TiO2 Network Research Articles

Template‐Free Macro‐Mesoporous TiO2/Carbon Nitride Interface for Visible‐Light‐Driven Photocatalysis

Porous titanium dioxide (TiO2) materials are pivotal in broad catalytic applications. However, design and development of macro‐mesoporous TiO2 involve a hard or soft templating method, which impedes the additional cost and complexity. Herein, a simple, scalable, and template‐free synthesis route to ordered uniform macro‐mesoporous TiO2 (mTiO2) layers is presented. The controlled aging of titanium tetraisopropoxide precipitates self‐assemble into an extended macroporous TiO2 network. As proved by X‐ray diffraction, the prepared materials are of anatase TiO2. Morphological analysis of mTiO2 displays a uniform macroporous network. To delight, the obtained mTiO2 possesses an extensive specific surface area of 257 m2 g−1, with an average pore size of ≈2 nm. This simplistic method provides a high yield of mTiO2 products. To find the practical utility of mTiO2, visible light active graphitic carbon nitride (g‐C3N4) is combined with mTiO2 and examined for photo fixation of dinitrogen (N2) and decomposition of organic pollutants. Remarkably, the mTiO2/g‐C3N4 interface shows drastically enhanced NH4+ production and dye degradation photocatalytic activity as compared with g‐C3N4 and commercial TiO2. This versatile and eco‐friendly preparation process of surface‐rich mTiO2 is scalable and may find widespread potential in large‐scale photocatalytic applications.

Investigating the impact of catalysts on N2 rotational and vibrational temperatures in low pressure plasmas

Future application of plasma assisted catalysis (PAC) requires a thorough understanding of energy partitioning within the plasma-catalyst system. We have studied the impact of adding a catalyst (TiO2 and zeolites) on the plasma energetics within an N2 low-temperature, radio frequency (RF) plasma by measuring the vibrational and rotational temperatures of the gas-phase N2. The presence of either micro- or nano-structured materials (zeolite and TiO2 substrates, respectively) within the plasma significantly decreases N2 (g) vibrational temperature, suggesting these materials promote vibrational relaxation within the discharge upon interaction with a catalytic substrate. In addition to evaluating the spectroscopic characteristics of the N2 discharge, we have assessed material morphology and chemical composition before and after plasma exposure. The porous network of both TiO2 and zeolite substrates were maintained after plasma exposure, although small amounts of nitrogen incorporation occurred at the surface of both materials.

Defect analysis of TiO2 doped with ytterbium and nitrogen by ab initio calculations

Different defects are studied in the network of anatase TiO2 to improve the utilization of the material for photoelectrochemical applications. With the ab initio calculations, defect-induced TiO2 models with different doping concentrations and oxidation states of Yb and N dopants are studied. Oxygen-deficient systems are modeled, and the interaction of oxygen vacancy with the Yb and N dopant in the bulk of TiO2 is elucidated. Yb 4f states are coupled with the O 2p states reducing the band gap and shifting the absorption edge of the TiO2 toward visible regime. Increasing Yb doping concentration reduced the band gap, and the 2.08% Yb doping concentration is considered as an optimal Yb doping. Comparing the band structures of mono-doped and codoped samples, Yb, N codoping reduced the band gap while creating isolated states in the forbidden region. Compensated and non-compensated systems of Yb- and/or N-doped TiO2 models are studied. Charge compensation in Yb, N-codoped TiO2 stabilized the system, reduced the band gap without having isolated states and provided broader absorption band. The Ti16−xYbxNyO31−y, x = 2, y = 1, model provided minimum structure modification with the suitable band structure for photoelectrochemical applications explaining the experimental results for the synergistic effect of Yb, N codoping in TiO2.

Preparation of a SnO2-Sb electrode on a novel TiO2 network structure with long service lifetime for degradation of dye wastewater.

Developing effective electrodes with long service lifetime for electrochemical degradation of dyes is of paramount importance for their practical industrial applications. We constructed a novel SnO2–Sb electrode (Ti/TiO2-NW/SnO2–Sb electrode) based on a uniform TiO2 network structure decorated Ti plate (Ti/TiO2-NW) for a long-term electrocatalytic performance. The SnO2–Sb coating layer on this electrode was grown on the Ti/TiO2-NW by pulse electrodeposition. The introduction of the three-dimensional TiO2-NW enhances the bonding strength between the Ti substrate and the SnO2–Sb surface coating. An accelerated life test shows that the service life of Ti/TiO2-NW/SnO2–Sb electrode is 11.15 times longer than that of the traditional Ti/SnO2–Sb electrode. The physicochemical properties of the electrodes were characterized through SEM, EDS, XRD and HRTEM. In addition, through LSV, EIS, CV and voltammetric charge analysis, it is found that compared with the traditional electrode, the Ti/TiO2-NW/SnO2–Sb electrode possesses a higher oxygen evolution potential, a lower charge transfer resistance and a larger electrochemical active surface area. Besides, this novel electrode also exhibits an outstanding electrocatalytic oxidation ability for degradation of acid red 73 in simulated sewage. After a 5 hours' test, the removal efficiency of acid red 73 and the COD reached 98.6% and 71.8%, respectively, which were superior to those of Ti/SnO2–Sb electrode (89.1% and 58.8%). This study highlights the excellent stability of the Ti/TiO2-NW/SnO2–Sb electrode and provides an energy-efficient strategy for dye degradation.

Polyimide-TiO2 nanocomposites and their corresponding membranes: Synthesis, characterization, and gas separation applications

In this work at firstly a new class of polyimide/TiO2 (PI/TiO2) composites containing amino acid (Methionine) and benzimidazole fractions in the structure with chemical bonds between the polyimide and TiO2 network has been synthesized by the sol-gel reaction and characterized and then secondly carbon (C) membranes were prepared by carbonization of PI/TiO2 at different temperatures under vacuum and nitrogen flow. Carbonization temperature was critical in the modification of membrane structure. At the same temperature, the carbon membranes fabricated under nitrogen atmosphere had higher gas permeations than those fabricated under vacuum. With the introduction of the TiO2, there was no significant enhancement of the gas separation in the resulting PI/TiO2 composite membranes over the PI membrane, which still suffered a ‘‘trade-off’’ relationship between permeability and selectivity. However, the derived C/PI/TiO2 composite membranes exhibited better gas separation properties. The C/PI/TiO2 15% composite membrane produced the highest permeations of 650.47, 520.56, 187.92 and 115.22 GPU for He, CO2, O2 and N2, respectively. The bionanocomposites were analyzed for antibacterial activity against Gram-positive and Gram-negative bacteria. The results indicated that the synthesized bionanocomposites is effective against all of the studied bacteria, and its effectiveness is higher for Pseudomonas aeruginosa.

Increase of power conversion efficiency in dye-sensitized solar cells through ferroelectric substrate induced charge transport enhancement

Ferroelectric functionalized dye-sensitized solar cells were fabricated by using a positively-poled LiNbO3 substrate coated with ITO (ITO-LiNbO3) as a collector electrode and demonstrated enhanced power conversion efficiency. Surface potential properties of TiO2 nanoparticle film coated on the ITO-LiNbO3 (TiO2/ITO-LiNbO3) examined by Kelvin probe force microscopy (KPFM) confirmed that a large electric field (a few 10 V/µm) generated from LiNbO3 can penetrate through the ITO layer and is applied to TiO2 film. This polarization-induced electric field leads to an increased photocurrent density by attracting and promoting electrons to direct transport through the mesoporous TiO2 network toward the collector electrode and a decreased charge recombination by facilitating electrons to pass through fewer boundaries of nanoparticles, resulting in high power conversion efficiency. The power conversion efficiency was enhanced by more than 40% in comparison with that without polarization-induced electric field. Incorporating functional ferroelectrics into photovoltaic cells would be a good strategy in improving photovoltaic performance and is applicable to other types of photovoltaic devices, such as perovskite solar cells.

Hierarchical submicroflowers assembled from ultrathin anatase TiO2 nanosheets as light scattering centers in TiO2 photoanodes for dye-sensitized solar cells

Hierarchical TiO2 submicroflowers (HTFs) were mixed with anatase TiO2 nanoparticles (TNPs) to fabricate efficient photoanodes for dye-sensitized solar cells (DSSCs). HTFs with high specific surface area (98 m2 g−1) and favorable dimension (∼660 nm) facilitate dye loading and light scattering of photoanodes, boosting the light harvesting capability of DSSCs. Moreover, due to the high crystallinity and reduced defects or grain boundaries, the HTFs embedded in the photoanodes act as efficient conductive pathways for charges. With excessive incorporation of HTFs, on the other hand, inevitable shrink of HTFs after calcination causes poor connectivity of TiO2 network and even cracks in the photoanodes, deteriorating the charge collection capability of DSSCs. As a result, a balance should be made between optical and photoelectrochemical characteristics of photoanodes via HTF introduction. The DSSC based on 10 wt% HTFs embedded photoanode exhibits the highest power conversion efficiency of 6.4%, which is 52.4% higher than that of DSSCs with TNP-based photoanodes.

Improving optoelectrical properties of photoactive anatase TiO2 coating using rGO incorporation during plasma electrolytic oxidation

In this study, photoactive anatase titanium dioxide (TiO2)/reduced graphene oxide (rGO) composite coatings were successfully developed on titanium substrate using plasma electrolytic oxidation (PEO) process to evaluate the effect of incorporated rGO on their photoactivity and optoelectrical properties. Structural and morphological examinations verified the incorporation of the rGO sheets in the anatase TiO2 coatings with pancake-like morphology. Optical spectroscopy revealed that a higher content of rGO enhanced the light absorption in UV and visible regions due to the high photo-trapping behavior of the composite samples. Photoluminescence declined by increasing rGO content, which suggested that the recombination of the photogenerated electron-hole pairs was suppressed. As a result, the photocurrent response of the TiO2/rGO coatings was enhanced significantly under UV radiation compared to the pure one, by which the photoactivity response revealed an over twenty-fold increase for a high concentration of rGO. Moreover, the electron transfer pathways created by the 2D structure of rGO sheets inside the anatase TiO2 network, promoted the photoresponsivity and photoswitching behavior more than 60%, interestingly.

Ag@SiO₂ Core-shell Nanoparticles Embedded in a TiO₂ Mesoporous Layer Substantially Improve the Performance of Perovskite Solar Cells.

In this study, Ag@SiO2 nanoparticles were synthesized by a modified Stöber method for preparing the TiO2 mesoporous layer of carbon counter electrode-based perovskite solar cells (PSCs) without a hole transporting layer. Compared with normal PSCs (without Ag@SiO2 incorporated in the TiO2 mesoporous layer), PSCs with an optimal content of Ag@SiO2 (0.3 wt. % Ag@SiO2-TiO2) show a 19.46% increase in their power conversion efficiency, from 12.23% to 14.61%, which is mainly attributed to the 13.89% enhancement of the short-circuit current density, from 20.23 mA/cm2 to 23.04 mA/cm2. These enhancements mainly contributed to the localized surface Plasmon resonance effect and the strong scattering effect of Ag@SiO2 nanoparticles. However, increasing the Ag@SiO2 concentration in the mesoporous layer past the optimum level cannot further increase the short-circuit current density and incident photon-to-electron conversion efficiency of the devices, which is primarily ascribed to the electron transport pathways being impeded by the insulating silica shells inside the TiO2 network.

Titanium Dioxide Doped with Nitrogen Nanopowder Prepared by Hydrothermal Method

Nitrogen-doped titanium dioxide (N-doped TiO2) nanopowder was successfully prepared by the hydrothermal method. Titanium isopropoxide and urea were used as the starting precursors. The hydrothermal reaction was controlled at 200 °C for 2, 4 and 6 h. The white powder was obtained and dried for 24h. The crystal structure was identified by X-ray diffraction (XRD). A single phase of anatase structure was obtained without calcination steps. The morphology was investigated by field emission scanning electron microscopy (FESEM). The particles were irregular in shape and highly agglomerated. The chemical composition was determined by energy dispersive X-ray spectrometry (EDXS). The characteristic X-ray energy of titanium (Kα = 4.51 keV and Kβ = 4.93 keV), oxygen (Kα = 0.52 keV) and nitrogen (Kα = 0.39 keV) were observed. The functional group was identified by Fourier transform infrared spectrophotometry (FTIR). The wavenumbers in the range 668 to 1389 cm-1 corresponded to vibrations of Ti–O–Ti bond. The wavenumber in the range of 1442 to 1500 cm-1 could be attributed to the nitrogen species in the TiO2 network.

Photodegradation of the herbicide imazapyr over mesoporous In2O3-TiO2 nanocomposites with enhanced photonic efficiency

Abstract Mesoporous In2O3-TiO2 nanocomposites were successfully synthesized through sol gel approach in the presence of a suitable template. Their photonic efficiencies have been determined by measuring photooxidation of the herbicide imazapyr and compared with those of porous TiO2 and of the commercial photocatalyst Hombikat UV-100. The X-ray diffraction patterns indicated the formation of tetragonal anatase and rutile phases of TiO2. The highest photonic efficiency values of 13.5% were obtained for the 0.1% In2O3-TiO2 nanocomposite in comparison with a value of 12% for mesoporous TiO2 and of 10.8% for commercial UV-100. As a result of the synergetic effect of TiO2 (anatase & rutile) and In2O3 nanoparticles, it is suggested that heterostructured In2O3-TiO2 nanocomposites are formed and are promoting the separation of photogenerated charge carriers. Moreover, the In2O3-TiO2 nanocomposites could be easily reused up to 5 times with only a slight decrease in their photocatalytic activity. The design for merging unique In2O3 nanoparticles with a mesoporous TiO2 network can be regarded as an innovation to further promote the photocatalytic properties of the latter.

Self-organized TiO2 network decorated with SnO2 nanoparticles as an anode for lithium-ion batteries

An efficient synthesis method is developed by combining hydrothermal and pyrolysis methods to fabricate the TiO2@SnO2 film electrode with a special three-dimensional (3D) nano-network structure. In the present study, the TiO2 network can offer a 3D scaffold for the SnO2 nanoparticles against the drastic pulverization problem and also support a fast charge transport path. When being assessed as an anode for lithium-ion batteries (LIBs), the unique structure eventually brings relatively stable cycling processes (834 mAh g−1 can be maintained after 100 cycles with high capacity retention of 120%) and a remarkable rate performance (796, 705, 564 and 464 mAh g−1 at 200, 400, 800 and 1600 mA g−1, respectively).

Effects of rapid thermal annealing on the local environment, electronic structure and magnetic properties of Mn doped TiO2 thin films

In this report, nanostructure thin films of Ti0.97 Mn0.03 O2−δ were deposited on Si (001) substrate by pulsed laser deposition (PLD) technique, followed by rapid thermal annealing (RTA) in different ambient gases; O2, N2 and Ar. The RTA treatment dramatically affected the surface defects, local environment, electronic structure and magnetic properties of the films, where, annealing of the film in O2 gas recovered the dislocated atoms of oxygen vacancy (Vo) at the surfaces and induced a diamagnetic phase, whereas the annealing of film in N2 gas introduced Vo, reduced the valence state of Ti4+ (TiO2) into Ti3+ (Ti2O3) and induced ferromagnetic (FM) signal. Therefore, the change in the local atomic defects of Vo at the surface texture is likely to be responsible for the magnetic response. The presence of oxygen vacancy has been traced by means of Raman scattering and near edge X-ray absorption fine structure (NEXAFS) spectroscopy measurements. The NEXAFS spectra were carried out in synchrotron facility at Ti/Mn L3,2 and O-K edges. The spectral feature at Ti L3,2 edges revealed the rutile phase of TiO2. The asymmetry of t2g and eg bands at the O-K edge confirmed the formation of Vo and reflected the modification in the O co-ordination around Ti4+cations. The Mn L3,2 edges revealed the incorporation of Mn ions in the TiO2 network with Mn2+ and Mn3+ mixed valence states. This different valance states (Mn2+ and Mn3+) is the basis for the ferromagnetism induced by Stoner spin-splitting of the local density of defects state at Fermi level (EF). The change in the magnetic moment of the films after RTA process is related to the change in the local density of defects band. Crystal field and charge transfer parameters were extracted roughly by using atomic multiplet calculations (theory).

Photocatalytic performance of plasmonic Au/Ag-TiO2 aerogel nanocomposites

Porous nanocomposites of Au/Ag-TiO2 aerogels benefit from unique morphological characteristics of aerogels, intrinsic physical, chemical and also photocatalytic required properties of TiO2 and strong plasmonic effects of metal nanoparticles, i.e., Au and Ag. Therefore, they are able to be applied as an ideal photocatalyst overcoming the intrinsic weak points of conventional TiO2. In this research, several composites of TiO2 aerogel modified with Au, Ag and mixture of Au-Ag nanoparticles were prepared by applying two different methods of synthesis. The photocatalytic efficiency of the samples was established by using salicylic acid (SA) as a standard pollutant molecule. The photodegradation rate of SA under irradiation was investigated by the decrease in its concentration in time, and found to depend on specific surface area and type of metal nanoparticles trapped into the TiO2 network. The larger surface area and the presence of the two metal nanoparticles lead to the best results. Furthermore, the morphological characterizations and physical properties of the nanocomposites were investigated through BET results, FESEM and TEM images.

DFT Insights into the Role of Relative Positions of Fe and N Dopants on the Structure and Properties of TiO₂.

The location and nature of the doped elements strongly affect the structural, electronic and optical properties of TiO2. To tailor the band structure and modify the photoelectrochemical properties of TiO2, a pair of dopants is selected. Fe and N atoms are inserted in the TiO2 network at substitutional and interstitial sites with different relative distances. The main objective behind the different locations and sites of the doped elements is to banish the isolated unoccupied states from the forbidden region that normally annihilates the photogenerated carriers. Fe at the Ti site and N at the O site doped in the TiO2 network separated at a distance of 7.805 Å provided a suitable configuration of dopant atoms in terms of geometry and band structure. Moreover, the optical properties showed a notable shift to the visible regime. Individual dopants either introduced isolated unoccupied states in the band gap or disturbed the fermi level and structural properties. Furthermore, the other co-doped configurations showed no remarkable band shift, as well as exhibiting a suitable band structure. Resultantly, comparing the band structure and optical properties, it is argued that Fe (at Ti) and N (at O) doped at a distance of 7.805 Å would strongly improve the photoelectrochemical properties of TiO2.

Efficiency Enhancement in Plasmonic Dye-Sensitized Solar Cell Employing High Performance TiO2 Photoanode Doped with Silver Nanoparticles

High performance TiO2 photoanodes undoped and doped with silver nanoparticels of size about 15 nm were fabricated. Employing these electrodes dye-sensitized solar cells (DSSCs) were fabricated using N719 dye as sensitizer and iodide-triodide as redox couple. Current-voltage measurements were performed under the illumination of 100 mW cm-2 (AM 1.5). The electrical parameters of the fabricated cells were extracted from the current-voltage data that include open-circuit voltage, short-circuit current, shunt resistance, series resistance, fill-factor, ideality factor and solar energy-to-electricity conversion efficiency. The comparison of parameters revealed improvement in both the photovoltaic and electrical parameters of the plasmonic cell. The conversion efficiency measured for the reference cell without Ag NPs in TiO2 was 7.43 %, whereas the efficiency of plasmonic device with TiO2:Ag NPs was 9.26 %, resulting an overall efficiency improvement of 23% with Ag NPs. The increased performance of the plasmonic DSSC can be assigned to the improvement of its photovoltaic and electrical parameters. The improved short-circuit photocurrent density appears to be boosted due the enhanced light harvesting capability of the photoanode caused by the localized surface plasmon resonance effect induced in Ag nanoparticles. While, the rise in Voc can be credited to the upward shift of Fermi level of TiO2 due to the dopping of Ag nanoparticles in TiO2 network.

Multifunctional Moth-Eye TiO2/PDMS Pads with High Transmittance and UV Filtering.

This work reports a facile fabrication method for constructing multifunctional moth-eye TiO2/polydimethylsiloxane (PDMS) pads using soft nano-imprinting lithography and a gas-phase-deposited thin sacrificial layer. Mesoporous TiO2 nanoparticles act as an effective UV filter, completely blocking high-energy UVB light and partially blocking UVA light and forming a robust TiO2/PDMS composite pad by allowing the PDMS solution to easily fill the porous TiO2 network. The paraboloid-shaped moth-eye nanostructures provided high transparency in the visible spectrum and also have self-cleaning effects because of nanoroughness on the surface. Furthermore, we successfully achieved a desired multiscale-patterned surface by partially curing select regions using TiO2/PDMS pads with partial UVA ray blockers. The ability to fabricate multifunctional polymeric pads is advantageous for satisfying increasing demands for flexible and wearable electronics, displays, and solar cells.

Conventional hydrothermal synthesis of titanate nanotubes: Systematic discussions on structural, optical, thermal and morphological properties

Titanate nanotubes were successfully synthesized by hydrothermal technique under acidic-base medium. The anatase and titanate phase of the starting TiO2 and tubular titanate was confirmed by powder XRD technique. The UV–vis-NIR spectroscopy was used to study the absorption nature of titanate nanotubes and the band gap was calculated as 3.3eV. Infrared technique was employed to detect the presence of all the functional groups in the synthesized titanate nanotube material. Thermal properties of the title material were studied by TG-DTA analyses. The shrinkage of interlayer distance of TiO2 network confirms the nanotube formation. Morphology and size information about the synthesized material were carried out using FESEM and TEM analysis. Titanate nanotubes are having the maximum length of 2.24µm and the average diameter of 169.73nm. EDX analysis gives out the elemental composition of the as synthesized product. This report may fetch an efficient way to synthesize TiO2 nanotubes using TiO2 nanoparticles.

Effects of Nanoscale Interfacial Design on Photocatalytic Hydrogen Generation Activity at Plasmonic Au–TiO2 and Au–TiO2/Pt Aerogels

We demonstrate that composite catalytic aerogels represent a superior materials design motif for the creation of solar fuels photocatalysts. We couple surface plasmon resonant (SPR) guests to the inherent compositional and interfacial design flexibility of catalytic aerogels to photogenerate molecular hydrogen (H2). We investigate the effects of synthetically modifying the TiO2 aerogel network and the nanoparticulate Au||TiO2 interfaces in plasmonic Au–TiO2 aerogels on H2 evolution under both broadband (i.e., UV + visible light) and visible excitation. We also introduce non-plasmonic Pt co-catalyst nanoparticles into our composite aerogels, creating Au–TiO2/Pt aerogels that perform visible light SPR-driven photocatalytic reduction of water to generate H2. The fuels production achieved with this multicomponent photocatalytic nanoreactor demonstrates that the nanostructured high-surface-area network in the aerogel can spatially and effectively separate charge while electrochemically connecting plasmonic nanoparticle sensitizers and metal nanoparticle. In doing so, we prove several crucial concepts: (1) integration of a plasmonic sensitizer with a separate water reduction co-catalyst within the ultraporous aerogel nanoarchitecture; (2) wiring the electron–hole pairs generated under visible light at the plasmonic Au||TiO2 interface to the co-catalyst via the nanoscale TiO2 network; and (3) combining both the photocatalytic oxidation and reduction reactions critical to solar fuels photocatalysis into one composite material at length scales compatible with the reaction kinetics. This work is supported by the Office of Naval Research.

Cocatalyzing Pt/PtO Phase-Junction Nanodots on Hierarchically Porous TiO2 for Highly Enhanced Photocatalytic Hydrogen Production.

Phase-junctions between a cocatalyst and its semiconductor host are quite effective to enhance the photocatalytic activity and are widely studied, while reports on the phase-juncted cocatalyst are still rare. In this work, we report the deposition of the Pt/PtO phase-juncted nanodots as cocatalyst via NaOH modification of an interconnected meso-macroporous TiO2 network with high surface area and inner-particle mesopores to enhance the performance of photocatalytic H2 production. Our results show that NaOH modification can largely influence Pt/PtO phase-juncted nanodot formation and dispersity. Compared to the TiO2 nanoparticles, the hierarchically meso-macroporous TiO2 network containing 0.18 wt % Pt/PtO phase-juncted cocatalyst demonstrates a highest photocatalytic H2 rate of 13 mmol g-1 h-1 under simulated solar light, and possesses a stable cycling activity without obvious decrease after five cycles. Such high H2 production performance can be attributed to both the phase-juncted Pt/PtO providing more active sites while PtO suppresses the undesirable hydrogen back reaction, and the special hierarchically porous TiO2 network with inner-particle mesopores presenting short diffusion path lengths for photogenerated electrons and enhanced light harvesting efficiency. This work suggests that Pt/PtO phase-juncted cocatalyst on hierarchically porous TiO2 nanostructures is a promising strategy for advanced photocatalytic H2 production.