Graphene Modified TiO₂ Composite Photocatalysts: Mechanism, Progress and Perspective

Harnessing coal and coal waste for environmental conservation: A review of photocatalytic materials

Fossil fuels, especially coal, have played a pivotal role in driving technological and economic advancements over the past century, though accompanied by numerous environmental challenges. Rapid progress in green and sustainable energy sources, including tidal, wind, and solar energy, coupled with growing environmental concerns, the conventional coal industry is experiencing a sustained decline in both size and financial viability. This situation necessitates the urgent adoption of advanced approaches to coal utilization. Beyond serving as an energy source, coal and its by-products, known as coal waste, can serve as valuable resources for the development of advanced materials, including photocatalysts. The advancement of photocatalytic materials derived from coal and coal waste can capitalize on these natural carbon and mineral sources, providing a viable solution to numerous environmental challenges. Currently, research in this domain remains in its early stages, with existing studies primarily focusing on specific types of photocatalysts or particular aspects of the fabrication process. Therefore, available coal-based and coal waste-based photocatalytic materials were systematically examined and categorized into six types according to their composition and dimensional/structural characteristics. Each type of photocatalytic material was introduced, along with common fabrication and characterization technologies. Representative works were discussed in detail to highlight the unique features of different types of coal-based and coal waste-based photocatalytic materials. Furthermore, the promising applications of these materials in environmental protection and pollution treatment were summarized, while also addressing the challenges and prospects in this research field. This review comprehensively overviews the fundamental knowledge and recent advancements in photocatalytic materials derived from coal and coal waste, with the goal of catalyzing the development of next generation photocatalysts and contributing to the transformation of the conventional coal industry.

A Review of Synthesis Methods, Modifications, and Mechanisms of ZnO/TiO2-Based Photocatalysts for Photodegradation of Contaminants

The environment being surrounded by accumulated durable waste organic compounds has become a critical crisis for human societies. Generally, organic effluents of industrial plants released into the water source and air are removed by some physical and chemical processes. Utilizing photocatalysts as cost-effective, accessible, thermally/mechanically stable, nontoxic, reusable, and powerful UV-absorber compounds creates a new gateway toward the removal of dissolved, suspended, and gaseous pollutants even in trace amounts. TiO2 and ZnO are two prevalent photocatalysts in the field of removing contaminants from wastewater and air. Structural modification of the photocatalysts with metals, nonmetals, metal ions, and other semiconductors reduces the band gap energy and agglomeration and increases the affinity toward organic compounds in the composite structures to expand their usability on an industrial scale. This increases the extent of light absorbance and improves the photocatalytic efficiency. Selecting a suitable synthesis method is necessary to prepare a target photocatalyst with distinct properties such as high specific surface area, numerous surface functional groups, and an appropriate crystalline phase. In this Review, significant parameters for the synthesis and modification of TiO2- and ZnO-based photocatalysts are discussed in detail. Several proposed mechanistic routes according to photocatalytic composite structures are provided. Some electrochemical analyses using charge carrier trapping agents and delayed recombination help to plot mechanistic routes according to the direction of photoexcited species (electron-hole pairs) and design more effective photocatalytic processes in terms of cost-effective photocatalysts, saving time and increasing productivity.

Highly Conductive and Long-Term Stable Phosphorene-Based Nanocomposite for Radio-Frequency Antenna Application

In this study, an omnidirectional and high-performance free-standing monopole patch radio-frequency antenna was fabricated using a urea-functionalized phosphorene/TiO2/polypyrrole (UTP) nanocomposite. The UTP nanocomposite antenna was fabricated via ball milling of urea-functionalized phosphorene, chemical oxidative polymerization of the UTP nanocomposite, and mechanical pelletizing of the composite. Based on experiments, the proposed UTP nanocomposite-based antenna exhibited long-term stability in terms of electrical conductivity. After 12 weeks, a slight change in surface resistance was observed. The proposed antenna exhibited high radiation efficiency (78.2%) and low return loss (-36.6 dB). The results of this study suggest the potential of UTP nanocomposite antennas for applications in 5G technology.

Optical and UV Shielding Properties of Inorganic Nanoparticles Embedded in Polymethyl Methacrylate Nanocomposite Freestanding Films

Polymethyl methacrylate (PMMA) is an interesting polymer employed in various applications due to its outstanding properties. However, its electrical and mechanical properties can be further improved by incorporating nanoparticles, and in particular, PMMA nanocomposite with nanoparticles provides various multifunctional properties. This work reports PMMA nanocomposite preparation and structural and optical characterizations incorporating carbon nanotubes (CNTs), TiO2 nanoparticles, and carbon quantum dots (CQDs). CNT/PMMA, TiO2/PMMA, and CQD/PMMA nanocomposite freestanding films were prepared using a simple solution method. Various properties of the prepared composite films were analyzed using scanning electron microscopy, X-ray diffraction, photoluminescence, Fourier transform infrared, and UV-Vis and Raman spectroscopy. Optical parameters and photocatalytic dye degradation for the films are reported, focusing on the properties of the materials. The CNT/PMMA, TiO2/PMMA, and CQD/PMMA films achieved, respectively, good electrical conductivity, photodegradation, and fluorescence compared with other composite films.

VIS-active TiO2 films decorated by expanded graphite: impact of the exfoliation time on the photocatalytic behaviour

TiO2/C nanocomposite films were applied on water treatment. Expanded graphite nanosheets (EG) were obtained by UVC-assisted liquid-phase exfoliation technique, without the addition of acids, surfactants, or aggressive oxidizing agents, which characterizes the process as an eco-friendly method. The carbon nanosheets were synthesized directly from graphite bulk at different times and deposited on TiO2 films surface by airbrush spray coating method, forming a TiO2/C heterojunction. The increase in the exfoliation time promoted a more efficient photocatalytic dye removal under visible light. Morphological modifications, changes in the electronic structure, and wide range of light absorption were observed from the TiO2/C heterojunction formation. The results showed that hybrid TiO2/C supported photocatalyst is a promise alternative for practical photocatalytic applications under sunlight.

Model Approach to Thermal Conductivity in Hybrid Graphene-Polymer Nanocomposites

The thermal conductivity of epoxy nanocomposites filled with self-assembled hybrid nanoparticles composed of multilayered graphene nanoplatelets and anatase nanoparticles was described using an analytical model based on the effective medium approximation with a reasonable amount of input data. The proposed effective thickness approach allowed for the simplification of the thermal conductivity simulations in hybrid graphene@anatase TiO2 nanosheets by including the phenomenological thermal boundary resistance. The sensitivity of the modeled thermal conductivity to the geometrical and material parameters of filling particles and the host polymer matrix, filler's mass concentration, self-assembling degree, and Kapitza thermal boundary resistances at emerging interfaces was numerically evaluated. A fair agreement of the calculated and measured room-temperature thermal conductivity was obtained.

Impact of the Precursor on the Physicochemical Properties and Photoactivity of TiO2 Nanoparticles Produced in Supercritical CO2

The synthesis of TiO2 nanoparticles (NPs) in supercritical media has been reported over the last two decades. However, very few studies have compared the physicochemical characteristics and photoactivity of the TiO2 powders produced from different precursors, and even fewer have investigated the effect of using different ratios of hydrolytic agent/precursor (HA/P) on the properties of the semiconductor. To bridge this knowledge gap, this research focuses on the synthesis and characterization of TiO2 NPs obtained in a supercritical CO2 medium from four different TiO2 precursors, namely diisopropoxytitanium bis (acetylacetonate) (TDB), titanium (IV) isopropoxide (TIP), titanium (IV) butoxide (TBO), and titanium (IV) 2-ethylhexyloxide (TEO). Further, the effect of various HA/P ratios (10, 20, 30, and 40 mol/mol) when using ethanol as a hydrolytic agent has also been analyzed. Results obtained have shown that the physicochemical properties of the catalysts are not significantly affected by these variables, although some differences do exist between the synthesized materials and their catalytic performances. Specifically, photocatalysts obtained from TIP and TEO at the higher HA/P ratios (HA/P = 30 and HA/P = 40) led to higher CO2 photoconversions (6.3-7 µmol·g-1·h-1, Apparent Quantum Efficiency < 0.1%), about three times higher than those attained with commercial TiO2 P-25. These results have been imputed to the fact that these catalysts exhibit appropriate values of crystal size, surface area, light absorption, and charge transfer properties.

Photocatalytic Activity of the Blends Based on TiO2 Nanoparticles and Reduced Graphene Oxide for Degradation of Acetaminophen

The aim of this work is to highlight the influence of blends based on TiO₂ nanoparticles and reduced graphene oxide (RGO) on the photodegradation of acetaminophen (AC). To this end, the catalysts of TiO₂/RGO blends with RGO sheet concentrations equal 5, 10, and 20 wt. % were prepared by the solid-state interaction of the two constituents. The preferential adsorption of TiO₂ particles onto the RGO sheets' surfaces via the water molecules on the TiO₂ particle surface was demonstrated by FTIR spectroscopy. This adsorption process induced an increase in the disordered state of the RGO sheets in the presence of the TiO₂ particles, as highlighted by Raman scattering and scanning electron microscopy (SEM). The novelty of this work lies in the demonstration that TiO₂/RGO mixtures, obtained by the solid-phase interaction of the two constituents, allow an acetaminophen removal of up to 95.18% after 100 min of UV irradiation. This TiO₂/RGO catalyst induced a higher photodegradation efficiency of AC than TiO₂ due to the presence of RGO sheets, which acted as a capture agent for the photogenerated electrons of TiO_2, hindering the electron-hole recombination. The reaction kinetics of AC aqueous solutions containing TiO₂/RGO blends followed a complex first-order kinetic model. Another novelty of this work is the demonstration of the ability of PVC membranes modified with Au nanoparticles to act both as filters for the removal of TiO₂/RGO blends after AC photodegradation and as potential SERS supports, which illustrate the vibrational properties of the reused catalyst. The reuse of the TiO₂/RGO blends after the first cycle of AC photodegradation indicated their suitable stability during the five cycles of pharmaceutical compound photodegradation.

Heterophase Polymorph of TiO2 (Anatase, Rutile, Brookite, TiO2 (B)) for Efficient Photocatalyst: Fabrication and Activity

TiO₂ exists naturally in three crystalline forms: Anatase, rutile, brookite, and TiO₂ (B). These polymorphs exhibit different properties and consequently different photocatalytic performances. This paper aims to clarify the differences between titanium dioxide polymorphs, and the differences in homophase, biphase, and triphase properties in various photocatalytic applications. However, homophase TiO₂ has various disadvantages such as high recombination rates and low adsorption capacity. Meanwhile, TiO₂ heterophase can effectively stimulate electron transfer from one phase to another causing superior photocatalytic performance. Various studies have reported the biphase of polymorph TiO₂ such as anatase/rutile, anatase/brookite, rutile/brookite, and anatase/TiO₂ (B). In addition, this paper also presents the triphase of the TiO₂ polymorph. This review is mainly focused on information regarding the heterophase of the TiO₂ polymorph, fabrication of heterophase synthesis, and its application as a photocatalyst.

Kinetic analysis of p-rGO/n-TiO2 nanocomposite generated by hydrothermal technique for simultaneous photocatalytic water splitting and degradation of methylene blue dye

In this study, the nanocomposites of reduced graphene oxide/TiO₂ (rGO/TiO₂ with different percentages) have been synthesized using a modified Hummers' method followed by hydrothermal treatment. The morphology and bonding structure of the prepared samples have been characterized by Fourier-transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffractometry (XRD), and X-ray photoelectron spectroscopy (XPS). The photo-characteristic aspects of the prepared samples have been indicated by photoluminescence (PL) emission spectroscopy and ultraviolet-visible diffuse reflection spectroscopy (DRS). The photocatalytic performance of rGO/TiO₂ demonstrated that it is an effective photocatalyst for methylene blue (MB) dye decomposition through illumination by a mercury lamp. Within 60 min of continuous irradiation, the nanocomposite-induced MB decomposition reached a rate of over 99%. Different MB concentrations and optimal percent loadings in catalysts have been investigated. Furthermore, the results showed that as the amount of catalyst increased, the decomposition of MB enhanced. Finally, the loading percentage of rGO with TiO₂ has been studied, and an empirical equation relating the reaction rate constant until the mass of the photocatalyst and dye concentration has been proposed. The results showed that the prepared nanocomposites had good photocatalytic activity toward water splitting and photo-decomposition of MB.

Photocatalytic activity of graphene oxide-TiO2 nanocomposite on dichlorvos and malathion and assessment of toxicity changes due to photodegradation

Heterogeneous photocatalysis was used for the removal of two widely used organophosphorus pesticides, dichlorvos, and malathion from water. Graphene oxide-TiO₂ nanocomposite (GOT) was synthesized and used as a photocatalyst for the removal of these pesticides. Batch studies for optimizing photocatalytic degradation and mineralization of pesticides over 80 min were conducted by varying the pH (2-10), catalyst dose (20 mg/L-200 mg/L), and initial pesticide concentration (0.5 mg/L-20 mg/L), and the irradiation source (125 W UV and visible lamp). Degradation kinetics for the pesticides were evaluated. Ellman assay was used to estimate the toxic effect of pesticides and evaluate toxicity reduction due to treatment. The highest degradation and mineralization of dichlorvos and malathion was observed at pH 6 and the optimum catalyst dose was 60 mg/L. Under UV irradiation, 80% and 90% degradation were observed for dichlorvos and malathion, respectively for 0.5 mg/L initial pesticide concentration. The photocatalytic degradation reaction followed Langmuir-Hinshelwood kinetics. A high degree of mineralization was achieved for both the pesticides. Analysis of the results revealed that the residual toxic effect after photocatalysis was primarily due to the residual parent compound. A comparative study revealed that GOT yielded better pesticide degradation compared to commercially available TiO₂ under both UV and visible irradiation.

Rapid Microwave-Assisted Synthesis of N/TiO2/rGO Nanoparticles for the Photocatalytic Degradation of Pharmaceuticals

Nanocomposites comprising nitrogen-doped TiO₂ and reduced graphene oxide (N/TiO₂/rGO), with different rGO loading qualities, were prepared by a cost-effective microwave-assisted synthesis method. The synthesized materials were broadly characterized by Raman spectroscopy, X-ray diffraction (XRD), infrared spectroscopy (FTIR), photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS), electron microscopy (SEM-EDS), and nitrogen adsorption/desorption isotherms. Anatase was the only crystalline phase observed for all synthesized materials. The rGO loading did not affect the morphological properties, but it positively influenced the photocatalytic activity of the nanocomposite materials, especially at low rGO loading. Photocatalysts were evaluated via the degradation of specific organic micropollutant (OMP) pharmaceuticals: ciprofloxacin (CIP), diclofenac (DCF), and salicylic acid (SA), under different radiation sources: ultraviolet A (UVA), solar light simulator (SLS), blue visible light (BVL) and cold visible light (CVL). CIP and SA were removed effectively via the synergy of adsorption and photocatalysis, while DCF degradation was achieved solely by photocatalysis. After implementing scavenger agents, photocatalytic degradation processes mainly depended on the specific pollutant type, while irradiation sources barely defined the photocatalytic mechanism. On the other hand, changes in irradiation intensity significantly influenced the photolysis process, while photocatalysis was slightly affected, indicating that irradiation spectra are more relevant than intensity.

Graphene-based nanocomposites and nanohybrids for the abatement of agro-industrial pollutants in aqueous environments

Incessant release of a large spectrum of agro-industrial pollutants into environmental matrices remains a serious concern due to their potential health risks to humans and aquatic animals. Existing remediation techniques are unable to remove these pollutants, necessitating the development of novel treatment approaches. Due to its unique structure, physicochemical properties, and broad application potential, graphene has attracted a lot of attention as a new type of two-dimensional nanostructure. Given its chemical stability, large surface area, electron mobility, superior thermal conductivity, and two-dimensional structure, tremendous research has been conducted on graphene and its derived composites for environmental remediation and pollution mitigation. Various methods for graphene functionalization have facilitated the development of different graphene derivatives such as graphene oxide (GO), functional reduced graphene oxide (frGO), and reduced graphene oxide (rGO) with novel attributes for multiple applications. This review provides a comprehensive read on the recent progress of multifunctional graphene-based nanocomposites and nanohybrids as a promising way of removing emerging contaminants from aqueous environments. First, a succinct overview of the fundamental structure, fabrication techniques, and features of graphene-based composites is presented. Following that, graphene and GO functionalization, i.e., covalent bonding, non-covalent, and elemental doping, are discussed. Finally, the environmental potentials of a plethora of graphene-based hybrid nanocomposites for the abatement of organic and inorganic contaminants are thoroughly covered.

Synthesis and Property Examination of Er2FeSbO7/BiTiSbO6 Heterojunction Composite Catalyst and Light-Catalyzed Retrogradation of Enrofloxacin in Pharmaceutical Waste Water under Visible Light Irradiation

A new photocatalyst, Er2FeSbO7, was prepared by solid phase sintering using the high-temperature synthesis method for the first time in this paper. Er2FeSbO7/BiTiSbO6 heterojunction (EBH) catalyst was prepared by the solvent thermal method for the first time. Er2FeSbO7 compound crystallized in the pyrochlore-type architecture and cubelike crystal system; the interspace group of Er2FeSbO7 was Fd3m and the crystal cellular parameter a of Er2FeSbO7 was 10.179902 Å. The band gap (BDG) width of Er2FeSbO7 was 1.88 eV. After visible light irradiation of 150 minutes (VLGI-150min) with EBH as a photocatalyst, the removal rate (RR) of enrofloxacin (ENR) concentration was 99.16%, and the total organic carbon (TOC) concentration RR was 94.96%. The power mechanics invariable k toward ENR consistency and visible light irradiation (VLGI) time with EBH as a photocatalyzer attained 0.02296 min−1. The power mechanics invariable k which was involved with TOC attained 0.01535 min−1. The experimental results showed that the photocatalytic degradation (PCD) of ENR within pharmaceutical waste water with EBH as a photocatalyzer under VLGI was in keeping with the single-order reactivity power mechanics. The RR of ENR with EBH as a photocatalyzer was 1.151 times, 1.269 times or 2.524 times that with Er2FeSbO7 as a photocatalyst, BiTiSbO6 as a photocatalyst, or N-doping TiO2 (N-TO) as a photocatalyst after VLGI-150min. The photocatalytic activity, which ranged from high to low among above four photocatalysts, was as follows: EBHP > Er2FeSbO7 > BiTiSbO6 > N-TO. After VLGI-150min toward three periods of the project with EBH as a photocatalyst, the RR of ENR attained 98.00%, 96.76% and 95.60%. The results showed that the stability of EBH was very high. With appending trapping agent, it could be proved that the oxidative capability for degrading ENR, which ranged from strong to weak among three oxidic radicals, was as follows: superoxide anion > hydroxyl radicals (HRS) > holes. This work provides a scientific basis for the research and oriented leader development of efficient heterojunction catalysts.

A Graphene Acid - TiO2 Nanohybrid as Multifunctional Heterogeneous Photocatalyst for the Synthesis of 1,3,4-Oxadiazoles

The immobilization of TiO₂ nanoparticles on graphene acid (GA), a conductive graphene derivative densely functionalized with COOH groups, is presented. The interaction between the carboxyl groups of the surface and the titanium precursor leads to a controlled TiO₂ heterogenization on the nanosheet according to microscopic and spectroscopic characterizations. Electronic communication shared among graphene and semiconductor nanoparticles shifts the hybrid material optical features toward less energetic radiation but maintaining the conductivity. Therefore, GA-TiO₂ is employed as heterogeneous photocatalyst for the synthesis of 2,5-disubstituted 1,3,4-oxadiazoles using ketoacids and hydrazides as substrates. The material presented enhanced photoactivity compared to bare TiO₂, being able to yield a large structural variety of oxadiazoles in reaction times as fast as 1 h with full recyclability and stability. The carbocatalytic character of GA is the responsible for the substrates condensation and the GA-TiO₂ light interaction ability is able to photocatalyze the cyclization to the final 1,3,4-oxadiazoles, demonstrating the optimal performance of this multifunctional photocatalytic material.

Graphene-based TiO2 composites for photocatalysis & environmental remediation: synthesis and progress

Photoactive nanomaterials constitute an emerging field in nanotechnology, finding an extensive array of applications spanning diverse areas, including electronics and photovoltaic devices, solar fuel cells, wastewater treatment, etc. Titanium dioxide (TiO₂), in its thin-film form, has been exhaustively surveyed as potential photocatalysts for environmental remediation owing to its innocuousness, stability, and photocatalytic characteristics when subjected to ultraviolet (UV) irradiation. However, TiO₂ has some shortcomings associated with a large bandgap value of around 3.2 eV, making it less efficient in the visible spectral range. TiO₂ is often consolidated with various carbon nanomaterials to overcome this limitation and enhance its efficiency. Graphene, a 2-dimensional allotrope of carbon with a bandgap tuned between 0 and 0.25 eV, exhibits unique properties, making it an attractive candidate to augment the photoactivity of semiconductor (SC) oxides. Encapsulating graphene oxide onto TiO₂ nanospheres demonstrates intensified photocatalytic properties and exceptional recyclability for the degeneration of certain dyes, including Rhodamine B. This review encompasses various techniques to synthesize graphene-based TiO₂ photoactive composites, emphasizing graphene capsulized hollow titania nanospheres, nanofibers, core/shell, and reduced graphene oxide-TiO₂-based nanocomposites. It also consolidates the application of the aforestated nanocomposites for the disintegration of various synthetic dyes, proving efficacious for water decontamination and degradation of chemicals and pharmaceuticals. Furthermore, graphene-based TiO₂ nanocomposites used as lithium (Li)-ion batteries manifesting substantial electrochemical performance and solar fuel cells for energy production are discussed here.

Photocatalytic Fuel Cells for Simultaneous Wastewater Treatment and Power Generation: Mechanisms, Challenges, and Future Prospects

Technological advancement is accompanied by excessive consumption of fossil fuels and affluent uses of chemical substances in many sectors, including transportation and manufacturing companies, and so on. Being an exhaustible resource, the excessive use of fossil fuels and of chemical substances may lead to a serious energy crisis in the long run, and it may additionally impose environmental pollution. Attempts have been made in the solution of such serious issues from every nook and corner. Nonetheless, no method has been found to be a panacea in waste water treatment and subsequent beneficiaries. One of the attempts in the solution to such issues is the application of photocatalytic technology, which could serve as a dual function in environmental remediation and clean energy production. A photocatalytic fuel cell is a tool developed for the recovery of energy from organic wastes. A rational cell construction needs the fabrication of photoelectrodes, the design of a photoanode and a photocathode chamber, in addition to an ion-transport membrane for pollution treatment and electricity generation. In this review, comprehensive fundamental assessments and recent developments in the design of photocatalytic fuel cells, their applications, future prospects, and challenges are covered.

Ag-Doped TiO2 Composite Films Prepared Using Aerosol-Assisted, Plasma-Enhanced Chemical Vapor Deposition

TiO2 is a promising photocatalyst, but its large bandgap restricts its light absorption to the ultraviolet region. The addition of noble metals can reduce the bandgap and electron-hole recombination; therefore, we prepared TiO2-Ag nanoparticle composite films by plasma-enhanced chemical vapor deposition (PECVD) using a mixture of aerosolized AgNO3, which was used as a Ag nanoparticle precursor, and titanium tetraisopropoxide, which acted as the TiO2 precursor. Notably, the use of PECVD enabled a low process temperature and eliminated the need for pre-preparing the Ag nanoparticles, thereby increasing the process efficiency. The structures and morphologies of the deposited films were characterized by ultraviolet (UV)—visible spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray spectroscopy, and the effects of the AgNO3 concentration on the photocatalytic activity of the deposited films were determined by assessing the degradation of methylene blue under UV light irradiation. The Ag ions were successfully reduced to metallic nanoparticles and were embedded in the TiO2 film. The best photocatalytic activity was achieved for a 1 wt% Ag-loaded TiO2 composite film, which was 1.75 times that of pristine TiO2.

Magnetic Resonance Studies of Hybrid Nanocomposites Containing Nanocrystalline TiO2 and Graphene-Related Materials

Nanocomposites based on nanocrystalline titania modified with graphene-related materials (reduced and oxidized form of graphene) showed the existence of magnetic agglomerates. All parameters of magnetic resonance spectra strongly depended on the materials’ modification processes. The reduction of graphene oxide significantly increased the number of magnetic moments, which caused crucial changes in the reorientation and relaxation processes. At room temperature, a wide resonance line dominated for all nanocomposites studied and in some cases, a narrow resonance line derived from the conduction electrons. Some nanocomposites (samples of titania modified with graphene oxide, prepared with the addition of water or butan-1-ol) showed a single domain magnetic (ferromagnetic) arrangement, and others (samples of titania modified with reduced graphene oxide) exhibited magnetic anisotropy. In addition, the spectra of EPR from free radicals were observed for all samples at the temperature of 4 K. The magnetic resonance imaging methods enable the capturing of even a small number of localized magnetic moments, which significantly affects the physicochemical properties of the materials.

Wavelength Dependence of the Transformation Mechanism of Sulfonamides Using Different LED Light Sources and TiO2 and ZnO Photocatalysts

The comparison of the efficiency of the commercially available photocatalysts, TiO2 and ZnO, irradiated with 365 nm and 398 nm light, is presented for the removal of two antibiotics, sulfamethazine (SMT) and sulfamethoxypyridazine (SMP). The •OH formation rate was compared using coumarin, and higher efficiency was proved for TiO2 than ZnO, while for 1,4-benzoquinone in O2-free suspensions, the higher contribution of the photogenerated electrons to the conversion was observed for ZnO than TiO2, especially at 398 nm irradiation. An extremely fast transformation and high quantum yield of SMP in the TiO2/LED398nm process were observed. The transformation was fast in both O2 containing and O2-free suspensions and takes place via desulfonation, while in other cases, mainly hydroxylated products form. The effect of reaction parameters (methanol, dissolved O2 content, HCO3- and Cl-) confirmed that a quite rarely observed energy transfer between the excited state P25 and SMP might be responsible for this unique behavior. In our opinion, these results highlight that "non-conventional" mechanisms could occur even in the case of the well-known TiO2 photocatalyst, and the effect of wavelength is also worth investigating.

Ultrasound-Assisted Synthesis of Nonmetal-Doped Titanium Dioxide Photocatalysts for Simultaneous H2 Production and Chemical Oxygen Demand Removal from Industrial Wastewater

A series of nonmetal-doped titanium dioxide (Nm _x /TiO₂, where x is the weight fraction of nonmetal elements) photocatalysts was prepared via ultrasonic-assisted impregnation for simultaneous hydrogen (H₂) production and chemical oxygen demand (COD) removal from industrial wastewater. Three types of Nm elements, carbon (C), silicon (Si), and phosphorus (P), were explored. The P₁/TiO₂ exhibited a higher photocatalytic activity for H₂ production and COD removal than the C₁/TiO₂ and Si₁/TiO₂ photocatalysts. Approximately 6.43 mmol/g photocatalyst of H₂ was produced, and around 26% COD removal was achieved at a P₁/TiO₂ loading of 4.0 g/L, a light intensity of 5.93 mW/cm², and a radiation time of 4 h. This is because the P₁/TiO₂ photocatalyst exhibited lower point of zero charge values and a more appropriate band position compared with other Nm _x /TiO₂ photocatalysts to produce H⁺, which can consequently form H₂, and reactive oxygen species (HO^· and O₂ ^{· -}), which serve as oxidizing agents to degrade the organic pollutants. Increasing the content of the P element doped into the TiO₂-based material up to 7.0% by weight enhanced the H₂ production and COD removal up to 8.34 mmol/g photocatalyst and 50.6%, respectively. This is attributed to the combined effect of the point of zero charge value and the S _BET of the prepared photocatalysts. The photocatalytic activity of the P₇/TiO₂ photocatalyst was still higher than the TiO₂-based material after the fourth use.

Photocatalysis and Li-Ion Battery Applications of {001} Faceted Anatase TiO2-Based Composites

Anatase TiO2 are the most widely used photocatalysts because of their unique electronic, optical and catalytic properties. Surface chemistry plays a very important role in the various applications of anatase TiO2 especially in the catalysis, photocatalysis, energy conversion and energy storage. Control of the surface structure by crystal facet engineering has become an important strategy for tuning and optimizing the physicochemical properties of TiO2. For anatase TiO2, the {001} crystal facets are the most reactive because they exhibit unique surface characteristics such as visible light responsiveness, dissociative adsorption, efficient charge separation capabilities and photocatalytic selectivity. In this review, a concise survey of the literature in the field of {001} dominated anatase TiO2 crystals and their composites is presented. To begin, the existing strategies for the synthesis of {001} dominated anatase TiO2 and their composites are discussed. These synthesis strategies include both fluorine-mediated and fluorine-free synthesis routes. Then, a detailed account of the effect of {001} facets on the physicochemical properties of TiO2 and their composites are reviewed, with a particular focus on photocatalysis and Li-ion batteries applications. Finally, an outlook is given on future strategies discussing the remaining challenges for the development of {001} dominated TiO2 nanomaterials and their potential applications.

Recent progress on Ag/TiO2 photocatalysts: photocatalytic and bactericidal behaviors

For many decades, titanium dioxide (TiO₂) semiconductor has been extensively applied in several environmental applications due to its higher photocatalytic performances toward different organic pollutants, pharmaceutical compounds, and bacteria. However, its shortfall response to visible light, and the expeditious recombination rate of the photogenerated electron-hole pairs, hampers its utilization. Doping TiO₂ semiconductor with silver nanoparticles is a sound strategy to (1) extend its photocatalytic activity to visible light, (2) prevent the electron/holes pairs recombination due to the formation of the Schottky barrier at the interfaces with TiO₂ that act as an electron-trapping center, and (3) enhance its bactericide performances. This review focuses on the recent progress on silver-doped titanium dioxide (Ag/TiO₂)-based photocatalysts. It addresses a wide range of Ag/TiO₂ synthesis techniques, their physicochemical properties and discusses thoroughly the important role of silver (Ag) nanoparticles in enhancing the removal capacity and antibacterial performances of the Ag/TiO₂ photocatalysts.

Photocatalysis of dichlorvos using graphene oxide-TiO2nanocomposite under visible irradiation: process optimization using response surface methodology

Graphene oxide-TiO₂nanocomposite (GOT) was used for degradation and mineralization of dichlorvos, an organophosphorus pesticide, from aqueous solution under visible irradiation. The nanocomposite was characterized by scanning electron microscopy, transmission electron microscopy, UV-DRS, Fourier-transform infrared spectroscopy, Raman spectroscopy, and x-ray photoelectron spectroscopy. Anatase phase TiO₂nanoparticles (10-20 nm in diameter) were present in the nanocomposite. The nanoparticles were uniformly distributed on reduced GO sheets. A three-factor face-centered central composite design with response surface methodology was used for modeling and optimization of various variables that may potentially affect photodegradation, i.e. pH, catalyst loading, and initial dichlorvos concentration. A quadratic model was built to predict degradation, mineralization efficiency, and reaction rate constant. The experimental and predicted values depicted a good correlation and the utility of the models was confirmed by the highF-values observed for the degradation and mineralization models. High coefficient of determination (R²) was obtained for the degradation (R² = 0.95) and mineralization (R² = 0.93) models. Pareto analysis was carried out to determine the effect of each variable on photocatalytic degradation and mineralization. The predicted results suggested that the optimum conditions for obtaining maximum degradation (69%) and mineralization (64%) were: initial dichlorvos concentration of 0.5 mg l^-1with a catalyst dose of 110 mg l^-1at pH 6.5. The main effect plots also suggested a significant influence of the variables used in the photocatalysis of dichlorvos by GOT.

Noble-Metal-Free Multicomponent Nanointegration for Sustainable Energy Conversion

Global energy and environmental crises are among the most pressing challenges facing humankind. To overcome these challenges, recent years have seen an upsurge of interest in the development and production of renewable chemical fuels as alternatives to the nonrenewable and high-polluting fossil fuels. Photocatalysis, photoelectrocatalysis, and electrocatalysis provide promising avenues for sustainable energy conversion. Single- and dual-component catalytic systems based on nanomaterials have been intensively studied for decades, but their intrinsic weaknesses hamper their practical applications. Multicomponent nanomaterial-based systems, consisting of three or more components with at least one component in the nanoscale, have recently emerged. The multiple components are integrated together to create synergistic effects and hence overcome the limitation for outperformance. Such higher-efficiency systems based on nanomaterials will potentially bring an additional benefit in balance-of-system costs if they exclude the use of noble metals, considering the expense and sustainability. It is therefore timely to review the research in this field, providing guidance in the development of noble-metal-free multicomponent nanointegration for sustainable energy conversion. In this work, we first recall the fundamentals of catalysis by nanomaterials, multicomponent nanointegration, and reactor configuration for water splitting, CO₂ reduction, and N₂ reduction. We then systematically review and discuss recent advances in multicomponent-based photocatalytic, photoelectrochemical, and electrochemical systems based on nanomaterials. On the basis of these systems, we further laterally evaluate different multicomponent integration strategies and highlight their impacts on catalytic activity, performance stability, and product selectivity. Finally, we provide conclusions and future prospects for multicomponent nanointegration. This work offers comprehensive insights into the development of cost-competitive multicomponent nanomaterial-based systems for sustainable energy-conversion technologies and assists researchers working toward addressing the global challenges in energy and the environment.

(Ti,Sn) Solid Solution Based Gas Sensors for New Monitoring of Hydraulic Oil Degradation

The proper operation of a fluid power system in terms of efficiency and reliability is directly related to the fluid state; therefore, the monitoring of fluid ageing in real time is fundamental to prevent machine failures. For this aim, an innovative methodology based on fluid vapor analysis through metal oxide (shortened: MOX) gas sensors has been developed. Two apparatuses were designed and realized: (i) a dedicated test bench to fast-age the fluid under controlled conditions; (ii) a laboratory MOX sensor system to test the headspace of the aged fluid samples. To prepare the set of MOX gas sensors suitable to detect the analytes’ concentrations in the fluid headspace, different functional materials were synthesized in the form of nanopowders, characterizing them by electron microscopy and X-ray diffraction. The powders were deposited through screen-printing technology, realizing thick-film gas sensors on which dynamical responses in the presence of the fluid headspace were obtained. It resulted that gas sensors based on solid solution Ti_xSn_1–xO₂ with x = 0.9 and 0.5 offered the best responses toward the fluid headspace with lower response and recovery times. Furthermore, a decrease in the responses (for all sensors) with fluid ageing was observed.

Hydrothermal Synthesis of rGO-TiO2 Composites as High-Performance UV Photocatalysts for Ethylparaben Degradation

A series of reduced graphene oxide-TiO2 composites (rGO-TiO2) were prepared by hydrothermal treatment using graphite and titanium isopropoxide as raw materials. The structural, surface, electronic, and optical properties of the prepared composites were extensively characterized by N2 adsorption, FTIR, XRD, XPS, Raman spectroscopy, and DRS. GO was found to be effectively reduced and TiO2 to be in pure anatase phase in all composites obtained. Finally, experiments were performed to evaluate the effectiveness of these new materials as photocatalysts in the degradation of ethylparaben (EtP) by UV radiation. According to the band-gap energies obtained (ranging between 3.09 eV for 4% rGO-TiO2 to 2.55 eV for 30% rGO-TiO2), the rGO-TiO2 composites behave as semiconductor materials. The photocatalytic activity is highest with a rGO content of 7 wt% (7% rGO-TiO2), being higher than observed for pure TiO2 (Eg = 3.20 eV) and achieving 98.6% EtP degradation after only 40 min of treatment. However, the degradation yield decreases with higher percentages of rGO. Comparison with rGO-P25 composites showed that a better photocatalytic performance in EtP degradation is obtained with synthesized TiO2 (rGO-TiO2), probably due to the presence of the rutile phase (14.1 wt %) in commercial P25.

Nanoarchitectonics of Nanoporous Carbon Materials in Supercapacitors Applications

High surface area and large pore volume carbon materials having hierarchical nanoporous structure are required in high performance supercapacitors. Such nanoporous carbon materials can be fabricated from organic precursors with high carbon content, such as synthetic biomass or agricultural wastes containing cellulose, hemicellulose, and lignin. Using recently developed unique concept of materials nanoarchitectonics, high performance porous carbons with controllable surface area, pore size distribution, and hierarchy in nanoporous structure can be fabricated. In this review, we will overview the recent trends and advancements on the synthetic methods for the production of hierarchical porous carbons with one- to three-dimensional network structure with superior performance in supercapacitors applications. We highlight the promising scope of accessing nanoporous graphitic carbon materials from: (i) direct conversion of single crystalline self-assembled fullerene nanomaterials and metal organic frameworks, (ii) hard- and soft-templating routes, and (iii) the direct carbonization and/or activation of biomass or agricultural wastes as non-templating routes. We discuss the appealing points of the different synthetic carbon sources and natural precursor raw-materials derived nanoporous carbon materials in supercapacitors applications.

Steering the Charge Kinetics in Dual-Functional Photocatalysis by Surface Dipole Moments and Band Edge Modulation: A Defect Study in TiO2-ZnS-rGO Composites

Developing an efficient photocatalyst for concurrent hydrogen production and environmental remediation by using solar energy is a challenge. Defect engineering, although it offers a strategical promise to enhance the photocatalytic performance, has limitations that come from the ambiguity surrounding its role. In the current work, a comprehensive study on defects in promoting the charge transfer, band edge modulation, and surface reaction was carried out. The excess electrons springing from defects act like donor states and cause band bending at the junction interface. Characterization techniques such as X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, electron spin resonance, and photoluminescence were employed to investigate defect functionality, and its ultimate effect on photocatalytic performance was studied by simultaneous H₂ production and methylene blue degradation. The role of graphene in optoelectronics and defect formation in the composite catalysts was explored. In addition, efforts have been made to unveil the reaction pathway for hydrogen evolution reaction and oxygen evolution reaction where excess defect density greatly hampered the quantum yield of the process. Results suggest that maintaining optimal defect concentration aborts the undesired thermodynamically favored back reactions. The conduction band and valence band values of the catalysts indicate that the photocatalytic mechanism was dominated by the electron pathway. Graphene acted as an effective electron sink when its concentration was around 2.5-3%. The superior activity of TiO₂-ZnS-rGO was attributed to the narrow bandgap, rapid separation of photo-excited charge carriers, and favorable conduction band position for photocatalytic reactions. This work may assist in exploring the fundamental role of defects in driving the photocatalytic reactions and improve the selectivity in heterogeneous photocatalysis.

Simultaneous Detection and Photocatalysis Performed on a 3D Graphene/ZnO Hybrid Platform

The synergy between graphene foam (Gf) and ZnO nanoparticles (NPs) allows the detection of analytes at low conentrations, which can be subsequently photocatalyzed on the hybrid surface as well as in the liquid phase upon illumination with low-power UV-vis light-emitting diode (LED) lamps. Detection of methylene blue (MB) and bisphenol A (BPA) is monitored either by graphene-enhanced Raman scattering (GERS) or molecular doping/sensing upon analyte adsorption. Using GERS, we were able to detect concentrations as low as 0.3 ppm of MB, which remained adsorbed on the graphene surface after a photocatalytic conversion of 88% (total conversion). The photocatalysis performances of BPA and MB performed in the liquid phase were lower and corresponded to 73 and 33% as indicated by gas chromatography-mass spectrometry (GC/MS) and UV-vis, respectively. The kinetics of photocatalysis was fitted with a quasi-first-order reaction, and the apparent rate constant (k_app) was calculated according to the Langmuir-Hinshelwood model. The fastest kinetics was achieved with the hybrid platform named "Gf-ZnO400", which was thermally treated at high temperatures and with most of the Ni etched away. This is consistent with the excellent electronic interaction between ZnO and graphene foam as indicated by photoelectrochemistry analysis. We mainly employed Raman scattering and UV-vis spectroscopy analyses for detection and photocatalysis applications; however, we also used other complementary techniques such as focused ion-beam scanning electron microscopy (FIB-SEM), X-ray photoelectron spectroscopy (XPS), diffuse reflectance, GC/MS, and photoelectrochemistry to explore the synergetic behavior of these two nanomaterials. This work brings about new insights into the detection of analyte molecules followed by photocatalysis performed in the solid and liquid states.

Manganese and Graphene Included Titanium Dioxide Composite Nanowires: Fabrication, Characterization and Enhanced Photocatalytic Activities

We report the detailed microstructural, morphological, optical and photocatalytic studies of graphene (G) and manganese (Mn) co-doped titanium dioxide nanowires (TiO₂(G–Mn) NWs) prepared through facile combined electrospinning–hydrothermal processes. The as-prepared samples were thoroughly characterized using X-ray diffraction (XRD), transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and diffuse reflectance spectroscopy (DRS). XRD studies reveal the formation of mixed anatase-rutile phases or rutile phase depending on the dopant (Mn) precursor concentrations in the electrospinning dope and calcination temperature. The evaluation of lattice parameters revealed that the incorporation of Mn species and carbon atoms in to the lattice of anatase or rutile TiO₂ could occur through substituting the sites of oxygen atoms. XPS results confirm the existence of Mn²⁺/Mn³⁺ within the TiO₂ NW. Raman spectroscopy provides the evidence for structural modification because of the graphene inclusion in TiO₂ NW. The optical band gap of G–Mn including TiO₂ is much lower than pristine TiO₂ as confirmed through UV-vis DRS. The photocatalytic activities were evaluated by nitric oxide (NOx) degradation tests under visible light irradiation. Superior catalytic activity was witnessed for rutile G–Mn-co-doped TiO₂ NW over their anatase counterparts. The enhanced photocatalytic property was discussed based on the synergistic effects of doped G and Mn atoms and explained by plausible mechanisms.

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.

Visible-Light Active Titanium Dioxide Nanomaterials with Bactericidal Properties

This article provides an overview of current research into the development, synthesis, photocatalytic bacterial activity, biocompatibility and cytotoxic properties of various visible-light active titanium dioxide (TiO2) nanoparticles (NPs) and their nanocomposites. To achieve antibacterial inactivation under visible light, TiO2 NPs are doped with metal and non-metal elements, modified with carbonaceous nanomaterials, and coupled with other metal oxide semiconductors. Transition metals introduce a localized d-electron state just below the conduction band of TiO2 NPs, thereby narrowing the bandgap and causing a red shift of the optical absorption edge into the visible region. Silver nanoparticles of doped TiO2 NPs experience surface plasmon resonance under visible light excitation, leading to the injection of hot electrons into the conduction band of TiO2 NPs to generate reactive oxygen species (ROS) for bacterial killing. The modification of TiO2 NPs with carbon nanotubes and graphene sheets also achieve the efficient creation of ROS under visible light irradiation. Furthermore, titanium-based alloy implants in orthopedics with enhanced antibacterial activity and biocompatibility can be achieved by forming a surface layer of Ag-doped titania nanotubes. By incorporating TiO2 NPs and Cu-doped TiO2 NPs into chitosan or the textile matrix, the resulting polymer nanocomposites exhibit excellent antimicrobial properties that can have applications as fruit/food wrapping films, self-cleaning fabrics, medical scaffolds and wound dressings. Considering the possible use of visible-light active TiO2 nanomaterials for various applications, their toxicity impact on the environment and public health is also addressed.

Electrospun reduced graphene oxide/TiO2/poly(acrylonitrile-co-maleic acid) composite nanofibers for efficient adsorption and photocatalytic removal of malachite green and leucomalachite green

Electrospun reduced graphene oxide/TiO₂/poly(acrylonitrile-co-maleic acid) composite nanofibers (E-spun RGO/TiO₂/PANCMA NFs) were fabricated using electrospinning of the dispersive solution of PANCMA, GO and TiO₂ followed by post-chemical reduction. The obtained composite nanofibers were compressed in a dialyzer and then used to absorb and degrade malachite green (MG) and leucomalachite green (LMG) from aqueous solution. Compared to the E-spun TiO₂/PANCMA and GO/TiO₂/PANCMA NFs, the E-spun RGO/TiO₂/PANCMA NFs exhibited higher adsorption capacity and photocatalytic degradation ability. Under optimized conditions, 90.6% of MG and 93.7% of LMG from 50 mL aqueous sample solution were adsorbed on the RGO/TiO₂/PANMA NFs (3.0 mg fibers) in 2.0 min, and subsequent the 91.4% and 95.2% of MG and LMG adsorbed on the NFs were degradated in 60 min under UV irradiation, respectively. In addition, the E-spun RGO/TiO₂/PANMA NFs exhibited good reusability and could be reused in multiple cycles of operations for adsorption and photocatalytic degradation of MG and LMG. This work demonstrated that the electrospun composite nanofibers are promising materials for removal of pollutants from environmental water samples.

New insights into the synergistic effect of active radicals and adsorptive ability on the photodegradation of gaseous acetaldehyde over reduced graphene Oxide/P25 composite

Although coupling reduced graphene oxide (RGO) with TiO₂ is believed to enhance the photocatalysis through the light utilization, studies on its photothermal conversion effect are rarely reported. Herein, RGOP (reduced graphene oxide/P25) was synthesized to explore roles of the enhanced light adsorption and photothermal conversion in the photocatalytic process. It was found that although RGOP had increased absorbance, it actually possessed lower available light utilization compared with P25. In the synergistic effect of available light utilization, transfer resistance and hydrophilicity, RGOP exhibited less superoxide radicals but more hydroxyl radicals. In the presence of scavenger experiments, O₂^- was proved to play the predominant role in the photocatalytic process, while OH was the secondary one. In comparison to P25, the change of active radicals of RGOP was adverse to its photocatalysis. However, due to the superior adsorptive property of RGOP, it exhibited higher photocatalytic activity than P25. The improved photocatalytic activity of RGOP was ascribed to its superior adsorptive ability aside from active radicals (O₂-, OH).