English

• 0.   Introduction

  Ammonia plays a unique role in the agricultural production, biomedicine, electronic technology, energy and chemical industries^[1-2]. Meanwhile, ammonia pos-sesses the advantages of high hydrogen content (mass fraction of 17.6%) and energy density^[3-5], which is also considered as an ideal hydrogen storage material and can be used as a fuel source for fuel cells. The demand for ammonia is rising with the social development. Tra-ditionally, the N₂ fixation process requires both H₂ and other harsh conditions^[4-6], while H₂ is mainly obtained from the decomposition of fossil fuels, leading to a large energy consumption and release of greenhouse gases^[7]. Thus, it is necessary to develop a green, cheap route to synthesize ammonia.

  Recently, much attention has been paid onto pho-tocatalytic N ₂ fixation, which is a cheap, green route for synthesizing the ammonia. The triple bond of N₂ owns a large bond energy of 941 kJ·mol^-1[8], thus the N ₂ reduction is much more difficult than water splitting into H₂. As is well known to all, TiO₂ is one of the earliest photocatalysts for N₂ fixation. Early in 1977, Schrauzer et al. discovered that NH ₃ could be produced by using a Fe - doped TiO₂ NPs photocatalyst under ultraviolet (UV) illumination^[9].

  So far, a series of photocatalysts have been demonstrated to own photocatalytic N₂ fixation capability, including TiO ₂^[10], BiOBr^[11], CdS^[12], g-C₃N₄^[13] and so on. Li et al. have illustrated that the BiOBr nanosheets of oxygen vacancies possessed a capability to reduce N₂ under visible light irradiation, leading to a photocatalyt-ic NH₃ yield rate of 104.2 mmol·h^{-1 [11]}. Wang et al. reported a visible -light- driven photocatalytic N ₂ reduc-tion process by using a type of K⁺ doped g-C₃N₄ nanoribbon photocatalysts, producing a considerable NH₃ yield of 3.42 mmol·g^-1·h^{-1 [14]}.

  Compared to the powders-based photocatalytic N₂ fixation system, the photoelectrochemical (PEC) cell might own a larger advantage for the N₂ fixation. Under a bias, the carrier separation could be more efficient, leading to a higher activity of photoexcited electrons to reduce N₂. Li et al. firstly used Au NPs to modify TiO₂ nanorod arrays electrode since 2017, and 13.4 nmol·cm^-2·h^-1 of photocatalytic ammonia production rate was achieved under 1 sun illumination^[15]. The local surface plasmon resonance (LSPR) effect induced by the Au NPs enhances visible light absorption and makes up for the shortcoming of TiO₂ poor absorption of visible light^[16-17]. Hu et al. also demonstrated that Au NPs could facilitate activating the N₂ molecules under a natural solar irradiation, and confirmed that Au nanoparticles could convert photons into a strong elec-tric field from a theoretical viewpoint, thereby inducing more photoexcited charges in the photocatalyst^[18].

  Recently, metal chalcogenides have attracted a great deal of attention in the photocatalysis fields, ow-ing to their advantages of visible light absorption and excellent charge transport properties^[19]. However, bina-ry chalcogenides (e. g., CdS, MoS₂) always exhibit poor stability during the photocatalysis reaction. In contrast, the polymetallic chalcogenides display excellent photo-catalytic stability in both the water reduction or oxidiza-tion reactions. So far, ZnIn₂S₄ has been applied widely in many fields, including photocatalytic water splitting into H₂^[20-21], photoreduction of CO₂^[22], photodegrada-tion, and so on. Nevertheless, there haven't any reports on the PEC N₂ fixation performance of ZnIn₂S ₄ nano-structured arrays.

  In this study, the ZnIn₂S₄/Au (namely, ZnIn₂S₄ modified with ultrathin Au NPs) arrays were synthe-sized for PEC N₂ fixation, while the effect of Au loading amounts was investigated on the PEC activity, too. The kinetic process of photoexcited carrier separation was characterized by the photoluminescence (PL) and PEC measurements. The results show that the Au NPs not only produce the LSPR to enhance the visible light absorption of the ZnIn₂ S ₄, but also reduces the carrier recombination rate and increases the lifetime of photo-generated charges under strong optical electric field. The photocatalytic ammonia production rate of ZnIn₂S₄/ Au(40) NSA reached 2.26 μg·cm^-2·h^-1, which was twice higher than the pure ZnIn₂S₄ without any Au loading.

  1.   Experimental

  1.1   Synthesis of ZnIn₂S₄ NSAs

  ZnIn₂S₄ NSAs were prepared on the FTO glass substrates by a facile hydrothermal method^[23-24]. Initially, a precursor solution was prepared by adding Zn(NO₃)₂· 6H₂O (0.5 mmol, 99%, Sinopharm), InCl₃·4H₂O (1 mmol, 99%, Macklin) and CH₄N₂S (2 mmol, 98%, Macklin) into 20 mL distilled water (DI water), and then stirred for 20 min. The pH value of this mixture solution was adjusted to 2.0 by adding the dilute HCl solution dropwise. Afterwards, the above solution and two same FTO substrates were transferred to 50 mL Teflon kettles. These Teflon kettles were sealed up and kept at 180 ℃ for 180 min in an oven. After cooling down, ZnIn₂S ₄ NSAs samples were taken out and washed by DI water for 2 min, dried at 60 ℃ overnight in a vacuum oven.

  The ZnIn₂S₄/Au NSAs were prepared via a photo-deposition route. Typically, a ZnIn₂S₄ NSAs sample was immersed into a diluent HAuCl₄ aqueous solution. After 30 min illumination by a 500 W Xe lamp, the ZnIn₂ S₄/Au samples were taken out, washed by DI water and dried at 60 ℃ in a vacuum oven. Three ZnIn₂S₄/Au NSAs samples were prepared and labeled as ZnIn₂S₄/Au(X), depending on the adding volume (μL) values (X=20, 40, 60) of the primary HAuCl₄ so-lution (50 mmol·L^-1) into 100 mL DI water.

  1.2   Materials characterization

  The crystal structure of the samples was measured by X-ray diffraction (XRD, Haoyuan DX-2700) with a Cu Kα source (λ =0.154 06 nm) and a scanning range from 10° to 80°. X - ray tube voltage and current were set at 40 kV and 30 mA, respectively. The morphology was investigated by both the field - emission scanning electron microscopy (FESEM, SU8220, Hitachi) operat-ing at 10 kV. The chemical composition and valence states were evaluated by X-ray photoelectron spectros-copy (XPS, EscaLab 250Xi). The absorption spectra were recorded on a Varian Cary 300 UV - Vis spectro-photometer with an integrating sphere. The steady-state PL spectra were tested on a FS5 fluorescence spectrom-eter with an excitation wavelength of 370 nm, while the time-resolved photoluminescence (TRPL) spectra were acquired on FLS980. All the above measurements were carried out at room temperature (RT).

  1.3   PEC N₂ fixation

  As displayed in Fig. 1, the photocatalytic N₂ fixa-tion was carried out in a quartz tube, which contained 30 mL methanol aqueous solution (V_water:V_methanol=4:1). Herein, a ZnIn₂S₄/Au NSAs and a Pt foil were applied as the working and the counter electrodes, respectively. And also, two electrodes were connected with a wire di-rectly, without any external bias in the whole measure-ment process. Prior to the reaction, the pure N₂ was bubbled into the solution with a flow of 20 mL·min^-1 for 30 min to exhaust air. The ZnIn₂S₄/Au samples were irradiated by a full solar spectrum light source with an intensity of 100 mW·cm^-2, which was provided by a 500 W Xe lamp without any filters. The 1.0 mL reac-tion solution was taken out from the tube every 25 min and mixed with Nessler's reagent for evaluating the concentration of NH₄⁺, since NH₃ was totally dissolved in water. The NH₄⁺ concentration was determined by monitoring the absorbance at 420 nm timely with a UV-Vis spectrometer.

  Figure 1

  

  Figure 1.  Schematic illustration on a PEC N₂ fixation install

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  1.4   PEC measurements

  A three-electrode system was used for PEC mea-surements in a cubic quartz cell, which was connected with an electrochemical workstation (CHI660E). A 500 W Xe lamp was employed for providing a full-spectrum irradiation, and also a Pt foil, saturated calomel (Hg/ HgCl₂, SCE) and a ZnIn₂S₄/Au(X) sample were em-ployed as the counter, reference and the working elec-trodes, respectively. A 0.1 mol·L^-1 Na₂SO₄ aqueous so-lution (pH=7.0) was used as the electrolyte. The poten-tials versus SCE (V_SCE) were converted to those versus reversible hydrogen electrode (V_RHE) through the Nernst equation:

  $ {V_{{\rm{RHE}}}} = {V_{{\rm{SCE}}}} + 0.059{\rm{pH + 0}}{\rm{.242}} $

  (1)

  The PEC measurements have the same illumination intensity with the fixation of N₂. The electrochemical impedance spectra (EIS) were recorded under irradia-tion at 0.2 V (vs SCE) with the frequency varying from 10-2 Hz to 50 kHz, while the Mott-Schottky (M-S) plots were measured at a constant frequency of 1 kHz. The photocurrentdensity-time (J-t) plots were obtained under a bias of 0 V (vs SCE) with an alternative irradiation.

  2.   Results and discussion

  2.1   Structure and morphology

  Fig. 2 compares the FESEM images of ZnIn₂S₄ and ZnIn ₂S₄/Au NSAs samples. It shows that ZnIn₂S₄ NSAs with uniform thickness were distributed on FTO glass substrates. Apparently, ZnIn₂S₄ NSAs has a typical pet-al-shaped structures, which not only could provide mul-tiple reflection to increase the visible- light utiliza-tion^[25-26], but also have a larger specific surface area and active sites. The edges of ZnIn ₂ S₄ NSAs have high-er photoelectric activity than the planes^[20], thus the Au3⁺ is more easily reduced to Au NPs on the ZnIn₂S₄ NSAs edges. The deposition amount of Au NPs increas-es with increasing the HAuCl₄ concentration, while the ZnIn₂S₄ NSA morphology did not change after Au load-ing. Fig. 3 displays the TEM images of ZnIn₂S₄/Au(40) NSAs and the corresponding energy dispersive X-ray spectroscopy (EDS) pattern. Apparently, the average size of these Au NPs was about 5 nm, while the Au NPs were mainly distributed in the regions near the edges. The existence of Au elements is also confirmed in the EDS spectrum indicated in Fig. 3c.

  Figure 2

  

  Figure 2.  FESEM images of ZnIn₂S₄ (a), ZnIn₂S₄/Au(20) (b), ZnIn₂S₄/Au(40) (c) and ZnIn₂S₄/Au(60) (d) NSAs samples

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  Figure 3

  

  Figure 3.  TEM images (a, b) and EDS spectrum (c) of ZnIn₂S₄/Au(40) NSAs

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  Fig. 4a also shows the XRD patterns of the sam-ples with various Au loading amounts. All the peaks can be indexed to the hexagonal phase of ZnIn₂S₄ (PDF No.65-2023), except a few peaks from the cubic phase SnO₂ (the main composition of FTO substrate). It should be noted that three main peaks of ZnIn₂S₄ appeared at 27.69°, 30.45°, 47.19° and were assigned to the (102), (104), (110) planes, respectively. No any Au related peaks were found in the ZnIn₂S₄/Au NSAs due to their ultralow Au loading amounts. It is worth mentioning that the Au mass loading content of ZnIn₂ S₄/ Au(40) could be determined to approximately 1.6% by using an inductively coupled plasma mass spectrome-try (ICP-MS).

  Figure 4

  

  Figure 4.  XRD patterns of ZnIn₂S₄/Au NSAs samples with various Au loading amounts

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  The light absorption properties of the samples are analyzed by UV-Vis absorption spectroscopy, as shown in Fig. 5. Apparently, ZnIn₂S₄ displays a clear absorp-tion edge around 500 nm, corresponding to its direct bandgap of ~2.33 eV. After loading Au NPs, the visible - light absorption of ZnIn₂S₄ was enhanced efficiently, which might be ascribed to the unique LSPR^{[15, 27]}. And also, it is noted that the visible light absorption capaci-ty increased with improving the Au loading amounts. Namely, the ZnIn₂S₄/Au(60) NSAs displays the highest visible - light absorption of all. The LSPR peak of Au was not clearly observed in Fig. 5, which might be over-lapped with the absorption peak of ZnIn₂S₄ (located around 530 nm).

  Figure 5

  

  Figure 5.  UV-Vis absorption spectra of ZnIn₂S₄/Au NSAs samples with different Au loadings

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  XPS measurements were carried out to examine the chemical valence status and elemental composition of the ZnIn₂S ₄/Au samples (Fig. 6). S2p core level spec-trum of ZnIn₂S₄/Au(40) samples before and after Au loading were compared in Fig. 6a. It is worth noting that both the S2p_3/2 and S2p_1/2 peaks show significant shifts with regard to the ZnIn₂S₄ one, implying that there might be a large interaction between Au and ZnIn₂S₄. Such an interaction should be favor of carrier transfer between two components (Au and S). Further investiga-tion is underway, the core level spectra of the In and Zn elements are discussed in Fig. 6b and 6c. It is note-worthy that no any changes in the peak positions are detected for two elements (In and Zn), except a small change in the peak intensity. It means that Au doesn't form a chemical bond with Zn or In, but its existence would decrease the XPS signals of these elements. Fur-thermore, the existence of Au has been confirmed by the appearance of Au related peaks (i. e., Au4f_7/2 and Au4 f_5/2) in Fig. 6d, which proves that Au NPs exist in a zero valence state.

  Figure 6

  

  Figure 6.  XPS spectra of ZnIn₂S₄/Au(40) sample: (a) S2p, (b) Zn2p, (c) In3d and (d) Au4f

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  2.2   PEC N₂ fixation performance and stability

  Fig. 7a compares the PEC N₂ fixation performance of ZnIn₂S₄ NSAs loaded with various Au amounts under a full-spectrum irradiation. When a suitable amount of Au NPs was loaded onto the NSAs, the PEC N₂ fixation performance reached the maximum value of 2.26 μg· cm^-2·h^-1 at an Au loading amount of 40 μL. The ZnIn₂S ₄ /Au(40) sample also displays a positive correlation of NH₄⁺ production rate with the N₂ fixation time, which is also another evidence for clarifying the excel-lent photocatalytic stability. Nevertheless, with a further increase of Au loading amounts, the PEC N₂ fixa-tion performance gets reduced to 1.59 μg·cm^-2 ·h^-1 for ZnIn₂S ₄/Au(60), which might be attributed to a reduc-tion of the contact area with electrolyte. It would hinder the adsorption of N₂ on the surface of ZnIn₂S₄ NSAs. Otherwise, the stability test of ZnIn₂S ₄ /Au(40) is shown in Fig. 7b, where the N ₂ photofixation activity remains nearly unchanged even after five runs of the PEC mea-surements, showing that ZnIn₂S₄/Au has an excellent photostability for N₂ photofixation under irradiation.

  Figure 7

  

  Figure 7.  PEC N₂ fixation (a) and cyclic stability plots (b) of ZnIn₂S₄/Au(40) NSAs samples

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  Fig. 8 also compares the PEC N₂ fixation performance of ZnIn₂S₄ NSAs with and without adding hole scavenger into the solution. As could be seen in Fig. 8a, the ZnIn₂S₄ shows a poor N₂ fixation activity without adding any hole scavenger (i. e., methanol). After add-ing methanol into the electrolyte, the NH₄⁺ generation rate of ZnIn₂S ₄ was improved significantly, which due to an efficient suppression of the photoexcited electron-hole recombination. In details, the holes are swallowed by methanol, which provides more chances for elec-trons to reduce N₂. Furthermore, Fig. 8a exhibits the PEC N₂ fixation performance is also very weak when bubbled Ar into the solution, which indicating that the reaction depends on external N₂. In Fig. 8b, the calibra-tion curve was obtained for quantitative analysis of the NH₃ concentration, showing a better linear relationship between absorbance and ammonia concentration.

  Figure 8

  

  Figure 8.  PEC N₂ fixation with ZnIn₂S₄/Au(40) sample under different conditions (a); Linear relationship between peak absorbance value and NH₃ concentration (b)

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  Fig. 9 also indicates the XRD patterns and SEM images of ZnIn₂S₄/Au after the PEC N₂ fixation mea-surements for 100 min. It is found clearly that XRD patterns remained unchanged, while the Au NPs were deposited on the edges of ZnIn₂S₄ NSAs even after fin-ishing PEC fixation measurements. Both the ZnIn₂S₄ NSAs and Au NPs displayed excellent photostability, while no any photocorrosion phenomena was observed.

  Figure 9

  

  Figure 9.  XRD patterns and SEM images of pure ZnIn₂S₄ (a, b) and ZnIn₂S₄/Au(40) (c, d) NSAs after PEC N₂ fixation reactions

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  2.3   Steady-state and time-resolved PL spectra

  Fig. 10a also compares the steady -state PL spectra of ZnIn₂S₄ and ZnIn₂S₄/Au(40). Both the samples dis-play a broad peak centered at approximately 530 nm, which could be assigned to the near band edge emis-sion of ZnIn₂S ₄ (i. e., band -to-band transition). Com-pared to single ZnIn₂S₄, the ZnIn₂S₄/Au(40) displays an obvious PL quenching, suggesting that there might be a significantly enhanced carrier separation from the load-ing of Au NPs. Otherwise, the TRPL spectra of two samples were also compared in Fig. 10b to analyze the charge carrier dynamics process. Obviously, the ZnIn₂S₄/Au(40) displays a longer average carrier life-time (9.40 ns) than the single ZnIn₂S₄ (6.89 ns), indicat-ing that the Au loading could lower the electron - hole recombination or improve the carrier separation effi-ciently. The result is consistent with the phenomenon indicated in Fig. 10a, too. Thus, both the experimental results could explain why the N₂ fixation performance of ZnIn₂S₄ was enhanced efficiently by Au loading.

  Figure 10

  

  Figure 10.  Steady-state (a) and time-resolved (b) PL spectra of the ZnIn₂S₄/Au samples

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  2.4   PEC performance

  In order to further understand the charge transport mechanism, the PEC measurements were carried out by constructing a three- electrode cell under a simulat-ed solar irradiation. As displayed in the Nyquist plots of Fig. 11a, the ZnIn ₂ S₄/Au electrode displayed a small-er semicircle than pure ZnIn₂ S₄, implying that the charge transfer resistance across the solid-liquid inter-face was lowered efficiently after loading the Au NPs^[22]. However, with a further increase of Au NPs, the semicircle of ZnIn₂S₄/Au(60) was higher than that of ZnIn₂S ₄/Au(40), indicating that excessive Au NPs may hinder the charge carrier transfer across solid -liquid in-terface again. In Fig. 11b, the J - t plots were compared to analyze the PEC performance of different Au loaded ZnIn ₂ S₄ NSA samples. Obviously, all the ZnIn₂S ₄/Au ones displayed higher photocurrent density than the pure ZnIn₂S₄. Of them, the highest PEC activity ap-pears in the ZnIn₂S₄/Au(40) sample, which was in a good agreement with the aforementioned PEC N₂ fixation results. It could be explained by a fact that the loading of Au NPs on the ZnIn₂S₄ NSAs enhanced both the visible - light harvesting and solid - liquid charge transfer process.

  Figure 11

  

  Figure 11.  Nyquist plots under irradiation (a), J-t plots at 0 V vs SCE (b), M-S plots in dark (c) and M-S plots under irradiation (d) of ZnIn₂S₄/Au NASs samples with various Au loading amounts

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  Fig. 11c and 11d show the M - S plots of ZnIn₂S₄ and ZnIn₂ S₄/Au(X) obtained in dark and light, respec-tively. As is well known, both the flat - band potential (V_FB) and donor density (N_d) could be determined by fit-ting the M-S plots linearly with the following formula^[28]:

  $ \frac{1}{{{C^2}}} = \frac{2}{{{e_0}\varepsilon {\varepsilon _0}{N_{\rm{d}}}}}\left( {V - {V_{{\rm{FB}}}} - \frac{{kT}}{{{e_0}}}} \right) $

  (2)

  where C is the interfacial capacitance; ε is the dielec-tric constant of the semiconductor; ε₀ is the permittivi-ty of free space; V_FB represents the flat band potential; N_d represents the charge carrier density; V is the applied voltage; k is Boltzmann's constant; T is the absolute temperature; e₀ is the electronic charge of ZnIn₂S₄ or ZnIn₂ S₄/Au NSAs sample. The fitted results are displayed in Table 1. The positive slope means that all the samples are n-type semiconductors. For the M-S plots obtained in dark, the V_FB value shifts towards the more positive side from -0.48 to -0.40 V (vs RHE) with increasing the Au loading amounts, while there aren't obvious changes in the plot slope (or said, N_d). It is well known that there a direct relationship between V_FB and Fermi - level (E_F) ^[29], the more positive the V_FB value is, the lower the E_F position is. The positive shift-ing of V_FB might be related with an enhanced charge transfer from ZnIn₂S₄ to Au and electrolyte, leading to a lowering E_F of ZnIn₂S₄. Furthermore, the M-S plots un-der a solar irradiation show almost the same variation trend with those measured in dark, with the V_FB shifting from -0.35 to -0.28 V (vs RHE). It is worth mentioning that all the ZnIn₂S₄/Au samples under irradiation also display more positive V_FB values than those in dark, im-plying that there isn't any electron accumulation in ZnIn₂S₄ even under a solar irradiation. The result fur-ther confirms that the photoexcited electron transfer pathway is from ZnIn₂S₄ to Au NPs, as would be illus-trated in Fig. 12.

  Table 1

  

  Table 1.  Calculated electronic parameters from the M-S plots in dark and Nyquist plots under irradiation

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  Sample	In dark			Under irradiation
  	Slope	VFB/ V (vs RHE)		Slope	VFB/ V (vs RHE)
  ZnIn2S4	1.639×109	-0.48		1.467×109	-0.35
  ZnIn2S4/Au(20)	2.169×109	-0.43		1.116×109	-0.32
  ZnIn2S4/Au(40)	1.022×109	-0.42		1.024×109	-0.29
  ZnIn2S4/Au(60)	1.243×109	-0.40		1.412×109	-0.28

  Figure 12

  

  Figure 12.  Schematic illustration of (a) ZnIn₂S₄ and (b) ZnIn₂S₄/Au

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  Fig. 12 also shows a schematic model to clarify the mechanism for the variation of N ₂ fixation activity. For the pure ZnIn₂S₄ NSAs, a number of electron-hole pairs are generated by the photo-excitation. After loading with ultrathin Au NPs, the visible light absorption of ZnIn₂S₄ NSA was enhanced by the LSPR effect, leading to the increased number of photogenerated carriers in the semiconductor ZnIn₂S ₄ electrodes^[30]. Meanwhile, these Au NPs also act as electrons trap, leading to a lowering of the interface resistance across solid elec-trode/liquid electrolyte. Finally, the above various fac-tors result in a significant enhancement of both the PEC activity and N₂ fixation performance.

  3.   Conclusions

  In summary, ZnIn₂S₄/Au NSAs were synthesized by depositing ~5 nm Au NPs on the surface of ZnIn₂S₄ NSAs for realizing a PEC N₂ fixation (into NH₃ or NH₄⁺). It was demonstrated that the Au NPs decoration facilitated enhancing the N₂ fixation activity of ZnIn₂S₄, reaching a maximum NH₃ generation rate of 2.26 μg· cm^-2·h^-1 in the ZnIn₂S₄ /Au(40) sample. Loading the excessive Au NPs would result in a lowering of the N₂ fixation performance, which might be attributed to the Au NPs blocking N₂ adsorption of ZnIn₂S₄. The varia-tion of the N₂ fixation performance is consistent with that of the PEC activity. It also means that the enhanced PEC performance after Au loading might be ascribed to the enhanced visible-light absorption, charge carrier separation and plasmon-induced reso-nance energy transfer. This work not only develops a new photoanode for the N ₂ photoreduction or NH₃ syn-thesis, but also expands the application of ZnIn₂S₄ NSAs in the PEC fields.

  
  
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  Figure 1  Schematic illustration on a PEC N₂ fixation install

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  Figure 2  FESEM images of ZnIn₂S₄ (a), ZnIn₂S₄/Au(20) (b), ZnIn₂S₄/Au(40) (c) and ZnIn₂S₄/Au(60) (d) NSAs samples

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  Figure 3  TEM images (a, b) and EDS spectrum (c) of ZnIn₂S₄/Au(40) NSAs

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  Figure 4  XRD patterns of ZnIn₂S₄/Au NSAs samples with various Au loading amounts

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  Figure 5  UV-Vis absorption spectra of ZnIn₂S₄/Au NSAs samples with different Au loadings

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  Figure 6  XPS spectra of ZnIn₂S₄/Au(40) sample: (a) S2p, (b) Zn2p, (c) In3d and (d) Au4f

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  Figure 7  PEC N₂ fixation (a) and cyclic stability plots (b) of ZnIn₂S₄/Au(40) NSAs samples

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  Figure 8  PEC N₂ fixation with ZnIn₂S₄/Au(40) sample under different conditions (a); Linear relationship between peak absorbance value and NH₃ concentration (b)

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  Figure 9  XRD patterns and SEM images of pure ZnIn₂S₄ (a, b) and ZnIn₂S₄/Au(40) (c, d) NSAs after PEC N₂ fixation reactions

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  Figure 10  Steady-state (a) and time-resolved (b) PL spectra of the ZnIn₂S₄/Au samples

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  Figure 11  Nyquist plots under irradiation (a), J-t plots at 0 V vs SCE (b), M-S plots in dark (c) and M-S plots under irradiation (d) of ZnIn₂S₄/Au NASs samples with various Au loading amounts

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  Figure 12  Schematic illustration of (a) ZnIn₂S₄ and (b) ZnIn₂S₄/Au

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  Table 1.  Calculated electronic parameters from the M-S plots in dark and Nyquist plots under irradiation

  Sample	In dark			Under irradiation
  	Slope	VFB/ V (vs RHE)		Slope	VFB/ V (vs RHE)
  ZnIn2S4	1.639×109	-0.48		1.467×109	-0.35
  ZnIn2S4/Au(20)	2.169×109	-0.43		1.116×109	-0.32
  ZnIn2S4/Au(40)	1.022×109	-0.42		1.024×109	-0.29
  ZnIn2S4/Au(60)	1.243×109	-0.40		1.412×109	-0.28

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