Construction of Z-scheme heterojunction of PANI-Ag-CN sandwich structure with efficient photocatalytic hydrogen evolution

Applied Surface Science 509 (2020) 145296

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Construction of Z-scheme heterojunction of PANI-Ag-CN sandwich structure with efficient photocatalytic hydrogen evolution

T

Yuanyuan Liu, JiFang Cui, Yinghua Liang , Weijia An, Huan Wang, Li Liu, Jinshan Hu, ⁎ Wenquan Cui ⁎

College of Chemical Engineering, Hebei Key Laboratory for Environment Photocatalytic and Electrocatalytic Materials, North China University of Science and Technology, Tangshan 063210, PR China

ARTICLE INFO

ABSTRACT

Keywords: Two-dimensional ultra-thin nanosheets Z-scheme system Sandwich structure Photocatalytic hydrogen evolution

In this work, we report a sandwich structure of PANI-Ag-CN photocatalyst, which is stacked layer by layer with g-C3N4 and PANI ultra-thin two-dimensional nanosheets and Ag nanoparticles are embedded as interlayers. The Z-scheme heterojunction can be formed in this sandwich structure of PANI-Ag-CN photocatalyst, where Ag nanoparticles act as the charge transfer intermediate, leading to the annihilation of the holes of g-C3N4 and the electrons of PANI, and thus the strong reductive electrons and the strong oxidative holes can be left at the CB of g-C3N4 and VB of PANI, respectively. Meanwhile, the short charge transfer and the large number of active sites on the ultra-thin two-dimensional nanosheets are proved beneficial for to the photocatalytic hydrogen evolution from water. The PANI-Ag-CN photocatalyst exhibits an increased hydrogen evolution activity of 5048 μmol·g−1·h−1, which is 43.52-fold higher than that of g-C3N4 and 58.02-fold higher than that of bulk gC3N4. Moreover, the photocurrent of PANI-Ag-CN is 1.9 × 10−6 A/cm2, which is 5.94 times higher than that of g-C3N4.

1. Introduction The application of photocatalysis to hydrogen evolution is an efficient way of converting solar energy and generating renewable energy [1] that are of great significance and practical value in photocatalytic technology in terms of clean energy production and reduction of carbon emissions [2]. Thus, the development of compound photocatalysts with high activity is critical to achieve hydrogen evolution in high efficiency [3]. Many methods have been adopted so far to improve the photocatalytic performance such as structural surface defects [4–6], elemental doping [7–9], semiconductor heterojunction [10–12], material self-assembly [13–15], and so on. Heterojunction can obviously improve the utilisation of sunlight, and promote the separation of charge carriers. However, the redox capability of charge carriers is reduced when traditional heterojunctions are formed. In addition, the catalytic driving force of heterojunction under visible light is weakened and may be unable to trigger specific photocatalytic reactions. The Z-scheme heterojunction has thus an advantage over the traditional type II heterojunction mode. Z-scheme heterojunction photocatalysts exhibit space separation ability and strong photocatalytic redox capability

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[16]. Two-dimensional (2D) nanosheets with large specific surface areas may favour the photocatalytic performance of Z-scheme heterojunctions, provide more active sites and higher charge mobility and expose the active crystal faces [17]. Moreover, ultra-thin 2D nanosheets contribute to the acceleration of photo-generated charges transfer from the interior to the surface and effectively facilitate the reaction [18]. Zhu et al. [19] reported the 2D Z-scheme heterojunction of BP and BiVO4, where the staggered arrangement of its band structure exhibited rapid charge separation efficiency. Chen et al. [20] utilised the hydrothermal method to generate 2D Z-scheme MnIn2S4/g-C3N4 nanocomposites. This complex substance could handle pharmaceutical wastewater and trigger hydrogen evolution under visible light. In addition, its high-efficiency photocatalytic performance could cause rapid charge separation due to the tight heterogeneous interface and the wellmatched energy band structure construction. Nevertheless, this conventional 2D Z-scheme heterojunction is susceptible to ions diffusion in solution, leading to slow transfer rate of charge carriers. Therefore, it is necessary to select a material that can be the driving force between 2D nanosheets and promote the direct and quick transfer of the photogenerated charges to the active surface sites [21]. Compared to conventional p-n heterojunction photocatalysts, the

Corresponding author. E-mail addresses: [email protected] (Y. Liang), [email protected] (W. Cui).

https://doi.org/10.1016/j.apsusc.2020.145296 Received 15 July 2019; Received in revised form 30 December 2019; Accepted 4 January 2020 Available online 08 January 2020 0169-4332/ © 2020 Elsevier B.V. All rights reserved.

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sandwich structure system not only maintains the strong reducibility and oxidation of photocatalyst but also improves the separation efficiency of system carriers. In this reaction, the photo-generated charges can be quickly separated and slowly compounded [22–25]. Through the synergistic reaction, this sandwich system considerably promotes the photocatalytic activity and enhances the stability [26]. At the same time, metal particles can efficiently transfer the photo-generated charge carriers of compound photocatalyst and improve the photocatalytic stability [27]. Lu et al. [28] successfully synthesised a Z-scheme gC3N4/Ag/MoS2 photocatalyst, which exhibited an excellent improvement in the visible light absorption and the photo-generated electronhole pair separation efficiency. Moreover, it was highly active in the hydrogen evolution and the degradation of pollutants. However, PANIAg-CN Z-scheme sandwich structure photocatalyst has not yet been reported in the literature. Graphite carbon nitride (g-C3N4) is a typical ultra-thin 2D nanosheet material [29,30] and its internal charges can rapidly move underlight irradiation. Furthermore, it has high chemical stability, thermal stability and unique electronic band structure with moderate band gap [1]. Conductive polymers are also useful to improve the performance of compound catalysts for the photocatalytic degradation of hazardous chemicals and for applications of the photocatalytic hydrogen evolution [31]. Especially the 2D nanostructure polyaniline (PANI) is a conductive polymer that is widely studied. PANI is a conjugated pi bond macromolecule with strong interactions [32,33] and its unique electron and hole transport properties and good chemical stability can significantly enhance the catalytic activity of the compound catalyst [34,35], while in contrast MoS2 exploits only the interface transfer. The double excitation of Z-scheme heterojunction sandwich structure and 2D nanosheet semiconductor materials could thus optimise the performance of compound catalysts, whose photogenerated electron-hole pairs can be separated, resulting in a new photocatalyst with promising application prospects in hydrogen evolution. Consequently, PANI-Ag-CN sandwich structured Z-scheme heterojunction system was proposed. While PANI and g-C3N4 nanosheets were combined to provide a large number of active sites, Ag nanoparticles were anchored between them, acting as charge quenching centres that could trigger the quick electron move. Through the Z-scheme chargecarrier transfer mechanism, the redox ability of catalyst’s electron-hole pairs could be improved and the charge separation efficiency could be accelerated. The phase structure, chemical composition, optical properties, morphology, and hydrogen evolution activity of PANI-Ag-CN composite photocatalyst were studied in detail and the hydrogen evolution mechanism was also discussed.

2.3. Synthesis of the PANI-Ag-CN photocatalyst Chemical reduction was applied to attach the Ag nanoparticles on the synthesised g-C3N4 nanosheets. 8 mL of PEG600 aqueous solution, which was used as the dispersing and protective agent, was mixed with an equal volume of AgNO3 solution at the temperature in an AgNO3/ PEG600 mass ratio of 1:5. Afterwards, 9.6 mL of NaBH4 aqueous solution was quickly poured in the stirring solution and stirring continued for 3 h, resulting in an orange solution. The AgNO3/NaBH4 mass molar ratio was 5:8. 0.1 g of g-C3N4 to was then added and the mixture was ultrasonicated for 1 h and stirred, yielding the Ag-CN. The same method was used to prepare Ag-CN compound catalysts with Ag contents of 0.5 wt%, 1 wt%, 3 wt%, 7 wt%, and 10 wt%. Next, 0.25 g of PANI was added to 250 mL of water, and sonicated for 20 h. The uniformly dispersed solution was then dried in an oven at 60 °C to afford the PANI nanosheets. These were added to the Ag-CN solution in a specified proportion, resulting in a green solution, which was sonicated for 2 h and stirred. After washing three times with distilled water, it was allowed to stand for 5 h, and the precipitated product was collected and lyophilised for 10 h to obtain the PANI-Ag-CN compound catalyst. Following the above method, PANI-Ag-CN compound catalysts were prepared with PANI of 0.5 wt%, 3 wt%, 7 wt%, 10 wt%, 15 wt%, 20 wt%, 50 wt%, and 80 wt%. 2.4. Catalyst characterization The surface and interior patterns of the catalyst were observed by field emission scanning electron microscopy (FESEM, S-4800) and field emission transmission electron microscope (FETEM, JEM-2800F). The elements were qualitatively and quantitatively analysed with energy dispersive X-ray spectroscopy (EDS). The crystal structure of the catalyst was analyzed by an X-ray diffractometer (XRD) under a Cu target Kα X-ray radiation source While the functional groups were qualitatively analysed by KBr pelleting using Fourier transform infrared spectroscopy (FTIR, Spectrum 100). The composition, valence and bonding of the synthesised catalysts were investigated using multifunctional surface analysis photo-generated electron spectroscopy (XPS, Thermo ESCALAB 250xi). The optical properties of each catalyst were analysed by a UV 1901 UV–vis diffuse reflectance spectrometer, using BaSO4 as the substrate and a scanning range of 200–800 nm. The fluorescence steady-state spectra of the catalysts at normal temperature and pressure conditions were recorded with an F-7000 fluorescence spectrometer. The transient fluorescence spectra of the catalysts were measured with a spectrofluorometer FS5 and the average lifetime of the electrons was calculated using an excitation wavelength of 380 nm. The Brunauer–Emmett–Teller (BET) specific surface area and the pore size distribution of the catalyst analysis were performed using an Auto-sorbIqa 3200-4 specific surface and pore size analyser.

2. Experimental 2.1. Materials and reagents Urea, hydrochloric acid (HCl), silver nitrate (AgNO3), polyethylene glycol (PEG600), sodium borohydride (NaBH4), and polyaniline (PANI) were analytical grade reagents and were used without further purification. The corresponding solutions were prepared using distilled water.

2.5. Photocurrent measurements The photoelectrochemical property of the catalysts was measured in a quartz photoelectrochemical cell (5 × 5 × 5 cm3) in a three-electrode system using a CHI660E electrochemical workstation. The light source was a 350 W Xenon lamp. A saturated calomel electrode was used as the reference electrode and a Pt wire served as the counter electrode. An indium tin oxide (ITO) glass supported catalyst was used as the working electrode. The distance between the light source and the working electrode was approximately 10 cm, and 100 mL of 0.1 M Na2SO4 solution was used as the electrolyte solution. To prepare the working electrode, the catalyst was evenly spread on the sand chip, which was firmly fixed on the conductive surface of the ITO glass piece using a conductive paste. The amperometric i-t curve mode was selected for the photocurrent response. The open circuit voltage was set, and the frequency range was 103–105 Hz. For the AC impedance spectrum, the AC impedance mode

2.2. Preparation of g-C3N4 nanosheets The synthesis of g-C3N4 nanosheets was performed using the proton acidising thermal polymerization method, based on previous reports [36–38]. Here, 10 g of urea was added in 15 mL distilled water and stirred till complete dissolution. 1 mL of HCl and 11 mL of distilled water were prepared to mix which was then added dropwise to the urea solution and the solution pH was adjusted to 4–5. The resulting solution was dried at 60 °C for 12 h, after which the solid matter was milled and transferred to an alumina crucible with a lid. The crucible was heated to 550 °C in a muffle furnace at 10 °C·min−1 for 2 h, yielding g-C3N4 nanosheets. 2

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was selected with an initial voltage of 0 V, while the frequency range was 1–105 Hz, and the amplitude was 5 mV. The flat band potential of the catalyst was determined by the Mott-Schottky curve and the impedance-potential mode was selected to prepare a suitable potential interval. The linear sweep volt-ampere used the linear sweep voltammetry (LSV) mode. The appropriate potential interval and the frequency range were determined, and the scan speed was 5 mV/s. 2.6. Photocatalytic activity for hydrogen evolution The hydrogen evolution was tested in a custom cylindrical closed reactor using an HPS-500XA 500 W Xenon lamp as the simulated sunlight. The optical power density was 100 mW/cm2 and the reaction was performed in a 24 °C circulating cooling water. 13 vol% of triethanolamine, 2 wt% of chloroplatinic acid aqueous solution, 15 mg of the catalyst and 23 mL of distilled water were placed in a 50 mL reactor. This solution was stirred for 30 min to achieve a uniform dispersion. The closed reactor was treated with vacuum and purged with argon before the reaction. Gas chromatography (GC-2014C) was used to measure the hydrogen production and analyse the photocatalytic hydrogen evolution. The calculation of the apparent quantum efficiency (AQE) was based on the following formula:

AQE =

Fig. 2. XRD spectra of g-C3N4, PANI and PANI-Ag-CN.

amorphous form. The broad peaks observed at 20.3° and 25.3° were the Bragg characteristic diffraction peaks of PANI, related to the periodicity of the PANI parallel polymer chains. The broad peak indicated that the PANI layer was very thin. In the PANI-Ag-CN curve, the diffraction peaks around 13.0° and 27.4° indicated that the complex did not change the original structure of g-C3N4. Besides, a diffraction peak appeared at 38°, which corresponded to the (111) Bragg reflection of Ag and suggested the presence of face-centred cubic Ag crystals in the compound (JCPDS No. 04-0783). There was no evident diffraction peak for PANI because the amount of PANI was small and was masked by the diffraction peak of g-C3N4. Fig. 3 illustrates the FTIR spectra of g-C3N4, PANI-CN, Ag-CN, and PANI-Ag-CN. The characteristic peak at 807 cm−1 in the spectrum of gC3N4 was attributed to the bending vibration mode of the triazine structure [41]. The typical stretching mode of the CeN heterocycle was divided in a series of peaks between 1200 and 1650 cm−1 [42]. In particular, the peak at 1637 cm−1 was attributed to the CeN stretching, the peak centered at 1570 cm−1 was caused by C]C stretching vibrations of the Quinone ring, the peak centered at 1424 cm−1 was attributed to aromatic CeN stretching vibrations and the benzene ring that bent in the plane. In addition, the peak at 1247 cm−1 was related to the CeN stretching mode. In the region from 3000 to 3500 cm−1, the absorption band at 3185 cm−1 was due to the stretching vibrations of NeH and the hydroxyl groups of the adsorbed water. It should be also noted that the peak positions of the compound catalyst PANI-Ag-CN coincided with those of g-C3N4. The morphology, microstructure and composition of the PANI-AgCN catalyst are characterized by SEM, TEM and EDS. Fig. 4(a) illustrates the SEM image of g-C3N4, which possessed a 2D thin layer structure, while the TEM image in Fig. 4(b) revealed that the edges of the g-C3N4 nanosheets were regular. Moreover, a mesoporous structure was formed on the nanosheet layer, which exhibited a mesh distribution [43,44], while the nanosheet layer was transparent, indicating that the g-C3N4 nanosheets were very thin. Fig. 4(c) and (d) depict the SEM and TEM images of the PANI nanosheets, which could be distinguished

2 × nhydrogen molecules n incident photons

2.7. Hydroxyl radical detection The fluorescent coumarin experiment was used to detect the content of hydroxyl radicals (%OH). 15 mg of the compound catalyst was dispersed in 30 mL of a 0.001 M coumarin solution. After reacting for 10 min in the dark, the solution was irradiated with light for 1 h using a 350 W Xenon lamp. 5 mL of the reaction solution was abstracted through the filter membrane to test its fluorescence spectroscopy. Moreover, an electron spin resonance (ESR) spectrometer (JESFA200) was used to determine the presence of %OH in the catalyst and investigate the effect of reactive oxgen species on the reaction mechanism. 3. Result and discussion The synthetic procedure of the PANI-Ag-CN catalyst is schematically presented in Fig. 1. The g-C3N4 nanosheets were firstly prepared using the proton acidification thermal polymerization method. Ag nanoparticles were then attached on the g-C3N4 surface by the chemical reduction method, resulting in Ag-CN mixture. Combination of PANI and Ag-CN via stripping afforded ultimately the Z-scheme sandwich structured catalyst. Fig. 2 depicts the XRD spectra of g-C3N4, PANI and PANI-Ag-CN. The XRD curve of g-C3N4 exhibited diffraction peaks at 13.0° and 27.4°, corresponding to the (100) and (002) planes of the g-C3N4 nanosheet [39], respectively, while the diffraction peak at 13.0° also corresponded to the 3-s-triazine ring structural unit [40]. PANI mainly existed in an

Fig. 1. Schematic representation of the synthesis process of heterostructured PANI-Ag-CN nanosheets. 3

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from the g-C3N4 nanosheets due to their wrinkled edges. Furthermore, the SEM image of PANI-Ag-CNconfirmed its 2D sheet structure in Fig. 4(e). Based on the TEM image in Fig. 4(f), the Ag nanoparticles were attached on the g-C3N4 layers and the irregular thin layer of PANI was above them. Fig. 4(g) shows the TEM image of PANI-Ag-CN, and Fig. 4(h) depicts its enlarged high resolution transmission electron micrograph (HRTEM). The observed lattice fringes of the Ag nanoparticles corresponded to the (1 1 1) crystal plane, while its lattice spacing was 0.24 nm [45,46]. To further confirm the sandwich structure of the PANI-Ag-CN catalyst, its elemental composition and distribution were tested. Hence, according to the surface sweep and EDS spectrum tests of the compound catalyst in Fig. 5(a) and (b), respectively, the Ag nanoparticles were uniformly dispersed on the nanosheet layer, while three elements (C, N and Ag) could be detected, supporting thus the successful preparation of the compound catalyst. The UV–Vis spectrum of g-C3N4 in Fig. 6 indicated that the

Fig. 3. FTIR spectra of g-C3N4, PANI-CN, Ag-CN and PANI-Ag-CN.

Fig. 4. (a) SEM and (b) TEM of g-C3N4. (c) SEM and (d) TEM of PANI. (e) SEM, (f, g) TEM and (h) HRTEM of PANI-Ag-CN. 4

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Fig. 5. (a) Elemental mapping images and (b) EDS spectra of PANI-Ag-CN.

absorption band edge was about 456 nm. Instead, the absorption band edges of the compound photocatalysts were red shifted, implying that the formation of the Z-scheme heterojunction could effectively expand the range of the visible light absorption. The photocatalytic performance and the light absorption efficiency of the mixture were thus greatly improved. The band gap of g-C3N4 was calculated using the Kubelka–Munk equation (αhν = A(hν − Eg)n/2) to 2.6 eV. X-ray photo-generated electron spectroscopy (XPS) is employed to further investigate the surface chemical composition and valence state of the PANI-Ag-CN samples. The full spectrum in Fig. 7(a) revealed that C, N and Ag were the major elements of the composite surface. The two identified peaks in the Gaussian fit in Fig. 7(b) corresponded to the Ag 3d5/2 and Ag 3d3/2 levels of the Ag atom, indicating the presence of metallic Ag [42]. Moreover, the diffraction peaks of Ag+ were not observed, denoting that no Ag+ was present in the synthesised samples, which was consistent with the XRD results. The binding energy of Ag 3d in PANI-Ag-CN was 0.26 eV higher than that of g-C3N4, illustrating that the chemical state of the compound catalystwas altered, probably due to the strong conjugate interactions of the system that enabled the electron transfer. This phenomenon improved also the charge separation efficiency [47]. In Fig. 7(c), two distinct peaks at 285.0 and 288.4 eV were observed in the C 1s spectra of g-C3N4 and PANI-Ag-CN. The peak centred at 285.0 eV could be attributed to the binding bonds of the sp2 C atom of the aromatic ring and the N atom

[48], while the C 1s peak that was dominant at 288.4 eV corresponded to the three coordinated C(N)3 groups [49]. Instead, the N 1s spectra of g-C3N4 and PANI-Ag-CN were divided into three characteristic peaks in Fig. 7(d). Specifically, the peaks at 399.0 eV and 400.9 eV corresponded to the N(C)2 groups and the N(C)3 group, respectively [50], while the peak at 404.7 eV was generated by the pi bond excitation from the weak binding energy, implying that the change in the binding energy significantly affected the charge separation. Fig. 8(a) shows the photocatalytic activities of bulk g-C3N4, PANIbulk CN, Ag-bulk CN and PANI-Ag-bulk CN. Compared with those in Fig. 8(b), the hydrogen evolution efficiency of the different bulk photocatalysts was substantially lower. For instance, the hydrogen release rate of g-C3N4 was 1.33 times greater than that of bulk g-C3N4, because the 2D g-C3N4 nanosheets contained a conjugated pi bond owing to the 3-s-triazine ring structure, which provided a fast migration channel for the photo-generated electron holes. This structure enhanced also the charge separation efficiency and prolonged the carriers’ lifetime. The activity tests to evaluate the hydrogen evolution rates were performed using visible light. Fig. 8(b) displays the photocatalytic hydrogen evolution rates of g-C3N4, PANI, PANI-Ag, PANI-CN, Ag-CN and PANI-Ag-CN. Specifically, PANI and PANI-Ag were not active, while the hydrogen evolution activity of the PANI-CN compound catalyst did not significantly differ from that of g-C3N4. The corresponding photocatalytic hydrogen evolution rates were 132 and 116 μmol·g−1·h−1, respectively. Moreover, the hydrogen release rate of PANI-Ag-CN increased to 5048 μmol·g−1·h−1, which was 2.65 times greater than that of Ag-CN, implying that PANI-Ag-CN had the highest photocatalytic activity owing to the attachment of the Ag nanoparticles. In fact, the Ag nanoparticles acted as a bridge, allowing thus the charge transfer between the PANI and g-C3N4 nanosheets, which in turn effectively improved the separation rate of the electron-hole pairs. Moreover, the hydrogen evolution activity was significantly enhanced due to the conjugated pi bonds that resulted in strong interactions in the PANI nanosheet structure. During the photocatalytic hydrogen evolution, chloroplatinic acid was used as a co-catalyst and could rapidly transfer photo-generated electrons. Many metal-based materials, such as MoS2 [51], and Ni [52], could also be used as co-catalysts in photocatalysis. However, Ptwas more favourable for the hydrogen evolution. The hydrogen evolution activity of the PANI-Ag-CN compound catalyst was optimised by adjusting the PANI and Ag proportion. In Fig. 9(a), the increase in the mass ratio of PANI resulted first in an increase in the photocatalytic hydrogen evolution rate, which was then

Fig. 6. UV–vis of the different composite photocatalysts. 5

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Fig. 7. (a) Survey XPS spectra for g-C3N4 and PANI-Ag-CN. (b) Ag 3d of Ag-CN and PANI-Ag-CN. (c) C 1s and (d) N 1s for g-C3N4 and PANI-Ag-CN.

reduced. For PANI quantities less than 10 wt.%, the hydrogen evolution activity was improved with the PANI content increase, because more active sites were generated in the compound catalyst. Electrons could be quick transferred and separated from the holes and thus light could efficiently reduce water to form hydrogen. However, when the PANI content was higher than 10 wt%, the hydrogen evolution rate of the compound catalyst gradually decreased as the PANI content increased, as shown in Fig. 9(b), because the PANI

excess hindered the g-C3N4 absorption of light and its contact with water masked the activity of g-C3N4. It also inhibited the separation of electron-hole pairs in the compound catalyst and reduced the hydrogen release rate. Similarly, the mass ratio of Ag increased, the hydrogen evolution rate was initially increased and then decreased in Fig. 9(a). The Ag nanoparticles acted as electron transfer mediators in the compound photocatalyst. When the Ag content was less than 3 wt%, the Ag

Fig. 8. Comparison of photocatalytic activities of (a) bulk g-C3N4, PANI-bulk CN, Ag-bulk CN and PANI-Ag-bulk CN and (b) g-C3N4, PANI, PANI-Ag, PANI-CN, Ag-CN and PANI-Ag-CN for hydrogen evolution. 6

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Fig. 9. Photocatalytic activity of hydrogen evolution for composite photocatalysts of (a) (x wt.%)PANI-(3 wt%)Ag-CN and (b) (10 wt%)PANI-(y wt.%)Ag-CN (x = 0.5/3/7/10/15/20/50/80, y = 0.5/1/3/5/7/10).

nanoparticles could not efficiently capture the photo-generated electrons, resulting in significantly low significantly low electron transfer and hydrogen evolution rates. When the Ag amount between the nanosheets was increased. However, further increase in the Ag amount reduced the combination of free electrons and H+ in water, decreasing thus the catalytic activity of the compound catalyst. The highest hydrogen generation rate (5048 μmol·g−1·h−1) of PANIAg-CN was achieved using 10 wt% PANI and 3 wt% Ag. Attachment of the Ag nanoparticles changed the distribution of the carriers in the PANI-Ag-CN system. Since the work function of the noble metal was higher than that of the semiconductor material, the photo-generated electrons migrated from the semiconductor material to the precious metal, whose Fermi level was lower, until an equal Fermi level between the materials was achieved, leading to a better photo-generated charge separation effect. In addition, a charge transfer intermediate was formed near the surface and prolonged the life of the excited electrons, improving in turn the electron mobility of the Z-scheme system [53]. Moreover, the combination of PANI and Ag-CN provided a disordered structure of PANI with a certain order, which prevented the agglomeration of PANI to some extent. This structure also increased the number of active sites of the compound photocatalyst, while the involvement of the photo-generated charges in the photocatalytic reaction was improved. Under the same experimental conditions, and after recycling g-C3N4 and PANI-Ag-CN for five times, each lasting 3 h under simulated sunlight, the hydrogen evolution activities were still above 80%, as shown in Fig. 10(a). The catalyst exhibited only minor losses in the recovery process of the solid-liquid separation due to the nanoscale powder sizes, which led to a slight reduction in the hydrogen evolution rate. However, the PANI-Ag-CN compound catalyst still possessed an excellent stability, leading to a hydrogen evolution capacity of 15144 μmol·g−1 for 3 h. Wang et al. [39] reported a higher activity, namely 1200 μmol·g−1 for 5 h, over Ag/polyaniline heterostructured nanosheets loaded with g-C3N4 nanoparticles. According to the XRD patterns of PANI-Ag-CN before and after the photocatalytic hydrogen evolution, the diffraction peak of PANI-Ag-CN remained unchanged, indicating that its crystal phase structure was preserved, as shown in Fig. 10(b). Thus, the PANI-Ag-CN compound catalyst exhibited high stability and recyclability and had great potential in the field of clean energy production. Fig. 11(a) shows the transient photocurrent response spectra of gC3N4, Ag-CN, PANI-CN and PANI-Ag-CN in an on/off lamp state. Under visible light irradiation, the photocurrent of PANI-Ag-CN composite catalyst was 1.9 × 10−6 A/cm2, which was 5.94 times higher than that of g-C3N4 (3.2 × 10−7 A/cm2). The significant improvement in the

photocurrent intensity indicated that the formation of the Z-scheme heterojunction favoured the charge transfer efficiency, shortened the transmission distance of the photo-generated charges, and effectively promoted their separation. Electrochemical AC impedance spectroscopy (EIS) is employed to examine the transfer efficiency and the interfacial reaction properties of the photo-generated charges. Fig. 11(b) presents the EIS spectra of gC3N4, Ag-CN, PANI-CN and PANI-Ag-CN, where the high-frequency arch represents the charge transfer process, while the diameter of the semicircle represents the charge transfer resistance [54]. As the radius of the Nyquist curve decreased, its electrode surface resistance was weakened and the catalytic response rate was increased, indicating that the charge transfer efficiency of the compound catalyst was improved [55]. The curve radius in the EIS spectrum was the smallest for PANIAg-CN, indicating its faster charge transfer rate. The injection lifetime of the photo-generated charges was analysed based on Fig. 11(c). Compared to g-C3N4, the characteristic frequency peak of the PANI-Ag-CN compound catalyst decreased from 1001 to 463.9 Hz. Moreover, the calculations in Table 1 showed that the electron lifetime of PANI-Ag-CN (343.081 ps) was 2.16 times higher than that of g-C3N4 (158.996 ps), proving that the formation of the Z-scheme heterojunction inhibited the recombination of the photo-generated charges and promoted the separation of the electron-hole pairs. LSV is a common electrochemical testing technique for exploring the carrier properties of semiconductors. As shown in Fig. 11(d), the photocurrent intensity of the PANI-Ag-CN compound catalyst was 7.65 mA·cm−2 at a −1.7 V applied bias, which was 1.4 times greater than that of g-C3N4. Hence, the increase in the external bias voltage further suppressed the recombination of the photo-generated charges and facilitated the electron transfer to the catalyst surface. In conclusion, the formation of the Z-scheme heterojunction improved the redox ability of the electron-hole pairs, prolonged the photo-generated electron lifetime of g-C3N4 and enhanced the catalytic ability of the complex. Fig. 12(a) displays the fluorescence spectra of g-C3N4, Ag-CN, PANICN and PANI-Ag-CN with an excitation and emission wavelength of 380 and 467 nm, respectively. The fluorescence peak intensity of the PANIAg-CN compound catalyst was significantly lower than that of g-C3N4. It exhibited also higher photo-generated charge separation efficiency and lower photo-generated electron-hole binding rate. The fluorescence lifetime generally refers to the average time that electrons need to pass from an excited to the ground state. As shown in Fig. 12(b), PANI-AgCN had a longer fluorescence lifetime than g-C3N4, implying that the photo-generated charge separation efficiency was higher in the PANIAg-CN catalyst, which was beneficial for the hydrogen evolution 7

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Fig. 10. (a) Photocatalytic hydrogen evolution cycle experiment of g-C3N4 and PANI-Ag-CN. (b) XRD patterns of PANI-Ag-CN before and after the reaction.

reaction [56–60]. The data analysis is presented in detail in Table 2. The BET specific surface areas of g-C3N4, bulk CN, PANI and PANIAg-CN are obtained by nitrogen adsorption-desorption experiments, as shown in Fig. 13. The catalysts exhibited IV-type isotherms and H3 hysteresis loops with distinct mesoporous structures [61]. Based on Table 3, the BET specific surface area of g-C3N4 was 47.096 cm3/g, which was about two times higher than that of bulk g-C3N4. Although the specific surface area of PANI-Ag-CN was not as large as that of gC3N4, its photocatalytic activity was greatly enhanced. This might be

Table 1 Bode data summary. Catalyst

Frequency (Hz)

Electron injection life (ps)

g-C3N4 PANI-CN PANI-Ag Ag-CN PANI-Ag-CN

1001 825.2 683.6 561.5 463.9

158.996 192.868 232.819 283.446 343.081

Fig. 11. (a) Transient photocurrent response. (b) EIS Impedance spectrum. (c) Bode-phase plots. (d) LSV curves of g-C3N4, Ag-CN, PANI-CN and PANI-Ag-CN. 8

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Fig. 12. (a) Fluorescence spectra. (b) PL lifetime decay curves of g-C3N4, Ag-CN, PANI-CN and PANI-Ag-CN.

due to the higher molecular weight of Ag, which reduced the atomic number mass per unit, affecting in turn the results of the specific surface area test. A similar phenomenon has been already reported in the literature [54]. Moreover, the nanosheets accumulation led to an increased interfacial contact, which correspondingly reduced the specific surface area. Consequently, the Z-scheme heterojunction effectively promoted the carriers’ separation. In order to further evaluating the photocatalytic activity of the compound catalyst, the AQE of PANI-Ag-CN was studied at different wavelengths, as shown in Fig. 14. The quantum efficiency of PANI-AgCN at 400, 450, and 500 nm was 1.795%, 0.568%, and 0.288%, respectively. When the wavelength exceeded 600 nm, the quantum efficiency remained unchanged, close to 0%. Thus, the complex absorbed photons with wavelengths over 600 nm but did not produce hydrogen. Therefore, it was concluded that the photocatalytic hydrogen evolution was mainly realised by the light absorption of g-C3N4. To elucidate the PANI-Ag-CN photocatalytic mechanism of the hydrogen evolution, the photoexcitation process of separating electrons and holes was described. The Mott-Schottky curve of the semiconductor was obtained by EIS. Fig. 15(a) and (c) shows the Mott-Schottky curves of g-C3N4 and PANI, respectively, with the positive slopes indicating that g-C3N4 and PANI were n-type semiconductor materials. The band potential of the n-type semiconductor conduction was approximately 0.1 V lower than the flat band potential. At different frequencies, the conduction band potential of g-C3N4 was −1.0 eV (vs. the normal hydrogen electrode (NHE)), while the conduction potential of PANI was 0.1 eV (vs. the NHE). Moreover, as shown in Fig. 15 (b), the calculated

Table 2 Transient fluorescence parameters.

a

Sample

Lifetime 〈τ〉 (μs)

Pre-exponential factors B%

〈τav〉 (μs)a

g-C3N4

τ1 = 1.1866 τ2 = 2.6383

B1 = 1369.417 B2 = 448.346

1.80

Ag-CN

τ1 = 2.0945 τ2 = 1.5736

B1 = 276.655 B2 = 1629.600

1.54

PANI-CN

τ1 = 2.7510 τ2 = 1.6679

B1 = 952.206 B2 = 1665.107

1.55

PANI-Ag-CN

τ1 = 1.1476 τ2 = 3.0747

B1 = 1513.108 B2 = 441.136

1.99

Average lifetime < τav > was determined using the equation < τav > = i =n Bi i2/ i = 1 Bi i .

i =n i=1

Fig. 13. N2 adsorption-desorption isotherms of g-C3N4, bulk g-C3N4, PANI and PANI-Ag-CN.

Table 3 Comparison of specific surface areas and hydrogen evolution rate. Sample 3

BET (cm /g) H2 (μmol·g−1·h−1)

bulk g-C3N4

g-C3N4

PANI-Ag-CN

PANI

28.4201 87

47.0957 116

37.6711 5048

9.7244 0

Fig. 14. Quantum efficiency of PANI-Ag-CN compared with g-C3N4, PANI and PANI-Ag-CN at different wavelengths. 9

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Fig. 15. Mott-Schottky plots of (a) g-C3N4 and (c) PANI. (b) Kubelka-Munk of g-C3N4. (d) VB spectrum for XPS spectra of PANI.

band gap of g-C3N4 was 2.6 eV. Fig. 15(d) shows the VB spectrum of the PANI XPS test, which indicated the VB potential of PANI was 2.5 eV. Fig. 16(a) illustrates the separation ability of the photo-generated electron-holes by the %OH content. Coumarin can easily react with %OH to generate a 7-hydroxycoumarin with fluorescent properties. The fluorescence peak of PANI-Ag-CN was significantly stronger than that of g-C3N4, implying that there were more %OH present in the complex. If the VB potential was greater than 2.46 eV, it was more likely to

generate %OH [62]. However, the band gap position of g-C3N4 did not satisfy this condition. To better understand the band gap position of PANI, the ESR spectroscopy was performed on PANI-Ag-CN under simulated sunlight. According to the recorded ESR spectrum in Fig. 16(b), the signal intensity ratio of the characteristic %OH peaks was 1:2:2:1. Thus, the presence of %OH in the photocatalytic hydrogen evolution further proved that the estimation on the band gap position of PANI was correct.

Fig. 16. (a) %OH fluorescence spectra. ESR Spectra of (b) PANI-Ag-CN for %OH under Dark and simulated Solar conditions. 10

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Fig. 17. Photocatalytic hydrogen evolution mechanism diagram.

The mechanism of the photocatalytic hydrogen evolution is shown in Fig. 17. The Ag nanoparticles inserted between g-C3N4 and PANI could participate in the transformation process of the photo-generated carriers. It was an excellent electron-trapping agent and could effectively capture the photo-generated electrons and inhibit the recombination of the photo-generated electron-hole pairs. Under light irradiation, weak holes excited by g-C3N4 could quickly be transferred to the Ag nanoparticles along with weakly photo-generated electrons excited by PANI. These photo-generated electrons and holes would be then annihilated when reaching Ag. Meanwhile, the strong reductive photo-generated electrons would move to the CB of g-C3N4, while strongly oxidised photo-generated holes would move to the VB of PANI. These photo-generated electrons would take part in the H+ reduction to H2 in H2O, while the photo-generated holes could participate in the oxidation of triethanolamine (TEOA) [63]. TEOA is generally used to capture photo-holes in photocatalytic hydrogen evolution systems, while chloroplatinic acid serves as an auxiliary electron transfer [64,65]. In addition, the formation of the Z-scheme structure could enhance the redox ability of the photo-generated holes and electrons, strengthen the photo-generated carriers lifetime of g-C3N4 and PANI, hinder the recombination process of electron-hole pairs and attain the charge separation and stabilisation. Due to the existence of conjugated pi bonds, the 2D nanosheet material could also provide a fast transmission channel for photo-generated electron-hole pairs and significantly improve the electron mobility, promoting thus the photocatalytic ability of the PANI-Ag-CN catalyst.

surface. Thus, the formation of the Z-scheme heterojunction greatly improved the redox ability of the electron-hole pairs and effectively promoted the photocatalytic reduction of the hydrogen evolution. CRediT authorship contribution statement Yuanyuan Liu: Data curation, Formal analysis, Writing-original draft, Writing-review & editing. JiFang Cui: Data curation, Formal analysis. Yinghua Liang: Funding acquisition, Supervision, Validation, Writing-review & editing. Weijia An: Data curation, Formal analysis, Writing-original draft, Writing-review & editing. Huan Wang: Data curation, Formal analysis, Writing-original draft, Writing-review & editing. Li Liu: Supervision, Validation, Writing-review & editing. Jinshan Hu: Supervision, Validation, Writing-review & editing. Wenquan Cui: Funding acquisition, Supervision, Validation, Writingoriginal draft, Writing-review & editing. Declaration of Competing Interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 51672081), the Natural Science Foundation of Hebei Province, China (No. B2018209356), One Hundred Excellent Talents of Innovation in Hebei Provincial Universities (III) (No. SLRC2017049). The second author JiFang Cui is an undergraduate student, who is studying towards a bachelor degree in applied chemistry from College of Chemical Engineering.

4. Conclusion In this work, PANI-Ag-CN Z-scheme sandwich structure photocatalyst was prepared. The Ag nanoparticles acted as an effective charge transfer intermediate between g-C3N4 and PANI. In addition, the determination of the PANI band gap position proved the establishment of the Z-scheme heterojunction. When the PANI content in PANI-Ag-CN was 10 wt% and the Ag content was 3 wt%, the optimum hydrogen generation rate (5048 μmol·g−1·h−1) was achieved. This rate was 43.52 times higher than that of g-C3N4 and 58.02 times higher than that of bulk g-C3N4. In addition, the ultra-thin 2D nanosheet structure facilitated the transfer of the photo-generated carriers to the catalyst

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