Nitrogen-Doped Titanium Dioxide as Visible-Light-Sensitive Photocatalyst: Designs, Developments, and Prospects

Synthesis, properties, and applications of black titanium dioxide nanomaterials

Photocatalysis has been regarded as one of best solutions to using the sunlight to produce hydrogen from water and to removing organic pollutants from the environment, and titanium dioxide (TiO₂) nanomaterials have been treated as the primary photocatalyst for these purposes. However, their large band gap has largely limited the activity to the UV region of the solar spectrum. The discovery of black TiO₂ in 2011 has triggered world-wide research interests with new hope to overcome this problem. This review briefly summarizes the recent progresses of black TiO₂ nanomaterials, including their synthesis, properties and applications, to provide a timely update and to inspire more ideas in the related research.

Semiconductor Surface Chemistry as Holy Grail in Photocatalysis and Photovoltaics

The trail of semiconductor surface photochemistry during the past four decades has led to the emergence of new areas in chemistry (e.g., photocatalysis, solar cells, solar fuels). How can one now exploit the richness of surface chemistry of hybrid architectures and make a transformative leap in light energy conversion and other applications?

Modification of N-doped TiO2 photocatalysts using noble metals (Pt, Pd) - a combined XPS and DFT study

Nitrogen-doped TiO₂ (N-TiO₂) is considered as one of the most promising materials for various photocatalytic applications, while noble metals Pd and Pt are known as good catalysts for hydrogen evolution. This work focuses on the determination of structural and electronic modifications of N-TiO₂, achieved by noble metal deposition at the surface, as a starting indicator for potential applications. We focus on the properties of easily synthesized nanocrystalline nitrogen-doped anatase TiO₂, modified by depositing small amounts of Pd (0.05 wt%) and Pt (0.10 wt%), aiming to demonstrate efficient enhancement of optical properties. The chemical states of dopants are studied in detail, using X-ray photoemission spectroscopy, to address the potential of N-TiO₂ to act as a support for metallic nanoparticles. DFT calculations are used to resolve substitutional from interstitial nitrogen doping of anatase TiO₂, as well as to study the combined effect of nitrogen doping and oxygen vacancy formation. Based on the binding energies calculated using Slater's transition state theory, dominant contribution to the N 1s binding energy at 399.8 eV is ascribed to interstitially doped nitrogen in anatase TiO₂. Given that both structure and photocatalytic properties depend greatly on the synthesis procedure, this work contributes further to establishing correlation between the structure and optical properties of the noble metal modified N-TiO₂ system.

A Metal-Organic Framework Approach toward Highly Nitrogen-Doped Graphitic Carbon as a Metal-Free Photocatalyst for Hydrogen Evolution

Zeolitic imidazolate framework-8 (ZIF-8)-derived N-doped graphene analogous polyhedrons (ZNGs) obtained via the direct carbonation of ZIF-8 are applied to photocatalytic hydrogen evolution for the first time. The contents of different types of nitrogen atoms in ZNGs can be fine-tuned via the calcination temperature, which significantly influences the hydrogen evolution rate of the ZNGs.

Carrier Step-by-Step Transport Initiated by Precise Defect Distribution Engineering for Efficient Photocatalytic Hydrogen Generation

Semiconductor photocatalysts have been widely used for solar-to-hydrogen conversion; however, efficient photocatalytic hydrogen generation still remains a challenge. To improve the photocatalytic activity, the critical step is the transport of photogenerated carriers from bulk to surface. Here, we report the carrier step-by-step transport (CST) for semiconductor photocatalysts through precise defect engineering. In CST, carriers can fast transport from bulk to shallow traps in the defective subsurface first, and then transfer to the surface active acceptors. The key challenge of initiating CST lies in fine controlling defect distribution in semiconductor photocatalysts to introduce the special band matching between the crystalline bulk and defect-controllable surface, moderate bridgelike shallow traps induced by subsurface defects, and abundant surface active sites induced by surface defects. In our proof-of-concept demonstration, the CST was introduced into typical semiconductor TiO₂ assisted by the fluorine-assisted kinetic hydrolysis method, and the designed TiO₂ can exhibit the state-of-the-art photocatalytic hydrogen generation rate among anatase TiO₂ up to 13.21 mmol h^-1 g^-1, which is 120 times enhanced compared with crystalline anatase TiO₂ under sunlight. The CST initiated by precise defect distribution engineering provides a new sight on greatly improving photocatalytic hydrogen generation performance of semiconductor catalysts.

Noble-Metal-Free Photocatalytic Hydrogen Evolution Activity: The Impact of Ball Milling Anatase Nanopowders with TiH2

Ball milling TiO₂ anatase together with TiH₂ can create an effective photocatalyst. The process changes the lattice and electronic structure of anatase. Lattice deformation created by mechanical impact combined with hydride incorporation yield electronic gap-states close to the conduction band of anatase. These provide longer lifetimes of photogenerated charge carriers and lead to an intrinsic cocatalytic activation of anatase for H₂ evolution.

Photocatalytic behaviour of WO3/TiO2-N for diclofenac degradation using simulated solar radiation as an activation source

In this study, the photocatalytic removal of an emerging contaminant, diclofenac (DCF) sodium, was performed using the nitrogen-doped WO₃/TiO₂-coupled oxide catalyst (WO₃/TiO₂-N). The catalyst synthesis was accomplished by a sol-gel method using tetrabutyl orthotitanate (C₁₆H₃₆O₄Ti), ammonium p-tungstate [(NH₄)₁₀H₂W₁₂O₄₂·4H₂O] and ammonium nitrate (NH₄NO₃) as the nitrogen source. For comparison, TiO₂ and WO₃/TiO₂ were also prepared under similar conditions. Analysis by X-ray diffraction (XRD), N₂ adsorption-desorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), diffuse reflectance UV-Vis spectroscopy (DRS) and X-ray photoelectron spectroscopy (XPS) were conducted to characterize the synthesized materials. The photocatalytic efficiency of the semiconductors was determined in a batch reactor irradiated with simulated solar light. Residual and mineralized DCF were quantified by high-performance liquid chromatography, total organic carbon analysis and ion exchange chromatography. The results indicated that the tungsten atoms were dispersed on the surface of TiO₂ as WO₃. The partial substitution of oxygen by nitrogen atoms into the lattice of TiO₂ was an important factor to improve the photocatalytic efficiency of WO₃/TiO₂. Therefore, the best photocatalytic activity was obtained with the WO₃/TiO₂-N_0.18 catalyst, reaching 100% DCF transformation at 250 kJ m^-2 and complete mineralization at 400 kJ m^-2 of solar-accumulated energy.

Titanium dioxide nanotube membranes for solar energy conversion: effect of deep and shallow dopants

Here we show the effect of shallow and deep dopants on titanium dioxide (TiO₂) nanotube membranes, for applications in photocatalytic, photoelectrochemical, photovoltaic, and other photosensitized devices for converting light into chemical feedstocks or electricity.

Single- and few-layer BiOI as promising photocatalysts for solar water splitting

Theoretical studies suggest that BiOI nanosheets can be efficient photocatalysts for solar water splitting.

Piezotronic-effect-enhanced Ag2S/ZnO photocatalyst for organic dye degradation

A promising strategy for enhancing the photocatalytic performance of Ag₂S@ZnO composite on common carbon fiber cloth using strain-induced piezopotentials/piezocharges.

C–N–S tridoping into TiO2matrix for photocatalytic applications: observations, speculations and contradictions in the codoping process

C–N–S-tridoped TiO₂: behind and beyond the codoping process.

The effects of transition-metal doping and chromophore anchoring on the photocurrent response of titanium-oxo-clusters

Titanium oxo-clusters with both doped metals and anchored chromophores were synthesized and characterized. The photocurrent densities of the clusters were improved by redox active metals and charge transfer chromophores.

A biomass carbon mass coated with modified TiO2 nanotube/graphene for photocatalysis

A floating mass constructed with fungus and Fe/N-TNTs/NG for the photocatalysis on the surface of solution.

Ultrathin TiO2nanosheets synthesized using a high pressure solvothermal method and the enhanced photoresponse performance of CH3NH3PbI3–TiO2composite films

Highly crystalline ultrathin (2–3 nm) TiO₂nanosheets are synthesized using a high pressure solvothermal method. The perovskite–TiO₂films exhibit strikingly enhanced photoresponse performance.

Eco-friendly one-pot synthesis of well-adorned mesoporous g-C3N4 with efficiently enhanced visible light photocatalytic activity

We report a facile and eco-friendly one-pot synthesis of well-adorned mesoporous g-C₃N₄ material through a bubble templating strategy and involving controlling the surface area from 17 to 195 m² g⁻¹ by simply adjusting the mass ratio of melamine/NH₄Cl.

Synthesis and characterization of Z-scheme In2S3/Ag2CrO4 composites with an enhanced visible-light photocatalytic performance

Efficient charge transfer at the interfaces of an In₂S₃/Ag₂CrO₄ composite, due to the formation of a Z-scheme system between In₂S₃ and Ag₂CrO₄, effectively facilitates photogenerated electron–hole pair separation.

Computational design of enhanced photocatalytic activity of two-dimensional cadmium iodide

The recent synthesis of two-dimensional cadmium iodide (CdI₂) opens up the questions of its properties and potential applications in optoelectronic and photovoltaic devices.

Green sol–gel route for selective growth of 1D rutile N–TiO2: a highly active photocatalyst for H2 generation and environmental remediation under natural sunlight

We report selective growth of N–TiO₂ 1D nanorods using a green aqueous sol–gel method followed by hydrothermal treatment.

Enhanced photoelectrochemical performance of anatase TiO2 for water splitting via surface codoping

We explore the (Rh + F) surface codoping effect on anatase TiO₂ (101) and (001) facets for solar water splitting by performing extensive density functional theory calculations.

Polydopamine nanotubes-templated synthesis of TiO2 and its photocatalytic performance under visible light

As templates for generation of TiO₂ photocatalysts, polydopamine nanotubes can provide codopants (carbon and nitrogen) and graphene-like carbon coverings simultaneously.

Tuning optical band gap by electrochemical reduction in TiO2 nanorods for improving photocatalytic activities

Tuning of the optical band gap of TiO₂ nanorods (TiO₂ NRs) was investigated by electrochemical methods for improving their photocatalytic activities.

Flame-made ultra-porous TiO2 layers for perovskite solar cells

We report methyl ammonium lead iodide (MAPbI₃) solar cells with an ultra-porous TiO₂ electron transport layer fabricated using sequential flame aerosol and atomic layer depositions of porous and compact TiO₂ layers. Flame aerosol pyrolysis allows rapid deposition of nanostructured and ultra-porous TiO₂ layers that could be easily scaled-up for high-throughput low-cost industrial solar cell production. An efficiency of 13.7% was achieved with a flame-made nanostructured and ultra-porous TiO₂ electrode that was coated with a compact 2 nm TiO₂ layer. This demonstrates that MAPbI₃ solar cells with a flame-made porous TiO₂ layer can have a comparable efficiency to that of the control MAPbI₃ solar cell with the well-established spin-coated porous TiO₂ layer. The combination of flame aerosol and atomic layer deposition provides precise control of the TiO₂ porosity. Notably, the porosity of the as-deposited flame-made TiO₂ layers was 97% which was then fine-tuned down to 87%, 56% and 35% by varying the thickness of the subsequent compact TiO₂ coating step. The effects of the decrease in porosity on the device performance are discussed. It is also shown that MAPbI₃ easily infiltrates into the flame-made porous TiO₂ nanostructure thanks to their high porosity and large pore size.

Biocompatibility and antibacterial activity of nitrogen-doped titanium dioxide nanoparticles for use in dental resin formulations

The addition of antibacterial functionality to dental resins presents an opportunity to extend their useful lifetime by reducing secondary caries caused by bacterial recolonization. In this study, the potential efficacy of nitrogen-doped titanium dioxide nanoparticles for this purpose was determined. Nitrogen doping was carried out to extend the ultraviolet absorbance into longer wavelength blue light for increased biocompatibility. Titanium dioxide nanoparticles (approximately 20-30 nm) were synthesized with and without nitrogen doping using a sol-gel method. Ultraviolet-Visible spectroscopy indicated a band of trap states, with increasing blue light absorbance as the concentration of the nitrogen dopant increased. Electron paramagnetic resonance measurements indicated the formation of superoxide and hydroxyl radicals upon particle exposure to visible light and oxygen. The particles were significantly toxic to Escherichia coli in a dose-dependent manner after a 1-hour exposure to a blue light source (480 nm). Intracellular reactive oxygen species assay demonstrated that the particles caused a stress response in human gingival epithelial cells when exposed to 1 hour of blue light, though this did not result in detectable release of cytokines. No decrease in cell viability was observed by water-soluble tetrazolium dye assay. The results show that nitrogen-doped titanium dioxide nanoparticles have antibacterial activity when exposed to blue light, and are biocompatible at these concentrations.

Effect of N and F content on structural, optical and photocatalytic methylene blue degradation properties of TiO2

Photocatalytic systems based on fluorine- and nitrogen-doped titanium dioxide were obtained via the sol–gel method. Titanium tetraisopropoxide and ammonium fluoride were used as precursors. Transmission electron microscopy and measurement of Brunauer–Emmett–Teller surface area and porosity were used to establish that the concentration of ammonium fluoride has an impact on the structural parameters of titanium dioxide produced in the sol–gel process. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy revealed that the dopants (F and N) are not incorporated into the crystal lattice of titanium dioxide, but affect the optical properties of titanium dioxide due to the emergence of additional bands in the absorption spectra in the range of 380–450 nm. Sorption and photoactivity were found using methylene blue photodecomposition under visible light. It was found that dopant introduction increases both sorption ability and photoactivity of the materials.

Preparation, characterization, and photocatalytic activity evaluation of Fe-N-codoped TiO2/fly ash cenospheres floating photocatalyst

Nitrogen-doped titanium dioxide (TiO₂) and Fe-N-codoped TiO₂ layers on fly ash cenospheres (FAC) as floating photocatalyst were successfully prepared through sol-gel method. Photocatalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), ultraviolet (UV)-Vis diffuse reflectance spectroscopy (DRS), and nitrogen adsorption analyses for Brunauer-Emmett-Teller (BET) specific surface area. Photocatalytic efficiency of the prepared catalyst was evaluated through using the decomposition of Rhodamine B (RhB) as a model compound under visible light irradiation. Photocatalytic activity and kinetics of catalyst under visible light were detected in details from different Fe/Ti mole ratios by detecting photodegradation of RhB. Experimental results show that when the calcination temperature was 550 °C, the dosage of FAC was 3.0 g, and the mole ratio of Fe/Ti was 0.71 %; the synthesized Fe-N-TiO₂/FAC photocatalyst presented as anatase phase and that N and Fe ions were doped into TiO₂ lattice. The material's specific surface area was 34.027 m²/g, and UV-Vis diffuse reflectance spectroscopy shows that the edge of the photon absorption has been red shifted up to 400-500 nm. Fe-N-codoped titanium dioxide on FAC had excellent photocatalytic activity during the process of photodegradation of RhB under visible light irradiation.

Standalone anion- and co-doped titanium dioxide nanotubes for photocatalytic and photoelectrochemical solar-to-fuel conversion

Several strategies are currently being investigated for conversion of incident sunlight into renewable sources of energy, and photocatalytic or photoelectrochemical production of solar fuels can provide an important alternative. Titanium dioxide (TiO₂) has been heavily investigated as a material of choice due to its excellent optoelectronic properties and stability, and anion-doping proposed as a pathway to improve light absorption as well as improving the efficiency of oxygen production. While several studies have used morphological tuning, elemental doping, and surface engineering in TiO₂ to extend its absorption, there is a need to optimize simultaneously charge transport and improve interfacial chemical reaction kinetics. Here we show anion-doped (nitrogen, carbon) standalone TiO₂ nanotube membranes that absorb visible light for the water-splitting reaction, using both wireless (photocatalysis) and wired (photoelectrochemical) solar-to-fuel conversion (STFC) cells. Using simulated solar radiation, we show generation of hydrogen as a solar fuel using visible light photocatalysis. Furthermore, using a model we elucidate detailed photophysics and photoelectrochemical properties of these nanotubes, and explain the kinetics of photogenerated charge carriers following light absorption. We show that while visible light induces a superlinear photoresponse for catalytic reduction and may benefit from higher incident light intensity, ultraviolet light shows a linear photoresponse and saturation with higher light flux due to trapping of photogenerated charges (mainly electrons). These results can have important implications for design of other metal-oxide membranes for solar fuel generation, and appropriate design of dopants and induced energy levels in these photocatalysts.

Magnetic properties of nitrogen-doped ZrO2: Theoretical evidence of absence of room temperature ferromagnetism

N-dopants in bulk monoclinic ZrO2 and their magnetic interactions have been investigated by DFT calculations, using the B3LYP hybrid functional. The electronic and magnetic properties of the paramagnetic N species, substitutionals and interstitials, are discussed. Their thermodynamic stability has been estimated as a function of the oxygen partial pressure. At 300 K, N prefers interstitial sites at any range of oxygen pressure, while at higher temperatures (700-1000 K), oxygen poor-conditions facilitate substitutional dopants. We have considered the interaction of two N defects in various positions in order to investigate the possible occurrence of ferromagnetic ordering. A very small magnetic coupling constant has been calculated for several 2N-ZrO2 configurations, thus demonstrating that magnetic ordering can be achieved only at very low temperatures, well below liquid nitrogen. Furthermore, when N atoms replace O at different sites, resulting in slightly different positions of the corresponding N 2p levels, a direct charge transfer can occur between the two dopants with consequent quenching of the magnetic moment. Another mechanism that contributes to the quenching of the N magnetic moments is the interplay with oxygen vacancies. These effects contribute to reduce the concentration of magnetic impurities, thus limiting the possibility to establish magnetic ordering.

In situ loading of Ag(2)WO(4) on ultrathin g-C(3)N(4) nanosheets with highly enhanced photocatalytic performance

The g-C3N4 nanosheets (g-C3N4NS) exhibit more excellent property than common bulk g-C3N4 (g-C3N4-B) due to their large surface areas, improved electron transport ability and well dispersion in water. In this work, ultrathin g-C3N4NS with a thickness of about 2.7nm have been synthesized by a simple thermal exfoliation of bulk g-C3N4, and then Ag2WO4 nanoparticles are in situ loaded on their surface to construct the Ag2WO4/g-C3N4NS heterostructured photocatalysts. Due to their unique physicochemical properties, the as-prepared heterostructures possess a fast interfacial charge transfer and increased lifetime of photo-excited charge carriers, and exhibit much higher photocatalytic activity. Under visible light irradiation, the optimum photocatalytic activity of Ag2WO4/g-C3N4NS composites is almost 53.6 and 26.5 times higher than that of pure g-C3N4-B and Ag2WO4/g-C3N4-B heterostructures towards the degradation of rhodamine B, respectively, and is almost 30.6 and 9.8 times higher towards the degradation of methyl orange, respectively. In addition, the natural sunlight photocatalytic activities of the as-prepared samples are also investigated.

Transition-Metal-Free Decarboxylative Photoredox Coupling of Carboxylic Acids and Alcohols with Aromatic Nitriles

A transition-metal-free protocol for the redox-neutral light-induced decarboxylative coupling of carboxylic acids with (hetero)aromatic nitriles at ambient temperature is presented. A broad scope of acids and nitriles is accepted, and alcohols can be coupled in a similar fashion through their oxalate half esters. Various inexpensive sources of UV light and even sunlight can be used to achieve this C-C bond formation proceeding through a free radical mechanism.

Enhanced photocatalytic activity of a self-stabilized synthetic flavin anchored on a TiO2 surface

Synthetic flavin molecules were anchored on Degussa P25 titanium dioxide (TiO2). The effect of their presence on the photocatalytic (PC) activity of TiO2 was studied. Under UV light, an increase in the degradation rate of ethanol was observed. This increase was accompanied by stabilization of the anchored flavin against self-degradation. The unprecedented stabilization effect was found also in the absence of a reducing agent such as ethanol. In contrast, under the less energetic visible light, fast degradation of the anchored flavin was observed. These rather surprising observations were attributed to the propensity for charge transport from excited flavin molecules to the semiconductor and to the role that such charge transfer may play in stabilizing the overall assembly. Anchored flavins excited by UV light to their S2, S3 electronic states were able to transfer the excited electrons to the TiO2 phase whereas anchored flavin molecules that were excited by visible light to the S1 state were less likely to transfer the photo-excited electrons and therefore were destabilized. These findings may be relevant not only to anchored flavins in general but to other functionalized photocatalysts, and may open up new vistas in the implementation of sensitizers in PC systems.

Probing the Effects of Templating on the UV and Visible Light Photocatalytic Activity of Porous Nitrogen-Modified Titania Monoliths for Dye Removal

Porous nitrogen-modified titania (N-titania) monoliths with tailored morphologies were prepared using phase separation and agarose gel templating techniques. The doping and templating process were simultaneously carried out in a one-pot step using alcohol amine-assisted sol-gel chemistry. The amount of polymer used in the monoliths that were prepared using phase separation was shown to affect both the physical and optical properties: higher poly(ethylene glycol) content increased the specific surface area, porosity, and visible light absorption of the final materials. For the agarose-templated monoliths, the infiltration conditions affected the monolith morphology. A porous monolith with high surface area and the least shrinkage was obtained when the N containing alkoxide precursor was infiltrated into the agarose scaffolds at 60 °C. The effect of the diverse porous morphologies on the photocatalytic activity of N-titania was studied for the decomposition of methylene blue (MB) under visible and UV light irradiation. The highest visible light activity was achieved by the agarose-templated N-titania monolith, in part due to higher N incorporation. This sample also showed better UV activity, partly because of the higher specific surface area (up to 112 m(2) g(-1)) compared to the phase separation-induced monoliths (up to 103 m(2) g(-1)). Overall, agarose-templated, porous N-titania monoliths provided better features for effectively removing the MB contaminant.