Visible Light Photocatalytic Water Splitting Research Articles

Graphitic Carbon Nitride Enters the Scene: A Promising Versatile Tool for Biomedical Applications.

Graphitic carbon nitride (g-C3N4), since the pioneering work on visible-light photocatalytic water splitting in 2009, has emerged as a highly promising advanced material for environmental and energetic applications, including photocatalytic degradation of pollutants, photocatalytic hydrogen generation, and carbon dioxide reduction. Due to its distinctive two-dimensional structure, excellent chemical stability, and distinctive optical and electrical properties, g-C3N4 has garnered a considerable amount of interest in the field of biomedicine in recent years. This review focuses on the fundamental properties of g-C3N4, highlighting the synthesis and modification strategies associated with the interfacial structures of g-C3N4-based materials, including heterojunction, band gap engineering, doping, and nanocomposite hybridization. Furthermore, the biomedical applications of these materials in various domains, including biosensors, antimicrobial applications, and photocatalytic degradation of medical pollutants, are also described with the objective of spotlighting the unique advantages of g-C3N4. A summary of the challenges faced and future prospects for the advancement of g-C3N4-based materials is presented, and it is hoped that this review will inspire readers to seek further new applications for this material in biomedical and other fields.

Synergistic effect of noble metal modified LaNiO3 perovskites for photocatalytic water splitting

Harnessing the solar energy to produce valuable fuels is a promising solution to the global energy crisis. One such vital fuel is hydrogen and it can be produced through the efficient evolution from water using a judiciously chosen perovskite photocatalyst. Perovskite materials, with their distinctive structural and optical properties, emerge as potential catalysts for visible light photocatalytic water splitting. Among these, LaNiO3 stands out as a promising photocatalyst due to its small bandgap and well-suited band edges that enable efficient visible light absorption. Strategic modifications with noble metals further elevate the photocatalytic performance. Utilizing the solution combustion method, catalysts are synthesized and extensively characterized through techniques such as XRD, XPS, ICP-OES, SEM, BET, DRS, UPS, Photoluminescence, and Time-resolved photoluminescence. Photocatalytic hydrogen evolution studies are conducted under solar simulator illumination. Notably, the addition of noble metals increased the photocatalytic activity from 1.37 mmol g-1 h-1 to 20.86 mmol g-1 h-1 while the apparent quantum efficiency varied from 2.58% to 39.42%. The TOF on the basis of the active sites varied from 214.44 h-1 to 1685.24 h-1. The catalysts demonstrate stable and consistent performance over a 12-hour duration. The photocatalytic H2 yield trend was found to be Rh/LaNiO3 > Ru/LaNiO3 > Ir/LaNiO3 > Pt/LaNiO3 > LaNiO3.

Nb-Ta3N5 protected with PANI nanocomposite for enhanced photocatalytic water-splitting towards hydrogen production under visible light irradiation

The photocatalytic production of hydrogen (H2) from water to convert solar energy to chemical energy is a promising way to solve energy crisis issues. In this study, we synthesized niobium-doped tantalum nitride protected by polyaniline (Nb-Ta3N5@PANI) for visible light photocatalytic water-splitting. Nb was successfully doped into Ta3N5 lattice confirmed by XRD. Nb as a dopant increases e−- h+ pair separation by acting as an intermediate band between the valence band (VB) and conduction band (CB) of Ta3N5. PANI works as a conducting polymer to further improve the photogenerated e− and h+ charge transfer. XRD, UV–vis, FT/IR, and FE-SEM were used to confirm their structure and properties. Based on our modification, the optimal solar H2 evolution on the Nb-Ta3N5@PANI photocatalyst reaches 71.3 mmol/g, ca. 3.1 folds that of the Ta3N5 photocatalyst.

A direct Z-scheme BS/PtO2 van der Waals heterojunction for enhanced visible-light photocatalytic water splitting: a first-principles study.

A direct Z-scheme heterostructure holds a unique advantage in solar-driven overall water splitting, while the rational design of efficient photocatalysts for water splitting in such heterostructures remains a challenge. Based on first-principles calculations, this study proposes a novel direct Z-scheme two-dimensional (2D) van der Waals (vdW) heterostructure photocatalyst, denoted as BS/PtO2. Its band edges match the oxidation-reduction potentials of water, satisfying the conditions for the oxidation and reduction of water. Under acidic conditions (pH = 0), the results of the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) indicate that BS/PtO2 can drive the OER without the need for an external bias, while the HER requires catalytic assistance. Interestingly, compared to single-layer materials, this heterostructure exhibits a significant enhancement in visible light absorption, implying a more efficient solar energy conversion capability. Therefore, the BS/PtO2 heterostructure holds the potential to become a promising direct Z-scheme photocatalyst with efficient visible light activity.

2H–MoSe2 nanosheet as noble-metal-free co-catalyst over Cd0·7Zn0·3S: optimizing interfacial charge transfer for enhanced photocatalytic hydrogen evolution

Developing highly efficient and low-cost noble-metal-free cocatalysts is significant for visible-light photocatalytic water splitting. Herein, 2H phase MoSe2 nanosheets were successfully synthesized via a solvent-assisted hydrothermal method and used as cocatalysts to construct heterostructure with hexagonal Cd0·7Zn0·3S nanoparticles. This well-designed heterostructure will significantly shorten the distance of charge migration to achieve efficient electron transfer between Cd0·7Zn0·3S and MoSe2. As expected, the optimized composites exhibited excellent photocatalytic hydrogen evolution (PHE) activity (3363.4 μmol g−1 in 5 h), which was 3.3 times than that of pristine Cd0·7Zn0·3S, and higher than that of 1%Pt–Cd0.7Zn0.3S. Based on electrochemical and photoluminescence results, the enhanced activity could be attributed to the lower overpotential (−0.27 V) and superior carrier separation of Cd0·7Zn0·3S/MoSe2 heterojunction. Meanwhile, 2H–MoSe2 nanosheets with large specific surface area can afford highly active Se-edge sites to absorb the protons for boosting hydrogen evolution. Moreover, Cd0·7Zn0·3S/MoSe2 photocatalysts also displayed a long-term stability without apparent debasement. This work provides new inspiration that transition metal dichalcogenides (TMDCs) as cocatalysts are promising to effectively enhance the PHE performance of sulfide solid solutions.

Embedding Aromatic Conjugated Monomer within Carbon Nitride for Efficient Photocatalytic Reduction Reactions

Due to a growing number of significant vitality and environmental issues, the standardized variation of carbon nitride (CN) for visible-light photocatalytic water splitting is an encouraging scientific topic. By revealing this, the functionalized monomer 2,6-dibromobenzimidazole (BI) was successfully embedded within the heptazine units of CN via a molecular engineering (Co-polymerization process) approach, and the as-synthesized product was named CN/BIx. Thereafter, as-synthesized materialswere employed in the photocatalytic production of hydrogen (H2) via water splitting and CO2 reduction into CO under visible light irradiance (λ = 420 nm). Surprisingly, the substituent framework of CN, which was intimidated by the description of BI monomer, acted as a substitution reaction material and lubricated the electronic structure of CN by endorsing charge transition dissociation, which in turn boosted its photocatalytic performance under visible irradiation. The CN/BI10.0 yields 62.8 mol of CO and 18.1 mol of H2 for 4 h of the catalyzed reaction upon photooxidation under light irradiation, emphasizing the maximum photocatalytic performance with response to CO2+. Correspondingly, the H2 evolution rate (HER) for bulkCN was estimated as 17.6 mol/h1, whereas it was approximated as 203.7 mol/h1 for CN/BI10.0, which is 10 times higher than that ofCN. Such a phenomenon also predicts a substantial encroachment in the surface area, energy gap, and chemical properties, along with promotes the effective segregation of photoinduced charge separation from the valence band (VB) to the conduction band (CB)of CN, thereby, making it a good alternative for the photocatalytic water andCO2 reduction reaction process.

Conduction band tuning by strengthening s-p hybridization of novel layered oxyhalide Bi4SbO8Cl for efficient visible-light photocatalytic water splitting

Bismuth-based Sillen–Aurivillius oxyhalide Bi4MO8Cl are promising visible-light photocatalysts. The element M of [MO4]3- perovskite layer in the Bi4MO8Cl is always a transition element. Due to d orbitals in transition element weakening the Bi 6s and O 2p orbitals hybridization, the bandgap energy would be decreased and the CBM would be lowered. Thus, Bi4MO8Cl always shows feeble reducibility for photocatalytic hydrogen generation. In this paper, we choose the main group element Sb substitute for transition element M in the Sillen–Aurivillius Bi4MO8Cl. Due to the s-p-d hybridization in Bi4NbO8Cl being replaced by only s-p hybridization in Bi4SbO8Cl, the strengthened Bi 6s and O 2p orbitals hybridization would eventually lead to the enlarged energy bandgap and elevated CBM. The bandgap of Bi4SbO8Cl was about 2.53 eV with the CBM estimated about −0.33 eV. Because of the high carrier mobility and strong reducibility brought by the dispersed Bi 6s states in the whole unoccupied states and the raised position of Bi 6p states in the CBM and linearity Sb–O–Sb, Bi4SbO8Cl shows enhanced photocatalytic H2 generation compared to Bi4NbO8Cl under the visible-light irradiation.

Puckered Penta-like PdPX (X = O, S, Te) Semiconducting Nanosheets: First-Principles Study of the Mechanical, Electro-Optical, and Photocatalytic Properties.

The atomic, electronic, optical, and mechanical properties of penta-like two-dimensional PdPX (X = O, S, Te) nanosheets have been systematically investigated using density functional theory calculations. All three PdPX nanosheets exhibit dynamic and mechanical stability on the basis of an analysis of phonon dispersions and the Born criteria, respectively. The PdPX monolayers are found to be brittle structures. Our calculations demonstrate that the PdPX nanosheets exhibit semiconducting characteristics with indirect band gaps of 0.93 (1.99), 1.34 (2.11), and 0.74 (1.51) eV for X = O, S, Te, respectively, using the PBE (HSE06) functional, where PdPTe is the best material for visible-light photocatalytic water splitting. Our findings give important basic characteristics of penta-like two-dimensional PdPX materials and should motivate further theoretical and experimental investigations of these interesting materials.

Hollow polymeric ionic liquid spheres with hierarchical electron distribution: A novel composite of g-C3N4 for visible light photocatalytic water splitting enhancement

Graphitic carbon nitride (g-C3N4) has been considered as a promising organic semiconductor for photochemistry but under great challenges of severe recombination rate of photogenerated electron-hole pairs, the weak ability of light harvesting, and other inherent defects. Herein, uniformed hollow polymeric ionic liquid spheres with layer gradients of electron structure were developed innovatively combining with g-C3N4 nanosheets driven by sonication and electrostatic forces. These two components can form a novel system due to the distinction of electron between poly(ionic liquids) (PILs) and g-C3N4. The novel as-developed PILs-x loaded on the g-C3N4 surface benefits the transfer and supplement of photoinduced electrons resulting from the effects of “electronic reservoir”. Meanwhile, the light harvesting of the PILs-x/g-C3N4 is also enhanced significantly. In addition, the “ion modulated effect” was studied by tuning the species of anion in PILs structure, achieving controllable regulation of photocatalytic activity. Benefiting from the above fascinating properties, the PILs-x/g-C3N4 system exhibit a significantly enhanced hydrogen evolution efficiency with Pt as co-catalyst under visible light irradiation compared with the Pt/PCN. This study opens a new sight to strengthen the photocatalytic activity of g-C3N4 by using functional hollow PILs with layer gradients of electron structure for various applications in energy and environmental fields.

Two dimensional twin T-graphene: monolayer for visible-light photocatalytic water splitting and bulk for anode material of magnesium batteries.

Two-dimensional (2D) carbon allotropes with all-sp2-hybridization have demonstrated great potential in nano-photoelectric devices, but the application of semiconductor photocatalysts for water splitting and anodes in magnesium batteries remains unoptimistic. Motivated by this, we theoretically study a highly stable all-sp2-hybridized 2D carbon allotrope twin T-graphene (TTG) via first-principles simulations. And through the calculations of the HSE06 functional, we find that TTG has a wide bandgap (2.70 eV) and suitable band edge positions satisfying the criteria of photocatalysts for overall water splitting. Additionally, TTG exhibits excellent photocatalytic properties for overall water splitting reflecting a high STH efficiency (12.34%), strong absorption coefficient in the visible-light region and the carrier mobility being high for electrons but low for holes. By investigating the strain effects, we get that, with biaxial compressive strain, the ability of overall photocatalytic water splitting can be effectively improved including STH up to ∼30%. Moreover, the bulk TTG also exhibits great potential as an anode material of magnesium batteries with a theoretical capacity of 556 mA h g-1, average voltage of 0.74 V and diffusion energy barrier of ∼0.96 eV. Our results would broaden the application of all-sp2-hybridized 2D carbon allotropes in the semiconductor photocatalytic and magnesium batteries field.

Band-engineered Zn2TiO4 nanowires for hydrogen generation from water using visible light: A first-principles study

The electronic structure and optical properties of inverse-spinel Zn2TiO4 nanowires and bulk material were investigated for hydrogen generation from water by visible-light photocatalysis using first-principles density calculations. In our theoretical studies, the bandgap of the Zn2TiO4 nanowires was much smaller than that of the bulk material. New intermediate states appeared in the mid-bandgap as a result of Zn and Ti atoms on the surface of ZnO-terminated and ZnTiO-terminated nanowires. These deep-level states could become recombination centers for photogenerated electron–hole pairs, indicating that these two types of nanowires would no longer meet the requirements for photocatalytic water splitting. In contrast, the electronic states arising from oxygen elements on the surface of the TiO-terminated nanowires resulted in an upward movement of the edge of the valence band, whereas the surface electronic states made no difference to the edge of the conduction band. Moreover, the optical property calculations showed that the optical absorption edge of the nanowire would be red-shifted. These calculations revealed that inverse-spinel Zn2TiO4 nanowires were appropriate for visible-light photocatalytic water splitting reactions.

Enhancing interfacial charge transfer in mesoporous MoS2/CdS nanojunction architectures for highly efficient visible-light photocatalytic water splitting

Mesoporous MoS2-modified CdS nanojunction networks possessing advantageous electronic connectivity and charge transfer behavior at the interfaces deliver highly efficient visible-light photocatalytic H2 production activity from water splitting.

First-principles study of configurations, electronic and photocatalytic properties of carbon-doped anatase TiO2

C-Doping TiO2 could significantly enhance the photocatalytic activity, as found in experiments, but the mechanism behind is elusive. We systematically investigated the configurations, electronic and photocatalytic properties of C-doped anatase TiO2 by first-principles calculation. The results show that the structures for the C-anion (Ca) and C-cation (Cc) doped TiO2 could exist in different form, and the Ca and substitutional Cc doping are favored under O-poor and O-poor conditions, respectively. The calculated electric and optical properties demonstrate that the substitutional Cc doping play the vital role in narrowing the bandgap, responsible for enhanced visible-light absorption of the TiO2. Finally, the calculated band edges using the hybrid functional of Heyd, Scuseria and Ernzerhof (HSE) suggested that substitutional Cc doped TiO2 could exhibit the small band gap and appropriate band edges for visible-light photocatalytic water splitting. The findings could not only give new insights into atomistic origin of visible-light response and photocatalysis of the C-doped TiO2.

Organic Conjugation of Polymeric Carbon Nitride for Improved Photocatalytic CO2 Conversion and H2 Fixation

The systematic alteration of a carbon nitride unit (CNU) for visible light photocatalytic water splitting is a promising research subject owing to the increasingly serious energy and environmental complications. Herein, the conjugated monomer 3,6‐dibromopyridazine (DBP) is integrated within polymeric carbon nitride (PCN named as CNU = carbon nitride containing urea precursor) via thermal condensation, which is designated as CNU‐DBP. These samples are used for the first time in the photocatalytic conversion of CO2 reduction and hydrogen (H2) evolution through water splitting. Such integration intimidates the electron density, promoting charge transfer separation and elevating the photocatalytic activity of CNU under visible light illumination. The superior sample such as CNU‐DBP9.0 after 4 h of photooxidation generates 65.7 μmol of CO and 17.3 μmol of H2 of the reaction system, emphasizing the highest photocatalytic activity. The H2 evolution rate (HER) for pristine CNU is found as 11.9 μmol h−1, whereas for CNU‐DBP9.0 it is estimated at 178.2 μmol h−1 with 15 times greater activity. This process predicts a significant diversion in the specific area, bandgap, and chemical composition and promotes the efficient separation of photogenerated charge carriers from the ground state to the excited state of CNU, thereby considering it a best candidate for the photoreduction of CO2 source and water splitting into H2.

Metal-doped KNbO3 for visible light photocatalytic water splitting: A first principles investigation

Materials suitable for visible light photocatalytic water splitting provide sustainable green energy production and environmental pollution solution. Potassium niobate, KNbO3, is not widely used in photocatalytic applications because of its large bandgap, which is not appropriate for the visible range of the solar spectrum. However, doping of semiconductors may help reduce their bandgaps by pinning a dopant level near the top/bottom of the valence/conduction band. We employ first-principles calculations to gain insight into the electronic and optical properties of KNbO3 doped with a number of 3d and 4d transition metals to design and enhance its photocatalytic behavior. We demonstrate the substitutional doping with these elements at the Nb site decreases the bandgap and improves the optical and photocatalytic activities of KNbO3. Our calculations prove that the best candidates for water splitting and CO2 gas reduction are Ag- and Mn-doped KNbO3, respectively. Computational outcomes are compared and discussed with existing experimental ones for doped KNbO3 and KTaO3 structures. Meanwhile, we found out Tc-doped KNbO3 can be beneficial for spintronic applications. The results achieved in this study will initiate a number of experimental investigations for the full exploration of the cubic perovskites, especially in green energy production.

Solid-phase synthesis of Bi3−xYxO4Cl solid solution for visible-light photocatalytic hydrogen generation

The solid solution constructed by two or more semiconductors with different bandgaps can effectively modulate the energy band, satisfying the redox potential for both H+/H2 and O2/H2O. In this paper, Sillen-Aurivillius compounds with layered structure Bi3O4Cl and Y3O4Cl solid solution (Bi3−xYxO4Cl) were successfully synthesized by the solid-phase method. The absorption edge of Bi3−xYxO4Cl could be dramatically changed from about 500–740 nm with the increase of Bi elements, which accounted for the entire UV-Vis region of the solar spectrum. Furthermore, the bandgaps for Bi3−xYxO4Cl could be adjusted from about 2.6–2.0 eV with the decreased Y/Bi ratio from 2 to 0.2. Significantly, the conduction band minimum energy (CBM) of Bi3−xYxO4Cl all satisfied the redox potential for H+/H2, which showed excellent visible-light photocatalytic water splitting.

Homogeneous dual-site P lattice doping in CdS quantum rods for visible-light photocatalytic water splitting

Dual-site lattice doping in semiconductor materials is highly promising for photocatalysis as it can tune the sub-band electronic structure and charge distribution of the photocatalyst. However, it is challenging to precisely regulate the band structure by dual-site doping using the same heteroatom. Herein, we report the homogeneous dual-site P lattice doping in CdS quantum rods, i.e. P is homogeneously doped into both substitutional site of S and interstitial site, leading to negative shift of valence and conduction bands with extraordinary visible-light excitation efficiency. The electrons generated in the newly designed photocatalyst have strong redox potential in reducing H+ into H2 in photocatalysis. By introducing nontrivial co-catalyst Ni3C quantum dots onto the surface of P-CdS quantum rods, the apparent quantum efficiency of the catalyst achieves 15.7% at 435 nm, which is 5 times higher than that of pristine CdS.

Introducing spin polarization into atomically thin 2D carbon nitride sheets for greatly extended visible-light photocatalytic water splitting

Atomically thin 2D carbon nitride sheets (CNs) became one of the most promising solar energy conversion materials. However, the application of CNs is still limited due to two reasons: (i) the bandgap of CNs is wider than its counterpart due to the quantum size effect, which reduces its effective utilization of the entire solar spectrum, and (ii) the visible-light photocatalytic activity of CNs is still low due to its faster recombination of photogenerated carriers than photocatalytic reaction. Here, we achieve a strong visible-light absorption band in CNs through fluorination followed by thermal defluorination in Se vapor (Se-CNs). Experimental results and theoretical calculations confirm that the formation of cyano groups accompanied with in-situ Se doping expands the absorption edge of CNs from 416 to 584 nm. More importantly, a downward electron spin polarization in the CNs structure improves dramatically the efficiency of charge separation and surface catalysis reaction. The hydrogen generation rate of Se-CNs with 3 wt% Pt under visible-light irradiation (> 420 nm) reaches up to 5411.2 μmol h−1 g−1 that is 176.5 times of the hydrogen generation of CNs. Additionally, the visible-light photocatalytic oxygen evolution of Se-CNs acquires tremendous improvements. This work provides a new approach for improving electron structure of atomically thin 2D non-metal semiconductor materials.

Pt-decorated CuO nanosheets and their application in the visible light photocatalytic water splitting reaction

CuO nanocompositions found many applications in chemistry and physics. However, its large band gap and charge carrier recombination when used as a photo catalyst hinder its effectiveness. In this paper we report a simple sol–gel method for the synthesis of mesoporous CuO nanosheets followed by the application of photo-assisted Pt incorporation to produce a uniformly Pt-decorated mesoporous CuO nanosheets. The nanosheet structure, crystallinity, morphology, and particle-size were confirmed employing XRD measurements, transition electron microscopy. The synthesized mesoporous Pt/CuO nanosheets showed high pore volumes of 0.350 cm3/g and a large surface area of 250 m2/g. The effectiveness of the photocatalyst was tested via application in the water splitting reaction under visible light and the use of glycerol as a positive hole scavenger. Pt/CuO yielded ~ 5400 µmol/g of H2, 7-times higher compared to pure mesoporous CuO. Higher efficiency is explained by narrower band gap, superior light harvesting capacity, and the efficient charge-carrier separation due to the use of glycerol. Photocurrent and photoluminescence were used to show the effect of Pt decoration on the photocatalytic efficiency of the material through the electron transfer from CuO to Pt atoms.

Phosphorus-based metal-free Z-scheme 2D van der Waals heterostructures for visible-light photocatalytic water splitting: a first-principles study.

Direct splitting of water over semiconductors under sunlight irradiation would be a promising approach for hydrogen production and solar energy utilization. In this work, BlueP/PN with a 2D van der Waals (vdW) heterostructure is proposed as a novel catalyst for the Z-scheme photocatalytic system. Its electronic structures, optical properties, and combined configuration were systematically evaluated by hybrid density functional theory (DFT) calculations. It was revealed that the 2D vdW heterostructure of BlueP/PN can play an important role in water splitting under visible light irradiation. This predicts a novel design of P-based vdW heterostructures for efficient photocatalysts.