Ag clusters anchored conducting polyaniline as highly efficient

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Ag clusters anchored conducting polyaniline as highly efficient cocatalyst for Cu2ZnSnS4 nanocrystals towards enhanced photocatalytic hydrogen generation Xiufang Wang, Yaner Ruan, Shaojie Feng, Shaohua Chen, and Kaixuan Su ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01364 • Publication Date (Web): 23 Jul 2018 Downloaded from http://pubs.acs.org on July 23, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Sustainable Chemistry & Engineering

Ag clusters anchored conducting polyaniline as highly

efficient

cocatalyst

for

Cu2ZnSnS4

nanocrystals towards enhanced photocatalytic hydrogen generation Xiufang Wang,﹡,† Yaner Ruan,‡ Shaojie Feng,† Shaohua Chen,† and Kaixuan Su† †

School of Materials and Chemical Engineering, Anhui Jianzhu University, 292 Ziyun Road, Hefei Anhui,

230601, P.R. China ‡

College of Materials Science and Engineering, Donghua University, 2999 Renmin North Road, Shanghai,

201620, P.R. China

﹡E-email address:[email protected]

ABSTRACT:

Accelerating

the

photogenerated

carries

separation

of

semiconductor

photocatalysts is still a big challenge to develop highly effective hydrogen evolution reaction (HER) systems. Here, a novel ternary photocatalytic system of Cu2ZnSnS4 nanocrystals/Ag clusters/conducting polyaniline (CZTS/Ag/PANI) was successfully synthesized via facile solvothermal and hydrothermal method for the first time. The PANI, serving as the support, efficiently prevent the aggregating of the Cu2ZnSnS4 and Ag clusters, so CZTS nanocrystals (30-

ACS Paragon Plus Environment

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50 nm) and Ag clusters (2-5 nm) disperse equably in macromolecule PANI. PANI with extended π-conjugated electron system and Ag clusters with plasmon resonance effect are used as sensitizer/cocatalyst to enhance the light absorption and electrical conductivity and excite the separation of photo-generated electron and hole pairs. As expected, the CZTS/Ag/PANI heterostructured photocatalyst exhibits significantly enhanced photocatalytic activity for hydrogen evolution from water splitting in the presence of visible light. The optimal hydrogen evolution rate over CZTS/Ag/PANI with the apparent quantum yield (AQE) ～30.5% at 450 nm is 859.6 µmol h-1, which is also higher than previously reported for CZTS-based photocatalysts. Recycling experiments confirms that the CZTS/Ag/PANI composite exhibits remarkably stable photocatalytic performance and can be reused in four successive cycles. This research indicates the application of Ag nanoclusters and PANI semiconductor as cocatalyst can provide a new approach to design and synthesize stable and high efficiency CZTS-based visible-light-induced photocatalysts. KEYWORDS: Cu2ZnSnS4; Polyaniline; Ag clusters; Cocatalyst; Photocatalytic hydrogen production INTORDUCTION In order to solve the energy crisis and environmental problems caused by the consumption of fossil fuels and meet the growing global energy demands, the development of renewable and clean energy is a promising approach. Hydrogen is deemed as one of the most potential energy carriers in the future, as it has the highest specific energy of combustion and produces only water as a combustion product. Semiconductor-based photocatalytic hydrogen (H2) production from water is of great significance for solar-to-chemical conversion courses to boost and accelerate the


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future hydrogen economy.1,2 In spite of lots of efforts have been made, photocatalytic H2 production by a robust and highly effective system driven by visible light all the same remains a great challenge. The major limiting factors affecting the efficiency of hydrogen production are as follow: (i) charge separation and transport, (ii) light absorption and (iii) surface chemical reaction. Thus, to achieve highly effective photochemical conversion, it is very necessary to maximize the separation of photogenerated electron-hole pairs and enhance the absorption of light.3 Among semiconductor photocatalysts, sulfide photocatalysts including ternary chalcogenides and binary sulfides (ZnIn2S4, SnIn4S8 CdS, Cu2S and so on) with narrow band gaps, have been demonstrated to be wonderful candidates for photocatalytic H2 evolution from water under visible light irradiation.4-7 Especially, recently, a kind of direct-gap semiconductor, Cu2ZnSnS4 with narrow band-gap of 1.5 eV, large absorption coefficient of over 104 cm-1 in visible spectrum range and low toxic of the composition elements has attracted much attention.8 The environmental friendly CZTS has been demonstrated to possess photocatalytic activity for H2 production under visible light radiation,9 which offers new insights into development of inexpensive and visible‐light driven photocatalysts for H2 production. At the same time, the CZTS photocatalyst is still facing the same challenges encountered by most photocatalysts, such as plenty of lattice and surface defects, limited catalytic efficiency associated with the fast recombination rate of photogenerated carriers, photocorrsion issue owing to the oxidation of S– metal bond by photo-generated holes and poor electrical conductivity.10 For improving the photocatalytic efficiency of CZTS, a variety of strategies have been utilized to enhance the H2 production efficiency of CZTS, for example controlling crystallinity,11 different synthesis methods,12 and forming heterostructure with cocatalysts. Among these strategies, the


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construction of heterostructure in CZTS structures with cocatalysts is consistently regarded to be a common way. The introduction of cocatalysts not only effectively separates the photogenerated carriers but also provides more active sites to promote the adsorption of light and oxidation/reduction reactions.13,14 For example, using Au or Pt as cocatalyst, Yu et al. studied the enhancement of photocatalytic H2 evolution of CZTS/Au and CZTS/Pt under visible light irradiation.15 Yuan et al. observed that CdS cocatalyst on the surface of CZTS crystals played a crucial role in achieving very high efficiency for H2 evolution.16 Cabot et al. used PtCo as cocatalysts to prepare CZTS–PtCo heterostructures with enhanced catalytic H2 evolution activity.17 Although these cocatalysts have been explored to be efficient for promoting the activity of CZTS-based photocatalysts, the development of novel cocatalysts to achieve the high photocatalytic activity of CZTS is still concerned. Conducting polymers have given rise to a great interest on account of their simple synthesis, high stability and conductivity, and good environmental compatibility.18 Among conducting polymers, polyaniline as a typical delocalized conjugated material is a narrow band gap semiconductor with ultrahigh absorption coefficient in the visible light range, and it can also be deemed to good substrate for photocatalysts.19 PANI can be usually used to suppress the recombination of the electron-hole pairs and enhance the charge separation efficiency, so it is used in hybrid with other photocatalysts, and the photocatalytic efficiency is promoted.20 PANIbased composite semiconductor photocatalysts for hydrogen production have been recently reported and demonstrated superior catalytic activity. Ghaly et al. prepared plasmonic photocatalysts of Ag/AgCl–PANI by deposition–precipitation reaction and studied their photocatalytic hydrogen production.21 Sasikala et al. synthesized a heterostructure MoS2-PANICdS catalyst, which assumed increased catalytic activity for H2 generation from water in the


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visible light.22 Through a green liquid probe sonication and convenient hydrothermal-assisted methods, our group designed and synthesized novel MoS2 quantum dot-modified Ag nanoparticle/polyaniline nanocomposites. The hybrid photocatalyst presents dramatically enhanced photocatalytic activity in visible light for H2 production from water splitting.23 Hence, construction of PANI-based visible light catalyst materials has been regarded as one of the viable strategies to improve the photocatalytic performance. In addition, not only due to their electronic effect promoting the separation and migration process of the photogenerated electrons and holes in metal-semiconductor interface,24 but also because of their unique surface plasmon resonance (SPR) effect which can prominently enhance the visible light harvesting of photocatalysts,25,26 loading silver nanoparticles (NPs) on the the surface of semiconductor photocatalysts has also been demonstrated as an effectual method to enhance photocatalytic activity.27-29 Therefore, an effective charge separation can be anticipated by preparation CZTS nanocrystals using plasmonic Ag NPs and PANI as cocatalyst. So far as we know, there are almost no reports on the preparation and photoactivity of the CZTS-Ag NPs-PANI ternary system for light-driven H2 evolution. Here we report a facile synthesis of ternary photocatalytic system consisting of Cu2ZnSnS4 nanocrystals and plasmonic Ag clusters supported on conducting polyaniline. The Ag/PANI as novel cocatalyst with superior electrical conductivity enhances the light absorption and active sites, and effectually separates the photogenerated electron-hole pairs. So the CZTS/Ag/PANI heterostructured photocatalyst exhibits significantly enhanced photocatalytic activity with the apparent quantum yield ～30.5% at 450 nm for water splitting H2 production under visible light irradiation. The mechanism of the improved photocatalytic performance of CZTS/Ag/PANI


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composites is also researched. This work may appord new insights for the design and preparation of novel high efficiency and stable visible–light–induced CZTS-based photocatalysts.

EXPERIMENTAL SECTION Materials. Zinc acetate dihydrate (Zn(CH3COO)2·2H2O), Cupric chloride dihydrate (CuCl2·2H2O), Tin(II) chloride dehydrate (SnCl2·2H2O), silver nitrate (AgNO3),Aniline monomer, ammonium persulfate and thiourea (H2NCSNH2) are of analytical grade, used without any further purification. Double distilled water was used in all the experiments to prepare the solutions. Preparation of CZTS Nanocrystals. Pure CZTS was synthesized using a traditional hydrothermal method. Briefy, CuCl2·2H2O (4 mmol), Zn(CH3COO)2·2H2O (2 mmol), SnCl2·2H2O (2 mmol), and H2NCSNH2 (8 mmol) were added into ethylene glycol (80 mL) under continous stirring to obtain a transparent yellow colloidal suspension, and then, the obtained transparent colloidal suspension was put in a Teflon autoclave (100 mL). The tightly closed autoclave was heated for 24 h at 200 ℃ and followed by, left to cool at room temperature. The precipitates were collected by centrifugation, washed with deionized water and ethanol three times and then dried under vacuum at 70 ℃ for 24 h and used for further experiments. Synthesis of CZTS/Ag/PANI nanocomposites. The CZTS/Ag//PANI nanocomposites were synthesized as follows: different amount of CZTS was first added into 30 mL distilled water. 0.18 mL of aniline, 0.051 g AgNO3 and 0.05 g PVP were mixed with above suspension with vigorous stirring under a nitrogen atmosphere. The mixture was sealed in a 50 mL teflonlined autoclave and heated at 140 ℃ for 4 h. After the autoclave was cooled to room temperature, the prepared sample was collected and washed three times each with ethanol and double distilled


6

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water, and dried at 60 ℃ for 24 h. The resulting products were referred to as CZTS/Ag/PANI-1,

CZTS/Ag/PANI-2,

CZTS/Ag/PANI-3,

CZTS/Ag/PANI-4

and

CZTS/Ag/PANI-5 when the mass of CZTS nanocrystals is 5 mg, 10 mg, 20 mg, 30 mg and 50 mg, respectively. In addition, for comparison, Ag/PANI was prepared as shown above in the absence of CZTS, and the pure PANI and CZTS/PANI were synthesized using ammonium persulfate as the oxidant for aniline. Characterization. The UV-vis absorption spectra in the range of 200-900 nm of the sample dispersed in distilled water through ultrasonic irradiation were obtained with U-4100 UV spectrophotometer. The fourier transform infrared (FTIR, frequency range from 500 to 3600 cm1

) spectra were obtained using a NEXUS-870 spectrophotometer. Raman spectra were measured

with a Renishaw 2000 model confocal microscopy Raman spectrometer (Renishaw Ltd., Gloucestershire, UK) with a CCD detector and a holographic notch filter. The scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) function were both applied to characterize the surface morphology and element contents of the samples and taken using a JSM7500F field-emission scanning electron microscope at an accelerating voltage of 5 kV. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were measured by using a JEM-2100F apparatus with a field emission gun operating at 200 kV. X-ray diffraction (XRD) characterizations were completed on a Bruker D8 ADVANCE X-ray diffraction device using Cu Ka radiation (50 kV). X-ray photoelectron spectroscopy (XPS) characterization was operated on an ESCALAB-MKII spectrometer with Al Kr X-ray radiation as the X-ray source for excitation. The photocurrent tests were performed on an electrochemical workstation in a traditional three-electrode cell(CHI660C, China). It is composed of a reference electrode (an Ag/AgCl electrode), the working electrode (the glass with


7


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sample) and the counter electrode (a Pt sheet). The electrolyte solution is Na2SO4 (0.5 mol L-1, pH = 6.8). The catalyst suspensions were smeared evenly on the precleaned indium tin oxide glass surfaces (1×2 cm) and then dried at 100 ℃ for 1h to prepare the working electrodes. The surface area of the working electrode exposed to the electrolyte was about 0.95 cm2. The photocurrent densities were gained under visible light irradiation of a 300 W Xe-lamp (light on/off cycles: 50 s) equipped with a 420 nm cut-off filter at a bias of 0 V vs. Ag/AgCl. Photocatalytic activity for hydrogen evolution. The photocatalytic hydrogen evolution experiments were carried out in a 100 mL flask with stirring at ambient temperature using a 300 W Xe lamp furnished with a cut-off filter (λ﹥420 nm). 50 mg of photocatalysts were dispersed in a 50 mL mixed aqueous solution containing Na2S and Na2SO3 as the sacrificial donor and stirred for 30 min using a magneton. In order to ensure the reaction system in an inertial condition, the system was bubbled with nitrogen for 30 min to remove the air before irradiation. A cooling water circulating and continuous stirring were necessary in the whole reaction to make certain the suspension homogeneity at normal temperature. Using high purity N2 as the carrier gas, the H2 was analyzed by gas chromatography (Shimazu GC2010) equipped with a 5 Å molecular sieve column and a thermal conductive detector. The AQE was measured on the basis of the following formula:

RESULTS AND DISCUSSION


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CZTS Ag

Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CZTS/Ag/PANI

002 100

110 103 112

101

CZTS

10

20

30

40

50

60

70

80

90

2 θ (degree) Figrue 1. XRD patterns of the pure CZTS and CZTS/Ag/PANI-3 nanocomposites.

XRD patterns of prepared samples are recorded and shown in Figure 1. For pure CZTS, the major diffraction peaks at 2θ values of 26.96°, 28.49°, 30.50°, 47.51°, and 51.46° are observed, which can be due to the (100), (002), (101), (110), (103), and (112) planes of a hexagonal structure and match with those of the simulated wurtzite CZTS. No impurity peaks are found, indicating a high purity of CZTS in the prepared sample. It can be seen that the CZTS/Ag/PANI nanocomposite exhibits similar XRD patterns to pure CZTS nanospheres, and the peaks and shapes have no change. This indicates that loading of Ag and PANI may not destroy the main crystal structure of CZTS. Due to the small size of Ag, several weak peaks of metallic Ag in the CZTS/Ag/PANI nanocomposites are observed at about 77.43°, 64.33°44.30°, and 38.05°. They correspond to Bragg’s reflections from (311), (220), (200) and (111) planes of a face-centered cubic lattice Ag, which are agreement with the literature values (JCPDS No. 04-0783). The presence of metallic Ag NPs in composite has been verified in the later section. PANI has a broad band at 28°, and it is ascribed to the periodicity parallel to the polymer chains of PANI.30 However, the band of PANI is covered by the high intensity of the [002] diffraction peak of


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CZTS with maximum intensity at around 28.49°. There are no other crystalline impurities in the synthesized nanocomposites.

Figure 2. SEM images of pure CZTS (a), and CZTS/Ag/PANI-3 nanocomposites (b and c), and EDS spectrum (d).

The morphology of the as-prepared CZTS and CZTS/Ag/PANI-3 samples is investigated by SEM. As shown in Figure 2a, pure CZTS has spherical shapes with the diameter of about 3050nm. These small size nanocrystals are conducive to photocatalytic reaction. But these nanospheres aggregate together, which can be attributed to high surface energy of the NPs. Figrue 2b reveals the low-magnification SEM image of CZTS/Ag/PANI-3. It is observed that the


10

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nanocomposite consists of CZTS nanopheres and thin flakes of PANI. The CZTS nanopheres are decentralized in the PANI flakes. The high-magnification SEM of Figrue 2c indicrates that the CZTS nanopheres are wrapped by PANI. However, the Ag NPs are not clearly observed in the CZTS/Ag/PANI-3 composite, and it may put down to the ultra small size of the Ag NPs, and it will further characterization by TEM and HRTEM. EDS analysis data of the CZTS/Ag/PANI-3 sample is shown in Figrue 2d. Ag, C, Cu, Zn, Sn, and S element are detected over the entire area of the CZTS/Ag/PANI sample. The Cu:Zn:Sn:S is 2:1:1:4, which is very close to that of stoichiometric CZTS. 60

50

50

40

Percentage(%)

Percentage(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40

30

30

20

20

10

10 0 1

0 20

30

40

50

Diameter (nm)

60

70

2

3

4

5

Diameter (nm)

6

7

Figure 3. TEM images of CZTS (a), CZTS/Ag/PANI (b), and HRTEM images of CZTS/Ag/PANI-3 (c and d).


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Figrue 3 shows the transmission electron microscopy analysis of the synthesized samples, which reflects the structural information and microscopic morphology. Figrue 3a shows the TEM image of as-prepared CZTS nanocrystals, which have a spherical geometry. As shown in the inset of Figrue 3a, the diameter of the CZTS nanospheres ranges from 30 nm to 50 nm and mainly locates at about 40 nm. A TEM image of the CZTS/Ag/PANI-3 is shown in Figrue 3b, and it can be seen clearly that two different nanosized materials coexist in the PANI. One with smaller size is Ag NPs and the other should be CZTS nanospheres. Energy-dispersive X-ray spectroscopy mapping also shows a uniform distribution of Ag NPs in the CZTS/Ag/PANI composites (Figure S1). The insert figure reveals that the size of Ag NPs is primarily 2-5 nm with average diameter about 4 nm. Ag nucleates preferentially at the PANI surface under the detailed reaction conditions, and no independent particles are observed. In our system, CZTS/Ag/PANI nanocomposites are synthesized using AgNO3 as the oxidant for aniline in the presence of CZTS nanospheres by one-step hydrothermal method, and aniline is oxidized to PANI while AgNO3 is reduced to Ag clusters. PANI molecules have a great deal of amine and imine functional groups, and the N atoms have a lone pair of electrons, which can involve coordination between metal ions and N atoms.31 The multiple nucleation sites present at the surface of PANI make CZTS and Ag NPs distribute homogeneously in the CZTS/Ag/PANI composites. The PANI efficiently protects the CZTS nanocrystals and Ag NPs from aggregation and also endows the system with enhanced stability.


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a

b ECZTS/Ag/PANI-2= 2.10 eV

CZTS/Ag/PANI-1

(ahv) (a.u.)

CZTS/Ag/PANI-2

ECZTS/Ag/PANI-3= 2.02 eV

2

Absorbance(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CZTS/Ag/PANI-3


CZTS/Ag/PANI-4

CZTS



CZTS/Ag/PANI-5

200

300

400

500

ECZTS= 1.52 eV

600

700

Wavelength(nm)

800

900

1

2

3

4

hν (eV)

5

6

7

Figure 4. Optical absorption spectra of CZTS and CZTS/Ag/PANI samples (a), and Plots of (ahv)2 vs. photon energy (hν) (b).

The optical absorption behavior of photocatalysts is essential for photocatalytic activity, and the UV-vis absorbance spectra of pure CZTS and CZTS/Ag/PANI samples are studied and shown in Figrue 4. The pure CZTS exhibits typical broad absorption from ultraviolet light to visible light region (Figrue 4a), which is consistent with the literature report.32 The excellent photoabsorption ability provides a favorable basis for visible light photocatalysis. The CZTS/Ag/PANI samples display additional visible light absorption peak from 410 nm to 600 nm region, which derives from the intense surface plasmon resonance of nanostructured silver and light absorption of PANI. This favors the photocatalytic performance of CZTS/Ag/PANI catalyst, and it also testifies the successful synthesis of Ag nanocrystalline and PANI. The light harvesting capacity of CZTS/Ag/PANI enhances gradually as increasing PANI and Ag amount in composites. The enhanced light absorption property will result in more effective taking advantage of the solar energy for the CZTS/Ag/PANI composite system. From the UV-vis spectrum, the direct band gap of the CZTS nanospheres has been calculated to be around 1.52 eV by extrapolation of the linear region of a plot of (αhν)2 versus energy (Figrue 4b), where hν is the


13


photon energy and α represents the absorption coefficient. The band gap energies of CZTS/Ag/PANI-1,

CZTS/Ag/PANI-2,

CZTS/Ag/PANI-3,

CZTS/Ag/PANI-4,

and

CZTS/Ag/PANI-5 are, 2.48, 2.02, 2.10 1.83, and 1.60 eV, respectively. Band gaps of the CZTS/Ag/PANI composites broaden with the modification of Ag NPs and PANI. It is well known that a higher energy band gap for the composite could be good for improving the redox potential of photocatalytic reaction, and it is very important for water splitting. In conclusion, this CZTS/Ag/PANI hybrid system presents its great potential to turn into a practical catalytic material in visible light.

CZTS/Ag/PANI

Transmittance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Ag/PANI

823

pure PANI

1150 1580 1240 12971495

500

1000

1500

2000

2500

3000

Wavenumber(cm-1)

3500

Figure 5. FTIR spectra of pure PANI, Ag/PANI and CZTS/Ag/PANI-3.

The molecular structure of the resulting samples is represented by FTIR spectroscopy. For comparison, the FTIR spectra of pure PANI and Ag/PANI composite are characterized and shown in the Figrue 5. In the FTIR spectrum of pure PANI, the bands at 823 cm-1 and 1150 cm-1 are the distinctive features of C-H out-of-plane bending and C-H in-plane, and peaks at 1240 cm1

and 1297 cm-1 are related to the C=N and C-N stretching modes. The peaks at 1495 cm-1 and


14

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1580 cm-1 are due to the C=C stretching vibration of benzenoid and quinoid rings, respectively. The FTIR spectra of the Ag/PANI and CZTS/Ag/PANI-3 composite resembles that of pure PANI and also show all the characteristic bands of PANI, confirming PANI existence. In the FTIR spectrum of the Ag/PANI, the band at 1297 cm-1 shifts to 1285 cm-1, which is by reason of the interactions between Ag nanoclusters and N atoms on PANI molecules through sharing lone pairs of electrons.33 After combining with CZTS, the absorption bands demonstrate some changes. The representative signals of C-N and C=N bond stretching vibration reduce apparently, and the bands at 1580 cm-1 shifts to 1600 cm-1. These changes derive from the interactions between CZTS and amine and imine functional groups of PANI, which results in a good dispersion of CZTS nanosphers (see Figrue 2 and 3).

Figure 6. Raman spectra of CZTS, and CZTS/Ag/PANI-3 (532 nm laser). Raman spectroscopy characterization is used to further verify the structure and composition of the samples. Raman scattering result of CZTS over 200-500 cm-1 is displayed in the inset of Figure 6, and the peak at 331 cm-1 is consistent well with the value reported for wurtzite CZTS.16 The result excludes the existence of other compounds and affirms the composition of the asobtained CZTS nanoparticles by only wurtzite CZTS. There some new peaks in the


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CZTS/Ag/PANI-3 composite apart from peaks of CZTS, and they are characteristic peaks of PANI and reveal the existence of PANI.23 The 868 cm-1 and 639 cm-1 signals are characteristic of the out-of-plane vibrations of aromatic rings and benzenoid ring deformation, respectively. The signal at 523 cm-1 is assigned to out-of-plane PANI ring bend. The C–N stretching vibrations of various quinonoid or polaronic forms, benzenoid and benzene ring deformations identify with the band at 1239 cm-1. The peak at 1310 cm-1 in the spectrum offers the information of the C–N+ vibration of delocalized polaronic structures. A band at 1340 cm-1 is observed in the spectrum, which is assigned to the phenazine structures. The peak at 1430 cm-1 can put down to the C=N stretching vibration in quinonoid units. The band of about 1518 cm-1 may belong to an N–H deformation vibration affiliated to the semi-quinonoid structures. The peak observed at 1578 cm1

is linked to C=C stretching vibration in the quinonoid ring of polyaniline.


16

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Figure 7. High-resolution XPS analysis of CZTS/Ag/PANI-3.

The XPS is a escpecially helpful instrument for studying the electronic structure and surface composition of materials, which can provide information on the chemical environment of the elements in the composite. The XPS survey spectrum identifies the existence of Ag, Cu, C, Zn, Sn, S and N in the CZTS/Ag/PANI (Figrue S2). Figrue 7a is the part of the same spectrum, and two characteristic copper 2p peaks with a split orbit of 19.8 eV locate at 933.0 eV and 952.8 eV, which indicate the Cu (I) in as-synthesized heterostructured catalyst. Zn(II) state is ascertained from the peaks at 1044.2 eV (2p 1/2) and 1021.7 (2p 3/2) eV with the peak splitting of 22.5eV


17


(Figrue 7b). The peaks of Sn 3d that emerge at 487.6 eV and 496.1 eV with the characteristic peak separation of 8.5 eV affirm Sn(IV) state (Figrue 7c). The S 2p peaks locate at 162.7 (2p 1/2 ）eV and 161.4 (2p 3/2) eV with an energy diversity of 1.3 eV (Figrue 7d), which agree well with the existence of sulfur element. Thus, the results are consistent with the reported data of CZTS in literatures.

15-17,34

In the Figrue 7e, two peaks corresponding to the Ag 3d5/2 and Ag

3d3/2 energy levels appear of Ag atom at 368.3 eV and 374.5 eV, indicating the presence of metallic Ag. The peak of C 1s is divided into three Gaussian peaks at 284.7 eV, 285.7 eV, and 288.8 eV, identifying with C=C/C-C in aromatic rings, N–C=N, and C–(N)3 (Figure 7f), and they are the characteristic peaks of PANI.23 Based on the above analysis, it can be inferred that the CZTS/Ag/PANI has been successfully synthesized. 1.0

a

Hydrogen/µmol h-1

800 600 CZTS/PANI

400 200

b

CZTS/Ag/PANI

PANI

CZTS

Ag/PANI

Hydrogen/µmol h-1

1000

CZTS/Ag/PANI-5 CZTS/Ag/PANI-4 CZTS/Ag/PANI-3 CZTS/Ag/PANI-2 CZTS/Ag/PANI-1

0.8 0.6 0.4 0.2 0.0

0 1

2

3

4

5

0

1

2

3

4

5

6

c 800

Hydrogen/µmol h-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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600

400

200

0

Ag cluster

Pt

Au

Figure 8. Photocatalytic H2 evolution rates from PANI, CZTS, Ag/PANI, CZTS/PANI and CZTS/Ag/PANI-3 (a), with different amounts of CZTS (b), photocatalytic H2 evolution over


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CZTS/Ag/PANI, CZTS/Pt/PANI and CZTS/Au/PANI (c), and. wavelength-dependent quantum efficiency over the CZTS/Ag/PANI-3 sample (d).

The photocatalytic performance is evaluated by a photocatalytic H2 evolution reaction under visible light irradiation (λ > 420 nm), and Na2S and Na2SO3 act as a hole-scavenger. Figrue 8a compares the photocatalytic H2 evolution rate of different samples. No H2 could be detected when pure PANI is used as catalyst, and Ag/PANI produces H2 at an average rate of 7.8 µmol h1

. The pure CZTS nanospheres display relatively higher photocatalytic hydrogen production rate

(109.7 µmol/h) because of lower band-gap than that of PANI and Ag/PANI. But the narrow band-gap of CZTS leads to rapid recombination of photogenerated electrons and holes, hence the photocatalytic hydrogen evolution rate of CZTS is still rather low. For comparison, the CZTS/PANI sample is also tested for photocatalytic H2 evolution, the average rate of H2 evolution is 420.6 µmol h−1, The PANI cocatalyst demonstrates good enhancement for photocatalytic H2 production compared with pure CZTS. The CZTS/Ag/PANI-3 composite reaches the optimum H2 production rate of 859.6 µmol h−1 (17.192 mmol g-1 h−1), and it is 2 times higher than that of CZTS/PANI composite and 4.1 times better than that of original CZTS. This value is also the highest compared with previous literature reports for CZTS-based photocatalysts.15,

16

The present results clearly indicate that the extraordinary hydrogen

production rate is owing to the synergetic effect of the PANI and Ag NPs as cocatalyts, which results in the efficient separation of photogenerated electron-hole pairs for efficient H2 production.


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Figure 8b reveals the effect of different content of CZTS in CZTS/Ag/PANI nanocomposites on the hydrogen production activity. The photocatalytic hydrogen production efficiency of the nanocomposites increases firstly and then decreases gradually with the growth in introduced CZTS quantity. Lower weight percentage of CZTS results in lower rates of hydrogen production owing to the fewer number of catalytic active sites. Increasing the weight percentage of CZTS beyond the optimum level (CZTS/Ag/PANI-3) leads to the decrease of photocatalytic hydrogen production rate. The excess CZTS possibly acts as a recombination center for the photogenerated electron-hole pairs,35 which results in the decline in the activity. To illuminate the effects of Ag clusters as cocatalysts in photocatalytic hydrogen production, we synthesized CZTS/Pt/PANI and CZTS/Au/PANI samples using H2PtCl6 and HAuCl4 as the oxidant for aniline, and inspected their photocatalytic H2 evolution activity, as shown in Figrue 8c. It can be seen that the H2 evolution rate of Pt and Au cocatalysts modified CZTS/PANI decreases as compared to CZTS/Ag/PANI. The oxidation ability of H2PtCl6 and HAuCl4 is stronger than that of AgNO3, so they can oxidize quickly aniline in aqueous solution. However, Pt and Au NPs with larger size in composites were obtained instead of Pt and Au clusters, which is bad for photocatalytic hydrogen generation. This result indicates that the electrons in Ag clusters are excited more easily to the higher energy state owing to the SPR effect for H2 evolution. To work out more details on the enhanced catalytic activity in visible light, the apparent quantum efficiency (AQE) has been investigated at different irradiation wavelength. As shown in Figure 8d, AQE is still very high when the wavelength reaches 500 nm. The action spectrum threshold of CZTS/Ag/PANI is up to 550 nm. The photons at wavelengths over 550 nm


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absorbed by CZTS/Ag/PANI cannot lead to hydrogen generation, which implies that the excitation light at λ>550 nm could not produce sufficient energy electrons for H+ reduction.

70

CZTS/Ag/PANI CZTS Ag/PANI PANI

-2

1 st run 2 nd run 3 rd run 4 th run

Current density/µA cm

a

1000

Hydrogen/µmol h-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

800

b

50 40

600

30

400

20

200

10 0

0 0

5

10 15 Time/h

20

25

0

50

100

150

200

Time/s

250

300

350

Figure 9. recyclability in H2 production with CZTS/Ag/PANI-3 (a) and photocurrent response vs. time profiles of PANI, CZTS, Ag/PANI, and CZTS/Ag/PANI-3(b).

To investigate the stability of the photocatalyst, we carry out the photocatalytic hydrogen production experiment using the CZTS/Ag/PANI-3 for 20 h of visible light irradiation (λ ≥ 420 nm). As shown in Figrue 9a, the H2 evolution of CZTS/Ag/PANI-3 heterostructure increases linearly. During the continuous 4 runs over 20 h, CZTS/Ag/PANI-3 does not indicate the obvious decrease in the photocatalytic activity, indicating the stable photocatalytic activity of CZTS/Ag/PANI-3 photocatalyst. The very slight decrease can be attribute to the loss of the sacrificial reagents during the long‐range reaction. Furthermore, after the reaction, the catalyst is recovered and characterized by XRD and no distinct alterations in its structure are found, which again reflects the robust stability of our photocatalyst (see Figrue S3). We also calculate the apparent quantum yield for the


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optimized CZTS/Ag/PANI-3 nanocomposite under visible light irradiation using a 300 W Xe lamp with a 450 nm band pass filter. The quantum yield is estimated to around 30.5 %. Photoelectrochemical performances of different samples are compared using chronoamperometric technique on FTO electrodes under the intermittent visible light, and the results are shown in Figrue 9b. It can be seen that the pure PANI does not demonstrate any current response under either visible light or dark conditions. The weak photocurrent response is observed for the Ag/PANI under the same on/off condition. However, the current signal of the CZTS exhibits a decrease and increase with light off and on, respectively. The CZTS nanospheres can strongly absorb visible light because of narrow band gap, resulting in the formation of electron-hole pairs. However, the ternary CZTS/Ag/PANI-3 nanocomposite shows the best photocurrent performance with a photocurrent density of 0.050 A/m2 in the presence of visible light, because the existence of Ag and PANI in the CZTS/Ag/PANI-3 nanocomposite greatly enhances the conductivity and increases the separation effect electron-hole pairs. The results are almost consistent

with

their

photocatalytic

performances,

affirming

that

the

ternary

CZTS/Ag/PANI-3 nanocomposite possesses highest visible light conversion efficiency.


22

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Scheme 1. Photocatalytic mechanism of CZTS/Ag/PANI nanocomposites under visible light irradiation.

Based on the above analyses, a photocatalytic H2 production mechanism is described and shown in Scheme 1. The band gaps of PANI and CZTS are ～2.8 eV,36 and ～1.5 eV (Figrue 4), and they can be excited when the CZTS/Ag/PANI photocatalyst is irradiated by the visible light. Based on the relative energy levels of PANI (-2.14 eV and +0.62 eV)36 and CZTS (-0.7 eV and +0.8 eV),16 the excited state electrons produced from PANI are immitted into the CB of CZTS. Afterwards, the holes on the VB of CZTS transfer to the VB of PANI. The recombination process of the photogenerated carriers is prevented, and charge separation and stabilization are achieved. Moreover, once the visible light is absorbed, the electrons in Ag nanoclusters are stimulated to a higher energy state because of the SPR effect. These plasmon hot electrons of Ag NPs transfer directly or via conducting PANI to the CB of CZTS. So the photo-induced electrons on the CB of CZTS are captured by H+ and used to reduce H+ to H2, meanwhile the holes are applied to oxidize the sacrificial agents (S2− and SO32-) to finish the catalytic cycle. Therefore, this significantly separation of photogenerated electron-hole pairs leads to the improvement of photocatalytic H2 production. CONCLUSIONS In summary, we have successfully synthesized a novel ternary photocatalytic system of CZTS/Ag/PANI through a simple hydrothermal process. The Ag clusters and conducting PANI can be used as efficient cocatalysts for CZTS nanocrystals, achieving extraordinary photocatalytic hydrogen production in visible light. The existence of Ag and PANI leads to


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enhance the light absorption, electrical conductivity and efficient separation of the photogenerated electron-hole pairs. Among all the discussed photocatalysts above, the CZTS/Ag/PANI-3 ternary photocatalytic system with outstanding stability possesses the highest HER as 859.6 µmol h-1 with the AQE ～30.5% at 450 nm. We hope the finding in this work can illuminate the reasonable design of CZTS-based low-cost, environmental friendliness and efficient photocatalysts for solar hydrogen generation. ASSOCIATED CONTENT Supporting Information. EDS map of Ag, XPS result of CZTS/Ag/PANI nanocomposite and XRD patterns of before and after catalytic reaction CZTS/Ag/PANI-3 nanocomposite. AUTHOR INFORMATION Corresponding Author *Xiufang Wang. E-email address: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Anhui Provincial Natural Science Research Project (KJ2018A0512), and Anhui provincial key research and development project (1704g07020113).


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Table of Contents Graphic and Synopsis

A novel ternary photocatalytic system of Cu2ZnSnS4 nanocrystals/Ag clusters/ polyaniline was fabricated towards enhanced photocatalytic hydrogen generation.


31


Table of Contents Graphic and Synopsis

! A novel ternary photocatalytic system of Cu2ZnSnS4 nanocrystals/Ag clusters/ polyaniline was fabricated towards enhanced photocatalytic hydrogen generation.


Page 32 of 32

Ag clusters anchored conducting polyaniline as highly efficient

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