Photothermal Catalytic CO2 Conversion: Beyond Catalysis and Photocatalysis.

In recent years, the combination of both thermal and photochemical contributions has provided interesting opportunities for solar upgrading of catalytic processes. Photothermal catalysis works at the interface between purely photochemical processes, which involve the direct conversion of photon energy into chemical energy, and classical thermal catalysis, in which the catalyst is activated by temperature. Thus, photothermal catalysis acts in two different ways on the energy path of the reaction. This combined catalysis, of which the fundamental principles will be reviewed here, is particularly promising for the activation of small reactive molecules at moderate temperatures compared to thermal catalysis and with higher reaction rates than those attained in photocatalysis, and it has gained a great deal of attention in the last years. Among the different applications of photothermal catalysis, CO2 conversion is probably the most studied, although reaction mechanisms and photonic-thermal synergy pathways are still quite unclear and, from the reaction route point of view, it can be said that photothermal-catalytic CO2 reduction processes are still in their infancy. This article intends to provide an overview of the principles underpinning photothermal catalysis and its application to the conversion of CO2 into useful molecules, with application essentially as fuels but also as chemical building blocks. The most relevant specific cases published to date will be also reviewed from the viewpoint of selectivity towards the most frequent target products.

Improved Methane Production by Photocatalytic CO2 Conversion over Ag/In2O3/TiO2 Heterojunctions.

In this work, the role of In2O3 in a heterojunction with TiO2 is studied as a way of increasing the photocatalytic activity for gas-phase CO2 reduction using water as the electron donor and UV irradiation. Depending on the nature of the employed In2O3, different behaviors appear. Thus, with the high crystallite sizes of commercial In2O3, the activity is improved with respect to TiO2, with modest improvements in the selectivity to methane. On the other hand, when In2O3 obtained in the laboratory, with low crystallite size, is employed, there is a further change in selectivity toward CH4, even if the total conversion is lower than that obtained with TiO2. The selectivity improvement in the heterojunctions is attributed to an enhancement in the charge transfer and separation with the presence of In2O3, more pronounced when smaller particles are used as in the case of laboratory-made In2O3, as confirmed by time-resolved fluorescence measurements. Ternary systems formed by these heterojunctions with silver nanoparticles reflect a drastic change in selectivity toward methane, confirming the role of silver as an electron collector that favors the charge transfer to the reaction medium.

Irradiance-Controlled Photoassisted Synthesis of Sub-Nanometre Sized Ruthenium Nanoparticles as Co-Catalyst for TiO2 in Photocatalytic Reactions.

Photoassisted synthesis is as a highly appealing green procedure for controlled decoration of semiconductor catalysts with co-catalyst nanoparticles, which can be carried out without the concourse of elevated temperatures, external chemical reducing agents or applied bias potential and in a simple slurry reactor. The aim of this study is to evaluate the control that such a photoassisted method can exert on the properties of ruthenium nanoparticles supported on TiO2 by means of the variation of the incident irradiance and hence of the photodeposition rate. For that purpose, different Ru/TiO2 systems with the same metal load have been prepared under varying irradiance and characterized by means of elemental analysis, transmission electron microscopy and X-ray photoelectron spectroscopy. The photocatalytic activity of the so-obtained materials has been evaluated by using the degradation of formic acid in water under UV-A light. Particles with size around or below one nanometer were obtained, depending on the irradiance employed in the synthesis, with narrow size distribution and homogeneous dispersion over the titania support. The relation between neutral and positive oxidation states of ruthenium could also be controlled by the variation of the irradiance. The obtained photocatalytic activities for formic acid oxidation were in all cases higher than that of undecorated titania, with the sample obtained with the lowest irradiation giving rise to the highest oxidation rate. According to the catalysts characterization, photocatalytic activity is influenced by both Ru size and Ru0/Ruδ+ ratio.

Synergism in TiO2 photocatalytic ozonation for the removal of dichloroacetic acid and thiacloprid.

The synergistic effect of the photocatalytic ozonation process (PH-OZ) using the photocatalyst TiO2 is usually attributed to influences of the physicochemical properties of the catalyst, pollutant type, pH, temperature, O3 concentration, and other factors. It is also often claimed that good adsorption on the TiO2 surface is beneficial for the occurrence of synergism. Herein, we tested these assumptions by using five different commercial TiO2 photocatalysts (P25, PC500, PC100, PC10 and JRC-TiO-6) in three advanced oxidation systems - photocatalysis (O2/TiO2/UV), catalytic ozonation (O3/TiO2) and PH-OZ (O3/TiO2/UV) - for the degradation of two pollutants (dichloroacetic acid - DCAA and thiacloprid) simultaneously present in water. The synergistic effect in PH-OZ was much more pronounced in the case of thiacloprid, a molecule with low adsorption on the surface of the catalyst - in contrast to DCAA with stronger adsorption. The faster kinetics of catalytic ozonation (O3/TiO2) correlated with the higher exposed surface area of TiO2 agglomerates, independent of the (lower) BET surfaces of the primary particles. Nevertheless, DCAA mineralization on the TiO2 surface was much faster than thiacloprid degradation in solution. Therefore, we propose that a high BET surface area of the photocatalyst is crucial for fast surface reactions (DCAA mineralization), while good dispersion - the high exposed surface area of the (small) agglomerates - and charge separation play an important role in photocatalytic degradation or PH-OZ of less adsorbed organic pollutants (thiacloprid).

Ti-Modified LaFeO3/ß-SiC Alveolar Foams as Immobilized Dual Catalysts with Combined Photo-Fenton and Photocatalytic Activity.

Ti-modified LaFeO3/ß-SiC alveolar foams were used as immobilized, highly robust dual catalysts with combined photocatalytic wet peroxide oxidation and photocatalytic activity under UV-A light. They were prepared by incipient wetness impregnation of a ß-SiC foam support, by implementing a sol-gel Pechini synthesis at the foam surface in the presence of dried amorphous sol-gel titania as a titanium source. The physicochemical and catalytic features suggest the stabilization at the foam surface of a substituted La1-xTixFeO3 catalyst analogous to its powdery counterpart. Taking 4-chlorophenol removal in water as a model reaction, its dual nature enables both high reaction rates and full total organic carbon (TOC) conversion because of a synergy effect, while its macroscopic structure overcomes the drawback of working with powdery catalysts. Further, it yields photonic efficiencies for degradation and mineralization of ca. 9.4 and 38%, respectively, that strongly outperform those obtained with a reference TiO2 P25/ß-SiC foam photocatalyst. The enhancement of the catalyst robustness upon Ti modification prevents any Fe leaching to the solution, and therefore, the optimized macroscopic foam catalyst with 10 wt % catalyst loading operates through pure heterogeneous surface reactions, without any activity loss during reusability test cycles.

Activity enhancement pathways in LaFeO3@TiO2 heterojunction photocatalysts for visible and solar light driven degradation of myclobutanil pesticide in water.

LaFeO3@TiO2 heterojunction composites with a core-shell porous structure and LaFeO3 contents in the 2.5-25 wt.% range have been synthesized via consecutive sol-gel syntheses and tested for the photocatalytic oxidation of the myclobutanil pesticide in water under solar light and pure visible light. Whatever the light spectrum, the kinetic rate constants for both myclobutanil degradation and TOC conversion exhibited a volcano-like profile with increasing the narrow band-gap (2.1 eV) LaFeO3 content, the optimum composite strongly overperforming both single phases, with full myclobutanil mineralization achieved in 240 min in the best case. The light spectrum influenced the optimum LaFeO3 content in the composite, being observed at 5 wt.% and 12.5 wt.% under solar and visible light, respectively. This has been attributed to the existence of different light-mediated reaction mechanisms. The optimum LaFeO3/TiO2 composite photocatalyst was active and stable after several runs under solar light with leached iron concentration below 0.1 mg/L in solution.

Recent Achievements in Development of TiO2-Based Composite Photocatalytic Materials for Solar Driven Water Purification and Water Splitting.

Clean water and the increased use of renewable energy are considered to be two of the main goals in the effort to achieve a sustainable living environment. The fulfillment of these goals may include the use of solar-driven photocatalytic processes that are found to be quite effective in water purification, as well as hydrogen generation. H2 production by water splitting and photocatalytic degradation of organic pollutants in water both rely on the formation of electron/hole (e-/h+) pairs at a semiconducting material upon its excitation by light with sufficient photon energy. Most of the photocatalytic studies involve the use of TiO2 and well-suited model compounds, either as sacrificial agents or pollutants. However, the wider application of this technology requires the harvesting of a broader spectrum of solar irradiation and the suppression of the recombination of photogenerated charge carriers. These limitations can be overcome by the use of different strategies, among which the focus is put on the creation of heterojunctions with another narrow bandgap semiconductor, which can provide high response in the visible light region. In this review paper, we report the most recent advances in the application of TiO2 based heterojunction (semiconductor-semiconductor) composites for photocatalytic water treatment and water splitting. This review article is subdivided into two major parts, namely Photocatalytic water treatment and Photocatalytic water splitting, to give a thorough examination of all achieved progress. The first part provides an overview on photocatalytic degradation mechanism principles, followed by the most recent applications for photocatalytic degradation and mineralization of contaminants of emerging concern (CEC), such as pharmaceuticals and pesticides with a critical insight into removal mechanism, while the second part focuses on fabrication of TiO2-based heterojunctions with carbon-based materials, transition metal oxides, transition metal chalcogenides, and multiple composites that were made of three or more semiconductor materials for photocatalytic water splitting.

TiO2 Nanocolumn Arrays for More Efficient and Stable Perovskite Solar Cells.

Organic-inorganic hybrid perovskite solar cells have attracted much attention due to their high power conversion efficiency (>25%) and low-cost fabrication. Yet, improvements are still needed for more stable and higher-performing solar cells. In this work, a series of TiO2 nanocolumn photonic structures have been intentionally fabricated on half of the compact TiO2-coated fluorine-doped tin oxide substrate by glancing angle deposition with magnetron sputtering, a method particularly suitable for industrial applications due to its high reliability and reduced cost when coating large areas. These vertically aligned nanocolumn arrays were then applied as the electron transport layer into triple-cation lead halide perovskite solar cells based on Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3. By comparison to solar cells built onto the same substrate without nanocolumns, the use of TiO2 nanocolumns can significantly enhance the power conversion efficiency of the perovskite solar cells by 7% and prolong their shelf life. Here, detailed characterizations on the morphology and the spectroscopic aspects of the nanocolumns, their near-field and far-field optical properties, solar cells characteristics, as well as the charge transport properties provide mechanistic insights on how one-dimensional TiO2 nanocolumns affect the performance of perovskite halide solar cells in terms of charge transport, light harvesting, and stability, knowledge necessary for the future design of higher-performing and more stable perovskite solar cells.

Ferrite Materials for Photoassisted Environmental and Solar Fuels Applications.

Ferrites are a large class of oxides containing Fe3+ and at least another metal cation that have been investigated for and applied to a wide variety of fields ranging from mature technologies like circuitry, permanent magnets, magnetic recording and microwave devices to the most recent developments in areas like bioimaging, gas sensing and photocatalysis. In the last respect, although ferrites have been less studied than other types of semiconductors, they present interesting properties such as visible light absorption, tuneable optoelectronic properties and high chemical and photochemical stability. The versatility of their chemical composition and of their crystallographic structure opened a playground for developing new catalysts with enhanced efficiency. This article reviews the recent development of the application of ferrites to photoassisted processes for environmental remediation and for the synthesis of solar fuels. Applications in the photocatalytic degradation of pollutants in water and air, photo-Fenton, and solar fuels production, via photocatalytic and photoelectrochemical water splitting and CO2 reduction, are reviewed paying special attention to the relationships between the physico-chemical characteristics of the ferrite materials and their photoactivated performance.

Influence of Post-Synthesis Modifications of Ti1-xZrxO2 Nanocrystallites on Their Photocatalytic Activity for Toluene and Methylcyclohexane Degradation.

The modification of the structural and surface characteristics of Ti1-xZrxO2 nanocrystallites by postsynthesis treatments is revealed as an effective way to enhance their photocatalytic activity. Starting with the same batch of mixed oxide prepared from reverse microemulsions, different photocatalysts have been obtained by either solvothermal treatment, calcination or a combination of both. Extensive physicochemical characterization of the resulting materials shows that solvothermally treated oxides present lower crystallite size and larger surface area, although without previous calcination these samples appear to have a higher degree of structural disorder. These differences are sharply reflected in the changes in photocatalytic activity for the removal of methylcyclohexane (MCH) and toluene vapours at relatively high concentrations. Thus, the best performance for MCH elimination is obtained with the photocatalysts prepared by calcination and subsequent solvothermal treatment. Following this procedure, the resulting Ti1-xZrxO2 material presents larger surface area and high Zr surface concentration with minimal disturbance of the anatase structure. In contrast, for toluene photooxidation, the solvothermally prepared sample shows improved performance, most likely due to its larger surface area, which contributes to hinder deactivation.

Evaluation of photoassisted treatments for norfloxacin removal in water using mesoporous Fe2O3-TiO2 materials.

We report the synthesis of mesoporous TiO2 and mesoporous Fe2O3-TiO2 catalysts by using a structure-directing-surfactant method, their characterization and their employment as photocatalysts for norfloxacin degradation in aqueous solution. The main findings show that in the presence of both O2 and H2O2, Fe-containing mesoporous titania (Fe2O3-TiO2), with iron percentages between 1 and 3 wt%, exhibited norfloxacin degradation rates more than 60% greater than otherwise identical mesoporous titania without iron. Furthermore, the activity of the mesoporous composite catalysts also exceeds that of titania when illuminated with 405 nm light-emitting diodes. Iron loading improved the photocatalytic activity for norfloxacin degradation with values of apparent reaction rate constants of 0.037 min-1 and 0.076 min-1 with 1 and 3 surface wt.% of iron, respectively. An optimum of activity was found with the 3 wt% Fe2O3-TiO2 catalyst. Under these conditions, 10 mg/L of norfloxacin is reacted essentially to completion and 90% of total organic carbon conversion was obtained within 120 min of reaction. This higher organic carbon conversion degree was reached due to the photo-oxidation of short-chain organic acids. The high activity of the as-synthesized mesoporous composites is attributed to the additional iron phase which led to the different reactions for H2O2 decomposition, but also due to the improvement in light absorbance. Finally, the activity of the most active catalyst was found to be stable over multiple sequential runs, which was related to a negligible amount of iron leaching (<0.1%) from these materials.

Mechanistic View of the Main Current Issues in Photocatalytic CO2 Reduction.

After 40 years of research on photocatalytic CO2 reduction, there are still many unknowns about its mechanistic aspects even for the most common TiO2-based photocatalytic systems. These uncertainties include the pathways inducing visible-light activity in wide-band gap semiconductors, the charge transfer between semiconductors and plasmonic metal nanoparticles, the unambiguous determination of the origin of C-bearing products, the very first step in the activation of the CO2 molecule, the factors determining the selectivity, the reasons for photocatalyst deactivation, the closure of the catalytic cycle by the hole-scavenging reagent, and the detailed reaction pathways and the most suitable techniques for their determination. This Perspective discusses these controversial issues based on the most relevant investigations reported so far. For that purpose, we have tried to view the complex CO2 reduction in a holistic manner, considering today's state-of-the-art approaches, strategies, and techniques for the study of one of the hottest topics in energy research.

Unravelling the effect of charge dynamics at the plasmonic metal/semiconductor interface for CO2 photoreduction.

Sunlight plays a critical role in the development of emerging sustainable energy conversion and storage technologies. Light-induced CO2 reduction by artificial photosynthesis is one of the cornerstones to produce renewable fuels and environmentally friendly chemicals. Interface interactions between plasmonic metal nanoparticles and semiconductors exhibit improved photoactivities under a wide range of the solar spectrum. However, the photo-induced charge transfer processes and their influence on photocatalysis with these materials are still under debate, mainly due to the complexity of the involved routes occurring at different timescales. Here, we use a combination of advanced in situ and time-resolved spectroscopies covering different timescales, combined with theoretical calculations, to unravel the overall mechanism of photocatalytic CO2 reduction by Ag/TiO2 catalysts. Our findings provide evidence of the key factors determining the enhancement of photoactivity under ultraviolet and visible irradiation, which have important implications for the design of solar energy conversion materials.

Ga-Promoted Photocatalytic H2 Production over Pt/ZnO Nanostructures.

Photocatalytic H2 generation is investigated over a series of Ga-modified ZnO photocatalysts that were prepared by hydrothermal methods. It is found that the structural, textural, and optoelectronic properties remarkably depend on the Ga content. The photocatalytic activity is higher in samples with Ga content equal to or lower than 5.4 wt %, which are constituted by Zn1-xGaxO phases. Structural, textural, and optoelectronic characterization, combined with theoretical calculations, reveals the effect of Ga in the doped ZnO structures. Higher Ga incorporation leads to the formation of an additional ZnGa2O4 phase with spinel structure. The presence of such a phase is detrimental for the textural and optoelectronic properties of the photocatalysts, leading to a decrease in H2 production. When Pt is used as the cocatalyst, there is an increase of 1 order of magnitude in the activity with respect to the bare photocatalysts. This is a result of Pt acting as an electron scavenger, decreasing the electron-hole recombination rate and boosting the H2 evolution reaction.

Highly active photocatalytic coatings prepared by a low-temperature method.

Photocatalytic properties of titanium (IV) oxide (TiO2) in anatase form can be used for various purposes, including photocatalytic purification of water. For such an application, suspended or fixed photocatalytic reactors are used. Those with fixed phase seem to be preferred due to some advantages, one of which is the avoidance of photocatalyst filtration. To avoid leaching and exfoliation of the fixed phase, an immobilization procedure leading to a good adhesion of a catalyst to a substrate is crucial. Within this work, we present physical and photocatalytic characterization results of five commercially available TiO2 photocatalysts (P25, P90, PC500, KRONOClean 7000, VPC-10) and one pigment (Hombitan LO-CR-S-M), which were successfully immobilized on glass slides by a "sol suspension" procedure. Different mechanical tests and characterization methods were used to evaluate the stability and morphology of the layers. Evaluation of photocatalytic activity was done by tests under UVA and UV-vis irradiation, using a method based on the detection of the fluorescent oxidation product of terephthalic acid (TPA), i.e., hydroxyterephthalic acid (HTPA). Aeroxide® P90 incorporated into the silica-titania binder was the most photocatalytically active layer and, unlike the others, showed significant increase of photocatalytic activity through the entire range of tested UVA irradiation intensities (2.3 mW/cm(2)-6.1 mW/cm(2)). The high mechanical stability of some photocatalytic layers allows using them in water photocatalytic purification reactions.

Synthesis of BiVO4/TiO2 composites and evaluation of their photocatalytic activity under indoor illumination.

BiVO4/TiO2 composites with different weight ratios have been prepared by coprecipitation-based reactions followed by either thermal or hydrothermal treatment with the aim of evaluating the TiO2 photosensitization by BiVO4. The obtained materials present in all cases the desired monoclinic phase of BiVO4 and anatase phase of TiO2. Visible light absorption increased with increasing amount of bismuth vanadate. XPS results reveal the surface enrichment of Ti with respect to the bulk composition in samples characterised by a higher content of BiVO4. The photocatalytic activity of the prepared materials was tested for the degradation of isopropanol in the gas phase under indoor illumination conditions. Although none of the composites was able to improve the activity of TiO2, the low BiVO4 containing samples appear as more suitable for further synthesis tuning.

Surface functionalization of nanostructured Fe2O3 polymorphs: from design to light-activated applications.

Nanostructured iron(III) oxide deposits are grown by chemical vapor deposition (CVD) at 400-500 °C on Si(100) substrates from Fe(hfa)2TMEDA (hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate; TMEDA = N,N,N',N'-tetramethylethylenediamine), yielding the selective formation of α-Fe2O3 or the scarcely studied ε-Fe2O3 polymorphs under suitably optimized preparative conditions. By using Ti(OPr(i))4 (OPr(i) = iso-propoxy) and water as atomic layer deposition (ALD) precursors, we subsequently functionalized the obtained materials at moderate temperatures (<300 °C) by an ultrathin titanomagnetite (Fe3-xTixO4) overlayer. An extensive multitechnique characterization, aimed at elucidating the system structure, morphology, composition and optical properties, evidenced that the photoactivated hydrophilic and photocatalytic behavior of the synthesized materials is dependent both on iron oxide phase composition and ALD surface modification. The proposed CVD/ALD hybrid synthetic approach candidates itself as a powerful tool for a variety of applications where semiconductor-based nanoarchitectures can benefit from the coupling with an ad hoc surface layer.

V-doped SnS2: a new intermediate band material for a better use of the solar spectrum.

Intermediate band materials can boost photovoltaic efficiency through an increase in photocurrent without photovoltage degradation thanks to the use of two sub-bandgap photons to achieve a full electronic transition from the valence band to the conduction band of a semiconductor structure. After having reported in previous works several transition metal-substituted semiconductors as able to achieve the electronic structure needed for this scheme, we propose at present carrying out this substitution in sulfides that have bandgaps of around 2.0 eV and containing octahedrally coordinated cations such as In or Sn. Specifically, the electronic structure of layered SnS(2) with Sn partially substituted by vanadium is examined here with first principles quantum methods and seen to give favourable characteristics in this respect. The synthesis of this material in nanocrystalline powder form is then undertaken and achieved using solvothermal chemical methods. The insertion of vanadium in SnS(2) is found to produce an absorption spectrum in the UV-Vis-NIR range that displays a new sub-bandgap feature in agreement with the quantum calculations. A photocatalytic reaction-based test verifies that this sub-bandgap absorption produces highly mobile electrons and holes in the material that may be used for the solar energy conversion, giving experimental support to the quantum calculations predictions.