Fabrication of one-dimensional titanium oxide and tungsten oxide nanostructures and their applications on separated gate field-effect transistor pH sensors

Abstract

This dissertation aims at fabricating one-dimensional (1D) nanostructured metal oxide materials and applying them into pH sensor application due to theirs higher surface-to-volume (SV) ratio and physical-chemical stability 1D titanium oxide (TiOx) nanorods (NR) and tungsten oxide (WOx) nanotape (NT) are fabricated by hydrothermal growth method (HTG) using autoclave and thermal oxidation method in a furnace respectively In the former the morphology evolution and possible growth mechanism of the processes of 1D TiOx nanostructures synthesis are investigated by controlling varied precursor concentrations in the same growing times For the past many papers had been used the ‘non-classical growth mechanism’ to explain the process of the relevant TiOx nanostructures fabricated by solution method But the more direct evidences are need to be found to clarify the morphology evolution and possible growth mechanism for the case of 1D TiOx nanostructures hydrothermally grown on fluorine-doped tin oxide (FTO) transparent thin-film substrate In the initial growth stage TiOx based precursors can be precipitated and agglomerated on the substrate by heterogeneous nucleation and amorphous TiOx nanoparticles (NPs) may be simultaneously formed in the solution by homogeneous nucleation respectively Then the nanowires (NWs) are formed with time and it is noted that they are always parallel arranged by six to form nanotapes (NTs) These morphologies and sizes are intimately dependent on the growth conditions With growing up these nanostructures proceeded nanosheets (NSs) and final into nanorods (NRs) morphology Furthermore the growth characteristics such as oriented attachment (OA) and ‘iso-oriented crystal’ formation which occur in the synthesis of TiOx nanostructures by HTG stands that the growth process is dominant by ‘non-classical growth mechanism’ The latter some growth mechanisms had been proposed in the early studies and indicated that the parallel-growth WOx NTs can be cohered along preferential growth direction [010] to form NSs Due to stress relaxation between the cohered interface the 1D periodic defects are presented Different crystal growth characteristics are appeared in TiOx and WOx nanostructures which are fabricated by HTG and thermal oxidation methods respectively To realize the crystal growth mechanism can help us tailor the morphology size crystal phase or the type/number of defects for nanostructures and whether they can be used as the sensing part of pH sensors or discover the sensing mechanism Finally these fabricated 1D TiOx NRs and WOx NTs are applied to separated-gate field effect transistor (SGFET) pH sensors We expect that when they are used as the sensing electrodes of pH sensors the sensitivity can be effectively improved Furthermore the possible pH sensing mechanism and degradation factors for sensitivity are discussed In the first chapter the development of nowadays nanostructured metal oxides will be introduced with the unique optical electrical thermal physical and chemical characteristics in which compared to that counterpart bulk material With the devices nanoscaling the ‘quantum confinement effect (QCE)’ is occurred and widely used to modify optoelectronic devices However for sensor applications the band gap widening effect could reduce the visible light interference to decrease noise Furthermore the exponential enhanced material adsorption ability of nanostructured metal oxides will be favorable for improving pH sensitivity The relevant theories of 1D metal oxide growth and sensing mechanism of potentiometric based pH sensor are introduced in the second chapter In the beginning we theoretical describe the presented exponential enhanced material adsorption ability through nanostructuring the metal oxides or semiconductors Then classical and non-classical crystal growth mechanisms are introduced to explain the processes of metal oxide nanostructures syntheses Generally non-classical crystal growth mechanism is occurred on solution-based methods in which using organic compounds as precursors or adding surfactants According to thermodynamic concept for classical growth mechanism when the sizes of nanostructures grow overwhelming the critical size of material stable state the net Gibbs free energy will continue to decrease (thermodynamic control) On the other hand non-classical growth mechanism can be proceeded multi-steps of metastable states and final form the single crystal (kinetic control) Then for pH sensors application the sensing mechanisms of electrodes of pH sensors such as electrochemical potential Nernst equation and electrical double layer (EDL) are introduced Finally the operation principle and performance evaluation for the SGFET pH sensor are demonstrated In the third chapter to prepare the analyzing samples based on TiOx nanostructures which hydrothermally grown on transparent FTO glass substrate the varied precursor concentrations with the same growth times respectively are controlled The morphology evolution and possible crystal growth mechanism are investigated by scanning-electron microscopy (SEM) transmission-electron microscopy (TEM) selected area electron diffraction (SAED) pattern and X-ray diffraction (XRD) instruments These results indicate that the morphology evolution can be classified into four stages i e heterogeneous nucleation on substrate and simultaneous amorphous NPs formation in solution then formation the morphologies of NWs/NTs in the same time Successively the NSs and NRs were formed with the growth time These crystal phases of TiOx nanostructures analyzed by XRD all present rutile characteristic Furthermore by TEM images and SAED patterns investigation the amorphous NPs can be preformed and composed into successive TiOx nanostructures and we also find that the possible metastable state of ‘iso-oriented crystal’ As our best knowledge this process and phenomenon are first proposed in the experiment Thus these growth processes of nanostructures through orientated attachment (OA) phenomenon and iso-oriented crystal pathways can be ascribed to non-classical growth mechanism Summary the experimental results we construct a simplified model used to explaining that the morphology evolution and possible growth mechanism of TiOx nanostructures which are fabricated in autoclave by HTG method In the fourth chapter the controllable morphology and growth process of TiOx nanostructures and many merits of SGFET pH sensor are first combined and shows Nernstian response The enhanced SV ratio of nanostructured TiOx can facilitate the pH sensitivity by adsorbing/absorbing more detecting ions/molecules Due to the intrinsic anti-corrosion property of TiOx it is suitable for strong acid-base measuring environment In the experiments the measuring scales are in the wide range of pH=2~12 In order to elucidate the effect of enhanced SV ratio in which resulting the better pH sensitivity Thus based on the same HTG condition we merely tune the placing angle of FTO glass substrate in autoclave and successively fabricate the TiOx thin-film morphology as the sensing electrode of pH sensor According to experimental results 1D TiOx NRs as sensing electrode of SGFET pH sensor shows superior pH sensitivity of 62 mV/pH in which compared to thin-film type (50 mV/pH) The pH sensitivity has been improved by 24% and it can be ascribed to 1D rutile TiOx NRs with anisotropic growth direction along c-axis higher SV ratio and the better crystallinity In the fifth chapter the metallic W film is firstly deposited on n-Si substrate by DC sputter and then put it into horizontal furnace to synthesize 1D W18O49 NTs by thermal oxidation method This 1D W18O49 NTs is first employed into SGFET pH sensor in the experiment After applying it as the sensing electrode of SGFET pH sensor it shows 33 1 mV/pH (r2=0 97522) pH sensing response For comparison purpose three metallic W films without and with thermal annealing of 350oC and 550oC are fabricated in the same growth condition and then applied as sensing electrodes of SGFET pH sensors They present the pH sensitivity of 65 86 mV/pH (r2=0 99722) 61 4 mV/pH (r2=0 99511) and 44 3 mV/pH (r2=0 95158) respectively Although the metallic W film as sensing electrode of SGFET pH sensor presents super-Nernstian response the stability is poor than on the state-of-art of metal oxide based pH sensors after carrying out the stability measurements Furthermore in order to demonstrate the degradation factors of pH sensitivity material analyses such as XRD energy dispersive spectroscopy (EDS) hall-effect measurement and contact-mode conducting atomic force microscopy (c-AFM) are carried out The results show that the numbers of grain boundaries play a key role in degradation the pH sensitivity based on metallic bulk samples On the other hand for semiconductor 1D W18O49 NTs the poor sensitivity can be ascribed to intrinsic monoclinic crystal structure and not along c-axis preferential growth direction which can possibly reduce the equivalent total charge distribution of EDL Notwithstanding 1D W18O49 NTs is applied as the sensing part of SGFET pH sensor and it can’t acquire the good pH sensitivity But the possible degradation factors for sensing mechanism is demonstrated we believe that this data can facilitate the development of SGFET pH sensor Finally the advantages and disadvantages of the SGFET pH sensors operation performance by using TiOx and WOx nanostructures respectively are summarized Furthermore some suggestions and future works are listed to facilitate the development of metal oxide based SGFET pH sensors

Date of Award	2017 Jan 12
Original language	English
Supervisor	Shui-Jinn Wang (Supervisor)