Photofixation of atmospheric nitrogen to ammonia with a novel ternary metal sulfide catalyst under visible light

Abstract

A novel ternary metal sulfide photocatalyst Mo_0.1Ni_0.1Cd_0.8S was prepared for photofixation of atmospheric nitrogen to ammonia under visible light. Characterization results indicate that the obtained catalyst is Mo and Ni co-doped CdS with many sulfur vacancies, not a mixture of MoS₂, NiS and CdS. The sulfur vacancies on Mo_0.1Ni_0.1Cd_0.8S not only serve as active sites to adsorb and activate N₂ molecules but also promote interfacial charge transfer from Mo_0.1Ni_0.1Cd_0.8S to N₂ molecules, thus significantly improving the nitrogen photofixation ability. The concentration of sulfur vacancies plays a significantly important role in the N₂ photofixation ability. A possible mechanism was proposed.

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Introduction

Nitrogen is an essential building element of plants and animals. Although molecular nitrogen (N₂) as the ultimate source of the element N is the major component of the atmosphere on earth, most organisms are unable to metabolize N₂ for the synthesis of biomolecules like proteins and nuclei acids. Therefore, nitrogen fixation is the second most important chemical process in nature, next to photosynthesis, the process through which atmospheric N₂ is transformed to its fully reduced form of ammonia that living organisms can use. Thermodynamically, N₂ fixation is perfectly accessible (ΔH_298K = −92.22 kJ mol⁻¹), but it does not happen spontaneously under ambient conditions. This phenomenon can partly be explained by the stubborn triple bond of N₂ toward dissociation (944 kJ mol⁻¹), but it is more related to the kinetic inertia of N₂.¹

Artificial nitrogen fixation is carried out through the Haber–Bosch process, ranking as one of the most important technological inventions in the 20^th century. During this process, hydrogen gas reacts with nitrogen gas to yield ammonia in the presence of catalysts under high pressure and temperature. Rather than directly reacting with H₂ up against unsurmountable kinetic limitations, N₂ is chemisorbed on the Fe-based catalyst to generate reactive surface-bound nitrides with their subsequent protonation processes being both thermodynamically and kinetically promoted.² However, both the energy consumption and raw material costs are high for this process. Therefore, because of the reduced input energy during the fixation process and no use of hydrogen gas, artificial nitrogen fixation under milder conditions is of considerable significance from the perspectives of cost and environmental protection.

In the past decades, photocatalytic nitrogen fixation technology has become the best alternative to traditional industrial nitrogen fixation techniques due to its advantages of green cleaning, mild conditions, low power consumption, and low cost, among others. In 1977, Schrauzer et al. first reported that N₂ can be reduced to NH₃ over Fe-doped TiO₂ under UV light.³ The reaction is as follows:

2N₂ + 6H₂O → 4NH₃ + 3O₂

Since then, many novel nitrogen-photofixation systems are reported successively.^4–12 N₂ photofixation can be conducted via successive self-excited electron and water-derived proton transfer. However, the nitrogen fixation efficiency is poor due to the absence of catalytic centers to tune the electronic states of adsorbed N₂ with the thermodynamic driving force only being controlled by electrons.¹¹

More recently, Li et al. reported the photofixation of N₂ on BiOBr nanosheets.¹³ They discovered that the introduction of oxygen vacancies to mimic the catalytic centers of Fe-based catalyst for N₂ adsorption could activate N₂ and promote interfacial electron transfer, thus significantly improving the nitrogen photofixation ability.¹³ We hypothesize that sulfur vacancies in metal sulfide semiconductors may have a similar effect on nitrogen photofixation because oxygen and sulfur have the similar chemical properties. CdS has long been one of the most attractive visible light active photocatalysts due to its efficient light absorption and suitable band edge position.^14–16 However, it is hard to be reduced to form the sulfur vacancies. Thus, in this work, the Mo and Ni codoped CdS was prepared. The doping of Mo and Ni causes the crystal lattice distortions, thus leading to the formation of sulfur vacancies in as-prepared ternary metal sulfide. The effects of sulfur vacancies on the properties and N₂ photofixation ability of as-prepared ternary metal sulfide are discussed.

Experimental

Preparation and characterization

Desired amounts of ammonium molybdate, nickel nitrate and cadmium acetate (Mo : Ni : Cd molar ratio = 1 : 1 : 8) were dissolved in deionized water to form a solution. A stoichiometric amount of Na₂S (0.5 M) was added dropwise to the solution, which was then stirred for 12 h at room temperature. The suspension was poured into a 100 mL Teflon-lined container, sealed in a stainless steel autoclave and maintained at 160 °C for 16 h. The solids were washed several times and dried at 80 °C. The obtained products were denoted as Mo_0.1Ni_0.1Cd_0.8S. In order to investigate the effect of sulfur vacancies, the as-prepared Mo_0.1Ni_0.1Cd_0.8S was calcined at 300 °C for 2 h in saturated O₂ atmosphere and denoted as Mo_0.1Ni_0.1Cd_0.8SO. When the Mo : Ni : Cd molar ratios are 0.5 : 0.5 : 9, 2 : 2 : 6 and 3 : 3 : 4 following the same procedure mentioned above, the obtained catalysts were denoted as Mo_0.05Ni_0.05Cd_0.9S, Mo_0.2Ni_0.2Cd_0.6S and Mo_0.3Ni_0.3Cd_0.4S. In order to investigate the effect of sulfur vacancy concentration on nitrogen photofixation ability, CdS was prepared using the same method mentioned above in the absence of ammonium molybdate and nickel nitrate. The single Mo or Ni doped CdS was also prepared using the same method and metal molar ratio.

The XRD patterns of the prepared samples were recorded on a Rigaku D/max-2400 instrument using Cu-Kα radiation (λ = 1.54 Å). The scan rate, step size, voltage and current were 0.05° min⁻¹, 0.01°, 40 kV and 30 mA, respectively. UV-Vis spectroscopy was carried out on a JASCO V-550 model UV-Vis spectrophotometer using BaSO₄ as the reflectance sample. The morphologies of prepared catalyst were observed by using a scanning electron microscope (SEM, JSM 5600LV, JEOL Ltd.). TEM images were taken on a Philips Tecnai G220 model microscope. Nitrogen adsorption was measured at −196 °C on a Micromeritics 2010 analyser. All the samples were degassed at 393 K prior to the measurement. ICP was performed on a Perkin-Elmer Optima 3300DV apparatus. Temperature programmed desorption (TPD) studies were performed using a CHEMBET-3000 (Quantachrome, U.S.A.) instrument. The photoluminescence (PL) spectra were measured at room temperature with a fluorospectrophotometer (FP-6300) using a Xe lamp as the excitation source. Electron paramagnetic resonance (EPR) spectrum was monitored using a digital X-band spectrometer (EMX-220, Bruker, USA) equipped with a Bruker ER 4121VT temperature controller within the temperature range 113–273 K. The Mott–Schottky plots were obtained using an electrochemical analyzer (LK2006A, Lanlike) using a three-electrode cell. The photocurrents were measured using an electrochemical analyzer (CHI 618C Instruments) equipped with a rectangular-shaped quartz reactor (20 × 40 × 50 mm) using a standard three-electrode system. The prepared sample film was used as the working electrode, a Pt flake was used as the counter electrode, and Ag/AgCl was used as the reference electrode. A 500 W Xe lamp was used to irradiate the working electrode from the back side. The light intensity on the working electrode was 120 mW cm⁻². In addition, a mechanical shutter was used to minimize the exposure of the sample to light. A 1.0 M Na₂SO₄ solution was used as the electrolyte. The applied potential was 0.00 V vs. Ag/AgCl. All the measurements were performed at room temperature (298 K).

Photocatalytic reaction

The nitrogen photofixation property was evaluated according to previous literature.⁸ The photoreactor setup is shown in Fig. 1. The nitrogen photofixation experiments were performed in a double-walled quartz reactor in air. For these experiments, 0.2 g of photocatalyst was added to a 500 mL solution of 0.789 g L⁻¹ ethanol as a hole scavenger.⁸ The suspension was dispersed using an ultrasonicator for 10 min. During the photoreaction under visible light irradiation, the suspension was exposed to a 250 W high-pressure sodium lamp with main emission in the range of 400 to 800 nm, and N₂ was bubbled at 100 mL min⁻¹ through the solution. The UV light portion of the sodium lamp was filtered by a 0.5 M NaNO₂ solution. All runs were conducted at ambient pressure and 30 °C. At given time intervals, 5 mL aliquots of the suspension were collected and immediately centrifuged to separate the liquid samples from the solid catalyst. The concentration of ammonia was measured using the Nessler's reagent spectrophotometry method (JB7478-87) with a UV-2450 spectrophotometer (Shimadzu, Japan).^8,13

Fig. 1 The photoreactor setup for N₂ photofixation.

Results and discussion

Fig. 2a shows the nitrogen photofixation performance over the as-prepared catalysts under visible light. The results of the control experiment indicate that the NH₄⁺ generation rate can be ignored in the absence of irradiation, N₂ or photocatalyst, indicating that nitrogen photofixation occurs via a photocatalytic process. The as-prepared ternary metal sulfides exhibit outstanding nitrogen photofixation ability. The NH₄⁺ generation rates are 1.2, 3.2, 2.4 and 2.0 mg L⁻¹ h⁻¹ g_cat⁻¹ for Mo_0.05Ni_0.05Cd_0.9S, Mo_0.1Ni_0.1Cd_0.8S, Mo_0.2Ni_0.2Cd_0.6S and Mo_0.3Ni_0.3Cd_0.4S, respectively. Mo_0.1Ni_0.1Cd_0.8S shows the highest NH₄⁺ generation rate. The Fig. 1 insert shows the photocatalytic stabilities of Mo_0.1Ni_0.1Cd_0.8S. No obvious decrease in nitrogen photofixation ability is observed after 20 h, hinting its good stability. For Mo_0.1Ni_0.1Cd_0.8SO, the NH₄⁺ generation rate sharply decreases to 0.31 mg L⁻¹ h⁻¹ g_cat⁻¹, only one tenth of that of Mo_0.1Ni_0.1Cd_0.8S. Using aprotic solvents (DMF and DMSO) instead of water shows that no NH₄⁺ is generated, confirming the necessity of H₂O as the proton source for the nitrogen photofixation (not shown here). The Mo, Ni, Cd and S concentrations of Mo_0.1Ni_0.1Cd_0.8S obtained by ICP are 7.6, 5.1, 63.9 and 23.4 wt%, close to the theoretical values. The actual atomic ratio is Mo_0.12Ni_0.13Cd_0.86S_1.1 for the as-prepared ternary metal sulfide. Under this metal ratio, the number of sulfur atoms should be 1.23. Thus it is deduced that many sulfur vacancies are formed in ternary metal sulfide. After 20 h reaction, the Mo, Ni, Cd and S concentrations of reused Mo_0.1Ni_0.1Cd_0.8S obtained by ICP are 7.7, 5.0, 63.7 and 23.6 wt%, close to fresh catalyst. For Mo_0.1Ni_0.1Cd_0.8SO, the actual atomic ratio is Mo_0.12Ni_0.12Cd_0.87S_1.1O_0.12 obtained by ICP, hinting the sulfur vacancies are oxidized during calcination process. Thus it is proposed that sulfur vacancies may play important role on nitrogen photofixation process. In order to further investigate the nitrogen source of NH₄⁺, the N₂ photofixation under ¹⁵N isotope-labeled N₂ (purity > 98%) was performed. The produced ¹⁵NH₄⁺ reacts with phenolic and hypochlorite to form ¹⁵N labeled indophenol, which was analysed by LC-MS. A strong ¹⁵N labeled indophenol anion mass spectroscopy signal presents at 199 m/z in LC-MS studies (Fig. 2b). It is noted that the intensity of this signal is obviously higher than that of the ¹⁴N : ¹⁵N natural abundance ratio. This observation further confirms that N₂ is the source of generated ammonium ion in this N₂ photofixation process.

Fig. 2 Nitrogen photofixation performance over as-prepared catalysts under visible light (a) and the mass spectra of the indophenol prepared from different atmosphere (b).

The XRD patterns of the as-prepared catalysts are shown in Fig. 3a. Three diffraction peaks located at 26.8, 44.2 and 52.3° are observed for Mo_0.1Ni_0.1Cd_0.8S. It is known that the diffraction peaks of cubic phase CdS are located at 26.6, 43.9 and 52°, which are very close to those of the as-prepared Mo_0.1Ni_0.1Cd_0.8S.¹⁷ In addition, no diffraction peaks corresponding to MoS₂ and NiS are observed.^18,19 Thus it is deduced that Mo and Ni were doped into the crystal lattice of CdS to form the ternary metal sulfide but not the mixture of MoS₂, NiS and CdS. The doping of Mo and Ni causes the crystal lattice distortions, thus leading to the formation of sulfur vacancies in as-prepared ternary metal sulfide. This is consistent with the ICP results. No obvious difference between Mo_0.1Ni_0.1Cd_0.8S and Mo_0.1Ni_0.1Cd_0.8SO is shown, probably due to the low oxygen content. EPR can provide direct information on monitoring various behaviors of native defects, such as oxygen and nitrogen vacancies.^20,21 As shown in Fig. 3b, Mo_0.1Ni_0.1Cd_0.8SO shows no peaks, suggesting that no localized unpaired electrons present. However, for Mo_0.1Ni_0.1Cd_0.8S, a resonance signal at g = 2.003 is observed, which should be due to the presence of sulfur vacancies. To confirm this resonance signal, the sulfur vacancy doped CdS was prepared by H₂ reduction method (500 °C for 2 h, H₂ : Ar = 1 : 9). The obtained catalyst is denoted as R-CdS. We test the EPR of CdS and R-CdS (not shown here). CdS shows no resonance signal, whereas R-CdS exhibits a resonance signal at g = 2.003. This signal should be attributed to the sulfur vacancies which formed by the H₂ reduction. This result confirms the resonance signal at g = 2.003 is assigned to the sulfur vacancies.

Fig. 3 The XRD patterns (a), EPR spectra (b), UV-Vis spectra (c) and VB XPS of Mo_0.1Ni_0.1Cd_0.8S and Mo_0.1Ni_0.1Cd_0.8SO (d).

Fig. 3c compares the UV-Vis spectra of the as-prepared Mo_0.1Ni_0.1Cd_0.8S and Mo_0.1Ni_0.1Cd_0.8SO. Both catalysts show typical semiconductor absorption. The band gap energy calculated based on the method of Oregan and Gratzel indicates that Mo_0.1Ni_0.1Cd_0.8S has an absorption edge at about 599 nm, corresponding to the band gap of 2.07 eV.²² It is known that the band gaps of MoS₂, NiS and CdS are approximately 1.8, 2.1 and 2.4 eV, respectively.^17,23,24 The absorption edge of Mo_0.1Ni_0.1Cd_0.8S is located among three metal sulfides, which confirms the Mo and Ni are doped into CdS crystal lattice. For Mo_0.1Ni_0.1Cd_0.8SO, the absorption edge shifts to 560 nm, corresponding to the band gap of 2.2 eV. This is probably due to the oxygen doping, which causes changes in the electronic structure and optical properties of the ternary metal sulfide. It is noted that the absorption tail in the whole visible light region is observed in the spectrum of Mo_0.1Ni_0.1Cd_0.8S, confirming the presence of sulfur vacancies. After calcination in saturated O₂ atmosphere, the absorption tail disappears for Mo_0.1Ni_0.1Cd_0.8SO due to the oxidation of sulfur vacancies during calcination process.

In Fig. 3d, the VB positions of Mo_0.1Ni_0.1Cd_0.8S and Mo_0.1Ni_0.1Cd_0.8SO are +1.0 and +0.58 eV. It is obtained from the UV-Vis results that the band gaps for Mo_0.1Ni_0.1Cd_0.8S and Mo_0.1Ni_0.1Cd_0.8SO are 2.07 and 2.2 eV. Thus the E_CB for Mo_0.1Ni_0.1Cd_0.8S and Mo_0.1Ni_0.1Cd_0.8SO is −1.07 and −1.62 eV, respectively. It is reported that the standard redox potential for N₂/NH₃ = −0.0922 V against NHE.⁶ The reduction potential of CB electrons in as-prepared catalysts is more negative than the redox potential for N₂/NH₃. Thus the CB electrons can reduce the N₂ molecule and form the NH₃.

The morphologies of the representative samples were examined using SEM analysis. Fig. 4a shows that the as-prepared Mo_0.1Ni_0.1Cd_0.8S is composed of uniform spherical nanoparticles with a diameter of 30–50 nm. Calcination in saturated O₂ atmosphere does not change the morphology (Fig. 4b). The typical HRTEM image of Mo_0.1Ni_0.1Cd_0.8S exhibits the clear lattice fringe (Fig. 4c). The measured lattice spacing is 0.330 nm, very close to the (111) crystal face of cubic phase CdS.¹⁷ The elemental mapping has been conducted to investigate the element distribution in Mo_0.1Ni_0.1Cd_0.8S (Fig. 4d). The result indicates that all the elements are homogenously distributed in the whole sample. It is concluded from above results that calcination of Mo_0.1Ni_0.1Cd_0.8S in saturated O₂ atmosphere does not change the crystal structure and morphology but oxidizes the sulfur vacancies to form Mo_0.1Ni_0.1Cd_0.8SO.

Fig. 4 SEM images of Mo_0.1Ni_0.1Cd_0.8S (a), Mo_0.1Ni_0.1Cd_0.8SO (b), HRTEM image of Mo_0.1Ni_0.1Cd_0.8S (c) and elemental mapping images of Mo_0.1Ni_0.1Cd_0.8S (d).

TPD is a method to determine the state of the gas adsorbed on the catalyst surface. Under the program heating, the gas molecules beforehand adsorbed on the catalyst surface are desorbed at a certain temperature. The peak area and desorbed temperature of TPD curve stands for the adsorption capacity and adsorption strength of the gas, respectively. Because the chemical adsorption sites are considered to be reaction centers capable of activating N₂, chemisorption is an essential step in photocatalytic N₂ fixation. TPD investigations were performed to understand N₂ chemisorption on the surface of the Mo_0.1Ni_0.1Cd_0.8S and Mo_0.1Ni_0.1Cd_0.8SO (Fig. 5). Two adsorbed N₂ species in Mo_0.1Ni_0.1Cd_0.8S and only one adsorbed N₂ species in Mo_0.1Ni_0.1Cd_0.8SO are observed. The peak for both catalysts at ∼140 °C is related to physical adsorption. The peak at ∼270 °C, which is related to the strong chemisorption species of N₂, is observed for Mo_0.1Ni_0.1Cd_0.8S but not for Mo_0.1Ni_0.1Cd_0.8SO. This result indicates that nitrogen vacancies could introduce many chemical adsorption sites on the surface of Mo_0.1Ni_0.1Cd_0.8S. Because chemisorption is generally associated with activation, these chemical adsorption sites will activate N₂ for nitrogen photofixation.

Fig. 5 The N₂-TPD of Mo_0.1Ni_0.1Cd_0.8S and Mo_0.1Ni_0.1Cd_0.8SO.

Fig. 6a shows the PL spectra of Mo_0.1Ni_0.1Cd_0.8S and Mo_0.1Ni_0.1Cd_0.8SO. The broad PL bands are located at approximately 600 nm under N₂ atmospheres, which corresponds to the band–band PL phenomenon with the energy of light approximately equal to the band gap. The PL emission of Mo_0.1Ni_0.1Cd_0.8S is broader than that of Mo_0.1Ni_0.1Cd_0.8SO, which should be ascribed to the presence of S vacancies. Fig. 6b compares the PL intensity of Mo_0.1Ni_0.1Cd_0.8S and Mo_0.1Ni_0.1Cd_0.8SO under Ar and N₂ atmospheres. The emission peak of Mo_0.1Ni_0.1Cd_0.8S is much weaker than that of Mo_0.1Ni_0.1Cd_0.8SO under both atmospheres. In general, at a lower PL intensity, the separation rate of the photogenerated electron–hole pairs is higher.^25,26 This indicates that sulfur vacancies can act as electron trappers to improve carrier separation. Under N₂ atmosphere, the PL intensity of Mo_0.1Ni_0.1Cd_0.8S has been sharply reduced compared with that under an Ar atmosphere. By contrast, no difference is observed for Mo_0.1Ni_0.1Cd_0.8SO between the PL spectra under N₂ and Ar atmospheres. This hints that sulfur vacancies may promote photogenerated electron transfer from as-prepared ternary metal sulfide to adsorbed N₂. Thus, the possible nitrogen activation process over Mo_0.1Ni_0.1Cd_0.8S is as follows. First of all, N₂ molecules are chemisorbed on the sulfur vacancies of ternary metal sulfide. When Mo_0.1Ni_0.1Cd_0.8S is excitated by the irradiation, those photogenerated-electrons are transferred immediately from the catalyst to the adsorbed N₂. Because the bonding orbitals of N₂ molecule are occupied by four electrons, this photogenerated-electron has to occupy the anti-bonding orbitals, leading to the nitrogen activation.

Fig. 6 The PL spectra of Mo_0.1Ni_0.1Cd_0.8S and Mo_0.1Ni_0.1Cd_0.8SO under N₂ atmospheres (a) and the comparison of PL intensity of Mo_0.1Ni_0.1Cd_0.8S and Mo_0.1Ni_0.1Cd_0.8SO under Ar and N₂ atmospheres (b).

Considering the sulfur vacancies are regarded as the active centers for nitrogen photofixation, the NH₄⁺ generation rate should be highly dependent on the sulfur vacancies concentration. The ICP results indicate that the actual Cd/S atomic ratio for CdS is 1 : 1. The actual formula for as-prepared single Mo or Ni doped CdS are Mo_0.12Cd_0.92S_1.08 and Ni_0.13Cd_0.87S_0.95 respectively. The difference between the theoretical and actual number of sulfur atoms represents the concentration of sulfur vacancies in ternary metal sulfide. Fig. 7 shows the nitrogen photofixation performance of the as-prepared catalysts as a function of the sulfur vacancies concentration. As expected, the NH₄⁺ generation rate over as-prepared catalysts are linearly related to the sulfur vacancies concentration (R = 0.9754), confirming the concentration of sulfur vacancies plays the significantly important role on the N₂ photofixation ability. The role of metal doping seems to create sulfur vacancies rather than promote N₂ photofixation ability.

Fig. 7 Nitrogen photofixation performance of the as-prepared catalysts as a function of the sulfur vacancies concentration.

Fig. 8 shows the photocurrent densities of Mo_0.1Ni_0.1Cd_0.8S and Mo_0.1Ni_0.1Cd_0.8SO under both N₂ and Ar atmospheres. The photocurrent of Mo_0.1Ni_0.1Cd_0.8S is much higher than that of Mo_0.1Ni_0.1Cd_0.8SO under both atmospheres, confirming that sulfur vacancies can trap the electrons to promote the separation rate of electrons–holes. Note that, under N₂ atmosphere, the photocurrent generated on Mo_0.1Ni_0.1Cd_0.8SO electrode remains unchanged with irradiation time. However, the photocurrent density of Mo_0.1Ni_0.1Cd_0.8S gradually decreases at the beginning and then remains stable. This photocurrent decay is probably due to the competition between N₂ and FTO glass for trapped electrons. Li and his co-workers reported that oxygen vacancies can trap electrons and promote interfacial charge transfer from BiOBr nanosheets to N₂.¹³ We hypothesize that sulfur vacancies have a similar effect because they are the congeners. The photogenerated electrons that arrived at the surface of Mo_0.1Ni_0.1Cd_0.8S are trapped by the sulfur vacancies, and then transferred immediately from the catalysts to the adsorbed N₂, causing the photocurrent decay. Under Ar atmosphere, the photocurrent generated on Mo_0.1Ni_0.1Cd_0.8SO is almost the same as that under N₂ atmosphere. However, the photocurrent of Mo_0.1Ni_0.1Cd_0.8S does not decay. This result confirms the fast electron transfer process from the Mo_0.1Ni_0.1Cd_0.8S to the adsorbed N₂.

Fig. 8 Photocurrent densities of Mo_0.1Ni_0.1Cd_0.8S and Mo_0.1Ni_0.1Cd_0.8SO under both N₂ and Ar atmospheres.

Conclusions

A novel ternary metal sulfide photocatalyst Mo_0.1Ni_0.1Cd_0.8S with outstanding nitrogen photofixation ability under visible light was prepared. Characterization results indicate that the actual atomic ratio of as-prepared ternary metal sulfide is Zn : Sn : Cd : S of 0.12 : 0.13 : 0.86 : 1.1, not a mixture of MoS₂, NiS and CdS. The sulfur vacancies on Mo_0.1Ni_0.1Cd_0.8S not only serve as active sites to adsorb and activate N₂ molecules but also promote interfacial charge transfer from Mo_0.1Ni_0.1Cd_0.8S to N₂ molecules, thus significantly improving the nitrogen photofixation ability. The concentration of sulfur vacancies plays the significantly important role on the N₂ photofixation ability.

Acknowledgements

This work was supported by National Key Technology R & D Programme of China (no. 2007BAC16B07, 2012ZX07505-001), Education Department of Liaoning Province (no. L2014145), Environmental Science and Engineering Innovation Team of Liaoning Shihua University ([2014]-11), and Students' Innovation Fund Project of China.

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