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

Nitrogen doped titanium dioxide as an aesthetic antimicrobial filler in dental polymers

OBJECTIVE

To develop an aesthetic resin composite using a nitrogen-doped titanium dioxide (NTiO₂) filler that possesses antimicrobial properties against cariogenic bacteria.

METHODS

N-TiO₂ powder was manufactured by calcining commercial TiO₂ with urea. Free radical release from the N-TiO₂ powder under visible light irradiation was analysed using UV-Vis spectrophotometry. The N-TiO₂ powder was incorporated into a dental resin and the photocatalytic activity assessed using a dye under both visible light and dark conditions. Using XTT assay to measure the cellular metabolic activity, the antibacterial properties of the N-TiO₂ /resin composite discs were tested using Streptococcus mutans.

RESULTS

Doping nitrogen of TiO₂ resulted in a band gap shift towards the visible light spectrum, which enabled the powder to release reactive oxygen species when exposed to visible light. When incorporated into a dental resin, the N-TiO₂/resin composite still demonstrated sustained release of reactive oxygen species, maintaining its photocatalytic activity and showing an antibacterial effect towards Streptococcus mutans under visible light conditions.

SIGNIFICANCE

N-TiO₂ filled resin composite shows great promise as a potential aesthetic resin based adhesive for orthodontic bonding.

Comparative visible-light driven selective oxidation to aldehydes of phenylmethanol (benzyl alcohol) and 4-pyridinylmethanol (4-pyridinecarbinol) on N-TiO2 and some commercial TiO2 samples

Visible light (λ > 420 nm) selective photooxidation of phenylmethanol and 4-pyridinylmethanol in CH₃CN to the corresponding aldehydes on N-TiO₂ is compared with homemade undoped TiO₂ (U-TiO₂) and commercial undoped anatase specimens (such as PC105, PC500). Significant differences observed between N-TiO₂ and undoped TiO₂ are neither directly related to the surface area nor to the adsorbed amount of alcohol in the dark by surface area unit. FTIR and EPR spectroscopies are used to study the surface of TiO₂ samples and to deeply understand the phenomena intervening in the visible-light photocatalytic activation of the doped vs the undoped oxides. In particular, it is shown that on N-TiO₂ (and also on undoped PC105) strong Lewis acid sites (LAS) exist. The favorable role of LAS on the photocatalytic activity is illustrated by the higher photooxidation of 4-pyridinylmethanol vs phenylmethanol over N-TiO₂ and PC105 in contrast to the other undoped samples, whose visible light sensitivity originates from a charge transfer between the alcohol and the solid. EPR spectra of N-TiO₂ point out the presence of paramagnetic centers related to nitrogen that disappear when the photocatalyst is irradiated with visible light in the presence of alcohol, which starts its oxidative process. On the basis of presented results, we propose that doping with N introduces new intraband gap states that not only contribute to LAS and adsorption of alcohol but also are directly involved in the photochemical process occurring under visible light irradiation.

Gas-Phase Nitrogen Doping of Monolithic TiO2 Nanoparticle-Based Aerogels for Efficient Visible Light-Driven Photocatalytic H2 Production

The development of visible light-active photocatalysts is essential for increasing the conversion efficiency of solar energy into hydrogen (H₂). Here, we present a facile method for nitrogen doping of monolithic titanium dioxide (TiO₂) nanoparticle-based aerogels to activate them for visible light. Plasma-enhanced chemical vapor deposition at low temperature enables efficient incorporation of nitrogen into preformed TiO₂ aerogels without compromising their advantageous intrinsic characteristics such as large surface area, extensive porosity, and nanoscale properties of the semiconducting building blocks. By balancing the dopant concentration and the defects, the nitridation improves optical absorption and charge separation efficiency. The nitrogen-doped TiO₂ nanoparticle-based aerogels loaded with palladium (Pd) nanoparticles show a significant enhancement in visible light-driven photocatalytic H₂ production (3.1 mmol h^-1 g^-1) with excellent stability over 5 days. With this method, we introduce a powerful tool to tune the properties of nanoparticle-based aerogels after synthesis for a specific application, as exemplified by visible light-driven H₂ production.

High-Efficiency Photocatalytic Degradation of Tannic Acid Using TiO2 Heterojunction Catalysts

Photocatalysts have been extensively used for hydrogen evolution or organic degradation. In this work, two different heterojunction types of composite photocatalysts, 1T-MoS₂@TiO₂ with Schottky heterojunction and 2H-MoS₂@TiO₂ with type-II heterojunction, are synthesized via hydrothermal synthesis. These two composite materials exhibit excellent photocatalytic activity toward the degradation of tannic acid, which is a typical organic in nuclear wastewater. At an optimal loading of 16% 1T-MoS₂, the 1T-MoS₂@TiO₂ shows the highest degradation capacity of 98%, which is 3.2 times higher than that of pure TiO₂. The degradation rate of 16% 1T-MoS₂@TiO₂ is much higher than that of 13% 2H-MoS₂@TiO₂. The enhanced photocatalytic activity might be attributed to the improved charge transfer according to the mechanism investigation, supported by the X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS) analyses. This work provides new opportunities for constructing highly efficient catalysts for nuclear waste disposal.

Facile Vacuum Annealing-Induced Modification of TiO2 with an Enhanced Photocatalytic Performance

In this work, the photocatalytic performance enhancement of hydrothermally prepared TiO₂ was achieved by facile vacuum annealing treatment. Calcination of TiO₂ powder in air (CA-TiO₂) maintained its white color, while gray powder was obtained when the annealing was performed under vacuum (CV-TiO₂). Fourier transform infrared, total organic carbon, X-ray photoelectron spectroscopy, and electron paramagnetic resonance analyses proved that vacuum annealing transformed ethanol adsorbed on the surface of TiO₂ into carbon-related species accompanied by the formation of surface oxygen vacancies (Vo). The residual carbon-related species on the surface of CV-TiO₂ favored its adsorption of organic dyes. Compared with TiO₂ and CA-TiO₂, CV-TiO₂ exhibited an improved charge carrier separation with surface Vo as trapping sites for electrons. Vacuum annealing-induced improvement of crystallinity, enhancement of adsorption capacity, and formation of surface Vo contributed to the excellent photocatalytic activity of CV-TiO₂, which was superior to that of commercial TiO₂ (P25, Degussa). Obviously, vacuum annealing-triggered decomposition of ethanol played an important role in the modification of TiO₂. In the presence of ethanol, vacuum annealing was also suitable for the introduction of Vo into P25. Therefore, the current work offers an easy approach for the modification of TiO₂ to enhance its photocatalytic performance by facile vacuum annealing in the presence of ethanol.

Acesulfame K Photodegradation over Nitrogen-Doped TiO2

Acesulfame K is a zero-calorie alternative to sugar used worldwide. There is contradictory information on the toxicity of the compound, but its accumulation in the aquatic environment is undeniable. In this study, one-pot sol-gel synthesis was used to obtain nitrogen-doped TiO2 photocatalysts. Doping up to 6.29 wt % of nitrogen caused an increase in the surface area of the catalysts (48.55–58.23 m2∙g−1) and a reduction of the pHPZC value (5.72–5.05). Acesulfame K photodegradation was tested at the initial concentration of 20–100 ppm and the catalyst concentration at the level of 1 g∙L−1. Compared to the pure anatase, 4.83–6.29 wt % nitrogen-doped TiO2 showed an effective photodegradation of Acesulfame K. Ninety percent molecule removal was obtained after ~100 min, ~90 min, and ~80 min for initial concentrations of 20 ppm, 50 ppm, and 100 ppm, respectively. The increased activity of the catalysts is due to the modification of the TiO2 lattice structure and probably the limitation of the photogenerated electron/hole charge carrier recombination. It was shown that the electrostatic interactions between Acesulfame K and the catalyst surface play an important role in the photodegradation efficiency.

Tailoring the defects of sub-100 nm multipodal titanium nitride/oxynitride nanotubes for efficient water splitting performance

Deciphering the photocatalytic-defect relationship of photoanodes can pave the way towards the rational design for high-performance solar energy conversion. Herein, we rationally designed uniform and aligned ultrathin sub-100 nm multipodal titanium nitride/oxynitride nanotubes (TiON _x NTs) (x = 2, 4, and 6 h) via the anodic oxidation of Ti-foil in a formamide-based electrolyte followed by annealing under ammonia gas for different durations. XPS, XPS imaging, Auger electron spectra, and positron annihilation spectroscopy disclosed that the high nitridation rate induced the generation of a mixture of Ti-nitride and oxynitride with various vacancy-type defects, including monovacancies, vacancy clusters, and a few voids inside TiO _x NTs. These defects decreased the bandgap energy to 2.4 eV, increased visible-light response, and enhanced the incident photon-to-current collection efficiency (IPCE) and the photocurrent density of TiON _x NTs by nearly 8 times compared with TiO₂NTs, besides a quick carrier diffusion at the nanotube/electrolyte interface. The water-splitting performance of sub-100 nm TiON₆NT multipodal nanotubes was superior to the long compacted TiON _x NTs with different lengths and TiO₂ nanoparticles. Thus, the optimization of the nitridation rate tailors the defect concentration, thereby achieving the highest solar conversion efficiency.

Computational Study of Novel Semiconducting Sc2CT2 (T = F, Cl, Br) MXenes for Visible-Light Photocatalytic Water Splitting

Seeking candidate photocatalysts for photocatalytic water splitting, via visible light, is of great interest and importance. In this study, we have comprehensively explored the crystal structures, electronic properties, and optical absorbance of two-dimensional (2D) Sc₂CT₂ (T = F, Cl, Br) MXenes and their corresponding photocatalytic water splitting, under the visible-light region, by first-principles calculations. Herein, we have proposed that 2D Sc₂CT₂ MXenes can be fabricated from their layered bulk compounds, alternatively to the traditional chemical etching method. Creatively, we proposed Sc₂CT₂ (T = F, Br) as new materials; the band edge alignments of Sc₂CF₂ can be tuned to meet the water redox potentials at pH = 8.0. It is highlighted that Sc₂CF₂ shows outstanding optical spectra harvested under visible-light wavelength regions, and efficient separation of photo-induced electrons and holes in different zones. These present results provide eloquent evidence and open a new door on the photocatalysis applications of such novel semiconducting MXenes.

Photocatalytic Properties of Amorphous N-Doped TiO2 Photocatalyst under Visible Light Irradiation

Amorphous TiO2 doped with N was characterized by its photocatalytic activity under visible light irradiation. The amorphous N-doped TiO2 was prepared by the sol-gel method through heat treatment at a low temperature. The photocatalyst showing activity in visible light despite heat treatment at low temperature can be applied to plastics and has excellent utility. The N-doped TiO2 appeared amorphous when heat-treated at 130 °C. It was converted into an anatase-type N-doped TiO2 when this was calcined at 500 °C. The photocatalyst showed photocatalytic activities in the photocatalytic decomposition of formaldehyde and methylene blue under visible light irradiation. The photocatalyst exhibited a higher rate of hydrogen production than that of TiO2 in photocatalytic decomposition of water under liquid-phase plasma irradiation. The bandgap of the amorphous N-doped TiO2 measured by investigation of optical properties was 2.4 eV. The lower bandgap induced the photocatalytic activities under visible light irradiation.

Multiphoton Upconversion Materials for Photocatalysis and Environmental Remediation

Solar-driven photocatalysis holds great potential for energy conversion, environmental remediation, and sustainable chemistry. However, practical applications of conventional photocatalytic systems have been constrained by their insufficient ability to harvest solar radiation in the infrared spectrum. Lanthanide-doped upconversion materials possess high photostability, tunable absorption, and the ability to convert low-energy infrared radiation into high-energy emission, making them attractive for infrared-driven photocatalysis. This review highlights essential principles for rational design of efficient photocatalysts. Particular emphasis is placed on current state-of-the-arts that offer enhanced upconversion luminescence efficiency. We also summarize recent advances in lanthanide-doped upconversion materials for photocatalysis. We conclude with new challenges and prospects for future developments of infrared-driven photocatalysts.

Defective Dopant-Free TiO2 as an Efficient Visible Light-Active Photocatalyst

Pristine and modified/doped titania are still some of the most widely investigated photocatalysts due to its high activity, stability, abundance and proper redox properties to carry out various reactions. However, modifiers and/or dopants resulting in visible-light activity might be expensive or work as recombination centers under UV irradiation. It seems that defective titania, known as “self-doped” TiO2, might be the best solution since it can be obtained under mild conditions without the addition of expensive materials and methods. This review discusses various methods of defective titania preparation, characterization of defect types, their localization (surface vs. bulk) and their function, as well as proposed mechanisms of photocatalytic reactions in the presence of self-doped titania. Although many kinds of defective titania samples have already been prepared with different colors, color intensities and defect kinds (mainly Ti3+ and oxygen vacancies), it is difficult to conclude which of them are the most recommended as the preparation conditions and activity testing used by authors differ. Furthermore, activity testing under solar radiation and for dyes does not clarify the mechanism since bare titania can also be excited and sensitized, respectively, in these conditions. In many reports, authors have not considered the possible influence of some impurities originated from the synthesis method (e.g., H, Al, Zn, Cl, F) that could co-participate in the overall mechanism of photocatalytic reactions. Moreover, some reports indicate that defective titania, especially black ones, might decrease activity since the defects might work as recombination centers. Despite some unproven/unclear findings and unanswered questions, there are many well-conducted studies confirmed by both experimental and theoretical studies that defective titania might be a promising material for various photocatalytic reactions under both UV and visible-light irradiation. Based on available literature, it could be proposed that optimal defects’ concentration, the preferential role of surface defects, a higher surface-to-bulk ratio of defects in rutile than in anatase, and the beneficial impact of disordered surface are the most important aspects to be considered during the preparation of defective titania.

Particulate photocatalytic reactors with spectrum-splitting function for artificial photosynthesis

We have applied spectrum splitting, which is the most reliable way for highly efficient solar energy utilization, to particulate photocatalytic reactors. We have elucidated that the spectrum splitting is feasible using plural cells/compartments, in which photocatalyst particles of different bandgaps are suspended respectively, arranged optically in series. When the particles are sufficiently small (≤20 nm in diameter), high-energy photons are absorbed in the wide-gap cell/compartment on the solar illumination side while low-energy photons reach the backside narrow-gap cell/compartment with being scarcely diffuse-reflected. We have proposed two concrete configurations of the reactors: wide-gap cell/narrow-gap Z-scheme cell (WG/Z), and wide-gap cell/two-compartment cell of middle-gap and narrow-gap (WG/MG-NG), based on the previous configuration of a two-compartment cell of wide-gap and narrow-gap (WG-NG). We have constructed a new model of the carrier supply process from the semiconductor photocatalysts to the active sites, and calculated the practical upper limits of the carrier supply rates and solar-to-chemical conversion efficiencies. The spectrum-splitting reactors can yield higher efficiencies of artificial photosynthetic H2 and CO production by up to 1.5-1.6 times than the conventional Z-scheme reactors. The newly proposed WG/Z reactor widens the room of the material developments and improves the robustness against solar spectrum variation, and hence would be a promising practical solution, although the efficiency is slightly lower than that for the ideal WG-NG reactor. The WG/MG-NG reactor yields the highest efficiency among the three configurations, with high spectral robustness.

Photocatalytic study of Ni-N-codoped TiO2 nanoparticles under visible light irradiation

In present paper pure and Ni-N-codoped TiO2 nanoparticles have been synthesized via sol gel technique. Crystal phase formation of as synthesized nanoparticles was determined from x-ray diffraction which confirms the existence of anatase phase of TiO2. The average crystalline size was determined from x-ray diffraction and estimated from Transmission Electron Micrographs found to vary from 24.8 nm to 10.2 nm. The morphology was studied by Field Emission Scanning Electron Microscopy and reveals that the synthesized nanoparticles are highly crystalline, spherical and small agglomerated. It is observed that on doping the agglomeration decreases and is due to relative rates of growth process. The band gap energy was calculated from UV–visible absorption spectroscopy and found to be 3.12, 1.81, 1.69 and 1.53 eV respectively. The appearance of emission bands at 453, 470, 483 and 494 nm in Photoluminescence spectra could be arising from defect energy states caused by oxygen vacancies within the forbidden region of TiO2.The structural formation of the synthesized nanoparticles is investigated from Fourier-transform-infrared and Energy dispersive x-ray spectroscopy measurements. Photocatalytic degradation efficiency of as synthesized nanoparticles against two different dyes (Congo red and Methyl orange) was investigated under visible light source of wavelength 420–520 nm and is found to increase with dopant concentration (x). It is observed that the increase in Photocatalytic degradation efficiency of synthesized nanoparticles is attributed to decrease in carrier recombination rate arises from the decrease in band gap energy. On the basis of these observations it is concluded that the increase in Photocatalytic activity is due to increase in surface area arises from the decrease in average crystalline size of the synthesized nanoparticles.

Synthesis and NOx removal performance of anatase S-TiO2/g-CN heterojunction formed from dye wastewater sludge

In this study, sludges generated from Ti-based flocculation of dye wastewater were used to retrieve photoactive titania (S-TiO₂). It was heterojunctioned with graphitic carbon nitride (g-CN) to augment photoactivity under UV/visible light irradiance. Later the as-prepared samples were utilized to remove nitrogen oxides (NO_x) in the atmospheric condition through photocatalysis. Heterojunction between S-TiO₂ and g-CN was prepared through facile calcination (@550 °C) of S-TiO₂ and melamine mix. Advanced sample characterization was carried out and documented extensively. Successful heterojunction was confirmed from the assessment of morphological and optical attributes of the samples. Finally, the prepared samples' level of photoactivity was assessed through photooxidation of NO_x under both UV and visible light irradiance. Enhanced photoactivity was observed in the prepared samples irrespective of the light types. After 1 h of UV/visible light-based photooxidation, the best sample STC4 was found to remove 15.18% and 9.16% of atmospheric NO, respectively. In STC4, the mixing ratio of S-TiO₂, to melamine was maintained as 1:3. Moreover, the optical bandgap of STC4 was found as 2.65 eV, where for S-TiO₂, it was 2.83 eV. Hence, the restrained rate of photogenerated charge recombination and tailored energy bandgap of the as-prepared samples were the primary factors for enhancing photoactivity.

TiO2 Inverse Opals Modified by Ag Nanoparticles: A Synergic Effect of Enhanced Visible-Light Absorption and Efficient Charge Separation for Visible-Light Photocatalysis

This work reports on the synthesis, characterization, and photocatalytic performance of the TiO2 inverse opal nanostructure (IP-TiO2) and the IP-TiO2 modified by Ag nanoparticles (Ag@IP-TiO2). The IP-TiO2 is fabricated using polystyrene spheres as the template and TiCl4 as the precursor, and the Ag@IP-TiO2 is realized by photoreduction method. The morphological, structural, and optical properties of the materials are investigated by scanning electron microscopy, X-ray diffraction, ultraviolet–visible (UV-VIS) absorption spectroscopy, and photoluminescence spectroscopy. Their photocatalytic performances are studied by the degradation of rifampicin antibiotic under the visible-light irradiation generated by an LED lamp. The results demonstrate that the IP-TiO2 is composed of mesopores arranged in the honeycomb structure and strongly absorbs visible light in the wavelength range of 400–500 nm. This facilitates the visible-light catalytic activity of IP-TiO2, which is further enhanced by the surface modification by Ag nanoparticles. Our studies on the UV-VIS absorption and photoluminescent properties of the materials reveal that the presence of Ag nanoparticles not only enhances the visible-light absorption of IP-TiO2, but also reduces the recombination of photogenerated electrons and holes. These two factors create a synergic effect that causes the enhanced photocatalytic performance of Ag@IP-TiO2.

A critical review on bismuth oxyhalide based photocatalysis for pharmaceutical active compounds degradation: Modifications, reactive sites, and challenges

Pharmaceutical active compounds (PhACs), as a kind of widely used pharmaceutical drugs, has attracted much attention. The bismuth oxyhalides (BiOX)-based photocatalysis can remove PhACs efficiently due to its unique layered structure, optical and electronic properties. Nevertheless, the rapid recombination of photogenerated electron-hole pairs, and the inherent instability of structure have limited its practical application. In order to solve these problems, recent modification studies tend to focus on facet control, elemental doping, bismuth-rich strategies, defect engineering and heterojunction. Therefore, the objective of this review is to summarize the recent developments in multiply modified strategies for PhACs degradation. The synthesis methods, photocatalytic properties and the enhancement mechanism are elaborated. Besides, based on theoretical calculation, the reactive sites of typical PhACs attacked by different reactive oxygen species were also proposed. Subsequently, challenges and opportunities in applications are also featured which include factors, viz., dissolution of halogen ions, instability under visible light, applications of real water/wastewater, intermediates and byproducts toxicity analysis of BiOX-based photocatalysis. Finally, the perspectives of BiOX-based photocatalysis for PhACs photodegradation in actual water applications are highlighted.

Efficient integration of plasmonic Ag/AgCl with perovskite-type LaFeO3: Enhanced visible-light photocatalytic activity for removal of harmful algae

A novel plasmonic Ag/AgCl@LaFeO₃ (ALFO) photocatalyst was successfully synthesized by a simple in-situ synthesis method with enhanced photocatalytic activity under visible light for harmful algal blooms (HABs) control. The structure, morphology, chemical states, optical and electrochemical properties of the photocatalyst were systematically investigated using a series of characterization methods. Compared with pure LaFeO₃ and Ag/AgCl, ALFO-20% owned a higher light absorption capacity and lower electron-hole recombined rate. Therefore, ALFO-20% had higher photocatalytic activity with a near 100% removal rate of chlorophyll a within 150 min, whose kinetic constant was 15.36 and 9.61 times faster than those of LaFeO₃ and Ag/AgCl. In addition, the changes of zeta potential, cell membrane permeability, cell morphology, organic matter, total soluble protein, photosynthetic system and antioxidant enzyme system in Microcystis aeruginosa (M. aeruginosa) were studied to explore the mechanism of M. aeruginosa photocatalytic inactivation. The results showed that ALFO-20% could change the permeability and morphology of the algae cell membrane, as well as destroy the photosynthesis system and antioxidant system of M. aeruginosa. What's more, ALFO could further degrade the organic matters flowed out after algae rupture and die, reducing the secondary pollution and avoiding the recurrence of HABs. Finally, the species of reactive oxygen species (ROS) (mainly •O₂^- and •OH) produced by ALFO were determined through quenching experiments, and a possible photocatalytic mechanism was proposed. Overall, ALFO can efficiently remove the harmful algae under the visible light, providing a promising method for controlling HABs.

An Overview of the Recent Progress in Polymeric Carbon Nitride Based Photocatalysis

Recently, polymeric carbon nitride (g-C₃ N₄ ) as a proficient photo-catalyst has been effectively employed in photocatalysis for energy conversion, storage, and pollutants degradation due to its low cost, robustness, and environmentally friendly nature. The critical review summarized the recent development, fundamentals, nanostructures design, advantages, and challenges of g-C₃ N₄ (CN), as potential future photoactive material. The review also discusses the latest information on the improvement of CN-based heterojunctions including Type-II, Z-scheme, metal/CN Schottky junctions, noble metal@CN, graphene@CN, carbon nanotubes (CNTs)@CN, metal-organic frameworks (MOFs)/CN, layered double hydroxides (LDH)/CN heterojunctions and CN-based heterostructures for H₂ production from H₂ O, CO₂ conversion and pollutants degradation in detail. The optical absorption, electronic behavior, charge separation and transfer, and bandgap alignment of CN-based heterojunctions are discussed elaborately. The correlations between CN-based heterostructures and photocatalytic activities are described excessively. Besides, the prospects of CN-based heterostructures for energy production, storage, and pollutants degradation are discussed.

Zn and N Codoped TiO2 Thin Films: Photocatalytic and Bactericidal Activity

We explore a series of Zn and N codoped TiO₂ thin films grown using chemical vapor deposition. Films were prepared with various concentrations of Zn (0.4-2.9 at. % Zn vs Ti), and their impact on superoxide formation, photocatalytic activity, and bactericidal properties were determined. Superoxide (O₂^•-) formation was assessed using a 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-5-[(phenylamino)carbonyl]-2H-tetrazolium sodium salt (XTT) as an indicator, photocatalytic activity was determined from the degradation of stearic acid under UVA light, and bactericidal activity was assessed using a Gram-negative bacterium E. coli under both UVA and fluorescent light (similar to what is found in a clinical environment). The 0.4% Zn,N:TiO₂ thin film demonstrated the highest formal quantum efficiency in degrading stearic acid (3.3 × 10^-5 molecules·photon^-1), while the 1.0% Zn,N:TiO₂ film showed the highest bactericidal activity under both UVA and fluorescent light conditions (>3 log kill). The enhanced efficiency of the films was correlated with increased charge carrier lifetime, supported by transient absorption spectroscopy (TAS) measurements.

Critical review of photocatalytic disinfection of bacteria: from noble metals- and carbon nanomaterials-TiO2 composites to challenges of water characteristics and strategic solutions

This critical review covers ways to improve TiO₂-based photocatalysts, how water characteristics may affect photocatalytic disinfection, and strategies to tackle the challenges arising from water characteristics. Photocatalysis has shown much promise in the disinfection of water/wastewater, because photocatalysis does not produce toxic by-products, and is driven by green solar energy. There are however several drawbacks that are curbing the prevalence of photocatalytic disinfection applications: one, the efficiency of photocatalysts may limit popular utilization; two, the water characteristics may present some challenges to the process. TiO₂-based photocatalysts may be readily improved if composited with noble metals or carbon nanomaterials. Noble metals give TiO₂-based composites a higher affinity for dissolved oxygen, and induce plasmonic and Schottky effects in the TiO₂; carbon nanomaterials with a tunable structure, on the other hand, give the composites an improved charge carrier separation performance. Other than photocatalyst materials, the characteristics of water/wastewater is another crucial factor in the photocatalysis process. Also examined in this review are the crucial impacts that water characteristics have on photocatalysts and their interaction with bacteria. Accordingly, strategies to address the challenge of water characteristics on photocatalytic disinfection are explored: one, to modify the semiconductor conduction band to generate long-lifetime reactive species; two, to improve the interaction between bacteria and photocatalysts.

Photocatalytic Lime Render for Indoor and Outdoor Air Quality Improvement

This article reports a novel photocatalytic lime render for indoor and outdoor air quality improvement that is composed of a lime binder and doped TiO2 (KRONOClean 7000®) nanoparticles. These nanoparticles were distributed throughout the bulk of the finishing render, instead of as a thin coating, thus ensuring the durability of the photocatalytic properties upon superficial damage. The physical properties of these renders were not affected by the addition of nanoparticles except in the case of surface area, which increased significantly. In terms of their photocatalytic activity, these novel lime renders were shown to degrade up to 12% NOx under UV light and up to 11% formaldehyde under visible light.

Structure of the amorphous titania precursor phase of N-doped photocatalysts

Amorphous titania samples prepared by ammonia solution neutralization of titanyl sulphate have been characterized by chemical and thermal analyses, and with reciprocal-space and real-space fitting of wide-angle synchrotron X-ray scattering data. A model that fits both the chemical and structural data comprises small segments of lepidocrocite-type layer that are offset by corner-sharing as in the monoclinic titanic acids H2Ti n O2n+1·mH2O. The amorphous phase composition that best fits the combined chemical and scattering data is [(NH4)3H21Ti20O52]·14H2O, where the formula within the brackets is the cluster composition and the H2O outside the brackets is physically adsorbed. The NH4 + cations are an integral part of the clusters and are bonded to layer anions at the corners of the offset layers, as occurs in the alkali metal stepped-layer titanates. The stepped-layer model is shown to give a consistent mechanism for the reaction of aqueous ammonia with solid hydrated titanyl sulphate, in which the amorphous product retains the exact size and shape of the reacting titanyl sulphate crystals.

Coproduction of hydrogen and lactic acid from glucose photocatalysis on band-engineered Zn1-xCdxS homojunction

Photocatalytic transformation of biomass into value-added chemicals coupled with co-production of hydrogen provides an explicit route to trap sunlight into the chemical bonds. Here, we demonstrate a rational design of Zn_1-xCd_xS solid solution homojunction photocatalyst with a pseudo-periodic cubic zinc blende (ZB) and hexagonal wurtzite (WZ) structure for efficient glucose conversion to simultaneously produce hydrogen and lactic acid. The optimized Zn_0.6Cd_0.4S catalyst consists of a twinning superlattice, has a tuned bandgap, and displays excellent efficiency with respect to hydrogen generation (690 ± 27.6 μmol·h⁻¹·g_cat.⁻¹), glucose conversion (~90%), and lactic acid selectivity (~87%) without any co-catalyst under visible light irradiation. The periodic WZ/ZB phase in twinning superlattice facilitates better charge separation, while superoxide radical (⋅O₂^-) and photogenerated holes drive the glucose transformation and water oxidation reactions, respectively. This work demonstrates that rational photocatalyst design could realize an efficient and concomitant production of hydrogen and value-added chemicals from glucose photocatalysis.

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Zn_1-xCd_xS ZB-WZ homojunction was designed to improve charge separation efficiency

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Bandgap engineering improved the hydrogen production from glucose photoreforming

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Optimized Zn_0.6Cd_0.4S ZB-WZ exhibited high lactic acid yield and selectivity

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Rational photocatalyst design realizes biomass valorization and H₂ coproduction

Effect of the Microstructure of the Semiconductor Support on the Photocatalytic Performance of the Pt-PtOx/TiO2 Catalyst System

The influence of the semiconductor microstructure on the photocatalytic behavior of Pt-PtOx/TiO2 catalysts was studied by comparing the methanol-reforming performance of systems based on commercial P25 or TiO2 from sol-gel synthesis calcined at different temperatures. The Pt co-catalyst was deposited by incipient wetness and formed either by calcination or high-temperature H2 treatment. Structural features of the photocatalysts were established by X-ray powder diffraction (XRD), electron spin resonance (ESR), X-ray photoelectron spectroscopy (XPS), optical absorption, Raman spectroscopy and TEM measurements. In situ reduction of Pt during the photocatalytic reaction was generally observed. The P25-based samples showed the best H2 production, while the activity of all sol-gel-based samples was similar in spite of the varying microstructures resulting from the different preparation conditions. Accordingly, the sol-gel-based TiO2 has a fundamental structural feature interfering with its photocatalytic performance, which could not be improved by annealing in the 400-500 °C range even by scarifying specific surface area at higher temperatures.

The Cerium/Boron Insertion Impact in Anatase Nano-Structures on the Photo-Electrochemical and Photocatalytic Response

Boron- and cerium-doped titania (Anatase) were prepared via sol-gel method. Phase composition and morphology were assessed by X-ray diffraction (XRD), scanning electronic microscopy (SEM), BET, diffuse reflectance spectra (DRS), and XPS. Photo-electrochemistry of these materials, deposited onto fluorine-doped SnO2 (FTO), was investigated in acid and acid-containing methanol. The boron-doped sample showed the best opto-electronic properties among the investigated samples. On the other hand, the cerium-doped titania samples annihilate to a certain extent the titania surface states, however, photogenerated charge separation was limited, and certainly associated to surface Ce3+/Ce4+ species. The substitutional effect of boron ions for O sites and interstitial sites was confirmed by XRD and XPS analyses.

Green photocatalytic disinfection of real sewage: efficiency evaluation and toxicity assessment of eco-friendly TiO2-based magnetic photocatalyst under solar light

To evaluate the green photocatalytic disinfection for practical applications, disinfection of different types of real sewage using magnetic photocatalyst RGO/Fe,N-TiO₂/Fe₃O₄@SiO₂ (RGOFeNTFS) under simulated solar light was investigated: low-salinity sewage after tertiary treatment, low-salinity sewage after secondary biological treatment, high-salinity sewage after secondary biological treatment, and high-salinity sewage after chemically enhanced primary treatment. The classification of the sewage as high and low-salinity is based on the regions of sewage source that use seawater and freshwater for toilet flushing, respectively. It shows potential of solar-light-driven photocatalytic disinfection in low-salinity sewage: around 20 min (for sewage after tertiary treatment) and 45 min (for sewage after secondary treatment) of photocatalytic disinfection are required for sewage to meet the discharge standard, and no bacterial regrowth is observed in the treated sewage after 48 h. However, due to the poorer water quality, the high-salinity sewage requires a relatively long reaction time (more than 240 min) to meet the discharge standard, showing minimal practical significance. Further, the complex characteristics of real sewage, such as organic matter, suspended matter, multivalent-ions, pH and DO level significantly influence photocatalytic disinfection, and should be carefully reviewed in evaluating the photocatalytic disinfection of sewage. Besides, RGOFeNTFS shows a good reusability over three cycles for photocatalytic disinfection of low-salinity sewage samples. Moreover, the non-toxicity, indicated by phytoplankton in seawater, of both RGOFeNTFS (<= 3 g/L) and treated low-salinity sewage demonstrates the feasibility of the practical application of photocatalytic disinfection using RGOFeNTFS under irradiation of solar light.

Pt(II)-Decorated Covalent Organic Framework for Photocatalytic Difluoroalkylation and Oxidative Cyclization Reactions

A new covalent organic framework (COF) based on imine bonds was assembled from 2-(4-formylphenyl)-5-formylpyridine and 1,3,6,8-tetrakis(4-aminophenyl)pyrene, which showed an interesting dual-pore structure with high crystallinity. Postmetallation of the COF with Pt occurred selectively at the N donor (imine and pyridyl) in the larger pores. The metallated COF served as an excellent recyclable heterogeneous photocatalyst for decarboxylative difluoroalkylation and oxidative cyclization reactions.

Visible Light Driven Photocatalytic Decolorization and Disinfection of Water Employing Reduced TiO2 Nanopowders

Defect-engineering of TiO2 can have a major impact on its photocatalytic properties for the degradation of persisting and non-biodegradable pollutants. Herein, a series of intrinsic and extrinsic defects are induced by post annealing of crystalline TiO2 under different reducing atmospheres. A detailed optoelectronic characterization sheds light on the key characteristics of the defect-engineered TiO2 nanopowders that are linked to the photocatalytic performance of the prepared photocatalysts. The photodegradation of a model dye, malachite green, as well as the inactivation of bacterial endospores of the Geobacillus stearothermophilus species were studied in the presence of the developed catalysts under visible light illumination. Our results indicate that a combination of certain defects is necessary for the improvement of the photocatalytic process for water purification and disinfection under visible light.

Blue TiO2 nanotube arrays as semimetallic materials with enhanced photoelectrochemical activity towards water splitting

In the past years there has been a great interest in self-doped TiO₂ nanotubes (blue TiO₂ nanotubes) compared to undoped ones owing to their high carrier density and conductivity. In this study, blue TiO₂ nanotubes are investigated as photoanode materials for photoelectrochemical water splitting. Blue TiO₂ nanotubes were fabricated with enhanced photoresponse behavior through electrochemical cathodic polarization on undoped and annealed TiO₂ nanotubes. The annealing temperature of undoped TiO₂ nanotubes was tuned before cathodic polarization, revealing that annealing at 500 °C improved the photoresponse of the nanotubes significantly. Further optimization of the blue TiO₂ nanotubes was achieved by adjusting the cathodic polarization parameters. Blue TiO₂ nanotubes obtained at the potential of –1.4 V (vs. SCE) with a duration of 10 min exhibited twice more photocurrent response (0.39 mA cm^-2) compared to the undoped TiO₂ nanotube arrays (0.19 mA cm^-2). Oxygen vacancies formed through the cathodic polarization decreased charge recombination and enhanced charge transfer rate; therefore, a high photoelectrochemical activity under visible light irradiation could be achieved.

C-,N- and S-Doped TiO2 Photocatalysts: A Review

This article presents an overview of the reports on the doping of TiO2 with carbon, nitrogen, and sulfur, including single, co-, and tri-doping. A comparison of the properties of the photocatalysts synthesized from various precursors of TiO2 and C, N, or S dopants is summarized. Selected methods of synthesis of the non-metal doped TiO2 are also described. Furthermore, the influence of the preparation conditions on the doping mode (interstitial or substitutional) with reference to various types of the modified TiO2 is summarized. The mechanisms of photocatalysis for the different modes of the non-metal doping are also discussed. Moreover, selected applications of the non-metal doped TiO2 photocatalysts are shown, including the removal of organic compounds from water/wastewater, air purification, production of hydrogen, lithium storage, inactivation of bacteria, or carbon dioxide reduction.

A TiO2 Coated Carbon Aerogel Derived from Bamboo Pulp Fibers for Enhanced Visible Light Photo-Catalytic Degradation of Methylene Blue

Carbon aerogels (CA) derived from bamboo cellulose fibers were coupled with TiO2 to form CA/TiO2 hybrids, which exhibited extraordinary performance on the photo-catalytic degradation of methylene blue (MB). The structure and morphology of CA/TiO2 were characterized by field emission scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and Raman spectrum. The CA displayed a highly porous and interconnected three-dimensional framework structure, while introducing the catalytic active sites of TiO2 onto the aerogel scaffold could remarkably enhance its photo-catalytic activity. The adsorption and photo-catalytic degradation of MB by the CA/TiO2 hybrid were investigated. The maximum adsorption capacity of CA/TiO2 for MB was 18.5 mg/g, which outperformed many similar materials reported in the literature. In addition, compared with other photo-catalysts, the present CA/TiO2 demonstrated superior photo-catalytic performance. Almost 85% of MB in 50 mL solution with a MB concentration of 10 mg/L could be effectively degraded by 15 mg CA/TiO2 in 300 min.

Optimization of N doping in TiO2 nanotubes for the enhanced solar light mediated photocatalytic H2 production and dye degradation

Herein, we report the optimization of nitrogen (N) doping in TiO₂ nanotubes to achieve the enhanced photocatalytic efficiencies in degradation of dye and H₂ gas evolution under solar light exposure. TiO₂ nanotubes have been produced via hydrothermal process and N doping has been tuned by varying the concentration of urea, being the source for N, by solid-state dispersion process. The structural analysis using XRD showed the characteristic occupancy of N into the structure of TiO₂ and the XPS studies showed the existence of Ti-N-Ti network in the N-doped TiO₂ nanotubes. The obtained TEM images showed the formation of 1D tube-like structure of TiO₂. Diffuse reflectance UV-Vis absorption spectra demonstrated that the N-doped TiO₂ nanotubes can efficiently absorb the photons of UV-Vis light of the solar light. The optimized N-doped TiO₂ nanotubes (TiO₂ nanotubes vs urea @ 1:1 ratio) showed the highest degradation efficiency over methyl orange dye (∼91% in 90 min) and showed the highest rate of H₂ evolution (∼19,848 μmol h^-1.g^-1) under solar light irradiation. Further, the recyclability studies indicated the excellent stability of the photocatalyst for the durable use in both the photocatalytic processes. The observed efficiency was ascribed to the optimized doping of N-atoms into the lattices of TiO₂, which enhanced the optical properties by forming new energy levels of N atoms near the valence band maximum of TiO₂, thereby increased the overall charge separation and recombination resistance in the system. The improved reusability of photocatalyst is attributed to the doping-induced structural stability in N-doped TiO₂. From the observed results, it has been recognized that the established strategy could be promising for synthesizing N-doped TiO₂ nanotubes with favorable structural, optical and photocatalytic properties towards dye degradation and hydrogen production applications.

Photocatalytic materials and light-driven continuous processes to remove emerging pharmaceutical pollutants from water and selectively close the carbon cycle

Past and present technologies for wastewater purification and future research directions are critically discussed in this review.

Z-scheme TiO2@Ti3C2/Cd0.5Zn0.5S nanocomposites with efficient photocatalytic performance via one-step hydrothermal route

Photocatalytic degradation of pollutants has been proved to be an effective strategy for wastewater treatment. Herein, TiO₂ nanoparticles were synthesized on a Ti₃C₂ matrix by in situ growth, forming Z-scheme TiO₂@Ti₃C₂/Cd_0.5Zn_0.5S (TO/CZS) multilevel structured nanocomposites via one-step hydrothermal route. The effects of hydrothermal temperature and Cd_0.5Zn_0.5S content on microstructure and properties of composites were assessed. TO/CZS nanocomposites were probed into phase composition, morphological and optical properties with x-ray diffractometer, infrared radiation, scanning electron microscope and UV-vis reflective spectra. Following the hydrothermal reaction at 160 °C for 12 h, TiO₂ nanoparticles of 30 nm in diameter were generated in situ on Ti₃C₂ lamina and Cd_0.5Zn_0.5S particles were evenly distributed on the Ti₃C₂ matrix. The photocatalytic activity of TO/CZS composites were evaluated, which found that degradation rate constant (k = 0.028 min^-1) of TO/CZS-40 on Rhodamine B was 5.19 times that of pure TiO₂ and 4.48 times that of Cd_0.5Zn_0.5S. Through anchoring Ti₃C₂ as an electron transition mediator and combination with TiO₂ and Cd_0.5Zn_0.5S, the new Z-scheme between TiO₂ oxidized by Ti₃C₂ and Cd_0.5Zn_0.5S establishes a multilevel structure of separating electron-hole pairs. This work demonstrates a valid way to control electrons and hole transfer directions efficiently through designing multilevel semiconductor structural designs.

Pyrochlore oxides as visible light-responsive photocatalysts

This perspective describes the use of pyrochlore oxides in photocatalysis with focus on the strategies to enhance their activity.