Conventional TiO2 Photocatalysts Research Articles

Efficient treatment of surfactant containing wastewater by photocatalytic ozonation with BiPO4 nanorods

In this study, monoclinic BiPO4 nanorods were fabricated by one-pot solvothermal method. Its catalytic capability in photocatalytic ozonation process was tested by degradation and mineralization of sodium dodecyl benzene sulfonate (SDBS) solution. The results demonstrated that the TOC removal rate was dramatically improved to 90.0% at 75 min for UV/O3/BiPO4 process, which was 4.9 and 3.8 times more than that of UV/BiPO4 and O3. Moreover, the pseudo-first-order kinetic constant (0.337 min−1) and mineralization rate (90.0%) for SDBS degradation using BiPO4 in UV/O3 process were 1.6 and 1.3 times as great as that of conventional TiO2 photocatalyst (0.206 min−1, 67.3%). The influence of BiPO4 dosage, O3 concentration initial pH and coexisted ions on SDBS degradation in UV/O3/BiPO4 process were also investigated. The outcome of quenching studies illustrated both ·OH and h+ contributed prominently to SDBS degradation in UV/O3/BiPO4 process, implying that high valence band position of BiPO4 could promote the synergism between photocatalysis and ozonation. The degradation pathway of SBDS was proposed by combination of intermediates analysis and DFT calculation. Real carwash wastewater was chosen as typical surfactant containing wastewater to explore the practical application of UV/O3/BiPO4 technology. During 30 min, COD and LAS removal efficiency reached 59.7% and 70.6%, respectively. The quality indices of effluent could meet the requirements for reuse of carwash water in Water Quality Standard for Urban Miscellaneous Use in China. Energy consumption in the process was calculated as 13.9 kW h m−3, which was about 3.6 and 2.2 times less than that of UV/BiPO4 and O3 process, respectively. The results suggest that UV/O3/BiPO4 system has an application potential for surfactant containing wastewater treatment or recycle.

Plant-based synthesis of TiO2 – rGO nanocomposite for the efficient photocatalytic degradation of an emerging contaminant: Experimental and theoretical investigations

The need to develop sustainable and efficient photocatalysts for the degradation of contaminants of emerging concern has gained considerable attention. In this study, plant-based titanium dioxide-reduced graphene oxide (TiO2-rGO) photocatalyst was synthesized by a hydrothermal method using Azadirachta indica (AI) leaf extract. Morphological and chemical properties of the AI-TiO2-rGO photocatalyst were evaluated in detail. The clusters of AI-TiO2 wrapped in AI-rGO were identified with TEM analysis, and from the XRD, the purity of AI-TiO2-rGO was confirmed. The low band gap (2.80 eV) of AI-TiO2-rGO made it a suitable photocatalyst under visible light. The X-ray photoelectron spectroscopy (XPS) results indicated the establishment of strong Ti-O-C chemical linkage within the AI-TiO2-rGO photocatalyst. The efficiency of AI-TiO2-rGO was assessed for the degradation of carbamazepine (CBZ) under UV-A irradiation (94.28%) and visible light (59.97%) in a batch photocatalytic reactor for 60 min. The optimized conditions such as AI-TiO2-rGO dosage (50 mg/L), initial CBZ concentration (4.5 mg/L), and light source (UV-A (60 W)), were validated with response surface methodology (RSM). The mechanism of degradation of CBZ by AI-TiO2-rGO was explained with the identification of intermediate compounds, and three plausible CBZ degradation pathways were proposed. The reusability study suggested that the AI-TiO2-rGO photocatalyst could be effectively used for 8 cycles. The evaluation of toxicity of treated water with E.Coli growth curve (reduction in Iind to 30.12% (UV) and 27.65% (visible)) signified that the treated water was less toxic. The comparison of AI-TiO2-rGO with chemically synthesized TiO2-rGO indicated that even though AI-TiO2-rGO exhibited a slightly higher band gap (2.80 eV), it can be the better alternative for the photocatalysts synthesized by chemical route. The economic analysis of the overall process proved that the application of AI-TiO2-rGO was more cost-effective and sustainable than the conventional TiO2 photocatalyst. Overall, the study provides valuable insight into synthesizing recyclable plant-based photocatalysts to degrade recalcitrant emerging contaminants in the water.

Feasibility of Solar Updraft Towers as Photocatalytic Reactors for Removal of Atmospheric Methane-The Role of Catalysts and Rate Limiting Steps.

Due to the alarming speed of global warming, greenhouse gas removal from atmosphere will be absolutely necessary in the coming decades. Methane is the second most harmful greenhouse gas in the atmosphere. There is an emerging technology proposed to incorporating photocatalysis with solar updraft Towers (SUT) to remove methane from the air at a planetary scale. In this study, we present a deep analysis by calculating the potential of methane removal in relation to the dimensions and configuration of SUT using different photocatalysts. The analysis shows that the methane removal rate increases with the SUT dimensions and can be enhanced by changing the configuration design. More importantly, the low methane removal rate on conventional TiO2 photocatalyst can be significantly improved to, for example, 42.5% on a more effective Ag-doped ZnO photocatalyst in a 200 MW SUT while the photocatalytic reaction is the rate limiting step. The factors that may further affect the removal of methane, such as more efficient photocatalysts, night operation and reaction zone are discussed as possible solutions to further improve the system.

Oxygen vacancy engineering of cerium oxide for the selective photocatalytic oxidation of aromatic pollutants

The engineering of oxygen vacancies in CeO2 nanoparticles (NPs) allows the specific fine-tuning of their oxidation power, and this can be used to rationally control their activity and selectivity in the photocatalytic oxidation (PCO) of aromatic pollutants. In the current study, a facile strategy for generating exceptionally stable oxygen vacancies in CeO2 NPs through simple acid (CeO2-A) or base (CeO2-B) treatment was developed. The selective (or mild) PCO activities of CeO2-A and CeO2-B in the degradation of a variety of aromatic substrates in water were successfully demonstrated. CeO2-B has more oxygen vacancies and exhibits superior photocatalytic performance compared to CeO2-A. Control of oxygen vacancies in CeO2 facilitates the adsorption and reduction of dissolved O2 due to their high oxygen-storage ability. The oxygen vacancies in CeO2-B as active sites for oxygen-mediated reactions act as (i) adsorption and reduction reaction sites for dissolved O2, and (ii) photogenerated electron scavenging sites that promote the formation of H2O2 by multi-electron transfer. The oxygen vacancies in CeO2-B are particularly stable and can be used repeatedly over 30 h without losing activity. The selective PCOs of organic substrates were studied systematically, revealing that the operating mechanisms for UV-illuminated CeO2-B are very different from those for conventional TiO2 photocatalysts. Thus, the present study provides new insights into the design of defect-engineered metal oxides for the development of novel photocatalysts.

Enhanced photocatalytic performance of tungsten-based photocatalysts for degradation of volatile organic compounds: a review

Photocatalytic oxidation process for the degradation of volatile organic compounds (VOCs) contaminants is a promising technology. But until now, the low photocatalytic activity of the conventional TiO2 photocatalyst under visible-light irradiation hinders the deployment of this technique for VOCs degradation. WO3 has been proved to be a suitable photocatalytic material for degradation of various VOCs as its appropriate band-gap, high stability and great capability. Nevertheless, the actual implementation of WO3 is still restricted by short lifetime of photoexcited charge carriers and low light energy conversion efficiency: its photocatalytic performance is needed to be improved. This review discusses the process of tungsten-based photocatalyst for removal of VOCs and summarizes a variety of strategies to improve the VOCs oxidation performances of WO3, such as controlling the morphology structure, engendering chemical defects, coupling heterojunction, doping suitable dopants and loading a co-catalyst. In addition, the practical application of tungsten-based photocatalyst is discussed.

Tandem CdS/TiO2(B) nanosheet photocatalysts for enhanced H2 evolution

A conventional TiO2 photocatalyst cannot respond to visible light due to its large band gap. Here, we describe the construction of tandem CdS/TiO2(B) nanosheet architectures with visible light H2 evolution. A type II heterojunction of hexagonal CdS/TiO2(B) (PCT) was constructed by photodeposition and CdS dots, with size about 6 nm, were anchored on TiO2(B) nanosheets. A second type I heterojunction of cubic CdS/TiO2(B) (HCT) was fabricated by a hydrothermal method and CdS nanoparticles, with size about 50 nm, were dispersed on TiO2(B) nanosheets. Both heterojunction systems display remarkable enhancement of H2 evolution activity under full spectrum irradiation, with highest H2 evolution rate of 1776 μmol·g−1·h−1 for the photodeposited catalyst and 1494 μmol·g−1·h−1 for the hydrothermal process. The different heterojunction systems show very different H2 evolution under just visible light irradiation. The highest H2 evolution (1577 μmol·g−1·h−1) for the type II system is far much higher than the type I (48 μmol·g−1·h−1). The type II system shows more effective utilization of full spectrum with the apparent quantum efficiency in the wavelength range extended from 300 to 500 nm, matching well with the optical absorbance spectrum of the composite catalyst. The structure, morphology, and photocatalytic mechanism of the CdS/TiO2(B) composites are discussed.

Facile fabrication and photocatalytic activity of Ag/AgI/rGO films

The composite material, Ag/AgX/graphene (X=Br, Cl, I), is considered a promising photocatalyst for photocatalytic degradation of organic pollutants. Its photocatalytic activity is superior to that of the conventional TiO2 photocatalyst; the enhanced photocatalytic activity is attributed to its effective charge separation ability and wide visible light absorption. However, the Ag/AgX/graphene composite is often prepared in the powder form, limiting its wide-spread application. In addition, the simple fabrication of Ag/AgX/graphene composite films is highly challenging. In this study, a simple solution-based process based on meniscus-dragging deposition is demonstrated for the fabrication of Ag/AgI/rGO composite films. Uniform catalyst films with reasonable photocatalytic activities can be easily fabricated by using this microliter-scale solution process.

Degradation behaviors of naproxen by a hybrid TiO2 photocatalyst system with process components

A hybrid system combining microwave and a microwave discharge electrodeless lamp (MDEL) was proposed to overcome the limitations of conventional TiO2 photocatalysts. The degradation efficiency and mechanism of naproxen were determined using a series of single processes, including conventional TiO2 photocatalyst reactors and a hybrid system that fuses them. Although the degradation efficiency tended to increase after changing the experimental condition of a single process, the optimal conditions existed for these experimental conditions. On the other hand, remarkable synergy was observed in the fused process, whose efficiency was significantly higher than that of the unit process. In particular, the optimal degradation ability was obtained by adding hydrogen peroxide together with microwave irradiation. The seven intermediates in the proposed photocatalytic degradation pathway were generated by the demethylation and hydroxylation by hydroxyl radicals. These results are expected to provide new data on the design of high efficiency photocatalytic systems at low cost.

Preparation of novel high performance recoverable and natural sunlight-driven nanocomposite photocatalyst of Fe3O4/C/TiO2/N-CQDs

This work describes a sunlight-active photocatalyst capable of magnetically separating prepared by depositing various amounts of N-doped carbon quantum dots (N-CQDs) on the Fe3O4/C/TiO2 (FCT) nanocomposite. The photocatalytic activity of the resulting composites was investigated in the decolorization of methylene blue (MB), under UV, visible and sunlight irradiation. In the composites, the amorphous carbon interlayer is devised to prevent photodissolution of the Fe3O4 core. To coat the TiO2 nanoparticles on the insulated carbon surface of the magnetic core, a vapor phase hydrolysis method was used while a hydrothermal method was used to deposit N-CQDs on the Fe3O4/C/TiO2 nanocomposite surface. The prepared nanocomposites were characterized by photoluminescence (PL) spectroscopy, X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray (EDX) spectroscopy, Fourier transform infrared (FT-IR) spectrometry, diffuse reflectance spectroscopy (DRS) and vibrating sample magnetometry (VSM). The results revealed that the N-CQDs content of the FCT composite has a great influence on the photocatalytic activity of the nanocomposite. So that, the photocatalyst FCT/N-CQDs−4.5 wt% resulted in 95.5% MB decolorization within 80 min sunlight irradiation while pure TiO2 and FCT provided 20.3% and 31.8% decolorization, respectively. In addition, the results showed that sunlight has a significant effect on the photocatalytic activity of the composite, compared to visible and UV light irradiation. Moreover, the composite photocatalysts possessed superparamagnetic properties due to their good saturation magnetization value, i.e. 24.9 emu g−1. As a result, the developed composite photocatalyst was recovered by applying an external magnetic field without losing its activity even after five cycles. In general, this study provides new insights into the fabrication of a new generation of TiO2-based photocatalysts to overcome the main drawbacks of conventional TiO2 photocatalysts.

Novel blended catalysts consisting of a TiO2 photocatalyst and an Al2O3 supported Pd-Au bimetallic catalyst for direct dehydrogenative cross-coupling between arenes and tetrahydrofuran.

Dehydrogenative cross-coupling (DCC) is a clean methodology to make C–C bonds by using abundant C–H bonds. The blended catalyst, developed in this study, consists of a TiO2 photocatalyst and an Al2O3 supported Pd–Au bimetallic catalyst and shows superior activity to the conventional TiO2 photocatalyst loaded with the corresponding metal co-catalyst for the direct DCC between various arenes and tetrahydrofuran, with concomitant evolution of hydrogen gas. The reactions were done under mild conditions without consuming any oxidising agent or other additional chemicals. This new approach of separating the photocatalyst and the metal catalyst parts allows their independent modification to improve the overall catalytic performance.

Wide band gap Ga2O3 as efficient UV-C photocatalyst for gas-phase degradation applications.

α, β, γ, and δ polymorphs of 4.6-4.8eV wide band gap Ga2O3 photocatalysts were prepared via a soft chemistry route. Their photocatalytic activity under 254nm UV-C light in the degradation of gaseous toluene was strongly depending on the polymorph phase. α- and β-Ga2O3 photocatalysts enabled achieving high and stable conversions of toluene with selectivities to CO2 within the 50-90% range, by contrast to conventional TiO2 photocatalysts that fully deactivate very rapidly on stream in similar operating conditions with rather no CO2 production, no matter whether UV-A or UV-C light was used. The highest performances were achieved on the high specific surface area β-Ga2O3 photocatalyst synthesized by adding polyethylene glycol (PEG) as porogen before precipitation, with stable toluene conversion and mineralization rate into CO2 strongly overcoming those obtained on commercial β-Ga2O3. They were attributed to favorable physicochemical properties in terms of high specific surface area, small mean crystallite size, good crystallinity, high pore volume with large size mesopore distribution and appropriate surface acidity, and to the possible existence of a double local internal field within Ga3+ units. In the degradation of hydrogen sulfide, PEG-derived β-Ga2O3 takes advantage from its high specific surface area for storing sulfate, and thus for increasing its resistance to deactivation and the duration at total sulfur removal when compared to other β-Ga2O3 photocatalysts. So, we illustrated the interest of using high surface area β-Ga2O3 in environmental photocatalysis for gas-phase depollution applications.

N–TiO2-coated polyester filters for visible light—Photocatalytic removal of gaseous toluene under static and dynamic flow conditions

The present work aims to utilize N-TiO2 coating for decontamination of gaseous pollutants under normal room light irradiation. N-TiO2 nanoparticles were prepared by incomplete thermal oxidation of TiN at different temperatures to achieve self-doping of TiO2 to improve simultaneously the structural, morphological, and electronic properties of conventional TiO2 photocatalyst. The thermal evolution of TiN to oxygen-rich titanium oxynitride (N-TiO2) and subsequently TiO2 at different temperatures is characterized systematically by different tools including XRD, TEM, XPS, UV–vis diffuse reflectance and SPV responses. The synthesized N-TiO2 was coated onto polyester filters using a facile spray coating method. The excellent photocatalytic performance was successfully demonstrated on mineralization of gaseous toluene with fluorescent light irradiation under both static and dynamic flow conditions in a comprehensive custom-built photocatalytic test system. Therefore, it shows promising potential to be utilized in practical air purification applications.

AgCl-loaded mesoporous anatase TiO2 with large specific surface area for enhancing photocatalysis

Conventional TiO2 photocatalyst possesses excellent stability, but requires higher activity for effective photocatalysis. It is highly desirable to develop an effective photocatalyst with enhanced activity. In this work, we developed a novel two-step strategy to synthesize AgCl-loaded mesoporous anatase TiO2 (AgCl/TiO2) photocatalyst. The Ag/Na-titanate samples were prepared by dealloying a ternary Al–Ti–Ag precursor, and further acid treatment resulted in the formation of AgCl/TiO2. The AgCl/TiO2 nanocomposites had a mesoporous structure and large specific surface area (222.86m2/g). In comparison to the commercial P25 photocatalyst, the AgCl/TiO2 nanocomposites exhibited an excellent photocatalytic activity toward the degradation of methyl orange (MO). In addition, the photocatalytic mechanism of AgCl/TiO2 was also rationalized.

Innovative three-dimensional (3D) eco-TiO₂ photocatalysts for practical environmental and bio-medical applications.

It is known that water purified by conventional TiO2 photocatalysts may not be safe enough for drinking, due to the toxicity by tiny existence of TiO2 nanoparticles after water treatment. We herein demonstrate a facile design of a three-dimensional (3D) TiO2 photocatalyst structure with which both the efficiency of purification and the safety level of the final purified water can be improved and ensured, respectively. The structure, consisting of 3D sulfur-doped TiO2 microtubes in nanotubes (eco-TiO2), is suitable for both environmental and bio-medical applications. Investigation of its formation mechanism reveals that anodic aluminum oxide (AAO), owing to a spatial constraint, causes a simple, nanoparticles-to-nanotubes structural rearrangement as a template for nanotube growth. It is found that eco-TiO2 can be activated under visible-light irradiation by non-metal (sulfur; S) doping, after which it shows visible-light photocatalytic activities over a range of solar energy. Importantly, an in vitro cytotoxicity test of well-purified water by eco-TiO2 confirms that eco-TiO2 satisfies the key human safety conditions.

One-step fabrication of N-doped mesoporous TiO2 nanoparticles by self-assembly for photocatalytic water splitting under visible light

A facile synthesis route is reported for preparation of N-doped mesoporous TiO2 nanoparticles (NMT-x) through evaporation induced self-assembly by using pluronic F127, titanium isopropoxide, and various amounts of urea as templating agents, titanium and nitrogen sources, respectively. A variety of different spectroscopic and analytical techniques, such as powder X-ray diffraction (XRD), N2 adsorption–desorption isotherms, transmission electron microscopy (TEM), Raman, UV–vis and X-ray photoelectron (XPS) spectroscopies were used to characterize the physicochemical properties of various NMT-x catalysts. Among the samples, NMT-1.25 catalysts, which may be due to moderate N-doping (3.46at.%) and surface area (67m2g−1) of mesoporous TiO2, were found to possess surpassing photocatalytic activities of hydrogen production from water splitting under visible light if compared with the conventional TiO2 photocatalyst.

Synthesis, characterization and photocatalytic performance of self-assembled mesoporous TiO2 nanoparticles

A facile synthesis route is reported for preparation of mesoporous TiO2 nanoparticles (MT-x) through evaporation induced self-assembly by using Pluronic F127, titanium isopropoxide, and various amounts of ethanol as templating agents, titanium sources and solvents, respectively. A variety of different spectroscopic and analytical techniques, such as small- and large-angle powder X-ray diffraction (XRD), N2 adsorption–desorption isotherms, transmission electron microscopy (TEM), Raman and Fourier transform infrared (FTIR) spectroscopies were used to characterize the physicochemical properties of various MT-x catalysts. Among the catalysts, MT-20 was found to have better mesostructures formed by the arrangement of anatase TiO2 nanoparticles of ca. 17.3nm with broad interparticle pore size distribution. Hydrogen generation from water splitting on MT-20 using visible light was enhanced by at least 8.7 times if compared with the conventional TiO2 photocatalyst. The superior photocatalytic performances observed for the synthesized MT-20 may be attributed to the presence of unique nanostructures in the TiO2 photocatalysts.

Fabrication and Characterization of Visible-Light-Driven Plasmonic Photocatalyst Ag/AgCl/TiO2 Nanotube Arrays

Conventional TiO2 photocatalyst possesses excellent activities and stabilities, but requires near-ultraviolet (UV) irradiation (about 4% of the solar spectrum) for effective photocatalysis, thereby severely limiting its practical application. It is highly desirable to develop a photocatalyst that can use visible light in high efficiency under sunlight irradiation. In this work, we prepare new visible-light-driven plasmonic photocatalyst Ag/AgCl/TiO2 nanotube arrays (NTs) by depositing AgCl nanoparticles (NPs) into the self-organized TiO2 NTs, and then reducing partial Ag+ ions in the surface region of the AgCl particles to Ag0 species under xenon lamp irradiation. The prepared metal−semiconductor nanocomposite plasmonic photocatalyst exhibits a highly visible-light photocatalytic activity for photocatalytic degradation of methyl orange in water and stability. A new plasmonic photocatalytic mechanism, which is proposed on the basis of the fact that the Ag NPs are photoexcited due to plasmon resonance and c...

Characteristics of Titanate Nanotube and the States of the Confined Sodium Ions

The titanate nanotubes (TNTs) prepared by the reaction of titanium dioxide (TiO2) with NaOH were treated with acid (HNO3) solutions of various concentrations and/or heat treated at different temperatures. The properties of the treated TNTs were investigated in relation to the photocatalytic activities. The state and the role of Na ions confined in the TNTs were analyzed by means of 23Na and 1H NMR spectroscopy. It was revealed that there exist two states of Na ions, i.e. strongly and weakly interacting with the TNT system. The Na ions strongly bound to the interlayer of TNT play an important role to stabilize the tube structure. On the other hand, Na ions weakly interacting with the surface of the TNT suppress the photocatalytic activity but can be easily removed by a weak acid treatment with 50 mM HNO3 solution to lead the optimum photocatalytic activity. In addition, the Na ions on the surface of the TNT interfere with the crystalline transition to anatase on calcinations. Although the photocatalytic activity of TNT has been believed to be lower than that of conventional TiO2 photocatalysts of nanoparticles, it was also found that TNT showed specifically high photocatalytic activities, even higher than P25 TiO2 (Degussa P25), for some cationic reactant such as trimethylamine.