Study of the electrochemical properties of nanostructured TiO2 electrodes

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INTRODUCTION This Ph.D thesis is focused on the preparation of nanostructured TiO2 electrodes with different crystalline phase (anatase and rutile) and morphology (nanowires, nanoparticles, nanotubes, nanocolumns and nanofibers) and on the investigation of their optical, electronic, and photoelectrochemical properties. The electronic structure (distribution of electronic states) and its influence on the photoelectrochemical properties has been studied employing electrochemical (cyclic voltammetry, chronoamperometry and chronopotentiometry) and spectroscopic techniques (UV-vis, electron paramagnetic resonance and surface photovoltage spectroscopy). This rational study has evidenced independently the relevance of the crystalline structure of TiO2 and the effect of the nanoparticle size and morphology on the electrode photoactivity toward water and organic model molecule photooxidation. In addition, the effect of the presence of gold nanoparticles on the TiO2 surface is studied under UV-visible and visible illumination. This Ph.D tries to pave the way towards the design of optimized TiO2 nanostructured electrodes. The main objectives of this thesis can be summarized as follows: To prepare anatase and rutile nanostructures with virtually the same size and morphology (nanowires) in order to study the electronic structure (distribution of electronic states) of the two crystal phases. Additionally, these nanostructures can be used to study the effect of the crystal structure on the photocatalytic activity of TiO2. To study and compare the electronic properties of different TiO2 nanostructured electrodes with different nanoparticle morphology (nanowires, nanoparticles, nanotubes, nanocolumns and nanofibers), paying particular attention to the energetic and spatial location of trap states. These objectives have been pursued employing mainly electrochemical (cyclic voltammetry, chronoamperometry and chronopotentiometry) and spectroscopic techniques (UV-vis spectroscopy and surface photovoltage spectroscopy). To study in a rational way the influence of the particle morphology on the photoelectrochemical properties in acidic (HClO4) and alkaline (NaOH) media and in the presence of various organic substrates (methanol, formic acid, hydrazine). To prepare TiO2 nanotubes and hierarchically organized nanostructures based on TiO2 nanotubes decorated with anatase and rutile nanowires and to study the advantages of having an ordered structure with enhanced interfacial area. To prepare TiO2 nanotube electrodes on Ti and on FTO and to investigate the role of the substrate. To investigate the activity of TiO2 in the visible range of the spectrum after modification of TiO2 nanoparticles with gold nanoparticles employing spectroscopic methods. CONCLUSIONS The general conclusions extracted from this thesis are: 1) The electrochemical techniques (cyclic voltammetry, chronoamperometry, chronopotentiometry) can be successfully employed to obtain information of the reactions taking place at TiO2 nanostructured electrodes. Thermodynamic and kinetic information can be obtained in a simple and rapid manner. An independent study of the two half-reactions, oxidation (anodic) and reduction (cathodic), occuring on the photocatalyst surface can be performed. On the other hand, the application of a bias can be used to diminish electron-hole recombination, being possible to manipulate the rate of the determining step of the overall photocatalytic process (i.e., hole transfer or electron transfer). 2) The electrochemical techniques have been proven to be useful for studying the distribution of electron trap states associated either with surface states or with grain boundaries for nanostructured TiO2 electrodes of different crystalline phase and morphology. 3) The most important parameters influencing the photoelectrochemical properties of TiO2 nanostructures are the crystalline phase, the size and the morphology of the nanoparticles, which are dependent on the preparation method. A right combination of crystal structure (anatase or rutile) and morphology (nanoparticles, nanowires, nanocolumns) is fundamental when applying TiO2 as a photocatalyst. 4) A high surface area of TiO2 electrodes is beneficial for the photooxidation of organic molecules, especially when they are adsorbed on its surface. 5) A thermal treatment of the TiO2 nanostructures can have a great impact on their final photoelectrochemical properties, including the photooxidation of water and also organic compounds such as methanol and formic acid. The specific conclusions extracted from this thesis are: 1) Rutile or anatase transparent thin films composed of nanowires, 2 nm in diameter, assembled in bundles (secondary units) were prepared employing the same chemical bath procedure with different titanium precursors (TiF4 in the case of anatase and TiOSO4 for rutile). Raman and X-ray diffraction data of the TiO2 nanowires confirmed that, in each sample, only one crystalline phase was present. 2) The electronic structure of TiO2 nanowires (band gap energy, conduction band edge and distribution of surface trap states) was successfully determined by employing electrochemical methods (photoinduced chronoamperometry and cyclic voltammetry). An exponential surface state distribution just below the conduction band was revealed in the case of anatase nanowires, while for rutile nanowires, these states were absent. In both cases, additional band gap monoenergetic states associated to grain boundaries are present, whose energetic location is dependent on the TiO2 crystalline phase. The existence of surface states in the case of anatase was also confirmed by electron spin resonance spectroscopy and surface photovoltage. 3) The electrochromic properties of anatase and rutile nanowire electrodes were studied by employing a combination of spectroscopic (UV-vis spectroscopy) and electrochemical methods (cyclic voltammetry and chronoamperometry). Their electrochromic behavior is influenced by their electronic structure. Although both samples show fast electrochromic properties, the coloration efficiency is significantly higher for anatase nanowires. According to this result the electronic structure has an important influence on the coloration efficiency, being the extinction coefficient of the electrons at surface states significantly larger than that of the conduction band electrons. 4) The density and energetic location of grain boundary trap states were highly dependent on both the TiO2 crystalline structure (whether anatase or rutile) and the electrode morphology (determining the facets that meet at the grain boundaries). The trap density was also sensitive to pH changes and to the presence of some adsorbates. The variation of the number of traps with the electrolyte indicates that, on the one hand, an apparent electronic density of states is actually measured, and on the other, that the traps are surface-related. This indicates that their location is likely at the perimeter of the grain boundaries. 5) Similar photoelectrocatalytic properties for methanol and formic acid photooxidation for both anatase and rutile nanowire electrodes with equal active surface areas were observed. For electrodes with the same thickness, the photocurrent obtained for rutile nanowire electrodes in the presence of methanol was higher. The photoelectrocatalytic properties of anatase nanowire electrodes were greatly improved after performing thermal treatments. The photocurrent was enhanced for water, formic acid and methanol photooxidation in acidic media. The photoelectrocatalytic properties of rutile nanowire electrodes for water photooxidation were also improved after the thermal treatments, while they had a deleterious effect on the photooxidation of organic molecules such as methanol and formic acid in acidic media. 6) The influence of the size and the morphology of the nanoparticles on TiO2 photoelectrochemical properties was investigated by comparing on one hand rutile nanowires with different diameter and on the other, rutile nanowires with rutile nanoparticles. The photocurrents obtained for methanol photooxidation with rutile NW electrodes of ~ 5 nm in diameter were significantly smaller than those obtained with nanowire electrodes of ~ 2 nm in diameter. On the other hand, the photocurrents for nanoparticulate electrodes were significantly smaller than those obtained with nanowire electrodes having similar inner surface area in the presence of methanol or formic acid. 7) The decoration of nanotubes with nanowires induced an important enhancement of the interfacial area of the tubular structure. Upon decoration with nanowires, larger photocurrents were measured for both water and oxalic acid photooxidation in comparison with the bare nanotubes. The nanotubes modified with anatase nanowires had superior photoelectrochemical properties than those modified with rutile. This is probably due to a proper band alignment and to a diminished number of electron traps at the nanowire/nanotube boundaries. 8) The nanotube electrodes transferred on FTO are characterized by a higher efficiency for water and metanol photooxidation than those composed of commercial anatase nanoparticles. The transfer of TiO2 nanotubes to FTO eliminate a compact underlayer of rutile that has a deleterious effect on their photoelectrochemical properties. 9) The photogenerated charge transfer in gold modified TiO2 nanostructures was investigated by Surface Photovoltage Spectroscopy and Electron Paramagnetic Resonance spectroscopy. Electron transfer from the gold nanoparticles to the TiO2 conduction band in the case of rutile and to surface states in the case of anatase under visible light illumination is observed. Under UV light illumination an enhanced charge separation was observed in the case of Au-modified rutile and anatase samples.