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

Recent Advances in the Design and Photocatalytic Enhanced Performance of Gold Plasmonic Nanostructures Decorated with Non-Titania Based Semiconductor Hetero-Nanoarchitectures

Plasmonic photocatalysts combining metallic nanoparticles and semiconductors have been aimed as versatile alternatives to drive light-assisted catalytic chemical reactions beyond the ultraviolet (UV) regions, and overcome one of the major drawbacks of the most exploited photocatalysts (TiO2 or ZnO). The strong size and morphology dependence of metallic nanostructures to tune their visible to near-infrared (vis-NIR) light harvesting capabilities has been combined with the design of a wide variety of architectures for the semiconductor supports to promote the selective activity of specific crystallographic facets. The search for efficient heterojunctions has been subjected to numerous studies, especially those involving gold nanostructures and titania semiconductors. In the present review, we paid special attention to the most recent advances in the design of gold-semiconductor hetero-nanostructures including emerging metal oxides such as cerium oxide or copper oxide (CeO2 or Cu2O) or metal chalcogenides such as copper sulfide or cadmium sulfides (CuS or CdS). These alternative hybrid materials were thoroughly built in past years to target research fields of strong impact, such as solar energy conversion, water splitting, environmental chemistry, or nanomedicine. Herein, we evaluate the influence of tuning the morphologies of the plasmonic gold nanostructures or the semiconductor interacting structures, and how these variations in geometry, either individual or combined, have a significant influence on the final photocatalytic performance.

A Mixed-Metal Porphyrinic Framework Promoting Gas-Phase CO2 Photoreduction without Organic Sacrificial Agents

A photoactive porphyrinic metal-organic framework (MOF) has been prepared by exchanging Ti into a Zr-based MOF precursor. The resultant mixed-metal Ti/Zr porphyrinic MOF demonstrates much-improved efficiency for gas-phase CO₂ photoreduction into CH₄ and CO under visible-light irradiation using water vapor compared to the parent Zr-MOF. Insightful studies have been conducted to probe the photocatalysis processes. This work provides the first example of gas-phase CO₂ photoreduction into methane without organic sacrificial agents on a MOF platform, thereby paving an avenue for developing MOF-based photocatalysts for application in CO₂ photoreduction and other types of photoreactions.

Statistical parameter optimization and modeling of photodegradation of methyl orange using a composite photocatalyst prepared by thermal synthesis

Doping a transition metal into photocatalysts enhances the photocatalytic activity drastically. In the first part of this study, Taguchi design of experiment is applied to evaluate and optimize the efficiency of the Fe₂O₃/TiO₂ photocatalyst synthetized by thermal method assisted by UV radiation. The contribution percentages of Fe:TiO₂ mass ratio, Fe₂O₃/TiO₂ dosage, and pH on the total organic carbon (TOC) removal are determined using analysis of variance (ANOVA). In the second part of this study, in order to model the photocatalytic degradation process, the optical properties of the photocatalyst, including the extinction, absorption, and scattering coefficients, are determined. Subsequently, the radiation transfer equation (RTE) is solved numerically based on the surface emission model using the discrete ordinate method. Furthermore, a rigorous model, including chemical reaction rates, radiation transfer, and mass transfer is proposed and validated by a set of experimental data. A satisfactory correlation between the predicted and experimental data with less than 5% error confirms the reliability of the model. The intrinsic kinetic parameters are also determined by comparing predicted values to those of the experimental results by applying non-linear regressions.

Integration of machine learning-based prediction for enhanced Model's generalization: Application in photocatalytic polishing of palm oil mill effluent (POME)

In predicting palm oil mill effluent (POME) degradation efficiency, previous developed quadratic model quantitatively evaluated the effects of O₂ flowrate, TiO₂ loadings and initial concentration of POME in labscale photocatalytic system, which however suffered from low generalization due to the overfitting behaviour. Evidently, high RMSE (131.61) and low R₂ (-630.49) obtained indicates its insufficiency in describing POME degradation at unseen factor ranges, hence verified the fact of poor generalization. To overcome this issue, several models were developed via machine learning-assisted techniques, namely Gaussian Process Regression (GPR), Linear Regression (LR), Decision Tree (DT), Supported Vector Machine (SVM) and Regression Tree Ensemble (RTE), subsequently being assessed systematically. To achieve high generalization, all models were subjected to 'train-all-test-all' strategy, 5-fold and 10-fold cross validation. Specifically, GPR model was furnished with high accuracy in 'train-all-test-all' strategy, judging from its low RMSE (1.0394) and high R₂ (0.9962), which however menaced by the risk of overfitting. In contrast, despite relatively poorer RMSE and R₂ (1.7964 and 0.9886) obtained in 5-fold cross validation, GPR model was rendered with highest generalization, while sufficiently preserving its accuracy in development process. Besides, SVM and RTE models were also demonstrated promising R₂ (0.9372 and 0.9208), which however shadowed by their high RMSEs (4.2174 and 4.7366). Furthermore, the extraordinary generalization of GPR model was coincidentally verified in 10-fold cross validation. The lowest RMSE (2.1624) and highest R₂ (0.9835) obtained with feature number of 36 asserted its sufficiency in both generalization and accuracy prospect. Other models were all rendered with slight lower R₂ (> 0.9), plausibly due to the higher RMSE (> 4.0). According to GPR model, optimized POME degradation (52.52%) can be obtained at 70 mL/min of O₂, 70.0 g/L of TiO₂ and 250 ppm of POME concentration, with only ∼3% error as compared to the actual data.

Visible-Light Photocatalysts and Their Perspectives for Building Photocatalytic Membrane Reactors for Various Liquid Phase Chemical Conversions

Photocatalytic organic synthesis/conversions and water treatment under visible light are a challenging task to use renewable energy in chemical transformations. In this review a brief overview on the mainly employed visible light photocatalysts and a discussion on the problems and advantages of Vis-light versus UV-light irradiation is reported. Visible light photocatalysts in the photocatalytic conversion of CO2, conversion of acetophenone to phenylethanol, hydrogenation of nitro compounds, oxidation of cyclohexane, synthesis of vanillin and phenol, as well as hydrogen production and water treatment are discussed. Some applications of these photocatalysts in photocatalytic membrane reactors (PMRs) for carrying out organic synthesis, conversion and/or degradation of organic pollutants are reported. The described cases show that PMRs represent a promising green technology that could shift on applications of industrial interest using visible light (from Sun) active photocatalysts.

Atomic Layer Deposition of Mixed-Layered Aurivillius Phase on TiO2 Nanotubes: Synthesis, Characterization and Photoelectrocatalytic Properties

For the first time, one-dimensional phase-modulated structures consisting of two different layered Aurivillius phases with alternating five and six perovskite-like layers were obtained by atomic layer deposition (ALD) on the surface of TiO2 nanotubes (Nt). It was shown that the use of vertically oriented TiO2 Nt as the substrate and the ALD technology of a two-layer Bi2O3-FeOx sandwich-structure make it possible to obtain a layered structure due to self-organization during annealing. A detailed study by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed that the coating is conformal. Raman spectroscopic analysis indicated the structure of the layered Aurivillius phases. Transient photocurrent responses under Ultraviolet-Visible (UV-Vis) light irradiation show that the ALD coating benefits the efficiency of photon excitation of electrons. The results of the photoelectrocatalytic experiments (PEC) with methyl orange degradation as a model demonstrate the significant potential of the synthesized structure as a photocatalyst. Photoluminescent measurement showed a decrease in the probability of recombination of photogenerated electron-hole pairs for ALD-coated TiO2 Nt, which demonstrates the high potential of these structures for use in photocatalytic and photoelectrochemical applications.

Photocatalytic CO2 Reduction and Electrocatalytic H2 Evolution over Pt(0,II,IV)-Loaded Oxidized Ti Sheets

Energy recycling and production using abundant atmospheric CO₂ and H₂O have increasingly attracted attention for solving energy and environmental problems. Herein, Pt-loaded Ti sheets were prepared by sputter-deposition and Pt⁴⁺-reduction methods, and their catalytic activities on both photocatalytic CO₂ reduction and electrochemical hydrogen evolution were fully demonstrated. The surface chemical states were completely examined by X-ray photoelectron spectroscopy before and after CO₂ reduction. Gas chromatography confirmed that CO, CH₄, and CH₃OH were commonly produced as CO₂ reduction products with total yields up to 87.3, 26.9, and 88.0 μmol/mol, respectively for 700 °C-annealed Ti under UVC irradiation for 13 h. Pt-loading commonly negated the CO₂ reduction yields, but CH₄ selectivity was increased. Electrochemical hydrogen evolution reaction (HER) activity showed the highest activity for sputter-deposited Pt on 400 °C-annealed Ti with a HER current density of 10.5 mA/cm² at −0.5 V (vs. Ag/AgCl). The activities of CO₂ reduction and HER were found to be significantly dependent on both the nature of Ti support and the oxidation states (0,II,IV) of overlayer Pt. The present result could provide valuable information for designing efficient Pt/Ti-based CO₂ recycle photocatalysts and electrochemical hydrogen production catalysts.

Unique S-scheme heterojunctions in self-assembled TiO2/CsPbBr3 hybrids for CO2 photoreduction

Exploring photocatalysts to promote CO2 photoreduction into solar fuels is of great significance. We develop TiO2/perovskite (CsPbBr3) S-scheme heterojunctions synthesized by a facile electrostatic-driven self-assembling approach. Density functional theory calculation combined with experimental studies proves the electron transfer from CsPbBr3 quantum dots (QDs) to TiO2, resulting in the construction of internal electric field (IEF) directing from CsPbBr3 to TiO2 upon hybridization. The IEF drives the photoexcited electrons in TiO2 to CsPbBr3 upon light irradiation as revealed by in-situ X-ray photoelectron spectroscopy analysis, suggesting the formation of an S-scheme heterojunction in the TiO2/CsPbBr3 nanohybrids which greatly promotes the separation of electron-hole pairs to foster efficient CO2 photoreduction. The hybrid nanofibers unveil a higher CO2-reduction rate (9.02 μmol g-1 h-1) comparing with pristine TiO2 nanofibers (4.68 μmol g-1 h-1). Isotope (13CO2) tracer results confirm that the reduction products originate from CO2 source.

Three-Dimensional Grids of Optimized Ti-Compounds on Si for Ultra-Wideband Optical Absorption

Typically, the optical applications of silicon (Si) are limited to wavelengths below ∼1100 nm. However, there is significant research on Si surface modification, which tries to extend the optical properties of Si further into the infrared (IR) region. In this work, we present an ultra-wideband complementary metal-oxide-semiconductor (CMOS)-biocompatible Si-based optical absorber with a hydrophobic surface. It consists of patterned three-dimensional grid-like structures of optimized compounds of titanium (Ti) on n-type Si (n-Si). Here, the Ti-compounds on Si were formed by subsequent deposition of patterned Ti and annealing. Moreover, we have shown that there are two possible Ti-compounds formed on Si, depending on the thickness of Ti deposited and the annealing time. The composition and the corresponding absorbance spectra for the two possibilities of Ti-compounds on n-Si, that is, Ti-O/Ti-O-Si/Ti-Si/n-Si (type 1) and Ti-O/Ti-O-Si/n-Si (type 2), were confirmed using an X-ray photoelectron spectroscopy depth profiler and ultraviolet-visible-near-infrared spectrometer. We also illustrate how type 1 improves the absorption of radiation in the IR region. Further, we experimentally demonstrate that our fabricated absorber has an average reflectance (R) of <25% and an average absorbance of approximately 60% for wavelengths ranging from 200 to 3300 nm. The average % R for wavelengths from 400 to 1500 nm is <10%. The surface hydrophobicity for the fabricated absorbers was confirmed using a water contact angle (WCA) measurement system with WCAs >100°, which makes the surface hydrophobic.

Recent Advances in Functional Polyurethane and Its Application in Leather Manufacture: A Review

Over last few years, polyurethane (PU) has been applied in a number of areas because of its remarkable features, such as excellent mechanical strength, good abrasion resistance, toughness, low temperature flexibility, etc. More specifically, PU can be easily "tailor made" to meet specific demands. This structure-property relationship endows great potential for use in wider applications. With the improvement of living standards, ordinary polyurethane products cannot meet people's growing needs for comfort, quality, and novelty. This has recently drawn enormous commercial and academic attention to the development of functional polyurethane. Among the major applications, PU is one of the prominent retanning agents and coating materials in leather manufacturing. This review gives a summary of academic study in the field of functional PU as well as its recent application in leather manufacture.

Solution-Processed Sb2Se3 on TiO2 Thin Films Toward Oxidation- and Moisture-Resistant, Self-Powered Photodetectors

Semiconductor-sensitized TiO₂ thin films with long-term air stability are attractive for optoelectronic devices and applications. Herein, we demonstrate the potential of the TiO₂ thin film (∼800 nm in thickness) sensitized with a Sb₂Se₃ layer (∼350 nm) grown from solution spin coating and processed by annealing recrystallization at 300 °C for high-performance optical detection. The type-II band alignment, p-Sb₂Se₃/n-TiO₂ heterojunction, and narrow band gap of Sb₂Se₃ (∼1.25 eV) endow the film photodetector with a large photocurrent, high switching stability and on/off ratio (>10³), and fast response speeds (<20 ms) under the broadband visible-near-infrared irradiation in a zero-bias self-powered photovoltaic mode. In particular, the photodetector shows notable resistance to oxidation and moisture for long-term operation, which is linked to the modest surface oxidation (Sb-O) of Sb₂Se₃, as verified by X-ray photoelectron spectroscopy. The first-principles calculations show that a low and medium concentration of oxygen substitution for Se (O_Se) and oxygen interstitial (O_i) with negative formation energies can lead to such a moderate surface oxidation but do not generate impurity states or just introduce a shallow-level acceptor state in the electronic structures of Sb₂Se₃ without degrading its optoelectronic performance. Our theoretical results offer a rational explanation for the air-stable and oxidation/moisture-resistant characteristics in moderately oxidized Sb₂Se₃ and may shed light on the surface oxidation-property relationship studies of other nonoxide semiconductor-sensitized devices.

A Blinking Mesoporous TiO2-x Composed of Nanosized Anatase with Unusually Long-Lived Trapped Charge Carriers

A mesoporous TiO_2-x material comprised of small, crystalline, vacancy-rich anatase nanoparticles (NPs) shows unique optical, thermal, and electronic properties. It is synthesized using polymer-derived mesoporous carbon (PDMC) as a template. The PDMC pores serve as physical barriers during the condensation and pyrolysis of a titania precursor, preventing the titania NPs from growing beyond 10 nm in size. Unlike most titania nanomaterials, during pyrolysis the NPs undergo no transition from the anatase to rutile phase and they become catalytically active reduced TiO_2-x . When exposed to a slow electron beam, the NPs exhibit a charge/discharge behavior, lighting up and fading away for an average period of 15 s for an extended period of time. The NPs also show a 50 nm red-shift in their UV/Vis absorption and long-lived charge carriers (electrons and holes) at room temperature in the dark, even long after UV irradiation. The NPs as photocatalysts show a good activity for CO₂ reduction.

Photodegradation of ibuprofen and four other pharmaceutical pollutants on natural pigments sensitized TiO2 nanoparticles

Pharmaceuticals and personal care products (PPCPs) in water system have drawn increasing concerns in recent years. TiO₂ -based photodegradation has shown great potential as a low-cost and sustainable technology in water treatment, however, can only use the UV light range of solar radiation which makes the system less efficient. Dyes have been studied to improve the TiO₂ system light-harvesting range, but studies on environmental friendly natural dyes are rare. In this study, a screening method using UV-Vis spectra analysis was carried out on a group of 22 different tropical natural plants for the potential applications on dye-sensitized TiO₂ in PPCP treatment. As a result, Begonia "Martin's Mystery" significantly increased TiO₂ photodegradation efficiency toward ibuprofen treatments which is first time reported in literature as our best knowledge. Moreover, the promising discovery of Begonia application in ibuprofen treatment has been successfully applied to warfarin and famotidine treatment. Similar results were expanded to many other Begonia species which indicate that Begonia extracts could be excellent sensitizers for TiO₂ -based photodegradation of PPCPs. Our discovery suggested that the screening process may potentially open a brand-new way for future TiO₂ photodegradation studies before the complex and time-consuming detailed mechanism studies. PRACTITIONER POINTS.: Natural dyes were screened as sensitizers for TiO₂ photodegradation of ibuprofen. Ibuprofen photodegradation efficiency was increased twice using Begonia "Martin's Mystery." The Begonia applications were extended to warfarin, trimethoprim, and famotidine. Promising results were also observed using five other Begonia species.

Enhanced visible light photocatalytic performance with metal-doped Bi2WO6 for typical fluoroquinolones degradation: Efficiencies, pathways and mechanisms

To clarify the photocatalytic mechanisms of metal-doped Bi₂WO₆ for fluoroquinolones (FQs) degradation, the effects of the chemical characteristics of four metals, molar ratios of the doped metals, morphology of the catalysts, and electrostatic interactions on the degradation of norfloxacin (NOR) and ciprofloxacin (CIP) were evaluated under visible light irradiation. Experimental results implied that the doping of Mg²⁺, Fe³⁺, Zn²⁺ and Cu²⁺ dramatically improved the photodegradation of Bi₂WO₆ for NOR and CIP removal, in which 1% Mg/Bi₂WO₆ exhibited the highest degradation rate of 89.44% for NOR and 99.11% for CIP. Photodegradation of NOR fitted to the pseudo-first-order model (k₁ value of 0.02576 min^-1), whereas that of CIP be better described by pseudo-second-order model. Moreover, the two FQs photodegradation pathways and the possible intermediates were summarized. The mechanisms of the metal dopants for the enhancement of photocatalytic activity were attributed to its enhanced specific surface area, electrostatic absorption, as well as the significant photogeneration of ·O₂^- and h⁺. Also, the photocatalyst exhibited a high stability with 78.5% photocatalytic performance after four cycles of repeated usage.

Light-driven catalysis with engineered enzymes and biomimetic systems

Efforts to drive catalytic reactions with light, inspired by natural processes like photosynthesis, have a long history and have seen significant recent growth. Successfully engineering systems using biomolecular and bioinspired catalysts to carry out light-driven chemical reactions capitalizes on advantages offered from the fields of biocatalysis and photocatalysis. In particular, driving reactions under mild conditions and in water, in which enzymes are operative, using sunlight as a renewable energy source yield environmentally friendly systems. Furthermore, using enzymes and bioinspired systems can take advantage of the high efficiency and specificity of biocatalysts. There are many challenges to overcome to fully capitalize on the potential of light-driven biocatalysis. In this mini-review, we discuss examples of enzymes and engineered biomolecular catalysts that are activated via electron transfer from a photosensitizer in a photocatalytic system. We place an emphasis on selected forefront chemical reactions of high interest, including CH oxidation, proton reduction, water oxidation, CO2 reduction, and N2 reduction.

PMMA-TiO2 Fibers for the Photocatalytic Degradation of Water Pollutants

Titanium dioxide (TiO2) is a promising photocatalyst that possesses a redox potential suitable for environmental remediation applications. A low photocatalytic yield and high cost have thus far limited the commercial adoption of TiO2-based fixed-bed reactors. One solution is to engineer the physical geometry or chemical composition of the substrate to overcome these limitations. In this work, porous polymethyl methacrylate (PMMA) substrates with immobilized TiO2 nanoparticles in fiber forms were fabricated and analyzed to demonstrate the influence of contaminant transport and light accessibility on the overall photocatalytic performance. The influences of (i) fiber porosity and (ii) fiber architecture on the overall photocatalytic performance were investigated. The porous structure was fabricated using wet phase inversion. The core-shell-structured fibers exhibited much higher mechanical properties than the porous fibers (7.52 GPa vs. non-testability) and maintained the same degradation rates as porous structures (0.059 vs. 0.053/min) in removing methylene blue with comparable specific surface areas. The highest methylene blue (MB) degradation rate (kMB) of 0.116 min-1 was observed due to increases of the exposed surface area, pointing to more efficient photocatalysis by optimizing core-shell dimensions. This research provides an easy-to-manufacture and cost-efficient method for producing PMMA/TiO2 core-shell fibers with a broad application in water treatment, air purification, and volatile sensors.

Enhanced photocatalytic efficiency of layered CdS/CdSe heterostructures: Insights from first principles electronic structure calculations

Metal sulfides are emerging as an important class of materials for photocatalytic applications, because of their high photo responsive nature in the wide visible light range. In this class of materials, CdS with a direct band gap of 2.4 eV, has gained special attention due to the relative position of its conduction band minimum, which is very close to the energies of the reduced protons. However, the photogenerated holes in the valence band of CdS are prone to oxidation and destroy its structure during photocatalysis. Thus constructing a CdS based heterostructure would be an effective strategy for improving the photocatalytic performance. In this work we have done a detail theoretical investigation based on hybrid density functional theory calculation to get insight into the energy band structure, mobility and charge transfer across the CdS/CdSe heterojunction. The results indicate that CdS/CdSe forms type-II heterostructure that has several advantages in improving the photocatalytic efficiency under visible light irradiation.

Recent Progresses on Metal Halide Perovskite-Based Material as Potential Photocatalyst

Recent years have witnessed an incredibly high interest in perovskite-based materials. Among this class, metal halide perovskites (MHPs) have attracted a lot of attention due to their easy preparation and excellent opto-electronic properties, showing a remarkably fast development in a few decades, particularly in solar light-driven applications. The high extinction coefficients, the optimal band gaps, the high photoluminescence quantum yields and the long electron–hole diffusion lengths make MHPs promising candidates in several technologies. Currently, the researchers have been focusing their attention on MHPs-based solar cells, light-emitting diodes, photodetectors, lasers, X-ray detectors and luminescent solar concentrators. In our review, we firstly present a brief introduction on the recent discoveries and on the remarkable properties of metal halide perovskites, followed by a summary of some of their more traditional and representative applications. In particular, the core of this work was to examine the recent progresses of MHPs-based materials in photocatalytic applications. We summarize some recent developments of hybrid organic–inorganic and all-inorganic MHPs, recently used as photocatalysts for hydrogen evolution, carbon dioxide reduction, organic contaminant degradation and organic synthesis. Finally, the main limitations and the future potential of this new generation of materials have been discussed.

Recent Advances in Solar-Driven Carbon Dioxide Conversion: Expectations versus Reality

Solar-driven carbon dioxide (CO₂) conversion to fuels and high-value chemicals can contribute to the better utilization of renewable energy sources. Photosynthetic (PS), photocatalytic (PC), photoelectrochemical (PEC), and photovoltaic plus electrochemical (PV+EC) approaches are intensively studied strategies. We aimed to compare the performance of these approaches using unified metrics and to highlight representative studies with outstanding performance in a given aspect. Most importantly, a statistical analysis was carried out to compare the differences in activity, selectivity, and durability of the various approaches, and the underlying causes are discussed in detail. Several interesting trends were found: (i) Only the minority of the studies present comprehensive metrics. (ii) The CO₂ reduction products and their relative amount vary across the different approaches. (iii) Only the PV+EC approach is likely to lead to industrial technologies in the midterm future. Last, a brief perspective on new directions is given to stimulate discussion and future research activity.

Water Depollution and Photo-Detoxification by Means of TiO2: Fluoroquinolone Antibiotics as a Case Study

Photocatalysis by semiconductors is considered one of the most promising advanced oxidation processes (AOPs) and TiO2 is the most well-studied material for the removal of contaminants from the aquatic system. Over the last 20 years, pharmaceuticals have been the most investigated pollutants. They re-enter the environment almost unmodified or slightly metabolized, especially in the aquatic environment, since the traditional urban wastewater treatment plants (WWTPs) are not able to abate them. Due to their continuous input, persistence in the environment, and unpleasant effects even at low concentrations, drugs are considered contaminants of emerging concern (ECs). Among these, we chose fluoroquinolone (FQ) antibiotics as an environmental probe for assessing the role of TiO2 photocatalysis in the degradation of recalcitrant pollutants under environmental conditions and detoxification of surface waters and wastewaters. Due to their widespread diffusion, their presence in the list of the most persistent pollutants, and because they have been deeply investigated and their multiform photochemistry is well-known, they are able to supply rich information, both chemical and toxicological, on all key steps of the oxidative degradation process. The present review article explores, in a non-exhaustive way, the relationship among pollution, toxicity and remediation through titanium dioxide photocatalysis, with particular attention to the toxicological aspect. By using FQs as the probe, in depth indications about the different phases of the process were obtained, and the results reported in this paper may be useful in the improvement of large-scale applications of this technology, and—through generally valid methods—they could be deployed to other pharmaceuticals and emerging recalcitrant contaminants.

Synthesis of Titanium Dioxide via Surfactant-Assisted Microwave Method for Photocatalytic and Dye-Sensitized Solar Cells Applications

In this study, titania nanoparticles were obtained using the microwave-assisted technique. Moreover, different surfactants (PEG (Mn = 400), Pluronic P123 and Triton X−100) were used during the synthesis in order to determine their impact on the crystallinity and morphology of the final products. Subsequently, techniques such as XRD, SEM and TEM (performed in high contrast and high-resolution mode), diffuse reflectance spectroscopy (DRS), low temperature N2 sorption (BET model), FTIR and TGA were carried out. Based on the crystallinity analysis of the obtained materials, it was established that the addition of surfactants results in greater (PEG and Triton X−100) or smaller (Pluronic P123) average crystallite size. The main purpose of this study was to use the synthesized nanomaterials in the photodegradation process (in the UV light range) of the model organic pollutants – phenol (20 mg/L) and etodolac (15 mg/L). Furthermore, it was also pointed out that the dye-sensitized solar cells can be a second application for the synthesized titania nanomaterials. The photo-oxidation and photovoltaic tests have shown that the titanium dioxide obtained using the surfactant-assisted microwave method is characterized not only by better photodegradation efficiency of phenol and etodolac, but also by higher photocurrent density compared to the reference titania samples—the pristine TiO2 and commercial P25.

Covalent Organic Frameworks: A Promising Materials Platform for Photocatalytic CO2 Reductions

Covalent organic frameworks (COFs) are a kind of porous crystalline polymeric material. They are constructed by organic module units connected with strong covalent bonds extending in two or three dimensions. COFs possess the advantages of low-density, large specific surface area, high thermal stability, developed pore-structure, long-range order, good crystallinity, and the excellent tunability of the monomer units and the linking reticular chemistry. These features endowed COFs with the ability to be applied in a plethora of applications, ranging from adsorption and separation, sensing, catalysis, optoelectronics, energy storage, mass transport, etc. In this paper, we will review the recent progress of COFs materials applied in photocatalytic CO₂ reduction. The state-of-the-art paragon examples and the current challenges will be discussed in detail. The future direction in this research field will be finally outlooked.

Covalent organic framework photocatalysts: structures and applications

In the light of increasing energy demand and environmental pollution, it is urgently required to find a clean and renewable energy source. In these years, photocatalysis that uses solar energy for either fuel production, such as hydrogen evolution and hydrocarbon production, or environmental pollutant degradation, has shown great potential to achieve this goal. Among the various photocatalysts, covalent organic frameworks (COFs) are very attractive due to their excellent structural regularity, robust framework, inherent porosity and good activity. Thus, many studies have been carried out to investigate the photocatalytic performance of COFs and COF-based photocatalysts. In this critical review, the recent progress and advances of COF photocatalysts are thoroughly presented. Furthermore, diverse linkers between COF building blocks such as boron-containing connections and nitrogen-containing connections are summarised and compared. The morphologies of COFs and several commonly used strategies pertaining to photocatalytic activity are also discussed. Following this, the applications of COF-based photocatalysts are detailed including photocatalytic hydrogen evolution, CO₂ conversion and degradation of environmental contaminants. Finally, a summary and perspective on the opportunities and challenges for the future development of COF and COF-based photocatalysts are given.

Impact of Pd Loading on CO2 Reduction Performance over Pd/TiO2 with H2 and H2O

This study investigated the impact of molar ratio of CO₂ to reductants H₂O and H₂, as well as Pd loading weight on CO₂ reduction performance with Pd/TiO₂ as the photocatalyst. The Pd/TiO₂ film photocatalyst is prepared by the sol-gel and dip-coating process to prepare TiO₂ film and the pulse arc plasma method is used to dope Pd on TiO₂ film. The prepared Pd/TiO₂ film was characterized by SEM, EPMA, STEM, EDS, and EELS. This study also investigated the performance of CO₂ reduction under the illumination condition of Xe lamp with or without ultraviolet (UV) light. As a result, it is revealed that when the molar ratio of CO₂/H₂/H₂O is set at 1:0.5:0.5, the best CO₂ reduction performance has been obtained under the illumination condition of Xe lamp with and without UV light. In addition, it is found that the optimum Pd loading weight is 3.90 wt%. The maximum molar quantities of CO and CH₄ produced per unit weight of photocatalyst are 30.3 μmol/g and 22.1 μmol/g, respectively, for the molar ratio of CO₂/H₂/H₂O = 1:0.5:0.5 under the condition of Xe lamp illumination with UV light. With UV light, C₂H₄ and C₂H₆, as well as CO and CH₄ are also produced by the Pd/TiO₂ film photocatalyst prepared in this study.

Photocatalytic Reforming for Hydrogen Evolution: A Review

Hydrogen is considered to be an ideal energy carrier to achieve low-carbon economy and sustainable energy supply. Production of hydrogen by catalytic reforming of organic compounds is one of the most important commercial processes. With the rapid development of photocatalysis in recent years, the applications of photocatalysis have been extended to the area of reforming hydrogen evolution. This research area has attracted extensive attention and exhibited potential for wide application in practice. Photocatalytic reforming for hydrogen evolution is a sustainable process to convert the solar energy stored in hydrogen into chemical energy. This review comprehensively summarized the reported works in relevant areas, categorized by the reforming precursor (organic compound) such as methanol, ethanol and biomass. Mechanisms and characteristics for each category were deeply discussed. In addition, recommendations for future work were suggested.

Synthesis and Surface Modification of TiO2-Based Photocatalysts for the Conversion of CO2

Among all greenhouse gases, CO2 is considered the most potent and the largest contributor to global warming. In this review, photocatalysis is presented as a promising technology to address the current global concern of industrial CO2 emissions. Photocatalysis utilizes a semiconductor material under renewable solar energy to reduce CO2 into an array of high-value fuels including methane, methanol, formaldehyde and formic acid. Herein, the kinetic and thermodynamic principles of CO2 photoreduction are thoroughly discussed and the CO2 reduction mechanism and pathways are described. Methods to enhance the adsorption of CO2 on the surface of semiconductors are also presented. Due to its efficient photoactivity, high stability, low cost, and safety, the semiconductor TiO2 is currently being widely investigated for its photocatalytic ability in reducing CO2 when suitably modified. The recent TiO2 synthesis and modification strategies that may be employed to enhance the efficiency of the CO2 photoreduction process are described. These modification techniques, including metal deposition, metal/non-metal doping, carbon-based material loading, semiconductor heterostructures, and dispersion on high surface area supports, aim to improve the light absorption, charge separation, and active surface of TiO2 in addition to increasing product yield and selectivity.

Metal-Free Carbon Quantum Dots Implant Graphitic Carbon Nitride: Enhanced Photocatalytic Dye Wastewater Purification with Simultaneous Hydrogen Production

The use of photocatalysts to purify wastewater and simultaneously convert solar energy into clean hydrogen energy is of considerable significance in environmental science. However, it is still a challenge due to their relatively high costs, low efficiencies, and poor stabilities. In this study, a metal-free carbon quantum dots (CQDs) modified graphitic carbon nitride photocatalyst (CCN) was synthesized by a facile method. The characterization and theoretical calculation results reveal that the incorporation of CQDs into the g-C₃N₄ matrix significantly improves the charge transfer and separation efficiency, exhibits a redshift of absorption edge, narrows the bandgap, and prevents the recombination of photoexcited carriers. The hydrogen production and simultaneous degradation of methylene blue (MB) or rhodamine B (RhB) in simulated wastewaters were further tested. In the simulated wastewater, the CCN catalyst showed enhanced photodegradation efficiency, accompanied with the increased hydrogen evolution rate (1291 µmol·h⁻¹·g⁻¹). The internal electrical field between the g-C₃N₄ and the CQDs is the main reason for the spatial separation of photoexcited electron-hole pairs. Overall, this work could offer a new protocol for the design of highly efficient photocatalysts for dye wastewater purification with simultaneous hydrogen production.