Review of material design and reactor engineering on TiO2 photocatalysis for CO2 reduction

Towards visible-light photocatalysis for environmental applications: band-gap engineering versus photons absorption-a review

A range of different studies has been performed in order to design and develop photocatalysts that work efficiently under visible (and near-infrared) irradiation as well as to improve photons absorption with improved reactor design. While there is consensus on the importance of photocatalysis for environmental applications and the necessity to utilized solar irradiation (or visible-light) as driving force for these processes, it is not yet clear how to get there. Discussion on the future steps towards visible-light photocatalysis for environmental application is of great interest to scientific and industrial communities and the present paper reviews and discusses the two main approaches, band-gap engineering for efficient solar-activated catalysts and reactor designs for improved photons absorption. Common misconceptions and drawbacks of each technology are also examined together with insights for future progress.

Titania-coated 2D gold nanoplates as nanoagents for synergistic photothermal/sonodynamic therapy in the second near-infrared window

The development of efficient nanomedicines to improve anticancer therapeutic effects is highly attractive. In this work, we firstly report titania-coated Au nanoplate (Au NPL@TiO2) heterostructures, which play dual roles as nanoagents for synergistic photothermal/sonodynamic therapy in the second near-infrared (NIR) window. On the one hand, because the controlled TiO2 shells endow the Au NPL@TiO2 nanostructures with a red shift to the NIR II region, the as-prepared Au NPL@TiO2 nanostructures possess a high photothermal conversion efficiency of 42.05% when irradiated by a 1064 nm laser and are anticipated to be very promising candidates as photothermal agents. On the other hand, the Au nanoplates (Au NPLs), as electron traps, vastly improve the generation of reactive oxygen species (ROS) by the Au NPL@TiO2 nanostructures in contrast with pure TiO2 shell nanoparticles upon activation by ultrasound (US) via a sonodynamic process. Moreover, the toxicity and therapeutic effect of the Au NPL@TiO2 nanostructures were relatively systemically evaluated in vitro. The Au NPL@TiO2 nanostructures generate a large amount of intracellular ROS and exhibit laser power density-dependent toxicity, which eventually induces apoptosis of cancer cells. Furthermore, a synergistic therapeutic effect, with a cell viability of only 20.3% upon both photothermal and sonodynamic activation, was achieved at low concentrations of the Au NPL@TiO2 nanostructures. Experiments on mice also demonstrate the superiority of the combination of PTT and SDT, with the total elimination of tumors. This work provides a way of applying two-dimensional (2D) gold nanoplate core/TiO2 shell nanostructures as novel nanoagents for advanced multifunctional anticancer therapies in the second NIR window.

Titanium Dioxide (TiO2) Mesocrystals: Synthesis, Growth Mechanisms and Photocatalytic Properties

Hierarchical TiO2 superstructures with desired architectures and intriguing physico-chemical properties are considered to be one of the most promising candidates for solving the serious issues related to global energy exhaustion as well as environmental deterioration via the well-known photocatalytic process. In particular, TiO2 mesocrystals, which are built from TiO2 nanocrystal building blocks in the same crystallographical orientation, have attracted intensive research interest in the area of photocatalysis owing to their distinctive structural properties such as high crystallinity, high specific surface area, and single-crystal-like nature. The deeper understanding of TiO2 mesocrystals-based photocatalysis is beneficial for developing new types of photocatalytic materials with multiple functionalities. In this paper, a comprehensive review of the recent advances toward fabricating and modifying TiO2 mesocrystals is provided, with special focus on the underlying mesocrystallization mechanism and controlling rules. The potential applications of as-synthesized TiO2 mesocrystals in photocatalysis are then discussed to shed light on the structure–performance relationships, thus guiding the development of highly efficient TiO2 mesocrystal-based photocatalysts for certain applications. Finally, the prospects of future research on TiO2 mesocrystals in photocatalysis are briefly highlighted.

Dye Degrading and Fouling-Resistant Membranes Formed by Deposition with Ternary Nanocomposites of N-Doped Graphene/TiO₂/Activated Carbon

A ternary nanocomposite consisting of N-doped graphene (NGR)/TiO₂/activated carbon (NGRT@AC) was prepared, and the components' synergetic effect on dye degradation was investigated after deposition on the surface of a polysulfone membrane (PSF). As far as we know, this ternary composite catalyst has never previously been used to degrade dyes nor been used as a functional layer for separation membranes. The surface morphology and structure of the as-prepared membranes were analyzed by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The NGRT@AC-modified PSF membrane (NGRT@AC-PSF) presents excellent photodegradation efficiency to methyl orange (MO) under both UV (95.2%) and sunlight (78.1%) irradiation, much higher than those values of PSF, TiO₂- modified PSF (TiO₂-PSF), and N-doped graphene/TiO₂ (NGRT)-modified PSF membranes (NGRT-PSF) under the same conditions. The high flux recovery ratio (95.5%) demonstrates that the NGRT@AC-PSF membrane shows improved antifouling performance. The photocatalytic results prove that surface deposition method (95.2%) was better than the blending method (31.1%) for forming high-performance membranes. Therefore, the NGRT@AC-PSF membrane has the potential for broad applications in dye degradation to treat waste water from textile industries.

A microfluidic photoelectrochemical cell for solar-driven CO2 conversion into liquid fuels with CuO-based photocathodes

Schematic representation of photoelectrochemical CO₂ reduction set-up.

Fabrication of surface hydroxyl modified g-C3N4with enhanced photocatalytic oxidation activity

Photocatalytic activity of C₃N₄in the decomposition of phenolic compounds in water was significantly improved with hydroxyl surface modification.

Systematic study of TiO2/ZnO mixed metal oxides for CO2 photoreduction

A novel example using a systematic design of experiments mixture design for developing mixed metal oxide photocatalysts for CO₂ photoreduction.

Engineering of highly active Au/Pd supported on hydrogenated urchin-like yolk@shell TiO2 for visible light photocatalytic Suzuki coupling

An engineered hydrogenated urchin-like yolk@shell TiO₂ structure decorated with Au/Pd nanoparticles was designed via sequential steps and employed in visible light photocatalytic Suzuki coupling.

TiO₂ Modification with Transition Metallic Species (Cr, Co, Ni, and Cu) for Photocatalytic Abatement of Acetic Acid in Liquid Phase and Propene in Gas Phase

The commercial P25 titania has been modified with transition metallic species (Cr, Co, Ni, and Cu), added by impregnation with aqueous solutions of the corresponding nitrates. The preparation procedure also includes a heat treatment (500 °C) in argon to decompose the nitrates, remove impurities and to strengthen the metal–TiO₂ interaction. The catalysts have been thoroughly characterized using N₂ adsorption, scanning electron microscopy (SEM), X-ray diffraction (XRD), UV-visible diffuse-reflectance spectroscopy (UV-vis DRS) and X-ray photoelectron spectroscopy (XPS), and have been tested in the aqueous phase decomposition of acetic acid and in the gas phase oxidation of propene, using an irradiation source of 365 nm in both cases. The photocatalytic activity of the four metal-containing catalysts varies with the nature of the metallic species and follows a similar trend in the two tested reactions. The effect of the nature of the added metallic species is mainly based on the electrochemical properties of the supported species, being Cu/P25 (the sample that contains copper) the best performing catalyst. In the photodecomposition of acetic acid, all the metal-containing samples are more active than bare P25, while in the gas phase oxidation of propene, bare P25 is more active. This has been explained considering that the rate-determining steps are different in gas and liquid media.

In Situ FTIR Reactor for Monitoring Gas-Phase Products during a (Photo)catalytic Reaction in the Liquid Phase

Various catalytic and photocatalytic reactions in the liquid phase give rise to gas products. Therefore, the identification and quantification of these products are of high importance and are even essential for some reactions. In this paper, a new in situ FTIR reactor is designed and used for analyzing the gas headspace of a (photo)catalytic reaction in solution. It allows the identification and quantification of the gas-phase products of a liquid reaction under operating conditions and in real time. The new reactor has been tested in three representative photocatalytic reactions widely studied as model reactions in the liquid phase: i.e., (i) decomposition of formic acid, (ii) oxidation of methylene blue, and (iii) reduction of CO₂. The validity of the results has been confirmed by analyzing the headspace at the end of the reaction using gas chromatography technique. The new reactor opens the possibility to follow online the (photo)catalyst activity. This is useful for ensuring the stability of the catalyst and studying the evolution of the selectivity during the reaction. The nondestructive behavior of the FTIR technique allows its coupling with other techniques for obtaining complementary results. The new reactor setup is easy to handle and to ship and is very efficient, which makes it very suitable for performing complementary, fast and/or preliminary studies.

The solvent-driven formation of multi-morphological Ag-CeO2 plasmonic photocatalysts with enhanced visible-light photocatalytic reduction of CO2

Ag-CeO2 plasmonic photocatalysts with multiple morphologies were synthesized via a simple solvent-driven method. The phase compositions, morphologies and optical properties of the samples were systematically investigated. A combination of noble metal Ag and semiconductor CeO2 in certain solvents (such as methanol and ethylene glycol) enhanced surface plasmon resonance (SPR), which was attributed to the good dispersion of Ag particles on CeO2 and high Ag0 ratios on the surface. The enhanced SPR effect boosted absorption of incident light and facilitated charge carrier separation and transport efficiency caused by the formation of Schottky barriers, thus promoting VLPCR performance. The optimum ACG sample (ethylene glycol was adopted as the solvent) exhibited the maximum VLPCR activity, achieving a CH4 yield of 100 μmol and a CH3OH yield of 35 μmol per gram of catalyst per hour during 6 h visible-light irradiation.

Photocatalytic reduction of carbon dioxide over Cu/TiO2 photocatalysts

The photocatalytic reduction of CO₂ with H₂O was investigated using Cu/TiO₂ photocatalysts in aqueous solution. For this purpose, Cu/TiO₂ photocatalysts (with 0.2, 0.9, 2, 4, and 6 wt.% of Cu) have been synthesized via sol-gel method. The photocatalysts were extensively characterized by means of inductively coupled plasma optical emission spectrometry (ICP-OES), N₂ physisorption (BET), XRD, UV-vis DRS, FT-IR, Raman spectroscopy, TEM-EDX, and photoelectrochemical measurements. The as-prepared photocatalysts contain anatase as a major crystalline phase with a crystallite size around 13 nm. By increasing the amount of Cu, specific surface area and band gap energy decreased in addition to the formation of large agglomeration of CuO. Results revealed that the photocatalytic reduction of CO₂ decreased in the presence of Cu/TiO₂ in comparison to pure TiO₂, which might be associated to the formation of CuO phase acting as a recombination center of generated electron-hole pair. Decreasing of photoactivity can also be connected with a very low position of conduction band of photocatalysts with high Cu content, which makes H₂ production necessary for CO₂ reduction more difficult.

Visible light driven photo-degradation of Congo red by TiO2ZnO/Ag: DFT approach on synergetic effect on band gap energy

In this paper, we report the combination of two metal oxides (TiO₂ZnO) that allows mixed density of states to reduce band gap energy, facilitating the photo-oxidation of Congo red dye under visible light. For the oxidation, a possible mechanism is proposed after analyzing the intermediates by GC-MS, and it is consistent with Density Functional Theory (DFT). The nanohybrids were characterized comprehensibly by several analytical techniques such as X-Ray diffraction (XRD), Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM), and X-ray Photoelectron Spectroscopy (XPS). For the addition of ZnO to TiO₂, a dominance of anatase phase was found rather than other phases (rutile or brookite). A broad band (∼550 nm) is observed in UV-Visible spectra for TiO₂ZnO/Ag NPs nm because of Surface Plasmon properties of Ag NPs. The band gap energy was calculated for TiO₂ZnO/Ag system, and then it has been further studied by DFT in order to show why the convergence of two semiconductors allows a mixed density of states, facilitating the reduction of the energy gap between occupied and unoccupied bands; ultimately, it improves the performance of catalysts under visible light. Significantly, the interaction of crystal planes (0 0 Ī) of TiO₂ anatase and (0 0 1) of ZnO crucially plays as an important role for the reduction of energy band-gap. Additionally, TiO₂ZnOAg NPs were used recognize Saccharomyces cerevisiae cells by con-focal fluorescence microscope, showing that it develops bright bio-images for the cells; while for TiO₂ or ZnO or TiO₂ZnO NPs, no fluorescent response was seen within the cells.

Sustainable Recovery of CO2 by Using Visible-Light-Responsive Crystal Cuprous Oxide/Reduced Graphene Oxide

A simple solution-chemistry method has been investigated to prepare crystal cuprous oxide (Cu2O) incorporated with reduced graphene oxide (designated as Cu2O-rGO-x, where x represents the contents of rGO = 1%, 5% and 10%) in this work. These Cu2O-rGO-x composites combine the prospective advantages of rhombic dodecahedra Cu2O together with rGO nanosheets which have been studied as visible-light-sensitive catalysts for the photocatalytic production of methanol from CO2. Among the Cu2O-rGO-x photocatalysts, the methanol yield photocatalyzed by Cu2O-rGO-5% can be observed to be 355.26 μmol g−1cat, which is ca. 36 times higher than that of pristine Cu2O nanocrystal in the 20th hour under visible light irradiation. The improved activity may be attributed to the enhanced absorption ability of visible light, the superior separation of electron–hole pairs, well-dispersed Cu2O nanocrystals and the increased photostability of Cu2O, which are evidenced by employing UV-vis diffuse reflection spectroscopy, photoluminescence, scanning electron microscopy/transmission electron microscopy and X-ray photoelectron spectroscopy, respectively. This work demonstrates an easy and cost-effective route to prepare non-noble photocatalysts for efficient CO2 recovery in artificial photosynthesis.

Preparation of Ni-Pt/Fe-TiO2 nanotube films for photoelectrochemical cathodic protection of 403 stainless steel

Iron-doped TiO₂ nanotubes (Fe-TNTs) were prepared by a one step anodization method. Nickel-platinum was then deposited on the surface of these Fe-TNTs samples by an electroless process during different time periods. SEM, XRD and UV-vis absorption spectroscopy was used to study the surface morphologies, composition and optical properties of the synthetic samples. In addition, the impact of coupling the samples prepared with 403 stainless steel on the corrosion of metal under illumination was studied. The corrosion behavior was studied by electrochemical techniques (choronoamperometery, open circuit potential and Tafel). The photocathodic protection was remarkably enhanced by Ni-Pt deposition on the Fe-TNTs photoanodes, as clearly indicated by the results.

Ag-doping on ZnO support mediated by bio-analytes rich in ascorbic acid for photocatalytic degradation of dipyrone drug

The analytes such as ascorbic acid (AA) present in Sechium edule were extracted (294 mg AA kg^-1 fruit) in an aqueous media for its potential application for Ag-doping onto wurtzite ZnO. The bandgap of ZnO was decreased to 2.85 eV at the optimal Ag-loading of 1.18% (w/w) against 3.13 eV for the control catalyst without using the analytes and, the commercial AA only could reduce the bandgap to 2.91 eV. The saturation photo-electrochemical current density (46.68 mA cm^-2) at E_anode ≥ 0.31 V vs. Ag/AgCl was almost double than pristine ZnO under visible light illumination (λ_mean = 525 nm, 18 K lux) and, the current density was insignificant in the dark. The doped catalyst exhibited the maximum 79.5% degradation (71% COD removal) of an anti-analgesic drug, dipyrone (100 μg L^-1 dipyrone, catalyst 100 mg L^-1) resulted from the formation of O₂^•- radical (g-factor of 2.002-2.008) and paramagnetic oxygen vacancies (g-factor of 2.020) and, no effect of dye-sensitization was noted. The highest quantum yield was found to be 34.7%. The catalyst loss was 6% after the fourth cycle and the dipyrone degradation was reduced to 70.8%.

Solar Fuels by Heterogeneous Photocatalysis: From Understanding Chemical Bases to Process Development

The development of sustainable yet efficient technologies to store solar light into high energy molecules, such as hydrocarbons and hydrogen, is a pivotal challenge in 21st century society. In the field of photocatalysis, a wide variety of chemical routes can be pursued to obtain solar fuels but the two most promising are carbon dioxide photoreduction and photoreforming of biomass-derived substrates. Despite their great potentialities, these technologies still need to be improved to represent a reliable alternative to traditional fuels, in terms of both catalyst design and photoreactor engineering. This review highlights the chemical fundamentals of different photocatalytic reactions for solar fuels production and provides a mechanistic insight on proposed reaction pathways. Also, possible cutting-edge strategies to obtain solar fuels are reported, focusing on how the chemical bases of the investigated reaction affect experimental choices.

Review on nanoscale Bi-based photocatalysts

Nanoscale Bi-based photocatalysts are promising candidates for visible-light-driven photocatalytic environmental remediation and energy conversion. However, the performance of bulk bismuthal semiconductors is unsatisfactory. Increasing efforts have been focused on enhancing the performance of this photocatalyst family. Many studies have reported on component adjustment, morphology control, heterojunction construction, and surface modification. Herein, recent topics in these fields, including doping, changing stoichiometry, solid solutions, ultrathin nanosheets, hierarchical and hollow architectures, conventional heterojunctions, direct Z-scheme junctions, and surface modification of conductive materials and semiconductors, are reviewed. The progress in the enhancement mechanism involving light absorption, band structure tailoring, and separation and utilization of excited carriers, is also introduced. The challenges and tendencies in the studies of nanoscale Bi-based photocatalysts are discussed and summarized.

Defective Anatase TiO2-x Mesocrystal Growth In Situ on g-C3 N4 Nanosheets: Construction of 3D/2D Z-Scheme Heterostructures for Highly Efficient Visible-Light Photocatalysis

Environmental remediation by employing visible-light-active semiconductor heterostructures provides effective solutions for handling emerging contaminants by a much greener and lower cost approach compared with other methods. This report demonstrates that the in situ growth of nanosized single-crystal-like defective anatase TiO_2-x mesocrystals (DTMCs) on g-C₃ N₄ nanosheets (NSs) can produce a 3D/2D DTMC/g-C₃ N₄ NS heterostructure with the two components held together by chemical bonds to form tight interfaces. This nanostructured heterostructure displayed remarkably improved photocatalytic activity toward the removal of the model pollutants Methyl Orange (MO) and Cr^VI under visible-light irradiation in comparison with the pristine DTMC and g-C₃ N₄ NS components, which suggests that both the oxidation and reduction abilities of the DTMC/g-C₃ N₄ NSs were simultaneously enhanced after fabrication. On the basis of the results of a systematic characterization, a reasonable mechanism for the photocatalytic activity based on a direct Z-scheme heterojunction is proposed and further verified by the measurement of ^. OH. This novel Z-scheme heterojunction endows the heterostructure with improved photogenerated electron/hole pair separation and a strong redox ability for the efficient degradation of wastewater pollutants. This work will be useful for the design and fabrication of direct Z-scheme heterostructured photocatalysts with novel architectures for applications in energy conversion and environmental remediation.

Tailoring Surface Frustrated Lewis Pairs of In2O3-x (OH)y for Gas-Phase Heterogeneous Photocatalytic Reduction of CO2 by Isomorphous Substitution of In3+ with Bi3

Frustrated Lewis pairs (FLPs) created by sterically hindered Lewis acids and Lewis bases have shown their capacity for capturing and reacting with a variety of small molecules, including H2 and CO2, and thereby creating a new strategy for CO2 reduction. Here, the photocatalytic CO2 reduction behavior of defect-laden indium oxide (In2O3-x (OH) y ) is greatly enhanced through isomorphous substitution of In3+ with Bi3+, providing fundamental insights into the catalytically active surface FLPs (i.e., In-OH···In) and the experimentally observed "volcano" relationship between the CO production rate and Bi3+ substitution level. According to density functional theory calculations at the optimal Bi3+ substitution level, the 6s2 electron pair of Bi3+ hybridizes with the oxygen in the neighboring In-OH Lewis base site, leading to mildly increased Lewis basicity without influencing the Lewis acidity of the nearby In Lewis acid site. Meanwhile, Bi3+ can act as an extra acid site, serving to maximize the heterolytic splitting of reactant H2, and results in a more hydridic hydride for more efficient CO2 reduction. This study demonstrates that isomorphous substitution can effectively optimize the reactivity of surface catalytic active sites in addition to influencing optoelectronic properties, affording a better understanding of the photocatalytic CO2 reduction mechanism.

Nitrogen Oxides Mitigation Efficiency of Cementitious Materials Incorporated with TiO₂

We explored the photocatalytic capacities of cementitious materials (cement paste and mortar) incorporating titanium dioxide (TiO₂). P-25 is a commercial TiO₂ preparation which, if incorporated into large civil buildings, is extremely expensive. It is essential to produce low-cost TiO₂. A cheap anatase form of TiO₂ powder, NP-400, manufactured under relatively low burning temperature, was considered in this paper. Addition of NP-400 to 0, 5, 10, and 20 wt % did not significantly affect the compressive strengths of mortar or cement paste. However, the compressive strengths of P-25-containing specimens were more consistent than those of NP-400-containing materials. The nitrogen oxide (NO) removal efficiencies by mortar with 5 and 10 wt % TiO₂ were similar at ca. 14–16%; the removal efficiency by mortar with 20 wt % NP-400 was ca. 70%. Although the NP-400 cluster size was almost halved by ultrasonication, NO removal efficiency was not enhanced. Removal was enhanced by the presence of accessible surface area: NP-400 dispersed in these surfaces readily adsorbed NO, aided by the large surface areas of the top and bottom faces. Scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM–EDX) confirmed that NP-400 tended to sink when added to cement, fine aggregates, and water because the true densities of P-25, NP-400, and cement powder differed (3.41, 3.70, and 3.15 g/mL). The true density of NP-400 was thus the highest of all ingredients. The relatively low apparent density of P-25 compared to that of NP-400 was associated with a more bulky distribution of P-25 within cementitious materials. Nevertheless, NP-400 could be a viable alternative to the definitive product, P-25.

White zein colloidal particles: synthesis and characterization of their optical properties on the single particle level and in concentrated suspensions

Growing interest in using natural, biodegradable ingredients for food products leads to an increase in research for alternative sources of functional ingredients. One alternative is zein, a water-insoluble protein from corn. Here, a method to investigate the optical properties of white zein colloidal particles is presented in both diluted and concentrated suspensions. The particles are synthesized, after purification of zein, by anti-solvent precipitation. Mean particle diameters ranged from 35 to 135 nm based on dynamic light scattering. The value of these particles as white colorant is examined by measuring their optical properties. Dilute suspensions are prepared to measure the extinction cross section of individual particles and this was combined with Mie theory to determine a refractive index (RI) of 1.49 ± 0.01 for zein particles dispersed in water. This value is used to further model the optical properties of concentrated suspensions. To obtain full opacity of the suspension, comparable to 0.1-0.2 wt% suspensions of TiO2, concentrations of 2 to 3.3 wt% of zein particles are sufficient. The optimal size for maximal scattering efficiency is explored by modeling dilute and concentrated samples with RI's matching those of zein and TiO2 particles in water. The transport mean free path of light was determined experimentally and theoretically and the agreement between the transport mean free path calculated from the model and the measured value is better than 30%. Such particles have the potential to be an all-natural edible alternative for TiO2 as white colorant in wet food products.

Photocatalytic degradation of aqueous Methyl Orange using nitrogen-doped TiO2 photocatalyst prepared by novel method of ultraviolet-assisted thermal synthesis

A nitrogen-doped titanium dioxide composite photocatalyst (N-TiO₂) with heterojunction structures is synthesized by three different approaches: a novel UV-assisted thermal synthesis, annealing, and microwave technique. Photocatalytic activities of synthesized photocatalysts are evaluated by the degradation of Methyl Orange under ultraviolet light types A (UV-A), B (UV-B), and C (UV-C), visible light, and direct sunlight irradiation. Results show that by using N-TiO₂ photocatalyst prepared by the UV-assisted thermal synthesis and annealing, the degradation increases by 16.5% and 20.4%, respectively, compared to that by bare TiO₂. The best results are obtained at a nitrogen to TiO₂ mass ratio of 0.15 (N:TiO₂). The enhancement of the photocatalytic activity observed in the visible range is mainly attributed to the increasing separation rate of photogenerated charge carriers. The novel UV-assisted thermal synthesis has produced encouraging results as a preparation method for the nitrogen-doped TiO₂ photocatalyst; thus, further studies are recommended for process optimization, immobilization, and scale-up to evaluate its applicability in wastewater treatment.

Hydrothermal in situ synthesis of Rb and S co-doped Ti-based TiO2 sheet with a thin film showing high photocatalytic activities

TiO2 is considered as one of the most promising semiconductor photocatalysts used to degrade organic pollutants. Element doping has a good effect on improving the properties of TiO2. Herein, by using Rb2SO4, we explored the in situ synthesis of Ti-based TiO2 sheets with a thin film through a hydrothermal reaction. Then, the photocatalyst was successfully fabricated by calcination. All samples were characterized by FT-IR, XRD, SEM, XPS, PL and UV-vis DRS measurements. The results indicate that the S doping together with surface hydroxyl groups lead to the band gap narrowing. S and a trace amount of Rb element can enable the formation of uniform microspheres on the surface of the Ti plate and the major phase transformed from titanium to anatase. The band gap absorption extended from 400 nm to 600 nm. The photocatalytic properties were investigated by performing the degradation of methyl orange (MO) and 4-chlorophenol (4-CP) in the aqueous solutions under UV and simulated sunlight. In the series of TiO2 photocatalysts, Rb/S/TiO2-48 shows the best photocatalytic efficiency and good photocatalytic performance on recycling. Interestingly, when H2O2 was added to the MO aqueous solution, a synergistic effect of the TiO2 thin film and H2O2 on degrading the pollutant was observed.

Cocatalysts in Semiconductor-based Photocatalytic CO2 Reduction: Achievements, Challenges, and Opportunities

Ever-increasing fossil-fuel combustion along with massive CO₂ emissions has aroused a global energy crisis and climate change. Photocatalytic CO₂ reduction represents a promising strategy for clean, cost-effective, and environmentally friendly conversion of CO₂ into hydrocarbon fuels by utilizing solar energy. This strategy combines the reductive half-reaction of CO₂ conversion with an oxidative half reaction, e.g., H₂ O oxidation, to create a carbon-neutral cycle, presenting a viable solution to global energy and environmental problems. There are three pivotal processes in photocatalytic CO₂ conversion: (i) solar-light absorption, (ii) charge separation/migration, and (iii) catalytic CO₂ reduction and H₂ O oxidation. While significant progress is made in optimizing the first two processes, much less research is conducted toward enhancing the efficiency of the third step, which requires the presence of cocatalysts. In general, cocatalysts play four important roles: (i) boosting charge separation/transfer, (ii) improving the activity and selectivity of CO₂ reduction, (iii) enhancing the stability of photocatalysts, and (iv) suppressing side or back reactions. Herein, for the first time, all the developed CO₂ -reduction cocatalysts for semiconductor-based photocatalytic CO₂ conversion are summarized, and their functions and mechanisms are discussed. Finally, perspectives in this emerging area are provided.