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Photocatalytic hydrogen production under solar light irradiation is an attractive and appealing technology to produce green and renewable hydrogen fuel to reduce CO2 emission and air pollution. Due to its special physicochemical properties, TiO2 photocatalysts have been commonly used as a promising photocatalyst for hydrogen production. However, its wide bandgap energy, which can be only activated under UV-light and fast charge carriers recombination restricts their photocatalytic applications. Nevertheless, several approaches have been proposed to reduce its bandgap energy for visible light activity. Among them, the doping TiO2 with a dopant can be regarded as an effective method not only to cause a shift of the light absorption toward the visible region but also to diminish the charge recombination. This review concentrated on the influence of different types of dopants on the enhancement of the photocatalytic activity of TiO2. Furthermore, the methods of synthesis and the characterization techniques used for doped-TiO2 were carefully checked. Importantly, this review examined that the photocatalytic hydrogen production activity of the doped-TiO2 depends heavily on the type and concentration of the dopants. Furthermore, a metal-like Cu, noble metals like Au and Pt are the most widely studied dopants for TiO2 due to their superior work function. In addition, nitrogen was also widely used as a nonmetal owing to its ability to reduce the bandgap energy of TiO2. The findings also showed that the sol-gel method is still widely used in the preparation of doped-TiO2 photocatalysts due to the high crystallinity and surface area of the obtained nanoparticles.

The g-C3N4/NaNbO3 photocatalyst was synthesized by simply calcining the mixture of NaNbO3 and melamine. The synthesized composite exhibits high photocatalytic performance in the selective oxidation of 5-Hydroxymethylfurfural (HMF) to 5-formyl-2-furancarboxylic acid (FFCA) when using water as solvent. The structure and composition of g-C3N4/NaNbO3 photocatalysts were characterized by TG, XRD, SEM, UV–Vis, FT-IR, and XPS methods, and the optical and electrochemical properties were investigated by EIS, PC, and PL techniques. O2− was inferred to be the primary active species in this process based on the active species trapping experiment. Heterostructure formation of g-C3N4/NaNbO3 composites efficiently promoted the separation of photo-generated electron-hole pairs and accelerated the electron transfer rate, thus reduced the formation of OH, and sequentially improved the selectivity of FFCA. The highest HMF conversion of 35.8% with FFCA selectivity of 87.4% was achieved on C/N-59.6 photocatalyst under the irradiation of visible light. The possible mechanism and reaction route were also proposed.

The objective of this paper was to prepare, characterize and evaluate the degradation efficiency of organic pollutants on a perovskite-type LaFeO3-coupled graphitic carbon nitride (g-C3N4) photocatalyst under visible light irradiation. The photocatalyst was synthesized by a simple calcination method. The physical and photophysical properties of the LaFeO3/g-C3N4 composite photocatalyst with various weight ratios of LaFeO3 were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), N2 adsorption–desorption isotherm measurement, UV-vis diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy, and electrochemical measurements. According to the flat band potential estimated by the Mott–Schottky plot and the optical band gap measurements, both semiconductors (LaFeO3 and g-C3N4) can absorb visible light and have band edge positions that allow the transfer of photoelectrons from g-C3N4 to LaFeO3. As compared to g-C3N4, the C 1s and N 1s peaks of the LaFeO3/g-C3N4 composite showed an obvious shift to lower binding energy, implying the existence of a synergistic effect between g-C3N4 and LaFeO3. Furthermore, photocurrent measurement and electrochemical impedance spectroscopy revealed the higher photo-induced charge carrier separation efficiency of the LaFeO3/g-C3N4 composite. The results show that the visible light activity of the composite photocatalyst LaFeO3/g-C3N4 for the degradation of Rhodamine B (RhB) and 4-chlorophenol (4-CP) is higher than that of pure LaFeO3 and g-C3N4, respectively. However, the main reason for the enhanced activity was attributed to the interfacial transfer of photogenerated electrons and holes between LaFeO3 and g-C3N4, leading to the effective charge separation in the composite, inhibited recombination of electron–hole pairs and, finally, enhanced photocatalytic performance of the composite. It was found that the holes, hydroxyl radicals (•OH) and superoxide radical ions (O2−) are the main reactive species in the degradation reaction of RhB and 4-CP over the LaFeO3/g-C3N4 composite photocatalyst. More importantly, on the basis of the estimated conduction band (CB) from the Mott–Schottky plots, LaFeO3 and the composite show no activity toward hydrogen production under visible light. This work can be applied for the production of other visible-light-responsive photocatalysts based on g-C3N4 that have potential in environmental purification applications.

This paper reports the synthesis of bare TiO2 and various molar concentrations of ruthenium (Ru)-doped TiO2 nanoparticles by the precipitation method. The as-synthesized photocatalysts were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS), N2 adsorption/desorption techniques, X-ray photoelectron spectroscopy (XPS), energy dispersive spectroscopy (EDS), Raman spectroscopy, photoluminescence spectroscopy (PL), and electrochemical measurements. The results from the UV-vis spectroscopy clearly indicated a red-shift of the optical response toward the visible region owing to the reduced band gap energy, thus showing an enhancement of the absorption in the visible spectrum. The XRD study showed that the samples were crystallized with the photoactive anatase phase of TiO2. The microstructural study using Raman spectroscopy indicated that the Ru dopant occupied the substitutional sites in the TiO2 lattice. According to the Mott–Schottky analysis, the flat band potential of the Ru-doped TiO2 was shifted to the negative potential. The photocurrent, electrochemical impedance spectroscopy, and photoluminescence revealed a higher photogenerated charge-carriers separation efficiency of the doped sample. The photocatalytic activities of the bare TiO2 and (0.05–0.2 mol%) Ru-doped TiO2 nanoparticles were examined by studying the hydrogen production from water using methanol as a sacrificial reagent and Pt nanoparticles as a cocatalyst under light of λ ≥ 320. The optimal iron content was determined to be 0.1 mol% and the corresponding hydrogen production rate was 3400 μmol h−1 in aqueous methanol, which is enhanced by more than 2 times compared to bare TiO2 (1500 μmol h−1) under the same reaction conditions. The higher activity for the doped materials was attributed to the presence of the Ru dopant to facilitate the visible-light-driven activity by introducing the electron donor/acceptor level of ruthenium and the mid-band energy level of defects between the conduction band minimum and valence band maximum of TiO2.

Herein, porous graphitic carbon nitrides (g-C3N4) were synthesized via a one-step pyrolysis process using different low-cost, environmentally benign, nitrogen-rich organic precursors, namely, urea, thiourea, and melamine. The physicochemical and photophysical properties of the obtained g-C3N4 samples were characterized by X-ray diffraction (XRD), solid-state MAS NMR, X-ray photoelectron microscopy (XPS), transmission electron microscopy (TEM), N2 adsorption/desorption, thermogravimetric analysis (TGA), UV-Vis diffuse reflectance absorption spectra (DRS), photoluminescence and electrochemical measurements. This study shows that the photocatalytic hydrogen evolution under visible light is around five times higher if urea is used as the g-C3N4 precursor instead of thiourea or melamine. This finding is confirmed by the lower degree of polymerization of the g-C3N4 sample formed from urea, leading to an altered porous structure, higher pore volume, and enhanced surface area. Moreover, the resulting structural imperfections lead to slightly more active sites as indicated by the most negative conduction band potential in urea-based g-C3N4.

YFeO3/g-C3N4 composites with varying weight ratios were prepared following three different methods The optimum weight ratio and the method of preparation for enhanced visible photocatalytic activity towards the degradation of organic contaminants were identified. The successful formation of the composites was proven by XRD and IR measurements. XPS analysis shows that the inactive Y2O3 passive layer on the hexagonal YFeO3 (h-YFeO3) disappeared during the composite formation, resulting in a direct and intimate contact of h-YFeO3/g-C3N4 which is crucial for the photocatalytic activities. The loading of h-YFeO3 varied from 1 to 20% and the highest activity was achieved at 2.4%. At the optimum loading 80% degradation of methyl orange (MO) was obtained under 5 h visible light illumination. The composite photocatalyst shows also modest activity for the photodegrading of 4-chlorophenol (4-CP). Additionally the synthesized composites were characterized by high resolution transmission electron microscopy (HR-TEM), Fourier transform infrared spectroscopy (FT-IR) and diffuse reflection UV–vis spectroscopy (DR-UV–vis). In order to determine the relative band positions of the composite photocatalysts their flat band potentials were estimated from Mott-Schottky analysis. Improved charge separation in the composite is mainly responsible for the enhanced photoactivity and supported by photoluminescence experiments.

Ferrites have been widely known as excellent photocatalysts thanks to their excellent properties such as relatively narrow bandgaps, chemical, and thermal stability, excellent magnetic property, and low-cost. They have recently been explored for their visible-light activity for various photocatalysis applications. In this review, the classification of ferrites according to their crystalline structure is discussed. Several ferrites synthesis approaches have been reviewed including the citric acid-assisted sol-gel method, hydrothermal methods, coprecipitation, and solid-state reactions. Besides, optical, structural, electronic, and morphological properties of ferrites were characterized using several physical and analytical methods are highlighted. A comprehensive survey on their photocatalytic applications with more emphasis on their novelties is briefly reviewed. These applications not only include the degradation of organic and inorganic pollutants and bacterial disinfection, but also on hydrogen production via water splitting, and CO2 photoreduction. In this review, we summarize the latest developments in designing and developing highly efficient ferrite photocatalysts. We conclude with an overview and some observations on challenges and future directions in the exploration as a photocatalyst of ferrite-based materials.

In recent years, heterogeneous photocatalysis using semiconductor and solar energy has gained a lot of attention owing to its potential applications in many fields such as the generation of renewable and sustainable hydrogen fuel, degradation of organic pollutants, and CO2 photoreduction. Due to its photochemical stability, fascinating electronic band structures, and effective light harvesting accompanied by suitable bandgap energy of 2.7 eV, graphitic carbon nitride (g-C3N4) has been considered as a promising metal-free photocatalyst for solving the energy crisis and environmental problems. Nevertheless, due to the low-charge carrier mobility, and low surface area, its photocatalytic efficiency is limited. Because of its unique band structure, coupling it with a large bandgap semiconductor to produce a heterojunction composite offers a promising way to improve the charge separation, expand the surface area, and boost light absorption. For that, g-C3N4 as a robust photocatalyst together with numerous synthesis methods has been highlighted. Three primary systems of g-C3N4-based nanocomposites have been classified and summarized: namely metal oxides, sulfides, and ferrites. We have summarized the latest applications of g-C3N4/nanocomposites in solar energy conversion and environmental remediation. This review ends with an overview and several insights on the problems and future avenues in investigating advanced nanomaterials based on g-C3N4.

In the present work, bare TiO2 and different molar concentrations of iron (Fe3+)-doped TiO2 photocatalysts (0.05−1 mol % Fe) were prepared by a simple precipitation method for photocatalytic hydrogen production and degradation of organic pollutants (MO and 4-CP) under light irradiation of λ ≥ 320 nm. XRD revealed the presence of anatase phase for both bare TiO2 and Fe doped titania. Based on the XPS results, Fe3+ ions are hardly seen may be attributed to the location of iron inside the titania matrix rather than their appearance at the surface. UV–vis-DRS of the doped titania demonstrated a redshift toward visible region owing to the reduced bandgap energy. N2 adsorption results showed that the doped materials have higher surface area compared to bare TiO2 due to decrease particle size. Importantly, electrochemical experiments (electrochemical impedance spectroscopy (EIS), photocurrent (PC), and photoluminescence (PL)) revealed that higher photogenerated charge carrier separation efficiency of iron-doped titania. Results showed that the 0.1 mol % Fe doped TiO2 exhibited the highest photocatalytic activity for both degradation and H2 evolution reactions. Furthermore, high surface area, small particle size and enhanced visible-light absorption as well as improved charge transfer and separation are believed to be responsible for the improvement of photocatalytic activity of the doped materials.

In the present work, ZnO/g-C3N4 photocatalysts were synthesized via a simple calcination method. The physical properties of the obtained ZnO/g-C3N4 nanocomposites were characterized by various physicochemical techniques. The photocatalytic activity of the photocatalysts was investigated by the degradation of methyl orange (MO) and 4-chlorophenol (4-CP) under visible light irradiation. Especially, the photocatalytic efficiency was the highest when the percentage of ZnO was 10%. However, an increase of the photocatalytic activities was related to the interfacial transfer of photogenerated electrons and holes, leading to the effective charge separation. More importantly, the nanocomposite showed no activity toward hydrogen.

Hexagonal perovskite YFeO3 was synthesized by a complex-assisted sol-gel technique allowing crystallization at calcination temperatures below 700 °C. As determined by diffuse reflectance spectroscopy (DRS) and Tauc plots, the hexagonal YFeO3 exhibits a lower optical band gap (1.81 eV) than the orthorhombic structure (about 2.1 eV or even higher) being typically obtained at elevated temperatures (>700 °C), and thus enables higher visible light photocatalysis activity. Structure and morphology of the synthesized YFeO3 perovskites were analyzed by powder X-ray diffraction (XRD) and nitrogen adsorption, proving that significantly smaller crystallite sizes and higher surface areas are obtained for YFeO3 with a hexagonal phase. The photocatalytic activity of the different YFeO3 phases was deduced via the degradation of the model pollutants methyl orange and 4-chlorophenol. Experiments under illumination with light of different wavelengths, in the presence of different trapping elements, as well as photoelectrochemical tests allow conclusions regarding band positions of YFeO3 and the photocatalytic degradation mechanism. X-ray photoelectron spectroscopy indicates that a very thin layer of Y2O3 might support the photocatalysis by improving the separation of photogenerated charge carriers.