Photocatalytic Water Splitting Reaction Research Articles

The Emergence of 2D Building Units in Metal‐Organic Frameworks for Photocatalytic Hydrogen Evolution: A Case Study with COK‐47

AbstractMetal‐organic frameworks (MOFs) are promising materials for photocatalytic water splitting reactions, but examples of visible light‐responsive, catalytically active, and stable MOFs are still rare. A detailed investigation is conducted for COK‐47 – a recently described MOF comprising 2D Ti‐O6 secondary building units (SBUs) – toward a photocatalytic hydrogen evolution reaction (HER), showing how overall particle morphology, surface area, and missing ligand defects are central parameters governing the material's ultimate performance. The newly synthesized COK‐47ISO is among the most active MOFs to date, yielding HER‐rates of 8.6 µmol h−1, and an apparent quantum yield (AQY) of 0.5% under visible light illumination. Optoelectronic and photoluminescence investigations, supported by theoretical calculations, enable the unraveling of its electronic structure along with charge transfer and recombination kinetics. A wavelength‐dependent reaction mechanism is proposed involving ligand to metal charge transfer (LMCT) and the main challenges for visible or UV photoexcitation are identified, demonstrating that the unique 2D layered structure aids charge separation and is key to the high performance. This work introduces COK‐47 as a promising alternative to the well‐known MIL‐125 family and offers directions for future studies

Rh/Cr2O3 and CoOx Cocatalysts for Efficient Photocatalytic Water Splitting by Poly (Triazine Imide) Crystals

AbstractIn situ photo‐deposition of both Pt and CoOx cocatalysts on the facets of poly (triazine imide) (PTI) crystals has been developed for photocatalytic overall water splitting. However, the undesired backward reaction (i.e., water formation) on the noble Pt surface is a spontaneously down‐hill process, which restricts their efficiency to run the overall water splitting reaction. Herein, we demonstrate that the efficiency for photocatalytic overall water splitting could be largely promoted by the decoration of Rh/Cr2O3 and CoOx as H2 and O2 evolution cocatalysts, respectively. Results reveal that the dual cocatalysts greatly extract charges from bulk to surface, while the Rh/Cr2O3 cocatalyst dramatically restrains the backward reaction, achieving an apparent quantum efficiency (AQE) of 20.2 % for the photocatalytic overall water splitting reaction.

Rh/Cr2 O3 and CoOx Cocatalysts for Efficient Photocatalytic Water Splitting by Poly (Triazine Imide) Crystals.

In situ photo-deposition of both Pt and CoOx cocatalysts on the facets of poly (triazine imide) (PTI) crystals has been developed for photocatalytic overall water splitting. However, the undesired backward reaction (i.e., water formation) on the noble Pt surface is a spontaneously down-hill process, which restricts their efficiency to run the overall water splitting reaction. Herein, we demonstrate that the efficiency for photocatalytic overall water splitting could be largely promoted by the decoration of Rh/Cr2 O3 and CoOx as H2 and O2 evolution cocatalysts, respectively. Results reveal that the dual cocatalysts greatly extract charges from bulk to surface, while the Rh/Cr2 O3 cocatalyst dramatically restrains the backward reaction, achieving an apparent quantum efficiency (AQE) of 20.2 % for the photocatalytic overall water splitting reaction.

Pt-doped TiO2 nanotubes as photocatalysts and electrocatalysts for enhanced photocatalytic H2 generation, electrochemical sensing, and supercapacitor applications

Hydrogen production plays a major role in both the stationary and transportation energy sectors. Electrochemical sensing is an additional sensitive electrochemical application for the detection of trace analytes. Semiconductor-based catalysts can be used for both of these applications. While doping these semiconductors, it improves the pure semiconductor's electrical properties. TiO2 nanotubes (NTbs) and Pt-doped TiO2 NTbs were successfully synthesized by the hydrothermal method at various doping concentrations. They were subjected to different methodical characterization tools such as XRD, FT-IR, UV-DRS, SEM, and TEM. Out of various doping concentrations of Pt, 1.6% Pt-doped TiO2 NTbs shows superior H2 generation through photocatalytic water splitting reactions, which is attributed to the high electrical conductivity and stable electrical properties of it with a low impedance of 79 Ohms. The amount of hydrogen gas produced from 1.6% Pt-doped TiO2 was found to be 19.22 mmolh−1g−1, which is due to the reduced band gap of the material from the incorporation of Pt into pure TiO2. Prepared NTbs were examined for their electrochemical activity towards the detection of nitrite at very low concentrations. The enhanced electrochemical activity of 1.6% Pt-doped TiO2 NTbs is effective in detecting nitrite with a detection limit of 14.7 nM. A supercapacitor device has been constructed using 1.6% Pt-doped TiO2. The constructed device produced stable cyclic voltammograms up to the maximum scan rate of 10,000 mVs−1 with a potential window of 2 V. The maximum specific capacitance and Energy density (Ed) of a 1.6% Pt-doped TiO2-based supercapacitor were determined to be 750.01 F/g and 78.8 Wh/Kg, respectively. These results demonstrate that it is an excellent material for energy storage applications.

Theoretical Study on (n,n)-Nanotubes Rolled-up from B/N Substituted Me-Graphene

In this work, the n,n-type nanotube systems rolled up from the B/N substituted Me-graphene (i.e., Me-CBNT and Me-CNN, respectively) were investigated with the aid of the density functional theory (DFT). Due to the lattice dynamic instabilities until n=10, the n,0 and n,m nanotube systems were not involved in this study. According to our calculations at the Perdew-Burke-Ernzerhof (PBE) level, the n,n Me-CBNT and Me-CNNT systems possess excellent mechanical strengths. The Young’s moduli of Me-CBNTs can reach 60% of single-walled carbon nanotubes (SWCNTs), while their mass densities are only around 70% of SWCNTs. Based on the fully relaxed geometric configurations at the PBE level, the electronic configurations of the related nanotubes were evaluated by using the global hybrid functional B3LYP with 36% Fock exchanges. The n,n Me-CBNTs are metallic, while the n,n Me-CNNTs are semiconductors with the inherent band gaps in the range of 3.08 eV to 3.31 eV. The Bloch flat bands appear on both sides of their Fermi levels, indicating the localized charge carriers. Their band edge arrangements imply that these materials are promising candidates for the photocatalytic water splitting reactions at certain pH values.

Photocatalytic effects on Au@TiO2 confined in BMIm.NTf2 ionic liquid for hydrogen evolution reactions

We report a photocatalytic device based on gold nanoparticles (AuNPs/TiO2) coated with an ionic liquid (IL) as a very active, stable, and versatile system for hydrogen evolution reactions (HERs) via methanol photo reforming (MPR) and photocatalytic water-splitting (WSR) reaction. The devices prepared by magnetron sputtering yield hybrid materials with direct Au–TiO2 interactions that were subsequently coated with BMIm.NTf2 (IL@Au@TiO2). The IL presence critically enhances the photocatalytic performance given its interaction with the AuNPs and TiO2 surfaces by decreasing the flat band energy levels and the Schottky barrier thickness. The results show an increase in the number of charge carriers yielding a capacitor-like effect generated by interactions of IL and Au@TiO2 particles. Apparent quantum yield (AQY) up to 28.3% under UV irradiation (365 nm) and 22.9% at 405 nm irradiation were observed in the HER employing the as-prepared materials.

Two-dimensional [formula omitted] (X = Si, Ge; Y = S, Se, Te) family with potential application in photocatalysis

Photocatalysis is an attractive route for energy harvesting and pollution mitigation, however, designing an optimal catalyst is rather challenging. Motivated by fundamental interest and potential applications, the investigations of two-dimensional (2D) photocatalyst are important subjects in solar energy conversion. In this work, the stability, electronic, and optical properties of 2D Cr2X2Y6 (X = Si, Ge; Y = S, Se, Te) family are investigated to explore their photocatalytic activity for water splitting. Density functional theory (DFT) and ab-initio molecular dynamics (AIMD) calculations were employed. The electronic band structure calculations indicate that all these monolayers, except Cr2Ge2Te6 and Cr2Si2Te6, have an adequate bandgap, as well as suitable band-edge positions for photocatalytic water splitting reactions, specifically under acidic conditions. Moreover, the studied materials show strong capability of harvesting sunlight, that is essential for photocatalytic activity. Remarkably, Cr2Ge2Se6 and Cr2Si2Se6 monolayers exhibited high solar-to-hydrogen energy conversion efficiencies of 14.89% and 13.26%, respectively. The presented results pave the way for designing other electronically optimized layered photocatalysts for water splitting.

Overall water splitting: What’s next?

Photocatalytic overall water splitting reaction driven by sunlight has been studied as a means of producing renewable solar hydrogen on a large scale. The practicality of this technology will ultimately depend on the cost of the produced solar hydrogen. This cost, in turn, is determined by various factors such as solar-to-hydrogen energy conversion efficiency, scalability, construction/operating costs and the service life of such systems. This article both summarizes the current state of development of photocatalytic overall water splitting technology and presents issues that must be addressed in the future to allow such systems to have real-world utility.

Post-functionalization of graphitic carbon nitride for highly efficient photocatalytic hydrogen evolution

In this work we report the feasible modification of graphitic carbon nitride (g-C3N4) polymer through a post-functionalization progress. The resultant photocatalyst exhibits boron doping and mesoporous structure with a high surface area of 125 m2/g, leading in an increased surface activity for photocatalytic water splitting reaction. X-ray diffraction, X-ray photoelectron spectroscopy, PL emission spectra and UV-Vis spectra were used to detect the properties of as-prepared samples. Based on X-ray photoelectron spectroscopy analysis, boron is proposed to dope in the g-C3N4 lattice. Optical studies indicated that boron doped g-C3N4 exhibits enhanced and extended light absorbance in the visible-light region and a much lower intensity of PL emission spectra compared to pure g-C3N4. As a result, boron doped g-C3N4 shows activity of 10.2 times higher than the pristine g-C3N4 for photocatalytic hydrogen evolution. This work may provide a way to design efficient and mesoporous photocatalysts through post modification.

Excited‐state nonadiabatic dynamics simulations on the heptazine and adenine in a water environment: A mini review

AbstractStudying the excited‐state decay process is crucial for materials research because what happens to the excited states determines how effective the materials are for many applications, such as photoluminescence and photocatalysis. The high computational cost, however, limits the use of high‐accuracy theoretical approaches for analyzing research systems containing a significant number of atoms. Time‐dependent density functional theory is a practical approach to investigate the photorelaxation processes in these systems, as demonstrated in the studies of the excited‐state decays of heptazine‐water clusters and adenine in water described in this review. Here, we highlight the importance of conical intersections in the excited‐state decay processes of these systems using the aforementioned examples. In the heptazine‐water and adenine‐water systems, these intersections are associated with the photocatalytic water splitting reaction, caused by a barrierless reaction called water to adenine electron‐driven proton transfer. We expect the result would be helpful for researching the excited‐state decays of graphitic carbon nitride materials and DNA nucleotides.

Constructing Co-S interface chemical bonds over Co@NC/ZnIn2S4 for an efficient solar-driven photocatalytic H2 evolution.

Developing novel photocatalysts with an intimate interface and sufficient contact is significant for the separation and migration of photogenerated carriers. In this work, a novel Co@NC/ZnIn2S4 heterojunction with a strong Co-S chemical bond was formed at the interface between Co@NC and ZnIn2S4, which accelerated charge separation. Meanwhile, the recombination of the electron-hole pairs was further restricted by the Co@NC/ZnIn2S4 Schottky junction. The Co@NC (5 wt%)/ZnIn2S4 composite exhibited an H2 evolution rate of 33.3 μmol h-1, which is 6.1 times higher than that of the pristine ZnIn2S4, and Co@NC/ZnIn2S4 showed excellent stability in the photocatalytic water splitting reaction. Its apparent quantum yield reached 38% at 420 nm. Furthermore, the Kelvin probe test results showed that the interfacial electric field formed as the driving force for interface charge transfer was oriented from Co@NC to ZnIn2S4. In addition, the Co-S bond as a high-speed channel facilitated the interfacial electron transfer. This work reveals that in situ formed chemical bonds will pave the way for designing high-efficiency heterojunction photocatalysts.

Alternatives to water oxidation in the photocatalytic water splitting reaction for solar hydrogen production.

The photocatalytic water splitting process to produce H2 is an attractive approach to meet energy demands while achieving carbon emission reduction targets. However, none of the current photocatalytic devices meets the criteria for practical sustainable H2 production due to their insufficient efficiency and the resulting high H2 cost. Economic viability may be achieved by simultaneously producing more valuable products than O2 or integrating with reforming processes of real waste streams, such as plastic and food waste. Research over the past decade has begun to investigate the possibility of replacing water oxidation with more kinetically and thermodynamically facile oxidation reactions. We summarize how various alternative photo-oxidation reactions can be combined with proton reduction in photocatalysis to achieve chemical valorization with concurrent H2 production. By examining the current advantages and challenges of these oxidation reactions, we intend to demonstrate that these technologies would contribute to providing H2 energy, while also producing high-value chemicals for a sustainable chemical industry and eliminating waste.

A multi-factor adjustable PtSe2/GaN van der Waals heterostructure with enhanced photocatalytic performance.

Two-dimensional van der Waals heterostructures with multiple tunable approaches in electronic and optical properties are highly superior for photocatalysis and novel devices. By applying first-principles calculations, we systematically studied the electronic structure, optical absorption, carrier mobility and solar-to-hydrogen efficiency of PtSe2/GaN heterostructures, which are affected by different thicknesses, varying directions of polarization of the GaN nanosheets and applied mechanical strain of the whole system. The results indicate that these heterostructures exhibit thermodynamic stability at room-temperature (300 K), and most configurations have type-II band alignment, among which the heterostructure consisting of GaN trilayers and PtSe2 shows high visible-light absorption (1.71 × 105 cm-1) and ultra-wide range of pH values (pH = 0-14) for the photocatalytic water splitting reaction and exceedingly high overpotential for the hydrogen evolution reaction (3.375 eV). Simultaneously, being two of the most significant parameters of photocatalysis and devices, the carrier mobility and solar-to-hydrogen efficiency have also been calculated, respectively, reaching up to 1601 cm2 V-1 s-1 and 17.2%. Moreover, the photoelectrical properties can be highly tuned through further biaxial strain engineering; especially, the visible-light absorption can be enhanced to 2.85 × 105 cm-1 by applying 6% compression strain. Thus, the PtSe2/GaN heterostructure we proposed shows a broad prospect for photocatalytic water splitting.

Ultrafast dynamics in polymeric carbon nitride thin films probed by time-resolved EUV photoemission and UV-Vis transient absorption spectroscopy.

The ground- and excited-state electronic structures of four polymeric carbon nitride (PCN) materials have been investigated using a combination of photoemission and optical absorption spectroscopy. To establish the driving forces for photocatalytic water-splitting reactions, the ground-state data was used to produce a band diagram of the PCN materials and the triethanolamine electron scavenger, commonly implemented in water-splitting devices. The ultrafast charge-carrier dynamics of the same PCN materials were also investigated using two femtosecond-time-resolved pump-probe techniques: extreme-ultraviolet (EUV) photoemission and ultraviolet-visible (UV-Vis) transient absorption spectroscopy. The complementary combination of these surface- and bulk-sensitive methods facilitated photoinduced kinetic measurements spanning the sub-picosecond to few nanosecond time range. The results show that 400 nm (3.1 eV) excitation sequentially populates a pair of short-lived transient species, which subsequently produce two different long-lived excited states on a sub-picosecond time scale. Based on the spectro-temporal characteristics of the long-lived signals, they are assigned to singlet-exciton and charge-transfer states. The associated charge-separation efficiency was inferred to be between 65% and 78% for the different studied materials. A comparison of results from differently synthesized PCNs revealed that the early-time processes do not differ qualitatively between sample batches, but that materials of more voluminous character tend to have higher charge separation efficiencies, compared to exfoliated colloidal materials. This finding was corroborated via a series of experiments that revealed an absence of any pump-fluence dependence of the initial excited-state decay kinetics and characteristic carrier-concentration effects that emerge beyond few-picosecond timescales. The initial dynamics of the photoinduced charge carriers in the PCNs are correspondingly determined to be spatially localised in the immediate vicinity of the lattice-constituting motif, while the long-time behaviour is dominated by charge-transport and recombination processes. Suppressing the latter by confining excited species within nanoscale volumes should therefore affect the usability of PCN materials in photocatalytic devices.

Switch effect on controlled water splitting by biaxial strain regulating the promising two-dimensional Janus X2PAs (X = Si, Ge and Sn) photocatalyst.

Two-dimensional photocatalytic materials with unique properties have been well-reported in recent decades. However, strategies for controlling the photocatalytic process are still ongoing. Herein, Janus X2PAs (X = Si, Ge and Sn) monolayers have been explored by first-principles calculations to meet this challenge. All strain-free X2PAs monolayers exhibit excellent photocatalytic properties with high carrier mobility (2.39 × 102-1.34 × 104 cm2 V-1 s-1), suitable band edge positions straddling the standard redox potential of water and large visible light absorption coefficients (up to 105 cm-1). Most importantly, a reaction switch effect is proposed for the first time towards controlling the microscopic photocatalytic process of water splitting on X2PAs monolayers through macroscopic mechanical strain. This effect renders the Janus X2PAs photocatalytic switches among the states of only oxygen evolution reaction, only hydrogen evolution reaction and the full redox reaction for controlled water splitting. This work not only provides a new avenue for designing highly tunable photocatalysts but also offers new physical insights into controlling the photocatalytic water-splitting reaction.

Insight into the Effect of Anionic–Anionic Co-Doping on BaTiO3 for Visible Light Photocatalytic Water Splitting: A First-Principles Hybrid Computational Study

In this research, we thoroughly studied the electronic properties and optical absorption characteristics with double-hole coupling of anions–anion combinations for designing effective photocatalysts for water redox using first-principles methods within the hybrid Heyd–Scuseria–Ernzerhof (HSE06) exchange–correlation formalisms. The findings reveal that the values of formation energy of both the anion mono- and co-doped configurations increase monotonically as the chemical potential of oxygen decreases. The N–N co-doped BaTiO3 exhibits a more favorable formation energy under an O-poor condition compared with other configurations, indicating that N and N pairs are more likely to be synthesized successfully. Interestingly, all the co-doping configurations give a band gap reduction with suitable position for oxygen production and hydrogen evolution. The obtained results demonstrate that all the co-doped systems constitute a promising candidate for photocatalytic water-splitting reactions. Furthermore, the enhanced ability of the anionic-anionic co-doped BaTiO3 to absorb visible light and the positions of band edges that closely match the oxidation-reduction potentials of water suggest that these configurations are viable photocatalysts for visible-light water splitting. Therefore, the wide-band gap semiconductor band structures can be tuned by double-hole doping through anionic combinations, and high-efficiency catalysts for water splitting using solar energy can be created as a result.

Higher-than-common temperature short-time processed polymeric carbon nitride nanosheets as an efficient photocatalyst for H2 production

Two-dimensional polymeric carbon nitride nanosheets exhibiting unique optical properties are considered promising materials for visible light active photocatalysts. Here, we synthesized thin polymeric carbon nitride nanosheets via the higher-than-common temperature (710 °C) pyrolysis of a polymeric carbon nitride intermediate for 20 min. The exfoliation efficiency of the nanosheets was dramatically improved by the higher-than-common temperature treatment. Notably, compared with the pristine polymeric carbon nitride nanosheets, the synthesized thin polymeric carbon nitride nanosheets exhibited a reduced optical bandgap and more negative conduction band (ECB) positions, which ensured enhanced visible-light absorption and reductive ability. When applied to photocatalytic water-splitting reaction for H2 evolution, the thin polymeric carbon nitride nanosheets exhibited enhanced activity under visible light irradiation, accounting for an optimal H2-evolution rate of 8187 µmol·g−1·h−1 with a Pt loading of 1 wt%, which was approximately six-fold higher than that obtained with the urea-derived pristine polymeric carbon nitride nanosheets. This work provides a rapid, affordable and facile method for synthesizing polymeric carbon nitride nanosheets with high photocatalytic performances.

Enhanced charge separation in dual Z-scheme Au decorated LaFeO3-g-C3N4-BiFeO3 system for efficient H2 production

The artificial Z-scheme system that mimics natural photosynthesis is encompassed of two semiconductors which generates two sets of charge carriers at different locations under solar irradiation. Researchers are trying to improve the efficacy of these systems by engineering highly stable redox couples, enhancing their light absorption capability, and discovering new cocatalysts. Herein, we report the fabrication of Au decorated LaFeO3-g-C3N4-BiFeO3 (Au/LFO-CN-BFO) catalyst as dual Z-scheme system for photocatalytic water splitting reactions. The photocatalytic performances of Au/LFO-CN-BFO, LFO-CN-BFO, LaFeO3, g-C3N4 and BiFeO3 were compared and the mechanism on the symmetric double Z-scheme Au/LFO-CN-BFO photocatalytic system was proposed. The results display that Au/LFO-CN-BFO has more superior photocatalytic performance compared to its counter parts (LaFeO3, g-C3N4 and BiFeO3), which is due to the symmetric double Z-scheme nature of photocatalyst. The as-fabricated Au/LFO-CN-BFO photocatalyst reveals high performance for photocatalytic water reduction to evolve H2 (i.e., 698.4 µmol h-1g−1) under visible light irradiation (λ ≥ 420 nm), which is far greater compared to that of its counterparts. As confirmed from various observations, the improved H2 evolution performance of the dual Z-scheme system is accredited to the improved light absorption and significantly promoted charge carrier’s separation. The design of this highly efficient dual Z-scheme composite photocatalyst would trigger the development of new artificial Z-scheme scheme systems that will solve the energy and associated environmental issues.

Hydrogen Production from Water Splitting using TiO2/CoS Composite Photocatalyst

Photocatalytic water splitting reaction has been considered an ideal method for hydrogen generation. In this study, a composite of TiO2/CoS photocatalyst prepared by hydrothermal synthesis method assisted by ball milling crushing process was used. The TiO2/CoS composites prepared with three variation compositions of 90/10, 80/20, and 70/30 were named M-10, M-20, and M-30, respectively. Field-emission scanning electron microscopy images showed that the morphologies of the composites were porous and uniform of nanospheres. The X-ray diffraction and energy dispersive spectroscopy analyses confirmed the presence of CoS in the composites. Ultraviolet–visible absorption characterization demonstrated the smallest bandgap value of approximately 2.72 eV presented by sample M-30 with the photocurrent density of 0.32 mA cm−2 at 0.9 V vs. Ag/AgCl. The presence of CoS in this study could increase the PC hydrogen generation of TiO2 by nearly 2.5 times. The composites forming a p-n heterojunction between TiO2 and CoS could prevent electron–hole recombination and increase the overall photoactivity of TiO2.

Band alignment of PCN-222 via selection of the metal porphyrin linker for sunlight driven photocatalytic overall water splitting

Simulated sunlight driven overall water splitting has been carried out using porphyrin-based PCN-222 metal-organic frameworks having none or Zn, Fe, Mn and Co metal ion at the porphyrin linker. Influence of the transition metal on the photocatalytic water splitting reaction was systematically studied filling the existing gap. Under simulated sunlight irradiation, PCN-222(Zn) shows the highest H2 evolution reaction (HER) efficiency, reaching almost 1.5 mmol of H2 per gram of photocatalyst by using MeOH, which corresponds to the HER rate of 65.8 μmol g−1 h−1 that is higher than some examples of porphyrinic MOFs with Pt. In addition to this, as the valence band (VB) levels of PCN-222(M) gets more positive than that of water oxidation potential, O2 evolution was detected in a quasi stoichiometric ratio respect to H2. To gain insights into the mechanism of the H2 and O2 evolution, transient absorption (TA) spectroscopy measurements were carried out, providing a strong evidence of the generation of a charge separated state and that the positive holes are able to oxidize H2O in the microsecond time. This study not only provides new insights for overall water splitting applications of porphyrin-based MOFs without using noble metal co-catalysts, but also shed light on future experiments for construct sustainable photocatalysts for important yet thermodynamically uphill reactions.