S-doped g-C3N4 Research Articles

Highly dispersed platinum-anchored g-C3N4 nanotubes for photocatalytic hydrogen generation

To overcome the shortcoming of rapid recombination of photoexcited electron-hole pairs of g-C3N4, noble metals are usually loaded as co-catalyst. However, conventional chemical reduction often brings about the loading of noble metal aggregates or nanoparticles that leads to a low atomic utilization efficiency. In this work, Pt atoms are highly dispersed on S-doped g-C3N4 nanotubes through in situ pyrolysis of H2PtCl6. Diffuse reflection infrared Fourier transform spectrum and XPS spectrum reveal that Pt exists in the state of +2 chemical valence and is anchored by coordination with diatomic N and S of g-C3N4, leading to a high dispersion and stabilization of Pt atoms in g-C3N4 nanotube. The optimal H2 production rate (0.3 wt% Pt) is 5304 μmol h−1g−1 and the turnover frequency (TOF) is 321.7 h−1 upon visible light illumination (λ > 420 nm), that is 3.2 times as high as Pt nanoparticle loaded g-C3N4 nanotube with the same loading content. Our work provides a new way to anchor noble metal on photocatalyst via diatomic coordination and enhance the atomic utilization efficiency of noble metal co-catalyst. Moreover, the unique electronic structure of Pt2+ with electron-deficient 5d orbitals decreases the kinetic energy barrier and promotes H2 evolution on the surface of photocatalyst.

In situ loading of ZnIn2S4 nanosheets onto S doped g-C3N4 nanosheets to construct type II heterojunctions for improving photocatalytic hydrogen production

In this work, type II heterojunctions were constructed by in suit loading of ZnIn2S4 (ZIS) nanosheets onto S-doped g-C3N4 (SCN) nanosheets by a simple hydrothermal synthesis method. The formation of heterojunction induced rapid electron-hole separation, while the introduction of SCN reduced the internal resistance of photogenerated carriers to transfer. When the mass fraction of SCN was 20 %, the best photocatalytic hydrogen production performance was 1.63 mmol g−1 h−1 under visible light irradiation (λ > 420 nm), which was 7.7 and 2.8 times higher than SCN (0.21 mmol g−1 h−1) and ZIS (0.57 mmol g−1 h−1), respectively. The enhanced photocatalytic activity was mainly due to greater light utilization achieved by band gap adjustment through constructing heterojunctions and improved electron-hole separation efficiency. This method can effectively improve the photocatalytic performance of ZIS-based photocatalysts in a simple way.

Degradation of tetracycline using persulfate activated by a honeycomb structured S-doped g-C3N4/biochar under visible light

In this work, a novel S-doped g-C3N4/biochar (SCNBC) nanocomposite was fabricated to activate persulfate (PS) under visible light for tetracycline (TC) abatement. The maximum photocatalytic degradation efficiency of TC by the SCNBC/PS system exhibited 2.34 and 2.32 times of individual biochar (BC) and g-C3N4, respectively. This was mainly because the honeycomb tubular structure facilitated electrons transfer along the linear direction, favoring electron-hole (e−-h+) pair separation. Meanwhile, BC could serve as an electron reservoir to further inhibit e−-h+ recombination. Radical scavenging tests and electron paramagnetic resonance measurement revealed that h+ and O2− were the dominant oxidizing radicals in TC removal. Three possible degradation pathways of TC were proposed, the mechanism of which mainly included dealkylation, deamidation, dehydration, hydroxylation and ring-cleavage reactions. Overall, this work provides a facile photocatalyst to activate PS for removal of organic pollutants.

Bamboo-like N,S-doped carbon nanotubes with encapsulated Co nanoparticles as high-performance electrocatalyst for liquid and flexible all-solid-state rechargeable Zn-air batteries

Developing nonprecious-metal carbon-based bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is greatly significant for practical applications of rechargeable Zn-air batteries. Herein, rod-like S-doped g-C3N4 has been used as the nitrogen-rich precursor to synthesize an excellent oxygen electrocatalyst Co@bCNTs by a facile two-step route including cobalt-infiltration and subsequent pyrolysis. In Co@bCNTs, Co nanoparticles are encapsulated within N, S-doped bamboo-like carbon nanotubes (bCNTs), which further are tangled with the doped graphene and hybridized with a small amount of amorphous CoSx. Benefiting from its effective N dopant, highly graphitized carbon and the synergistic effects of its components with different functionalities, the as-prepared catalyst has superior bifunctional ORR/OER electrocatalytic activities with a small potential difference of 0.72 V and outstanding durability. Its assembled liquid and all-solid-state flexible batteries show excellent performance with peak power densities up to 190.3 and 106.2 mW cm−2, respectively. This work provides a new dimension to design and prepare high-performance bifunctional oxygen eletrocatalysts using nitrogen-rich polymers for developing rechargeable zinc-air batteries.

One-step synthesis of S-doped and nitrogen-defects co-modified mesoporous g-C3N4 with excellent photocatalytic hydrogen production efficiency and degradation ability

S-doped and nitrogen defect co-modified mesoporous g-C3N4 (Tm-g-C3N4) is successfully prepared. Introduced mesopores result in an expansion of the surface area to 153.8 m2 g-1 which provides more active sites. S-doped and nitrogen defect co-optimize band structure, decrease the band gap energy and expand the light absorption capacity, simultaneously new excessive conduction band energy level is generated, which could accept more photogenerated electrons, promoting separation and migration of photo-generated carriers. Hence, its H2 evolution rate is 4441 μmol g-1 h-1, which is 48.8 folds more than previous one. Meanwhile, there is also a superior function in degradation (RhB, MO, MB, TC and CV), and keeps an excellent stability. Clearly, this work uses multiple modification methods for obtaining an efficient photocatalyst, which might exhibit marvelous effect on the fields of environmental treatment and energy development.

Efficient bacterial inactivation with S-doped g-C3N4 nanosheets under visible light irradiation.

The pathogenic bacteria in water that threatens the human health and photocatalytic disinfection have been proven to be a cost-effective and promising green technology. It is significant and necessary to develop efficient, safe, and visible light-driven photocatalysts. In this study, Escherichia coli was used as model bacterium and the disinfection performance of prepared S-doped g-C3N4 nanosheets (S-CNNs) under visible light was investigated. The results showed that the synergistic effects of S doping and the unique 2D structure of S-CNNs enhanced the visible light absorption, enlarged the specific surface area and reduced the recombination of photogenerated charge carriers which is beneficial for promoting the photocatalytic disinfection of the E. coli. Scavenger experiments indicated •O2- and h+ were the predominant reactive species in the photocatalytic disinfection process. In addition, the kinetics of disinfection activity were fitted by the modified Hom model and the k2 value of S-CNNs is 0.0219min-1, which is much higher than that of the bulk g-C3N4 (CN). This work has demonstrated efficient bacterial inactivation with S-CNNs under visible light irradiation.

Synergistic Effect of Honeycomb Tubular Structured S-Doped G-C3n4/Biochar and Persulfate Under Visible Light: Implication for Degradation of Tetracycline

Synergistic Effect of Honeycomb Tubular Structured S-Doped G-C3n4/Biochar and Persulfate Under Visible Light: Implication for Degradation of Tetracycline

Efficient solar-light photocatalytic H2 evolution of Mn0.5Cd0.5S coupling with S,N-codoped carbon

The low-cost and highly-efficient photocatalysts for solar-light induced water splitting had drawn great attention to develop the renewable hydrogen economy. To enhance the solar-light driven H2 evolution of Mn0.5Cd0.5S, Mn0.5Cd0.5S was modified by S,N-codoped carbon (3-MCS/SN-C) via the in-situ sulfurization at high temperature. In contrast with Mn0.5Cd0.5S (11.13 mmol g−1 h−1) and S-doped g-C3N4 (0.35 mmol g−1 h−1), the optimal 3-MCS/SN-C with a mass ratio of Mn0.5Cd0.5S to S,N-codoped carbon of 1:3 exhibited the best H2 evolution rate of 27.58 mmol g−1 h−1 and an apparent quantum yield of 27.43% at 400 nm. The photo-corrosion induced to the decreased H2 evolution capacity of 3-MCS/SN-C from 131.13 to 126.52 mmol g−1 after five cycles of 5 h. S,N-codoped carbon could efficiently capture visible light and absorb the photons, leading to the efficient transformation and separation of photo-generated carriers at the junction interface between Mn0.5Cd0.5S and S,N-codoped carbon.

Core-shell S-doped g-C3N4@P123 derived N and S co-doped carbon as metal-free electrocatalysts highly efficient for oxygen reduction reaction

The advantage of g-C3N4 as nitrogen-rich precursor for synthesizing nitrogen-doped carbon-based oxygen reduction reaction (ORR) catalyst has not been brought into full play. Herein, a simple approach with S-doped g-C3N4 (S-g-C3N4) and P123 as precursor for synthesizing nitrogen and sulfur co-doped carbon (N-S-C) catalysts highly efficient for ORR was developed. S-g-C3N4 synthesized via pyrolysis of a mixture of urea and thiourea was used as nitrogen and sulfur source, and P123 serving as carbon source was used to envelop S-g-C3N4 to form core–shell S-g-C3N4@P123 sample, whose pyrolysis yielded N-S-C catalyst. Characterization shows that the N-S-C catalyst synthesized at 900 °C with a S-g-C3N4/P123 mass ratio of 2:1 (N-S-C-2) exhibits the largest nitrogen and sulfur content and the highest half-wave potential of 0.863 V vs. RHE in alkaline media, which can be ascribed to the selection of the mixture of urea and thiourea for S-g-C3N4 synthesis, and the suitable S-g-C3N4/P123 mass ratio and pyrolysis temperature. N-S-C-2 also shows good ORR catalytic stability, excellent methanol tolerance, and high performance as the cathodic catalyst of Zn-air battery. This study gives full play to the advantage of g-C3N4 and provides a novel route for synthesizing N-S-C catalysts highly efficient for ORR.

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/(mLH2O.gcat) 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.

First-Principle study of lithium polysulfide adsorption on heteroatom doped graphitic carbon nitride for Lithium-Sulfur batteries

The intermediate lithium polysulfide species (LiPSs) suppressing dissolution into liquid electrolyte has been one of the important issues for large-scale commercial applications of Lithium-sulfur batteries (LSBs). Here, we perform periodic density functional theory calculations to reveal the potential of graphitic carbon nitride nanosheet (g-C3N4) and its heteroatoms (B, O, P and S) doped systems, to be used as chemical anchoring materials for LiPSs. From the calculations, the LiPSs are stabilized on g-C3N4 materials via lithium bond interaction between Li of LiPSs and N of g-C3N4. With the introduction of dopant atoms, tuning possibility of the LiPSs adsorption energies is observed. The interaction of LiPSs on B- and P-doped g-C3N4 provide optimum adsorption energies which is appropriate for LSBs anchoring. However, the O- and S-doped g-C3N4 have a weak adsorption which is similar to the pristine one. The anchoring performance of g-C3N4 materials for suppressing LiPSs in dissolving into the electrolytes is also highlighted by comparing their adsorption on g-C3N4 materials and on various electrolytic clusters. The adsorption energies of LiPSs on B- and P-doped g-C3N4 are found to be stronger than those on electrolytes. The electron transfers and van der Waals interaction are found to play significant roles for the LiPSs adsorption. The results provide the fundamental understanding g-C3N4 surface tuning by doping heteroatoms for the suppression materials to reduce the shuttle effect, which will be beneficial for the advanced LSBs development.

First-principles study of S-doped point defects with different charge states in monolayer g-C3N4

Non-metal-doped graphitic carbonitride (g-C3N4) can improve the redox capacity and photocatalytic activity, as a photocatalyst for the degradation of organic pollutants. In the present study, the theoretical defect models of S-doping with different doping sites and valence states were built in g-C3N4. The stability of charged defects was systematically investigated by the first-principles calculations based on the density functional theory. The effects of S-doping on the electronic structures and optical properties of g-C3N4 were revealed. According to the results, the g-C3N4 doped with positive divalent S is the most favorable defect form in energy. A new electron transfer pathway between adjacent heptazine rings on S-doped g-C3N4 was proposed. It is found that the special electronic states and the enhanced local charge density of S-doped g-C3N4 could effectively optimize the electron transport dynamic, leading to the increased reduction ability of photo-generated holes. As a result, the S-doped g-C3N4 shows better ability to separate photo-generated electron/hole pairs and improve photocatalytic efficiency with respect to the pristine g-C3N4.

One-pot sulfurized synthesis of ZnIn2S4/S,N-codoped carbon composites for solar light driven water splitting

To enhance the intrinsic active sites and to suppress the recombination of charge carriers, ZnIn2S4 modified with S,N-codoped carbon (ZIS/SN-C) composites were prepared for solar light driven water splitting via the one-pot sulfurized route. Compared with g-C3N4 and S-doped g-C3N4, the combined effect between S,N-codoped carbon and ZnIn2S4 can greatly enhance the photocatalytic activity of ZIS/SN-C. The optimal 4-ZIS/SN-C with the Zn(II) content of 13.78% and the calculated In/Zn molar ratio of 2.03:1 presents the H2 evolution rate of 2937.1 μmol g−1 h−1, which is 2.98 and 23.42 times higher than that of one-pot sulfurized ZnIn2S4 and S-doped g-C3N4, respectively. However, long-term photo-corrosion induces to the declined durability of 4-ZIS/SN/C for water splitting after three cycles. S vacancies of ZnIn2S4 serve as the efficient active sites of H2 evolution reaction, and S, N-codoped carbon acts as the photo-induced electrons trapper. The one-pot sulfurized approach is thus a potential strategy to fabricate metal sulfide-based photocatalysts.

Tuning the strength of built-in electric field in 2D/2D g-C3N4/SnS2 and g-C3N4/ZrS2 S-scheme heterojunctions by nonmetal doping

Single photocatalysts usually exhibit unsatisfactory performance due to the serious recombination of photogenerated electron‒hole pairs. Combining two photocatalysts to construct S-scheme heterojunction could solve this problem. In S-scheme mechanism, the interfacial built-in electric field (IEF) provides a vital driving force for efficient charge separation. Modifying the IEF is a feasible strategy to further improve the photocatalytic activity. Herein, a novel idea of tuning the strength of IEF in 2D/2D graphitic carbon nitride (g-C3N4)/MS2 (M = Sn, Zr) S-scheme heterojunctions by nonmetal doping was developed by employing density functional theory calculation. Three nonmetal elements (O, P, and S) were severally introduced into g-C3N4/MS2 composites. Charge density difference suggested that O and S doping led to increased interfacial electron transfer, while P doping had minimal influence. As expected, the calculated field strength of O- and S-doped g-C3N4/MS2 composites was significantly larger than that of pristine and P-doped g-C3N4/MS2 composites. Therefore, O and S doping endowed g-C3N4/MS2 S-scheme heterojunctions with enhanced IEF and more thorough charge transfer. Correspondingly, the experimentally synthesized O-C3N4/SnS2 composite exhibited better photocatalytic H2-production activity than g-C3N4/SnS2 composite. This work proposed an original idea of employing proper nonmetal doping to magnify the advantage of S-scheme heterojunction in accelerating charge separation.

Computational and experimental evidence of Pd supported P-doped porous graphitic carbon nitride as a highly efficient and exceptionally durable photocatalyst for boosted visible-light-driven benzyl alcohol oxidation

Here, we present the low cost and environmentally friendly photocatalyst, P-doped porous g-C3N4/Pd (P-pCN/Pd) for the first time. The designed photocatalyst was fabricated successfully via thermal copolymerization of melamine in the presence of H3PO4 as a phosphorous source. In pursuance of comparison, a series of photocatalysts including bulk g-C3N4, g-C3N4 nanosheets, porous g-C3N4, N-doped porous g-C3N4, S-doped porous g-C3N4, P-doped porous g-C3N4, P-doped porous g-C3N4/Ag, P-doped porous g-C3N4/Cu, and P-doped porous g-C3N4/Pd were synthesized. Also, as a result of the calculation, it has appeared that the presence of Pd nanoparticles significantly makes the band gap narrower, facilitates the electron transfer, and increases the catalyst efficiency. Developing photocatalysts for the oxidation of organic materials hold considerable promise for the generation of high-value-added products such as benzaldehyde under visible light irradiation. All materials were characterized physico-chemically and the efficiency of the synthesized photocatalysts was evaluated toward catalyzing benzyl alcohol oxidation to benzaldehyde derivatives. The experiments uncovered that P-doped porous g-C3N4/Pd exhibits significant visible light activity at room temperature.

Preparation of S-doped g-C3N4 with C vacancies using the desulfurized waste liquid extracting salt and its application for NOx removal

In this study, graphitized carbon nitride photocatalysts were successfully prepared by using the desulfurized waste liquid extracting salt from coking plants. The NOx removal efficiency of g-C3N4 synthesized using ammonium thiocyanate (NH4SCN), ammonium thiosulfate ((NH4)2S2O3) and ammonium sulfate ((NH4)2SO4) with different mass ratios were evaluated. The results showed that due to the addition of (NH4)2S2O3 and (NH4)2SO4 a large amount of sulfur-containing gas was released during pyrolysis, which significantly increased the specific surface area and pore volume of the photocatalysts. The release of sulfur-containing gas also resulted in the incorporation of sulfur, and due to this doping process the band gap of the photocatalyst was widened to 2.94 eV, thus enhancing its visible light response. C vacancies were formed in the hepazine ring, which increased the electron density around the N–(C)3 bridge bond and formed an electron trapping center, thus inhibiting the recombination of charge carriers. The photocatalysts had increased O2 adsorption capacity and increased number of amino groups on their surface. This facilitated the rapid generation and consumption of reactive oxygen species, inhibited the accumulation of NO2 on the surface, reduced the production of NO2, thus improving the NOx removal efficiency of the photocatalysts. The principle of using “waste for waste control” was achieved, and a new method for nitrogen oxide removal was provided.

Novel NiFe2O4 deposited S-doped g-C3N4 nanorod: Visible-light-driven heterojunction for photo-Fenton like tetracycline degradation

Fabrication of visible-light-active heterojunction photocatalysts for organic pollutant degradation plays a crucial role in the environmental remediation field. We have fabricated sulfur-doped carbon nitride nanorod (SCNNR) by template free urea assisted hydrothermal method. The facile wet chemical impregnation method was used to deposit nickel ferrite (NFO) nanoparticles on the SCNNR. Various analytical characterization methods were used to study the structural, morphological, optical, and chemical compositions of the prepared photocatalysts. Strong chemical interaction between the NFO and SCNNR nanoparticles enhances the visible-light utilization and also improves the suppression of electron-hole recombination. In particular, the composite 20NFO/SCNNR photocatalyst exhibits greater photo-Fenton-like catalytic degradation of tetracycline (TCN) in the presence of H2O2 under commercial LED light irradiation. The photo-Fenton reaction mechanism and reactive oxygen species for TCN degradation were investigated by the elemental trapping experiment. Moreover, the recycle test proves that the prepared composite photocatalyst is an efficient candidate for environmental remediation applications.

CoS2 engulfed ultra-thin S-doped g-C3N4 and its enhanced electrochemical performance in hybrid asymmetric supercapacitor

This work demonstrates a high-performance hybrid asymmetric supercapacitor (HASC) workable in very high current density of 30 A g−1 with in-situ pyrolytic processed sulfur-doped graphitic carbon nitride/cobalt disulfide (S-gC3N4/CoS2) materials and bio-derived carbon configuration and achievement of high electrochemical stability of 89% over 100,000 cycles with the coulombic efficiency of 99.6%. In the electrochemical studies, the S-gC3N4/CoS2-II electrode showed a high specific capacity of 180 C g−1 at 1 A g−1 current density in the half-cell configuration. The HASC cell was fabricated using S-gC3N4/CoS2-II material and orange peel derived activated carbon as a positive and negative electrode with a maximum operating cell potential of 1.6 V, respectively. The fabricated HASC delivered a high energy density of 26.7 Wh kg−1 and power density of 19.8 kW kg−1 in aqueous electrolyte. The prominent properties in specific capacity and cycling stability could be attributed to the CoS2 nanoparticles engulfed into the S-gC3N4 framework which provides short transport distance of the ions, strong interfacial interaction, and improving structural stability of the S-gC3N4/CoS2-II materials.

Bisphenol S-doped g-C3N4 nanosheets modified by boron nitride quantum dots as efficient visible-light-driven photocatalysts for degradation of sulfamethazine

Antibiotics may pose a great risk to ecosystem and human health. Photocatalysis, as a low-cost and environmentally friendly technology is widely used for the removal of antibiotics from wastewater. Graphitic carbon nitride (g-C3N4) has shown promising prospects in visible light photocatalysis, while its photocatalytic performance is greatly limited because of the sluggish charge separation and transfer. In this work, metal-free boron nitride quantum dots (BNQDs) modified bisphenol S (BPS)-doped g-C3N4 nanosheets (BNQDs/BPS-CN) heterojunction was synthesized to overcome these defects and applied in photodegradation of sulfamethazine (SMZ, a typical antibiotic) under the visible light. Multifarious characterization methods were used to explore the structure, porosity, elemental composition, optical performances, photo-electrochemical properties and photocatalytic performances of as-prepared BNQDs/BPS-CN composites. The degradation efficiency of SMZ with BNQDs/BPS-CN-4 composite reaches 100% within 60 min, and the rate constant is 13.7 times higher than that of pure g-C3N4. This phenomenon is because of the shrinking band gap width and the introduction of electronegative BNQDs, which is conducive to the absorption of visible light and high-efficiency separation of photoexcited charge carriers. Meanwhile, the results of free radical trapping experiments and electron spin resonance characterization prove that the photogenerated holes and superoxide radicals play predominant roles in the photodegradation of SMZ. This study proposes an effective mechanism for the construction of novel visible-light-driven photocatalysts using metal-free two-dimensional materials and quantum dots, which can be applied in the treatment of organic contaminants.

Low-temperature in situ fabrication of porous S-doped g-C3N4 nanosheets using gaseous-bubble template for enhanced visible-light photocatalysis

Developing one-step modification strategy for highly efficient metal-free g-C3N4 photocatalyst has always been challenging and yet demanding. In this study, we have demonstrated an in-situ low-temperature production of porous S-doped g-C3N4 nanosheets, using ammonium chloride (NH4Cl) as a gaseous-bubble template (GBT) to boost the visible-light photocatalytic degradation of organic pollutant dyes. The photocatalytic activities of GBT-induced porous S-doped g-C3N4 nanosheets (so-called p–SCN–x, x = mass ratio of NH4Cl/thiourea) were examined towards the degradation of Rhodamine B (Rh.B) under visible light. Importantly, both surface area and photocatalytic activities of the p-SCN–xsample were optimized by adjusting the mass fraction of thiourea/NH4Cl. The optimized p–SCN–2 exhibited a maximum photocatalytic degradation of 97.50%, which is 7.75-fold higher than that of GBT-free analogue (so-called bulk SCN-0). Moreover, the p–SCN–2 was tested for colorless tetracycline hydrochloride as an emerging pollutant, demonstrating 87% degradation under visible light irradiation. The enhanced performance of p-SCN–2photocatalyst was mainly attributed to an improved surface area, boosted light-harvesting range, and efficient electron-hole separation, and facilitated transfer of charge carriers by dye-sensitization mechanism. Moreover, the scavengers quenching study proved that the superoxide anion (•O2−) radicals and holes (h+) were the leading active sorts. In situ fabrication of p-SCN–x nanosheets at low temperature provided an inexpensive and green synthetic route for highly active g-C3N4–based photocatalysts that can resolve environmental pollution and clean energy issues.