Kinetics of liquid phase semiconductor photoassisted reactions: Supporting observations for a pseudo-steady-state model

The kinetics of liquid phase semiconductor photocatalytic and photoassisted reactions are an area of some debate, reignited recently by an article by Ollis(1) in which he proposed a simple pseudo- steady- state model to interpret the Langmuir- Hinshelwood type kinetics, commonly observed in such systems. In the current article, support for this model, over other models, is provided by a reinterpretation of the results of a study, reported initially in 1999,2 of the photoassisted mineralization of 4- chlorophenol, 4-CP, by titania films and dispersions as a function of incident light intensity, I. On the basis of this model, these results indicate that 4- CP is adsorbed more strongly on P25 TiO2 when it is in a dispersed, rather than a film form, due to a higher rate constant for adsorption, k(1). In addition, the kinetics of 4- CP removal appear to depend on I-beta where, beta = 1 or 0.6 for when the TiO2 is in a film or a dispersed form, respectively. These findings are discussed both in terms of the pseudo- steady- state model and other popular kinetic models.

Demonstration of a novel, flexible, photocatalytic oxygen-scavenging polymer film

The preparation of a novel, flexible, photocatalytic, oxygen-scavenging polymer film is described. The film incorporates nanocrystalline titania particles in an ethyl cellulose polymer film, with or without an added sacrificial electron donor of triethanolamine. When coated on the inside of a glass vessel its UV-driven light-scavenging action is demonstrated by platinum octaethyl porphyrin coated glass beads sealed inside, since their luminescence increases with increasing UV-irradiation time. When used as a flexible film, work with an oxygen electrode shows that the film is able to scavenge oxygen at an average rate of 0.017 cm(3) O-2 h(-1) cm(-2) over a 24 h period, which compares favourably to other, well-established oxygen-scavenger systems. The potential of using such as system for oxygen scavenging in packaging is discussed briefly. (c) 2005 Elsevier B.V. All rights reserved.

The photo-oxidation of water by sodium persulfate, and other electron acceptors, sensitised by TiO2

A number of different electron acceptors are tested for efficacy in the oxidation of water to oxygen, photocatalysed by titanium dioxide. The highly UV-absorbing metal ion electron acceptors, Ce4+ and Fe3+, appear ineffective at high concentration (10(-2) M), due to UV-screening, but more effective at lower concentrations (10(-3) M). The metal-depositing electron acceptor, Ag+, is initially effective, but loses activity upon prolonged irradiation due to metal deposition which promotes electron-hole recombination as well as UV-screening the titania particles. Most striking of the electron acceptors tested is persulfate, particularly in alkaline solution (0.1 M NaOH). The kinetics of the photo-oxidation of water by persulfate, photocatalysed by titania are studied as a function of pH, [S2O82-] and incident light intensity (I). The initial rate of water oxidation increases with pH, is directly proportional to the concentration of persulfate present and depends upon I-0.6. The TiO2/alkaline persulfate photosystem is robust and shows very little evidence of photochemical wear upon repeated irradiation. The results of this work are discussed with regard to previous work in this area and current mechanistic thinking. The formal quantum efficiency of the TiO2/alkaline persulfate photosystem was estimated as ca. 2%. (C) 2004 Elsevier B.V. All rights reserved.

Light-driven oxygen scavenging by titania/polymer nanocomposite films

We demonstrate that UV illumination of nanocrystalline TiO2 films in the presence of excess organic hole scavengers can result in the deoxygenation of a closed environment. The kinetics of deoxygenation are investigated under continuous UV illumination as a function of film preparation and hole scavenger employed. Optimum deoxygenation is observed using methanol as a hole scavenger, although efficient deoxygenation is also observed for a range of different polymer/TiO2 nanocomposite films deposited on glass and plastic substrates. Transient absorption spectroscopy is used to probe the kinetics of the deoxygenation reaction, focusing on the kinetics of the reduction of oxygen by photogenerated TiO2 electrons. Under aerobic conditions, this oxygen reduction reaction is observed to exhibit first order kinetics with a rate constant of 70 s(-1), more than one order of magnitude faster than alternative reaction pathways for the photogenerated electrons. These observations are discussed in terms of the Langmuir-Hinshelwood equation for photocatalytic action. (C) 2004 Elsevier B.V. All rights reserved.

Atmospheric Pressure Chemical Vapour Deposition of thin Films of Nb2O5 on Glass

A novel route involving atmospheric pressure chemical vapour deposition (APCVD) is reported for coating Nb2O5 onto glass substrates via the reaction of NbCl5 and ethyl acetate at 400-660degreesC. Raman spectroscopy is shown to be a simple diagnostic tool for the analysis of these thin films. The contact angle of water on Nb2O5-coated glass drops on UV irradiation from 60degrees to 5-20degrees. XPS Analysis showed that the Nb:O ratio of the film was 1:2.5. Glancing angle X-ray diffraction showed that all films were crystalline, with only a single phase being observed; this has some preferred orientation in the (201) plane of Nb2O5. The niobium(V) oxide materials show minimal photocatalytic ability to degrade organic material.

Alkaline hydrolysis of trinitrotoluene, TNT

The kinetics of the alkaline hydrolysis of trinitrotoluene, TNT, in an aqueous solution is a possible approach to destroying the active agent in unwanted munitions. The kinetics are shown to have a rapid initial step, step A, in which a highly coloured species, X (lambda(max) = 450 nm) is formed via an equilibrium reaction: TNT + OH- double left right arrow X. The bimolecular rate constant for the forward part of this equilibrium process, k(1), is: 0.099 +/- 0.004, 0.32 +/- 0.02 and 1.27 +/- 0.05 dm(3) mol(-1) s(-1), at 25, 40 and 60degreesC, respectively. The activation energy for the forward process is 60 kJ mol(-1). The first-order rate constant for the reverse of this process, k(-1), is: (5.3 +/- 2.6) x 10(-4), (1.2 +/- 1.0) x 10(-3) and (7.7 +/- 2.9) x 10(-3) s(-1) at 25, 40 and 60degreesC, respectively. The activation energy for the overall equilibrium process (k(1)/k(-1)) is ca. -5 kJ mol(-1). The subsequent alkaline hydrolysis of X to form the final product P, i.e. step B, is much slower than step A and appears to comprise two processes coupled in series, i.e. steps B1 (X +2OH(-) --> Z) and B2 (Z+OH- --> P). At 25degreesC, Step B1 appears rate determining throughout the decay process. At 45 degreesC and, more so, at 60degreesC, step B appears increasingly biphasic with increasing alkaline concentrations, as step B2 begins to compete with step B1 for position as the rate determining step. The trimolecular rate constant for step B1 is: 0.017 +/- 0.001, 0.0085 +/- 0.0002 and 0.0011 +/- 0.0001 dm(6) mol(-2) s(-1) at 25, 40 and 60degreesC, respectively, and the process has an activation energy of 64 kJ mol(-1). The transition from uniform kinetics, described by step B1, to mixed kinetics, described by steps B1 and B2, as the reaction temperature and alkali concentration are increased most likely occurs because (a) step B2 has a lower activation energy than B1, although it was not possible to measure the former parameter, and (b) step B2 has a lower (1st) order dependence upon [OH-] compared with that of step B1 (2nd). The bimolecular rate constant for step B2 is 0.0035 +/- 0.03 dm(3) mol(-1) s(-1) at 60degreesC. A brief NMR study of the initial hydrolysis product in water, acetone and chloroform, coupled with UV/visible spectra, provides evidence that species X is a Meisenheimer complex.

Thick titanium dioxide films for semiconductor photocatalysis

Thick paste TiO2 films are prepared and tested for photocatalytic and photoinduced superhydrophilic (PSH) activity. The films are effective photocatalysts for the destruction of stearic acid using near or far UV and all the sol-gel films tested exhibited a quantum yield for this process of typically 0.15 %. These quantum yields are significantly greater (4-8-fold) than those for titania films produced by an APCVD technique, including the commercial self-cleaning glass product Activ(TM). The films are mechanically robust and optically clear and, as photocatalysts for stearic acid removal, are photochemically stable and reproducible. The kinetics of stearic acid photomineralisation are zero order with an activation energy of ca. 2.5 Kj mol(-1). All titania films tested, including those produced by APCVD, exhibit PSH. The light-induced fall, and dark recovery, in the water droplet contact angle made with titania paste films are similar in profile shape to those described by others for thin titania films produced by a traditional sol-gel route. (C) 2003 Elsevier Science B.V. All rights reserved.

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

Pilkington Glass Activ(TM) represents a possible suitable successor to P25 TiO2, especially as a benchmark photocatalyst film for comparing other photocatalyst or PSH self-cleaning films. Activ(TM) 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(TM) 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(TM) 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 X 10(-5) and 10.2 x 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(TM) 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(TM) reveals a high water contact angle (67) before ultra-bandgap irradiation reduced to 0degrees 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(TM) appears a very suitable reference material for semiconductor film photocatalysis. (C) 2003 Elsevier Science B.V All rights reserved.

Preparation and characterisation of novel thick sol-gel titania film photocatalysts

The preparation and characterization of thick (9 mum), clear, mechanically robust and photocatalytically active films of nanocrystalline anatase titania are described. XRD and SEM analysis show the films comprise 13 nm particles of anatase TiO2. Thin (54 nm) films of the 'paste' TiO2, along with sol-gel titania films made by a more traditional route are also prepared and characterised. All titania films mediate the photocatalytic destruction of stearic acid with a quantum yield of 0.0016 +/- 0.0003. using either 365 nm (i.e. BLB) or 254 nm (germicidal) light. P25 TiO2 films also appear to mediate the same process with a similar formal quantum efficiency. Of all the films tested, the thick paste TiO2 films are the most ideally suited for use with near UV light, for reasons which are discussed. All the titania films tested exhibit photoinduced superhydrophilicity.

Titania and tungsten doped titania thin films on glass; active photocatalysts

Acidification of an isopropanol solution containing mixtures of [Ti(OPri)(4)] and [W(OEt)(5)] produced solutions from which various TiO2, WO3 and TiO2/WO3 thin films could be obtained by dip coating and annealing. The films were analysed by X-ray diffraction, SEM/EDAX, Raman, electronic spectra, contact angle and photoactivity with respect to destruction of an over layer of stearic acid. The TiO2/WO3 films were shown to be mixtures of two phases TiO2 and WO3 rather than a solid solution TixWyO2. The 2% tungsten oxide doped titania films were shown to be the most effective photocatalysts. All of the TiO2 and TiO2/WO3 films showed light induced superhydrophillicity. (C) 2002 Elsevier Science Ltd. All rights reserved.

Atmospheric pressure chemical vapour deposition of titanium dioxide coatings on glass

Atmospheric pressure chemical vapour deposition of titanium dioxide coatings on glass substrates was achieved by the reaction of TiCl4 and a co-oxygen source (MeOH, EtOH, (PrOH)-Pr-i or H2O) at 500-650degreesC. The coatings show excellent uniformity, surface coverage and adherence. Growth rates were of the order of 0.3 mum min(-1) at 500degreesC. All films are crystalline and single phase with XRD showing the anatase TiO2 diffraction pattern; a = 3.78(1), c = 9.51(1) Angstrom. Optically, the films show minimal reflectivity from 300-1600 nm and 50-80% total transmission from 300-800 nm. Contact angles are in the range 20-40degrees for as-prepared films and 1-10degrees after 30 min irradiation at 254 nm. All of the films show significant photocatalyic activity as regards the destruction of an overlayer of stearic acid.

A web-based overview of semiconductor photochemistry-based current commercial applications

The major current commercial applications of semiconductor photochemistry promoted on the world wide web are reviewed. The basic principles behind the different applications are discussed, including the use of semiconductor photochemistry to: photo-mineralise organics, photo-sterilise and photo-demist. The range of companies, and their products, which utilise semiconductor photochemistry are examined and typical examples listed. An analysis of the geographical distribution of current commercial activity in this area is made. The results indicate that commercial activity in this area is growing world-wide, but is especially strong in Japan. The number and geographical distribution of patents in semiconductor photocatalysis are also commented on. The trends in the numbers of US and Japanese patents over the last 6 years are discussed. (C) 2002 Elsevier Science B.V. All rights reserved.

Novel TiO2 CVD films for semiconductor photocatalysis

A novel CVD film of titanium(IV) oxide has been prepared on glass, via the reaction of titanium(IV) chloride and ethyl acetate, using a CVD technique. The film is clear, very robust mechanically and comprised of a thin (24 nm) layer of nanocrystalline anatase titania that absorbs light of lambda

The kinetics of semiconductor photocatalysis: Light intensity effects

The kinetics of the photomineralisation of 4-chlorophenol, 4-CP, by oxygen, sensitized by TiO2 as a function of incident light intensity are described. Degussa P25 TiO2 in the form of either a thin film or a dispersion is used as the photocatalyst. With a TiO2 dispersion the initial rate of photomineralisation, R-i, depends upon I-0.64, implying that electron-hole recombination is the dominant process with respect to photogenerated holes (where R-i is expected proportional to I-0.5), but that the light intensities used spanned both the high (R-i expected proportional to I-0.5) and low (R-i expected proportional to I) intensity regimes. With a TiO2 film R-i is proportional to I indicating that the photocatalytically active particles of the TiO2 film are shielded in some way and so operate under low intensity conditions. Most significantly, it was also found that the apparent value of the Langmuir adsorption coefficient, K4-CP, as determined from the kinetic data, was not independent of I for either a TiO2, film or dispersion photocatalyst. Rather K4-CP increased with decreasing light intensity. A possible mechanism is suggested as a rationale for the observed light intensity effects reported.

Photobleaching of methylene blue sensitised by TiO2: an ambiguous system?

The photobleaching of methylene blue MB, sensitised by TiO2, in an aqueous solution is studied in the absence and presence of oxygen, in the absence of oxygen and in the presence of a sacrificial electron acceptor (SED), MB is photoreduced to its colourless leuco form, LMB, by the TiO2 photocatalyst. This same photoreduction process is observed even if an SED is not present, indicating that MB itself can act as an SED. The oxidation of LMB by oxygen to regenerate MB is significantly slower if the aqueous solution is acidified (0.01 mol dm(-3) HClO4) and, at low partial pressures, the rate of reaction depends directly upon the concentration of dissolved oxygen. The TiO2-sensitised photobleaching of MB is irreversible in an oxygen-saturated aqueous solution, as expected, since the bleaching was due to an oxidative process. However, in an acidified solution (0.01 mol dm(-3) HClO4), the photobleaching process, in an oxygen-saturated solution, generates LMB initially. The latter situation arises because, under acidic conditions, LMB reacts only very slowly with oxygen to form MB. The significance of these findings with respect to the popular use of photobleaching of MB as a demonstration of semiconductor photomineralisation is discussed. (C) 1999 Elsevier Science S.A. All rights reserved.