Efficient Photocatalytic Water Splitting Research Articles

Zr-Al co-doped SrTiO3 with suppressed charge recombination for efficient photocatalytic overall water splitting.

Zr-Al co-doped SrTiO3 with reduced Ti3+ concentration demonstrates more than 2 times enhancement compared with Al-doped SrTiO3 in photocatalytic overall water splitting. Systematic studies reveal that the co-doping of Zr4+ can reduce the substitution of Ti4+ by Al3+ and effectively suppress the formation of charge carrier recombination centers (Ti3+).

Piezo-phototronic and plasmonic effect coupled Ag-NaNbO3 nanocomposite for enhanced photocatalytic and photoelectrochemical water splitting activity

Silver (Ag) nanoparticles decorated NaNbO3 nanorods (Ag-NaNbO3) based nanocomposite have been successfully synthesized by simple chemical solution method with the aim to couple the plasmonic and piezo-phototronic effect. The Ag-NaNbO3 nanocomposite showed much enhanced photocatalytic and photoelectrochemical water splitting properties as compared to bare NaNbO3. A ∼10 fold enhancement in the photodecomposition of organic MB dye and a ∼9 fold increment in the photocurrent density of photoelectrochemical water splitting was observed as compared to bare NaNbO3, which has been attributed to the combined plasmonic and piezo-phototronic effect. The plasmonic effect due to the presence of Ag nanoparticles on the NaNbO3 surface resulted in enhanced absorption of visible light and piezo-photoelectric effect resulted in better separation of the photogenerated charges due to the built-in electric field. This approach demonstrates a novel strategy for enhancing the performance of silver decorated semiconducting/piezoelectric material for achieving efficient photocatalytic dye degradation and PEC water splitting.

Photoinduced synthesis of ultrasmall amorphous NiWSx nanodots for boosting photocatalytic H2-evolution activity of TiO2

It is a great challenge to explore new, abundant and cost-effective H2-evolution cocatalysts for realizing highly efficient photocatalytic water splitting using renewable solar energy. In this study, the ultrasmall amorphous NiWSx nanodots (ca. 2 nm) as a new H2-evolution cocatalyst of bimetallic sulfide were evenly anchored onto TiO2 surface to prepare NiWSx nanodot-modified TiO2 (NiWSx-ND/TiO2) photocatalysts through a facile photodeposition strategy. The photocatalytic results showed that the amorphous NiWSx nanodots could obviously accelerate the hydrogen-production activity of TiO2. Especially, the as-prepared NiWSx-ND/TiO2 (3 wt%) photocatalyst manifested the maximum H2-generation efficiency of 4580 μmol h−1 g−1, corresponding to an apparent quantum efficiency (AQE) of 13%. Based on the present results, a H2-evolution mechanism of NiWSx cocatalyst is proposed, namely, the ultrasmall amorphous NiWSx nanodots first act as a capturer to speedily capture photoexcited electrons, and then play as an electron-transport medium to promote the hydrogen-generation activity. Considering its mild preparation, inexpensive cost, and admirable efficiency, the amorphous NiWSx nanodots can become a promising cocatalyst for potential various applications in photocatalytic field.

Optoelectronic properties of the novel perovskite materials LiPb(Cl:Br:I)3 for enhanced hydrogen production by visible photo-catalytic activity: Theoretical prediction based on empirical formulae and DFT

In order to first principle density functional theory, the effects of anionic changes on the opto-electronic characteristics and photo-catalyst efficiency of perovskite LiPb(Cl,Br,I)3 is determined for water splitting. In recent years, photo-catalytic activity splitting of water for the production of hydrogen is one of the emerging research routes. The thermodynamic and structural properties have been calculated using the PBEsol-generalized-gradient-approximation, while the TB-mBJ-LDA has been used for computing LiPb(Cl:Br:I)3 opto-electronic properties. The predicted formation energy indicate the thermodynamic stability of LiPb(Cl:Br:I)3 compounds. The anionic atom replacement at X site decreases band-gap of LiPb(Cl:Br:I)3. The calculated effective mass signifies high charge mobility of this cubic perovskite compounds, class LiPb(Cl:Br:I)3. The large difference between electron and hole effective mass decreases electron/hole recombination. The evaluation of the electronic structure and optical absorption of LiPb(Cl:Br:I)3 with respect to the oxidation/reduction potentials of H2O confirms that this system is a suitable candidate for efficient photo-catalytic water splitting in the visible and UV region of the electromagnetic spectrum.

Theoretical insights of codoping to modulate electronic structure of hbox {TiO}_2 and hbox {SrTiO}_3 for enhanced photocatalytic efficiency

hbox {TiO}_2 and hbox {SrTiO}_3 are well known materials in the field of photocatalysis due to their exceptional electronic structure, high chemical stability, non-toxicity and low cost. However, owing to the wide band gap, these can be utilized only in the UV region. Thus, it’s necessary to expand their optical response in visible region by reducing their band gap through doping with metals, nonmetals or the combination of different elements, while retaining intact the photocatalytic efficiency. We report here, the codoping of a metal and a nonmetal in anatase hbox {TiO}_2 and hbox {SrTiO}_3 for efficient photocatalytic water splitting using hybrid density functional theory and ab initio atomistic thermodynamics. The latter ensures to capture the environmental effect to understand thermodynamic stability of the charged defects at a realistic condition. We have observed that the charged defects are stable in addition to neutral defects in anatase hbox {TiO}_2 and the codopants act as donor as well as acceptor depending on the nature of doping (p-type or n-type). However, the most stable codopants in hbox {SrTiO}_3 mostly act as donor. Our results reveal that despite the response in visible light region, the codoping in hbox {TiO}_2 and hbox {SrTiO}_3 cannot always enhance the photocatalytic activity due to either the formation of recombination centers or the large shift in the conduction band minimum or valence band maximum. Amongst various metal-nonmetal combinations, hbox {Mn}_text {Ti}hbox {S}_text {O} (i.e. Mn is substituted at Ti site and S is substituted at O site), hbox {S}_text {O} in anatase hbox {TiO}_2 and hbox {Mn}_text {Ti}hbox {S}_text {O}, hbox {Mn}_text {Sr}hbox {N}_text {O} in hbox {SrTiO}_3 are the most potent candidates to enhance the photocatalytic efficiency of anatase hbox {TiO}_2 and hbox {SrTiO}_3 under visible light irradiation.

Integration of bio-inspired lanthanide-transition metal cluster and P-doped carbon nitride for efficient photocatalytic overall water splitting.

Photosynthesis in nature uses the Mn4CaO5 cluster as the oxygen-evolving center to catalyze the water oxidation efficiently in photosystem II. Herein, we demonstrate bio-inspired heterometallic LnCo3 (Ln = Nd, Eu and Ce) clusters, which can be viewed as synthetic analogs of the CaMn4O5 cluster. Anchoring LnCo3 on phosphorus-doped graphitic carbon nitrides (PCN) shows efficient overall water splitting without any sacrificial reagents. The NdCo3/PCN-c photocatalyst exhibits excellent water splitting activity and a quantum efficiency of 2.0% at 350 nm. Ultrafast transient absorption spectroscopy revealed the transfer of a photoexcited electron and hole into the PCN and LnCo3 for hydrogen and oxygen evolution reactions, respectively. A density functional theory (DFT) calculation showed the cooperative water activation on lanthanide and O−O bond formation on transition metal for water oxidation. This work not only prepares a synthetic model of a bio-inspired oxygen-evolving center but also provides an effective strategy to realize light-driven overall water splitting.

Noble-metal and cocatalyst free W2N/C/TiO photocatalysts for efficient photocatalytic overall water splitting in visible and near-infrared light regions

Photocatalysts that can not only harvest broader solar irradiation (from UV to near infrared (NIR) light region) but also achieve higher solar-to-hydrogen conversion efficiency are critical for solar hydrogen economy. In this research, we report a ternary bridge chain W2N/C/TiO photocatalyst for the first time to realize an efficient and stable visible and NIR light driven water splitting. This photocatalyst can realize overall water splitting using bifunctional mechanism with hydrogen (H2) generation part and oxygen (O2) generation part. The optimal H2 and O2 generation rates based on the W2N/C/TiO photocatalyst are 2.01 and 1.11 μmol g-1h−1 under the irradiation of NIR light (λ > 700 nm). In this new photocatalyst, W2N and TiO were integrated into nanosized carbon fibers that serve as H2 and O2 evolution active sites. And the hydrogen apparent quantum efficiency (AQE) for W2N/C/TiO composite photocatalyst for NIR light water splitting can reach as high as 2.14% at 700 ± 15 nm. This research indicates that W2N/C/TiO photocatalyst can be effective under broaden (Ultraviolet, Visible and Near-Infrared) light. Combining interfacial and electrochemical characterizations, we have found that W2N/C/TiO photocatalyst can efficiently separate charges through the interfaces between C/W2N and C/TiO and harvest low energy photons in NIR region. Because of the bridge chain structure among W2N, C, and TiO electron transfer from W2N to TiO via C can promote charge separation and impede the recombination of photo-generated electron and hole pairs. It is therefore that this bridge chain structured W2N/C/TiO nanofiber photocatalyst can be a promising material for overall water splitting due to the capacity to utilize a broader solar spectrum.

Gallium oxide nanoparticles prepared through solid-state route for efficient photocatalytic overall water splitting

Gallium oxide (Ga2O3) is one of the most efficient photocatalysts for the overall water splitting. Doping with appropriate metal cations substantially boosts the quantum yield to split water into H2 and O2. In this research, Ga2O3 nanoparticles in the β polymorph (β-Ga2O3) are successfully prepared through a solid-state route under heat treatment. The size of the photocatalyst particles is sufficiently small to shorten the migration distance of the photoexcited charge carriers from the bulk to the surface. To beneficially modify the bulk state of β-Ga2O3, doping with various metals including alkaline-earth metals and first row transition metals is performed. The alkaline-earth metal cations generally increase the water splitting activity. On the other hand, the first row transition metal cations decrease the activity, except for Zn. No systematic trend is observed on the water splitting activity of β-Ga2O3 doped with metal cations in the same group or in the same period. The oxidation state of the dopants plays a role in dictating the water splitting activity. Doping with metal cations having oxidation states lower and higher than that of Ga increases and decreases the activity, respectively. In particular, doping with Zn considerably increases the water splitting activity. At a Zn amount of 1 mol% in the presence of Rh0.5Cr1.5O3 (0.5 wt% Rh) co-catalyst, the H2 and O2 evolution rates can reach as high as 400 and 195 μmol h―1, respectively, with a H2/O2 molar ratio of 2 under UV irradiation with an average intensity of 58 mW cm−2 at 254 nm.

A new polyniobotungstate based on {GeW9Nb3O40} clusters and nickel cation with photocatalytic properties

A new polyniobotungstate, Cs11K7[(Ge4W36Nb12O156)Ni(H2O)2]·29H2O (1) based on {GeW9Nb3O40} fundamental building units, was successfully obtained. Photocatalytic study has shown that 1 has efficient photocatalytic water splitting activity.

Uniformly assembling n-type metal oxide nanostructures (TiO2 nanoparticles and SnO2 nanowires) onto P doped g-C3N4 nanosheets for efficient photocatalytic water splitting

Exploitation of g-C3N4 based photocatalysts capable of drastically boosting photoinduced electron-hole separation and solar-to-H2 conversion efficiency consequences is a hotspot topic. Herein, a construction of few-layer P-doped g-C3N4 (PCN) nanosheets decorated with n-type metal oxides (NMOs) nanostructures, i.e. TiO2 nanoparticles and SnO2 nanowires via a self-assembly method is reported. NMOs uniformly distributing on PCN act as electron transporting channels, dramatically enhancing extraction efficiency of photogenerated electrons from PCN. The resultant TiO2/PCN and SnO2/PCN in the absence of Pt under visible light irradiation exhibit drastically enhanced H2 evolution reaction (HER) rates of 1082 and 2090 μmol h−1 g−1 comparing with PCN (132 μmol h−1 g−1). Such HER activities are superb among noble metal-free g-C3N4 based photocatalysts. Under a very low Pt amount (0.33 wt%), the HER rates of TiO2-Pt/PCN and SnO2-Pt/PCN further climb up to 11,632 and 12,469 μmol h−1 g−1.

Recent Advances in Conjugated Polymers for Visible-Light-Driven Water Splitting.

With the ambition of solving the challenges of the shortage of fossil fuels and their associated environmental pollution, visible-light-driven splitting of water into hydrogen and oxygen using semiconductor photocatalysts has emerged as a promising technology to provide environmentally friendly energy vectors. Among the current library of developed photocatalysts, organic conjugated polymers present unique advantages of sufficient light-absorption efficiency, excellent stability, tunable electronic properties, and economic applicability. As a class of rising photocatalysts, organic conjugated polymers offer high flexibility in tuning the framework of the backbone and porosity to fulfill the requirements for photocatalytic applications. In the past decade, significant progress has been made in visible-light-driven water splitting employing organic conjugated polymers. The recent development of the structural design principles of organic conjugated polymers (including linear, crosslinked, and supramolecular self-assembled polymers) toward efficient photocatalytic hydrogen evolution, oxygen evolution, and overall water splitting is described, thus providing a comprehensive reference for the field. Finally, current challenges and perspectives are also discussed.

Facile synthesis of anatase/rutile TiO2/g-C3N4 multi-heterostructure for efficient photocatalytic overall water splitting

Rational design of high-efficiency heterostructure photocatalyst is an effective strategy to realize photocatalytic H2 evolution from pure water, but remains still a considerable challenge. Herein, an anatase/rutile TiO2/g-C3N4 (A/R/CN) multi-heterostructure photocatalyst was prepared by a facile thermoset hybrid method. The combination of two type-II semiconductor heterostructures (i.e., A/R and R/CN) significantly improve the separation and transfer efficiency of photogenerated carriers of anatase TiO2, rutile TiO2 and g-C3N4, and A/R/CN photocatalyst with high activity is obtained. The optimal A/R/CN photocatalyst exhibits significantly increased photocatalytic overall water splitting activity with a rate of H2 evolution of 374.2 μmol g−1h−1, which is about 8 and 4 times that of pure g-C3N4 and P25. Moreover, it is demonstrated to be stable and retained a high activity (ca. 91.2%) after the fourth recycling experiment. This work comes up with an innovative perspective on the construction of multi-heterostructure interfaces to improve the overall photocatalytic water splitting performance.

(Invited) Semiconductor Nanomaterials for Photoelectrochemical Solar Fuel Generation

Semiconducting materials hold the key for efficient photocatalytic and photoelectrochemical water splitting. In this talk, we will give a brief overview of our recent progresses in designing semiconductor metal oxides materials for photoelectrochemical energy conversion including photocatalytic solar fuel generation. In more details, we have been focusing the following a few aspects; 1) band-gap engineering of layered semiconductor compounds including layered titanate, tantalate and niobate-based metal oxide compounds for visible light phtocatalysis, and 2) facet-controlled TiO2, Fe2O3, BiVO4 and perovskites as building blocks for new photoelectrode design，and 3) the combination of a high performance photoelectrode BiVO4 with perovskite solar cells can lead to unassisted solar driven water splitting process with unassisted solar-to-hydrogen conversion efficiency of >6.5%; The resultant material systems exhibited efficient visible light photocatalytic performance and improved power conversion efficiency in solar energy, which underpin important solar-energy conversion applications including solar fuel generation and simultaneous environmental application.

MoS2@SnO2 core-shell sub-microspheres for high efficient visible-light photodegradation and photocatalytic hydrogen production

Two-step hydrothermal processes have been used to synthesize MoS2@SnO2 core-shell sub-microspheres. The effect of the different weight of citric acid for MoS2@SnO2 core-shell sub-microspheres on the microstructural and photocatalytic properties was studied. The results indicate that MoS2@SnO2 core-shell sub-microspheres can reveal the highly efficient photocatalytic water splitting and degradation of rhodamine B solution under visible-light irradiation. The enhanced photocatalytic performance was ascribed to MoS2 sub-microspheres decorated with SnO2 nanoparticles, improving their surface-to-volume ratio and photoinduced electron-hole pairs separation.

Function-switchable metal/semiconductor junction enables efficient photocatalytic overall water splitting with selective water oxidation products

A novel metal/semiconductor photocatalyst, Cu nanoparticles (NPs) modified TiO2 hollow spheres (Cu/TiO2), was designed for efficient photocatalytic overall water splitting (POWS) under both ultraviolet (UV) and visible (Vis) light. This Cu/TiO2 photocatalyst possesses excellent POWS performance under Vis light at the highest level among the reported TiO2-based photocatalysts. Interestingly, the metal/semiconductor junction formed between Cu and TiO2 enables controlled water-oxidation product selectivity (H2O2 or O2) via different reaction pathways regulated by irradiation wavelengths. Under UV light, the electrons excited in TiO2 are captured by Cu NPs through the Cu/TiO2 Schottky interface for H2 production, with the photoholes in TiO2 producing H2O2 through a two-electron process; whilst under Vis light, Cu NPs act as plasmon to inject hot electrons to TiO2 for H2 production, while O2 is produced by hot holes on Cu NPs via a four-electron process. This rational design of function-switchable metal/semiconductor junction may be helpful to understand the mechanisms for POWS with desired gas/liquid water-oxidation products.

Ag-doped BiVO4/BiFeO3 photoanode for highly efficient and stable photocatalytic and photoelectrochemical water splitting

In a conventional photoelectrochemical (PEC) water splitting system using BiVO4 (BVO), most of the charge carriers have very sluggish photocatalysis reaction kinetics because they are easily recombined from the defects developed from the bulk or the surface of the photoanodes before reaching the fluorine-doped tin dioxide (FTO). Herein, we present a facile design and fabrication technique for a Ag-BVO/BiFeO3 (BFO) heterostructure photoanode by Ag doping and surface passivation with BFO on the as-preparedBVO photoanode. Its photocatalytic properties for PEC water splitting and tetracycline (TC) degradation are compared to those of BVO/BFO, BVO, and Ag-BVO photocatalyst nanoparticle (NP) films. The effect of Ag-doping/BFO surface passivation on the morphological, structural, and optical properties and surface electronic structure of the as-obtainedBVO electrodes was investigated. The photocatalytic degradation of TC in aqueous solution by Ag-BVO/BFO was greatly increased (>1.5-fold) compared to that of BVO. The TC was completely photodegraded in 50 min of visible-light irradiation. The as-preparedAg-BVO/BFO heterojunction photoanode not only exhibited 4-fold higher PEC performance (0.72 mA cm−2 vs. RHE) and stability than those of the pure BVO components, but also the onset potential in the Ag-BVO/BFO photoanode was cathodically shifted by 600 mV compared to that of the bare BVO. The Ag-BVO/BFO photoelectrode with the highest donor density and the lowest charge transfer resistance exhibited a 4.46-fold higher carrier density than that of the pure BVO photoelectrode. More specifically, the Mott-Schottky (MS) and electrochemical impedance spectroscopy (EIS) results demonstrated that the Ag-doping not only effectively increased the carrier charge density of BVO, thus increasing the consumption rate of charge carriers, but also increased the charge transfer and transport efficiencies of the BVO photoanodes.

Efficient sequential harvesting of solar light by heterogeneous hollow shells with hierarchical pores.

In nature, sequential harvesting of light widely exists in the old life entity, i.e. cyanobacteria, to maximize the light absorption and enhance the photosynthesis efficiency. Inspired by nature, we propose a brand new concept of temporally-spatially sequential harvesting of light in one single particle, which has purpose-designed heterogeneous hollow multi-shelled structures (HoMSs) with porous shells composed of nanoparticle subunits. Structurally, HoMSs consist of different band-gap materials outside-in, thus realizing the efficient harvesting of light with different wavelengths. Moreover, introducing oxygen vacancies into each nanoparticle subunit can also enhance the light absorption. With the benefit of sequential harvesting of light in HoMSs, the quantum efficiency at wavelength of 400 nm is enhanced by six times compared with the corresponding nanoparticles. Impressively, using these aforementioned materials as photocatalysts, highly efficient photocatalytic water splitting is realized, which cannot be achieved by using the nanoparticle counterparts. This new concept of temporally-spatially sequential harvesting of solar light paves the way for solving the ever-growing energy demand.

Promoting visible-light photocatalytic activities for carbon nitride based 0D/2D/2D hybrid system: Beyond the conventional 4-electron mechanism

Photocatalysis is regarded as one of promising technology for future clean and sustaniable energy applications. Herein, we have fabricated Au-modified reduced graphene oxide coupled with carbon nitride (Au/rGO/g-C3N4) as novel 0D/2D/2D photocatalytic nanocomposites. The optimized sample 2Au/0.6rGO/g-C3N4 exhibits exceptional visible-light activity for water splititng and CO2 reduction with quantum efficiency of 3.82 % and 1.98 %, respectively. Electrochemistry and ultraviolent photoemission are combined to determine the band alignments and elaborate associated water splitting path-way mechanism. It is validated that due to intrinsic deep valence band position of g-C3N4, the obtained nanocomposites exhibit unusual two-step two-electron way of water oxidization through intermediate H2O2 catalyzed by rGO addition. The exceptional photoactivities are attributed to dual functions of enhanced charge separation and two-electron water oxidization facilitated by rGO and surface plasmon effect of decorated Au. Our work provides illuminations for low cost and high efficiency photocatalytic water splitting and CO2 reduction applications.

Molten‐Salt‐Mediated Synthesis of an Atomic Nickel Co‐catalyst on TiO2 for Improved Photocatalytic H2 Evolution

AbstractAtomic co‐catalysts offer high potential to improve the photocatalytic performance, of which the preparation with earth‐abundant elements is challenging. Here, a new molten salt method (MSM) is designed to prepare atomic Ni co‐catalyst on widely studied TiO2 nanoparticles. The liquid environment and space confinement effect of the molten salt leads to atomic dispersion of Ni ions on TiO2, while the strong polarizing force provided by the molten salt promotes formation of strong Ni−O bonds. Interestingly, Ni atoms are found to facilitate the formation of oxygen vacancies (OV) on TiO2 during the MSM process, which benefits the charge transfer and hydrogen evolution reaction. The synergy of atomic Ni co‐catalyst and OV results in 4‐time increase in H2 evolution rate compared to that of the Ni co‐catalyst on TiO2 prepared by an impregnation method. This work provides a new strategy of controlling atomic co‐catalyst together with defects for efficient photocatalytic water splitting.

Molten-Salt-Mediated Synthesis of an Atomic Nickel Co-catalyst on TiO2 for Improved Photocatalytic H2 Evolution.

Atomic co-catalysts offer high potential to improve the photocatalytic performance, of which the preparation with earth-abundant elements is challenging. Here, a new molten salt method (MSM) is designed to prepare atomic Ni co-catalyst on widely studied TiO2 nanoparticles. The liquid environment and space confinement effect of the molten salt leads to atomic dispersion of Ni ions on TiO2 , while the strong polarizing force provided by the molten salt promotes formation of strong Ni-O bonds. Interestingly, Ni atoms are found to facilitate the formation of oxygen vacancies (OV) on TiO2 during the MSM process, which benefits the charge transfer and hydrogen evolution reaction. The synergy of atomic Ni co-catalyst and OV results in 4-time increase in H2 evolution rate compared to that of the Ni co-catalyst on TiO2 prepared by an impregnation method. This work provides a new strategy of controlling atomic co-catalyst together with defects for efficient photocatalytic water splitting.