Preparation of hierarchical g-C3N4@TiO2 hollow spheres for enhanced visible-light induced catalytic CO2 reduction

TiO2-based photocatalysts from type-II to S-scheme heterojunction and their applications

Photocatalysis is a promising sustainable technology to remove organic pollution and convert solar energy into chemical energy. Titanium dioxide has drawn extensive attention in this field owing to its high activity under UV light, good chemical stability, large availability, low price and low toxicity. However, the poor quantum efficiency derived from fast electron/hole recombination, the limited utilization of sunlight, and a weak reducing ability still hinder its practical application. Among the modification strategies of TiO2 to enhance its performance, the construction of heterojunctions with other semiconductors is a powerful and versatile way to maximise the separation of photogenerated charge carriers and steer their transport toward enhanced efficiency and selectivity. Here, the research progress and current status of TiO2 modification are reviewed, focusing on heterojunctions. A rapid evolution of the understanding of the different charge transfer mechanisms is witnessed from traditional type II to the recently conceptualised S-scheme. Particular attention is paid to different synthetic approaches and interface engineering methods designed to improve and control the interfacial charge transfer, and several cases of TiO2 heterostructures with metal oxides, metal sulfides and carbon nitride are discussed. The application hotspots of TiO2-based photocatalysts are summarized, including hydrogen generation by water splitting, solar fuel production by CO2 conversion, and the degradation of organic water pollutants. Hints about less studied and emerging processes are also provided. Finally, the main issues and challenges related to the sustainability and scalability of photocatalytic technologies in view of their commercialization are highlighted, outlining future directions of development.

Enhancing photocatalytic CO2 reduction with TiO2-based materials: Strategies, mechanisms, challenges, and perspectives

The concentration of atmospheric CO2 has exceeded 400 ppm, surpassing its natural variability and raising concerns about uncontrollable shifts in the carbon cycle, leading to significant climate and environmental impacts. A promising method to balance carbon levels and mitigate atmospheric CO2 rise is through photocatalytic CO2 reduction. Titanium dioxide (TiO2), renowned for its affordability, stability, availability, and eco-friendliness, stands out as an exemplary catalyst in photocatalytic CO2 reduction. Various strategies have been proposed to modify TiO2 for photocatalytic CO2 reduction and improve catalytic activity and product selectivity. However, few studies have systematically summarized these strategies and analyzed their advantages, disadvantages, and current progress. Here, we comprehensively review recent advancements in TiO2 engineering, focusing on crystal engineering, interface design, and reactive site construction to enhance photocatalytic efficiency and product selectivity. We discuss how modifications in TiO2's optical characteristics, carrier migration, and active site design have led to varied and selective CO2 reduction products. These enhancements are thoroughly analyzed through experimental data and theoretical calculations. Additionally, we identify current challenges and suggest future research directions, emphasizing the role of TiO2-based materials in understanding photocatalytic CO2 reduction mechanisms and in designing effective catalysts. This review is expected to contribute to the global pursuit of carbon neutrality by providing foundational insights into the mechanisms of photocatalytic CO2 reduction with TiO2-based materials and guiding the development of efficient photocatalysts.

Tailored Covalent Organic Framework Platform: From Multistimuli, Targeted Dual Drug Delivery by Architecturally Engineering to Enhance Photothermal Tumor Therapy

Engineering bulk covalent organic frameworks (COFs) to access specific morphological structures holds paramount significance in boosting their functions in cancer treatment; nevertheless, scant effort has been dedicated to exploring this realm. Herein, silica core-shell templates and multifunctional COF-based reticulated hollow nanospheres (HCOFs) are novelly designed as a versatile nanoplatform to investigate the simultaneous effect of dual-drug chemotherapy and photothermal ablation. Taking advantage of the distinct structural properties of the template, the resulting two-dimensional (2D) HCOF, featuring large internal voids and a peripheral interconnected mesoporous shell, presents intriguing benefits over its bulk counterparts for cancer treatment, including a well-defined morphology, an outstanding drug loading capability (99.6%) attributed to its ultrahigh surface area (2087 m2/g), great crystallinity, improved tumor accumulation, and an adjustable drug release profile. After being loaded with hydrophilic doxorubicin with a remarkable loading capacity, the obtained drug-loaded HCOFs were coated with gold nanoparticles (Au NPs) to confer them with three properties, including pore entrance blockage, active-targeting capability, and improved biocompatibility via secondary modification, besides high near infrared (NIR) absorption for efficient photothermal hyperthermia cancer suppression. The resultant structure was functionalized with mono-6-thio-β-cyclodextrin (β-CD) as a second pocket to load docetaxel as the hydrophobic anticancer agent (combination index = 0.33). The dual-drug-loaded HCOF displayed both pH- and near-infrared-responsive on-demand drug release. In vitro and in vivo evaluations unveiled the prominent synergistic performance of coloaded HCOF in cancer elimination upon NIR light irradiation. This work opens up a new avenue for exciting applications of structurally engineered HCOFs as hydrophobic/hydrophilic drug carriers as well as multimodal treatment agents.

In-situ grown carbon as charge transfer medium for enhanced photoinduced electrons extraction from polymer carbon nitride toward TiO2

Interfacial charge transfer resistance is one of the main limiting factors for realizing high photocatalytic efficiency of heterostructures system. Herein, an activated carbon layer is successfully introduced between the interface of polymer carbon nitride (CN) and TiO2 heterostructure (CNP-x) as charge transfer medium by in situ pyrolysis carbonization method. Because of the lower spatial resistance of the crystalline/amorphous interface and the fast carrier transportation character of activated carbon, the efficiency of TiO2 in extracting photoinduced electrons from CN was significantly improved. That is, the separation/transport of photocarriers in CNP-x heterostructure is accelerated, and the recombination time of photogenerated electrons and holes is prolonged. The CNP-1 exhibits a H2 evolution rate of 1298.5 μmol h-1 with apparent quantum yield (AQY) of 34.5 %, 20.3 % and 12.6 % at 365 nm, 380 nm and 400 nm, respectively. This work offers a novel and unique strategy to promote interface charge separation and transport of CN-based heterostructures by accurately introduction charge transfer medium.

Synergistic adsorption and kinetic studies of heterostructured g-C3N4/TiO2 nano-photocatalyst under visible light for enhanced CO2 reduction

Graphitic carbon nitride (g-C3N4) and titanium dioxide (TiO2) were synthesized using sol-gel and ultrasonic impregnation technique followed by calcination for photocatalytic CO2 reduction. The nano-photocatalysts were analyzed for their morphological, structural, and optical characteristics. Scanning electron microscopy (SEM) revealed the presence of spherical and layered sheet-like nanoparticles, as well as the occurrence of minor aggregations. The ultraviolet-visible spectroscopy (UV-vis) revealed that g-C3N4 has good photocatalytic properties with a medium band gap (2.7 eV), and TiO2 has high charge transfer potentials, robust oxidation properties, and high band gap (3.20 eV). However, the larger band gap makes it unresponsive in the visible light spectrum. In order to circumvent this constraint, a hybrid heterostructured g-C3N4/TiO2 catalyst with different compositions, viz., 1:1, 1:2, and 2:1, were fabricated using the ultrasonic impregnation technique followed by calcination process. The optical band gap of g-C3N4/TiO2 nanocomposite shows a red shift towards 2.85 eV from 3.20 eV for bare TiO2, inferring enhanced absorption in the visible light region. Further, the photocatalytic experiments were performed using visible light sources for all the catalysts. The g-C3N4/TiO2 (2:1) reported higher photocatalytic activity due to its reduced crystallite size of 12.94 nm which were investigated using X-ray diffraction analysis (XRD) and lower band gap of 2.85 eV. The study infers that hybrid photocatalyst enhances the visible light absorption, electron-hole (e - /h +) pair separation rate, and photocatalytic reduction of CO2. In addition, two adsorption models Langmuir and Freundlich were used and adsorption kinetic data were fitted to pseudo-first-order reaction for all the five catalysts. The adsorption isotherm of CO2 by g-C3N4/TiO2 (2:1) well fitted by the Freundlich adsorption equation. On the basis of adsorption magnitude (n) values (1.74), it was found that the interaction between CO2 molecules and g-C3N4/TiO2 occurs according to the chemisorption mechanism. The kinetic study infers that the highest value of apparent rate constant (kapp) was exhibited by g-C3N4/TiO2 (2:1), which indicates that the products predominate at equilibrium.

Black Phosphorus-based Photocatalysts: Synthesis, Properties, and Applications

Photocatalysis is considered as an eco-friendly and sustainable strategy, since it uses abundant light for the advancement of the reaction, which is freely accessible and is devoid of environmental pollution. During the last decades, (nano)photocatalysts have gained broad industrial applications in terms of purification and detoxification of water as well as production of green fuels and hydrogen gas due to their special attributes. The degradation or remediation of toxic and hazardous compounds from the environment or changing them into non-toxic entities is a significant endeavor and necessary for the safety of humans, animals, and the environment. Black phosphorus (BP), a two-dimensional single-element material, has a marvelous structure, tunable bandgap, changeable morphology from bulk to nanosheet/quantum dot, and unique physicochemical properties, which makes it attractive material for photocatalytic applications, especially for sustainable development purposes. Since it can serve as a photocatalyst with or without coupling with other semiconductors, various aspects for multidimensional exploitation of BP are deliberated including their preparation via solvothermal, ball milling, calcination, and sonication methods to obtain BP from red phosphorus. The techniques for improving the photocatalytic and stability of BP-based composites are discussed along with their multifaceted applications for environmental remediation, pollution degradation, water splitting, N2 fixation, CO2 reduction, bacterial disinfection, H2 generation, and photodynamic therapy. Herein, most recent advancements pertaining to the photocatalytic applications of BP-based photocatalyst are cogitated, with a focus on their synthesis and properties as well as crucial challenges and future perspectives.

Mg/Li@GCN as highly active visible light responding 2D photocatalyst for wastewater remediation application

In this study, a highly visible light responding 2D photocatalytic material has been prepared and analysed for its potential for photodegradation of organic pollutants. The pristine GCN has been co-doped with Mg/Li using the facile synthesis route. The prepared photocatalytic materials were then analysed using characterisation techniques like X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, diffuse reflectance spectra (DRS) and photoluminescence spectroscopy (PL) analysis. The prepared samples were analysed for photocatalytic degradation analysis towards methylene blue dye. The apparent rate constant value increased up to 5.4 times in the case of the GCNML (0.5,2) sample in comparison to GCNP. In addition, the GCNML (0.5,2) sample was also analysed for degradation of crystal violet (CV) (97% in 80 min), rose bengal (RB) (84% in 120 min) and methyl orange (MO) (45% in 120 min) dyes. The result obtained from the study confirmed that GCNML (0.5,2) can act as a potential photocatalyst for wastewater remediation application.

Insights into engineered graphitic carbon nitride quantum dots for hazardous contaminants degradation in wastewater

Increased environmental pollution is a critical issue that must be addressed. Photocatalytic, adsorption, and membrane filtration methods are suitable in environmental governance because of their high selectivity, low cost, environment-friendly nature, and excellent treatment efficiency. Graphitic carbon nitride (g-C₃N₄) quantum dots (QDs) have been considered as photocatalysts, adsorbents, and membrane materials for wastewater treatments, owing to their stability, adsorption capacity, photochemical properties, and low toxicity and cost. This review summarizes g-C₃N₄ QD synthesis techniques, operating parameters affecting the removal performance in the treatment process, modification effects with other semiconductors, and benefits and drawbacks of g-C₃N₄ QD-based materials. Furthermore, this review discusses the practical applications of g-C₃N₄ QDs as adsorbents, photocatalysts, and membrane materials for organic and inorganic contaminant treatments and their value-added product formation potential. Modified g-C₃N₄ QD-based material adsorbents, photocatalysts, and membranes present potentially applicable effects, such as removal of most waterborne contaminants. Excellent results were obtained for the reduction of methyl orange, bisphenol A, tetracycline, ciprofloxacin, phenol, rhodamine B, E. coli, and Hg. Overall, this paper provides comprehensive background on g-C₃N₄ QD-based materials and their diverse applications in wastewater treatment, and it presents a foundation for the enhancement of similar unique materials in the future.

Spinach-derived boron-doped g-C3N4/TiO2 composites for efficient photo-degradation of methylene blue dye

Green synthesis of nanoparticles can be beneficial due to their low toxicity, cost-effectiveness, and environment-friendliness. Its synthesis involves the use of eco-friendly and biodegradable materials such as plant extracts, natural products, and microorganisms to reduce the negative environmental impacts of traditional nanoparticle synthesis methods. Herein, Spinacia oleracea leaves are used as a boron source, and a visible light active photo-catalyst is produced. The effect of Co-Catalyst Boron in Graphitic carbon nitride based nanocomposites for methylene blue dye photo-degradation in water is examined. Titanium dioxide (TiO₂) was activated by changing the hydrogen potential value while utilizing a typical orange dye as a sensitizer. The graphitic carbon nitride/TiO₂ nanocomposites were synthesized through a hydrothermal technique. To improve their performance, Boron used as a co-catalyst and B-doped g-C₃N₄/TiO₂nanocomposites prepared through wet chemical co-percipitate mathod. UV-visible spectroscopy, SEM and FTIR spectroscopy were used to analyze the photocatalyst and boron-doped composites in detail. The photocatalytic performance of pristine photocatalyst CNTx (x = 2%,4%,6%,8%) and B-doped CNTx composites were examined for Methylene Blue degradation in the presence of a light source. The spectroscopy analysis showed that B-doped g-C₃N₄/TiO₂ -8% nano-composites performed better than all other synthesized pristine catalysts and composites in this research. This research has demonstrated that B-doped g-C₃N₄/TiO₂ composites can provide an ideal solution for treating polluted water using visible light as a source to activate these photocatalysts.

Facile synthesis of hollow spherical g-C3N4@LDH/NCQDs ternary nanostructure for multifunctional antibacterial and photodegradation activities

Heterojunction nanostructure construction and morphology engineering are considered to be effective approaches to improve photocatalytic performance. Herein, ternary hierarchical hollow structures consisting of cobalt-aluminum-layered double hydroxide (CoAl-LDH) nanoplates grown on hollow carbon nitride spheres (HCNS) and decorated with N-doped carbon quantum dots (NCQDs) were prepared using a templating method and a subsequent solvothermal process. The obtained HCNS@LDH/NCQD composites presented an improved performance in photocatalytic degradation of tetracycline and inactivation of E. coli compared with pure HCNS and LDH under visible light illumination. The enhanced photocatalytic activity of the designed photocatalyst could be attributed to the following reasons: (1) A special hollow structure provides more active sites and has multiple capabilities of light reflection by helping with a high specific surface area that improves the harvesting efficiency of solar light and (2) the strong synergistic effect among the constituents, which promotes separation and transfer of charge carriers and broadens the photo-response range.

A Comprehensive Review on Graphitic Carbon Nitride for Carbon Dioxide Photoreduction

Inspired by natural photosynthesis, harnessing the wide range of natural solar energy and utilizing appropriate semiconductor-based catalysts to convert carbon dioxide into beneficial energy species, for example, CO, CH₄ , HCOOH, and CH₃ COH have been shown to be a sustainable and more environmentally friendly approach. Graphitic carbon nitride (g-C₃ N₄ ) has been regarded as a highly effective photocatalyst for the CO₂ reduction reaction, owing to its cost-effectiveness, high thermal and chemical stability, visible light absorption capability, and low toxicity. However, weaker electrical conductivity, fast recombination rate, smaller visible light absorption window, and reduced surface area make this catalytic material unsuitable for commercial photocatalytic applications. Therefore, certain procedures, including elemental doping, structural modulation, functional group adjustment of g-C₃ N₄ , the addition of metal complex motif, and others, may be used to improve its photocatalytic activity towards effective CO₂ reduction. This review has investigated the scientific community's perspectives on synthetic pathways and material optimization approaches used to increase the selectivity and efficiency of the g-C₃ N₄ -based hybrid structures, as well as their benefits and drawbacks on photocatalytic CO₂ reduction. Finally, the review concludes a comparative discussion and presents a promising picture of the future scope of the improvements.

Electrospun composite nanofibers as novel high-performance and visible-light photocatalysts for removal of environmental pollutants: A review

Environmental pollution caused by industries and human manipulations is coming a serious global challenge. On the other hand, the world is facing an energy crisis caused by population growth. Designing solar-driven photocatalysts which are inspired by the photosynthesis of plant leaves is a fantastic solution to use solar energy as green, available, and unlimited energy containing ∼50% visible light for the removal of environmental pollutants. The polymeric and non-polymeric-based electrospun composite nanofibers (NFs) are as innovative photocatalytic candidates which increase photocatalytic activity and transition from UV light to visible light and overcome the aggregation, photocorrosion, toxicity, and hard recycling and separation of the nanosized powder form of photocatalysts. The composite NFs are fabricated easily by either embedding the photocatalytic agents into the NFs during electrospinning or via their decorating on the surface of NFs post-electrospinning. Polyacrylonitrile-based, tungsten trioxide-based, zinc oxide-based, and titanium dioxide-based composite NFs were revealed as the most reported composite NFs. All the lately investigated electrospun composite NFs indicated long-term stability, high photocatalytic efficiency (∼> 80%) within a short time of light radiation (10-430 min), and high stability after several cycles of use. They were applied in various applications including degradation of dyes/antibiotics, water splitting, wastewater treatment, antibacterial usage, etc. The photogenerated species especially holes, O₂^∙-, and ^.OH were mostly responsible for the photocatalytic mechanism and pathway. The electrospun composite NFs have the potential to use in large-scale productions in condition that their thickness and recycling conditions are optimized, and their toxicity and detaching are resolved.

Nano-architectural design of TiO2 for high performance photocatalytic degradation of organic pollutant: A review

In the past several decades, significant efforts have been paid toward photocatalytic degradation of organic pollutants in environmental research. During the past years, titanium dioxide nano-architectures (TiO₂ NAs) have been widely used in water purification applications with photocatalytic degradation processes under Uv/Vis light illumination. Photocatalysis process with nano-architectural design of TiO₂ is viewed as an efficient procedure for directly channeling solar energy into water treatment reactions. The considerable band-gap values and the subsequent short life time of photo-generated charge carriers are showed among the limitations of this approach. One of these effective efforts is the using of oxidation processes with advance semiconductor photocatalyst NAs for degradation the organic pollutants under UV/Vis irradiation. Among them, nano-architectural design of TiO₂ photocatalyst (such as Janus, yolk-shell (Y@S), hollow microspheres (HMSs) and nano-belt) is an effective way to improve oxidation processes for increasing photocatalytic activity in water treatment applications. In the light of the above issues, this study tends to provide a critical overview of the used strategies for preparing TiO₂ photocatalysts with desirable physicochemical properties like enhanced absorption of light, low density, high surface area, photo-stability, and charge-carrier behavior. Among the various nanoarchitectural design of TiO₂, the Y@S and HMSs have created a great appeal given their considerable large surface area, low density, homogeneous catalytic environment, favorable light harvesting properties, and enhanced molecular diffusion kinetics of the particles. In this review was summarized the developments that have been made for nano-architectural design of TiO2 photocatalyst. Additional focus is placed on the realization of interfacial charge and the possibility of achieving charge carriers separation for these NAs as electron migration is the extremely important factor for increasing the photocatalytic activity.

Boosting the Photocatalytic H2 Evolution and Benzylamine Oxidation using 2D/1D g-C3N4/TiO2 Nanoheterojunction

The present research aims at the elevation of solar-to-chemical energy conversion with extortionate performance and sustainability. The nanostructured materials are revolutionizing the water splitting technology into decoupled hydrogen with simultaneous value-added organic chemical production. Yet, the bottleneck in semiconductor photocatalysis is rapid charge recombination and sluggish reaction kinetics. Herein, we demonstrate an efficient and non-noble metal-based catalyst for successful redox reaction with a theoretical modeling through density functional theory (DFT) study. Implementing this robust approach on 2D/1D ultrathin g-C₃N₄ nanosheets and TiO₂ nanowires heterojunction, we achieved H₂ production of 5.1 mmol g^-1 h^-1 with apparent quantum efficiency of 7.8% under visible light illumination and 93% of benzylamine conversion to N-benzylidene benzylamine in situ. The interface of 2D g-C₃N₄ nanosheets and 1D nanowires provide ample active sites and extends the visible light absorption with requisite band edge position for the separation of photoinduced charge carriers with superior stability. The electronic properties, band structure, and stability of the heterojunction are further investigated via DFT calculations which corroborate the experimental results and in good agreement for the enhanced activity of the heterojunction.

N-TiO2-δ/g-C3N4 Dual Photocatalysts for Efficient Oxytetracycline Hydrochloride Photodegradation and CO2 Photoreduction

A series of $x\%$ (wt) N-TiO2-δ/g-C3N4 composites was synthesized by calcination and hydrothermal methods (labeled $x$ TiCN, $x$ : 5, 10, and 15). All composites were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, UV-vis diffuse reflectance spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The photocatalytic activity of these composites was evaluated through oxytetracycline hydrochloride (denoted as OTC) photodegradation and CO2 photoreduction. The $x$ TiCN composites exhibited higher OTC photodegradation than bulk g-C3N4. 10TiCN was slightly more active than 5TiCN and 15TiCN, with a photodegradation yield of 97% after 5 h of light irradiation and constant rate of 0.647 h-1. For CO2 photoreduction, it was observed that 5TiCN exhibited the highest activity among the synthesized composites, with 7.0 ppm CH4 formed. This CH4 concentration was 7.8 times higher than the concentration formed by bulk g-C3N4 (0.9 ppm). A $Z$ -scheme mechanism was proposed to explain the enhanced photocatalysis by $x\%$ (wt) N-TiO2-δ/g-C3N4 composites. The $Z$ -scheme structure increased redox ability, caused better separation of photogenerated electron-hole pairs, and broadened the light absorption zone of the photocatalysts.

Template-mediated copper doped porous g-C3N4 for efficient photodegradation of antibiotic contaminants

Graphite carbon nitride (g-C₃N₄) has great potential to treat antibiotic wastewater, but limited by small specific surface area, rapid recombination of photogenerated carriers and narrow visible light absorption range. In order to solve above problems, we designed a simple template-mediated approach by supramolecular self-assembly (Cu-melamine-cyanuric acid) to prepare copper doped porous graphitic carbon nitride (Cu-pCN) photocatalyst. The pre-organized template self-assembly driven by hydrogen bonds and electrostatic interaction, resulted in highly porous structure. The specific surface area of Cu-pCN increased to 142.8 m²/g from 11.37 m²/g of conventional bulk g-C₃N₄. In addition, the doping of Cu endowed them with better light absorption, higher separation and transfer rate of photogenerated carriers. Consequently, the obtained Cu-pCN displayed the superior photocatalytic degradation rate for tetracycline (TC) and high recycling stability.

Insight into the activity of TiO2@nitrogen-doped hollow carbon spheres supported on g-C3N4 for robust photocatalytic performance

Designing an advance nanostructure semiconductor is an efficient strategy to promote the charge separation and thus improve the photocatalytic activity. Herein, a relatively high recombination rate of electron-hole pairs and low specific surface area of g-C₃N₄ (GCN) were subjected to the surface deposition of the core shell nanoparticles composed of nitrogen doped hollow carbon spheres (N-HCSs) as the supporting scaffold and TiO₂ nanoparticles as the photoactive layer. The ternary composites with different TiO₂@N-HCS content were prepared through a simplified nanocasting method followed by the two consecutive hydrothermal process. The effects of nitrogen doping in carbon framework, and nanoparticles amount were evaluated on the photocatalytic ability through the photodegradation of tetracycline (TC) molecules under the visible light irradiation. At the optimum content of core shell nanoparticles (7 wt%), the solar-driven TC photocatalytic degradation for ternary composite was approximately 85%, which was much better (about three times) than that of the pure GCN. More interestingly, the experimental results revealed that doping of nitrogen atoms has a positive role on the charge separation and the resulting photocatalytic efficiency. The employed hollow carbon spheres here play three important roles: (1) providing a substrate to uniformly dispersion of TiO₂ nanoparticles without any aggregation; (2) reducing the combination of charge carriers and improving the separation of photoinduced carriers; (3) formation of larger surface area and more active sites on the photocatalyst surface. Furthermore, the underlying photocatalytic degradation mechanism was introduced by the controlled experiments using photoluminescent and radical scavenger tests.

Photocatalytic Reduction of Carbon Dioxide on TiO2 Heterojunction Photocatalysts-A Review

The photocatalytic reduction of carbon dioxide to renewable fuel or other valuable chemicals using solar energy is attracting the interest of researchers because of its great potential to offer a clean fuel alternative and solve global warming problems. Unfortunately, the efficiency of CO₂ photocatalytic reduction remains not very high due to the fast recombination of photogenerated electron–hole and small light utilization. Consequently, tremendous efforts have been made to solve these problems, and one possible solution is the use of heterojunction photocatalysts. This review begins with the fundamental aspects of CO₂ photocatalytic reduction and the fundamental principles of various heterojunction photocatalysts. In the following part, we discuss using TiO₂ heterojunction photocatalysts with other semiconductors, such as C₃N₄, CeO₂, CuO, CdS, MoS₂, GaP, CaTiO₃ and FeTiO₃. Finally, a concise summary and presentation of perspectives in the field of heterojunction photocatalysts are provided. The review covers references in the years 2011–2021.

Effect of nonmetals (B, O, P, and S) doped with porous g-C3N4 for improved electron transfer towards photocatalytic CO2 reduction with water into CH4

Photocatalytic conversion of carbon dioxide (CO₂) into gaseous hydrocarbon fuels is an auspicious way to produce renewable fuels in addition to greenhouse gas emission mitigation. In this work, non-metals (B, O, P, and S) doped graphitic carbon nitride (g-C₃N₄) was prepared via solid-state polycondensation of urea for photocatalytic CO₂ reduction into highly needed methane (CH₄) with water under UV light irradiation. The various physicochemical characterization results reveal the successful incorporation of B, O, P, and S elements in the g-C₃N₄ matrix. The maximum CH₄ yield of 55.10 nmol/(mL_H2O.g_cat) over S-doped g-C₃N₄ has been obtained for CO₂ reduction after 7 h of irradiation. This amount of CH₄ production was 1.9, 1.4, 1.7, and 2.4-folds higher than B, O, P and bare g-C₃N₄ samples. The doping of S did not enlarge the surface area and photon absorption ability of the g-C₃N₄ sample, but this significant improvement was evidently due to effective charge separation and migration. The observed results imply that the doping of non-metal elements provides improved charge separation and is an effective way to boost photocatalyst performance. This work offers an auspicious approach to design non-metal doped g-C₃N₄ photocatalysts for renewable fuel production and would be promising for other energy application.

A Critical Review on Black Phosphorus-Based Photocatalytic CO2 Reduction Application

Energy shortages and greenhouse effects are two unavoidable problems that need to be solved. Photocatalytically converting CO₂ into a series of valuable chemicals is considered to be an effective means of solving the above dilemmas. Among these photocatalysts, the utilization of black phosphorus for CO₂ photocatalytic reduction deserves a lightspot not only for its excellent catalytic activity through different reaction routes, but also on account of the great preponderance of this relatively cheap catalyst. Herein, this review offers a summary of the recent advances in synthesis, structure, properties, and application for CO₂ photocatalytic reduction. In detail, the review starts from the basic principle of CO₂ photocatalytic reduction. In the following section, the synthesis, structure, and properties, as well as CO₂ photocatalytic reduction process of black phosphorus-based photocatalyst are discussed. In addition, some possible influencing factors and reaction mechanism are also summarized. Finally, a summary and the possible future perspectives of black phosphorus-based photocatalyst for CO₂ reduction are established.

Single layered hollow NiO-NiS catalyst with large specific surface area and highly efficient visible-light-driven carbon dioxide conversion

A sea urchin-shaped, single-layer, and hollow NiO-NiS photocatalyst with a large surface area was designed for carbon dioxide (CO₂) conversion in this study. A d-glucose polymeric hollow frame was fabricated using a d-glucose monomer, and NiO particles were stably grown on it using the hydrothermal method to form a hollow NiO surface. The d-glucose frame was removed by heat treatment to create hollowed NiO; hollowed NiO-NiS (h-NiO-NiS) was subsequently obtained through ion exchange between the O ions in NiO and S ions in the sulfur powder. Additionally, we attempted to determine the correlation among the surface area of the h-NiO-NiS catalyst, CO₂ gas adsorption capacity, and catalyst performance. The surface area of the h-NiO-NiS catalyst was ten times larger than that of the nanometer-sized NiO-NiS (n-NiO-NiS, 21.2 m² g^-1) catalyst. The CO₂ photocatalytic conversion performance of the hollowed catalyst was approximately seven times larger than that of the nanosized catalyst. As the amount of ion-exchanged S increased, methane selectivity increased, and optimal methane production was obtained when the weight ratio of NiO and sulfur powder was 1 : 4. Using temperature-programmed desorption (TPD) analyses of CO₂ and H₂O, the adsorption of water molecules on the Ni-S surface and that of CO₂ gas on the Ni-O surface during CO₂ conversion reaction were confirmed. The h-NiO-NiS catalyst facilitated an effective charge separation through a well-developed interfacial transition between the linked NiS and NiO, and resulted in increased CO₂ photoreduction performance under sunlight.

Recent advances in photocatalytic decomposition of water and pollutants for sustainable application

Photoinduced reduction and oxidation, the important processes in photocatalytic water splitting and organic degradation, have generated increasing interest to address the energy and environmental issues. In this review, the recent developments in bandgap and interfacial engineering for enhanced light absorption, efficient charge separation and interfacial reaction are focused toward the applications in photocatalytic water splitting and organic degradation. In photoinduced reduction for hydrogen evolution, three major strategies are discussed: cocatalysts, sacrificial agents and heterojunctions. In photoinduced oxidation for organic degradation, three types of emerging pollutants of current concerns are highlighted: organic dyes, pharmaceuticals and volatile organic compounds. The key challenges of promising photocatalysts are discussed for future development and practical application.

Recent advances in g-C3N4 composites within four types of heterojunctions for photocatalytic CO2 reduction

Studies of photocatalytic conversion of CO₂ into hydrocarbon fuels, as a promising solution to alleviate global warming and energy issues, are booming in recent years. Researchers have focused their interest in developing g-C₃N₄ composite photocatalysts with intriguing features of robust light harvesting ability, excellent catalysis, and stable performance. Four types of heterojunctions (type-II, Z-scheme, S-scheme and Schottky) of the g-C₃N₄ composites are widely adopted. This review aims at presenting and comparing the photocatalytic mechanisms, characteristics, and performances of g-C₃N₄ composites concerning these four types of heterojunctions. Besides, perspectives and undergoing efforts for further development of g-C₃N₄ composite photocatalysts are discussed. This review would be helpful for researchers to gain a comprehensive understanding of the progress and future development trends of g-C₃N₄ composite heterojunctions for photocatalytic CO₂ reduction.

Amplified photocurrent signal for fabricating photoelectrochemical sulfadimethoxine aptasensor based on carbon nitride photosensitization with visible/near-infrared light responsive zinc phthalocyanine

Developing effective analytical method for sulfadimethoxine (SDM) detection is highly desirable and vitally crucial for protecting environment safety and human health. Herein, a highly selective and sensitive photoelectrochemical (PEC) aptasensor for accurate detection of SDM was proposed, which employed zinc phthalocyanine/graphitic carbon nitride (ZnPc/CN) nanocomposite as photosensitive material. The ZnPc/CN nanocomposite was constructed by modification of CN nanosheet with visible/near-infrared light responsive photosensitizer ZnPc. The introduction of ZnPc into CN exhibited amplified PEC response, which was 5.7 and 18.3 times than pure ZnPc and CN, attributed to the enhanced light harvesting ability and improved photoelectric conversion efficiency of such nanocomposite. By using ZnPc/CN and sulfadimethoxine (SDM) aptamer as PEC response material and specific probe, a PEC aptasensor was established for SDM detection. The aptamer was connected to the surface of chitosan/ZnPc/CN/ITO through the formation of phosphoramidate bonds between the amino group of the chitosan and phosphate group of the aptamer at 5' end. The fabricated aptasensor displayed good detection linearity of 0.1 ~ 300 nM and low detection limit of 0.03 nM (S/N = 3) under optimized conditions, and the potential applicability of the PEC aptasensor was confirmed by detecting SDM in milk powder samples.

Mechanistic insight into photocatalytic CO2 reduction by a Z-scheme g-C3N4/TiO2 heterostructure

The Z-scheme g-C₃N₄/TiO₂ heterostructure has remarkable catalytic activity for reducing CO₂ to CH₄ and CH₃OH.

Boosting charge carrier separation efficiency by constructing an intramolecular DA system towards efficient photoreduction of CO2

The charge carrier excitation–recombination process between the donor and acceptor, and the photocatalytic reduction of CO₂ to CO over CN based DA composites.

Biomimetic design and synthesis of visible-light-driven g-C3N4 nanotube @polydopamine/NiCo-layered double hydroxides composite photocatalysts for improved photocatalytic hydrogen evolution activity

In the practical process of photocatalytic H₂ evolution, optimizing the ability of light absorption and charge spatial separation is the top priority for improving the photocatalytic performance. In this study, we elaborately engineer neoteric g-C₃N₄ nanotube@polydopamine(pDA)/NiCo-LDH (LPC) composite photocatalyst by combining hydrothermal and calcination method. In the LPC composite system, the one-dimensional (1D) g-C₃N₄ nanotubes with larger specific surface area can afford more active sites and conduce to shorten the charge migration distance, as well as the high-speed mass transfer in the nanotube can accelerate the reaction course. The g-C₃N₄/NiCo-LDH type-II heterojunction can efficaciously stimulate the spatial separation of photo-produced charge. In addition, pDA as heterojunction metal-free interface mediums can provide multiple action (π-π* electron delocalization effect, adhesive action and photosensitization). The optimized LPC nanocomposite displays about 3.3-fold high photoactivity for H₂ evolution compared with the g-C₃N₄ nanotube under solar light irradiation. In addition, the cycle experiment result shows that the LPC composite photocatalyst possesses superior stability and recyclability. The resultant g-C₃N₄@pDA/NiCo-LDH composite photocatalyst displays the potential practical application in the field of energy conversion.