Photocatalytic CO2 Reduction with H2O on TiO2 Nanocrystals: Comparison of Anatase, Rutile, and Brookite Polymorphs and Exploration of Surface Chemistry

Why Is Water More Reactive Than Hydrogen in Photocatalytic CO2 Conversion at Higher Pressures? Elucidation by Means of X-Ray Absorption Fine Structure and Gas Chromatography-Mass Spectrometry

Photocatalytic conversion of CO2 into mainly methane using Pd/TiO2 photocatalyst proceeded faster at 0.80 MPa using water rather than hydrogen as a reductant. The former reaction (CO2 + water) consists of two steps: first, water photosplitting and second, the latter reaction (CO2 + hydrogen). It was paradoxical that total steps proceeded faster than each step based on simple kinetics. To elucidate the reason, Pd and Ti K-edge X-ray absorption fine structure (XAFS) was monitored during CO2 photoconversion using H2 or moisture and the exchange reaction of 13CO2 at Pd/TiO2 surface was also monitored. As a result, the coordination number, N(Ti-O) and N[Ti(-O-)Ti] values, decreased from original values for TiO2 crystalline (6 and 12) to 4.9-5.7 and 9.7-10.6 under CO2 and moisture, respectively, in contrast to significantly smaller decreases under CO2 and H2 and under Ar. The exchange of gas-phase 13CO2 with preadsorbed 12CO2 reached the equilibrium in ~20 h with a rate constant of 0.20 h-1. The reason of the higher activity using water rather than H2 could be explained owing to the oxygen vacancy (O v ) sites as confirmed by XAFS. The reaction of TiO2 surface with water formed O v sites responsible for water oxidation, specially separated from Pd nanoparticle sites for CO2 reduction. In contrast, Pd nanoparticle sites were competed by CO2 and H species, and the photoconversion of CO2 was suppressed at the elevated pressure of CO2 + H2.

The Work Function of TiO2

Polycrystalline anatase thin films, (001)- and (101)-oriented anatase TiO 2 single crystals and (001)- and (110)-oriented rutile TiO 2 single crystals with various surface treatments were studied by photoelectron spectroscopy to obtain their surface potentials. Regardless of orientations and polymorph, a huge variation of the Fermi level and work function was achieved by varying the surface condition. The most strongly oxidized surfaces are obtained after oxygen plasma treatment with a Fermi level ∼2.6 eV above the valence band maximum and ionization potentials of up to 9.5 eV (work function 7.9 eV). All other treated anatase surfaces exhibit an ionization potential independent of surface condition of 7.96 ± 0.15 eV. The Fermi level positions and the work functions vary by up to 1 eV. The ionization potential of rutile is ∼0.56 eV lower than that of anatase in good agreement with recent band alignment studies.

Metal-Organic-Framework-Based Catalysts for Photoreduction of CO2

Photoreduction of CO₂ into reusable carbon forms is considered as a promising approach to address the crisis of energy from fossil fuels and reduce excessive CO₂ emission. Recently, metal-organic frameworks (MOFs) have attracted much attention as CO₂ photoreduction-related catalysts, owing to their unique electronic band structures, excellent CO₂ adsorption capacities, and tailorable light-absorption abilities. Recent advances on the design, synthesis, and CO₂ reduction applications of MOF-based photocatalysts are discussed here, beginning with the introduction of the characteristics of high-efficiency photocatalysts and structural advantages of MOFs. The roles of MOFs in CO₂ photoreduction systems as photocatalysts, photocatalytic hosts, and cocatalysts are analyzed. Detailed discussions focus on two constituents of pure MOFs (metal clusters such as Ti-O, Zr-O, and Fe-O clusters and functional organic linkers such as amino-modified, photosensitizer-functionalized, and electron-rich conjugated linkers) and three types of MOF-based composites (metal-MOF, semiconductor-MOF, and photosensitizer-MOF composites). The constituents, CO₂ adsorption capacities, absorption edges, and photocatalytic activities of these photocatalysts are highlighted to provide fundamental guidance to rational design of efficient MOF-based photocatalyst materials for CO₂ reduction. A perspective of future research directions, critical challenges to be met, and potential solutions in this research field concludes the discussion.

Influence of natural organic matter in porous media on fine particle transport

Although extensive research has been conducted to understand the effects of dissolved organic matter (DOM) on fine particle transport, less attention has been paid to natural organic matter (NOM) in the transport medium (i.e., immobile rock and sediment grains). The objective of this study is to elucidate the roles of NOM in the transport medium in mediating particle transport. We conducted experimental and modelling study on the transport of nanoscale titanium dioxide (nTiO₂) and illite colloid in columns packed with quartz sand under water-saturated conditions. Peat moss was used as an example NOM and packed in some of the columns to investigate its influence on particle transport. Experimental results showed that NOM may either increase or decrease particle transport depending on the specific conditions. NOM in the transport medium was found to attract particles and reduce particle mobility when the energy barrier between particle and NOM is low or non-existent. NOM also adsorb to Fe and Al oxyhydroxides and promote the transport of negatively-charged particles at low pH. Partial dissolution of NOM releases DOM, and the DOM adsorbs to and increases the transport of positively-charged particles. Additionally, NOM changes pore water pH, which influences particle mobility by affecting the interaction energy between the particle and transport medium. Modelling results showed that the deposition sites provided by peat moss are very heterogeneous, and the NOM from peat moss may reduce particle deposition rate by adsorbing to the particle and/or transport medium. Findings from this study demonstrate that NOM in the transport medium not only changes property of the medium, but also may alter water chemistry. Therefore, the role of NOM in mediating particle transport is complicated and dependent on the property of the particle, NOM, and mineralogical composition of the medium.

Ultrathin WO3·0.33H2O Nanotubes for CO2 Photoreduction to Acetate with High Selectivity

Artificial photosynthesis from CO₂ reduction is severely hampered by the kinetically challenging multi-electron reaction process. Oxygen vacancies (Vo) with abundant localized electrons have great potential to overcome this limitation. However, surface Vo usually have low concentrations and are easily oxidized, causing them to lose their activities. For practical application of CO₂ photoreduction, fabricating and enhancing the stability of Vo on semiconductors is indispensable. Here we report the first synthesis of ultrathin WO₃·0.33H₂O nanotubes with a large amount of exposed surface Vo sites, which can realize excellent and stable CO₂ photoreduction to CH₃COOH in pure water under solar light. The selectivity for acetum generation is up to 85%, with an average productivity of about 9.4 μmol g^-1 h^-1. More importantly, Vo in the catalyst are sustainable, and their concentration was not decreased even after 60 h of reaction. Quantum chemical calculations and in situ DRIFT studies revealed that the main reaction pathway might be CO₂ → ^•COOH → (COOH)₂ → CH₃COOH.

Ultrafast Processing of Hierarchical Nanotexture for a Transparent Superamphiphobic Coating with Extremely Low Roll-Off Angle and High Impalement Pressure

Low roll-off angle, high impalement pressure, and mechanical robustness are key requirements for super-liquid-repellent surfaces to realize their potential in applications ranging from gas exchange membranes to protective and self-cleaning materials. Achieving these properties is still a challenge with superamphiphobic surfaces, which can repel both water and low-surface-tension liquids. In addition, fabrication procedures of superamphiphobic surfaces are typically slow and expensive. Here, by making use of liquid flame spray, a silicon dioxide-titanium dioxide nanostructured coating is fabricated at a high velocity up to 0.8 m s^-1 . After fluorosilanization, the coating is superamphiphobic with excellent transparency and an extremely low roll-off angle; 10 µL drops of n-hexadecane roll off the surface at inclination angles even below 1°. Falling drops bounce off when impacting from a height of 50 cm, demonstrating the high impalement pressure of the coating. The extraordinary properties are due to a pronounced hierarchical nanotexture of the coating.

RGD-modified dihydrolipoamide dehydrogenase conjugated to titanium dioxide nanoparticles - integrin-targeted photodynamic treatment of melanoma cells

The photocytotoxic effect of UVA-excited titanium dioxide (TiO2), which is caused by the generation of reactive oxygen species (ROS), is often used in medical applications, such as cancer treatment. Photodynamic-therapy (PDT) is applied in several cancer models including cutaneous melanoma (CM), however the lack of selectivity causing damage to surrounding healthy tissues limits its applicability and novel targeted-delivery approaches are required. As cancer cells often overexpress integrin receptors (e.g. αvβ3) on their cell surface, targeted delivery of TiO2 nanoparticles (NPs) via an Arg-Gly-Asp (RGD) motif would make PDT more selective. We have recently reported that the mitochondrial enzyme dihydrolipoamide dehydrogenase (DLDH) strongly and specifically conjugates TiO2 via coordinative bonds. In this work we have modified DLDH with RGD moieties (DLDHRGD), creating a molecular bridge between the integrin-expressing cancer cells and the photo-excitable TiO2 nanoparticles. Physicochemical assays have indicated that the hybrid-conjugated nanobiocomplex, TiO2-DLDHRGD, is producing controlled-release ROS under UVA illumination, with anatase NPs being the most photoreactive TiO2 form. This drug delivery system exhibited a cytotoxic effect in αvβ3 integrin-expressing mice melanoma cells (B16F10), but not in normal cells lacking this integrin (HEK293). No cytotoxic effect was observed in the absence of UV illumination. Our results demonstrate the feasibility of combining the high efficiency of TiO2-based PDT, with an integrin-mediated tumor-targeted drug delivery for nanomedicine.

Imaging Catalytic Activation of CO2 on Cu2O (110): A First-Principles Study

Balancing global energy needs against increasing greenhouse gas emissions requires new methods for efficient CO₂ reduction. While photoreduction of CO₂ is promising, the rational design of photocatalysts hinges on precise characterization of the surface catalytic reactions. Cu₂O is a promising next-generation photocatalyst, but the atomic-scale description of the interaction between CO₂ and the Cu₂O surface is largely unknown, and detailed experimental measures are lacking. In this study, density-functional theory (DFT) calculations have been performed to identify the Cu₂O (110) surface stoichiometry that favors CO₂ reduction. To facilitate interpretation of scanning tunneling microscopy (STM) and X-ray absorption near-edge structures (XANES) measurements, which are useful for characterizing catalytic reactions, we present simulations based on DFT-derived surface morphologies with various adsorbate types. STM and XANES simulations were performed using the Tersoff-Hamann approximation and Bethe-Salpeter equation (BSE) approach, respectively. The results provide guidance for observation of CO₂ reduction reaction on, and rational surface engineering of, Cu₂O (110). They also demonstrate the effectiveness of computational image and spectroscopy modeling as a predictive tool for surface catalysis characterization.

CO2 Reduction by Photocatalysis on TiO2

This chapter focuses on the recent research progress on TiO2-based photocatalysts for CO2 reduction. The scope of this chapter for photoreduction of CO2 is set to focus on the most widely studied TiO2-based photocatalysts, composites, and systems since 1979. In addition, several important kinds of other related photocatalysts will be introduced briefly.

Selectivity reversal of photocatalytic CO2 reduction by Pt loading

The selectivity of semiconductor photocatalysts for CO₂ reduction was effectively reversed from CO to CH₄ due to the introduction of Pt.

Influence of substrate on the structure of predominantly anatase TiO2films grown by reactive sputtering

Revealing the crystalline structure of predominantly grown anatase TiO₂films.

Twin defects engineered Pd cocatalyst on C3N4 nanosheets for enhanced photocatalytic performance in CO2 reduction reaction

Photocatalytic conversion of CO₂ to value-added chemicals, a potential route to addressing the depletion of fossil fuels and anthropogenic climate change, is greatly limited by the low-efficient semiconductor photocatalyst. The integration of cocatalyst with light-harvesting semiconductor is a promising approach to enhancing the photocatalytic performance in CO₂ reduction reaction. The enhancement is greatly determined by the catalytic active sites on the surface of cocatalyst. Herein, we demonstrate that the photocatalytic performance in the CO₂ reduction reaction is greatly promoted by twin defects engineered Pd cocatalyst. In this work, Pd nanoicosahedrons with twin defects were in situ grown on C₃N₄ nanosheets, which effectively improve the photocatalytic performance in reduction of CO₂ to CO and CH₄ in comparison with Pd nanotetrahedrons without twin defects. It is proposed that the twin boundary (TB) terminations on the surface of Pd cocatalysts are highly catalytic active sites for CO₂ reduction reaction. Based on the proposed mechanism, the photocatalytic activity and selectivity in CO₂ reduction were further advanced through reducing the size of Pd icosahedral cocatalyst resulted from the increased surface density of TB terminations. The defect engineering on the surface of cocatalyst represents a novel route in realizing high-performance photocatalytic applications.

Surface modification of TiO2 particles with the sono-assisted exfoliation method

This study reported a novel approach to optimize the photocatalytic property of anatase TiO₂ particles by delaminating their outer surface into highly reactive nanosheets via the sono-assisted exfoliation method. To modify the surface, TiO₂ particles were dispersed in aqueous solution of 10M sodium hydroxide (NaOH) and tetrabutylammonium hydroxide (TBAOH), followed by irradiation with high intensity ultrasonic wave (20kHz, 150W/cm²) for 60min. The intercalation and exfoliation processes were accelerated with the driving force of the extreme acoustic cavitation leading to the delamination of TiO₂ nanosheets with highly reactive exposed {001} facets from the mother TiO₂ crystals. The presence of TBAOH increased yield of nanosheets formation and stabilized the nanosheet structure. The unique morphology of the surface modified TiO₂ particles provided benefits in increasing the specific surface area and lowering the optical band gap energy (E_g) and electron-hole recombination rate resulting in an enhancement of methylene blue dye degradation efficiency. The surface modification of TiO₂ particles by the sono-assisted exfoliation method can optimize the photocatalytic activity by yielding synergetic effects of the high surface reactive sites of the nanosheets and the high degree of crystallinity of the bulk structure.

CO2 Reduction: From the Electrochemical to Photochemical Approach

Increasing CO2 concentration in the atmosphere is believed to have a profound impact on the global climate. To reverse the impact would necessitate not only curbing the reliance on fossil fuels but also developing effective strategies capture and utilize CO2 from the atmosphere. Among several available strategies, CO2 reduction via the electrochemical or photochemical approach is particularly attractive since the required energy input can be potentially supplied from renewable sources such as solar energy. In this Review, an overview on these two different but inherently connected approaches is provided and recent progress on the development, engineering, and understanding of CO2 reduction electrocatalysts and photocatalysts is summarized. First, the basic principles that govern electrocatalytic or photocatalytic CO2 reduction and their important performance metrics are discussed. Then, a detailed discussion on different CO2 reduction electrocatalysts and photocatalysts as well as their generally designing strategies is provided. At the end of this Review, perspectives on the opportunities and possible directions for future development of this field are presented.

Structure sensitive photocatalytic reduction of nitroarenes over TiO 2

It is a subject of exploration whether the phase pure anatase or rutile TiO₂ or the band alignment due to the heterojunctions in the two polymorphs of TiO₂ plays the determining role in efficacy of a photocatalytic reaction. In this work, the phase pure anatase and rutile TiO₂ have been explored for photocatalytic nitroarenes reduction to understand the role of surface structures and band alignment towards the reduction mechanism. The conduction band of synthesized anatase TiO₂ has been found to be more populated with electrons of higher energy than that of synthesized rutile. This has given the anatase an edge towards photocatalytic reduction of nitroarenes over rutile TiO₂. The other factors like adsorption of the reactants and the proton generation did not play any decisive role in catalytic efficacy.

Structure sensitive photocatalytic reduction of nitroarenes over TiO 2

It is a subject of exploration whether the phase pure anatase or rutile TiO₂ or the band alignment due to the heterojunctions in the two polymorphs of TiO₂ plays the determining role in efficacy of a photocatalytic reaction. In this work, the phase pure anatase and rutile TiO₂ have been explored for photocatalytic nitroarenes reduction to understand the role of surface structures and band alignment towards the reduction mechanism. The conduction band of synthesized anatase TiO₂ has been found to be more populated with electrons of higher energy than that of synthesized rutile. This has given the anatase an edge towards photocatalytic reduction of nitroarenes over rutile TiO₂. The other factors like adsorption of the reactants and the proton generation did not play any decisive role in catalytic efficacy.

Synthesis and Photo-Catalytic Activity of Nanoparticles with Structure "Core/Shell": Fe3O4@SiO2@TiO2

In this work, the nanosized magnetic Fe3O4@SiO2@TiO2 photocatalyst was prepared by sol-gel methods. First the nuclei of magnetite nanoparticles were prepared by co-precipitation of iron (II&III) salts solutions. Secondly, the magnetite nanoparticles were dispersed in ethanol using sonication, and solutions of both ammonia and tetraethoxysilane were added to the suspension under intense stirring, since it was suggested that the introduction of an intermediate passive SiO2 layer between the Fe3O4 and TiO2 phases inhibits the direct electrical contact and hence prevents the photodissolution of the magnetite phase and deterioration of the surface photocatalytic properties. Finally, solution of tetrabutoxytitanium was added dropwise to the mixture of Fe3O4@SiO2 nanoparticles under sonication & intense stirring. The resulting particles were separated using a magnet, washed and dried to constant weight, the yield was 70%. The photocatalytic activity of Fe3O4@ SiO2@TiO2 nanoparticles was investigated by photodegradation of methyl orange in aqueous solution under UV light irradiation. The photodegradation dynamics revealed that even though the oxidation rate decreases over time, about 90% of methyl orange is oxidized during the first 35 min.

Investigating the Unrevealed Photocatalytic Activity and Stability of Nanostructured Brookite TiO2 Film as an Environmental Photocatalyst

Among three polymorphs of TiO₂, the brookite is the least known phase in many aspects of its properties and photoactivities (especially comparable to anatase and rutile) because it is the rarest phase to be synthesized in the standard environment among the TiO₂ polymorphs. In this study, we address the unrevealed photocatalytic properties of pure brookite TiO₂ film as an environmental photocatalyst. Highly crystalline brookite nanostructures were synthesized on titanium foil using a well-designed hydrothermal reaction, without harmful precursors and selective etching of anatase, to afford pure brookite. The photocatalytic degradation of rhodamine B, tetramethylammonium chloride, and 4-chlorophenol on UV-illuminated pure brookite were investigated and compared with those on anatase and rutile TiO₂. The present research explores the generation of OH radicals as main oxidants on brookite. In addition, tetramethylammonium, as a mobile OH radical indicator, was degraded over both pure anatase and brookite phases, but not rutile. The brookite phase showed much higher photoactivity among TiO₂ polymorphs, despite its smaller surface area compared with anatase. This result can be ascribed to the following properties of the brookite TiO₂ film: (i) the higher driving force with more negative flat-band potential, (ii) the efficient charge transfer kinetics with low resistance, and (iii) the generation of more hydroxyl radicals, including mobile OH radicals. The brookite-nanostructured TiO₂ electrode facilitates photocatalyst collection and recycling with excellent stability, and readily controls photocatalytic degradation rates with facile input of additional potential.