Colin Boxall

Photocatalyzed Degradation of a Pesticide Derivative Glyphosate in Aqueous Suspensions of Titanium Dioxide

The photocatalytic degradation of a herbicide derivative, glyphosate [(N-phosphonomethyl) glycine] has been investigated in aqueous suspensions of titanium dioxide at different pH values. This compound was found to degrade more efficiently under alkaline pH, where no adsorption takes place on the surface of the catalyst in the dark. The main degradation route involves the cleavage of the P–C bond giving rise to sarcosine and glycine as the intermediate products formed during the photooxidation process.

Micro-optical ring electrode: development of a novel electrode for photoelectrochemistry

The design of a novel photoelectrochemical sensor, the micro-optical ring electrode (MORE), is described. Based on a thin-ring microelectrode and using a fibre-optic light guide as the insulating material interior to the ring, the MORE is capable of delivering light directly to the region of electrochemical measurement and can therefore be used to conduct microelectrochemical studies of systems with complex photochemistry. A novel fabrication procedure is described, involving the coating of commercially available fibre optics (radius 1.25 × 10–4 m) with a 600 nm layer of gold, so allowing exploitation of the electroanalytical advantages peculiar to thin-ring microelectrodes. The dark electrochemistry of the thin-ring microelectrode is characterized by use of cyclic voltammetry and chronoamperometry and found to agree with previously published theoretical results. Preliminary exploration of the photoelectrochemical response of the MORE is reported, achieved via the interrogation of the photoelectrochemically active phenothiazine dye methylene blue (MB+). Photocurrent signals obtained during cyclic voltammetric and chronoamperometric studies of MB+, conducted with the MORE under illuminated conditions and in the absence of any deliberately added reducing agent, are attributed to the formation and subsequent detection of 3MB+ within the diffusion layer of the microring electrode. The data demonstrate that the use of the MORE for the direct electrochemical detection of photogenerated species with lifetimes of < 9 × 10–5 s is possible. The electrochemistry of 3MB+ over the applied potential range from –0.4 to +1.0 V versus SCE is elucidated and discussed in the context of the behaviour of photoexcited MB+ in the presence of the deliberately added reducing agent Fe2+.

Oxidation-reduction reactions of simple hydroxamic acids and plutonium(IV) ions in nitric acid

Summary Simple hydroxamic acids such as formo- and aceto-hydroxamic acids have been proposed as suitable reagents for the separation of either Pu and/or Np from U in modified or single cycle Purex based solvent extraction processes designed to meet the emerging requirements of advanced fuel cycles. The stability of these hydroxamic acids is dominated by their decomposition through acid hydrolysis. Kinetic studies of the acid hydrolysis of formo- and aceto-hydroxamic acids are reported in the absence and the presence of Pu(IV) ions. The slow reduction of these plutonium(IV) hydroxamate complexes to Pu(III) aquo-ions has been characterised by spectrophotometry and cyclic voltammetry. The reductions of Pu(IV) in the presence of FHA and AHA are consistent with a mechanism in which free hydroxamic acid in solution is hydrolysed whilst Pu(IV) ions remain fully complexed to hydroxamate ligands; then at some point close to a 1 : 1 Pu(IV) : XHA ratio, some free Pu4+ is released from the complex and reduction is initiated. Electrochemical and kinetic data suggest that the reductant is the hydroxamic acid rather than the hydroxylamine.

Photoelectrophoresis of colloidal semiconductors. Part 1.—The technique and its applications

A Malvern Zetasizer IIc laser Doppler electrophoresis apparatus has been modified to enable the effect of illumination on electrophoretic mobilities of aqueous dispersions of semiconductor particles to be studied. The electrophoretic mobilities of TiO2 particles were determined in the dark and whilst exposed to monochromatic light, the wavelength of which was varied between experiments. The results showed that a positive charge accumulated on the particles in the absence of easily reducible or oxidizable species in solution. This effect was observed only when the particles were illuminated with photons of ultra-band gap energy, demonstrating unambiguously that it was related to the generation of electron—hole pairs and their subsequent reactions with Ti—O surface groups. Similar shifts were observed for suspensions of SnO2, Ti4O7 and Fe(CN)4–6 photosensitized TiO2 particles. No such shift was observed for CdS particles in the absence of any excess lattice ions, probably owing to decomposition of the particles to generate a surface layer of photo-inactive sulphur.

An in situ electrochemical quartz crystal microbalance study of polycrystalline gold electrodes in nitric acid solution

The dissolution and redeposition processes of Au in HNO3 solution at pH 1 and 0 were investigated as a function of sweep rate by in situ EQCM experiments. The results demonstrate that, at pH 1 and vless than or equal to10 mV s(-1), An dissolves through a 3 e(-) oxidation at E > +1.0 V versus saturated calomel electrode (SCE), a process that, at vgreater than or equal to10 mV s(-1), is kinetically hindered with respect to upd of OH surface sublattices and subsequent 2-D-phase oxide formation. The inhibition arises from the presence of a close packed layer of nitrate oxyanions at the gold surface, although the layer is disrupted and the inhibition removed by the Au dissolution and reprecipitation processes that occur during potential cycling at low v

Photoelectrophoresis of colloidal semiconductors. Part 2.—Transient experiments on TiO2 particles

We report experimental data and a second-order kinetic model for the time dependence of photo-induced changes in electrophoretic mobilities of aqueous dispersions of TiO2 semiconductor particles when irradiated with photons of energy corresponding to greater than their band gap (ca. 3.2 eV), in the absence of hole and electron scavengers. This is the first time that (laser Doppler) electrophoresis has been used to obtain kinetic information, in this case for analysing photocatalytic processes relevant to solar energy harvesting for chemical synthesis/destruction.The second-order rate constant for intra-particle electron–hole recombination, kr′ was determined as > 106 times greater than either the pseudo-first-order rate constants for charged surface site generation from holes and electrons, k′A and k′B, respectively; this indicates that recombination was a dominant process in determining the efficiency of surface charge generation in the absence of charge carrier scavengers. The rate constants for positive and negative ionic surface charge removal by electrons and holes ke and kh, respectively, were ca. 1–20 m3 mol–1 s–1, and compare very closely with the value of kr′ indicating that these processes compete with direct electron–hole recombination. The quantum yield, ϕ, for hole/electron capture by oxidizable/reducible sites on the particle surface was in the range (5.2–7.9)× 106.

Fabrication and Characterisation of the Graphene Ring Micro Electrode (GRiME) with an Integrated, Concentric Ag/AgCl Reference Electrode

We report the fabrication and characterisation of the first graphene ring micro electrodes with the addition of a miniature concentric Ag/AgCl reference electrode. The graphene ring electrode is formed by dip coating fibre optics with graphene produced by a modified Hummers method. The reference electrode is formed using an established photocatalytically initiated electroless deposition (PIED) plating method. The performance of the so-formed graphene ring micro electrodes (GRiMEs) and associated reference electrode is studied using the probe redox system ferricyanide and electrode thicknesses assessed using established electrochemical methods. Using 220 μm diameter fibre optics, a ∼15 nm thick graphene ring electrode is obtained corresponding to an inner to outer radius ratio of >0.999, so allowing for use of extant analytical descriptions of very thin ring microelectrodes in data analysis. GRiMEs are highly reliable (current response invariant over >3,000 scans), with the concentric reference electrode showing comparable stability (current response invariant over >300 scans). Furthermore the micro-ring design allows for efficient use of electrochemically active graphene edge sites and the associated nA scale currents obtained neatly obviate issues relating to the high resistivity of undoped graphene. Thus, the use of graphene in ring microelectrodes improves the reliability of existing micro-electrode designs and expands the range of use of graphene-based electrochemical devices.

Photoassisted Degradation of a Herbicide Derivative, Dinoseb, in Aqueous Suspension of Titania

The titanium dioxide (TiO2) photoassisted degradation of herbicide dinoseb has been examined in aqueous suspensions under UV light irradiation. The degradation kinetics were studied under various conditions such as substrate concentration, type of catalyst, catalyst dosage, pH, and light intensity as well as in presence of electron acceptors such as hydrogen peroxide, potassium bromate, and potassium persulphate under continuous air purging, and the degradation rates were found to be strongly influenced by these parameters. The Degussa P25 was found to be more efficient photocatalyst as compared to other photocatalysts tested. Dinoseb was found to degrade efficiently in acidic pH and all the electron acceptors studied enhanced the degradation rate. The results manifested that the photocatalysis of dinoseb followed pseudo-first-order kinetics. A qualitative study of the degradation products generated during the process was performed by GC-MS, and a degradation mechanism was proposed.

Photoinduced superhydrophilicity: a kinetic study of time dependent photoinduced contact angle changes on TiO2 surfaces.

Transparent TiO(2) thin films were prepared on quartz substrates via a reverse micelle, sol-gel, spin-coating technique. The time dependence of the TiO(2) film photoinduced superhydrophilicity (PISH) was measured by goniometric observation of the contact angle, θ, of sessile water drops at the film surfaces. In these measurements, the TiO(2) substrate was illuminated by 315 nm light and drops were sequentially applied at a range of illumination times. Using a model for the wetting of heterogeneous surfaces derived by Israelachvili and Gee, these measurements were used to calculate the time dependence of f(2), the fractional surface coverage of the TiO(2) surface by adventitious contaminating organics (Israelachvili, J. N.; Gee, M. L. Contact angles on chemically heterogeneous surfaces. Langmuir 1989, 5, 288). Extending this model to include a Langmuir-Hinshelwood based kinetic analysis of f(2) as a function of time allowed for calculation of an expected value for θ immediately prior to illumination, that is, at illumination time t = 0. Such expected values of θ at t = 0 were calculated using two possible values of θ(1), the contact angle on a pristine unilluminated homogeneous TiO(2) surface: (i) θ(1) = 4° as suggested by, inter alia, Zubkov et al. (Zubkov, T.; Stahl, D.; Thompson, T. L.; Panayotov, D.; Diwald, O.; Yates, J. T. Ultraviolet Light-Induced Hydrophilicity Effect on TiO(2)(110)(1 × 1). Dominant Role of the Photooxidation of Adsorbed Hydrocarbons Causing Wetting by Water Droplets. J. Phys. Chem. B2005, 109, 15454); and (ii) where θ(1) = 25°, as suggested by Fujishima et al., representative of a more hydrophobic homogeneous TiO(2) surface that reconstructs upon exposure to ultraband gap illumination into a hydrophilic surface where θ(1) → 0° (Fujishima, A.; Zhang, X.; Tryk, D. A. TiO(2) photocatalysis and related surface phenomena Surf. Sci. Rep.2008, 63, 515). Analysis of data from our experiments and from selected literature sources demonstrates better agreement between these calculated and experimental values of θ at t = 0 when θ(1) is taken to be 4°, implying that an uncontaminated TiO(2) surface is inherently hydrophilic. The results of this study are discussed in the context of the current debate over the origin of the photoinduced superhydrophilic effect.

Photoelectrophoresis of colloidal iron oxides. Part 2.—Magnetite (Fe3O4)

Aqueous dispersions of colloidal magnetite have been prepared by aqueous precipitation and characterised using X-ray diffraction (XRD). Their surface chemistry was studied using photo-electrophoresis. Changes in the electrophoretic mobility of colloidal magnetite, indicative of the formation of net surface positive charge may be observed upon irradiation with ultra-band-gap energy photons at pH less than about 7. This was attributed to the hole driven photo-oxidation of surface > Fe—OH sites to form (> Fe—OH)+ sites. Photogenerated holes concomitantly oxidised the magnetite surface to maghaemite while the photogenerated electrons were removed from the particles by reductive dissolution of the Fe3O4 surface. Between pHs of ca. 7 and 12, the mobility change upon illumination was insignificant, reflecting reductive dissolution then being thermodynamically disallowed. At pH 12, photogenerated holes were removed from the particles by the photoanodic corrosion of the Fe3O4 surface. The remaining electrons reduce surface FeIIItet sites to FeIItet, with an attendant change in the observed electrophoretic mobility of colloidal magnetite congruous with the observed formation of net surface negative charge.The photoelectrophoretic mobility–illumination wavelength spectrum of colloidal Fe3O4 exhibited three distinct mobility change onsets: one each at 1.8, 2.2 and 3.1 eV, reflecting the band structure of magnetite. Comparison with the photoelectrophoretic mobility–illumination wavelength spectrum of α-Fe2O3 showed the latter two mobility change onsets to be common to both materials, a result of the structural similarities between magnetite and haematite.The oxidation of surface > Fe—OH groups responsible for the increase in net positive surface charge of colloidal magnetite at pH less than about 7, was found to occur at the same rate as the corresponding reaction on colloidal haematite under identical conditions. Further, the oxidation of > Fe—OH on both materials was found to occur ten times more slowly than the corresponding reaction on colloidal TiO2.