Enhanced H2 production activity under solar irradiation over N-doped TiO2 prepared using pyridine as a precursor: a typical sample of N-doped TiO2 series

A series of N-doped TiO₂ photocatalysts were synthesized by simple hydrolysis method using titanium isopropoxide and pyridine precusors. XRD studies of all the catalysts showed a single anatase phase and TEM results confirmed that the particles obtained by this procedure are of nano size and in the range of 3–5 nm. UV–vis DRS of N-doped TiO₂ catalysts clearly show an enhanced absorption in the visible light region, and the band gaps are seen decreasing as the content of doped nitrogen increases. The formation of oxynitride linkages is evidenced by the FTIR results. Further the XPS analysis confirms the presence of Ti-O-N linkage, and the binding energy values indicate that the doped nitrogen is either interstitial or chemisorbed. The photocatalytic activity of N-doped TiO₂ catalysts is evaluated for hydrogen production in methanol: water mixture under solar light irradiation and 0.5 wt% N-doped TiO₂ has shown a stable and high activity, 3500 μ mol/h/g.cat. The enhanced hydrogen production activity of N-doped TiO₂ catalysts is attributed to the high surface area of the catalysts containing doped nitrogen mostly at the interstitial positions and enhanced visible light absorption.     

Characterizations and photocatalytic activity comparisons of N-doped nc-TiO2 depending on synthetic conditions and structural differences of amine sources

N-doped TiO₂ nanoparticles were prepared by using 1° and 2° alkyl and alcohol amines [n-propylamine (nPRYL), ethanolamine (ETOH), propanolamine (PROH), diethylamine (DETYL), dipropylamine (DPRYL), diethanolamine (DETOH), and ammonium hydroxide (NH₄OH)] as nitrogen sources through microwave and hydrothermal growth (HT) methods. Characterization of the nanoparticles was done by X-ray diffraction, scanning electron microscopy, ultraviolet-visible absorption spectra, X-ray photoelectron spectroscopy, fourier transform infrared spectroscopy and BET analysis techniques. Nitrogen species in TiO₂ lattice were interstitial impurities. Nitrogen content of the particles depended on the preparation conditions and structural differences of nitrogen sources. Photocatalytic degradation of methylene blue was carried out under Xenon lamp irradiation. N-doped TiO₂ nanoparticles exhibited higher photocatalytic activity, compared to undoped ones. Acidity differences of amine sources and irradiation differences of synthetic conditions had an effect on photocatalytic activity. Although N doping into TiO₂ lattice was the least effective in the particle prepared by the use of nPRYL as the amine source and HT as the synthetic condition, its photocatalytic activity was slightly better compared to others.     

Photocatalytic activity of N, S co-doped and N-doped commercial anatase TiO2 powders towards phenol oxidation and E. coli inactivation under simulated solar light irradiation

Nitrogen and sulfur co-doped and N-doped TiO₂ anatase TKP 102 (Tayca) were prepared by manual grinding with thiourea and urea, respectively, and annealing at 400 °C. Both materials showed visible-light absorption as measured by Diffuse Reflectance Spectroscopy (DRS). Interstitial N-doping, anionic and cationic S-doping was found when the TiO₂ was doped with thiourea while TiO₂ doped with urea showed only the presence of interstitial N-doping as measured by X-ray Photo-electron Spectroscopy (XPS). The N content on the surface of N-doped TKP 102 photocatalyst was 2.85 at.% and higher than the N content in the N, S co-doped TiO₂ photocatalyst (0.6 at.%).The photocatalytic activity of the doped catalysts was tested using phenol and Escherichia coli as chemical and biological targets, respectively, using N, S co-doped, N-doped TiO₂, undoped Degussa P-25 and undoped TKP 102 powders under simulated solar light. It was found that undoped Degussa P-25 was the photocatalyst with the highest photocatalytic activity towards phenol oxidation and E. coli inactivation. N, S co-doped powders showed almost the same photocatalytic activity as undoped TKP 102 while N-doped TKP 102 was the less active photocatalyst probably due the N impurities on the TiO₂ acting as recombination centers.     

Fabrication of nano N-doped In2Ga2ZnO7 for photocatalytic hydrogen production under visible light

N-doped In₂Ga₂ZnO₇ photocatalysts were fabricated by solid state reaction route. All the prepared photocatalysts were successfully characterised by PXRD, optical absorption spectra, SEM, TEM, XPS, BET surface area and photoresponse studies. The formation of In₂Ga₂ZnO₇ was confirmed by the PXRD pattern. Optical absorption spectra showed that the visible light absorption of all the photocatalysts were enhanced by nitrogen doping. Among all the prepared photocatalysts, 1 wt% Pt loaded N-GaInZn-500 showed enhanced photocatalytic activity towards hydrogen evolution under visible light irradiation in presence of 10 vol% methanol solution as sacrificial agent. The excellent photocatalytic activity of N-GaInZn-500 is in agreement with N-content, bandgap energy, PL intensity and Surface area.     

Visible light-active nitrogen-doped TiO2 thin films prepared by DC magnetron sputtering used as a photocatalyst

The visible light-active nitrogen-doped TiO₂ has been prepared by dc-reactive magnetron sputtering using Ti target in an Ar+O₂/N gas mixture. The preparation of highly crystallized anatase TiO_xN_y thin films with various nitrogen concentrations allowed us to identify the optimum nitrogen flow ratio for the photocatalytic oxidation (PCO) of 2-propanol. At higher nitrogen flow rate, nitrogen is found to be difficult to substitute for oxygen having been predicted to contribute the band gap narrowing, giving rise to undesired deep level defects. In addition, Raman spectroscopy and X-ray diffraction (XRD) studies revealed that highly crystallized anatase growth of nitrogen-doped TiO_xN_y thin films are difficult at higher nitrogen flow rate. The optical band gap was found to be lower for the films deposited at 2 sccm of nitrogen flow rate. The PCO of 2-propanol was studied as a function of nitrogen flow rate using in situ FTIR spectroscopy. The PCO of 2-propanol found to proceed along two routes: one was through the chemisorbed species, 2-propoxide to form the CO₂ directly; the other was through conversion of 2-propanol to acetone, followed by formation of formate species, and finally CO₂.     

Photocatalytic performance of S doped TiO2 in relation to processing conditions: calcination temperature and heating rate

Abstract

The effect of different heating profiles on the photocatalytic performance of sulphur doped TiO₂ photocatalysts is reported. The photocatalysts were synthesised by a sol–gel method using thiourea as the dopant precursor and characterised using X-ray diffraction, elemental analysis and reflection measurements. The degradation of dichloroacetic acid, under indirect sunlight and visible light irradiation, was used to determine the photocatalytic performance of the synthesised materials. A number of different commercial photocatalysts were used as comparative standards. In all the studied specimens, anatase TiO₂ was the dominant crystalline type. Additionally, compared with undoped TiO₂ and commercial standards, significant absorption into the visible region (400–470 nm) was observed for the modified TiO₂.     

Hydrothermally stabilized Fe(III) doped titania active under visible light for water splitting reaction

Fe³⁺ doped TiO₂ photocatalysts were prepared by hydrothermal treatment for the photocatalytic water splitting to produce stoichiometric hydrogen and oxygen under visible light irradiation. It was found that hydrothermal treatment at 110 °C for 10 h was essential for the synthesis of highly stabilized Fe³⁺ doped TiO₂ photocatalysts. The synthesized photocatalysts were characterized by field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS) and BET surface area techniques. The doping of highly stabilized Fe³⁺ in the titania matrix leads to significant red shift of optical response towards visible light owing to the reduced band gap energy. Optimum amount of Fe³⁺ doped TiO₂, 1.0 wt% Fe/TiO₂, showed drastically improved hydrogen production performance of 12.5 μmol-H₂/h in aqueous methanol and 1.8 μmol-H₂/h in pure water, respectively. This Fe/TiO₂ photocatalyst was stable for 36 h without significant deactivation in the water splitting reaction.     

Physically and chemically synthesized TiO2 composite thin films for hydrogen production by photocatalytic water splitting

TiO₂ thin films have been synthesized by radio-frequency magnetron sputtering and sol–gel method to study the hydrogen generation by photocatalytic water splitting under visible light irradiation. Photoelectrochemical cell with chemical bias, involving photo-anode in form of TiO₂ film deposited on conducting indium tin oxide (ITO) film and Pt as cathode, is developed. The effect of conducting ITO layer on photo-voltage is studied by varying the thickness of ITO films. Constant H₂ generation rate is obtained for long period of time by both the TiO₂ films because of the separated evolution of H₂ and O₂ gas, thus eliminating the back-reaction effect. Sputter-deposited film as compared to sol–gel-synthesized film showed better H₂ generation rate, mainly explained in terms of the higher visible light absorption achieved by oxygen vacancies created in the TiO₂ film by the energetic target ions during deposition in pure Ar gas pressure.     

Hydrogen production by photocatalytic water-splitting using Cr- or Fe-doped TiO2 composite thin films photocatalyst

Cr- or Fe-ion-doped TiO₂ thin films have been synthesized by radio-frequency magnetron sputtering and a sol–gel method to study hydrogen generation by photocatalytic water-splitting under visible light irradiation. The doping method, dopant concentration, charge transfer from metal dopants to TiO₂, and type of dopants used for modification of TiO₂ were investigated for their ability to enhance photocatalytic activity. UV–Visible spectra show that the metal-doped-TiO₂ obtained by sputtering is much more efficient than that obtained by the sol–gel technique at inducing a red shift of the absorption edge in the visible light range. Low concentration metal ion doping must be done near the conducting indium tin oxide (ITO) – TiO₂ interface to avoid the formation of recombination centers for photo-generated electron–hole pairs. H₂ production rate (μmol/h) is higher for Fe-doped TiO₂ (15.5 μmol/h) than for Cr-doped TiO₂ (5.3 μmol/h) due to the ability of Fe ions to trap both electrons and holes, thus avoiding recombination, while Cr can only trap one type of charge carrier. A constant H₂ generation rate is obtained for long periods of time by all the investigated TiO₂ films because of the separate evolution of H₂ and O₂ gases, thus eliminating the back-reaction effect.     

A continuous valence band through NO orbital hybridization in NTiO2 and its induced full visible-light absorption for photocatalytic hydrogen production

Photocatalytic hydrogen production represents an effective approach for solar energy conversion, which can greatly ease the current energy crisis. Herein, we report a successful NO orbital hybridization in N-doped TiO₂ nanotube, the absorption wavelength is greatly red-shifted to visible light (from 400 to 800 nm) with large absorbance. The doping N element can partially replace the oxygen sites in TiO₂ lattice to form NTiN bonds. The hybridization effect of N 2p and O 2p makes a continuous valence band and the position up-shift from 1.99 to 1.67 eV, the band gap is subsequently narrowed from 3.21 to 2.77 eV for 1.85-NTiO₂ nanotube, which has been confirmed by ultraviolet–visible diffuse reflectance spectra and X-ray photoelectron spectroscopy valence band spectra. Benefiting from the enhanced visible light absorption ability and ultrathin shell feature, 1.85-NTiO₂ nanotube exhibits exciting photocatalytic hydrogen evolution performance with a rate of 10870 μmol h⁻¹ g⁻¹ under the selected visible light irradiation (λ > 400 nm). This work demonstrates an alternative strategy for tuning visible light absorption ability by doping for wide-band-gap semiconductors in photocatalysts design, and the philosophy can also be extended to other photocatalytic systems.     

In-situ growth of N–TiO2 on delaminated N–Ti3C2 with highly strengthened photocatalytic activity

Herein, a collection of N–TiO₂/delaminated N–Ti₃C₂ (NTTx) composites is designed and synthesized by one-step calcination of NH₄Cl–Ti₃C₂ precursors. The thermal decomposition of NH₄Cl not only serves as the gas template to make the delamination of Ti₃C₂, but also acts as N source to for doping. As expected, the N–TiO₂ nanoparticles uniformly anchor on the surface and interlayer of Ti₃C₂ nanosheets with intimate contact. Both the photocatalytic degradation rate of Rh–B and photocatalytic nitrogen fixation experiments of NTT2 composites show higher performance than that of pure P25 and NTT0 under visible light irradiation. The strengthened photocatalytic activity is due to the decrease of band gap by N doping in TiO₂ and excellent electrical conductivity of N–Ti₃C₂, which leads to enhanced light response and photogenerated electron-hole separation, respectively. This study develops a new strategy to design efficient photocatalysts for degradation of contaminants and fixation of nitrogen.     

Down-conversion Ce-doped TiO2 nanorod arrays and commercial available carbon based perovskite solar cells: Improved performance and UV photostability

Ce-doped TiO₂ nanorod arrays as the mesoporous supporting layer and TiO₂ compacted electron blocking layer was simultaneously fabricated by one-pot hydrothermal method. Ce-doped TiO₂ nanorod was utilized as down-conversion luminescence centers and commercial available carbon was used in hole-transport-free perovskite solar cells. SEM indicated that the doping of Ce reduced the diameter of TiO₂ nanorods. XRD and EDS spectra demonstrate the successful incorporation of Ce ions. UV–Vis absorption spectra and PL emission spectra show that the down-conversion center Ce⁴⁺ can effectively utilize ultraviolet light and extend the absorption into the ultraviolet region. The carbon-based device using the optimized Ce doping concentration achieved power conversion efficiency (PCE) of 10.1%, which was 24.6% higher than that of undoped device. Ce doping changes the band gap of TiO₂ and the Fermi energy level, and thus improve the open circuit voltage. The open circuit voltage decay curve show that Ce-doped TiO₂ prolongs electron lifetime. EIS analysis hints that more carriers are generated. The UV stability has also been significantly improved., PCE of the optimized Ce-doped device still retains 90% of the initial value, however, the undoped device decreases to 55% of the initial value under light irradiation at ambient atmosphere for 12 h.     

Solar photocatalytic degradation of methylene blue in carbon-doped TiO2 nanoparticles suspension

Carbon-doped TiO₂ nanoparticles were prepared by sol–gel auto-combustion method and characterized by X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), Brunauer–Emmett–Teller method (BET), UV–vis diffuses reflectance spectroscopy (DRS). UV–vis diffuse reflectance spectra showed that carbon-doped TiO₂ exhibited obvious absorption in the visible light range. The visible light photocatalytic activity of carbon-doped TiO₂ was ascribed to the presence of oxygen vacancy state between the valence and the conduction bands because of the formation of Ti³⁺ species in the as-synthesized carbon-doped TiO₂. The sample calcined at 873 K showed the highest photocatalytic activity under solar irradiation. The effects of photocatalyst concentration, initial concentration of methylene blue, and pH value in aqueous solution were also presented.     

Visible light driven nanocrystal anatase TiO2 doped by Ce from sol–gel method and its photoelectrochemical water splitting properties

Visible light driven nanocrystal anatase TiO₂ was prepared by doping rare earth element Ce through sol–gel method. UV–Vis diffusion reflectance spectrum indicated its absorption edge extended to about 550 nm, red shifting about 170 nm compared with that without doping. Ce doping TiO₂ showed obvious anodic photocurrent effect for water splitting under visible light irradiation (λ > 420 nm) in photoelectrochemical measurement with three electrodes configuration. Ce doping TiO₂ showed higher photocurrent density than that of without doping TiO₂ under full arc irradiation. Furthermore, the electronic structures for CeO₂ and TiO₂ were analyzed theoretically based on the first principle calculation. As a result, the electronic structure for Ce doping TiO₂ is proposed as the overlap and some degree of hybridization among splitting occupied Ce 4f and unoccupied Ce 4f with O 2p and Ti 3d respectively. The visible light responsive property is mainly due to the transition from O 2p hybridizing with occupied Ce 4f to unoccupied Ce 4f overlapping with Ti 3d.     

Investigation on the electron transfer between anatase and rutile in nano-sized TiO2 by means of surface photovoltage technique and its effects on the photocatalytic activity

Photoinduced electron transfer between anatase and rutile in nanosized TiO₂, prepared by a sol–gel method, was revealed by means of the surface photovoltage technique, and its effects on the photocatalytic performance in the degradation of a phenol solution were investigated. Also, the role of the surface states during the processes of photo-physics and photochemical reactions was discussed. In the as-prepared TiO₂ sample consisting of anatase and rutile, the photoinduced electrons can easily transfer from anatase surface states to rutile, as well as from anatase conduction band to rutile. These factors are responsible for the strong surface photovoltage response and high photocatalytic activity. Moreover, the surface states related to oxygen vacancies can induce photocatalytic reactions under visible irradiation, especially in the resulting biphase TiO₂, due to the electron transfer from anatase surface states to rutile.     

LED light-induced enhanced photocatalytic hydrogen evolution on Au@TiO2 core–shell modified by nitrogen doping and reduced graphene oxide

This study focused on the large band gap of TiO₂ for its use as a photocatalyst under light emitting diode (LED) light irradiation. The photocatalytic activities of core–shell structured Au@TiO₂ nanoparticles (NPs), nitrogen doped Au@TiO₂ NPs, and Au@TiO₂/rGO nanocomposites (NCs) were investigated under various light intensities and sacrificial reagents. All the materials showed better photocatalytic activity under white LED light irradiation than under blue LED light. The N-doped core–shell structured Au@TiO₂ NPs (Au@N–TiO₂) and Au@TiO₂/rGO NCs showed enhanced photocatalytic activity with an average H₂ evolution rate of 9205 μmol h^?1g^?1 and 9815 μmol h^?1g^?1, respectively. All these materials showed an increasing rate of hydrogen evolution with increasing light intensity and catalyst loading. In addition, methanol was more suitable as a sacrificial reagent than lactic acid. The rate of hydrogen evolution increased with increasing methanol concentration up to 25% in DI water and decreased at higher concentrations. Overall, Au@TiO₂ core–shell-based nanocomposites can be used as an improved photocatalyst in photocatalytic hydrogen production.     

Recent advances in non-metals-doped TiO2 nanostructured photocatalysts for visible-light driven hydrogen production,CO2 reduction and air purification

The generation of hydrogen and oxygen from the photocatalytic water splitting reaction under visible light is a promisingly renewable and clean source for H₂ fuel. The transition metal oxide semiconductors (e.g. TiO₂, WO_3, ZnO, and ZrO₂) are have been widely used as photocatalysts for the hydrogen generation. Because of safety, low cost, chemical inertness, photostability and other characteristics (bandgap, corrosion resistance, thermal and environmental stability), TiO₂ is considered as a most potential catalyst of the semiconductors being investigated and developed. However, the extensive applications of TiO₂ are hampered by its inability to exploit the solar energy of visible region. Other demerits are lesser absorbance under visible light, and recombination of photogenerated electron-hole pairs. In this review, we focus on the all the possible reactions taking place at the catalyst during photo-induced H₂ from water splitting reaction, which is green and promising technology. Various parameter affecting the photocatalytic water splitting reactions are also studied. Predominantly, this review is focussed on bandgap engineering of TiO₂ such as the upward shift of valence band and downward shift of conduction bands by doping process to extend its light absorption property into the visible region. Furthermore, the recent advances in this direction including various new strategies of synthesis, multiple doping, hetero-junction, functionalization, perspective and future opportunities of non-metals-doped TiO₂-based nanostructured photocatalysts for various photocatalytic applications such as efficient hydrogen production, air purification and CO₂ reduction to valuable chemicals have been discussed.     

Visible-light-driven TiO2 catalysts doped with low-concentration nitrogen species

Visible-light-driven nitrogen-doped TiO₂ was synthesized using a novel nitrogen-ion donor of hydrazine hydrate. Low-concentration (0.2 at%) nitrogen species and Ti³⁺ were detected in the TiO₂-based photocatalyst by X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) spectroscopy. The trace amount of Ti–N would contribute to the minor band-gap narrowing of about 0.02 eV. Those nitrogen-containing species, especially the NO₂²⁻ species, form surface states, which make the catalysts possible to degrade 4-chlorophenol (4-CP) under visible irradiation (λ>400 nm). Moreover, Ti³⁺ species induce oxygen vacancy states between the valence and the conduction bands, which would also contribute to the visible response. The photocatalytic activity of the nitrogen-doped TiO₂ catalyst was thought to be the synergistic effect of nitrogen and Ti³⁺ species. The catalysts showed higher photocatalytic activity for degradation of 4-CP than pure TiO₂ under not only visible but also UV irradiation. The visible response and the higher UV activity of the nitrogen-doped TiO₂ make it possible to utilize solar energy efficiently to execute photocatalysis processes.     

The preparation of TiO2–nitrogen doped by calcination of TiO2·xH2O under ammonia atmosphere for visible light photocatalysis

The investigation on incorporating nitrogen group into titanium dioxide in order to obtain powdered visible light-active photocatalysts is presented. The industrial hydrated amorphous titanium dioxide (TiO₂·xH₂O) obtained directly from sulphate technology installation was modified by heat treatment at temperatures of 100–800 °C for 4 h in an ammonia atmosphere. The photocatalysts were characterized by UV–VIS–DR and XRD techniques. The UV–VIS–DR spectra of the modified catalysts exhibited an additional maximum in the VIS region (, ) which may be due to the presence of nitrogen in TiO₂ structure. On the basis of XRD analysis it can be supposed that the presence of nitrogen does not have any influence on the transformation temperature of anatase to rutile. The photocatalytic activity of the modified photocatalysts was determined on the basis of decomposition rate of phenol and azo-dye (Reactive Red 198) under visible light irradiation. The highest rate of phenol degradation was obtained for catalysts calcinated at 700 °C (6.55%), and the highest rate of dye decomposition was found for catalysts calcinated at 500 and 600 °C (ca. 40–45%). The nitrogen doping during calcination under ammonia atmosphere is a very promising way of preparation of photocatalysts which could have a practical application in water treatment system under broader solar light spectrum.     

Band gap reduction of (Mo+N) co-doped TiO2 nanotube arrays with a significant enhancement in visible light photo-conversion: A combination of experimental and theoretical study

The present research aimed to evaluate the effects of co-doping TiO₂ nanotube aligned arrays (TNAs) with molybdenum and nitrogen on photocatalytic activity/performance under visible light irradiation. The surface morphology, electronic and optical properties of the pure and modified TNAs based on experimental characterization and theoretical calculations are reported. Both, pure and doped/modified TNAs were synthesized using a single step/low cost anodization method. Titanium sheets were immersed in ethylene glycol-based electrolytes containing NH₄F and NH₄F + K₂MoO₄ to fabricate highly ordered TNAs and Mo-doped TNAs, respectively. Mo–N-doped TNAs were fabricated by a thermal annealing process of Mo-doped samples in nitrogen environment (N₂-gas flow rate of 400 cc/min) for 2hr at 520 °C. Physical/chemical characteristics, structural and photo-electrochemical/electronic properties of the photo-electrodes were observed using several techniques including, field emission scanning electron microscopy (FESEM), Energy Dispersive X-Ray Spectroscopy (EDX), XRD, X-ray photoelectron spectroscopy (XPS), Raman and UV–Vis spectroscopy. We further used a full potential density functional theory (DFT) method to estimate the morphological and electronic structure of the synthesized photo-anodes and also observed a good agreement between theoretical calculations and characterization results. The characterization techniques confirm that Mo and N atoms have been incorporated into the lattice of anodized TNAs and molybdenum atoms partially substituted titanium atoms in the structure of TNAs. UV–Vis DRS spectroscopy experiments and theoretical results reveal that (Mo + N) co-doping creates a positive synergic effect on the band structure of TNAs which can enhance photo-conversion activity, compared to the single Mo/N-doped TNAs samples. In presence of sacrificial agent/electrolyte (aqueous solution of Na₂S/Na₂SO₃) and visible light irradiation, average photocurrent density of the co-doped TNAs photo-anode is 14 times greater than that of the undoped TNAs.