Research On The Valve-metal Anodization And Its Applications In Energy Conversion

Valve metal anodizaton is a facile and effective electrochemical process.Through anodization,oxide films with unique cellular nanochannels can be fabricated on valve metal,such as Al and Ti.And the morphology and structure of the nanochannels can be controlled by tuning the anodization parameters.Due to the distinctive nanostructure and chemical/physical properties,the anodic valve metal oxides have been widely used in many fields.However,till now,there is still no model or mechanism that can perfectly elaborate the formation of ordered anodic titanium oxide(ATO)nanotubes and porous anodic aluminum oxide(AAO),and this would hinder their further developments and applications.To address this issue,the anodization mechanism of valve metals was explored from the prospective of electronic current,and a deep insight into the self-assembly of ordered porous oxides was provided.And then,porous AAO and ATO nanotubes with novel structures and functions were developed,and their applications in the field of energy conversion were demonstrated.In this thesis,the morphology evolution of the ATO nanotubes was monitored through controlling the anodization conductions.The O2 bubble mold effect was proposed from the perspective of electronic current and ionic current.This model could not only explain the formation of ATO nanotubes with lotus-root shape,and also illustrate the relationship between anodic voltage and tube diameter.In addition,considering the existence of ionic and electronic currents,a mathematical model was established about the relationship between anodic current and time.From the experimental and calculated results,the linear relationship between the ionic current and tube length was confirmed.And meanwhile,the electronic current was responsible for the O2 evolution,which was close related to the formation of the initial pore of the nanotubes.The oxygen bubbles inside the growing oxide can act as a mold to guide the“viscous flow”of the barrier oxide from the pore bases to the pore walls.The O2 bubbles can stimulate the initial formation of pore embryos,and also reasonably lead to the ordered hemispherical bottoms of the nano tubes.The O2 bubble model provided a new way for understanding the formation of ordered anodic valve metal oxide,which had great significance in the fabrication and regulation of porous nanostructures.Upon understanding the formation mechanism of the anodic oxide,highly ordered AAO template was prepared through combining the nanoimprint and anodization technology.Then the through-hole AAO template was employed as a mask,silver nanodot arrays on both rigid and flexible substrates were achieved by evaporation method.Because of the periodic structure,the Ag nanodot arrays demonstrated unique optical properties.The Ag nanodots on Si substrate showed a multiscale core-satellites hybrid nanostructure.Between the satellites and the core islands,the gaps dropped down to?10 um,which would facilitate strong coupling of localized plasmon resonances in the core-satellite systems.Meanwhile,the Ag nanodot arrays on flexible PDMS demonstrated a broadband extraordinary optical transmission above 55%at wavelengths from 300 to 1100 nm,showing an enhancement factor of 2.7 when normalized to the area of exposed gaps.The Ag nanodot arrays on PDMS can be bended and attached to any curved surface.Moreover,the transmission and the diffractive properties can be tuned by simply stretching the PDMS.Further,AAO template with inverted nanocone channels was also fabricated by cyclic anodizing and etching process.The pore pitch was?500 nm,and the depth could be controlled.Then correspondingly,“moth eye”anti-reflection polymer film with nanocone arrays was fabricated by molding the AAO template.The anti-re flection film demonstrated excellent optical properties:high transparency,low reflection,high haze.Moreover,the contact angel on the anti-reflection film reached 140°,which endowed the film self-cleaning function And then,the“moth-eye”anti-reflection film was mounted on a tubular a-Si:H solar cell module.Due to the novel geometry design and the anti-re flection film,the tubular solar cells can effectively trap the sunlight,rendering omnidirectional and broadband enhancement in light collecting.The photoelectric conversion efficiency up to 9.86%was achieved which outperforms the bare tubular device by 8.1%.As to the application of ATO,we focused on two aspects:one was to enhance the adhesion of ATO nanotubes to the underlying Ti substrate;the other was to fabricate ATO nano tube arrays with ultra-large diameter.Due to the existence of the fluoride rich layer in the bottoms of ATO nanotubes,the adhesion between nano tubes and Ti substrate was poor,which would limit the application of ATO nanotubes.To overcome this issue,an additional compact oxide layer between ATO nanotubes and Ti substrate was introduced.This additional compact layer resulted in a more than threefold increase of the adhesion strength between nanotubes and the Ti substrate,and meanwhile,the additional compact oxide layer gave the ATO nano tubes better PEC performance.And then,ATO nanotubes with ultra-large diameter were prepared by optimizing the anodization parameters.There was an almost linear relationship between the nano tube diameter and the anodization voltage.The diameter ranged from?100 nm to?600 nm,and the anodization area was relatively large.Moreover,ATO nano tubes with different diameter demonstrated different PEC activities,among which nano tubes with diameter?310 nm showed the highest photo-response.At last,ATO nanotube arrays were removed from the Ti foil after simple ultrasonic treatment,leaving nanodent arrays in the surface of Ti foils correspondingly.Then the nanotextured Ti foils were creatively utilized as substrates,on which flexible a-Si:H solar cells were constructed.The size of the quasi-periodic nanodent can be tuned from-220 nm to-600 nm.The a-Si:H solar cells on nanotextured Ti foils can induce light scattering effectively,thus increase the optical path length and light absorption.As a result,The PEC of the nanostructured a-Si:H solar cells reached 6.73%.In addition,Cu,Au,Ag plasmonic back-reflectors were also constructed on the nanotextured Ti foils,and their influence on light absorption in solar cells was investigated,respectively.According to the finite different time domain(FDTD)simulation and experimental results,the three metallic back-reflectors all exited the surface plasmon resonances,and in the long wavelength region,the light absorption in solar cells all got improved.However,as plasmonic back-reflector in ?-Si:H solar cells,Ag showed advantages over Cu and Au because of less light loss.And the solar cells with Ag back-reflector achieved an efficiency of 7.26%.In conclusion,a-Si:H solar cells constructed on nanotextured Ti foils not only had light weight,high energy conversion efficiency,but also realized flexibility,which will provide new approaches for flexible and wearable devices.