LSPR-enhanced photocatalytic N₂ fixation over Z-scheme POMOF-derived Cu/WO₂ modified C-BiOBr with multiple active sites

Abstract

The conception and production of nitrogen-fixing photocatalysts with efficient charge separation rates and multiple active sites have been the focus of research. In this paper, we prepared Cu/WO₂ nanoparticles by high-temperature calcination of polyoxometalate based open frameworks (POMOFs) and then anchored them in interstitial carbon-doped BiOBr using a one-step hydrothermal method to obtain a novel Z-scheme Cu/WO₂/C-BOB ternary heterostructure. The ammonia generation rate over the Cu/WO₂/C-BOB heterojunction is 477.5 μmol g⁻¹ h⁻¹ in deionized water without any sacrificial reagents under the full solar spectrum, which is nearly 6.1 times greater than that of pure BiOBr. The synergistic effect of heteroatom doping, Z-scheme heterojunction and oxygen vacancies inhibits the recombination of photogenerated carriers and maintains their maximum redox capacity, providing more reaction sites and significantly improving the photocatalytic performance. In addition, the localized surface plasmon resonance (LSPR) effect of Cu NPs enhances light absorption and induces high-energy hot electrons to produce additional oxygen vacancies. Meanwhile, we explored the charge transfer pathways and possible reaction mechanisms of the heterojunctions through experimental characterization and DFT calculations, which provided a new idea to synergistically utilize the LSPR effect and Z-scheme heterostructures for the design of efficient photocatalysts.

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Introduction

Ammonia is a component in nitrogen fertilizer production, which is widely used in industry and agriculture.^1–3 At present, the main technique used for large-scale ammonia synthesis is the Haber–Bosch process. This process involves high temperatures and pressures, which results in significant energy usage and greenhouse gas emissions. To overcome the environmental damage and energy crisis, photocatalytic technology has been developed and applied in various catalytic fields.^4–6 Green photocatalytic nitrogen fixation under mild conditions has thus become the focus of research.^7–10

BiOBr is considered to be a promising photocatalyst for ammonia production because of its layered structure, low toxicity, and ease of defect generation (especially oxygen vacancies).^11,12 However, the weak interaction between BiOBr and N₂ makes it difficult to activate inert N₂ molecules. Rapid complexation of photogenerated carriers kinetically limits the catalytic ability of BiOBr. Therefore, modifications such as heterostructure building, defect engineering and heteroatom doping are necessary to enhance its photocatalytic performance.^13–15 Oxygen vacancies and heteroatom doping significantly increase the active sites for N₂ fixation, induce the creation of defect energy levels and improve charge-transfer kinetics. According to the previous literature, the introduction of OVs into bismuth-based materials can change their Fermi energy level positions, create defective energy levels in the bandgap, and capture photogenerated electrons as active sites. It lowers the frequency at which photogenerated carriers recombine and promotes the photocatalytic nitrogen fixation reaction.¹⁶ The introduction of heteroatoms into semiconductor photocatalysts can regulate their energy band structure and improve their redox properties, and the nonmetallic element carbon has been widely studied because of its excellent optimization in light absorption and carrier separation.^17,18 A few reports have been published thus far on C-doped BiOBr materials, which are valuable to study.

However, single semiconductor photocatalysts still suffer from the problems of rapid complexation of photogenerated carriers and limited photoresponse, and the strategy of constructing heterostructured photocatalysts has been proposed to solve this dilemma. Notably, the Z-scheme heterojunction can maximally retain the redox ability of the photogenerated carriers and exhibit excellent performance in suppressing the rapid electron–hole complexation, which greatly improves the photocatalytic reaction efficiency and is much superior to the conventional type II heterojunction.¹⁹ It has been demonstrated that adding metal nanoparticles to semiconductor–semiconductor heterojunction materials can improve photocatalytic performance. Nanoparticles such as gold (Au), palladium (Pd) and copper (Cu) have excellent electronic conductivity and can enhance the visible light response through the LSPR effect.^20–22 During LSPR excitation, hot electrons in the plasma that are vibrating leave the Fermi energy level of the metal and enter the conduction band of the adjacent semiconductor, promoting an effective charge transfer mechanism.²³ For example, Kong et al. prepared a plasma Ag/Na-doped defective graphitic carbon nitride/NiFe layered double hydroxide Z-scheme heterostructure by a hydrothermal method, which efficiently promotes charge separation and greatly improves the photocatalytic degradation efficiency.²⁴ Li et al. constructed Z-scheme heterostructured CdS/Au–Ag/B-TiO₂ photocatalysts, which facilitated directional charge transfer and accelerated the HER and CO₂RR processes.²⁵ However, the LSPR effect of non-precious metal copper nanoparticles coupled with a Z-scheme heterojunction for photocatalytic nitrogen reduction has not been reported so far.

The introduction of polyoxometalates (POMs) with unique capabilities into metal–organic frameworks (MOFs) with substantial specific surface area and abundant porosity to assemble stable polyoxometalate-based open framework (POMOF) materials prevents the self-aggregation of POMs and exposes more catalytically active centers to improve the catalytic activity effectively.^26–28 It is noteworthy that POMOFs can be employed as precursors or templates for the regulated synthesis of oxides, sulfides and porous carbon materials with excellent catalytic properties.^29,30 The multicomponent and multifaceted structures in these derived materials effectively promote electron transfer, expose significant metal activity centers, and enhance photocatalytic properties.^31–34

Combined with the above discussion, we designed a Z-scheme ternary heterostructure system with the LSPR effect, which effectively promotes light utilization and improves the efficiency of electron–hole separation. Cu/WO₂ nanoparticles were obtained by annealing treatment with NENU-3 as the precursor, whereas they were later anchored on the surface of the interstitial carbon-doped BiOBr material by ethylene glycol reduction, which effectively inhibited the oxidation of the Cu nanoparticles under the coexistence of ethylene glycol and glucose. In addition, glucose enables the introduction of interstitial carbon into BiOBr materials.¹⁸ A series of characterization studies proved that the Cu/WO₂/C-BOB ternary composites have the best photocatalytic nitrogen fixation performance. We believe that the increased efficiency is mostly caused by the abundant oxygen vacancies and C doping providing more sites for the activation of nitrogen molecules, which effectively enhance light absorption, and the two electron transfer pathways, Z-scheme heterojunction and LSPR-excited hot-electron injection, which efficiently improve the charge transfer and photogenerated carrier polarization efficiency. Finally, a reaction mechanism based on Cu/WO₂/C-BOB for efficient nitrogen fixation was proposed using the UPS test, ESR test and DFT calculations.

Experimental section

Synthesis of the NENU-3 precursor

The synthesis procedure of NENU-3 was slightly modified from the previously reported method.³⁵ In a typical synthesis procedure, 0.3993 g of copper acetate monohydrate, 0.3 g of phosphotungstic acid and 0.1471 g of L-glutamic acid were mixed in 70 mL of water and ultrasonicated for 30 min to acquire a homogeneous solution called solution A. Then 0.2816 g of trimesic acid was dispersed in 70 ml of anhydrous ethanol to formulate a clear solution. The above solution was poured into solution A and quickly assembled into NENU-3 crystals, and the solution immediately became turbid. A light blue precipitate was formed after 14 hours of mixing. The resultant product was dried after being cleaned with anhydrous ethanol.

Synthesis of Cu/WO₂

In a nitrogen-filled tube furnace, NENU-3 nano-octahedra were calcined for 6 h at 800 °C with a heating rate of 2 °C per minute. Reddish brown Cu/WO₂ nano-octahedra were obtained.

Synthesis of Cu/WO₂/C-BOB

1.46 g of CTAB, 5 mg of PVP and 0.2 g of glucose were added to 25 mL of ethylene glycol and dispersed by sonication for 20 min. Subsequently, 25 ml of ethylene glycol solution containing 1.94 g of Bi(NO₃)₃·5H₂O in a completely dissolved form was added dropwise to the aforementioned solution. After 10 min of mixing, 0.05 g of Cu/WO₂ was added and stirring was continued for one hour until it was completely dissolved. Finally, the solution was heated in an oven at 120 °C for 10 hours. The cooled product was dried at 60 °C after three water washes. Interstitial carbon-doped BiOBr materials, named C-BOB, were synthesized under the same synthesis conditions without the addition of Cu/WO₂. No glucose was added during the synthesis of pure BiOBr.

Synthesis of WO₂/C-BOB

15 mg of Cu/WO₂ was added to 10 mL of 4 M hydrochloric acid and washed to obtain pure WO₂ with Cu removed. WO₂/C-BOB was synthesized in a similar way to that of Cu/WO₂/C-BOB.

Characterization

Scanning electron microscopy (SEM) (Hitachi SU8010), transmission electron microscopy (TEM model: JEM-2100 PLUS), powder X-ray diffraction (XRD model: Siemens, D5005), Fourier transform infrared spectrometry (FTIR-650), and X-ray photoelectron spectroscopy (Thermoscience Escalab 250Xi) were utilized to examine the morphology, composition and chemical state of the photocatalysts. The light absorption capacity of the samples was examined using UV-vis diffuse reflectance spectroscopy (Varian Cary 7000 spectrophotometer). Photoluminescence (PL) spectra were obtained using an FLS1000 at an excitation wavelength of 590 nm.

Photocatalytic nitrogen reduction

The light supply for the photocatalytic reaction for fixing nitrogen was a 300 W xenon lamp with an AM 1.5 filter. The whole reaction was conducted under ambient conditions. First, 10 mg of photocatalyst was fully dissolved in 100 mL of purified water in a quartz reaction vessel, and then nitrogen was bubbled for 30 minutes under darkness. Subsequently, a xenon lamp was switched on to irradiate the reactor for the photocatalytic reaction, and the circulating condensate was allowed to fill the outer wall of the reactor to ensure a constant temperature, and 7 mL of the solution was removed after one hour of continuous stirring. After centrifugation and filtration, the concentration of NH₄⁺ was determined spectrophotometrically at 420 nm with Nessler's reagent. The procedure was repeated for photocycling tests.

Results and discussion

Morphology, composition and structure of photocatalysts

The Cu/WO₂/C-BOB synthesis method is shown in Fig. 1. First, NENU-3 nano-octahedra were synthesized by introducing phosphotungstic acid molecules into the pores of a copper-based MOF (HKUST-1, Cu₃(BTC)₂(H₂O)₃). Subsequently, they were subjected to an annealing process at a temperature of 800 °C under a N₂ atmosphere. The +6-valent W in phosphotungstic acid was reduced to +4-valent in situ to form tungsten dioxide during the pyrolysis process. The BTC ligand was carbonized into porous carbon at high temperature, and the +2-valent Cu ions were reduced to metallic copper, resulting in the Cu/WO₂ carbon-on-backbone nanocomposites. Subsequently, Cu/WO₂ was anchored in interstitial carbon-doped BiOBr using a hydrothermal method to synthesize the Cu/WO₂/C-BOB Z-scheme ternary heterostructure.

Fig. 1 Diagrammatic representation of the Cu/WO₂/C-BOB heterojunction preparation method.

The crystal structures of the photocatalysts were investigated using X-ray diffraction spectroscopy. The successful preparation of NENU-3 is displayed in Fig. S2a.† As seen in Fig. 2a, the diffraction peaks of pure BOB match those of tetragonal phase BiOBr (JCPDS 09-0393).³⁶ C-BOB, WO₂/C-BOB and Cu/WO₂/C-BOB all show the distinct peaks described above, demonstrating the continued preservation of the BiOBr crystal structure and no new substances being generated. As can be seen from Fig. S2b and S2c,† the characteristic peaks of the C-BOB series of materials obtained with glucose as the carbon source show a little rightward shift as compared to pure BOB, which suggests that carbon atoms have been introduced into the BiOBr lattice. Considering the smaller radius of the carbon atom, the higher energy required to replace the oxygen atom, the unique layered structure of BiOBr and the weak inter-cell interactions, it is evident that the carbon atoms have been introduced into the BiOBr lattice by interstitial doping.³⁷ From the XRD spectrum of Cu/WO₂ in Fig. 2b, it is evident that the two distinct diffraction peaks at 43.3° and 50.5° indicate the (111) and (200) crystal planes of Cu (JCPDS 85-1326), and the diffraction peaks at 25.8°, 36.8° and 52.9° match the (011), (200) and (−220) crystal planes of WO₂ (JCPDS 32-1393).^38,39 The peaks of Cu/WO₂ and C-BOB can be observed in the Cu/WO₂/C-BOB heterostructure, and after adding Cu/WO₂, the strength of the BiOBr diffraction peaks decreases, indicating the successful synthesis of the heterojunction. In addition, WO₂ can be obtained by treating Cu/WO₂ with dilute hydrochloric acid solution. The copper nanoparticles are corroded to Cu²⁺ by hydrochloric acid, which remains in the solution, and pure WO₂ is obtained by centrifugal washing. The peaks belonging to the metal copper in the XRD spectrum of the etched product disappear, whereas the characteristic peaks of WO₂ are still present, with a slight decrease in intensity (Fig. S2d†).

Fig. 2 (a and b) XRD patterns, (c) FTIR spectra and (d) EPR spectra of the samples.

The chemical structures and functional groups of photocatalysts were investigated using FT-IR spectroscopy (Fig. 2c). The peaks at 518 and 1450 cm⁻¹ represent the stretching vibrations of the Bi–O and Bi–Br bonds of BiOBr, respectively.⁴⁰ For Cu/WO₂, the peak at 808 cm⁻¹ belongs to the stretching vibration of the W–O bond, and the peak at 1627 cm⁻¹ matches the C=C bond corresponding to the graphite structural domain, which is produced by calcination of the BTC ligand.^41,42 It is noteworthy that these two peaks are found in both WO₂/C-BOB and Cu/WO₂/C-BOB, and in the heterojunction material, the peak positions are a little red-shifted compared to the base material. Combined with XRD analysis, we confirm the successful construction of the Cu/WO₂/C-BOB heterostructure and the existence of interfacial interactions between Cu/WO₂ and C-BOB.

The content of oxygen vacancies in the samples was studied qualitatively using electron spin resonance (ESR) spectroscopy (Fig. 2d). Significant oxygen vacancy signal peaks were seen at g = 2.002 in all cases. The oxygen vacancies in pure BOB and C-BOB were obtained by ethylene glycol reduction, and the oxygen vacancies in Cu/WO₂ came from high-temperature annealing with the inert gas and the LSPR effect of Cu NPs. It is obvious from the figure that Cu/WO₂/C-BOB has the highest content of oxygen vacancies. By generating defective energy levels inside the photocatalyst band gap, oxygen vacancies can increase the light absorption efficiency and promote carrier separation. In addition, oxygen vacancies can be active sites in the reaction to activate nitrogen molecules. This is the key factor causing Cu/WO₂/C-BOB to have the highest nitrogen fixing efficiency.⁴³

The microstructure and morphology of the photocatalysts were analyzed using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The synthesized NENU-3 sample has an octahedral structure with a smooth surface (Fig. S3†). Cu/WO₂ obtained by high-temperature annealing treatment still retained the original octahedral morphology (Fig. 3a), but the surface became rough with a size of roughly 100–200 nm. From the TEM images of Cu/WO₂ (Fig. 3b), it can be observed that a large number of WO₂ species (darker regions, marked with orange circles) are within the matrix of amorphous carbon, and Cu species are also clearly visible (isolated bright spots, indicated by blue circles).^36,44 The lattice stripes at 0.21 nm and 0.34 nm in the HRTEM image (Fig. 3c) match the (011) crystal plane of Cu and the (111) crystal plane of WO₂. C-BOB is a microsphere assembled from nanosheets, as seen by its SEM and TEM images (Fig. 3d and e). The HRTEM image (Fig. 3f) displays a lattice stripe of 0.28 nm, which is in alignment with the (110) crystal plane of BiOBr. It is evident from the SEM and TEM images of Cu/WO₂/C-BOB (Fig. 3g and h) that the Cu/WO₂ nano-octahedra were successfully anchored on the C-BOB microspheres during the hydrothermal process, transforming the ultrathin nanosheet structure of the surface into nanoparticles, and this change increased the specific surface area of the heterostructure, which is more favorable for the adsorption of N₂ molecules. As can be seen from Fig. S4a–g,† with the gradual increase in the content of Cu/WO₂, the amount of loaded particles on the surface of the C-BOB microspheres increased, and the specific surface area of the composites progressively increased; nevertheless, when the loading amount exceeded a specific threshold, the pores on the surface of the C-BOB microspheres were entirely occluded, which would be unfavorable for the adsorption of N₂. The HRTEM images of Cu/WO₂/C-BOB (Fig. 3i) simultaneously present the (111) crystalline surface of Cu, the (011) crystalline surface of WO₂, and the (110) crystalline surface of BiOBr, and the presence of distinct interfaces can be observed (indicated by red dashed lines). All the above results indicate that the Cu/WO₂/C-BOB heterostructure was successfully constructed, and the efficient ongoing electron transport between the interfaces effectively promotes the charge migration of the photocatalysts and accelerates the photocatalytic process. EDS analysis (Fig. S5†) and element mapping images (Fig. S6, S7,† and Fig. 3j) allow us to derive the elemental composition and the homogeneous distribution in the material. The elemental content was quantified using inductively coupled plasma atomic emission spectroscopy (ICP-AES) (Table S1†).

Fig. 3 (a, d and g) SEM images of the samples. (b, e and h) TEM images of the samples. (c, f and i) HRTEM images of the samples. (j) Element mapping images of Cu/WO₂/C-BOB.

The elemental makeup and chemical states of the photocatalysts were further investigated using X-ray photoelectron spectroscopy (XPS). From the XPS full spectrum, elemental peaks of Cu, W, O, Bi, Br and C were observed (Fig. 4a). In comparison with pure BOB, the Bi 4f peaks of C-BOB are shifted towards higher binding energies (Fig. S8a†), which indicates that the bismuth atoms lose their electrons due to the greater electronegativity of C, suggesting that carbon has been successfully introduced into the lattice of BiOBr. And the peaks located at 164.8/164.7 eV and 159.4/159.3 eV in C-BOB and Cu/WO₂/C-BOB were identified as Bi 4f_7/2 and Bi 4f_5/2 peaks (Fig. 4b). It indicates that there is no change in the chemical valence state of Bi in both where it exists as Bi³⁺.⁴⁵ In the Br 3d spectrum (Fig. S8b†), the two peaks of Br 3d_5/2 and Br 3d_3/2 at 69.5/69.3 eV and 68.5/68.5 eV can be attributed to Br⁻. The findings indicate that in comparison with C-BOB, the Bi 4f and Br 3d spectral peaks of Cu/WO₂/C-BOB are shifted to lower binding energies. Electrons from Cu/WO₂ are transferred to C-BOB after heterojunction building, thereby increasing the electron cloud density on the surface of C-BOB. The O 1s spectra of Cu/WO₂, C-BOB, and Cu/WO₂/C-BOB are shown in Fig. 4c, from which it can be concluded that the peaks at higher binding energies are attributable to surface hydroxyl groups and the peaks at lower binding energies are due to lattice oxygens.^46,47 Notably, the peaks at 531.6 eV for Cu/WO₂, 531.8 eV for C-BOB and 531.1 eV for Cu/WO₂/C-BOB are ascribed to the peaks at the oxygen vacancies.⁹ As can be seen from the specific values of the oxygen vacancy peak areas (Table S2†), with the formation of heterogeneous structures, the concentration of oxygen vacancies increased, increasing the active sites available for nitrogen adsorption, in line with the EPR findings. Fig. 4d displays the W 4f spectrograms and the peaks of Cu/WO₂ and Cu/WO₂/C-BOB located at 34.9/35.1 eV and 32.9/33.2 eV belong to the W⁴⁺ 4f_5/2 and W⁴⁺ 4f_7/2 peaks.⁴⁸ However, the unavoidable surface high valence oxidation state W⁶⁺ 4f_7/2 and W⁶⁺ 4f_5/2 peaks were also observed in the spectra. The W 4f spectra of Cu/WO₂/C-BOB were shifted towards higher binding energies compared to Cu/WO₂, further demonstrating that the electron transfer from Cu/WO₂ to C-BOB is responsible for the strong interaction involving Cu/WO₂ and C-BOB. As shown in Fig. 4e, the Cu 2p spectra showed two main peaks with the Cu 2p_1/2 peaks located at 952.6/952.7 eV and the Cu 2p_3/2 peaks located at 932.7/932.8 eV, which are assigned to Cu⁰/Cu⁺ species.⁴⁹ Similarly, weak peaks belonging to Cu²⁺ were found in the spectra.⁵⁰ We also obtained the Cu LMM X-ray-induced Auger peak spectra to characterize Cu⁰ and Cu⁺ (Fig. 4f), and the characteristic peaks of Cu⁰ and Cu⁺ are located at 569.9/570.2 eV and 572.4/572.5 eV, respectively. The remaining two peaks in the figure represent the transition state peaks of the Cu LMM spectra.⁵¹ It can be seen that most of the metallic copper in the heterogeneous structure still exists as Cu⁰ species. In summary, an internal electric field oriented from Cu/WO₂ to C-BOB is introduced by the directional migration of electrons at the Cu/WO₂/C-BOB heterojunction, as indicated by the binding energy changes of the respective peaks.

Fig. 4 (a) The XPS survey spectra of the samples. The high-resolution XPS spectra of (b) Bi 4f, (c) O 1s, (d) W 4f, (e) Cu 2p and (f) Cu LMM for the samples.

To investigate the BET specific surface area and pore size distribution of the photocatalysts, we executed N₂ adsorption–desorption isotherm tests. The adsorption–desorption isotherm of type IV exhibited a clear H3 hysteresis return line (Fig. S9a†). The specific surface areas of Cu/WO₂, C-BOB, and Cu/WO₂/C-BOB were 128.0, 10.4, and 20.12 m² g⁻¹, respectively. A clear hysteresis return line can be observed at P/P₀ = of 0.5–1.0, suggesting that the catalysts have mesopores, which is in agreement with the pore structure distribution map of Fig. S9b.† Compared with C-BOB, the specific surface area of Cu/WO₂ nanoparticles was increased after attaching them to its surface, which increased the number of nitrogen adsorption active sites and facilitated the photocatalytic ammonia production reaction.

In addition, to explore the adsorption of N₂ molecules on different photocatalysts, we implemented the N₂ temperature-programmed desorption (N₂-TPD) test. We can observe three desorption peaks of C-BOB and Cu/WO₂/C-BOB in Fig. S10,† corresponding to two physical adsorption peaks (<350 °C) and one chemical adsorption peak (450–550 °C), respectively. The figure clearly shows that the chemisorption peaks of Cu/WO₂/C-BOB showed a significant red-shift in the position and a significant increase in the peak intensity, indicating enhanced chemisorption of N₂. This is explained by the increased oxygen vacancies in the heterostructures, which facilitate a greater number of reaction sites where N₂ can be adsorbed. On the other hand, the weak adsorption of N₂ by Cu/WO₂ suggests that BiOBr is the main N₂ activation site.

Light absorption capacity and photoelectrochemical properties

We tested the UV-vis absorption spectra (DRS) of the photocatalysts to investigate their light absorption characteristics. As shown in Fig. 5a, pure BOB has a typical absorption edge around 420 nm.³⁶ Compared with pure BOB, WO₂/C-BOB exhibits higher light absorption and a subtle redshifting of the absorption edge is observed, which may be due to the doping of non-metallic carbon and the interaction at the WO₂ and C-BOB heterojunction interface. In contrast, Cu/WO₂/C-BOB possessed stronger light absorption than WO₂/C-BOB and showed a significant redshift; this was ascribed to Cu nanoparticles located on the catalyst surface, which increased the light absorption range by producing an intense LSPR effect.⁵² From Fig. 5b, the SPR characteristic peak of Cu nanoparticles is seen to have a clear absorption peak at approximately 580 nm in Cu/WO₂.⁵³ Using the Kubelka–Munk function, the band gap energies (E_g) of WO₂ and C-BOB were determined to be 1.65 eV and 2.75 eV, respectively (Fig. 5c). The valence band potentials of WO₂ and C-BOB were calculated to be 2.51 eV and 2.04 eV, by analyzing their valence band (VB)-XPS spectra (Fig. 5d). Based on the equation E_CB = E_VB − E_g, the conduction band point potentials were 0.86 eV and −0.71 eV. Non-metallic carbon doping resulted in a decrease in the bandgap width and an increase in the CB potential of C-BOB, as well as a notable improvement in photocatalytic N₂ fixing performance.

Fig. 5 (a and b) UV–vis DRS spectra, (c) Tauc plots and (d) XPS valence band spectra of the samples.

Photoluminescence (PL) spectra were examined to analyze the carrier separation and complexation efficiency of the samples (Fig. 6a). The intensity of the emission peaks of Cu/WO₂/C-BOB loaded with Cu/WO₂ nanoparticles was significantly decreased compared to that of C-BOB and WO₂/C-BOB. This is explained by the construction of the Z-scheme heterojunction and the hot electron injection produced by the LSPR effect, which prevented photogenerated electrons and holes from complexing and so enhanced the utilization of photocarriers in the N₂ photoreduction reaction.⁵⁴ The electrochemical impedance (EIS) plots in Fig. 6b demonstrate that Cu/WO₂/C-BOB has the greatest charge transfer efficiency due to its shortest arc radius, which effectively promotes the photocatalytic reaction. Fig. 6c depicts the transient photocurrent response curves under visible light irradiation (λ > 420 nm), and Cu/WO₂/C-BOB has the strongest photocurrent density, which suggests that the charge transfer efficiency can be greatly increased by building Z-scheme heterostructures. Moreover, the conduction band position of WO₂ can be injected with high-energy hot electrons generated by the LSPR effect of Cu NPs to enhance its electron density and promote the photocatalytic reaction. Transient photocurrent response tests under visible-near-infrared (vis-NIR) light (λ > 700 nm) irradiation (Fig. S11†) were conducted to demonstrate this claim, and it can be observed that the photocurrent undulation is barely observed for pure BOB, C-BOB, and WO₂/C-BOB, which is explained by the reality that the energy of the vis-NIR photons is lower than that of their of the photoinduced electrons. However, it is noteworthy that relatively distinct photocurrent profiles can be detected in Cu/WO₂ and Cu/WO₂/C-BOB, which is explained by the introduction of hot electrons from Cu NPs into the conduction band of WO₂. Fig. 6d shows the linear scanning voltammetry (LSV) plots of C-BOB and Cu/WO₂/C-BOB in saturated N₂ and saturated Ar electrolytes. Cu/WO₂/C-BOB in saturated N₂ electrolyte has a remarkably low overpotential, indicating that it has prominent N₂ reduction selectivity and photocatalytic nitrogen fixation activity. In summary, a series of photoelectrochemical experiments collectively demonstrated that the successful construction of the Z-scheme heterostructure and the LSPR effect of Cu nanoparticles synergistically promoted photocatalytic ammonia synthesis process.

Fig. 6 (a) PL spectra, (b) Nyquist plots, (c) transient photocurrent response and (d) LSV curves of different samples.

Photocatalytic N₂ fixing performance

The ability of the samples to be used for photocatalytic ammonia production was assessed in a natural environment using a presentational standard curve of NH₄⁺ concentration. With a 300 W xenon lamp serving as the light source, the photocatalytic tests were performed in purified water without the addition of any sacrificial agent. As shown in Fig. 7a and S12,† Cu/WO₂/C-BOB demonstrated the best nitrogen fixation capability with an ammonium root production of 477.5 μmol g⁻¹ h⁻¹, significantly greater than that of other single-component catalysts and binary photocatalysts. In addition, as can be seen from Fig. S13a,† we detected the ammonia yield of Cu/WO₂/C-BOB using ion chromatography, and the test result was 460.9 μmol g⁻¹ h⁻¹. This is in agreement with the test result of the Nessler reagent colorimetric method, which proves the reliability of the test result. We also detected the content of O₂, a by-product generated during photocatalytic nitrogen fixation, and the test results showed that water, as the main source of protons in the reaction process, inevitably produces a certain amount of O₂. We conducted photocatalytic nitrogen fixation experiments under monochromatic light irradiation at 420 nm and obtained the apparent quantum efficiency (AQE) of the different samples (Fig. S13b†). Cu/WO₂/C-BOB has a better photo-utilization rate with an apparent quantum efficiency of 0.66%. Compared with the reported bismuth-based nitrogen fixation photocatalysts (Table S3†), Cu/WO₂/C-BOB can achieve extremely superior nitrogen fixation efficiency without any sacrificial reagent addition. The LSPR effect of Cu nanoparticles and the design of Z-scheme heterostructures were attributed for the exceptional photocatalytic activity of Cu/WO₂/C-BOB. Additionally, Cu/WO₂/C-BOB exhibits good stability. After five cycling tests (Fig. 7b), the photocatalytic activity of Cu/WO₂/C-BOB could still reach 95.2% of the original efficiency. Meanwhile, the SEM, TEM and XRD images of the samples after the cycling tests showed (Fig. S13 and S14†) that the morphology and crystal structure of the samples before and after the reaction remained essentially unchanged, which indicated that the Cu/WO₂ particles were anchored on C-BOB to form an efficient and stable photocatalyst. We conducted comparative experiments of photocatalytic nitrogen fixation under catalyst-free, dark and Ar environment (Fig. 7c) conditions, and the presence of ammonia was almost undetectable, suggesting that the ammonia produced by the test came from the photocatalyst-immobilized nitrogen. Meanwhile, the common by-product hydrazine was detected in solution after the photocatalytic ammonia synthesis using the Watt and Chrisp method,⁵⁵ as shown in Fig. 7d, and the content of the by-product was negligible, indicating that the samples had a good selectivity for ammonia throughout the process of photocatalysis.

Fig. 7 (a) NH₄⁺ yield of nitrogen-fixing photocatalysts. (b) Cycling experiment of Cu/WO₂/C-BOB. (c) NH₄⁺ yield of Cu/WO₂/C-BOB under various reaction conditions. (d) N₂H₄ yield of the samples.

We determined the water contact angle of samples to ascertain their hydrophilicity to examine the kinetic parameters influencing the photocatalytic nitrogen fixation reaction (Fig. S15†). The water contact angle of C-BOB was 58.31°, and the water contact angle was significantly decreased after surface loading of Cu/WO₂ (52.87°). It could have resulted from the increase in the concentration of surface-defective oxygen vacancies in the heterogeneous structure that makes its hydrophilicity stronger. This change will make Cu/WO₂/C-BOB fully contact with N₂ in water, which will produce more H protons and favor the conversion of nitrogen to ammonia.

Potential photocatalytic nitrogen fixation reaction mechanism

From the energy band arrangement of WO₂ and C-BOB, it can be seen that the two can form type-II or Z-scheme heterostructures upon contact. To ascertain the precise pathways of charge movement in the WO₂/C-BOB heterostructure, we detected the ˙OH and ˙O₂⁻ species in WO₂/C-BOB using ESR experiments with DMPO as a trapping agent (Fig. 8a). The WO₂/C-BOB system detected distinct ESR signals for DMPO-˙OH and DMPO-˙O₂⁻ adducts. On the one hand, the E_VB value of C-BOB could not satisfy the conditions of oxidation of water to ˙OH since the typical reduction potential of H₂O/˙OH is 2.33 eV. On the other hand, the photogenerated electrons in the conduction band of WO₂ are unable to combine with O₂ to make O₂⁻, as evidenced by its E_CB value of 0.86 eV, which is greater than the typical redox potential of O₂/˙O₂⁻. The above analysis indicated that the migration of photogenerated carriers in WO₂/C-BOB could not follow the conventional type-II heterojunction mechanism; therefore, an efficient direct Z-scheme charge migration pathway was proposed to be applied to the study of improved photocatalytic nitrogen fixation on the heterostructure of WO₂/C-BOB. Combined with the UPS test (Fig. 8b), the values of the work function for WO₂ and C-BOB were obtained by the formula Φ = hν − E_cut-off. The comparison of the two work functions further demonstrated the migration pathway of the photogenerated carriers in the Z-scheme (Fig. 8c), which correlates with the XPS results. When WO₂ and C-BOB are in close contact, the electrons in the C-BOB semiconductor with a higher Fermi energy level spontaneously flow towards the WO₂ semiconductor with a lower Fermi energy level, and the WO₂ energy band at the interface bends downward due to the large amount of electrons gained, while the C-BOB loses electrons resulting in the bending of band edges upward until the equilibrium of Fermi energy levels is reached for the C-BOB and WO₂. Between C-BOB and WO₂, which have opposing positive and negative charges, an inherent electric field is created. The holes in the valence band of C-BOB and the electrons in the conduction band of WO₂ are complexed at the heterogeneous interface during the photocatalytic process, driven by the inherent electric field, coulombic interaction, and energy band bending. Concurrently, the photocatalytic nitrogen fixation reaction is efficiently aided by the holes with strong oxidizing ability that are retained on the E_VB level of WO₂ and the electrons with strong reducing capacity that are on the E_CB level of C-BOB.

Fig. 8 (a) ESR spectra of DMPO-˙O₂⁻ and DMPO-˙OH for WO₂/C-BOB. (b) UPS spectra of WO₂ and C-BOB. (c) Charge transfer paths of the Z-scheme heterojunctions.

By combining the aforementioned analyses, we suggest a potential photocatalytic nitrogen fixation reaction mechanism (Fig. 9) and investigate the following points as potential causes for the strong photocatalytic activity of Cu/WO₂/C-BOB. (1) When exposed to light, electrons in the valence bands of WO₂ and C-BOB migrated to their conduction bands, leaving holes in the valence bands. While driven by IEF, the electrons in the conduction bands of WO₂ interacted with the holes in the valence bands of C-BOB. As a result, the redox interaction between nitrogen and water is facilitated by a significant accumulation of photogenerated electrons and holes in the valence band of WO₂ and the conduction band of C-BOB. The charge transfer pathway of the Z-scheme greatly improves the ability of Cu/WO₂/C-BOB heterojunction to fix nitrogen while maintaining the redox characteristics of the semiconductor to the fullest. It also successfully increases the rate at which photogenerated carriers separate. (2) The Cu/WO₂/C-BOB heterojunction has a lot of oxygen vacancies. On the one hand, by trapping electrons, the defect energy levels caused by oxygen vacancies can effectively prevent the recombination of photogenerated carriers and significantly preserve the oxidation ability of the holes; on the other hand, oxygen vacancies in C-BOB can act as centers for nitrogen adsorption activation, which effectively promotes the process of photocatalytic reaction. (3) The LSPR effect that Cu NPs produced improved the use of visible light and the photocatalytic activity of the ternary heterojunction of Cu/WO₂/C-BOB was increased by introducing energetic hot electrons produced by light into the WO₂ conduction band and complexing with holes in the C-BOB valence band.

Fig. 9 Mechanism of photocatalytic nitrogen fixation over Cu/WO₂/C-BOB plasma Z-scheme heterojunction photocatalysts.

DFT calculations

Finally, the LSPR effect of Cu nanoparticles and the construction of Z-scheme heterojunctions on the mechanism of nitrogen fixation were further investigated using DFT calculations. To get a deeper understanding of the complex six-electron coupling and proton transfer processes during the N₂ fixing reaction, we optimized the structures of the C-BOB and Cu/WO₂/C-BOB intermediates for the nitrogen fixation process (Fig. S15 and S16†) and calculated the corresponding Gibbs free energies. Since no hydrazine was detected during the reaction, we concluded that the nitrogen fixation process followed a distal mechanism rather than an alternating mechanism (Fig. 10a). A contrasting trend can be seen from the Gibbs free energy spectra of the NRR on C-BOB and Cu/WO₂/C-BOB (Fig. 10b), where the rate-determining steps (RDS) during the C-BOB and Cu/WO₂/C-BOB reactions are *N₂ + H⁺ + e⁻ → *NNH and *N + H⁺ + e⁻ → *NH, respectively. The required reaction energy barriers are 1.91 eV and 1.26 eV. In addition, the hydrogenation of N₂ molecules to produce *N₂H is usually regarded as a key intermediate in the NRR, and photocatalysts with efficient nitrogen-fixing properties should contribute to the generation of *N₂H.⁷ The hydrogenation reaction energy barrier of *N₂ on Cu/WO₂/C-BOB is 0.88 eV, which is much lower than that of C-BOB. DFT calculations show that the Z-scheme heterojunctions and the LSPR impact of Cu nanoparticles in the Cu/WO₂/C-BOB composites efficiently lower the energy barrier for the nitrogen-fixation reaction and speed up the photocatalytic reaction process.

Fig. 10 (a) Reaction process diagram of the complete reaction of Cu/WO₂/C-BOB. (b) Free energy diagrams of the NRR over C-BOB and Cu/WO₂/C-BOB based on DFT calculations.

Conclusion

In this work, novel Cu/WO₂/C-BOB plasma Z-scheme heterojunctions were successfully synthesized for photocatalytic ammonia synthesis by calcination and hydrothermal methods. In pristine water without any sacrificial reagent present, Cu/WO₂/C-BOB exhibited an ammonia production rate of 477.5 μmol g⁻¹ h⁻¹, which was significantly greater than that of single semiconductors. The following factors are responsible for the increased catalytic performance. First, carbon doping shortens the bandgap, modifies the energy band structure of BOB and promotes light absorption. In addition, enhanced oxygen vacancies in the structure promote the photocatalytic reaction process as electron traps and active sites for the reaction. Ultimately, the separation and transfer of photogenerated carriers were greatly facilitated by the LSPR effect and the synergistic electron transfer mechanism of the Z-scheme, enhancing the photocatalytic activity. This paper offers fresh perspectives on the investigation of innovative plasma Z-scheme heterojunction photocatalysts.

Author contributions

Xue Yang: writing – original draft, investigation, formal analysis, and data curation. Donghui Cui: investigation. Tingting Zhang: investigation. Yu Liu: investigation. Fengyan Li: resources, project administration, supervision, and writing – review & editing.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

No conflicting financial interests were stated by the authors.

Acknowledgements

This work was financially supported by the Natural Science Foundation of China (Grant No. 22071018) and the Natural Science Foundation of Jilin Province (No. 20220101069JC).

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Footnote

† Electronic supplementary information (ESI) available: Additional XRD, SEM, EDS and elemental mapping figures. See DOI: https://doi.org/10.1039/d4qi02128b

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