Photocatalytic water splitting on Au/HTiNbO5 nanosheets

The layered potassium titanium niobate, KTiNbO₅, is known as a photocatalyst for hydrogen production from water splitting under UV light. Here we show that titanium niobate nanosheets with a slit like framework can be obtained by exfoliation of KTiNbO₅ followed by proton exchange. Gold nanoparticles were deposited on the titanium niobate nanosheets using deposition-precipitation (DP), photo-deposition (PD) and impregnation (IMP) method in order to improve photocatalytic hydrogen production from water splitting.     

Photo-assisted splitting of water into hydrogen using visible-light activated silver doped g-C3N4 & CNTs hybrids

Herein, highly efficient and cost effective solar photocatalytic water splitting for hydrogen (H₂) generation was achieved by modified g-C₃N₄. Visible light absorption of g-C₃N₄ was enhanced by decorating g-C₃N₄ matrix with silver nanoparticles (Ag). Moreover, incorporation of carbon nanotubes (CNTs) in Ag/g-C₃N₄ facilitated photocatalytic performance through efficient separation and transfer of photogenerated e-h pairs (charges) in Ag/g-C₃N₄ that consequently generated very pure and significant H₂. Among several tested ratios (wt. %) of Ag/g-C₃N₄/CNTs, 1.82 (Ag/g-C₃N₄) and 2.00 (and Ag/g-C₃N₄/CNTs) were found to be highly efficient that harvested maximum visible-light and produced H₂ @1.48 mmol h⁻¹ and 1.78 mmol h⁻¹. We witnessed distinctive role of CNTs as an electron collector and carrier to separate photogenerated e-h pairs to facilitate photocatalysis for H₂ generation together with possible utility of Ag and CNTs doped materials with regard to energy transformation.     

Computational exploration of magnesium-decorated carbon nitride (g-C3N4) monolayer as advanced energy storage materials

Density functional theory (DFT) computational studies were conducted to explore the hydrogen storage performance of a monolayer material that is built on the base of carbon nitride (g-C₃N₄, heptazine structure) with decoration by magnesium (Mg). We found that a 2 × 2 supercell can bind with four Mg atoms. The electronic charges of Mg atoms were transferred to the g-C₃N₄ monolayer, and thus a partial electropositivity on each adsorbed Mg atom was formed, indicating a potential improvement in conductivity. This subsequently causes the hydrogen molecules’ polarization, so that these hydrogen molecules can be efficiently adsorbed via both van der Waals and electrostatic interactions. To note, the configurations of the adsorbed hydrogen molecules were also elucidated, and we found that most adsorbed hydrogen molecules tend to be vertical to the sheet plane. Such a phenomenon is due to the electronic potential distribution. In average, each adsorbed Mg atom can adsorb 1–9 hydrogen molecules with adsorption energies that are ranged from ?0.25 eV to ?0.1 eV. Moreover, we realised that the nitrogen atom can also serve as an active site for hydrogen adsorption. The hydrogen storage capacity of this Mg-decorated g-C₃N₄ is close to 7.96 wt %, which is much higher than the target value of 5.5 wt % proposed by the U.S. department of energy (DOE) in 2020 [1]. The finding in this study indicates a promising carbon-based material for energy storage, and in the future, we hope to develop more advanced materials along this direction.     

Self-assembly of two-dimensional g-C3N4/rGO nanojunctions with enhanced charge separation and transfer for photocatalytic H2 production

Owing to the efficient transfer of charges between layers, two dimensional (2D) heterojunctions have attracted much attention in the development of photocatalysts. While how to achieve close interface contact between different materials is still a principal problem. In this work, the hydrogen bonding in graphene carbon nitride (CN) is fractured by alkali conditions, and CN is converted into highly dispersed water-soluble nanowires (S–CN). Thereby the self-assembly of S–CN on the rGO nanosheets is realized by the recombination of new hydrogen bonding to construct a 2D heterojunction with intimate interfacial contact. Because of the efficient transfer of charges in composite interface, S–CN/rGO (2%) exhibits an excellent photocatalytic hydrogen production performance, which is 5 times that of CN/rGO (2%). The efficient transportation of charges and photocatalytic mechanism in 2D heterostructure are studied by FTIR, TEM, XPS, i-t and other characterizations. It supplies a new approach for developing novel 2D heterojunction through intermolecular self-assembly.     

Self-assembly core-shell BixY1-xVO4@g-C3N4 as an S-scheme heterojunction photocatalyst for pure water splitting

The self-assembly core-shell Bi_xY_1-xVO₄@g-C₃N₄ (BYVO@PCN) photocatalyst was synthesized by in-situ polycondensation of melamine on the surface of Bi_xY_1-xVO₄. The formation mechanism of the core-shell structure was mainly attributed to the excessive unpaired O atoms existed on the surface of BYVO, which could absorb the intermediate products of polycondensation. In addition, the core-shell BYVO@PCN can achieve photocatalytic pure water splitting into H₂ and O₂ which is about 5 times higher than the Bi_xY_1-xVO₄. Compared with PCN, BYVO@PCN tackles the problem that little O₂ evolution in pure water splitting by PCN. Furthermore, BYVO@PCN forms an S-scheme heterojunction instead of a type Ⅱ heterojunction, which significantly accelerates the separation of charge carriers.     

The destabilization of LiBH4 through the addition of Bi2Se3 nanosheets

Efficient storage of hydrogen is a key issue to establish hydrogen infrastructure. In the efforts of searching suitable hydrogen storage alloys, several systems have been explored so far. All of them suffers from some drawbacks such as low gravimetric capacity, high stability, slow sorption kinetics, etc. Lithium borohydride (LiBH₄) is one of the leading contender among the hydrogen storage materials owing to its high hydrogen content of 18.5 wt%. However, its high stability needs a high operating temperature (>450 °C) for the decomposition. Recently, a thermochemical reaction between Bi₂X₃ and LiBH₄ was observed at 120 °C while performing experiments on the anode properties of Bi₂X₃ (X = S, Se, & Te) for Li-ion batteries. This indicated the possibility of destabilization of LiBH₄ and its low-temperature decomposition. This work presents the effect of Bi₂Se₃ addition to the decomposition properties of LiBH₄ using XRD and XPS techniques. The first step decomposition was observed to be initiated at around 180 °C, which is much lower than 450 °C for the pristine LiBH₄. A further reduction in the onset temperature is observed when the bulk Bi₂Se₃ is replaced by the nanosheets of this material. The mechanism of this destabilization is reported herein.     

Novel route of NiCr-layered double hydroxide nanosheets synthesis on nickel foam as a bifunctional electrocatalyst for water splitting process

In this study, we present a novel direct synthetic route for producing NiCr-layered double hydroxide (LDH) nanosheets on a nickel foam surface using the Successive Ionic Layer Deposition (SILD) method. The morphology of the nanolayers was characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Additionally, the electrocatalytic properties of the nanolayers were examined using electrochemical techniques. The NiCr-layered double hydroxide nanolayers produced through SILD using sodium hexahydroxochromate (III) anion precursor were found to be ultrathin “nanosheets” with a hydrotalcite-like structure and low crystallinity. The efficacy of electrodes based on these nanolayers as cathode and anode materials in electrocatalytic cells for hydrogen production through electrolytic water splitting in alkaline media were investigated. Our results showed that the electrode based on NiCr-LDH nanolayers exhibited good kinetics in both the cathode and anode areas.     

Integrating Co3O4 nanoparticles with MnO2 nanosheets as bifunctional electrocatalysts for water splitting

Developing high-efficiency and low-cost electrocatalyst is significant for the application of water splitting technology. Herein, Co₃O₄ nanoparticles and MnO₂ nanosheets are separately synthesized and subsequently assembled into a unique 0/2-dimensional heterostructure via van der Waals interactions. The consequent composites expose abundant accessible active sites and expedite the reaction kinetics, which can be testified by the superiorities in Tafel slope, exchange current density and double-layer capacitance, only requiring overpotentials of 355 and 129 mV for oxygen and hydrogen evolution reactions in 1.0 M KOH at 10 mA cm^?2, respectively. Moreover, a cell voltage of 1.660 V can drive the electrolyzer at 10 mA cm^?2. Benefitted from robust integration, the original aggregation and restacking of individual materials have been overcome, thereby leading to superior elelctrocatalysis durability. This facile and universal strategy may inspire the researchers on the design and construction of advanced functional composites.     

CO2 aided H2 recovery from water splitting processes

Solar energy can be utilized to produce H₂ via photocatalytic water splitting. One major drawback of the one-step approach is the co-production of H₂ and O₂ in the same reactor environment creating a potentially hazardous scenario. This obstacle can be avoided by utilizing CO₂ as a flammability suppressant which is proven to be more effective than N₂. In this case study, several membrane cascade designs were implemented to recover the H₂ while maintaining compositions outside the flammability range. The optimizations are based on the use of commercially available composite polymer membranes from Membrane Technology and Research, Inc. (MTR) in the spiral wound architecture. Both the H₂-selective membrane (Proteus?) and CO₂-selective membrane (Polaris?) were explored in three layouts. The process optimization is solved by nonlinear programming. The aim of the study was to minimize the present value of all outgoing cash flow (no income) for the separation process and achieving 99% product (H₂) purity. Optimization results showed that utilizing three membrane units of Proteus? material with one recycle stream is the optimum layout over a wide range of recovery values. Incorporating the CO₂-selective membrane (Polaris?) leads to more expensive process due to higher recycle flow rates to compensate for the low selectivity of this material. Overall, the best economic results for this process were obtained at 85% recovery rate with 99% product purity at a cost of 6.40 $/kg. Comparing to our previous study using a N₂ diluent, higher purity product with lower specific cost can be achieved with CO₂ diluent system but with slight decrease in recovery rate. As a final element to this study, comparative simulations were executed to demonstrate the potentially added value of using hollow fiber membranes versus spiral wound for this separation process.     

Effect of nonmetal element dopants on photo- and electro-chemistry performance of ultrathin g-C3N4 nanosheets

Increased charge transfer and light absorption as well as large specific surface area are effective ways to improve the catalytic efficiency of photocatalysts. In this paper, barbituric acid, urea and thiourea were separately freeze-dried to form a homogeneous precursor with melamine, followed by thermal polymerization at 750 °C, g-C₃N₄ was doped successfully with C, N and S. Among them, C-doped g-C₃N₄ revealed well photocatalytic hydrogen production which reached to 2.45 mmol g⁻¹ h⁻¹. N-doped g-C₃N₄ exhibited enhanced photocatalytic degradation properties for organic dye (RhB) which is completely degraded within 8 min while the performance S-doped sample is not satisfied. The introduction of C resulted in planarized sample which has a larger specific surface area and provides more active sites. N doping makes the valence band position moving in the direction of high energy. Thus, holes have stronger oxidizing ability. Accordingly, the effect of C and N ratios in g-C₃N₄ photocatalysis was fully discussed for more comprehensive understanding of g-C₃N₄.     

Controllable fabrication of 3D porous carbon nitride with ultra-thin nanosheets templated by ionic liquid for highly efficient water splitting

Modifying the texture of carbon nitride to adjust its physicochemical performance is a fascinating method for achieving high photocatalytic activity. Herein, we synthesized 3D porous carbon nitride with ultra-thin nanosheets by using cyanuric acid-melamine supramolecular and ionic liquid as precursor and template, respectively. The ionic liquid adjusts the morphology of materials and induces the carbon residue into the porous channels owing to its incomplete degradation. The 3D porous framework makes carbon nitride reflect the enhanced surface area, exposes adequate reaction sites, and offers a pathway for charge transport. And carbon residue and ultra-thin nanosheets further promote the photogenerated carriers transport and reduce the recombination rate of charge carriers. Consequently, 3D porous carbon nitride with ultra-thin nanosheets exhibit outstanding and stable hydrogen evolution under visible light irradiation. Significantly, as-fabricated sample CN-100 reflects an improved H₂ generation rate, up to 17,028 μmol h^?1 g^?1, which is 12 times higher than that of CN (1412 μmol h^?1 g^?1). The present work offers a unique synthesis strategy to develop the novel photocatalyst with efficient photocatalytic performance.     

Preparation of porous C doped g-C3N4 nanosheets controlled by acetamide for photocatalytic H2 evolution

Photocatalytic production H₂ of splitting water is an important branch of the application of photocatalytic materials. In this paper, porous C doped carbon nitride was successfully synthesized by calcining urea and acetamide at high temperature, and then analyzed by various characterization methods. XRD and FT-IR spectra show that the crystal structure of carbon nitride modified by acetamide are similar to bulk carbon nitride. SEM and TEM spectra show that the morphology and structure of carbon nitride and carbon nitride was modified by acetamide, which presents a porous nanosheets structure. XPS and EDS show that the content of carbon element in carbon nitride modified by acetamide increased significantly. The PL spectrum is obviously red shift, and the light absorption range is obviously enlarged, which is consistent with the absorption edge of UV–vis Spectrum. The BET of the optimized sample CN2-6 is 122.79 m²g^-1, which is 3.04 times higher than that of bulk carbon nitride CN1 (40.456 m²g^-1). In addition, the hydrogen production of CN2-6 is 13169.04 μmol/g/h under 300 W Xenon lamp, which is 14.95 times more than CN1 (880.82 μmol/g/h) hydrogen production rate. When the light source is a 10 W LED lamp, the apparent quantum efficiency (AQE) of sample CN2-6 reaches the maximum (2.91) under the wavelengths of 430 nm, which is 1.27 times more than CN1 under the same conditions. Meanwhile, prepared samples have excellent stability and durability under visible light.     

Accelerating photocatalytic hydrogen evolution of Ta2O5/g-C3N4 via nanostructure engineering and surface assembly

Despite that several strategies have been demonstrated to be effective for improving the catalytic hydrogen evolution activity of bulky g-C₃N₄, the large-scale hydrogen production over g–C₃N₄–based photocatalysts still confronts a big challenge. Here, a two-step calcination method is presented in constructing metal oxide/two-dimensional g-C₃N₄, i.e., Ta₂O₅/2D g-C₃N₄ photocatalyst. Thanks to the superiority of the synthetic method, nanostructure engineering forming 2D structure, and surface assembly with another semiconductor, can be realized simultaneously, in which ultrathin structure of 2D g-C₃N₄ and strong interfacial coupling between two components are two important characteristics. As a result, the structure engineered Ta₂O₅/2D g-C₃N₄ induces high photocatalytic hydrogen evolution half reaction rate of ~19,000 μmol g^?1 h^?1 under visible light irradiation (λ > 400 nm), and an external quantum efficiency (EQE) of 25.18% and 12.48% at 405 nm and 420 nm. The high photocatalytic performance strongly demonstrates the advance of the synchronous engineering of nanostructure and construction of heterostructure with tight interface, both of which are beneficial for the fast charge separation and transfer.     

In-Situ solid-state synthesis of 2D/2D interface between Ni/NiO hexagonal nanosheets supported on g-C3N4 for enhanced photo-electrochemical water splitting

Herein, we report a synthesis of 2D/2D interfaces between nickel/nickel oxide (Ni/NiO) hexagonal nanosheets with graphitic-carbon nitride (g-C₃N₄) using an in-situ solid-state heat treatment that shows enhanced activity for electrochemical as well as photo-electrochemical (PEC) water splitting. The transmission electron microscopy characterization confirms the homogenous dispersion of 2D hexagonal nanosheets of Ni/NiO on the surface of g-C₃N₄. The higher electrochemical and PEC water splitting activity of 2D/2D interface may be due to the more intimate contact between 2D sheets of NiO with g-C₃N₄. Moreover, the effect of NiO loading in nanoheterostructures have been studied towards overall water splitting by varying the ratio of precursors for NiO to that of g-C₃N₄ viz. 1:1, 1:8, and 1:16. A compositional ratio of 1:8 have been found to show the best PEC activity towards OER depicting a maximum photocurrent density of 20 mA cm⁻² at an over potential of 190 mV. Whereas, the highest ratio of NiO to g-C₃N₄ nanosheets (i.e. 1:1) was noted to demonstrate the best performance towards electrochemical hydrogen evolution reaction showing dramatically reduced over potential of 26 mV to drive a current density of 10 mA cm⁻².     

Photocatalytic overall water splitting by Z-scheme g-C3N4/BiFeO3 heterojunction

Solar water splitting by photocatalysis in the absence of sacrificial agent has been identified as a promising approach to produce green hydrogen. Increasing photocatalytic efficiency is the core issue in this process. Forming heterojunctions is one potential solution to improve photoactivity. Herein, we successfully synthesized several g-C₃N₄/BiFeO₃ composites containing different mass ratio of g-C₃N₄ by an uncomplicated and cost-effective method. The composite samples exhibited remarkably enhanced photocatalytic performance compared with the bare BiFeO₃ and g-C₃N₄. The highest hydrogen production rate obtained is ~ 160.75 μmol h⁻¹.g-¹ under UV irradiation and ~23.31 μmol h⁻¹.g-¹ under visible light irradiation. This enhanced photocatalytic activity is attributed to the synergistic effect of the junction and Z-scheme charge transfer mechanism between BiFeO₃ and g-C₃N₄, which can effectively accelerate the separation of photogenerated electron-hole pairs and also capability of carrying out redox reaction.     

MoO3/g-C3N4 Z-scheme (S-scheme) system derived from MoS2/melamine dual precursors for enhanced photocatalytic H2 evolution driven by visible light

A dual-precursors co-pyrolysis strategy was adopted to prepare photocatalysts of MoO₃/g-C₃N₄ composite from MoS₂/melamine through a facile one-pot method. By following this strategy, MoS₂ was in-situ transformed to MoO₃ accompanying with the pyrolysis and polymerization of melamine to g-C₃N₄ nanosheets. A set of samples about MoO₃/g-C₃N₄ composite containing different MoO₃ contents were prepared and the visible-light driven photocatalytic performance of samples were investigated. The photocatalytic activity was notably enhanced and the highest evolution rate for H₂ reached 13.9 times and 2.5 times that of the pristine g-C₃N₄ and the MoO₃/g-C₃N₄ derived from MoO₃/melamine, respectively. On the one hand, layered MoS₂ used as the precursor of MoO₃ contributed to the intimate contact with g-C₃N₄ nanosheets and the high dispersity of derived MoO₃, and on the other hand, the morphology and electronic structure of g-C₃N₄ were actually changed by the strong interaction between the MoO₃ and g-C₃N₄, thus inducing an efficient Z-scheme (S-scheme) system in the composite. The above-mentioned co-operation effect in the composite resulted in the significant improvement on the activity of photocatalytic H₂ evolution. This work presented a valuable reference, by taking MoO₃/g-C₃N₄ as an example, for the construction of composite-photocatalyst system in visible-light-driven H₂ evolution.     

Hydrogen gas sensing of nano-confined Pt/g-C3N4 composite at room temperature

In this study, we report the hydrogen sensing properties of the Pt dispersed graphitic carbon nitride (g-C₃N₄) nanocomposite at room temperature and inert atmosphere. The nanocomposite was synthesized by a wet-chemical approach, where melamine and chloroplatinic acid hexahydrate were used as precursors. The fabrication of the sensor was done by jet nebulizer-based spray pyrolysis setup. Various characterizations were performed for the analysis of the synthesized nanocomposite. Electrical resistance in the presence and absence of the analyte gas was drastically different. The results at different concentrations and film thickness show Pt/g-C₃N₄ to possess good sensitivity towards the hydrogen gas, suggesting that it could be used for reliable hydrogen gas sensing.     

SnO2 promoted carrier separation in superior thin g-C3N4 nanosheets for enhanced photocatalytic degradation and H2 generation

The construction of heterostructures is an efficient approach to improve the photocatalystic performance of semiconductors. In this paper, SnO₂-g-C₃N₄ (SnO₂–CN) nanocomposites were created via thermal polymerization using SnO₂ nanoparticles and layered g-C₃N₄ nanosheets. A mechano-chemical pre-reaction and the second thermal polymerization of bulk g-C₃N₄ play important roles for the formation of SnO₂/g-C₃N₄ heterostructures with improved interface nature. The heterostructures with an optimized SnO₂ weight ratio of 10% was obtained by adjusting parameters for enhanced photocatalytic reactions in visible light region. Hydrogen generation and the degradation of rhodamine B (Rh B) were tested to characterize the photocatalytic performance of the SnO₂–CN nanocomposites. The degradation of a 20 mg/L Rh B solution was finished within 15 min, in which the degradation rate was about twice compared with superior thin g-C₃N₄ nanosheets prepared by a two-step polymerization procedure. The SnO₂–CN nanocomposite with 10% SnO₂ revealed a H₂ generation rate of 2569.5 μmol g⁻¹L⁻¹. The enhanced photocatalytic performance is ascribed to a type II heterostructure formed and improved interface properties between g-C₃N₄ and SnO₂. In addition, the improved conductivity of SnO₂ promoted the photogenerated carrier separation and transfer. The result provided a new idea for the construction of g-C₃N₄ heterostructures with improved interface characterization and the improvement of photocatalytic properties.     

Phosphorus and sulphur co-doping of g-C3N4 nanotubes with tunable architectures for superior photocatalytic H2 evolution

Non-metal doping not only optimizes the energy band structure of g-C₃N₄ to improve the absorption of visible light, but also exacerbates the distortion of lowest and highest unoccupied molecular orbital plane, causing polarization, thereby improving photocatalytic activity. For the first time, S and P are co-introduced into g-C₃N₄ network to enhance photocatalytic performance and create various tubular morphologies. The ratio of S to P is crucial to control the tubular morphology and property. In the photocatalytic process, the separation of electrons and holes causes by the polarization of the S and P elements and the synergy of the tubular morphology results in new migration paths for photogenerated electrons and holes. Using optimized preparation conditions, g-C₃N₄ tubes co-doped with S and P (CNSP) reveal very high H₂ generation efficiency (163.27 μmol/h), which is two orders of magnitude higher compared to that of pure g-C₃N₄ and apparent quantum yield is 18.93% at 420 nm. Fast degradation of Rhodamine B by using CNSP occurs within 5 min under visible light irradiation. Because of the reproducible process, the synthetic strategy provides a novel method for controlling the morphology of g-C₃N₄-based materials with super activity.