Highly efficient hamburger-like nanostructure of a triadic Ag/Co3O4/BiVO4 photoanode for enhanced photoelectrochemical water oxidation

The combination of a semiconductor heterojunction and oxygen evolution cocatalyst (OEC) is an important strategy to improve photoelectrochemical (PEC) water oxidation. Herein, a novel hamburger-like nanostructure of a triadic photoanode composed of BiVO₄ nanobulks, Co₃O₄ nanosheets and Ag nanoparticles (NPs), that is, Ag/Co₃O₄/BiVO₄, was designed. In our study, an interlaced 2D ultrathin p-type Co₃O₄ OEC layer was introduced onto n-type BiVO₄ to form a p–n Co₃O₄/BiVO₄ heterojunction with an internal electric field (IEF) in order to facilitate charge transport. Then the modification with Ag NPs can significantly facilitate the separation and transport of photogenerated carriers through the surface plasma resonance (SPR) effect, inhibiting the electron–hole recombination. The resulting Ag/Co₃O₄/BiVO₄ photoanodes exhibit largely enhanced PEC water oxidation performance: the photocurrent density of the ternary photoanode reaches up to 1.84 mA cm⁻² at 1.23 V vs. RHE, which is 4.60 times higher than that of the pristine BiVO₄ photoanode. The IPCE value is 2.83 times higher than that of the pristine BiVO₄ at 400 nm and the onset potential has a significant cathodic shift of 550 mV for the ternary well-constructed photoanode.

A novel hamburger-like nanostructure of a triadic photoanode Ag/Co₃O₄/BiVO₄ was designed to enhance photoelectrochemical water splitting, providing a fascinating pathway to efficiently improve the PEC conversion efficiency.     

Synergetic effect of dual co-catalysts on the activity of BiVO4 for photocatalytic carbamazepine degradation

An efficient visible-light driven three components photocatalyst for carbamazepine (CBZ) degradation has been assembled by co-loading reduction cocatalyst Pt and oxidation cocatalyst Co₃O₄ (MnO_x) on BiVO₄. The apparent rate constant of the three components photocatalyst Pt/BiVO₄/Co₃O₄ for degradation of CBZ is 54 times that of Co₃O₄/BiVO₄ and 2.5 times that of Pt/BiVO₄, which shows a synergetic effect in the photocatalytic activity. The same synergetic effect is also observed for Pt/BiVO₄/MnO_x. The spatial separation of the reduction and oxidation cocatalysts could reduce the recombination of the photogenerated charges, which mainly accounts for the high photocatalytic activity of the three components photocatalyst. The photocatalytic intermediates of CBZ were detected by HPLC-ESI-MS, and a deductive degradation pathway of CBZ was proposed.

An efficient visible-light driven three components photocatalyst for carbamazepine (CBZ) degradation has been assembled by co-loading reduction cocatalyst Pt and oxidation cocatalyst Co₃O₄ (MnO_x) on BiVO₄. An obvious synergetic effect is observed.     

Ag2O-decorated electrospun BiVO4 nanofibers with enhanced photocatalytic performance

Semiconductor photocatalysts are emerging as tools for pollutant degradation in industrial wastewater, air purification, antibacterial applications, etc. due to their use of visible light, which is abundant in sunlight. Here, we report a new type of p–n junction Ag₂O/BiVO₄ heterogeneous nanostructured photocatalyst with enhanced photocatalytic performance. P-type Ag₂O nanoparticles were in situ reduced and assembled on the surface of electrospun BiVO₄ nanofibers using ultraviolet (UV) irradiation; this process hindered the recombination of localized photogenerated electron–hole pairs, and hence resulted in the enhanced photocatalytic activity of the BiVO₄/Ag₂O nanocomposites. The photocatalytic activities of the obtained BiVO₄ and BiVO₄/Ag₂O nanocomposites were assessed by measuring the degradation of rhodamine B (RhB) under visible light. The 10 wt% Ag₂O/BiVO₄ sample yielded the optimum degradation of RhB (98.47%), much higher than that yielded by pure BiVO₄ nanofibers (64.67%). No obvious change in the XRD pattern of an Ag₂O/BiVO₄ sample occurred as a result of its use in the photocatalytic reaction, indicating its excellent stability. The high photocatalytic performance observed was attributed to the large surface-to-volume ratio of the essentially one-dimensional electrospun BiVO₄ nanofibers and to the in situ growth of p-type Ag₂O on the surface of the n-type BiVO₄ nanofibers.

Ag₂O doped electrospun BiVO₄ nanofibers with p–n junction heterogeneous structures show enhanced photocatalytic activity under visible light (photocatalytic efficiency: 98.47% within 100 min) and good cycling stability.     

Photocatalytic properties and energy band offset of a tungsten disulfide/graphitic carbon nitride van der Waals heterojunction

Semiconductor heterojunctions have higher photocatalytic performance than a single photocatalytic material. However, the energy band offset and the photocatalytic reaction mechanism of these heterojunctions remain controversial. Here, tungsten disulfide (WS₂)/graphitic carbon nitride (GCN) heterojunction photocatalytic water splitting is investigated with the hybrid density functional method. The band structures and the density of states (DOS) indicate that the WS₂/GCN heterojunction is a type-II heterojunction, and its valence band offset and conduction band offset are 0.27 and 0.04 eV, respectively. The differential charge density distribution and the work function calculation indicate that a built-in electric field is formed in the WS₂/GCN heterojunction. The potential of the built-in electric field is 0.16 V, and its direction is from the GCN surface to the WS₂ surface. The built-in electric field separates the photogenerated electrons and the holes in space, effectively improving the photocatalytic efficiency of the WS₂/GCN heterojunction. Our work provides insights into the electronic properties and the photocatalytic hydrogen evolution mechanism of the WS₂/GCN heterojunction.

WS₂/GCN heterojunction is a type-II heterostructure and its electric field separates the electrons and the holes in space.     

Efficient photoelectrochemical water oxidation using a TiO2 nanosphere-decorated BiVO4 heterojunction photoanode

Constructing heterojunctions by coupling dissimilar semiconductors is a promising approach to boost charge separation and charge transfer in photoelectrochemical (PEC) water splitting. In this work, we fabricated a highly efficient TiO₂/BiVO₄ heterojunction photoanode for PEC water oxidation via a simple hydrothermal method. The resulting heterojunction photoanodes show enhanced PEC performance compared to the bare BiVO₄ due to the simultaneous improvements in charge separation and charge transfer. Under simulated sunlight illumination (AM 1.5G, 100 mW cm⁻²), a high photocurrent of 3.3 mA cm⁻² was obtained at 1.23 V (vs. the reversible hydrogen electrode (RHE)) in a neutral solution, which exceeds those attained by the previously reported TiO₂/BiVO₄ heterojunctions. When a molecular Co–cubane catalyst was immobilized onto the electrode, the performance of the TiO₂/BiVO₄ heterojunction photoanode can be further improved, achieving a higher photocurrent density of 4.6 mA cm⁻² at 1.23 V, an almost three-fold enhancement over that of the bare BiVO₄. These results engender a promising route to designing an efficient photoelectrode for PEC water splitting.

Constructing heterojunctions by coupling dissimilar semiconductors is a promising approach to boost charge separation and charge transfer in photoelectrochemical (PEC) water splitting.     

Efficient charge separation and transfer of a TaON/BiVO4 heterojunction for photoelectrochemical water splitting

The separation and transfer of photogenerated electron–hole pairs in semiconductors is the key point for photoelectrochemical (PEC) water splitting. Here, an ideal TaON/BiVO₄ heterojunction electrode was fabricated via a simple hydrothermal method. As BiVO₄ and TaON were in well contact with each other, high performance TaON/BiVO₄ heterojunction photoanodes were constructed. The photocurrent of the 2-TaON/BiVO₄ electrode reached 2.6 mA cm⁻² at 1.23 V vs. RHE, which is 1.75 times as that of the bare BiVO₄. TaON improves the PEC performance by simultaneously promoting the photo-generated charge separation and surface reaction transfer. When a Co-Pi co-catalyst was integrated onto the surface of the 2-TaON/BiVO₄ electrode, the surface water oxidation kinetics further improved, and a highly efficient photocurrent density of 3.6 mA cm⁻² was achieved at 1.23 V vs. RHE. The largest half-cell solar energy conversion efficiency for Co-Pi/TaON/BiVO₄ was 1.19% at 0.69 V vs. RHE, corresponding to 6 times that of bare BiVO₄ (0.19% at 0.95 V vs. RHE). This study provides an available strategy to develop photoelectrochemical water splitting of BiVO₄-based photoanodes.

TaON/BiVO₄ heterojunction electrodes exhibited significant enhancement in the photoelectrochemical water oxidation.     

Facile and rapid synthesis of a novel spindle-like heterojunction BiVO4 showing enhanced visible-light-driven photoactivity

A spindle-like monoclinic–tetragonal heterojunction BiVO₄ was successfully synthesized by a pressure-controllable microwave method. The as-prepared BiVO₄ samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy, transient photocurrent responses and electrochemical impedance spectroscopy (EIS). The visible-light-driven photocatalytic activity of the BiVO₄ samples was evaluated for the degradation of Rhodamine B (RhB) and tetracycline (TC). The synthesis process needs microwave irradiation for only 10 min without the addition of any auxiliary reagent, pH adjustment, and calcination. The as-prepared spindle-like monoclinic–tetragonal heterojunction BiVO₄ exhibits excellent photocatalytic activity for the degradation of both RhB and TC. The photocatalytic degradation rates of RhB and TC over spindle-like BiVO₄ are 1.77 and 1.64 times higher, respectively, than that measured over monoclinic BiVO₄. The enhanced photocatalytic activity is mainly attributed to the fact that the existence of a heterojunction effectively promotes the separation of photo-generated carriers and extends the visible-light absorption of BiVO₄.

A novel spindle-like monoclinic–tetragonal BiVO₄ heterojunction is rapidly synthesized via a pressure-controllable microwave method.     

Hydrothermal synthesis of cotton-based BiVO4/Ag composite for photocatalytic degradation of C.I. Reactive Black 5

Photocatalytic materials with high efficiency and convenient recyclability have attracted great interest for the treatment of printing and dyeing wastewater. In this paper, a narrow band gap BiVO₄ photocatalyst was loaded onto Ag modified cotton fabric by a hydrothermal method. The prepared composite materials were characterized by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and ultraviolet visible light absorption spectroscopy (UV-vis). The composite materials as prepared show superb photocatalytic activity and reusable performance for the degradation of C.I. Reactive Black 5 (RB5). The degradation rate can reach 99% within 90 min under 1 kW xenon lamp irradiation, and over 90% of the photocatalytic performance is preserved even after five recycles. Furthermore, the photocatalytic mechanism was proposed by spectral analysis and free radical trapping experiments.

BiVO₄/Ag/cotton-K composite material exhibited excellent photocatalytic activity and recyclability, and the photodegradation mechanism of RB5 by BiVO₄/Ag/cotton-K was also proposed.     

Mechanism analysis of Au,Ru noble metal clusters modified on TiO2 (101) to intensify overall photocatalytic water splitting

Accelerating the separation and migration of photo-carriers (electron–hole pairs) to improve the photo-quantum utilization efficiency in photocatalytic overall water splitting is highly desirable. Herein, the photo-deposition of Ru or Au noble metal clusters with superior electronic properties as a co-catalyst on the (101) facet of anatase TiO₂ and the mechanism of intensifying the photocatalysis have been investigated by calculation based density functional theory (DFT). As a result, the as-synthesized Ru/TiO₂ and Au/TiO₂ exhibit high hydrogen evolution reaction (HER) activity. Such a greatly enhanced HER is attributed to the interfacial interactivity of the catalysts due to the existence of robust chemical bonds (Ru–O–Ti, Au–O–Ti) as electron-traps that provide the photogenerated electrons. In addition, the formation of new degenerate energy levels due to the existence of Ru-4d and Au-5d electronic impurity states leads to the narrowing of the band gap of the catalysts. In addition, the as-synthesized Au/TiO₂ exhibits more faster HER rate than Ru/TiO₂, which is attributed to the effects of surface plasmon resonance (SPR) as a synergistic effect of plasmon-induced ‘hot’ electrons that enhance the harvesting of the final built-in electric field and promote the migration and separation of the photo-carriers, which efficiently facilitates hydrogen evolution from the photocatalytic overall water splitting reaction.

Enhancement mechanism of the hydrogen evolution reaction (HER) attributed to the synergistic effect of electron-traps and surface plasmon resonance (SPR).     

Z-scheme 2D-m-BiVO4 networks decorated by a g-CN nanosheet heterostructured photocatalyst with an excellent response to visible light

For economical water splitting and degradation of toxic organic dyes, the development of inexpensive, efficient, and stable photocatalysts capable of harvesting visible light is essential. In this study, we designed a model system by grafting graphitic carbon nitride (g-C₃N₄) (g-CN) nanosheets on the surface of 2D monoclinic bismuth vanadate (m-BiVO₄) nanoplates by a simple hydrothermal method. This as-synthesized photocatalyst has well-dispersed g-CN nanosheets on the surface of the nanoplates of m-BiVO₄, thus forming a heterojunction with a high specific surface area. The degradation rate for bromophenol blue (BPB) shown by BiVO₄/g-CN is 96% and that for methylene blue (MB) is 98% within 1 h and 25 min, respectively. The 2D BiVO₄/g-CN heterostructure system also shows outstanding durability and retains up to ∼95% degradation efficiency for the MB dye even after eight consecutive cycles; the degradation efficiency for BPB does not change too much after eight consecutive cycles as well. The enhanced photocatalytic activities of BiVO₄/g-CN are attributed to the larger surface area, larger number of surface active sites, fast charge transfer and improved separation of photogenerated charge carriers. We proposed a mechanism for the improved photocatalytic performance of the Z-scheme photocatalytic system. The present work gives a good example for the development of a novel Z-scheme heterojunction with good stability and high photocatalytic activity for toxic organic dye degradation and water splitting applications.

For economical water splitting and degradation of toxic organic dyes, the development of inexpensive, efficient, and stable photocatalysts capable of harvesting visible light is essential.     

Synthesis and characterization of BiVO4 nanoparticles for environmental applications

In the present study, a chemical precipitation method is adopted to synthesize bismuth vanadate nanoparticles. The calcination temperature dependent photocatalytic and antibacterial activities of BiVO₄ nanoparticles are examined. The structural analysis evidences the monoclinic phase of BiVO₄ nanoparticles, where the grain size increases with calcination temperature. Interestingly, BiVO₄ nanoparticles calcined at 400 °C exhibit superior photocatalytic behaviour against methylene blue dye (K = 0.02169 min⁻¹) under natural solar irradiation, which exhibits good stability for up to three cycles. The evolution of antibacterial activity studies using a well diffusion assay suggest that the BiVO₄ nanoparticles calcined at 400 °C can act as an effective growth inhibitor of pathogenic Gram-negative (P. aeruginosa & A. baumannii) and Gram-positive bacteria (S. aureus).

In the present study, a chemical precipitation method is adopted to synthesize bismuth vanadate nanoparticles.     

Preparation and photocatalytic activity characterization of activated carbon fiber–BiVO4 composites

Herein, we describe the hydrothermal immobilization of BiVO₄ on activated carbon fibers (ACFs) and characterize the obtained composite by several instrumental techniques, using Reactive Black KN-B (RB5) as a model pollutant for photocatalytic performance evaluation and establishing the experimental conditions yielding maximal photocatalytic activity. The photocatalytic degradation of RB5 is well fitted by a first-order kinetic model, and the good cycling stability and durability of BiVO₄@ACFs highlight the potential applicability of the proposed composite. The enhanced photocatalytic activity of BiVO₄@ACFs compared to those of BiVO₄ and ACFs individually was mechanistically rationalized, and the suggested mechanism was verified by ultraviolet-visible spectroscopy, attenuated total reflectance Fourier-transform infrared spectroscopy, and RB5 degradation experiments. Thus, this work contributes to the development of BiVO₄@ACF composites as effective photocatalysts for environmental remediation applications.

Herein, we describe the hydrothermal immobilization of BiVO₄ on activated carbon fibers, using Reactive Black KN-B photocatalytic performance evaluation and establishing the experimental conditions yielding maximalphotocatalytic activity.     

Two dimensional Ni2P/CdS photocatalyst for boosting hydrogen production under visible light irradiation

Two-dimensional (2D) semiconductor materials have attracted considerable attention in the field of photocatalysis due to the high interfacial charge separation efficiency and abundant surface active sites. Herein, we have fabricated 2D/2D sheets of Ni₂P/CdS heterostructure for photocatalytic H₂ evolution. The microscopic and photocatalytic activity results suggested that Ni₂P nanosheets were coupled with snowflake CdS. The optimal hydrogen production rate reached 58.33 mmol h⁻¹ g⁻¹ (QE = 34.38%, λ = 420 nm) over 5 wt% Ni₂P, which is equivalent to that of 1 wt% Pt/CdS. Compared with pure CdS, Ni₂P/CdS presented lower fluorescence intensity and stronger photocurrent density, which demonstrated that the 2D/2D Ni₂P/CdS heterojunction photocatalyst significantly improved the separation efficiency of photogenerated electrons and holes. The excellent performance of Ni₂P/CdS clearly indicated that Ni₂P was an excellent cocatalyst and could provide abundant active sites for hydrogen evolution.

The mechanism of photogenerated charges separation and hydrogen production of 2D/2D structure Ni₂P/CdS.     

Tuning the exposure of BiVO4-{010} facets to enhance the N2 photofixation performance

Effective separation of photoexcited carriers and chemisorption of the N₂ molecule are two key issues to efficient nitrogen photofixation. The spatial charge separation of BiVO₄ with anisotropic exposed facets, namely the transfer of photoexcited electrons and holes to {010} and {110} facets, respectively, helps to enhance the separation ability of photogenerated carriers. Theoretical calculation results predict that a surface oxygen vacancy is easier to form on the (010) facet than on the (110) facet of BiVO₄. Accordingly, in this study, enhanced N₂ photofixation performance has been achieved for the first time by tuning the exposure of {010} facets of BiVO₄.

Effective separation of photoexcited carriers and chemisorption of the N₂ molecule are two key issues to efficient nitrogen photofixation.

Nitrogen fixation to NH₃ is an important artificial synthesis in the chemical industry.¹ Nowadays, NH₃ is commonly produced through the Haber–Bosch process, which requires high temperature (400–500 °C) and high pressure (15–25 MPa).² This process accounts for ∼2% of the total global energy consumption and contributes ∼1.6% of the total global emissions.³ With the growing energy needs and demand for cleaner environment, a more environmentally friendly method is needed for NH₃ manufacture. Photocatalytic N₂ fixation, which employs solar energy and water to produce ammonium, is a promising sustainable and green strategy for NH₃ synthesis compared with the traditional Haber–Bosch process.^4–9 However, the photocatalytic performance of N₂ fixation is far from satisfactory due to the inefficient separation of photogenerated carriers, the high activation energy barriers and hard cleavage of the strong N N triple bond energy (941 kJ mol⁻¹) of the N₂ molecule.¹⁰Crystal facet engineering of semiconductors is a significant strategy for fine-tuning the charge separation of photocatalysts.¹¹ Facet engineering of anatase TiO₂ has been given considerable research attention for photocatalytic reaction by controlling the {001} exposure ratio.^12–14 Recently, different research groups reported that the exposure of anisotropic facets of BiOX (X = Cl, Br, or I) enabled the directional transfer of photoexcited electrons and holes for spatial charge separation, accordingly improved the photocatalytic activities.^15–17 Other semiconductors including SrTiO₃,¹⁸ LaNbON₂,¹⁹ and C₃N₄ (ref. 20) also demonstrated that the predominating anisotropic facet exposure could improve the separation of photogenerated carriers. Simultaneously, the spatial charge separation of BiVO₄ with anisotropic exposed facets has been observed by direct imaging²¹ and photo-reduction or photo-oxidation reactions on {010} or {110} facets, respectively.^22–24 These results indicated that the photoexcited electrons (e⁻) and holes (h⁺) would transfer to the {010} and {110} facets, respectively. Based on the special property of BiVO₄, photocatalytic water splitting for O₂ evolution has been greatly improved.²⁵ However, a few researches on BiVO₄ with anisotropic exposed facets have been reported for photocatalytic N₂ fixation.It is commonly understood that surface vacancies with abundant localized electrons play a critical role in N₂ photofixation by capturing and activating the inert N₂ molecule.^26,27 Efficient transfer of the photogenerated electrons to the inert N₂ molecule is also a key step for the effective photocatalytic N₂ fixation.²⁸ Considerable research results have revealed that surface oxygen vacancies could activate the N₂ molecule by chemisorption, and act as the transfer bridge of photoexcited electrons from photocatalysts to the activated N₂ molecule. For example, Pan et al. reported that the bond length of N N was elongated by the interaction with the surface O_VS on MoO_3−x nanobelts or W₁₈O₄₉ nanowires, and the photocatalytic activities for the N₂ fixation were directly related to the surface O_VS concentration.^29,30 The critical role of O_VS in the photofixation of N₂ was also revealed by other semiconductors including TiO₂,^10,31–33 BiOCl,³⁴ BiO quantum dots,³⁵ ultrafine Cu₂O,⁸ and amorphous CeO_x.³⁶ However, the surface O_VS on BiVO₄ for N₂ photofixation has been rarely studied.Theoretical calculation results predicted that the formation energy (E_f) of an O_V on the surface of the representative (010) facet was lower than that on the surface of the typical (110) facet, accordingly there were more O_VS on the surface of the (010) facet. Together with the spatial charge separation property, anisotropic exposed BiVO₄ was believed to exhibit a good N₂ photofixation performance. In this study, BiVO₄ with anisotropic {010} and {110} facets were synthesized by a solid–liquid state reaction.³⁷ It was first found that the as-prepared BiVO₄ with higher percentage of exposed {010} facets exhibited better performance for N₂ photofixation without any sacrificial reagent and cocatalyst under ambient conditions. The easier formation of a surface oxygen vacancy (O_V) on the (010) facet was experimentally proved by the enhanced chemisorption of the N₂ molecule based on the temperature programmed desorption (TPD) characterization. The enhanced separation of photogenerated carriers and more surface oxygen vacancies (O_VS) made BiVO₄ with a higher exposed ratio of {010} facets to be more efficient for photocatalytic N₂ fixation.The primitive unit cell consists of four units as shown by the side and top view of the optimized BiVO₄ (Fig. 1a and b), and the optimized lattice parameters are as follows: a = 7.33 Å, b = 11.77 Å, c = 5.18 Å, and β = 134.92°. They are in good agreement with the experimental values: a = 7.25 Å, b = 11.70 Å, c = 5.09 Å, and β = 134.225°. The formation energy of the oxygen vacancy (E_f) on the surface of (010) and (110) facets were calculated by following equation:E_f = E_OV + E_½O2 − E_surfacewhere E_OV, E_½O2, and E_surface represent the total energy of surface with one oxygen vacancy, the half of total energy of oxygen and the total energy of surface without oxygen vacancy. The calculated formation energies of an oxygen vacancy were 2.91 eV and 4.93 eV on (010) and (110) surface, respectively. It means that the formation of a surface oxygen vacancy on the (010) facet was more energetically favorable than that on the (110) facet.Open in a separate windowFig. 1The side view (a) and top view (b) of optimized monoclinic BiVO₄. The purple, blue and red atom indicates Bi, V and O, respectively; (c) the optimized structure of the (010) surface and (010) surface with one vacancy. (d) The optimized structure of the (110) surface and (110) surface with one vacancy.Based on the theoretical calculation result, BiVO₄ with different exposure ratios of {010} facets was fabricated by tuning the nitric concentration in the reaction solution according to a previous report (see ESI^†). The as-synthesized BiVO₄-0.50, BiVO₄-0.75, and BiVO₄-1.0 can be indexed to the monoclinic crystal structure (PDF# 14-0688) (Fig. 2a and S1^†). However, the BiVO₄-0.25 sample in Fig. S1^† consists of a hybrid monoclinic structure (PDF# 14-0688) and a tetragonal crystal phase (PDF# 14-0133), probably due to the low concentration of the HNO₃ solution. Furthermore, the (020) peak of BiVO₄-0.50 appears around 15° of 2θ (Fig. 2a). The as-synthesized BiVO₄ in different concentrations of HNO₃ shows different facet exposure ratios of {010} to {110}, and BiVO₄-0.50 exhibits the largest exposure ratio (Fig. 2b and S2^†), which is consistent with the previous report.³⁷ The largest exposure of {010} facets must cause the appearance of the (020) peak in Fig. 2a. The typical TEM image shows the regular shape of BiVO₄-0.50 (Fig. 2c). The HRTEM interplanar spacing is 0.47 nm, corresponding to the value of (110) facet (Fig. 2d). The selected area electron diffraction (SAED) rings in the inset of Fig. 2d corresponds to the (110) and (040) crystal facets of monoclinic BiVO₄.Open in a separate windowFig. 2Typical XRD pattern (a), SEM image (b), TEM image (c), and HRTEM image (d) of the as-synthesized BiVO₄-0.50. Inset in (d) SAED pattern. Fig. 3 exhibits the temperature programmed desorption (TPD) characterization of the N₂ molecule. A single peak centering at 220 °C is observed, which is ascribed to the desorption of the chemisorbed N₂, and BiVO₄-0.50 exhibits the strongest chemisorption of the N₂ molecule. It is consistent with the theoretical calculation result shown in Fig. 1.Open in a separate windowFig. 3N₂-TPD profiles of BiVO₄ synthesized in the aqueous solution with different HNO₃ concentrations.For the general photocatalytic N₂ photofixation process in pure water, the photoexcited electrons are injected into the chemisorbed N₂ molecule via oxygen vacancy, and then the activated N₂ molecule combines with H⁺ from water to form NH₃. Simultaneously, the photogenerated holes oxidize the OH⁻ from water to produce O₂. The standard curve from the different concentrations of NH₄⁺ is shown in Fig. S4.^† Photocatalytic performance test indicates that BiVO₄-0.50 exhibits the best activity for N₂ fixation among these four samples (Fig. 4a). The average NH₄⁺ evolution rate is about 15.5 μmol g⁻¹ L⁻¹ h⁻¹ for BiVO₄-0.50 during a 6 h test. The average NH₄⁺ evolution rates decrease to 13.1, 5.35, and 5.3 μmol g⁻¹ L⁻¹ h⁻¹ for BiVO₄-0.75, BiVO₄-1.00, and BiVO₄-0.25, respectively (Fig. 4a). The Brunauer–Emmett–Teller (BET) measurements indicate that the surface areas are 2.9 m² g⁻¹, 2.0 m² g⁻¹, 1.8 m² g⁻¹, and 1.5 m² g⁻¹ for BiVO₄-0.25, BiVO₄-0.50, BiVO₄-0.75, and BiVO₄-1.00, respectively. The corresponding ratios of the average NH₄⁺ evolution rate to surface area are 4.5 μmol L⁻¹ h⁻¹ m⁻², 7.8 μmol L⁻¹ h⁻¹ m⁻², 3.0 μmol L⁻¹ h⁻¹ m⁻², and 3.5 μmol L⁻¹ h⁻¹ m⁻². It is obvious that the surface area is not the determining factor of the photocatalytic activity. In order to confirm the origination of the nitrogen element in NH₄⁺, Ar gas was bubbled and the aqueous suspension of the BiVO₄-0.50 sample was irradiated by a 300 W xenon lamp during the test time. It is found that there is no detectable NH₄⁺ (Fig. 4a), and it can be concluded that the detected NH₄⁺ did not originate from environmental contamination, but from the photofixation reaction of N₂ molecule by the BiVO₄ photocatalyst. Fig. 4b shows the light-wavelength-dependent photofixation activity of BiVO₄-0.50. The photocatalytic performance decreases from 365 nm and exhibits almost no NH₄⁺ from 515 nm, and no NH₄⁺ is detected at 590 nm due to the light absorption range (Fig. S3^†). The photocatalyst possessed photoexcited electrons with higher energy under the irradiation of the shorter wavelength light, and the photofixation of N₂ was more easy to occur. However, the photofixation reaction of N₂ molecule could not occur when the longer wavelength light was unable to excite the BiVO₄ photocatalyst. This result further confirmed the photofixation of N₂ in our study. The calculated value of quantum efficiency (QE) at 365 nm was about 0.003%.Open in a separate windowFig. 4(a) Photocatalytic nitrogen fixation performance of BiVO₄ samples synthesized with different concentrations of nitric acid (light source: 300 W xenon lamp; photocatalyst: 0.05 g; reaction solution: 100 ml of pure water). (b) Nitrogen fixation performance of BiVO₄-0.50 illuminated by LED with different wavelengths (365 nm, 384 nm, 400 nm, 470 nm, 498 nm, 515 nm, and 590 nm).In order to evaluate the stability of BiVO₄-0.50 for N₂ photofixation, three cycle tests were carried out, as exhibited in Fig. 5. After each cycle, the photocatalyst was washed by vacuum filtration with pure water for several times and dried in a vacuum oven. The photocatalytic activity gradually decreased probably due to the loss of photocatalyst during the wash process.Open in a separate windowFig. 5Photocatalytic stability test for BiVO₄-0.50 (light source: 300 W xenon lamp; photocatalyst: 0.05 g; reaction solution: 100 ml of pure water).In summary, BiVO₄ with exposed anisotropic {010} and {110} facets was synthesized, and studied for the first time for ammonia production by photocatalytic N₂ fixation from pure water. It was found that the sample with the high facet exposure ratio of {010} to {110} exhibited a better photocatalytic performance for N₂ fixation, which resulted from the better separation ability of the photoexcited carriers and more surface O_VS on the {010} facet. The photoexcited electrons more effectively transferred to the more surface O_VS on the typical (010) facet, where the N₂ molecule can be activated, and accordingly benefited to the enhancement of the photocatalytic performance. Our study provides a good strategy to improve the photocatalytic activity for N₂ fixation.     

Band offset engineering at C2N/MSe2 (M = Mo,W) interfaces

Stacking layered two-dimensional materials in a type-II band alignment block has provided a high-performance method in photocatalytic water-splitting technology. The key parameters in such heterostructure configurations are the valence and conduction band offsets at the interface, which determine the device performance. Here, based on density functional theory calculations, the bandgap and band offsets at C₂N/MSe₂ (M = Mo, W) interfaces have been engineered. The main findings demonstrate that the C₂N monolayer interacts with both MoSe₂ and WSe₂ monolayers through weak van der Waals interactions. These heterostructures possess a narrower indirect bandgap and a typical type-II heterostructure feature, being suitable for promoting the separation of photogenerated electron–hole pairs. The calculated Gibbs free energy of hydrogen adsorption demonstrates a reduction in the overpotential, towards the hydrogen evolution reaction, upon forming heterostructures. To further tune the bandgap values and band offsets of heterostructures, the external perturbations are included through a vertical strain and finite electric field. It is found that both the vertical strain and electric field strongly modulate the bandgap values and the magnitude of the band offsets, while the typical type-II band alignment remains preserved. It is noticeable that the band offset magnitudes of the C₂N/MoSe₂ and C₂N/WSe₂ heterostructures are more sensitive to an external electric field than to a vertical interlayer strain.

Stacking layered two-dimensional materials in a type-II band alignment block has provided a high-performance method in photocatalytic water-splitting technology.     

Green fabrication of nanoporous BiVO4 films on ITO substrates for photoelectrochemical water-oxidation

A new green method was developed to prepare nanoporous BiVO₄ films on ITO substrates for photoelectrochemical (PEC) water-oxidation under visible light irradiation. The films can be prepared by simple drop-casting of a stable aqueous solution of Bi³⁺ and V⁵⁺ complexes with tartaric acid and ethylenediaminetetraacetic acid, followed by drying and calcination in air. Thanks to these ligands, the aqueous precursor solution is remarkably stable over a wide range of pH (pH 4–9). The BiVO₄ films on ITO substrates possess a 3D-network structure comprised of nanoparticles with a scheelite–monoclinic phase and a diameter of ca. <100 nm, after calcination at 450–500 °C for 1 h. The PEC performance clearly depended on the film thickness that can be controlled by coating times, and calcination conditions (temperature and time). The CoPi-loaded BiVO₄ electrodes exhibited relatively high performance for PEC water oxidation (ABPE of 0.35% at 0.8 V vs. RHE) under simulated sunlight irradiation.

Nanoporous BiVO₄ photoanodes for efficient water oxidation were directly fabricated on an ITO substrate using an aqueous solution of mild pH.     

Piezoelectric built-in electric field advancing TiO2 for highly efficient photocatalytic air purification

Photocatalytic air purification is a promising technology; however, it suffers from a limited rate of photocatalytic mineralization (easily inactivated surfactant sites of hydroxyls) and poor kinetics of degradation. Herein, we report a ferroelectric strategy, employing a polyvinylidene fluoride (PVDF) layer embedded with TiO₂, where the polarization field of stretched PVDF dramatically enhances and stabilizes active adsorption sites for the promotion of charge separation. The F (−) and H (+) atomic layers with distinct local structures in stretched PVDF increase the electron cloud density around Ti which simultaneously promotes the dissociation of water to form hydroxyl groups which are easier to activate for adsorption of formaldehyde molecules. Besides, the ferroelectric field of stretched PVDF effectively separates the photogenerated charge carriers and facilitates the carriers'' transportation of TiO₂/PVDF. The optimal stretched TiO₂/PVDF exhibits excellent photocatalytic mineralization for formaldehyde with considerable stability. This work may evolve the polarization field as a new method to enhance adsorption and activation of hydroxyls and disclose the mechanism by which hydroxyl radicals mineralize gaseous formaldehyde for photocatalytic air purification.

Ferroelectric built-in electric fields were used for photocatalytic air purification, where the stretched PVDF dramatically enhances and stabilizes active adsorption sites.     

Engineering exposed vertical nano-TiO2 (001) facets/BiOI nanosheet heterojunction film for constructing a satisfactory PEC glucose oxidase biosensor

In the field of photoelectrochemical (PEC) enzyme biosensors, constructing efficient photoelectrodes, in which the recombination of photogenerated carriers is an important factor affecting the performance, is of great significance. Herein, to enhance the separation efficiency of photogenerated carriers, titanium dioxide (TiO₂) nanosheet (NS)/bismuth oxyiodide (BiOI) NS/glucose oxidase (GOx) composites were prepared via hydrothermal and solvothermal methods. Single-crystal anatase TiO₂ NSs with a high percentage of (001) facets lead to better photocarrier separation due to heterojunctions between facets. After coupling with BiOI NSs, the photoelectrochemical performance of the electrode was greatly improved. The photogenerated electrons from TiO₂ and BiOI gathered at TiO₂ (101) and were exported through the fluorine-doped tin oxide (FTO) substrate to generate electrical signals. Photogenerated holes were transferred to TiO₂ (001) and BiOI to participate in the enzymatic reaction, showing the outstanding separation of electrons and holes. The prepared TiO₂ NS/BiOI NS/GOx glucose biosensor achieved satisfactory results, with sensitivity of 14.25 μA mM⁻¹ cm⁻², a linear measurement range of 0–1 mM, and a limit of detection (3S/N) of 0.01 mM in phosphate buffered saline (PBS) at a pH of 7.4. The mechanism for the efficient separation of photogenerated carriers based on the facet heterojunctions introduced in this paper also provides new insights into other optoelectronic biosensors.

Demonstration of the mechanism based on the synergistic effect of TiO₂ facet heterojunctions and TiO₂/BiOI heterojunctions to promote efficient separation of photogenerated carriers.     

Superior visible light photocatalytic performance of reticular BiVO4 synthesized via a modified sol–gel method

Reticular BiVO₄ catalysts were successfully synthesized via a modified sol–gel method. Here, citric acid (CA) was used as the chelating agent and ethylenediaminetetraacetic acid (EDTA) was used as the chelating agent and template. Furthermore, the effects of pH values and EDTA on the structure and morphology of the samples were studied. We determined that EDTA and pH played important roles in the determination of the morphology of the as-prepared BiVO₄ samples. Photocatalytic evaluation revealed that the reticular BiVO₄ exhibited superior photocatalytic performance characteristics for the degradation of methylene blue (MB) under visible-light (λ > 400 nm) exposure, about 98% of the MB was found to degrade within 50 min. Moreover, the degradation kinetics of MB was in good agreement with pseudo-first-order kinetics. The obtained apparent reaction rate constant k_app of reticular BiVO₄ was much higher than that of BiVO₄ synthesized by the citric acid sol–gel method.

Reticular BiVO₄ catalysts with superior visible light photocatalytic performance were successfully synthesized via a modified sol–gel method.     

Insight into the PEC and interfacial charge transfer kinetics at the Mo doped BiVO4 photoanodes

BiVO₄ is a promising photoanode material for the photoelectrochemical (PEC) oxidation of water; however, its poor charge transfer, transport, and slow surface catalytic activity limit the expected theoretical efficiency. Herein, we have investigated the effect of Mo doping on SnO₂ buffer layer coated BiVO₄ for PEC water splitting. SnO₂ and Mo doped BiVO₄ layers are coated with layer by layer deposition through a precursor solution based spin coating technique followed by annealing. At 5% doping of Mo, the sample (SBM5) shows a maximum current density of 1.65 mA cm⁻² at 1.64 V vs. RHEl in 0.1 M phosphate buffer solution under AM 1.5 G solar simulator, which is about 154% improvement over the sample without Mo (SBM0). The significant improvement in the photocurrent upon Mo doping is due to the improvement of various bulk and interfacial properties in the materials as measured by UV-vis spectroscopy, electrochemical impedance spectroscopy (EIS), Mott–Schottky analysis, and open-circuit photovoltage (OCPV). The charge transfer kinetics at the BiVO₄/electrolyte interface are investigated to simulate the oxygen evolution process in photoelectrochemical water oxidation in the feedback mode of scanning electrochemical microscopy (SECM) using 2 mM [Fe(CN)₆]³⁻ as the redox couple. SECM investigation reveals a significant improvement in effective hole transfer rate constant from 2.18 cm s⁻¹ to 7.56 cm s⁻¹ for the hole transfer reaction from the valence band of BiVO₄ to [Fe(CN)₆]⁴⁻ to oxidize into [Fe(CN)₆]³⁻ with the Mo doping in BiVO₄. Results suggest that Mo⁶⁺ doping facilitates the hole transfer and suppresses the back reaction. The synergistic effect of fast forward and backward conversion of Mo⁶⁺ to Mo⁵⁺ expected to facilitate the V⁵⁺ to V⁴⁺ which has an important step to improve the photocurrent.

BiVO₄ is a promising photoanode material for the photoelectrochemical (PEC) oxidation of water; however, its poor charge transfer, transport, and slow surface catalytic activity limit the expected theoretical efficiency.