Photoelectrochemical Methanol Oxidation Research Articles

(Invited) Photoelectrochemical Methanol Oxidation Under Visible and UV Excitation of TiO2-Supported TiN and Zrn Plasmonic Nanoparticles

TiN and ZrN nanoparticles (NPs) are an emerging class of plasmonic materials that can be used for harvesting energy of visible light in order to drive chemical reactions on the photocatalyst surface [1-3]. These refractory NPs effectively absorb light over l = 450-1200 nm, covering most of the solar spectrum. Both TiN and ZrN have the optical appearence of gold, but with the advantages of thermal stability, corrosion resistance and low cost. Previous theoretical and experimental research have shown that TiN does not exhibit as strong of a plasmonic resonance as gold (broader response, sample variability due to defect-dependent metallicity) [2, 4]. ZrN is expected to have a sharper blue-shifted local surface plasmon resonance (LSPR) response in comparison to TiN [4, 5]. Indeed, this theoretical prediction was verified by observation of a sharp plasmonic maximum for ZrN NPs decorated by a thin layer of SiN [4]. However, ZrN NPs appear to be less chemically inert owing to zirconium’s strong affinity for oxygen. A layer of dielectric ZrO2 forms on the surface of ZrN NPs even in the presence of trace oxygen. The oxide layer has a detrimental effect on the LSPR, leading to its broadening and red-shifting [5].Herein both experimental and computational approaches are used in order to optimize the performance of TiO2-supported TiN and ZrN NPs towards photoelectrochemical methanol (CH3OH) oxidation under visible excitation. UV excitation is utilized in order to provide complementary information on the interaction between photogenerated carriers at the plasmonic NP/semiconductor interface.The effects of plasmonic catalyst loading and applied potential were examined. Reaction products were determined by gas chromatography-mass spectrometry analysis of ZrN/TiO2 and TiN/TiO2 aqueous suspensions. Optical spectra for both TiN and ZrN NPs were computed using COMSOL including calculations of these NPs in solution and embedded in a TiO2 matrix. To examine the effect of oxide layer, the NPs were decorated with the corresponding oxide layers.ZrN NPs were synthesized by ammonolysis of Zr(NMe2)4. Clean Zr(NMe2)4 was prepared following a protocol from the literature [6]. In-house made ZrN powder and commercial TiN NPs (PlasmaChem) were dispersed into a P25 TiO2 matrix via the sonochemically mediated mixing of the transition metal nitride and TiO2 NPs in 50:50 (v/v) H2O:ethanol mixture overnight. The resulting photocatalysts contained 0.5 - 5 wt. % loadings of TiN and ZrN on TiO2. The ZrN/TiO2, TiN/TiO2 and bare TiO2 films for photoelectrochemical experiments were prepared by drop-casting 60 mL of ink comprising 20 mg of the photocatalyst, 30 ml of 5 wt% Nafion solution (Ion Power), 2.96 ml H2O and 0.74 ml iso-propanol, on FTO slide (coated surface area of 0.385 cm2). Experiments were conducted in a three-electrode photoelectrochemical cell. The photocatalyst films deposited on FTO-coated glass substrates served as working electrodes, while Pt foil and Ag/AgCl in 3 M NaCl (BioLogic, Inc) were used as counter and reference electrodes, respectively. Nine single-color LED lights were employed for illumination. The intensities varied between 10-100 mW/cm2, and 300-480 mW/cm2 in the wavelength range of 490-670 and 730-960 nm, respectively.Our results indicate that optical properties and photocatalytic activity of ZrN/TiO2 are strongly affected by ZrN surface oxidation and agglomeration. We found that under visible illumination, both in-house synthesized 17 nm ZrN and commercial 30 nm TiN NPs promote TiO2 activity for CH3OH oxidation, while under visible + UV excitation, an inhibition effect is observed. The differences between the TiN/TiO2 and ZrN/TiO2 interfaces are discussed and the mechanisms of promotion/inhibition of TiO2 photocatalytic activity by ZrN and TiN NPs are proposed.

Photoelectrochemical Methanol Oxidation Under Visible and UV Excitation of TiO2-Supported TiN and ZrN Plasmonic Nanoparticles

TiN and ZrN refractory transition metal nitride nanoparticles (NPs) have recently emerged as an alternative to noble metals in plasmonic applications. However, plasmon-driven photocatalysis by ZrN NPs is largely unexplored. In this study, optical properties, morphology, crystal structure and surface composition of in-house synthesized and commercial ZrN nanoparticles (NPs) are vigorously characterized in order to select the best candidate material for evaluation of activity towards CH3OH photoelectrochemical oxidation. The photocatalytic activity of TiO2-supported ZrN NPs is compared to that of TiN/TiO2 as a function of NP loading and illumination wavelength. Our results indicate that optical properties and photocatalytic activity of ZrN/TiO2 are strongly affected by ZrN surface oxidation and agglomeration. We found that under visible illumination, both in-house synthesized 17 nm ZrN and commercial 30 nm TiN NPs promote TiO2 activity for CH3OH oxidation, while under visible + UV excitation, an inhibition effect is observed. The differences between the TiN/TiO2 and ZrN/TiO2 interfaces are discussed and the mechanisms of promotion/inhibition of TiO2 photocatalytic activity by ZrN and TiN NPs are proposed. Electromagnetic simulations are used to facilitate interpretation of experimental extinctions and photocatalytic activities.

Platinum nanoparticles with TiO2–skin as a durable catalyst for photoelectrochemical methanol oxidation and electrochemical oxygen reduction reactions

Performance degradation of Pt nanoparticles is considered to be among the most severe problems for electrochemical reactions. Pt–TiO2 catalysts are widely studied due to the optical effect of TiO2, which is beneficial for electron transfer during methanol oxidation. However, different configurations of these catalysts are insufficient in research. Here, we report rationally designed Pt nanoparticles with TiO2-skin as electrocatalysts for methanol oxidations and oxygen reduction reactions. Methanol oxidation current densities of the Pt–TiO2 nanoparticles are largely improved under irradiation. When the catalysts are tortured successive at ultrahigh current up to 1.0 mA cm−2, the potential-increase is as small as 90 mV. It shows good stability with a peak current fades of 4% after 250 cycles under illumination, compared with 13% in the dark. It also exhibits a much better oxygen reduction durability compared with Pt/C. By mechanism analysis, we believe the excellence in photoelectrochemistry is attributed to the TiO2-skin. The electrons generated in TiO2 spontaneously migrate to Pt nanoparticles, which provide a partially reducing atmosphere to prevent Pt nanoparticles from ionization and dissolution.

Comparing Photoelectrochemical Methanol Oxidation Mechanisms for Gold versus Titanium Nitride Nanoparticles Dispersed in TiO2 Matrix

Using nanostructured TiO2 as the common matrix, TiN/TiO2 and Au/TiO2 photocatalysts are compared in their activity for photoelectrochemical methanol oxidation testing both visible and UV excitation. We demonstrate that both TiN and Au nanoparticles (NPs) act as photo-sensitizers extending TiO2 activity into the visible region of white light. Our results highlight two major differences between the TiN/TiO2 and Au/TiO2 photocatalysts. First, potential bias is required to initiate visible light-induced CH3OH oxidation at the TiN/TiO2 interface that forms an ohmic junction. Second, the reaction at the TiN/TiO2 interface is most likely driven by the highly reactive holes generated by interband transitions in TiN NPs at wavelengths shorter than 630 nm, while plasmonic effect peaking at 580 nm is thought to be a major driving force for the CH3OH oxidation on the Au/TiO2 material. Under UV excitation, an increase in the saturation photocurrent vs. bare TiO2 is observed both for TiN/TiO2 and Au/TiO2 photocatalysts. We attribute this behavior to (1) the participation of highly reactive holes generated by interband transitions in TiN NPs in photoelectrochemical CH3OH oxidation, and (2) better electron-hole separation at the Au/TiO2 interface due to back transfer of photoexcited electrons from the conduction band of TiO2 to Au. Theoretical modeling is performed to assist with interpretation of optical and photoelectrochemical results.

(Invited) The Benefits of Nanoscale Metal Oxide Films to Solar Fuels Research

Though the field of photoelectrochemical (PEC) solar fuel production focuses on a broad variety of materials, metal oxides are probably the most commonly employed materials in state-of-the-art photoelectrodes. Metal oxides such as bismuth vanadate and hematite are currently intensely studied as photoanode materials in our group and by the PEC water oxidation community.1-4 Stable metal oxides such as TiO2 have been successfully employed as protection layers in, for example, Cu2O photocathodes and are now advancing the field of organic photocathodes for hydrogen evolution.5, 6 Especially metal oxides grown by atomic layer deposition (ALD) offer many benefits to solar fuel research such as i) precise growth control on the atomic scale ii) conformal and pinhole-free coatings and ii) formation of high-quality interfaces. In our studies of recombination losses, we benefit from high-quality ALD hematite films and are able to gain new insights from electrochemical impedance spectroscopy that will be reported here. Furthermore, we could demonstrate stable organic photocathodes for hydrogen evolution and will report on new organic photocathode designs employing ALD TiO2 electron selective layers.6 Finally, I will discuss the importance of ALD surface treatments in the science of CO2 reduction.7 Le Formal, F.; Pastor, E.; Tilley, S. D.; Mesa, C. A.; Pendlebury, S. R.; Gratzel, M.; Durrant, J. R. Rate Law Analysis of Water Oxidation on a Hematite Surface. Journal of the American Chemical Society 2015, 137, 6629-6637.Mesa, C. A.; Kafizas, A.; Francas, L.; Pendlebury, S. R.; Pastor, E.; Ma, Y. M.; Le Formal, F.; Mayer, M. T.; Gratzel, M.; Durrant, J. R. Kinetics of Photoelectrochemical Oxidation of Methanol on Hematite Photoanodes. Journal of the American Chemical Society 2017, 139, 11537-11543.Ma, Y. M.; Mesa, C. A.; Pastor, E.; Kafizas, A.; Francas, L.; Le Formal, F.; Pendlebury, S. R.; Durrant, J. R. Rate Law Analysis of Water Oxidation and Hole Scavenging on a Bivo4 Photoanode. Acs Energy Letters 2016, 1, 618-623.Ma, Y. M.; Kafizas, A.; Pendlebury, S. R.; Le Formal, F.; Durrant, J. R. Photoinduced Absorption Spectroscopy of Copi on Bivo4: The Function of Copi During Water Oxidation. Advanced Functional Materials 2016, 26, 4951-4960.Luo, J. S.; Steier, L.; Son, M. K.; Schreier, M.; Mayer, M. T.; Gratzel, M. Cu2o Nanowire Photocathodes for Efficient and Durable Solar Water Splitting. Nano Letters 2016, 16, 1848-1857.Steier, L.; Bellani, S.; Rojas, H. C.; Pan, L. F.; Laitinen, M.; Sajavaara, T.; Di Fonzo, F.; Gratzel, M.; Antognazza, M. R.; Mayer, M. T. Stabilizing Organic Photocathodes by Low-Temperature Atomic Layer Deposition of Tio2. Sustainable Energy & Fuels 2017, 1, 1915-1920.Schreier, M.; Heroguel, F.; Steier, L.; Ahmad, S.; Luterbacher, J. S.; Mayer, M. T.; Luo, J. S.; Gratzel, M. Solar Conversion of Co2 to Co Using Earth-Abundant Electrocatalysts Prepared by Atomic Layer Modification of Cuo. Nature Energy 2017, 2.

Photoelectrochemical Methanol Oxidation by TiN Nanoparticles Supported on TiO2

Photocatalytic activity of TiO2-supported TiN and Au nanoparticles towards methanol oxidation is investigated under both visible and UV light excitation. We demonstrate that the TiN/TiO2 photocatalyst performs better than the Au/TiO2 under visible excitation at potentials more positive than 0.1 V vs a Ag/AgCl reference electrode. Under UV excitation, photocurrent enhancement is observed for both the TiN/TiO2 and the Au/TiO2 photocatalysts in the entire potential region from -0.6 to 0.5 V, as compared to pure TiO2. The photocurrent enhancement under UV excitation was assigned to (1) better charge separation in the Au/TiO2 photocatalyst, and (2) generation of highly reactive holes due to interband transitions in TiN and Au nanoparticles.

Oxidation−Stable Plasmonic Copper Nanoparticles in Photocatalytic TiO2 Nanoarchitectures

We describe the incorporation of plasmonic copper (Cu) nanoparticles into titania (TiO2) aerogels and demonstrate surface plasmon resonance (SPR)–driven photoelectrochemical oxidation of methanol under visible light. Copper could provide a more abundant, less expensive alternative to gold and silver as a visible light–active plasmonic metal, however, the propensity of Cu to oxidize at the expense of its plasmonic character greatly limits its potential applications. Most reports of supported, plasmonic Cu nanoparticles describe the necessity of preventing Cu oxidation by maintaining strictly anoxic environments or relying on surface-obscuring chemical stabilizers. We stabilize plasmonic Cu nanoparticles by establishing extensive interfacial contact between Cu nanoparticles and a TiO2 aerogel support. The multiple points of contact between 2–3 nm Cu nanoparticles photodeposited at the networked ~10 nm TiO2 particulates of the aerogel stabilizes Cu against oxidation to an extent that preserves the plasmonic behavior of the nanoparticles, even after long-term exposure to ambient air. The wavelength dependence of photoelectrochemical methanol oxidation at Cu/TiO2 aerogel photoanodes verifies the plasmonic origin of the photoelectrochemistry. Plasmonic behavior is not seen for Cu photodeposited at a commercially available, non-networked TiO2 nanopowder in which the primary particle size is ~10× greater than the size of the supported Cu particle. The divergent behavior of Cu when a 3D Cu||oxide interface is formed on aerogel supports compared to the 1D junction between Cu on commercial TiO2 nanopowder supports highlights the importance of the interfacial design motif on preserving plasmonic Cu.

Fabrication of a composite of platinum, N-g-C3N4 and Ketjen Black for photo-electrochemical methanol oxidation

Recently N-doped graphitic carbon-incorporated g-C3N4 (denoted as N-g-C3N4) exhibits better electroconductivity and photo-catalytic property compared with pure g-C3N4 as a support material for Pt nanoparticles (NPs). A composite catalyst consisting of Pt nanoparticles and N-g-C3N4-modified KB was designed and fabricated. Pt nanoparticles were prepared through a solution plasma process; N-g-C3N4 was prepared by direct heating the mixture of urea and citric acid monohydrate and subsequently treated by hydrothermal method. Their chemical composition, particle size and microstructure were investigated by X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron microscopy. Compared with the Pt/KB catalyst, ternary Pt/N-g-C3N4/KB nanocomposites exhibit excellent electrocatalytic properties for methanol oxidation owning to the significant synergistic effects of individual components. Once light irradiation was supplied, Pt/N-g-C3N4/KB catalyst exhibits exceptional forward peak current densities as high as 2800 mA mg Pt −1 . And Pt/N-g-C3N4/KB catalyst also shows better CO tolerance than Pt/KB. Such a Pt/N-g-C3N4/KB composite catalyst may bring new design ideas for photo-assisted direct methanol fuel cell in the future.

Chemically reduced graphene oxide-P25-Au nanocomposite materials and their photoelectrocatalytic and photocatalytic applications.

Visible light active photocatalysts consisting of gold nanoparticle (Au NP) decorated chemically reduced graphene oxide-P25 nanocomposite materials (CRGO-P25-Au NCMs) were prepared through a one-pot chemical reduction method. The nanocomposite materials were characterized using diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and electrochemical impedance spectroscopy (EIS) analyses. The performances of CRGO-P25-Au NCM modified ITO electrodes were evaluated towards the photoelectrochemical oxidation of methanol. The photoelectrode fabricated using CRGO-P25-Au NCM exhibited a higher photocurrent of 293 μA cm-2 compared to other control electrodes. The CRGO-P25-Au NCMs were also used for the photocatalytic reduction of highly toxic chromium(vi) ions to chromium(iii) ions in the presence of oxalic acid as a sacrificial electron donor. The results showed that around 75% of the Cr(vi) ions were photocatalytically reduced to Cr(iii) ions by the CRGO-P25-Au NCM within the light irradiation time of 1 h. In both applications, the enhanced catalytic activity of the CRGO-P25-Au NCM was attributed to the improved visible light absorption and the reduced charge recombination exerted by the interaction of CRGO and Au NPs with P25 and their synergistic effects.

Photoelectrochemical Hydrogen Production on α‐Fe2O3 (0001): Insights from Theory and Experiments

The photoelectrochemical (PEC) decomposition of organic compounds in wastewater is investigated by using quantum chemical (DFT) methods to evaluate alternatives to water splitting for the production of renewable and sustainable hydrogen. Methanol is used as a model organic species for the theoretical evaluations of electrolysis on the surface of the widely available semiconductor hematite, α-Fe2 O3 , a widely studied photocatalyst. Three different α-Fe2 O3 surface terminations were investigated, including the predominant surface found in aqueous electrolytes, (OH)3 R. The PEC oxidation of methanol is energetically downhill, producing CO2 and protons. The protons are reduced to hydrogen on the cathode. Experimental PEC measurements were also performed for several polyalcoholic compounds, glycerol, erythritol, and xylitol, on α-Fe2 O3 as the photocatalyst and showed high incident-photon-to-current-efficiencies (IPCE) that were much greater than those of water splitting. Interestingly, high IPCEs were observed for hydrogen production from polyalcohols in the absence of any applied bias, which was not thought to be possible on hematite. These results support the potential application of PEC for hydrogen production by using widely available hematite for the PEC oxidation of selected components of organic wastewater present in large quantities from anthropogenic and industrial sources.

Platinized titanium dioxide electrodes for methanol oxidation and photo-oxidation (Dedicated to 75th birthday of Professor Sergio Trasatti)

Platinized deposits have been formed on TiO 2 particulate films supported on Ti substrates, by means of galvanic replacement of pre-deposited metallic Cu and subsequent immersion of the Cu/TiO 2 coatings into a chloroplatinic acid solution. The spontaneous replacement of Cu by Pt results in Pt(Cu)/TiO 2 /Ti electrodes. Both the platinized and the precursor TiO 2 /Ti electrodes have been characterized by SEM micro­scopy/EDS spectroscopy, their surface electrochemistry has been assessed by cyclic voltammetry in the dark and their photoelectrochemical properties by photovolta­m­metry under UV illumination. It has been found that, although platinized rutile-rich electrodes exhibit typical Pt surface electrochemistry, the anatase-rich electrodes show only traces of oxide formation and stripping. The latter has been translated to a suppression of methanol oxidation at anatase-rich electrodes. On the contrary, methanol oxidation at platinized rutile-rich electrodes occurs at significant rates and can be further enhanced upon UV illumination, as a result of Pt and TiO 2 synergism in the photoelectrochemical oxidation of methanol.

Platinized titanium dioxide electrodes for methanol oxidation and photo-oxidation

Platinized deposits have been formed on TiO2 particulate films supported on Ti substrates, by means of galvanic replacement of pre-deposited metallic Cu and subsequent immersion of the Cu/TiO2 coatings into a chloroplatinic acid solution. The spontaneous replacement of Cu by Pt results in Pt(Cu)/TiO2/Ti electrodes. Both the platinized and the precursor TiO2/Ti electrodes have been characterized by SEM microscopy/EDS spectroscopy, their surface electrochemistry has been assessed by cyclic voltammetry in the dark and their photoelectrochemical properties by photovoltammetry under UV illumination. It has been found that, although platinized rutile-rich electrodes exhibit typical Pt surface electrochemistry, the anatase-rich electrodes show only traces of oxide formation and stripping. The latter has been translated to a suppression of methanol oxidation at anatase-rich electrodes. On the contrary, methanol oxidation at platinized rutile-rich electrodes occurs at significant rates and can be further enhanced upon UV illumination, as a result of Pt and TiO2 synergism in the photoelectrochemical oxidation of methanol.

Comparison of the photoelectrochemical oxidation of methanol on rutile TiO2 (001) and (100) single crystal faces studied by intensity modulated photocurrent spectroscopy

The photooxidation of methanol as a model substance for pollutants on rutile TiO(2) (001) and (100) surfaces was investigated using intensity modulated photocurrent spectroscopy (IMPS). The results are analyzed in view of the influence of the surface structure, the methanol concentration and the electrode potential on the rate constants of charge transfer and recombination. The obtained results have been explained with a model combining the theory of IMPS for a bulk semiconductor surface and the nature of the surface-bound intermediates (alternatively mobile or immobile OH˙ radicals). The results indicate that water photooxidation proceeds via mobile OH˙ radicals on both surfaces, while methanol addition gives rise to the involvement of immobile OH˙ radicals on the (100) surface. Detailed analysis in view of the surface structures suggests that the latter observation is due to efficient electron transfer from bridging OH˙ radicals on the (100) surface to methanol, while coupling of two of these radicals occurs in the absence of methanol, making them appear as mobile OH˙ radicals. In the case of the (001) surface, the coupling reaction dominates even in the presence of methanol due to the smaller distance between the bridging OH˙ radicals, leading to more efficient water oxidation, but less efficient methanol photooxidation on this surface.

Functionalized Silicate Sol−Gel-Supported TiO2−Au Core−Shell Nanomaterials and Their Photoelectrocatalytic Activity

The N-[3-(trimethoxysilyl)propyl]ethylenediamine (EDAS) derived silicate matrix supported core-shell TiO(2)-Au nanoparticles (EDAS/(TiO(2)-Au)(nps)) were prepared by NaBH(4) reduction of HAuCl(4) precursor on preformed TiO(2) nanoparticles in the presence of EDAS monomer. The core-shell (TiO(2)-Au)(nps) nanoparticles were stabilized by the amine functional group of the EDAS silicate sol-gel network. The potential application of this EDAS/(TiO(2)-Au)(nps) modified electrode toward the photoelectrochemical oxidation of methanol was explored. The EDAS/(TiO(2)-Au)(nps) modified electrode showed a 12-fold enhancement in the catalytic activity toward photoelectrooxidation of methanol when compared to TiO(2) dispersed in EDAS silicate sol-gel matrix. This improved photoelectrochemical performance is explained on the basis of beneficial promotion of interfacial charge transfer processes of the EDAS/(TiO(2)-Au)(nps) nanocomposite. A methanol oxidation peak current density of 12.3 mA cm(-2) was achieved at an optimum loading of Au(nps) on TiO(2) particles. This novel amine functionalized EDAS silicate sol-gel stabilized core-shell (TiO(2)-Au)(nps) nanomaterial could be an excellent candidate for the photocatalytic and photoelectrochemical applications.

Photoactive WO3 and Se-WO3 thin films for photoelectrochemical oxidation of organic compounds

Thin films of bare and Se-containing tungsten trioxide (WO3) on AISI304-type stainless steel were prepared by electrochemical deposition using peroxy-tungstate solutions. The obtained films were characterized by X-ray diffraction, photoelectron spectroscopy, scanning electron microscopy, thermal and photovoltammetry analyses. The oxidation of methylene blue, phenol, and methanol was used to evaluate the photoelectrocatalytic activity of the prepared films. It has been established that the incident photon-to-current efficiency (IPCE) in 0.1-M K2SO4 decreases as the concentration of methylene blue and phenol increases. On the contrary, the IPCE values increase with the increase in initial concentration of methanol. The bulk electrolysis experiments revealed that the prepared films are stable and can be used for photoelectrochemical oxidation of methanol.

Evolution of Physical and Photocatalytic Properties in the Layered Titanates A2Ti4O9 (A = K, H) and in Nanosheets Derived by Chemical Exfoliation

K2Ti4O9 has been known as a photocatalyst for the oxidation of methanol under UV irradiation. Here we study the evolution of morphology, optical, and photocatalytic properties of this titanate as it is converted into H2Ti4O9 and subsequently exfoliated into individual tetrabutylammonium (TBA)-supported [Ti4O9]2- nanosheets. We find that proton exchange and exfoliation are accompanied by a red shift of the optical absorption edge and fluorescence maximum, suggesting a reduction of the bandgap in the series K2Ti4O9 (3.54 eV), H2Ti4O9 (3.25 eV), TBA2Ti4O9 (3.00 eV). Neither compound is active for photochemical water splitting, even after photochemical deposition of platinum nanoparticles. However, in aqueous methanol, all platinated compounds are moderately active for H2 evolution upon bandgap irradiation, and in 0.01 M AgNO3, they all produce moderate quantities of O2. From the onset potentials for photoelectrochemical methanol oxidation, the values for the valence band edges at pH = 7 are deduced to lie be...

…and for titanium dioxide

The mechanism of the photoelectrochemical oxidation of methanol and salicylic acid on anatase film electrodes was studied as a function of the applied potential and pollutant concentration at pH 3. The dependencies of the steady state photocurrents on substrate concentration reflect the type of surface interaction: weak in the case of methanol, that leads to a simple saturation curve, and strong in the case of salicylate, that shows a steady state photocurrent peaking at intermediate concentrations. At 0.6 V vs. SCE the oxidation rate is largely enhanced as compared to open circuit conditions (Eoc=−0.3 V). Even under nitrogen, the reaction proceeds at an appreciable rate, and the ratio of circulated charge to the number of oxidized salicylate ions approaches 28 electrons per mol at low salicylate concentration: oxidized salicylate mineralizes almost totally, and the intermediates are rapidly destroyed. At higher substrate concentrations, the ratio decreases, and uv spectral evidence suggests the formation of some undefined oxidation products. Under oxygen at 0.6 V, the radicals generated in the initial photoelectrochemical step are mostly oxidized by O2, increasing the amount of salicylate destroyed for a given total circulated charge; at sufficiently high substrate concentration, the above ratio decreases to values below 4. No evidence of the presence of traces of partially oxidized molecules is found. Adequate control of the experimental conditions permits therefore to achieve substantially increased efficiencies of salicylate destruction per absorbed photon, and the build up of uncontrolled intermediates can be prevented. The results are discussed in terms of the oxidation length Y, defined as the number of oxidation steps that are triggered by a single hole transfer event, and of the oxidation efficiency ϵ, defined as the ratio of the oxidation length to the maximum possible oxidation length (the length achieved when one hole transfer suffices to trigger total mineralization).

Photoelectrochemical behaviour of methanol oxidation at nanoporous TiO2 film electrodes

A study of photoelectrochemical oxidation of methanol at nanoporous TiO2 film electrodes was carried out. The effect of variables such as applied potential, light intensity, methanol concentration and pH on the photoelectrochemical behaviour was investigated. It was found that the photoresponse was greatly influenced by these factors and a unique photocurrent/potential characteristic of the electrode in solution was observed which was similar to that of the single crystal semiconductor electrode. It was also found that the rate-limiting step for the overall photooxidation process varied depending on potential, light intensity and methanol surface coverage. In low potential region at a given light intensity and methanol concentration the migration of electron across the TiO2 film is the rate-limiting step. While in the relatively high potential region the interfacial charge transfer was found to be the rate-limiting step. A deviation of onset potential dependence on pH from the Nernstian relationship was also observed, and based on the energy band theory and the existence of deep electron traps an explanation was given. An unusual abrupt increase of photocurrent in strong basic methanol solution was observed, which was attributed to the existence of acidic hydroxyl group on TiO2 and its stronger adsorption to methanol molecule after its reaction with OH−, and the higher reactivity of the methanol thus adsorbed.

THE INFLUENCE OF APPLIED BIAS POTENTIAL ON THE PHOTOOXIDATION OF METHANOL AND SALICYLATE ON TITANIUM DIOXIDE FILMS

The mechanism of the photoelectrochemical oxidation of methanol and salicylic acid on anatase film electrodes was studied as a function of the applied potential and pollutant concentration at pH 3. The dependencies of the steady state photocurrents on substrate concentration reflect the type of surface interaction: weak in the case of methanol, that leads to a simple saturation curve, and strong in the case of salicylate, that shows a steady state photocurrent peaking at intermediate concentrations. At 0.6 V vs. SCE the oxidation rate is largely enhanced as compared to open circuit conditions ( E oc=−0.3 V). Even under nitrogen, the reaction proceeds at an appreciable rate, and the ratio of circulated charge to the number of oxidized salicylate ions approaches 28 electrons per mol at low salicylate concentration: oxidized salicylate mineralizes almost totally, and the intermediates are rapidly destroyed. At higher substrate concentrations, the ratio decreases, and uv spectral evidence suggests the formation of some undefined oxidation products. Under oxygen at 0.6 V, the radicals generated in the initial photoelectrochemical step are mostly oxidized by O 2, increasing the amount of salicylate destroyed for a given total circulated charge; at sufficiently high substrate concentration, the above ratio decreases to values below 4. No evidence of the presence of traces of partially oxidized molecules is found. Adequate control of the experimental conditions permits therefore to achieve substantially increased efficiencies of salicylate destruction per absorbed photon, and the build up of uncontrolled intermediates can be prevented. The results are discussed in terms of the oxidation length Y, defined as the number of oxidation steps that are triggered by a single hole transfer event, and of the oxidation efficiency ϵ, defined as the ratio of the oxidation length to the maximum possible oxidation length (the length achieved when one hole transfer suffices to trigger total mineralization).