Abstract
The chemistry of systems intended to produce metal-doped, nanostructured oxide materials containing titanium(IV) has been investigated. Manganese and iron were chosen as cheap, sustainable metal atom dopants with a view to possible use of product materials in electrochemical devices. In particular, the aim was to enhance electronic conductivity and to introduce psudocapacitance for investigation in supercapacitor devices. Previous literature in this area was found to be misleading; products of earlier synthetic studies have been incompletely characterised, with solid products being assumed to be single crystalline phases and metal atom contents being assumed on the basis of hypothetical doping levels rather than being determined analytically. Electrochemical properties of product materials in electrode films were determined in this study via use of EIS, CV and galvanostatic charge-discharge techniques. Conventional sol-gel and micro-emulsion synthetic methods were undertaken with the view to control of product particle sizes and yields for single phase, Mn-doped titanium(lV) oxide (ideally within the anatase structure). Other than for the pure binary oxides of manganese or titanium, the products obtained were mixtures of oxides rather than single phases, with titanium(lV) in the rutile polymorph within the mixed-phase products. "Doped" product materials produced by the emulsion method showed no improvements in conductivity or specific capacitance when compared with a standard anatase (undoped) sample. The mixed-oxide materials produced by the sol-gel method, containing MnO2, had enhanced conductivities but no improvement in specific capacitance. Single-phase, anatase structured, Mn-doped titanium dioxide materials, were prepared using the continuous hydrothermal synthesis (CHS) method. The maximum amount of Mn successfully incorporated into the anatase structure was found to be approximately 8%wt, as determined by AAS, ICP-MS and SEM-EDX analytical methods; high Mn-content resulted in phase segregation and irreversibihty within cyclic voltammograms, as evident in observation of an oxidation peak but no reduction peak. The highest recorded specific capacitance value for these samples was that for the CHSMn008 sample (theoretical composition Ti0.92Mn0.08O2), with a value of 4.5 F g-1 (oxide specific surface area, 34.8 m2 g-1). This compared to a similarly prepared anatase (undoped) material with a specific surface area of 48.6 m2 g-1 and a specific capacitance of 2.5 F g-1. No correlation between specific surface areas and specific capacitance values was observed, but the BET-determined specific surface areas were for powdered samples whereas capacitances refer to composite electrode films. The initial CHS product materials displayed no improvement in conductivity, but pH-adjusted synthesis led to samples (denoted CHS MnKOH) with higher conductivities consequent on higher Mn contents, and these samples also had the highest specific capacitance values. In view of the success of the CHS method in producing Mn-doped anatase materials, it was also used to synthesise Fe-doped titanium(IV) oxides. The products were a range of single phase materials, but syntheses aimed at high Fe-contents led to mixtures, TiO2 + Fe oxides + Fe2TiOs- the maximum amount of Fe incorporated whilst still maintaining a pure single phase product was approximately 7%wt. The conductivity of these materials showed marked improvement relative to the standard (undoped) anatase sample, comparable to those for CHS MnKOH samples, but the Mn-doped samples had higher specific capacitances than Fe-doped counterparts. The highest specific capacitance values recorded were between 25 and 32 F g-1, regardless of dopant (Mn/Fe) and/or phase purity, and were associated with a high specific surface area or a high concentration of “dopant” within the sample.