developing more sustainable methods. Concentrating on recent advances, this article covers industrial aspects, inorganic materials, organic synthesis, cocrystallisation, pharmaceutical aspects, metal complexes (including metal-organic frameworks), supramolecular aspects and characterization methods. The historical development, mechanistic aspects, limitations and opportunities are also discussed (314 references).

Self-cleaning coatings

This review summarises the key topics in the field of self-cleaning coatings, concentrating on the materials that have been used in commercial applications and recent research that aims to improve these materials. Hydrophobic and hydrophilic coatings are discussed, and the various mechanisms of self-cleaning are described and related to the material properties of the coatings. Although several multinational companies have released products incorporating self-cleaning coatings, there remains much potential in this field.

Robust self-cleaning surfaces that function when exposed to either air or oil

A robust paintlike repellent coating Superhydrophobic materials often depend on a particular surface patterning or an applied coating. However, these surfaces can be damaged by wear or fouled by oily materials. Lu et al. devised a suspension of coated titanium dioxide nanoparticles that can be spray-painted or dipcoated onto a range of hard and soft surfaces, including paper, cloth, and glass. The coatings resisted rubbing, scratching, and surface contamination. Science, this issue p. 1132 Robust, coated self-cleaning surfaces function after either abrasion or oil contamination. Superhydrophobic self-cleaning surfaces are based on the surface micro/nanomorphologies; however, such surfaces are mechanically weak and stop functioning when exposed to oil. We have created an ethanolic suspension of perfluorosilane-coated titanium dioxide nanoparticles that forms a paint that can be sprayed, dipped, or extruded onto both hard and soft materials to create a self-cleaning surface that functions even upon emersion in oil. Commercial adhesives were used to bond the paint to various substrates and promote robustness. These surfaces maintained their water repellency after finger-wipe, knife-scratch, and even 40 abrasion cycles with sandpaper. The formulations developed can be used on clothes, paper, glass, and steel for a myriad of self-cleaning applications.

Antimicrobial surfaces and their potential in reducing the role of the inanimate environment in the incidence of hospital-acquired infections

Environmental surfaces and their role in the epidemiology of hospital-acquired infections (HAIs) have become an area of great scientific interest, particularly in light of the much publicised cases of infections due to methicillin-resistant Staphylococcus aureus (MRSA) and Clostridium difficile in UK hospitals. This feature article sets out to examine the role of surfaces and the inanimate environment in the spread of HAIs, and looks at various antimicrobial techniques being researched to reduce microbial contamination of surfaces. Preventative measures such as coatings which reduce initial microbial adhesion to surfaces will be considered alongside actively antimicrobial measures which inactivate microorganisms already adherent to a surface. The principal focus of this feature article will be given to light-activated antimicrobial surfaces such as the photocatalyst TiO2 and surfaces with embedded photosensitisers. Surfaces which release antimicrobial compounds or metal ions such as silver and copper are also examined, alongside materials which kill microbes upon contact. The widespread research and development of these antimicrobial surfaces is of great importance in maintaining acceptable levels of hygiene in hospitals and will help to fight the spread of HAIs via the contamination of inanimate surfaces in the healthcare environment.

Titania and silver–titania composite films on glass—potent antimicrobial coatings

Titania (anatase) and Ag-doped titania (anatase) coatings were prepared on glass microscope slides by a sol - gel dip-coating method. The resultant coatings were characterised by X-ray diffraction, X-ray absorption near edge structure (XANES), Raman, scanning electron microscopy (SEM), wavelength dispersive X-ray (WDX) analysis, X-ray photoelectron spectroscopy (XPS) and UV-vis techniques and shown to consist of anatase with ca. 0.2 - 1 atom% Ag2O. Photocatalytic activity of the coatings was determined by photomineralisation of stearic acid, monitored by FT-IR spectroscopy. Photocatalytically-active coatings were screened for their antibacterial efficacy against Staphylococcus aureus (NCTC 6571), Escherichia coli ( NCTC 10418) and Bacillus cereus (CH70-2). Ag-doped titania coatings were found to be significantly more photocatalytically and antimicrobially active than a titania coating. No antimicrobial activity was observed in the dark - indicating that silver ion diffusion was not the mechanism for antimicrobial action. The mode of action was explained in terms of a charge separation model. The coatings also demonstrated significantly higher activity against the Gram-positive organisms than against the Gram-negative. The Ag2O - TiO2 coating is a potentially useful coating for hard surfaces in a hospital environment due to its robustness, stability to cleaning and reuse, and its excellent antimicrobial response.

Characterisation of the photocatalyst Pilkington Activ (TM): a reference film photocatalyst?

Pilkington Glass Activ™ represents a possible suitable successor to P25 TiO2, especially as a benchmark photocatalyst film for comparing other photocatalyst or PSH self-cleaning films. Activ™ is a glass product with a clear, colourless, effectively invisible, photocatalytic coating of titania that also exhibits PSH. Although not as active as a film of P25 TiO2, Activ™ vastly superior mechanical stability, very reproducible activity and widespread commercial availability makes it highly attractive as a reference photocatalytic film. The photocatalytic and photo-induced superhydrophilitic (PSH) properties of Activ™ are studied in some detail and the results reported. Thus, the kinetics of stearic acid destruction (a 104 electron process) are zero order over the stearic acid range 4-129 monolayers and exhibit formal quantum efficiencies (FQE) of 0.7×10−5 and 10.2×10−5 molecules per photon when irradiated with light of 365±20 and 254 nm, respectively; the latter appears also to be the quantum yield for Activ™ at 254 nm. The kinetics of stearic acid destruction exhibit Langmuir-Hinshelwood-like saturation type kinetics as a function of oxygen partial pressure, with no destruction occurring in the absence of oxygen and the rate of destruction appearing the same in air and oxygen atmospheres. Further kinetic work revealed a Langmuir adsorption type constant for oxygen of 0.45±0.16 kPa−1 and an activation energy of 19±1 kJ mol−1. A study of the PSH properties of Activ™ reveals a high water contact angle (67°) before ultra-bandgap irradiation reduced to 0° after prolonged irradiation. The kinetics of PSH are similar to those reported by others for sol-gel films using a low level of UV light. The kinetics of contact angle recovery in the dark appear monophasic and different to the biphasic kinetics reported recently by others for sol-gel films [J. Phys. Chem. B 107 (2003) 1028]. Overall, Activ™ appears a very suitable reference material for semiconductor film photocatalysis.

Preparation and Characterisation of Super-Hydrophobic Surfaces

The interest in highly water-repellent surfaces has grown in recent years due to the desire for self-cleaning surfaces. A super-hydrophobic surface is one that achieves a water contact angle of 150 degrees or greater. This article explores the different approaches used to construct super-hydrophobic surfaces and identifies the key properties of each surface that contribute to its hydrophobicity. The models used to describe surface interaction with water are considered, with attention directed to the methods of contact angle analysis. A summary describing the different routes to hydrophobicity is also given.

Solid state metathesis reaction for metal borides, silicides, pnictides and chalcogenides: ionic or elemental pathways

Solid rate metathesis reactions offer a rapid, low external energy route to a range of inorganic ceramic materials including nitrides, oxides, pnictides, chalcogenides, silicides and borides. The reactions can be filament or bulk thermally initiated and often proceed with a thermal flash or propagation wave. The reactions can follow either an ionic metathetical or a reductive recombination pathway.

The antimicrobial properties of light-activated polymers containing methylene blue and gold nanoparticles

We report the formation of polysiloxane polymers containing embedded methylene blue and gold nanoparticles incorporated by a swell-encapsulation-shrink method. These polymers show significant antimicrobial activity against methicillin-resistant Staphylococcus aureus and Escherichia coli with up to a 3.5 log(10) reduction in the viable count when exposed for 5 min to light from a low power 660 nm laser. The bacterial kill is due to the light-induced production of singlet oxygen and other reactive oxygen species by the methylene blue. Interestingly, the presence of 2 nm gold nanoparticles significantly enhanced the ability of the methylene blue to kill bacteria.