Nanocrystalline TiO2 Thin Films Research Articles

Reduced Work Function in Anatase ⟨101⟩ TiO2 Films Self-Doped by O-Vacancy-Dependent Ti3+ Bonds Controlling the Photocatalytic Dye Degradation Performance.

TiO2 has the proven capability of catalytically decomposing pollutants under light illumination, thereby embracing potential applications in wastewater management. The photocatalytic dye degradation activity is largely controlled by the optical band gap that dictates the extent of electron-hole pair generation via photon absorption, and the recombination kinetics of charges. In this context, the material's work function governs how easily the charge carriers can be transferred at the dye-adsorbed photocatalytically active sites. Accordingly, nanocrystalline TiO2 thin films are grown in the anatase phase with ⟨101⟩ orientation, using RF magnetron sputtering at 200 °C. Besides studying the film's structural morphology, optical band gap, and elemental composition, the electronic properties are extensively investigated. The work function of the material was controlled by varying the O-vacancy-dependent Ti3+ bonding configuration in the network. It has been demonstrated how the photocatalytic methylene blue dye degradation activity of the nanocrystalline TiO2 films of predominantly the anatase phase improves on reducing the sputtering pressure during deposition. At a low deposition pressure of 20 mTorr, a low work function of ∼4.2 eV of the film, resulting from the formation of a Ti3+-bond through the O vacancies in the network, potentially increases its carrier lifetime and delivers the superior photocatalytic activity (∼82.7% dye degradation with a rate constant of k ∼ 0.0073 min-1) via silently facilitating fast electron transfer from the photocatalyst to the dye in the aqueous solution. The higher stoichiometric film prepared at p = 40 mTorr exhibits an inferior photocatalytic activity (∼20.4% dye degradation with a rate constant of k ∼ 0.0009 min-1), as retarded by its higher work function of ∼4.62 eV, despite retaining a relatively low band gap. Thus, without using any heterojunction or extrinsically doped photocatalyst, the dye degradation can be controlled simply by reducing the work function of nanocrystalline TiO2 thin films via controlling the O-vacancy-dependent Ti3+ bonding in its self-doped network.

Fabrication of nanocrystalline TiO2 thin films using Sol-Gel spin coating technology and investigation of its structural, morphology and optical characteristics

Although TiO2 is a popular semiconductor photocatalyst but due to its large bandgap (3.2 eV), its light absorption capability is limited to UV range (about 3 – 5% of solar spectrum), making it inefficient for solar energy conversion. However, light absorption capability can be increased by narrowing down the bandgap through a technique known as doping, but the dopants which are used in this process induces defect. These defect states present in the TiO2 lattice may act as recombination centres for the photogenerated charge carriers, thereby reducing the efficiency of solar energy conversion. As a result, defects from the doping are the limitation towards increasing the efficiency of TiO2 for solar energy conversion. So, researchers have been searching for a dopant free technique to narrow down bandgap of anatase TiO2 below 3.2 eV. This paper demonstrates a new cost-effective technique called as Sol-Gel employing spin coating technology for the fabrication of nanocrystalline TiO2 thin films. Following that, freshly formed TiO2 thin film samples were annealed at temperatures ranging from 400 °C to 550 °C and a thorough investigation on their structures, optical characteristics and surface morphology, were analysed using X-Ray diffraction, Raman spectroscopy, UV–Vis spectroscopy, FESEM and AFM etc. and are presented in this paper. Unlike any other previously performed sol–gel method, we have managed to achieve dopant free purely anatase nanocrystalline TiO2 thin film with low band gap reaching up to 2.68 eV.

Photoelectrochemical and photocatalytic activity of nanocrystalline TiO2 thin films deposited by chemical bath deposition method

Photoelectrochemical and photocatalytic activity of nanocrystalline TiO2 thin films deposited by chemical bath deposition method

Highly sensitive and selective H2S gas sensor based on TiO2 thin films

Hydrogen Sulfide (H2S) gas sensors with very high sensitivity, selectivity, and quick responses are highly demanding for practical industrial applications. However, achieving it remains a challenging due to high-cost fabrication, poor selectivity, and stability of existing sensor materials. In this work, we report the nanocrystalline TiO2 thin film based highly selective H2S sensor on a silicon substrate, which exhibits superior sensor response of ~112000% and ~85% at 50 ppm and 100 ppb concentration of H2S gas, respectively when operated at 100 °C. Also, it exhibits quick response time (~48 s) and fast recovery time (5 min) with long term stability in H2S gas sensing. The sensitivity of H2S gas on the TiO2 film is related to its surface roughness and excessive chemisorbed oxygen on the surface of the film. Thus, it was found that TiO2 thin film with appropriate roughness and defects plays a significant role in achieving a much better H2S selectivity and sensitivity properties at lower ppm detection for the H2S gas sensor.

Microplotter-Printed On-Chip Combinatorial Library of Ink-Derived Multiple Metal Oxides as an "Electronic Olfaction" Unit.

Information about the surrounding atmosphere at a real timescale significantly relies on available gas sensors to be efficiently combined into multisensor arrays as electronic olfaction units. However, the array's performance is challenged by the ability to provide orthogonal responses from the employed sensors at a reasonable cost. This issue becomes more demanded when the arrays are designed under an on-chip paradigm to meet a number of emerging calls either in the internet-of-things industry or in situ noninvasive diagnostics of human breath, to name a few, for small-sized low-powered detectors. The recent advances in additive manufacturing provide a solid top-down background to develop such chip-based gas-analytical systems under low-cost technology protocols. Here, we employ hydrolytically active heteroligand complexes of metals as ink components for microplotter patterning a multioxide combinatorial library of chemiresistive type at a single chip equipped with multiple electrodes. To primarily test the performance of such a multisensor array, various semiconducting oxides of the p- and n-conductance origins based on pristine and mixed nanocrystalline MnOx, TiO2, ZrO2, CeO2, ZnO, Cr2O3, Co3O4, and SnO2 thin films, of up to 70 nm thick, have been printed over hundred μm areas and their micronanostructure and fabrication conditions are thoroughly assessed. The developed multioxide library is shown to deliver at a range of operating temperatures, up to 400 °C, highly sensitive and highly selective vector signals to different, but chemically akin, alcohol vapors (methanol, ethanol, isopropanol, and n-butanol) as examples at low ppm concentrations when mixed with air. The suggested approach provides us a promising way to achieve cost-effective and well-performed electronic olfaction devices matured from the diverse chemiresistive responses of the printed nanocrystalline oxides.

Effect of metal ions doping on structural, optical properties and photocatalytic activity of anatase TiO2 thin films

The metal ion‐doped (Fe3+, Co2+, Ni2+ and Ag+) TiO2 nanocrystalline thin films were spin‐coated on glass substrates. The grazing incidence X‐ray diffractometer (GIXRD), SEM, energy‐dispersive X‐ray Spectroscopy (EDX), Raman, UV–VIS and a fluorescence spectrometer were employed to study the structural, compositional and optical properties of metal‐doped TiO2 films. As revealed by GIXRD, all the metal‐doped TiO2 thin films were found to anatase tetragonal crystal structure. The deposited films were optimized using annealing temperature (550°C) to get high‐quality crystalline films with the anatase phase. The thin films with the specific surface area of Fe‐, Co‐, Ni‐ and Ag‐doped anatase TiO2 nanocrystals are 152.18, 88.25 96.68 and 178.38 m2/g, respectively, which has been calculated by GIXRD results. Optical transmittance spectra were used to determine the thickness, band gap energy and refractive index of metal‐doped TiO2 thin films. The characteristic vibrational Raman modes also revealed that the films were found to be anatase phase. The intensity of luminescence emission decreases with the doping of metal ions. Metal‐doped TiO2 films exhibit strong photocatalytic performance than undoped TiO2 films. The degradation efficiencies typically increase with the different dopants for the visible‐light degradation of methylene blue (MB) and methyl orange (MO). Moreover, the photocatalytic activity (PCA) of silver ion‐doped titania thin film is found to be better than that of Fe3+‐, Co2+‐, and Ni2+‐doped TiO2 films.

Turning Waste into Useful Products by Photocatalysis with Nanocrystalline TiO2 Thin Films: Reductive Cleavage of Azo Bond in the Presence of Aqueous Formate.

UV-photoexcitation of TiO2 in contact with aqueous solutions of azo dyes does not imply only its photocatalytic degradation, but the reaction fate of the dye depends on the experimental conditions. In fact, we demonstrate that the presence of sodium formate is the switch from a degradative pathway of the dye to its transformation into useful products. Laser flash photolysis experiments show that charge separation is extremely long lived in nanostructured TiO2 thin films, making them suitable to drive both oxidation and reduction reactions. ESR spin trapping and photoluminescence experiments demonstrate that formate anions are very efficient in intercepting holes, thereby inhibiting OH radicals formation. Under these conditions, electrons promoted in the conduction band of TiO2 and protons deriving from the oxidation of formate on photogenerated holes lead to the reductive cleavage of N=N bonds with formation and accumulation of reduced intermediates. Negative ion ESI–MS findings provide clear support to point out this new mechanism. This study provides a facile solution for realizing together wastewater purification and photocatalytic conversion of a waste (discharged dye) into useful products (such as sulfanilic acid used again for synthesis of new azo dyes). Moreover, the use of TiO2 deposited on an FTO (Fluorine Tin Oxide) glass circumvents all the difficulties related to the use of slurries. The obtained photocatalyst is easy to handle and to recover and shows an excellent stability allowing complete recyclability.

Singlet Fission and Electron Injection from the Triplet Excited State in Diphenylisobenzofuran–Semiconductor Assemblies: Effects of Solvent Polarity and Driving Force

Singlet fission has emerged as a promising way to overcome the Shockley-Queisser limit in solar energy conversion devices, and a few studies have claimed proof-of-principle results using dye-sensitized photoelectrodes. However, a detailed understanding of what factors govern the fate of the excited state on mesoporous surfaces is still lacking. Here, we have studied how the excitation progresses into singlet fission, electron injection, or formation of molecular charge separated states in diphenylisobenzofuran derivatives with flexible carbon linkers attached to nanocrystalline mesoporous ZrO2, TiO2, and SnO2 thin films. We show that singlet fission occurs for the molecule attached to ZrO2 films when the assembly is immersed in nonpolar solvents, and that singlet fission is hampered by the formation of a molecular charge separated state in more polar solvents. On TiO2 surfaces, direct electron injection from the singlet excited state outcompetes the singlet fission. Instead, triplet formation occurs via charge recombination from the conduction band of TiO2 in nonpolar solvents. When the molecule is attached to SnO2 films, singlet fission partly outcompetes electron injection from the singlet excited state and the two processes occur in parallel. Subsequent to singlet fission on SnO2, triplet injection into the conduction band of SnO2 is observed. The results presented here provide a detailed picture of the singlet fission dynamics in molecules attached to mesoporous semiconductor surfaces, demonstrating that both the semiconductor substrate as well as the environment around the molecules have a large impact, which can be useful in the design of future devices.

Optimization of physicochemical and optical properties of nanocrystalline TiO2 deposited on porous silicon by metal-organic chemical vapor deposition (MOCVD)

Titanium dioxide (TiO2) is very employed in solar cells due to its interesting physicochemical and optical properties allowing high device performances. Considering the extension of applications in nanotechnologies, nanocrystalline TiO2 is very promising for nanoscale components. In this work, nanocrystalline TiO2 thin films were successfully deposited on porous silicon (PSi) by metal organic chemical vapor deposition (MOCVD) technique at temperature of 550 °C for different periods of times: 5, 10 and 15 min. The objective was to optimize the physicochemical and optical properties of the TiO2/PSi films dedicated for photovoltaic application. The structural, morphological and optical properties of the elaborated TiO2/PSi samples were analyzed by means of x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX), atomic force microscopy (AFM), photoluminescence (PL) and UV-Visible absorption spectroscopy methods. The effect of deposition time on the microstructural properties which influences the optical characteristics of the obtained samples was also examined. The XRD analysis confirms the nanocrystalline structure of the deposited TiO2 composed only by anatase phase. The SEM characterization evidenced an increase in the TiO2 film thickness showing more uniform surfaces as the deposition time rises. Correspondingly, the surface roughness increases with the particle size and film thickness as indicated by AFM studies. The TiO2/PSi/Si sandwich structure evidenced by cross-sectional SEM confirms the good adherence of the TiO2 nanocrystalline film on the porous silicon forming with silicon a composite material. The UV-Vis measurements showed a considerable enhancement in optical absorption of porous silicon after the deposition of TiO2 films. Indeed, the TiO2 coatings deposited on PSi for 15 min with thickness of 200 nm have the best structure quality and exhibit, consequently, the highest absorption. From these interesting results, we demonstrate the viability of the use of the MOCVD as reproducible process for the elaboration of high-quality TiO2/PSi films.

Investigation of resistive switching effect in nanocrystalline TiO2 thin film for neuromorphic system manufacturing

Effect of resistive switching in TiO2 thin film was investigated. It was shown, resistive switching from high resistance state (HRS) to low resistance state (LRS) has occurred at 3.2±0.2 V, and from LRS to HRS at -2.8±0.5 V. Endurance test shown that HRS decreased from 42.31±5.26 kΩ to 26.45±6.14 kΩ, LRS increased from 2.25±1.15 kΩ to 3.45±1.18 kΩ. HRS/LRS coefficient has decreased from 18.8 to 7.6. Time-stability of TiO2 surface charge was investigated. It was shown, that voltage decreased from 320±21 to 22±5 mV during 90 minutes and square side increased from 3.43±0.12 to 4.12±0.14 µm during 90 minutes. The results can be useful for neuromorphic systems manufacturing based on nanocrystalline TiO2 films.

Comparative study of Ag, Sn or Zn doped TiO2 thin films for photocatalytic degradation of methylene blue and methyl orange

Nanocrystalline titania (TiO2) films with high optical transmittance and appropriate type and level of metal dopants offer promise for use in photocatalysis and environmental remediation purposes. In this study, a facile, cost-efficient sol-gel reflux synthesis route was devised to obtain smooth, nanocrystalline anatase TiO2 thin films doped with silver (Ag), tin (Sn) or zinc (Zn) with up to 5 mole percent. The microstructure, surface morphology and phase composition were determined using scanning electron microscope, atomic force microscope and x-ray diffraction techniques. The films were found to be nanocrystalline with average crystallite size of the order of ∼11 nm. Upon Ag or Sn doping, the films became superhydrophilic due to generation of surface defects and enhanced absorption via incident light scattering. The low extinction coefficient confirmed low surface roughness and uniform dispersion of the metal dopants into the TiO2 lattice. There was a decrease in the refractive index values of the Zn-doped films. The band gap values, as computed using Tauc plots, indicated a maximum reduction from 3.66 eV for the updoped TiO2 to 3.25 eV for Ag-doped TiO2 film. When assessed for photocatalytic degradation of methylene blue (MB) and methyl orange (MO), all the films were found to follow pseudo first-order kinetics in accordance with the Langmuir-Hinshelwood model. The apparent rate constant values were found to increase by 2 to 3 fold (4.30 × 10−3 s–1 for MB; 5.34 × 10−3 s–1 for MO) upon doping with 5 mole percent ZnO, presumably due to dual role of Zn as a reductant causing Ti3+ formation and a stabilizer of the oxygen vacancies.

Comparative Study of Thin-Film-Based Planar and Vertical Devices Towards Isopropyl Alcohol Sensing

The influence of the electrode position in devices based on nanocrystalline TiO2 thin film for sensing of isopropyl alcohol has been investigated. Two types of device structure, viz. planar and vertical, with similar dimensions (length and width) were fabricated, employing Al metal electrodes. Both sensors were tested in the concentration range from 20 ppm to 400 ppm in the temperature range of 50°C to 200°C. The comparative study revealed that both sensors (planar and vertical) exhibited an optimum operating temperature of 150°C, with an optimum response of 77% and 33.08%, respectively. The response time and recovery time were found to be faster for the vertical device compared with its planar counterpart. The dissimilar sensor response of the planar and vertical devices can be understood based on the surface states and interface kinetics of the TiO2 sensing layer. An electrical model describing the modulation of the barrier height, the effect of grain boundaries (GBs), and carrier trapping at interface states at GBs was applied to understand the response time and recovery time.

Effect of annealing temperature on structural, morphological and optical properties of nanocrystalline TiO2 thin films synthesized by sol–gel dip coating method

TiO2 is one of the common materials used for solar cells, diodes, different types of gas sensors, optoelectronic devices, etc., TiO2 is widely used in photovoltaic devices due to interesting optical and electrical properties. In this work, nanocrystalline TiO2 thin films have been prepared by sol gel dip coating method. X-ray diffraction pattern showed the formation of anatase phase of TiO2. The scanning electron microscopy (SEM) is used for analysis of morphology. The energy dispersive X-ray spectroscopy (EDX) analysis confirmed that Ti and O elements are present in the sample. The FT-IR spectrum revealed the strong presence of TiO2. Photoluminescence (PL) and UV–Vis spectroscopy was used for investigation of TiO2 optical properties.

Tailoring of TiO2 film crystal texture for higher photocatalysis efficiency

Abstract Nanocrystalline TiO2 thin films were deposited on borosilicate glass substrates using pulsed DC reactive magnetron sputtering in oxygen rich O2 Ar gas mixtures. The influence of sputtering power and argon‑oxygen gas flow ratio over crystallinity, crystal texture and surface morphology of the films was investigated. Interestingly, there was no correlation between the O/Ti concentration ratio in the films and their indirect band gap values. Based on experimentally observed band gap values and other properties of the films the model of the films consisting of nearly stoichiometric TiO2 crystals with a small fraction of weakly coordinated Ti impurities or amorphous low oxidation state titanium suboxide formations is proposed. Photocatalytic activity of the deposited films was analysed using inorganic markers and strong correlation between photocatalytic efficiency of TiO2 films with the (004) Crystalographic Preffered Orientation parameter was observed.

Functional nanocrystalline TiO2 thin films for UV enhanced highly responsive silicon photodetectors

In this paper, we have studied the effect of nanocrystalline TiO2 functional layer on the performance of Si photodetectors. It was found that the electron affinity of the deposited TiO2 films is much lower (3.2 eV) than the conventional films of the value 3.9 eV. In addition, the band gap of these newly developed films was found to be 3.6 eV which makes the films transparent for band to band optical absorption. However, a very high absorption co efficient of the value 4.6 × 105 cm−1 and excitonic dissociation facilitate charge generation and transfer mechanisms in such films. It was found that the photodetector parameters like internal gain, response time, responsivity, detectivity, sensitivity and noise can be improved by inserting functional TiO2 layer to make either anisotype or isotype heterojunction with Si photodetectors. The major phenomena for such enhancements were found to be decrease in tunneling length and increased charge generation/transfer due to TiO2 nanocrystals. By this modified design, TiO2/p-Si hybrid device offer high responsivity of the value 23.07 A/W and detectivity of 4.5 × 1012 Jones for red light (620 nm) at an incident power of 1 mW/cm2. Furthermore, it was confirmed that faster devices with rise time of only 30 μs in broadband region from 425 to 620 nm can be made out of TiO2/n-Si configuration which allows slower diffusion of charge carriers. This study will provide a pathway to design ultramodern, highly responsive/sensitive and broadband Si photodetectors.

Effect of doped TiO2 film as electron transport layer for inverted organic solar cell

Nanocrystalline TiO2 and Sn-doped TiO2 thin films were prepared by sol–gel spin coating method. The crystallinity and anatase phase of TiO2 and Sn-doped TiO2 were confirmed from X-ray diffraction analysis. The EDAX analysis also confirmed the presence of tin, oxygen and titania elements. By fabricating an inverted organic solar cell with device configuration of ITO/Sn-doped TiO2/active layer/MoO3/Al, power conversion efficiency (PCE) of the Sn-doped TiO2 was observed to be 3.08% compared to the TiO2 based solar cell of 2.64%.

Optical Characterizations of RF-Magnetron Sputtered Nanocrystalline TiO2 Thin Film

TiO2 nanocrystalline thin films are widely used as antireflection coatingin solar cell, in this paper, RF magnetron sputtering technique is used to prepareTiO2 thin film on glass substrates, TiO2 thin films deposited under differentpowers (75,100,125 and 150) Watt for (1.5) hour resulted in different layerthickness (62.5,88,118 and 132.6) nm respectively. The optical propertiesexamined by UV-VIS spectroscopy. TiO2 thin films exhibit a high transparency inthe region from about 350 nm above, we suggest that these results indicate themost suitable growing conditions for obtaining high quality sputtered TiO2 thinfilms with higher transparence performance for solar cell application. the opticalabsorbance coefficient for all films were genuinely high esteems coming to above104 cm-1, which implies that there is allowed direct transitions, the energy gabreach to the typical value of the bulk TiO2 (3.5) eV.

Structural, Morphological and Raman Studies of CdS/CdSe Sensitized TiO2 Nanocrystalline Thin Films for Quantum Dot Sensitized Solar Cell Applications

Background: It is well known that quantum dot-sensitized solar cells based on nanostructured semiconductor films are considered as a promising alternative to silicon-based solar cells. The aim of this paper is to investigate the structural and morphological properties of CdS/CdSe quantum dot sensitized photoanodes based on nanocrystalline TiO2 thin films considering their performance can reach an efficiency of 2.7%. Methods: TiO2 thin films were prepared on fluorine tin oxide (FTO) glass via the chemical route using commercial Degussa 25 and crystallized at 550°C. Furthermore, a layer of CdS and CdSe nanoparticles was deposited on the titania film by a sequence of successive ionic layer adsorption and reaction (SILAR) and chemical bath deposition (CBD) methods. After preparation, samples were analyzed using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy for their structural properties and composition. Scanning electron microscopy (SEM) was used to investigate their surface morphology, while energy dispersive X-ray spectrometry (EDS) was used to analyze the sample stoichiometry. Results: The structural properties and morphology of quantum dot sensitized photoanodes revealed that the titania thin films were highly crystalline belonging predominantly in the tetragonalanatase structure, while the CdS/CdS quantum dots were in the cubic phase. Furthermore, scanning electron microscopy (SEM) along with energy dispersive X-Ray mapping EDS showed little contamination. Conclusions: Combined analysis suggests that our preparation route leads to highly crystalline, stoichiometric photoanodes. This plays an important role in the performance of the quantum dot sensitized solar cells.

Fabrication of Perovskite Uniform Film in Air via Introduction of Aniline Cations

AbstractAlkylammonium lead(II) halide (APbX3) perovskite is developed to be the most potential light absorbing material in the photovoltaic field. However it is sensitive to water and oxygen, and the perovskite uniform film is difficult to be prepared in atmospheric environment. Here, CH3NH3PbI3 perovskite uniform layer was prepared in ambient environment via adding aniline cations in the methylamine deposition solution of the perovskite layer. XRD results indicate 3D crystalline structure of CH3NH3PbI3 perovskite is remained in the case of addition of the low concentration of aniline cations. With the addition increasing of aniline cations, 2D crystalline structure of (C6H5‐NH3)2PbI4 is gradually formed. SEM images indicate that the introduction of aniline cations obviously improves the coverage of CH3NH3PbI3 perovskite on nanocrystalline TiO2 thin film. The short circuit photocurrent and power conversion efficiency of perovskite solar cell based on the mixed perovskite were enhanced. The perovskite solar cell fabricated with the optimized molar ratio of aniline cations shows a short circuit photocurrent density of 17.1 mA⋅cm−2 and power conversion efficiency of 5.7%, which is 28% higher than that of the device based on pure CH3NH3PbI3 perovskite. The photovoltaic performance enhancement of perovskite solar cell is due to the formation of perovskite uniform layer as aniline cations are introduced, which suppresses the possible recombination processes.

Nano-crystalline TiO2 thin film: Synthesis and investigation of its optical switching characteristics

Optical-switching property of nano-crystalline TiO2 thin films of thickness ~ 100 nm synthesized using sol-gel assisted spin coating method on ~ 0.14 mm thick Al substrate were investigated in this work. The structural analysis of TiO2 thin film show its polycrystalline nature with anatase phase. Surface analysis of the film via scanning electron microscopy and atomic force microscopy show rough TiO2 surface with nearly crack free surface. Current-voltage measurement carried out on this sample (Pt/TiO2/Al) in dark and under optical illumination show enhancement in dark current under optical illumination resulting from photo generation of charge carriers in TiO2 thin film. In addition to this, it also gives rise to optical-switching in the current from low to high and high to low as the illumination was ON and OFF. These results suggest that nano-crystalline TiO2 thin film based optical-switching devices have potential for integrating current device technology towards exciting photo-active device technology.