The photocatalytic decolorization of organic dyes is considered one of the most efficient and promising methods. Bismuth Subcarbonate (Bi₂O₂CO₃) exhibits photocatalytic activity, although its wide bandgap limits its ability to absorb visible light. Therefore, it is important to extend the light-responsive range. In this work, Bi₂O₂CO₃ photocatalysts were prepared by adding bromide (Br) sources and subjecting them to nitric acid treatment under various conditions. The photocatalytic activity of the prepared Bi₂O₂CO₃ was evaluated by decolorization experiments using Rhodamine B (RhB) as a representative dye.

To provide pretreatment Bi₂O₂CO₃, Br sources were dissolved in nitric acid solutions of different concentrations. Then, Bi₂O₂CO₃ was added to the resulting solution, and the mixture was stirred for 2 hours. Finally, the products were washed with water and ethanol and vacuum-dried at 60°C for 12 hours.

The photocatalytic activity was assessed by the decolorization of RhB. In particular, 20 mg of the synthesized photocatalyst was dispersed within the RhB solution (10 ppm, 35 mL) and stirred in the dark for 60 min to achieve adsorption/desorption equilibrium. The solution was then irradiated with a 500 W Xe lamp (λ ≥ 420 nm). At regular intervals, the aliquots of the solution were collected and centrifuged. The decolorization rate of RhB was analyzed using a UV–visible spectrophotometer by measuring the absorbance at 554 nm.

Pure Bi₂O₂CO₃, as well as catalysts treated only with acid or only with a Br source, exhibited almost no photocatalytic activity under visible-light irradiation. Acid-treated Bi₂O₂CO₃ with cetrimonium bromide (CTAB) as the Br source enhanced RhB decolorization performance. The results showed that the combined effects of these modifications are essential.

Photocatalysis is considered an ideal energy conversion technology that can convert solar energy into clean hydrogen energy through water splitting. Hydrogen can be used directly as a source of energy rather than fossil fuels due to its higher energy yield and eco-friendly attributes. However, most photocatalysts exhibit low photocatalytic performance due to their lower light absorption capability, higher energy bandgap, poor charge separation and migration capability, and rapid electron–hole pair recombination properties. To overcome the above-mentioned problems, in this study, we performed the in situ simultaneous photodeposition of ammonia-treated graphene oxide (NGO) on CTAB-assisted ZnIn₂S₄ (ZIS) for photocatalytic hydrogen evolution from splitting under visible light irradiation (450 nm). Amounts of 1 wt% of both Pt and triethanolamine were used as the co-catalyst and hole-scavengers, respectively. The reaction conditions (catalyst and co-catalyst loading, ammonia concentration and mixing, and reaction temperature and pH) were optimized. The NGO/ZIS composite increased H₂ production by 3.25-fold and 67-fold compared to pure ZIS and NGO, respectively, indicating the achievement of a synergetic effect. Synthesized photocatalysts were characterized by XRD, XPS, FTIR, SEM, TEM, BET, TRPL, DRS, PL, and EIS analysis. Density functional theory indicated that the free energy, dipole moment, HOMO-LUMO bandgap, chemical reactivity, and surface charge were significantly increased in NGO compared to pure graphene oxide. Due to the NGO/ZIS composite formation, the light-harvesting capability, energy bandgap, charge separation and migration, surface area and morphology, and electron–hole pair recombination properties were significantly improved.

Introduction

Covalent organic frameworks (COFs) are promising photocatalysts due to their excellent visible light absorption, large surface area, and high chemical stability. However, their photocatalytic activity is often limited by the rapid recombination of photogenerated electron–hole pairs. In this study, we aimed to enhance hydrogen production activity by constructing a heterojunction between TpTSN-COF and Copper-doped ZnIn₂S₄.

Methods

Synthesis of TpTSN-COF/Cu-doped ZIS: The prepared Cu-doped ZIS was added to a mixture of dioxane, mesitylene, and acetic acid and then sonicated. Tp (1,3,5-triformylphloroglucinol) and TSN (o-Tolidine Sulfone) were subsequently added, and hydrothermal synthesis was performed. The resulting product was designated x%-TpTSN-COF/ Cu-doped ZIS, where x% indicates the weight ratio of TpTSN-COF to Cu-doped ZIS.

Hydrogen production test: A Pyrex reactor was charged with a photocatalyst, distilled water, sodium ascorbate (as a sacrificial agent), HCl (for pH adjustment), and H₂PtCl₆ (as a co-catalyst). Before light irradiation, the reaction system was purged with N₂ gas to remove any dissolved oxygen. An LED lamp (450 nm) was used as the light source, and the hydrogen production rate was calculated based on the amount of hydrogen production after 3 hours.

Results

The hydrogen production experiments were conducted with different ratios of TpTSN-COF and Cu-doped ZIS. As a result, 5%-TpTSN-COF/Cu-doped ZIS exhibited the highest hydrogen production rate of 40,000 μmol/g·h, which was approximately 1.5 times higher than that of TpTSN-COF and Cu-doped ZIS individually. The characterization revealed that the increased activity was attributed to improvements in light absorption, charge transfer properties, and an increase in the surface area.

Conclusions

The novel hybridized photocatalyst, TpTSN-COF/ Cu-doped ZIS, was successfully synthesized. The optimal photocatalyst (5%-TpTSN-COF/Cu-doped ZIS) showed approximately 1.5 times higher activity than that of TpTSN-COF and Cu-doped ZIS individually, and this increased activity was attributed to improvements in light absorption, charge transfer properties, and an increase in the surface area.

Heavy metal pollution, particularly Cr(VI) contamination, has recently garnered significant attention. Cr(VI), which is commonly discharged in industrial wastewater, is a serious environmental and health threat due to its carcinogenic and mutagenic properties. Developing effective methods to reduce Cr(VI) to Cr(III)—a less toxic form—is therefore highly desirable. In this study, we developed an environmentally friendly and highly efficient photocatalytic method for Cr(VI) reduction. The reduction efficiency was enhanced by using modified carbon nitride under visible-light irradiation. For catalyst preparation, urea was dissolved in pure water and heated in an electric furnace at a rate of 2 °C/min to 600 °C, and this temperature was held for 2 hours. A modified version, g-C3N4 (HB), was synthesized by adding 5 mg of 1,3,5-HB before calcination. For the reduction experiments, the reaction solution was prepared with 30 ppm of Cr(VI), 100 ppm of EDTA, and 15 mg of photocatalyst. The mixture was stirred in the dark for 30 minutes to achieve adsorption/desorption equilibrium, followed by 90 minutes of irradiation with blue light (450 nm). The Cr(VI) concentration was analyzed using the diphenylcarbazide method, with the absorbance measured by a UV–visible spectrophotometer. The incorporation of 1,3,5-HB into carbon nitride increased the reduction rate by 40%, likely due to the introduction of hydroxyl groups into the carbon nitride framework. Additionally, calcination at 550 °C yielded higher reduction rates after 90 minutes compared to 600 °C, possibly because the lower temperature minimized catalyst loss during calcination. In future studies, we plan to confirm the structural changes using techniques such as SEM and TEM and evaluate the catalytic performance through electrochemical measurements.

To achieve a sustainable society, it is essential to have a clean energy system that does not rely on depletable resources. Hydrogen production through water splitting using photocatalysts driven by sunlight is attracting attention as a promising solution. Covalent organic frameworks (COFs) are ideal for this application, as they have a high density of surface-active sites and are water-dispersible, thermally durable, and metal-free. However, current synthesis methods have industrial disadvantages due to the complex and harsh synthesis conditions, such as temperature and atmosphere. In this study, we attempted to synthesize a highly active photocatalyst under simple and mild conditions in order to introduce chlorine substituents into the imine-bonded COF, Tp-Pa-COF.

1,3,5-Trifluoromethyl fluoroglucinol (Tp) and 1,4-phenylenediamine (Pa) or 2,5-dichloro-1,4-phenylenediamine (Pa-Cl₂) were reacted to synthesize Tp-Pa-X-COF (X = H₂ or Cl₂). After 30 minutes of ultrasonic treatment, the mixture was stirred at different temperatures (room temperature to 120 °C) and times (6 to 24 hours) under air atmosphere. Photocatalytic hydrogen evolution was performed using hexachloroplatinic acid as a co-catalyst, sodium ascorbate as a sacrificial agent, and an LED lamp (λ=450 nm, 17.5 mW/cm²) as a light source. UV-vis DRS was used to measure visible-light absorption capacity, and gas chromatography (TCD) was used to quantify hydrogen.

As a result, it was found that even at room temperature and atmospheric pressure, chlorine substitution improved photocatalytic hydrogen production activity, improved visible-light absorption, and suppressed the recombination of electrons and holes. It was revealed that factors such as reaction time, temperature, and solvent affect the crystallization and photocatalytic activity of Tp-Pa-COF.

Dye wastewater is a highly toxic organic pollutant that is not easily degraded. For that reason, it causes serious environmental damage to nature and ecosystems. Therefore, the degradation research of organic matter by using photocatalysts is attracting attention because of their low cost and environmental friendliness. Graphitic carbon nitride (g-C₃N₄), a photocatalyst, has the advantages of being inexpensive, chemically and thermally stable, and easy to prepare. On the other hand, g-C₃N₄ possesses a small surface area, narrow visible light absorption, and a quick-recombination photogenerated electron–hole pair. Photocatalysts are also known to incorporate heteroatoms within their structures and to develop various characteristics depending on the atoms incorporated. In this research, we aim to enhance the dye degradation performance by synthesizing g-C₃N₄, incorporating heteroatoms.

The photocatalyst was obtained by mixing urea, a carbon nitride source, and a heteroatom source, followed by thermal treatment. Bulk g-C₃N₄ (CN-U), N-deficient g-C₃N₄ (CN-N), Br-doped g-C₃N₄ (CN-Br), S-doped g-C₃N₄ (CN-S), and O-doped g-C₃N₄ (CN-O) were prepared. Photocatalytic degradation was carried out under visible light irradiation using a Xenon lamp equipped with a cutoff filter. Methyl orange (MO) and methylene blue (MB) were selected as decolorizing targets. They were characterized in a variety of ways to clarify their photocatalytic properties.

A series of characterization results showed that the typical structure of g-C₃N₄ was maintained. It has been shown that doping carbon nitride with heteroatoms changes the optical properties of carbon nitride. Furthermore, we confirmed that doping carbon nitride with heteroatoms prevents the recombination of photogenerated electron–hole pairs. From the results of the dye degradation experiment, for MO, CN-N degraded the most. For MB, CN-U and CN-N degraded the most. We think this is due to the improved optical properties resulting from the incorporation of heteroatoms.

The utilization of clean energy is being explored to address energy and environmental challenges, with hydrogen emerging as a promising next-generation energy source. Research efforts are focused on hydrogen production through water splitting using graphitic carbon nitride (g-C₃N₄), a polymer semiconductor photocatalyst. In this process, a photocatalyst-dispersed aqueous solution is irradiated with light to generate hydrogen. g-C₃N₄ is characterized by its non-toxic nature, thermal and physical stability, and favorable bandgap [1]. However, its low specific surface area, limited light absorption range, and high photogenerated electron--hole recombination rate hinder its photocatalytic efficiency [2]. In this study, 1,3-benzothiazole-2-carbaldehyde is introduced into g-C₃N₄ to enhance the intramolecular charge transfer, thereby improving hydrogen production activity.

The photocatalyst was synthesized by the triple calcination of dicyanodiamide and 1,3-benzothiazole-2-carbaldehyde. Photocatalytic activity was evaluated by adding triethanolamine (sacrificial agent) and hexachloroplatinic acid to the catalyst in an aqueous solution, purging with nitrogen, and irradiating with visible light (λ ≥ 420 nm) for 6 h. Hydrogen production was quantified by gas chromatography (GC/TCD). The crystal structure, morphology, surface state, and optical properties of the photocatalysts were also characterized.

The optimized g-C₃N₄ modified with 1,3-benzothiazole-2-carbaldehyde demonstrated a hydrogen generation rate of 690 µmol h-1 g-1 through photocatalysis. This rate was roughly 14 times higher than that observed for unmodified g-C₃N₄. The characterization results showed that the benzothiazole-doped g-C₃N₄ exhibited exfoliation of the nanosheets and changes in surface structure. Additionally, a low fluorescence intensity and an extended visible light absorption range were observed. These effects were attributed to the introduction of 1,3-benzothiazole-2-carbaldehyde, which formed a donor--acceptor structure, enhancing the separation and transfer of photogenerated carriers. Consequently, the number of reactive electrons increased, leading to improved hydrogen production activity.

REFERENCES

[1] X. Zhang, F. Wu, G. Li, L. Wang, J. Huang, A. Song, A. Meng, Z. Li, Journal of Colloid and Interface Science, 655, 2024, 439-450

[2] F. Zhou, X. Chen, Y. Zhao, J. Cheng, G. Xu, Journal of Photochemistry & Photobiology, A: Chemistry, 449, 2024, 115378.

Graphitic carbon nitride (g-C₃N₄) is one photocatalyst that has garnered attention due to its good visible-light responsivity, chemical stability, and synthesis from low-cost materials. However, single g-C₃N₄ suffers from a high recombination rate of electrons and holes. To overcome this limitation, the hybridization of g-C₃N₄ with other materials has been studied. In this work, S-doped g-C₃N₄ photocatalysts were synthesized from urea and thiourea by calcination in an electric furnace. Photocatalytic methane production was conducted from an acetic acid solution with S-doped g-C₃N₄ photocatalyst under visible light irradiation (450 nm) for 6 hours. Cu was applied as a co-catalyst to enhance performance, and the evolved gas was analyzed using a GC-FID system. Optimal conditions for methane production were investigated, including the weight ratio of urea to thiourea, and the proportion of the Cu co-catalyst. S-doped g-C₃N₄ was synthesized by the calcination of a mixture of urea and thiourea. The characterization of S-doped g-C₃N₄ was performed by SEM, TEM, DRS, PL, XRD, and XPS. The S-doping could be confirmed by the color of photocatalyst materials and XPS analysis.

The photocatalyst synthesized with 30 wt% thiourea showed the best activity, with a production rate approximately 3.3 times higher compared with that of g-C₃N₄ synthesized from urea alone. The production rate with 1.0 wt% co-catalyst was about 2.3 times greater than that without the co-catalyst. The structural, morphological, and optical properties were analyzed through various characterizations. The analysis results showed the formation of g-C₃N₄ with characteristic peaks and structures, a redshift due to sulfur doping, a low recombination rate of electron--hole pairs, and improved charge separation. The enhanced photocatalytic activity was attributed to increased visible-light absorption induced by sulfur doping. These findings suggest that Cu-modified S-doped g-C₃N₄ can serve as a highly promising photocatalyst for methane production under visible light.

　Dyes are highly toxic organic pollutants that are difficult to decompose, causing significant environmental damage. In recent years, research has been conducted on dye degradation using photocatalysis, an environmentally friendly, efficient, and clean pollutant degradation technology. Among photocatalysts, bismuth tungstate (Bi₂WO₆) has been reported to possess photocatalytic activity under visible light irradiation due to its stable physicochemical properties and suitable band gap. However, the narrow light absorption region and high recombination rate of electron–hole pairs require further studies to improve their photocatalytic performance. In this work, the effect of surfactants in synthetic systems on the morphology and photocatalytic activity of Bi2WO6 is investigated .

　Bi₂WO₆ was synthesized by a simple one-pod solvothermal method using ethylene glycol. Various surfactants, including cationic, anionic, and nonionic types, were introduced during catalyst preparation, respectively. The photocatalytic efficiency was evaluated through the decolorization of Rhodamine B as a model dye.

　The decolorization rate of RhB after 120 minutes of light irradiation confirmed that adding surfactants to the synthetic system improved the photocatalytic activity. In particular, high decolorization rates were achieved when cationic (benzalkonium chloride, hexadecyl trimethylammonium bromide) and anionic (sodium oleate) surfactants were used. Various characterizations suggest that the enhanced photocatalytic activity could be attributed to the improved ability to separate and transfer photogenerated carriers.

As energy and environmental problems caused by economic development become more serious, hydrogen is attracting attention as the next generation of clean energy. Photocatalytic hydrogen generation is a method with low environmental impact because it uses natural energy sources such as water and sunlight. Indium zinc sulfide is a photocatalyst with visible light response and chemical stability. The aim of this study was to improve the photocatalytic activity of indium zinc sulfide while reducing the use of expensive indium.

Copper-modified zinc indium sulfide was synthesized by heating and stirring zinc chloride, indium chloride tetrahydrate, copper(I) chloride, and thioacetamide in an autoclave at 180 °C for 18 h. Six types of catalysts were prepared by varying the amount of copper added. To evaluate the photocatalytic activity, 40 mg of catalyst was added in 40 mL of an aqueous solution containing 20 mL of 0.25 M Na₂SO₃/0.35M Na₂S (sacrificial agent) and 1.2 mL of hexachloroplatinic acid solution (co-catalyst). The solution was purged with nitrogen for 30 min, with stirring, followed by irradiation with visible light (λ ≥ 420 nm) for 6 h. Hydrogen production was measured using gas chromatography every 3 h. Catalyst characterization was also performed.

Compared to the hydrogen production rate of Zn₆In₂S₉, the hydrogen production rate of Zn_5.7Cu_0.3In₂S₉ was about five times higher. Therefore, the catalysts were characterized, and the characterization showed that the addition of copper suppressed the recombination of electron–hole pairs, increased the optical absorption in the visible light region and narrowed the band gap. The results indicate that the addition of copper to indium zinc sulfide improves the photocatalytic activity. The quantum yields for Zn_5.7Cu_0.3In₂S₉ were found to be consistent with the DRS trend, indicating that hydrogen production occurs from the photocatalyst.