Browsing by Subject "TiO2"

Now showing 1 - 13 of 13

Open Access
Atomic layer deposition of metal oxides on self-assembled peptide nanofiber templates for fabrication of functional nanomaterials
(2016-08) Eren, Hamit
There are mainly two basic approaches in nanostructured materials synthesis. The rst one is the top-down approach and requires material removal from a bulk substrate material by chemical, physical, mechanical or thermal means; acid etching, focused ion milling, and laser ablation are among these top-down synthesis techniques. It is a straightforward { albeit poor in material architecture control { method that has established its niche in today's high-volume CMOS transistor fabrication technology which already produces single-digit nanometerscale device features. On the other hand, bottom-up approach exploits ne-tuned materials assembly. Bottom-up approach is realized via direct self-assembly of target nanostructures or material growth on synthetic or natural nanotemplates. Bottom-up nanostructured materials synthesis o ers considerably wider spectrum of achievable material architectures and structural hierarchies. Synthesis of nanostructured materials on self-assembled soft nanotemplates is of signi cant importance because many biological systems utilize this very similar approach to construct complex biomolecule-templated materials. Peptide amphiphile (PA) molecules with their intrinsic property to self-assemble into nanostructures such as bers, present a versatile tool in inorganic material templating. PAs were used as soft templates in several studies for fabrication of nanoscale inorganic materials. Most of these studies are focused on in-solution material deposition on the surface of a template. Even though this approach allows successful material deposition, precise control over material thickness, uniformity, and high conformality is di cult to achieve in a repeatable manner. In order to circumvent this challenge, in this thesis, atomic layer deposition (ALD) technique was deployed for conformal coating of PA nanonetwork templates. ALD involves low-temperature iterative vapor-phase material deposition in a self-limiting fashion. In each deposition half-cycle, Ti- or Zn- containing volatile metalorganic complexes form a self-limiting uniform monolayer that consequently reacts with water vapor (H2O) as an oxygen precursor in the subsequent process half-cycle. As each half-cycle is separated with purge cycles, no gas-phase reactions occurs and material growth proceeds only with surface chemical ligand-exchange reactions. ALD approach allowed obtaining TiO2 or ZnO nanonetworks with tunable wall thickness and ultimate conformality. Obtained metal oxide-peptide hybrid materials were further treated di erently. In the case of TiO2, organic template was removed upon calcination at 450 C, a temperature at which amorphous titania transforms to anatase form. ZnO-peptide hybrid materials on the other hand, did not undergo any thermal processing, as ZnO already grows in wurtzite crystalline form during ALD process. In principle, nanostructured anatase TiO2 and wurtzite ZnO are wide bandgap semiconductors which can be used as photoanode materials. Nanostructured anodic materials still attract a great interest as the matter at nanoscale regimes can provide considerable enhancement in charge carrier separation, charge carrier transport, and active surface area. Here we demonstrate the fabrication of nanostructured TiO2 and ZnO on self-assembled soft templates. As a proof of principle, we utilized semiconducting TiO2 and ZnO in assembly of dye sensitized solar cells and studied material thickness e ect on device performance parameters such as open circuit voltage (Voc), short circuit current (Jsc), and ll factor. Three sets of nanostructured photoanodes with di erent TiO2 deposition cycles (100, 150, and 200) and ZnO deposition cycles (100, 125 and 150) were fabricated. TiO2 and ZnO nanonetworks in photoanodes form a system of interconnected nanotubes, which can facilitate electron transfer. Moreover, these networks are porous high-surface area materials and they can drastically increase number of sensitizer molecules attached to the semiconductor material surface.
Open Access
Electrospun mesoporous composite CuO−Co3O4/N‐ TiO2 nanofibers as efficient visible light photocatalysts
(Wiley-Blackwell, 2017-08) Pradhan, A. C.; Senthamizhan A.; Uyar, Tamer
One-dimensional mesoporous composite CuO−Co3O4 /N-TiO2 nanofibers (CuCoNT NFs) have been fabricated by in situ sol−gel electrospinning technique. In our approach, both polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG) are used as dual polymeric carrier matrix for the fabrication of electrospun CuCoNT NFs. PVP chains assist the electrospinning of the uniform composite nanofibers whereas PEG is responsible for mesoporosity which is confirmed by N2 sorption analyses. Along with CuCoNT NFs, other nanofiber samples (TiO2 NFs, N-TiO2 NFs, CuO/N-TiO2 NFs, Co3O4/N-TiO2 NFs) have also been fabricated for comparative studies. The morphology and composition of the NFs have been confirmed by the HR-TEM and XPS analyses. The red shifting of band gap energy from anatase TiO2 NFs to composite CuCoNT NFs (1.57 eV) is suggesting formation of visible light response. Oxygen vacancies in CuCoNT NFs, leads to lowering the e− − h+ recombination. The lowering of photoluminescence spectrum and high photocurrent response in CuCoNT NFs makes CuO as low cost cocatalyst. The composite CuCoNT NFs is treated as an efficient photocatalyst for swift degradation of mixed dyes in visible light, an exemplary move. Exactly, 100% mixed dyes (30 mg/L) degradation is achieved at pH 10 in just 60 minutes.
Open Access
An experimental and first-principles study of the effect of B / N doping in TiO2 thin films for visible light photo-catalysis
(Elsevier, 2013) Uddin, M. N.; Shibly, S. U. A.; Ovali, R.; Saiful, I.; Islam, M. S.; Uddin, M. J.; Gulseren, O.; Bengu, E.; Mazumder, M. M. R.
Thin films of TiO2 and boron-nitrogen (B/N) co-doped TiO 2 on glass substrates have been prepared by a simple sol-gel dip coating route. Titanium (IV) isopropoxide, boric acid and urea have been used as titanium, boron and nitrogen sources, respectively. The films were characterized by X-ray diffraction, X-ray photo-electron spectroscopy, scanning electron microscopy, Raman spectroscopy and UV-vis spectroscopy. The TiO 2 thin films with co-doping of different B/N atomic ratios (0.27-20.89) showed better photo-catalytic degradation ability of methylene blue compared to that of bare-TiO2 under visible light. The TiO 2 film doped with the highest atomic concentration of N showed repeatedly the best photo-catalytic performance. The high activity of co-doped TiO2 thin films toward organic degradation can be related to the stronger absorption observed in the UV-vis region, red shift in adsorption edges and surface acidity induced by B/N doping. Furthermore, several atomic models for B/N doping have been used to investigate the effect of doping on electronic structure and density of states of TiO2 through ab-initio density functional theory calculations. The computational study suggested a significant narrowing of the band gap due to the formation of midgap states and the shift of Fermi-level for the interstitial N model supporting the experimental results. © 2013 Elsevier B.V.
Open Access
Finding an optimum surface chemistry for [Formula] systems as NOx storage materials
(2010) Şentürk, Göksu Seda
Titania promoted NOx storage materials in the form of BaO/TiO2/Al2O3 were synthesized via two different sol-gel preparation techniques, with varying surface compositions and morphologies [1, 2]. The influence of the TiO2 units on the NOx storage component (8 - 20 wt. % BaO), the nature of the crystallographic phases, thermal stabilities and the dispersion of the surface oxide/nitrate domains were investigated. The structural characterization of the synthesized NOx storage materials were analyzed by means of BET surface area analysis, X-ray diffraction (XRD), ex-situ Raman spectroscopy, scanning electron microscopy (SEM), energy dispersive X- ray (EDX) and transmission electron microscopy (TEM). Comparative analysis of the results showed that the TiO2/Al2O3 support material derived by the co-precipitation of the corresponding hydroxides via the sol-gel technique, exhibited distinctively more homogenous distribution of TiO2 domains. The functionality/performance of these materials upon NOx and SOx adsorptions were monitored by temperature programmed desorption (TPD) and insitu Fourier transform infrared (FTIR) spectroscopy. An improved Ba surface dispersion was observed for the BaO/TiO2/Al2O3 materials synthesized via the coprecipitation of alkoxide precursors which was found to originate mostly from the increased fraction of accessible TiO2/TiOx sites on the surface. These TiO2/TiOx sites functioned as strong anchoring sites for surface BaO domains and were tailored to enhance surface dispersion of BaO. The relative stability of the NOx species adsorbed on the BaO/TiO2/Al2O3 system was found to increase in the following order: NO+ /N2O3 on alumina << nitrates on alumina < surface nitrates on BaO < bridged/bidentate nitrates on large/isolated TiO2 clusters < bulk nitrates on BaO on alumina surface and bridged/bidentate nitrates on TiO2 crystallites homogenously distributed on the surface < bulk nitrates on the BaO sites located on the TiO2 domains. The detailed study of the interaction of SOx with BaO/TiO2/Al2O3 ternary oxide materials showed that titania (TiO2) was a promising candidate for improving the sulfur tolerance on these type of surfaces. Adsorption of SOx on both pure Al2O3 and TiO2 showed that Al2O3 formed strongly bound SOx species, that were thermally stable up to T > 1073 K. SOx adsorption directly altered stability of the nitrate species on the Ti/Al (Protocol 1, Protocol 2) samples. SOx uptake properties of the BaO/TiO2/Al2O3 materials were found to be strongly influenced by the morphology of the TiO2/TiOx domains and the BaO loadings (8/20 wt% BaO). Consequently, the presence of titania domains was seen to decrease the SOx desorption temperatures and enhance the sulfur-tolerance of these materials by destabilizing the accumulated sulfate species. SOx exposure on the synthesized materials led to a significant decrease in the NOx adsorption capacities. The results obtained from FT-IR spectra showed that the sulfur deposition on the NOx storage materials promoted by Ti
Open Access
Hierarchical synthesis of corrugated photocatalytic TiO2 microsphere architectures on natural pollen surfaces
(Elsevier BV, 2017) Erdogan, D. A.; Ozensoy, E.
Biomaterials are challenging, yet vastly promising templates for engineering unusual inorganic materials with unprecedented surface and structural properties. In the current work, a novel biotemplate-based photocatalytic material was synthesized in the form of corrugated TiO2 microspheres by utilizing a sol-gel methodology where Ambrosia trifida (Ab, Giant ragweed) pollen was exploited as the initial biological support surface. Hierarchically synthesized TiO2 microspheres were structurally characterized in detail via SEM-EDX, Raman spectroscopy, XRD and BET techniques in order to shed light on the surface chemistry, crystal structure, chemical composition and morphology of these novel material architectures. Photocatalytic functionality of the synthesized materials was demonstrated both in gas phase as well as in liquid phase. Along these lines, air and water purification capabilities of the synthesized TiO2 microspheres were established by performing photocatalytic oxidative NOx(g) storage and Rhodamine B(aq) degradation experiments; respectively. The synthetic approach presented herein offers new opportunities to design and create sophisticated functional materials that can be used in micro reactor systems, adsorbents, drug delivery systems, catalytic processes, and sensor technologies.
Open Access
Identification, stability, and reactivity of NOx species adsorbed on titania-supported manganese catalysts
(Elsevier, 2001-12-10) Kantcheva, M.
The nature of the NOx species obtained on NO adsorption and its coadsorption with O2 at room temperature on TiO2 and MnOx/TiO2 catalysts with two different manganese loadings has been studied by means ofin situFourier transform infrared spectroscopy. In order to obtain information about the potentials of titania-supported manganese materials as catalysts for selective catalytic reduction (SCR) of NO by hydrocarbons, the stability and reactivity of the adsorbed NOx species toward decane has been investigated. The adsorption of NO on the support and the catalysts leads to disproportionation of NO to anionic nitrosyl, NO−, its protonated form, NOH, and NO2. On coadsorption of NO and O2 at room temperature, various kinds of surface nitrates are observed differing in the mode of their coordination. The nitrates on the manganese-containing samples are characterized by significantly lower thermal stability than the NO− 3 species on the pure support, titania. The difference in the thermal stability of the nitrates parallels their reactivity toward the reducer (decane). The monodentate and bridged nitrates formed on the manganese catalysts studied are able to oxidize the adsorbed hydrocarbon at temperatures as low as 373 K. A mechanism for the interaction between the surface nitrates and the adsorbed decane is proposed in which the NO− 3 and NCO− species are considered as important intermediates leading to dinitrogen formation. The concentration of the reactive surface nitrates on the MnOx/TiO2 catalyst with manganese content corresponding to a monolayer is considerably greater than that on the sample with higher manganese loading. The former catalyst is promising for the SCR of NO by longer chain saturated hydrocarbons. °c 2001 Elsevier Science
Open Access
Nanofibrous nanocomposites via electrospinning
(2011) Deniz, Ali Ekrem
In recent years, numerous studies have been reported for fabrication of composite nanofibers from polymeric and inorganic materials by using electrospinning method. In the first part of this study, TiO2 and ZnO inorganic nanofibers were produced by electrospinning from their precursors by using polymeric carrier matrix and their photocatalytic activity of these metal oxide nanofibers were studied. Moreover, electrospun TiO2 nanofibers were crushed into short nanofibers (TiO2-SNF) and embedded in electrospun polymeric nanofiber matrixes such as poly(methyl methacrylate) (PMMA), polyacrylonitrile (PAN), polyethylene terephthalate (PET), polycarbonate (PC) and polyvinylidene fluoride (PVDF). Different weight loading of TiO2-SNF ranging from 2% to 8% (w/w, respect to polymer) incorporated into PVDF nanofibrous matrix was applied and the structural and morphological changes along with their photocatalytic activities were also examined. In the second part, metallic nanoparticles produced by laser ablation method were incorporated into nanofibrous polymeric matrix by using electrospinning technique. For example, gold (Au) and silver (Ag) nanoparticles (NPs) were produced from their metallic sources by laser ablation method directly in the polymer solutions. The NPs/polymer mixtures were electrospun and surface plasmon resonance effect of Au-NPs and Ag-NPs on optical properties of the nanofibers was studied. In addition, germanium nanocrystals produced by means of laser ablation were mixed with PVDF polymer solution and consequently electrospun into composite polymeric nanofiber matrix.
Open Access
Photocatalytic nanocomposites for increased optical activity
(2008) Tek, Sümeyra
To combat environmental pollution, photocatalytic decomposition provides degradation of organic and inorganic contaminants near the surface of the photocatalyst nanoparticles by converting optical energy of the absorbed light into chemical energy for the redox reactions. However, photocatalytic activities of such semiconductor metal-oxide nanoparticles are limited with their bandgap energy that allows for optical absorption typically in the ultraviolet spectral range. Yet another limitation is that the photocatalytic activity of these semiconductor nanoparticles is substantially reduced when they are immobilized in solid thin films, resulting from their effectively decreased active surface area. But such immobilized nanoparticles are much more desired in industrial applications, e.g., for mass environmental decontamination and outdoors/indoors self-cleaning on large surfaces. To address these issues, in this thesis, we investigated and demonstrated the spectral behavior and time evolution of optical activity curves of immobilized TiO2 and ZnO nanoparticles. We studied the nanoparticle size effect for the optical activity and demonstrated significant increase in the resulting photocatalysis with decreasing the size of such immobilized nanoparticles for the first time. We obtained optimal excitation conditions for TiO2 and ZnO nanocomposite films separately. We achieved maximum optical recovery levels of 93% for TiO2 nanoparticles and 55% for ZnO nanoparticles at the excitation wavelengths of 310 nm and 290 nm, respectively, after optical irradiation with an excitation density of 7.3 J/cm2 , where we observed no optical recovery for their respective negative control groups (with no nanoparticles). In these comparative spectral studies, we showed strong correlation between the differential optical recovery and the photocatalytic activity. For further substantial enhancement in the near ultraviolet and visible spectral ranges, we also proposed and demonstrated the use of a unique combination of TiO2-ZnO nanoparticles integrated together into the same resin. In this novel approach, we observed higher levels of photocatalytic activity under optical irradiation at and above 380 nm compared to the cases of only TiO2 or only ZnO nanocomposite films with the same total metal-oxide nanoparticle density. At 400 nm in the visible, we accomplished an optical recovery level of ~30% with the combination of TiO2-ZnO nanoparticles together while this level was only ~14% for the TiO2 nanoparticles alone and ~3% for the ZnO nanoparticles alone under identical conditions. Even at 440 nm, we obtained ~20% optical recovery using the TiO2-ZnO photocatalytic synergy, despite the optical activity of the single type of nanoparticles alone close to the zero base-line of their control group. These proof-of-concept experimental demonstrations indicate that such TiO2-ZnO combined nanocomposite films hold great promise for efficient environmental decontamination in daylight.
Open Access
Photocatalytic NOx oxidation and storage under ambient conditions for air purification
(2012) Soylu, Aslı Melike
Air pollution is one of the most serious environmental problems in both urban and rural settings with a direct impact on human health. A variety of chemical compounds can be associated with air pollution and gaseous nitrogen oxides (NOx), such as NO and NO2, are especially among the most hazardous environmental pollutants. NOx abatement can be efficiently performed at elevated temperatures (i.e. T > 300oC), however, an important challenge in air purification is the abatement of gaseous NOx species under ambient conditions (i.e. at room temperature and under regular atmospheric conditions). Photocatalytic systems offer promising opportunities in order to tackle this important environmental challenge, as these systems can be tailored to efficiently clean/purify air under ambient conditions with the help of ultraviolet (UV) and/or visible (VIS) light. In the current work, a hybrid technology for the photocatalytic oxidation and storage of gas phase NOx species is proposed where titania based powders are investigated as candidate photocatalytic materials. With this aim, various components of a thermally activated conventional NOx Storage/Reduction (NSR) catalyst is combined with a photocatalytically activated NOx oxidation catalyst to obtain a photocatalytically activated NOx oxidation and storage material. In this regard, three different sets of samples were prepared and investigated. The first set of photocatalysts was prepared by employing Al2O3, a high surface area support material, in order to disperse the photocatalytically active titania in an effective manner. Using a ―sol-gel co-precipitation method‖, TiO2/Al2O3 binary oxides were synthesized (where TiO2:Al2O3 mole ratio was chosen to be 0.25, 0.5, 1.0) and characterized by X-ray diffraction, Raman Spectroscopy and BET. For these samples, the effects of specific surface area, calcination temperature and the crystallinity of TiO2 were investigated in relevance to the photocatalytic NOx oxidation/storage reaction. Next, an alkali/alkaline earth oxide storage component is added to the TiO2- Al2O3 mixture and the incorporation of the storage component is achieved via two different routes; (i) either through ―incipient wetness impregnation‖ of 5 or 10% (w/w) metal nitrate [M(NO3)x] salts on TiO2-Al2O3 and a subsequent calcination to obtain alkali/alkaline earth oxides [MyO] or (ii) by physically grinding 5 or 10% (w/w) BaO powder with TiO2-Al2O3 binary oxide to obtain a ternary mixture. For these samples, the route of metal oxide incorporation (impregnation vs. physical mixture), the type of metal oxide storage component (alkali vs. alkaline earth metal) and the percentage of metal oxide loading (5% vs. 10%, w/w) were examined in photocatlytic NOx oxidation/storage reaction. The photonic efficiencies of these samples were tested using a continuous flow system, composed of mass flow controllers, a custom-made UVA-illuminated reaction cell and an ambient chemiluminescence NOx analyzer. Photocatalytic performance of all samples were compared with that of a commercially available Degussa P25 TiO2 benchmark catalyst. Photocatalytic preformance tests revealed that the TiO2-Al2O3 binary oxides had much higher NOx storage capacities compared to Degussa P25 and the further addition of an alkaline earth oxide (BaO) storage component on TiO2-Al2O3 by physical mixing significantly enhanced the NOx capture in solid state and decreased unwanted gaseous NO2 emission to an almost negligible level. On the other hand, the ―incipient wetness impregantion‖ of metal nitrates resulted in metal titanate (MxTiyOz) formation on TiO2-Al2O3 binary oxide and diminished the photooxidation ability of the catalyst.
Open Access
Thermal evolution of structure and photocatalytic activity in polymer microsphere templated TiO2 microbowls
(Elsevier, 2014) Erdogan, D. A.; Polat, M.; Garifullin, R.; Güler, Mustafa O.; Ozensoy, E.
Polystyrene cross-linked divinyl benzene (PS-co-DVB) microspheres were used as an organic template in order to synthesize photocatalytic TiO2 microspheres and microbowls. Photocatalytic activity of the microbowl surfaces were demonstrated both in the gas phase via photocatalytic NO(g) oxidation by O2(g) as well as in the liquid phase via Rhodamine B degradation. Thermal degradation mechanism of the polymer template and its direct influence on the TiO2 crystal structure, surface morphology, composition, specific surface area and the gas/liquid phase photocatalytic activity data were discussed in detail. With increasing calcination temperatures, spherical polymer template first undergoes a glass transition, covering the TiO 2 film, followed by the complete decomposition of the organic template to yield TiO2 exposed microbowl structures. TiO2 microbowl systems calcined at 600 °C yielded the highest per-site basis photocatalytic activity. Crystallographic and electronic properties of the TiO2 microsphere surfaces as well as their surface area play a crucial role in their ultimate photocatalytic activity. It was demonstrated that the polymer microsphere templated TiO2 photocatalysts presented in the current work offer a promising and a versatile synthetic platform for photocatalytic DeNOx applications for air purification technologies.
Open Access
TiO2-Al2O3 binary mixed oxide surfaces for photocatalytic NOx abatement
(Elsevier, 2014) Soylu, A. M.; Polat, M.; Erdogan, D. A.; Say, Z.; Yıldırım, C.; Birer, Ö.; Ozensoy, E.
TiO2-Al2O3 binary oxide surfaces were utilized in order to develop an alternative photocatalytic NOx abatement approach, where TiO2 sites were used for ambient photocatalytic oxidation of NO with O2 and alumina sites were exploited for NOx storage. Chemical, crystallographic and electronic structure of the TiO2-Al2O3 binary oxide surfaces were characterized (via BET surface area measurements, XRD, Raman spectroscopy and DR-UV-Vis Spectroscopy) as a function of the TiO2 loading in the mixture as well as the calcination temperature used in the synthesis protocol. 0.5 Ti/Al-900 photocatalyst showed remarkable photocatalytic NOx oxidation and storage performance, which was found to be much superior to that of a Degussa P25 industrial benchmark photocatalyst (i.e. 160% higher NOx storage and 55% lower NO2(g) release to the atmosphere). Our results indicate that the onset of the photocatalytic NOx abatement activity is concomitant to the switch between amorphous to a crystalline phase with an electronic band gap within 3.05-3.10 eV; where the most active photocatalyst revealed predominantly rutile phase together and anatase as the minority phase.
Open Access
Visible light-active non-metal doped titanium dioxide materials for photocatalytic oxidation
(2014) Altay, Pelin
One of the most important technologies for a better human life is environmental purification which has drawn attention and gained importance over the past years. Titanium dioxide has been the apple of the eye of both air and water purification systems for its strong ability of oxidation, low cost, nontoxicity, inertness and availability. However, being a wide band gap semiconductor, titanium dioxide can mostly absorb UV photons (nm) in the sun light, which is only about 3% of the total solar radiation. In this regard, sensitizing titanium dioxide based materials capable of visible light absorption via doping methods is a challenging but yet a rewarding effort. In the current work, a variety of doping protocols have been employed in conjunction with sol-gel titanium dioxide synthesis protocols in an attempt to prepare visible-active photocatalytic powders. This study has been a preliminary work to propose a simple sol-gel synthesis route for the preparation of visible-active titanium dioxide in order to combine with previously studied UV active titanium dioxide powders to create tandem systems that will harvest both visible and UV light for water and air purification. Along these lines, two different sets of samples were prepared and investigated. The first set of samples was prepared by a sol-gel route with the addition of non-metallic compounds of Ti which are TiN, TiC and TiS2. Non-metal atom to titanium mol ratio was kept at 0.1:1 and the syntesized powders were characterized by XRD, Raman Spectroscopy, BET, UV-VIS Diffuse Reflectance Spectroscopy in order to investigate the effect of calcination temperature, surface area and band gap on photocatalytic activity. Besides, these commercial TiN, TiC and TiS2 powders that were used as dopants, were also annealed in open air to prepare partially oxidized titanium materials. Secondly, inexpensive sources of non-metal compounds such as boric acid, diethanolamine (DEA), triethylamine (TEA), thiourea, urea and cyclohexanol were added in an alternative sol gel synthesis route. Dopant compound to titanium dioxide mol ratio was also kept at 0.5:1. Structural and electronic characterization of this family of materials were also carried out in addition to photocatalytic activity tests. Photocatalytic activity measurements were done in liquid phase via the degradation of an organic contaminant, Rhodamine B, in a custom-designed VISilluminated photocatalytic reaction cells. Photocatalytic performance of all samples were compared with that of a commercially available Degussa P25 TiO2 benchmark catalyst. Photocatalytic preformance tests revealed improved photocatalytic activity for non-metal compound added titanium dioxide compared to unmodified titanium dioxide prepared with the same method. Also, several samples presented even higher photocatalytic activity compared to Degussa P25. Characterization experiments showed hinderance in anatase to rutile transformation due to foreign atoms. It was also observed that although a small band gap is important for the photocatalytic activity, there are other critical parameters such as particle size, surface area, crystallinity, active facets, oxygen vacancies which have to be fine tuned for photocatalytic performance optimization.
Open Access
ZnO, TiO² and exotic materials for low temperature thin film electronic devices
(2012) Oruç, Feyza Bozkurt
The metal-oxide-semiconductor field-effect transistor (MOSFET) technology is the core of integrated circuit industry. Nearly all electronic devices around us contain transistors for various purposes like electronic switches, amplifiers or sensors. As the need for more complex and miniature circuits has arisen, scaling down transistor sizes become the top priority. As Moore’s law indicates, number of transistors on integrated circuits doubles every two years but in future fabrication challenges and limitations like quantum effects seen in small devices will block further miniaturization. New growth techniques are required for depositing conformal, high quality films -like high-k dielectrics instead of SiO2- with atomic thickness control to reduce possible problems. Atomic layer deposition techniques are developed to meet these requirements. The field of thin film transistors (TFT), which is a subset of MOSFET’s have first started to be used in flat panel displays but now they are used in various fields, since their functional properties make them powerful candidates for sensor applications. ALD technology is important also for TFT applications since its low temperature growth mechanism allows fabricating TFT’s on various substrates like flexible and/or transparent ones. With ALD technique, transistors can be built even on cloths which makes the dream of e-suits real. In this thesis, thin film transistors are designed and fabricated using atomic layer deposition technique both for channel and dielectric layer growth. Design and fabrication steps of the TFT devices are realized in a cleanroom environment. The fabricated TFT’s are mainly characterized by measuring their current-voltage relations. A parameter analyzer with a probe station is used for such measurements. ALD grown ZnO TFT’s and the effect of growth temperature on performance characteristics are examined. High performance devices having very high Ion/Ioff ratios are fabricated at a temperature low as 80°C. ALD grown TiO2 TFT’s are also fabricated and effects of annealing temperature on device performance are analyzed. This study is, to the best of our knowledge, the first demonstration of TiO2 TFT’s grown by a thermal-ALD system. GaN and pentacene TFT’s are also fabricated and showed promising results. Pentacene TFTs have a special importance since it is a p-type organic semiconductor which gives us the opportunity to work on hybrid organic-inorganic structures. In conclusion, TFT devices based on ALD grown channel and/or dielectric layers show very encouraging results in terms of low cost, low temperature fabrication opportunities and freedom of using any substrate that can handle ALD processing temperature.