Anodic tin oxide films: fundamentals and applications

Abstract

The study of the electrochemical oxidation behaviour of tin in alkaline media, the development of anodic self-ordered tin oxide nanostructures and its application to laboratory scale devices have been the main focus of this PhD Thesis.

The passivation process of Sn has been studied from an electrochemical approach covering the whole range from hydrogen evolution to the final passivation of the metal. The oxides formed in each relevant potential region have been characterized by ex situ techniques and a mechanism for the process has been proposed by correlating composition, morphology and electronic properties. Three main processes have been identified: the formation of a SnO·nH2O primary passive layer; the hydroxyl induced etching of tin and concurrent SnO precipitation; and the final passivation due to the growth of Sn(IV) based oxides. Also, in situ techniques like EC-STM were used to gain more knowledge on the growth mechanism, especially for the first oxidation process. Before the onset of the peak, the formation of islands and metal dissolution and redeposition phenomena were observed. The composition of these islands is still under study.

The knowledge gained while attaining this first goal is used for the design of protocols to develop self-ordered anodic tin oxide structures. In this view, several electrolytes were explored to find one that allows growing self-ordered tin oxide structures without the actual limitations of the films prepared in oxalic acid or NaOH: clogged pores and cracks on its cross-section. Once proper structures were attained in the new Na2S and NH4F electrolyte, applications such as gas sensing or photoelectrochemical water splitting were attempted in order to assess their value.

The as-grown nanochannelled tin oxide structures are in general amorphous so an appropriate thermal treatment is required to convert them into SnO2, which is a material commonly applied in hydrogen sensors. Our layers proved to have superior characteristics by detecting H2 concentrations as low as 9 ppm and by operating at relatively low temperatures (80 ºC). The performance is comparable to that of noble-metal doped SnO2 particles or single SnO2 nanowires and superior to that of other self-ordered structures prepared in non-optimized conditions or oxalic acid. The opening of the pores provides a higher specific area whereas the continuity of the channels improves the conducting properties and the transference of electrons through the structure.

By adjusting the annealing conditions, SnO2 nanochannelled structures with absorption in the visible range were prepared. A band gap of 2.4 eV was achieved for self-ordered tin oxide structures annealed at 200 ºC in Argon atmosphere due to the improved crystallinity, with respect to as-grown samples, together with the conservation of the Sn2+ defects and oxygen vacancies. These layers could have potential applications in photoelectrochemical water splitting or photocatalysis. Despite the enhancement in collecting visible light, the structures were found to be unstable in the photoanode working conditions. The further oxidation occurring during the photoelectrochemical performance seems to arise from the Sn2+ defects and vacancies themselves. An alternative approach to build up photoanodes is proposed. The system combines the good conducting properties and the high surface area of antinomy doped nanochannelled SnO2 structures with the stability and good visible absorption of hematite nanoparticles. By building up this host-guest system some of the limitations of hematite like its poor conductivity and extremely short hole diffusion length are overcome. Although the role of SnO2 here is just as charge collector, the overall performance of the photoanodes is shown to be strongly dependent on its characteristics. Photocurrents attained are in line with the values reported for other reported hematite nanostructured electrodes.

Aquesta tesi doctoral s’estructura en dos grans blocs: d’una banda es pretén estudiar el procés d’oxidació electroquímica i passivació de l’estany en solucions alcalines per tal d’entendre els processos que hi estan involucrats, i de l’altre desenvolupar nanoestructures anòdiques d’òxid d’estany amb aplicacions reals en dispositius com sensors de gasos o ànodes per a la fotòlisi de l’aigua.

L’estudi fonamental de la passivació de l’estany s’ha dut a terme combinant tècniques electroquímiques clàssiques amb mètodes de caracterització ex situ per a determinar la composició i morfologia dels òxids formats en la zona activa i passiva de potencial. Amb això s’ha pogut establir que el primer procés anòdic correspon a la formació d’una pel·lícula amorfa de SnO·nH2O mitjançant un mecanisme de dissolució-precipitació, seguidament té lloc la precipitació de cristalls de SnO com a producte de l’atac preferencial tant de la primera capa com del metall, i finalment l’elèctrode passiva gràcies a l’aparició d’un òxid a la interfase Sn/SnO que conté Sn(IV). El primer procés anòdic s’ha seguit també amb tècniques in situ com el EC-STM per a extreure informació sobre els canvis que tenen lloc a la superfície a nivell nanomètric.

L’anodització autoorganitzada és un mètode habitual per a obtenir nanotubs o nanopors de manera relativament senzilla en metalls com el Ti o el Al, però per al cas de l’estany es troba encara en una fase molt preliminar. Així doncs, es va treballar en el desenvolupament i l’optimització d’un nou electròlit basat en Na2S i NH4F fins a assolir estructures d’òxid d’estany en forma de nanocanals amb pors totalment oberts i sense fissures en la secció (estructures multicapa). Un cop tractades tèrmicament aquestes pel·lícules van demostrar una molt bona resposta per a la detecció de H2 (9 ppm a temperatures de 80 ºC). Altrament, es van construir fotoànodes combinant les nanoestructures de SnO2 dopades amb Sb i nanopartícules de Fe2O3. La eficiència final d’aquests fotoelèctrodes en la producció de H2 a partir de l’aigua depèn fortament del dopatge amb Sb.