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ZnS nanocomposites decorated g-C3N4 nanosheets as noble metal-free photocatalyst for superior photocatalytic water splitting
Accepted Manuscript Cauliflower-like CuS/ZnS nanocomposites decorated g-C3N4 nanosheets as noble metal-free photocatalyst for superior photocatalytic water splitting R. Rameshbabu, P. Ravi, M. Sathish PII: DOI: Reference:
S1385-8947(18)32141-7 https://doi.org/10.1016/j.cej.2018.10.180 CEJ 20243
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
27 July 2018 24 October 2018 25 October 2018
Please cite this article as: R. Rameshbabu, P. Ravi, M. Sathish, Cauliflower-like CuS/ZnS nanocomposites decorated g-C3N4 nanosheets as noble metal-free photocatalyst for superior photocatalytic water splitting, Chemical Engineering Journal (2018), doi: https://doi.org/10.1016/j.cej.2018.10.180
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Cauliflower-like CuS/ZnS nanocomposites decorated g-C3N4 nanosheets as noble metal-free photocatalyst for superior photocatalytic water splitting R. Rameshbabu, P. Ravi, M. Sathish* Functional Materials Division, CSIR-Central Electrochemical Research Institute, Karaikudi-630 003, Tamil Nadu, India Name and Mailing Address of Corresponding Author: Dr. M. Sathish Scientist, Functional Materials Division, CSIR-Central Electrochemical Research Institute, Karaikudi -630006 Tamilnadu, INDIA Phone: +91-4565-241410 Web: http://www.cecri.res.in/ Web: http://scholar.google.co.in/citations?user=4X-7WnsAAAAJ&hl=en Email: [email protected] [email protected] Abstract A promising cauliflower like morphology of CuS/ZnS/g-C3N4 is developed as an effective visible-light driven photocatalyst via a facile hydrothermal route followed by ultrasound assisted wet impregnation method and cation exchange treatment. As prepared CuS/ZnS/g-C3N4 nanocomposites were characterized using various analytical techniques to confirm the composite formation and study their morphology, structure and band gap. Na2S and Na2SO3 solutes illuminated by 250 W halogen lamp are employed to evaluate the hydrogen production capability of the as prepared photocatalyst. The results revealed the superior photocatalytic activity in hydrogen production reaction by as prepared CuS/ZnS/g-C3N4 nanocomposites against g-C3N4, ZnS and CuS/ZnS photocatalysts. We have achieved the highest hydrogen production rate of 9868 μmol h-1 g-1 by the composite containing 5 wt% of gC3N4 with excellent durability during photocatalytic hydrogen production. This excellent performance attributed to the increased absorption level of visible light in the nanocomposite and higher stability of electron-hole pairs created upon irradiation.
Keywords: Photocatalysts; g-C3N4 nanosheets; hydrogen evolution; CuS/ZnS nanocomposite; stability; Introduction To meet the growing energy demands and counter the pollution, many researchers have shown interest in the photocatalytic water splitting and organic dye degradation [1–8]. TiO2 is found to serve the purpose, as it is photo catalytically active with good stability and non-toxicity [9,10]. However, its large band gap of 3.2 eV restricts its use to fully utilize the photon energy it to utilize the photon energy at full potential thereby hindering its usability in photocatlysis. Thus, it’s a challenge to develop an alternative material which could show a promising photocatalytic activity for water splitting in obtaining a better hydrogen production [11,12]. As a good photocatalyst, g-C3N4 (graphitic carbon nitride) has drawn serious attention in recent times for its easy availability, environmentally benignity and high stability. g-C3N4 has been synthesized by the heat treatment to a mixture of urea, thiourea, melamine and dicyandiamide [13–17]. Interestingly, g-C3N4 has the appropriate band positions for absorbing visible light close to 460 nm by exhibiting optimum band gap of 2.6 eV which is highly desirable for water splitting. Hence, the selection of g-C3N4 as a photocatalyst can well serve the purpose of efficient hydrogen production. It possesses some drawbacks of instability in photogenerated electron-hole pairs and low surface area resulting in insufficient utilization of visible light [13,14,18]. On account of it, many efforts/measures have been taken to modify g-C3N4 for exploring its potential by combining it with other materials to prepare heterojunction composites [19–21], doping with metal or non-metal [22–24], increasing the porosity [25,26] and protonating with concentrated acids [27,28]. Among these, the preparation of heterojunction composites seems to be the most
viable, as heterojunction help in the separation of electron-hole pairs leading to improved stability and efficiency [29,30]. In the present work, we have used CuS/ZnS whose band gap can be tuned accordingly to considerably enhance the stability and water splitting capability upon halogen lamp irradiation [31–33]. Anyhow the water splitting H2-production capability of CuS/ZnS is still far behind from the expensive Pt-co catalyst and thus requires much improvement [32]. Doping of CuS/ZnS with metals [34] and making composite by attaching it with some other materials like graphene [35– 38], CdS [39,40] and NiS [41] could be a solution in this regard. It is expected that functionalizing of CuS/ZnS with g-C3N4 may provide new properties which favor for effective hydrogen production. Therefore, very first herein we report CuS/ZnS/g-C3N4 is being reported as an effective catalyst for hydrogen production for the first time. Moreover, the photocatalytic efficiency was optimized by varying the g-C3N4 content in the CuS/ZnS composite. In the present investigation, we have developed cauliflower shaped CuS/ZnS/g-C3N4 in cauliflower shape as an effective photocatalyst using three step processes which includes viz, hydrothermal, cation exchange and ultrasound assisted wet impregnation methods. Thus, the obtained CuS/ZnS/g-C3N4 nanocomposite showed excellent photocatalytic activity for H2production under halogen lamp irradiation. Experimental section Materials All chemicals such as Zinc acetate (Zn(O2CCH3)2(H2O)2), Cupric nitrate trihydrate (Cu(NO3)2.3H2O), Melamine (C3H6N6), Thiourea (CH4N2S), Triethylamine (C6H15N), Sodium sulfide (Na2S) and Sodium sulfite (Na2SO3) used in the present study are purchased from sigma Aldrich (USA).
ZnS cauliflowers synthesis ZnS cauliflowers were synthesized using triethylamine assisted hydrothermal method. In a typical synthesis, 0.1 M of Zinc acetate and 0.2 M of thiourea were dissolved in 100 ml deionized water. Then, 5 mL of triethylamine was added drop wise under magnetic stirring. The stirring was continued for 30 min. Then, as prepared solution was sealed inside a 200 ml Teflon lined autoclave and allowed for hydrothermal reaction at 200 °C for 12 h. Finally, the autoclave was left undisturbed to reach the room temperature. The final product was a white colored precipitate obtained after centrifugation followed by washing with ethanol and distilled water and drying at 60 °C overnight. CuS/ZnS nanocomposites synthesis Cation exchange method [42] was followed to synthesize CuS/ZnS nanocomposites. As synthesized ZnS (100 mg) was dispersed in distilled water via ultrasonication for 30 min and 1.97 mol % of Cu (NO3)2.3H2O aqueous solution was added into the above solution followed by magnetic stirring for 120 min. Thus obtained final product was washed with ethanol and distilled water, and dried at 70 °C for 12 h. g-C3N4 synthesis g-C3N4 was prepared by giving heat treatment to melamine [43]. The given amount of melamine (2g) was calcinated at ramping rate of 3 °C min-1, 500 °C. The temperature was then changed at a ramp rate of 2 °C min-1 to 520 °C, 2 h and was finally cooled naturally. The final product was in yellow colored powder form obtained after proper grinding. CuS/ZnS/g-C3N4 composites synthesis Ultrasonic assisted wet impregnation method was used to synthesize CuS/ZnS/g-C3N4 nanocomposites. As prepared g-C3N4 powder was re-dispersed in solution containing 20 ml
water and 20 ml ethanol by ultrasonication for 30 min. Then, 100 mg of as synthesized CuS/ZnS powder was added with vigorous stirring. The final product was obtained by drying the solution in hot air oven at 60 °C for 12 h. The weight of g-C3N4 in the CuS/ZnS/g-C3N4 nanocomposite was varied with 2.5, 5, 7.5 and 10 wt%, and the obtained final product are designated as CZ2.5CN, CZ-5CN, CZ-7.5CN and CZ-10CN nanocomposite in the subsequent discussions, respectively.
Characterization Phase and structural properties were studied via XRD (X-ray diffraction) measurements captured on D8-ADVANCE X-ray Diffractometer (BRUKER) at 1.5418Å Cu Kα. The infrared absorption and emission property were analyzed from FT-IR (Fourier Transform infrared) spectra captured on TENSOR 27 spectrometer (Bruker) in infrared region of 400 - 4000 cm-1. Physical overview of the sample were captured on Carl Zeiss AG (Supra 55VP) SEM (Scanning electron microscope). Further insight into physical dimension and elemental mapping were carried by utilizing Carl Zeiss AG (Supra 55VP) at 5-30 kV, a Field emission scanning electron microscope (FESEM). Morphology and structure were identified by TEM, Tecnai™ G2 20 instrument, a Transmission electron microscope (TEM) at 200 kV. Optical properties were analyzed through UV-Vis DRS spectra recorded on Varian CARY-5000 spectrophotometer at 175-3300 nm. Further insight into the elemental composition and VB-XPS was determined from XPS (X-ray photoelectron spectroscope) spectra captured on ESCALAB 250 Xi-XPS system with Al Kα radiation. BET (Brunauer-Emmett-Teller) and BJH (Barret-Joyner-Halenda) methods were followed to study the surface properties utilizing Micrometrics ASAP 2020
system. Electrochemical property was evaluated via EIS (electrochemical impedance spectroscopy) at 100 KHz-1 MHz Photocurrent measurements A three electrode system comprising counter, reference and working electrodes as Pt-wire, Ag/AgCl (in saturated KCl) and the prepared material, respectively was employed to evaluate the photocurrent measurements. For the preparation of working electrode, a homogenous slurry consisting of 5 mg of the CuS/ZnS/g-C3N4 sample and 10 mL Triton X-100 was prepared with the help of distilled water. FTO surface with area 0.5x0.5 cm2 was employed as substrate over which the as prepared slurry was coated smoothly by doctor blade. The coated surface served as working electrode after drying at 100 °C, 6 hrs. The measurements were captured on CHI608E electrochemical workstation at 0.1 M Na2SO4 electrolyte solution and 250 W Xe lamp (OSRAM, Germany) photon source. Photocatalytic hydrogen production activity For evaluating the efficiency of hydrogen production, the test solution was prepared by mixing 20 mg of the catalyst in 50 ml solution containing Na2S and Na2SO3 at 0.25 M and 0.35 M, respectively. The solution was sealed inside quartz reactor by rubber spectrum and stirred for 20 min. The dissolved oxygen was removed completely through purging of nitrogen. The reactor was kept at a distance of 20 cm from the light source, a 250 W halogen lamp. The amount of hydrogen produced during the course of the reaction is followed by injecting a 250 µl volume of gas collected through the septum at given intervals of time using offline gas chromatography (GC-14C, Shimadzu, Japan) equipped with TCD detector and molecular sieve 5 Å column, with nitrogen as carrier gas. Results and Discussion
Phase of the synthesized materials were determined using powder XRD measurements. XRD patterns of ZnS, CuS/ZnS, CZ-2.5CN, CZ-5CN, CZ-7.5CN and CZ-10CN nanocomposites have showed strong diffraction peaks at 2θ = 28.81, 47.85 and 56.75 (Fig.1) corresponds to the (002), (110) and (112) planes of the hexagonal phase of ZnS (JCPDS card No 80-0007), respectively. The broadness of the diffraction peaks reveals the presence of small crystallites and the crystallite size of the particle of ZnS was calculated by Scherrer’s formula is about 7 nm. In the case of CuS/ZnS nanocomposite, there were no additional lines have been observed due to the presence of CuS (Fig. 1). This may be due to the low concentration of CuS nanoparticles when compared to the ZnS that was uniformly distributed over the ZnS [42]. However, a significant reduction in the peak intensities was observed for CuS/ZnS compared to bare ZnS because the uniform deposition of CuS nanoparticle over ZnS and thereby exposing lesser possibility of ZnS in the composite. The XRD pattern of prepared g-C3N4 is in good agreement with reported literature (Fig. S1) [43]. The XRD patterns of CuS/ZnS, CZ-2.5CN, CZ-5CN, CZ7.5CN and CZ-10CN nanocomposites are similar to those of ZnS. Moreover, no other characteristic diffraction peak corresponding to g-C3N4 is observed in the CZ-2.5CN, CZ-5CN and CZ-7.5CN nanocomposites, which may be due to the lower loading of g-C3N4 in the metal sulfide composites [44]. The diffraction peaks of g-C3N4 invisible in the XRD pattern of CZ10CN even though its existence is about 10 wt%. Because, the diffraction peaks of ZnS and gC3N4 will exists at almost same 2 theta range. Thus, there is possibility to overlap the peaks of both compounds results invisibility of g-C3N4 diffraction peaks. The FT-IR spectrum of CZ-5CN is shown in Fig. 2. It is observed that the peak at 628 cm-1 is assigned to the ZnS band (i.e. corresponding to sulfides) for CZ-5CN sample. FTIR spectra of CZ-5CN sample yield the bands which are in good agreement with the reported
values [45]. The peak at 829 cm-1 belongs to triazine ring mode, which correspond to condensed CN heterocycles in g-C3N4 nanosheets [46]. The absorption peak at 1231, 1325 and 1391 cm-1 are attributed to -C-N stretching of g-C3N4. Nevertheless, the absorption peaks at 1507 and 1636 cm-1 correspond to the -C=N stretching of g-C3N4 [46]. Besides, a broad band near 3409 cm-1 corresponds to the O-H Stretching vibration [47]. The presence of all characteristic peaks of gC3N4 in the spectra exhibited by CZ-5CN sample confirmed the presence of g-C3N4. The surface topography and elemental composition of the ZnS, CuS/ZnS and CZ-5CN photocatalysts were investigated by SEM and EDX analysis. Regular, cauliflower like morphology of bare ZnS (Fig. 3a) was obtained from the SEM analysis. The typical SEM image of CuS/ZnS confirmed that the basic cauliflowers like morphology remains unchanged and the surface is loaded with CuS clusters (Fig. 3e). After introducing g-C3N4, the CuS loaded ZnS cauliflower-like morphology are perceptible at g-C3N4 surface (Fig. 3i). Interestingly, after incorporating g-C3N4 with the CuS/ZnS composite, the deposited CuS clusters are almost sandwiched and closely compacted between ZnS and g-C3N4 sheets to make a CuS/ZnS/g-C3N4 ternary dual-interface nanocomposite. The SEM-EDX analysis of ZnS, CuS/ZnS and CZ-5CN catalyst confirmed that the elements compositions are in the order as per the starting composition through Fig. (3d, 3h, 3m), respectively. TEM was carried out to investigate the morphology and microstructure of the as prepared for pure ZnS and CuS/ZnS catalyst. As shown in Fig. 3(b) and 3(c), the typical cauliflower-like structure of ZnS can be clearly observed with two different magnification. Fig. 3(f) and 3(g) shows a different magnification of TEM images of the CuS/ZnS sample. It can be observed that the uniform CuS clusters were homogeneously dispersed on ZnS surface. In order to further analyze the interaction between ZnS, CuS and g-C3N4 in the ZnS/CuS/g-C3N4 (CZ-5CN) sample were characterized with TEM and HR-TEM. The HRTEM
analysis of CZ-5CN sample is presented by Fig. 3(l). The dispersion of CuS/ZnS nanoparticles on the surface of g-C3N4 can be seen clearly. The observed d-spacing of 0.312 nm in (002) growth direction can be ascribed to the hexagonal ZnS and the d-spacing of 0.24 nm observed in small clusters corresponds to cubic CuS in (103) growth direction. To further investigate the existence of Zn, Cu, S, C and N elements in the resultant CuS/ZnS/g-C3N4 composite photocatalyst, EDS mapping analysis was performed. Fig. 4(b–f) display the individual elemental mapping images of Zn, Cu, S, C and N, which are indicated through different colours. The obtained mapping images clearly indicates that CuS/ZnS nanoparticles are uniformly distributed over the g-C3N4 support. Optical properties of synthesized photocatalysts were analyzed by UV-Vis diffuse reflectance spectroscopy. Fig. 5(a) shows the optical absorption spectra of ZnS, CuS/ZnS, CZ-2.5CN, CZ5CN, CZ-7.5CN and CZ-10CN photocatalysis. It has clearly seen that the sharp absorption edges observed at 361, 427, 471, 480, 476 and 461 nm are corresponding ZnS, CuS/ZnS, CZ-2.5CN, CZ-5CN, CZ-7.5CN and CZ-10CN photocatalyst, respectively. The absorption edge of pure ZnS at 361 nm corresponds to its bandgap energy of 3.43 eV. The presence of small amount of CuS in the ZnS enhances the visible light absorption characteristics of ZnS owing to bulk CuS adsorption at 565 nm (Fig. 5b). Furthermore, an enhanced absorbance in the visible light region was observed when the CuS/ZnS nanocomposite was loaded on g-C3N4 (Fig. 5a). The absorption edges observed for the CuS/ZnS/g-C3N4 composites at 471 (CZ-2.5CN), 480 (CZ-5CN), 476 (CZ-7.5CN) and 461 nm (CZ-10CN). It could be clearly seen that the extend of visible light absorption increased until 5 wt % loading of g-C3N4 then it decreased significantly. Thus, the observed red shift in the absorption edge of g-C3N4 supported CuS/ZnS photocatalyst is due to the introduction of CuS and g-C3N4. [48]. The UV-Vis diffuse reflectance spectrum pure g-C3N4
shown in Fig. 5c. Tauc plot of pure g-C3N4 shows a band gap of 2.6 eV (Fig. S2). VB-XPS have been carried out to reveal the exact valance band maximum positions of ZnS, g-C3N4 and CuS [49,50]. The calculated valance band positions for ZnS, g-C3N4 and CuS are +2.54 eV, +1.57 eV and +1.70 eV, respectively (Fig. 5d). By combining the obtained valance band positions with UV-DRS results, the conduction band position minimum of ZnS, g-C3N4 and CuS were calculated as -0.89 eV, -1.03 eV and -0.40 eV, respectively. Chemical composition and the chemical oxidation states of the CZ-5CN composite were obtained from the XPS Spectra. The survey scan spectrum (Fig. 6a) for CZ-5CN exhibited the peaks of Zn 2p, Cu 2p, S 2p, C 1s and N 1s, and the weak peaks of O from the adsorbed H2O, O2 and CO2 on the surface of the nanocomposite, which is in good agreement with EDS results [51]. Fig. 6 (b-f) shows the high resolution XPS spectra of the elements Zn 2p, Cu 2p, S 2p, C 1s and N 1s. The high resolution Zn 2p spectrum (Fig. 6b) displays two peaks at the binding energy values of 1022.17 and 1045.16 eV for CZ-5CN catalyst attributes the splitting respective to Zn 2p3/2 and Zn 2p1/2. Similarly, the Cu 2p (Fig. 6c) experiences peak splitting at 932.58 and 952.45 eV respective to Cu 2p3/2 and Cu 2p1/2 [52]. Fig. 6d displays the peaks at 163.85 and 162.58 eV accountable to S2− 2p1/2 and S2− 2p3/2 as per the report of CuS/ZnS catalysis [53]. In high resolution XPS spectrum of C 1s Fig. 6(e), two deconvolution peaks at 284.2 and 288.3 eV are observed, which are assigned to graphite C−C bonds and sp2-hybridized carbon in N-containing aromatic ring (N−C=N) [39], respectively. Fig 6(f) shows the high resolution N 1s spectrum comprising of three separate binding energies. The observed peaks at 399.67, 398.30 and 397.82 eV confirms the presence of C-NH, N–(C)3 and C–N–C groups, respectively [54]. In general, surface properties such as high surface area of the heterogeneous photocatalyst significantly impact its photocatalytic activity by providing more active sites at surface for
efficient visible light adsorption [55,56]. In addition, the high porosity is expected to offer promising interconnected porous network at interiors which prompts multiple scattering and favors more interaction with reactants thereby enhancing the photocatalytic process [57]. Fig. 7 illustrates BET curves with the respective pore dimensions of as prepared ZnS, CuS/ZnS, CZ2.5CN, CZ-5CN, CZ-7.5CN and CZ-10CN. Through the IUPAC classification, type IV can be ascribed, suggesting the existence of mesoporous structure [58]. The hysteresis loop scan is classified as type H3, for the slit-like pores in line with previously reported work [59]. Inset in Fig. 7 indicates broad range of pore dimensions (2-30 nm) which further confirms the existence of mesoporous and macroporous structure for ZnS, CuS/ZnS, CZ-2.5CN, CZ-5CN, CZ-7.5CN and CZ-10CN. The surface area of 149, 108, 115, 123, 100 and 94 m2g-1 is calculated for ZnS, CuS/ZnS, CZ-2.5CN, CZ-5CN, CZ-7.5CN and CZ-10CN, respectively. It can be clearly seen that the observed pure ZnS has a more surface area and it get decreases after CuS loading on ZnS surface. Then, there is increase in specific surface area when these composite was loaded over gC3N4 and it reaches a maximum for the CZ-5CN. Further increase in the CuS/ZnS loading decrease the surface area for CZ-7.5CN and CZ-10CN composite. A higher specific surface area of 123 m2g-1 was observed for the CZ-5CN nanocomposite photocatalyst, which can provide more active sites and facilitate the charge carrier transport process compare to other g-C3N4 loaded (CZ-2.5CN, CZ-7.5CN and CZ-10CN) photocatalyst. This results in the enhancement of reactants adsorption that is beneficial for the photocatalytic activity. Generally, the electron-hole pair generated at the semiconductor photocatalyst needs to be used effectively for the reduction and oxidation reaction during the photocatalytic process. However, the recombination of these electron-hole pair will results in poor catalytic performance. Thus, the extension of charge separation that is the electron-hole pair on the photocatalyst needs to be
studied for understanding the photocatalytic process. Thus, photo-electrochemical analysis was undertaken to evaluate the excitation and transfer of photogenerated charge carriers on the photocatalysis. Fig. 8 shows the transient photocurrent response of the bare ZnS, CuS/ZnS, CZ2.5CN, CZ-5CN, CZ-7.5CN and CZ-10CN and it was analyzed for several on-off cycles under solar simulator irradiation. Among the samples, CZ-5CN catalyst exhibited a higher transient photocurrent than bare ZnS, CuS/ZnS, CZ-2.5CN, CZ-7.5CN and CZ-10CN. This may be attributed to the more charge separation in CZ-5CN catalyst that could able to produce more hydrogen. In addition, the EIS was recorded to study the charge transfer resistance and the separation efficiency between photogenerated electrons and holes during photocatalytic reactions. The Nyquist plots of ZnS, CuS/ZnS, CZ-2.5CN, CZ-5CN, CZ-7.5CN and CZ-10CN photocatalysts are shown in Fig. 9. Generally, the smaller the arc radius higher the charge carrier separation at the interface [51]. It is observed that the CZ-5CN nanocomposites has a small arc radius on the impedance plot than ZnS, CuS/ZnS, CZ-2.5CN, CZ-7.5CN and CZ-10CN, which strongly suggests that the fastest charge transfer and lower rate of photon generated electrons/holes pair recombination which will favor for enhanced photocatalytic activity [60]. The above observed result is in good agreement with transient photocurrent studies [61]. The photocatalytic hydrogen production ability of the prepared samples such as bare g-C3N4, bare ZnS, CuS/ZnS, CZ-2.5CN, CZ-5CN, CZ-7.5CN and CZ-10CN nanocomposites were studied under halogen lamp irradiation via an aqueous solution containing 0.25 M Na2S and 0.35 M Na2SO3. Among them, g-C3N4 photocatalyst showed very poor and merely negligible hydrogen production rate, which could be ascribed to the fast recombination of photo induced charge carriers. Whereas, ZnS photocatalyst exhibited weak hydrogen production of about 1218 μmol h-1g-1, which is due to the wide band gap of ZnS that results poor visible light absorption.
Whereas, the CuS/ZnS showed significantly higher hydrogen production rate (4945 μmol h-1g-1) due to the small portion of Cu2+ that taking part in the photocatalytic water splitting reaction. The observed ~4 times improvement in the photocatalytic hydrogen evolution by addition of CuS in ZnS photocatalyst is already reported in literature [42,59]. Interestingly, when the g-C3N4 support was added, the hydrogen production rate of CuS/ZnS/g-C3N4 is further increased drastically. To optimize the hydrogen production rate of the CuS/ZnS/g-C3N4 nanocomposites, the content of gC3N4 is varied from 2.5, 5, 7.5 to 10 wt%. A maximum hydrogen production rates of 9309, 9868, 9636 and 5347 μmol h-1g-1 were observed for 2.5, 5, 7.5 and 10 wt% of g-C3N4 content in the CuS/ZnS/g-C3N4 nanocomposite (Fig. 10a), respectively. Among the various concentrations of g-C3N4, the 5 wt% of g-C3N4 incorporated sample (CZ-5CN) showed a high rate of hydrogen production of 9868 μmol h-1g-1. It is noted that the obtained hydrogen production rate was 2 fold higher than that of CuS/ZnS photocatalyst. However, when increasing the g-C3N4 weight loading in the nanocomposite to 7.5 and 10 wt%, the hydrogen production rate was decreased, this may be attributed to poor light absorption by CuS/ZnS due to g-C3N4 coverage. Also, the higher concentration of g-C3N4 may introduced additional states that will acts as recombination center for the excited electron-hole pairs. However, the hydrogen evolution rate of CZ-10CN is still higher than the CuS/ZnS photocatalyst. It may due to the intact contact between CuS/ZnS and gC3N4 which is crucial for the interfacial electron transfer between the two components. Notably, the photocatalytic hydrogen production activity of the CuS/ZnS/g-C3N4 composite is higher than the various CuS/ZnS based photocatalyst reported, so far, such as CuS/ZnS [42], CuS/ZnS nanosheets [59], ZnS-ZnO-CuS-CdS [62] and ZnS-CuS-CdS [63]. To the best of our knowledge, the observed higher hydrogen evolution rate of 9868 μmol h-1g-1 for the CZ-5CN is the highest value in the literature for the CuS/ZnS/g-C3N4 nanocomposite.
To further evaluate the stability of as synthesized photocatalyst for sustainable hydrogen production activity, continuous photocatalytic experiments were conducted for CZ-5CN. The same catalyst was used for five cycles without washing or regeneration, the hydrogen production activity for all the five cycles are shown in Fig. 10b. It could be clearly seen that there is no significant loss in the hydrogen production after the four consecutive cycles. It suggests that the synthesized CZ-5CN catalyst could be utilized for large scale commercial application since it has good photocatalytic stability. To confirm the physicochemical stability of the photocatalyst, XRD was measured after the five cycles. The XRD pattern of used catalyst (Fig. S3) retains its crystallinity ever after the five cycles, it confirms that CuS/ZnS/g-C3N4 nanocomposite is promising for sustainable hydrogen production using halogen lamp illumination. Based on the above obtained results and discussions, a plausible mechanism for photocatalytic hydrogen production over CuS/ZnS/g-C3N4 nanocomposites is proposed as shown in Fig. 11. Initially, the pure ZnS cannot respond under visible light due to its wide band gap energy, it will be active only in UV region. When irradiated with the halogen lamp, the ZnS will be active due to minimal amount of UV emission. Nevertheless, loading a small portion of CuS on the ZnS surface leads to injunction of charge carriers by the formation of hetero-junction between CuS and ZnS. The formation of hetero-junction will promote the photogenerated charge separation. To improve the efficiency by extending the absorbance range of composited photocatalyst, the CuS/ZnS composites were dispersed on the g-C3N4 nanosheets which is having the band gap of 2.6 eV and suitable for the absorption of visible light. Therefore, the conduction band of g-C3N4 is higher than that of CuS/ZnS. Photo-excited electrons from the conduction band of g-C3N4 will be transferred to the surface of flower like structured ZnS catalyst. Here, it is important to note that the conduction band potential of any
semiconductor should be more positive than the normal hydrogen potential to get effective reduction of H+ ions for the hydrogen generation from water. Then, dispersion of these composite nanoparticles on the surface of g-C3N4 nanosheets results in high energy conversion. Here, the conduction band potential of ZnS was relatively positive than the water reduction potential which is suitable to convert more number of H+ ions into hydrogen. Due to interparticle charge transfer, the photogenerated electrons from g-C3N4 (visible-active) will be transferred to CuS cluster via multistep charge transfer leads to high photocatalytic performance. Therefore, the photocatalytic activity of CuS/ZnS was gradually increased by assembling heterojunction with g-C3N4 nanosheets. Notably, the above mentioned nanocomposite photocatalyst is more effective for the absorption of both visible and minimal UV light produced by halogen lamp. Thus, the photogenerated electrons and holes are efficiently separated at the heterojunctions via multi-step charge transfer. The separated electrons reduce the adsorbed H+ into H2 and the separated holes at the valance band are simultaneously utilized by the sacrificial reagents. CONCLUSIONS It is demonstrated that a novel ternary ZnS cauliflower-like CuS/ZnS/g-C3N4 composite developed using hydrothermal, cation exchange treatment and ultrasound assisted wet impregnation methods, serves well as an efficient photocatalyst for hydrogen production. A maximum rate of 9868 μmol h-1g-1 in hydrogen production was obtained at 5 wt% of g-C3N4. The achieved value is considerably ahead of CuS/ZnS by 2 fold. This could be the maximum hydrogen production rate ever reported for the g-C3N4 based nanocomposite. This fete was achieved due to the unique heterostructure as developed for CuS/ZnS and g-C3N4 which might have restricted the recombination of electron/hole pairs. Additionally, the resultant ternary
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Figure captions
Fig.1 XRD patterns of pure ZnS, CuS/ZnS, CZ-2.5CN, CZ-5CN, CZ-7.5CN and CZ-10CN nanocomposites. Fig. 2 FTIR spectra of CZ-5CN nanocomposites. Fig. 3 (a) SEM image (b & C) TEM images of different magnification (d) EDX profile of Pure ZnS; (e) SEM image (f&g) TEM images of different magnification (h) EDX profile of CuS/ZnS catalyst; (i) SEM image, (j) TEM image and (k) HR-TEM image and (m) EDX profile of CZ-5CN photocatalyst. Fig. 4 (a) Elemental mapping image of the CZ-5CN composite and (b–f) individual elemental mapping images of Zn, Cu, S, C and N, respectively. Fig.5 UV-vis absorbance spectra (recorded in DRS mode) of (a) pure ZnS, CuS/ZnS, CZ-2.5CN, CZ-5CN, CZ-7.5CN, CZ-10CN nanocomposites, (b) pure CuS (c) pure gC3N4, (d) Valence-band XPS spectra of ZnS, g-C3N4 and CuS. Fig. 6 (a) XPS spectra of CZ-5CN catalyst; and the corresponding high-resolution XPS spectra of (b) Zn 2p (c) Cu 2p (d) S 2p (e) C 1s and (f) N 1s. Fig. 7 N2 adsorption-desorption isotherms and the corresponding pore size distribution curves of (a) ZnS, (b) CuS/ZnS, (c) CZ-2.5CN, (d) CZ-5CN, (e) CZ-7.5CN and (b) CZ-10CN. Fig. 8 Transient photocurrent responses of pure ZnS, CuS/ZnS, CZ-2.5CN, CZ-5CN, CZ-7.5CN and CZ-10CN under solar simulator irradiation. Fig. 9 Nyquist plots of pure ZnS, CuS/ZnS, CZ-2.5CN, CZ-5CN, CZ-7.5CN and CZ-10CN nanocomposites. Fig.10 (a) Photocatalytic hydrogen production rate of g-C3N4, ZnS, CuS/ZnS, CZ-2.5CN, CZ-5CN, CZ-7.5CN and CZ-10CN (b) Reusability of CZ-5CN photocatalysts for five run cycles. Fig.11 Plausible mechanism for hydrogen production over CuS/ZnS/g-C3N4 photocatalyst.
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Fig. 11 Highlights Cauliflower-like CuS/ZnS/g-C3N4 composites was developed for efficient activity Metal free composites for H2 evolution under halogen lamp illumination was achieved The improved charge separation, transfer and transport results highest H2 production Best activity results are observed for CuS/ZnS catalysts supported on g-C3N4 First time we report highest H2 production activity for CuS/ZnS/g-C3N4 comoposites
Graphical abstract Cauliflower-like CuS/ZnS nanocomposites decorated g-C3N4 nanosheets as noble metal-free photocatalyst for superior photocatalytic water splitting R. Rameshbabu, P. Ravi, M. Sathish*
Cauliflower-like CuS/ZnS/g-C3N4 photocatalyst was developed using hydrothermal and ultrasound assisted wet impregnation method followed cation exchange treatment. A highest hydrogen production of 9868 μmolh-1g-1 was obtained for the above nanocomposite containing 5 wt% of g-C3N4.
S1385-8947(18)32141-7 https://doi.org/10.1016/j.cej.2018.10.180 CEJ 20243
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Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
27 July 2018 24 October 2018 25 October 2018
Please cite this article as: R. Rameshbabu, P. Ravi, M. Sathish, Cauliflower-like CuS/ZnS nanocomposites decorated g-C3N4 nanosheets as noble metal-free photocatalyst for superior photocatalytic water splitting, Chemical Engineering Journal (2018), doi: https://doi.org/10.1016/j.cej.2018.10.180
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Cauliflower-like CuS/ZnS nanocomposites decorated g-C3N4 nanosheets as noble metal-free photocatalyst for superior photocatalytic water splitting R. Rameshbabu, P. Ravi, M. Sathish* Functional Materials Division, CSIR-Central Electrochemical Research Institute, Karaikudi-630 003, Tamil Nadu, India Name and Mailing Address of Corresponding Author: Dr. M. Sathish Scientist, Functional Materials Division, CSIR-Central Electrochemical Research Institute, Karaikudi -630006 Tamilnadu, INDIA Phone: +91-4565-241410 Web: http://www.cecri.res.in/ Web: http://scholar.google.co.in/citations?user=4X-7WnsAAAAJ&hl=en Email: [email protected] [email protected] Abstract A promising cauliflower like morphology of CuS/ZnS/g-C3N4 is developed as an effective visible-light driven photocatalyst via a facile hydrothermal route followed by ultrasound assisted wet impregnation method and cation exchange treatment. As prepared CuS/ZnS/g-C3N4 nanocomposites were characterized using various analytical techniques to confirm the composite formation and study their morphology, structure and band gap. Na2S and Na2SO3 solutes illuminated by 250 W halogen lamp are employed to evaluate the hydrogen production capability of the as prepared photocatalyst. The results revealed the superior photocatalytic activity in hydrogen production reaction by as prepared CuS/ZnS/g-C3N4 nanocomposites against g-C3N4, ZnS and CuS/ZnS photocatalysts. We have achieved the highest hydrogen production rate of 9868 μmol h-1 g-1 by the composite containing 5 wt% of gC3N4 with excellent durability during photocatalytic hydrogen production. This excellent performance attributed to the increased absorption level of visible light in the nanocomposite and higher stability of electron-hole pairs created upon irradiation.
Keywords: Photocatalysts; g-C3N4 nanosheets; hydrogen evolution; CuS/ZnS nanocomposite; stability; Introduction To meet the growing energy demands and counter the pollution, many researchers have shown interest in the photocatalytic water splitting and organic dye degradation [1–8]. TiO2 is found to serve the purpose, as it is photo catalytically active with good stability and non-toxicity [9,10]. However, its large band gap of 3.2 eV restricts its use to fully utilize the photon energy it to utilize the photon energy at full potential thereby hindering its usability in photocatlysis. Thus, it’s a challenge to develop an alternative material which could show a promising photocatalytic activity for water splitting in obtaining a better hydrogen production [11,12]. As a good photocatalyst, g-C3N4 (graphitic carbon nitride) has drawn serious attention in recent times for its easy availability, environmentally benignity and high stability. g-C3N4 has been synthesized by the heat treatment to a mixture of urea, thiourea, melamine and dicyandiamide [13–17]. Interestingly, g-C3N4 has the appropriate band positions for absorbing visible light close to 460 nm by exhibiting optimum band gap of 2.6 eV which is highly desirable for water splitting. Hence, the selection of g-C3N4 as a photocatalyst can well serve the purpose of efficient hydrogen production. It possesses some drawbacks of instability in photogenerated electron-hole pairs and low surface area resulting in insufficient utilization of visible light [13,14,18]. On account of it, many efforts/measures have been taken to modify g-C3N4 for exploring its potential by combining it with other materials to prepare heterojunction composites [19–21], doping with metal or non-metal [22–24], increasing the porosity [25,26] and protonating with concentrated acids [27,28]. Among these, the preparation of heterojunction composites seems to be the most
viable, as heterojunction help in the separation of electron-hole pairs leading to improved stability and efficiency [29,30]. In the present work, we have used CuS/ZnS whose band gap can be tuned accordingly to considerably enhance the stability and water splitting capability upon halogen lamp irradiation [31–33]. Anyhow the water splitting H2-production capability of CuS/ZnS is still far behind from the expensive Pt-co catalyst and thus requires much improvement [32]. Doping of CuS/ZnS with metals [34] and making composite by attaching it with some other materials like graphene [35– 38], CdS [39,40] and NiS [41] could be a solution in this regard. It is expected that functionalizing of CuS/ZnS with g-C3N4 may provide new properties which favor for effective hydrogen production. Therefore, very first herein we report CuS/ZnS/g-C3N4 is being reported as an effective catalyst for hydrogen production for the first time. Moreover, the photocatalytic efficiency was optimized by varying the g-C3N4 content in the CuS/ZnS composite. In the present investigation, we have developed cauliflower shaped CuS/ZnS/g-C3N4 in cauliflower shape as an effective photocatalyst using three step processes which includes viz, hydrothermal, cation exchange and ultrasound assisted wet impregnation methods. Thus, the obtained CuS/ZnS/g-C3N4 nanocomposite showed excellent photocatalytic activity for H2production under halogen lamp irradiation. Experimental section Materials All chemicals such as Zinc acetate (Zn(O2CCH3)2(H2O)2), Cupric nitrate trihydrate (Cu(NO3)2.3H2O), Melamine (C3H6N6), Thiourea (CH4N2S), Triethylamine (C6H15N), Sodium sulfide (Na2S) and Sodium sulfite (Na2SO3) used in the present study are purchased from sigma Aldrich (USA).
ZnS cauliflowers synthesis ZnS cauliflowers were synthesized using triethylamine assisted hydrothermal method. In a typical synthesis, 0.1 M of Zinc acetate and 0.2 M of thiourea were dissolved in 100 ml deionized water. Then, 5 mL of triethylamine was added drop wise under magnetic stirring. The stirring was continued for 30 min. Then, as prepared solution was sealed inside a 200 ml Teflon lined autoclave and allowed for hydrothermal reaction at 200 °C for 12 h. Finally, the autoclave was left undisturbed to reach the room temperature. The final product was a white colored precipitate obtained after centrifugation followed by washing with ethanol and distilled water and drying at 60 °C overnight. CuS/ZnS nanocomposites synthesis Cation exchange method [42] was followed to synthesize CuS/ZnS nanocomposites. As synthesized ZnS (100 mg) was dispersed in distilled water via ultrasonication for 30 min and 1.97 mol % of Cu (NO3)2.3H2O aqueous solution was added into the above solution followed by magnetic stirring for 120 min. Thus obtained final product was washed with ethanol and distilled water, and dried at 70 °C for 12 h. g-C3N4 synthesis g-C3N4 was prepared by giving heat treatment to melamine [43]. The given amount of melamine (2g) was calcinated at ramping rate of 3 °C min-1, 500 °C. The temperature was then changed at a ramp rate of 2 °C min-1 to 520 °C, 2 h and was finally cooled naturally. The final product was in yellow colored powder form obtained after proper grinding. CuS/ZnS/g-C3N4 composites synthesis Ultrasonic assisted wet impregnation method was used to synthesize CuS/ZnS/g-C3N4 nanocomposites. As prepared g-C3N4 powder was re-dispersed in solution containing 20 ml
water and 20 ml ethanol by ultrasonication for 30 min. Then, 100 mg of as synthesized CuS/ZnS powder was added with vigorous stirring. The final product was obtained by drying the solution in hot air oven at 60 °C for 12 h. The weight of g-C3N4 in the CuS/ZnS/g-C3N4 nanocomposite was varied with 2.5, 5, 7.5 and 10 wt%, and the obtained final product are designated as CZ2.5CN, CZ-5CN, CZ-7.5CN and CZ-10CN nanocomposite in the subsequent discussions, respectively.
Characterization Phase and structural properties were studied via XRD (X-ray diffraction) measurements captured on D8-ADVANCE X-ray Diffractometer (BRUKER) at 1.5418Å Cu Kα. The infrared absorption and emission property were analyzed from FT-IR (Fourier Transform infrared) spectra captured on TENSOR 27 spectrometer (Bruker) in infrared region of 400 - 4000 cm-1. Physical overview of the sample were captured on Carl Zeiss AG (Supra 55VP) SEM (Scanning electron microscope). Further insight into physical dimension and elemental mapping were carried by utilizing Carl Zeiss AG (Supra 55VP) at 5-30 kV, a Field emission scanning electron microscope (FESEM). Morphology and structure were identified by TEM, Tecnai™ G2 20 instrument, a Transmission electron microscope (TEM) at 200 kV. Optical properties were analyzed through UV-Vis DRS spectra recorded on Varian CARY-5000 spectrophotometer at 175-3300 nm. Further insight into the elemental composition and VB-XPS was determined from XPS (X-ray photoelectron spectroscope) spectra captured on ESCALAB 250 Xi-XPS system with Al Kα radiation. BET (Brunauer-Emmett-Teller) and BJH (Barret-Joyner-Halenda) methods were followed to study the surface properties utilizing Micrometrics ASAP 2020
system. Electrochemical property was evaluated via EIS (electrochemical impedance spectroscopy) at 100 KHz-1 MHz Photocurrent measurements A three electrode system comprising counter, reference and working electrodes as Pt-wire, Ag/AgCl (in saturated KCl) and the prepared material, respectively was employed to evaluate the photocurrent measurements. For the preparation of working electrode, a homogenous slurry consisting of 5 mg of the CuS/ZnS/g-C3N4 sample and 10 mL Triton X-100 was prepared with the help of distilled water. FTO surface with area 0.5x0.5 cm2 was employed as substrate over which the as prepared slurry was coated smoothly by doctor blade. The coated surface served as working electrode after drying at 100 °C, 6 hrs. The measurements were captured on CHI608E electrochemical workstation at 0.1 M Na2SO4 electrolyte solution and 250 W Xe lamp (OSRAM, Germany) photon source. Photocatalytic hydrogen production activity For evaluating the efficiency of hydrogen production, the test solution was prepared by mixing 20 mg of the catalyst in 50 ml solution containing Na2S and Na2SO3 at 0.25 M and 0.35 M, respectively. The solution was sealed inside quartz reactor by rubber spectrum and stirred for 20 min. The dissolved oxygen was removed completely through purging of nitrogen. The reactor was kept at a distance of 20 cm from the light source, a 250 W halogen lamp. The amount of hydrogen produced during the course of the reaction is followed by injecting a 250 µl volume of gas collected through the septum at given intervals of time using offline gas chromatography (GC-14C, Shimadzu, Japan) equipped with TCD detector and molecular sieve 5 Å column, with nitrogen as carrier gas. Results and Discussion
Phase of the synthesized materials were determined using powder XRD measurements. XRD patterns of ZnS, CuS/ZnS, CZ-2.5CN, CZ-5CN, CZ-7.5CN and CZ-10CN nanocomposites have showed strong diffraction peaks at 2θ = 28.81, 47.85 and 56.75 (Fig.1) corresponds to the (002), (110) and (112) planes of the hexagonal phase of ZnS (JCPDS card No 80-0007), respectively. The broadness of the diffraction peaks reveals the presence of small crystallites and the crystallite size of the particle of ZnS was calculated by Scherrer’s formula is about 7 nm. In the case of CuS/ZnS nanocomposite, there were no additional lines have been observed due to the presence of CuS (Fig. 1). This may be due to the low concentration of CuS nanoparticles when compared to the ZnS that was uniformly distributed over the ZnS [42]. However, a significant reduction in the peak intensities was observed for CuS/ZnS compared to bare ZnS because the uniform deposition of CuS nanoparticle over ZnS and thereby exposing lesser possibility of ZnS in the composite. The XRD pattern of prepared g-C3N4 is in good agreement with reported literature (Fig. S1) [43]. The XRD patterns of CuS/ZnS, CZ-2.5CN, CZ-5CN, CZ7.5CN and CZ-10CN nanocomposites are similar to those of ZnS. Moreover, no other characteristic diffraction peak corresponding to g-C3N4 is observed in the CZ-2.5CN, CZ-5CN and CZ-7.5CN nanocomposites, which may be due to the lower loading of g-C3N4 in the metal sulfide composites [44]. The diffraction peaks of g-C3N4 invisible in the XRD pattern of CZ10CN even though its existence is about 10 wt%. Because, the diffraction peaks of ZnS and gC3N4 will exists at almost same 2 theta range. Thus, there is possibility to overlap the peaks of both compounds results invisibility of g-C3N4 diffraction peaks. The FT-IR spectrum of CZ-5CN is shown in Fig. 2. It is observed that the peak at 628 cm-1 is assigned to the ZnS band (i.e. corresponding to sulfides) for CZ-5CN sample. FTIR spectra of CZ-5CN sample yield the bands which are in good agreement with the reported
values [45]. The peak at 829 cm-1 belongs to triazine ring mode, which correspond to condensed CN heterocycles in g-C3N4 nanosheets [46]. The absorption peak at 1231, 1325 and 1391 cm-1 are attributed to -C-N stretching of g-C3N4. Nevertheless, the absorption peaks at 1507 and 1636 cm-1 correspond to the -C=N stretching of g-C3N4 [46]. Besides, a broad band near 3409 cm-1 corresponds to the O-H Stretching vibration [47]. The presence of all characteristic peaks of gC3N4 in the spectra exhibited by CZ-5CN sample confirmed the presence of g-C3N4. The surface topography and elemental composition of the ZnS, CuS/ZnS and CZ-5CN photocatalysts were investigated by SEM and EDX analysis. Regular, cauliflower like morphology of bare ZnS (Fig. 3a) was obtained from the SEM analysis. The typical SEM image of CuS/ZnS confirmed that the basic cauliflowers like morphology remains unchanged and the surface is loaded with CuS clusters (Fig. 3e). After introducing g-C3N4, the CuS loaded ZnS cauliflower-like morphology are perceptible at g-C3N4 surface (Fig. 3i). Interestingly, after incorporating g-C3N4 with the CuS/ZnS composite, the deposited CuS clusters are almost sandwiched and closely compacted between ZnS and g-C3N4 sheets to make a CuS/ZnS/g-C3N4 ternary dual-interface nanocomposite. The SEM-EDX analysis of ZnS, CuS/ZnS and CZ-5CN catalyst confirmed that the elements compositions are in the order as per the starting composition through Fig. (3d, 3h, 3m), respectively. TEM was carried out to investigate the morphology and microstructure of the as prepared for pure ZnS and CuS/ZnS catalyst. As shown in Fig. 3(b) and 3(c), the typical cauliflower-like structure of ZnS can be clearly observed with two different magnification. Fig. 3(f) and 3(g) shows a different magnification of TEM images of the CuS/ZnS sample. It can be observed that the uniform CuS clusters were homogeneously dispersed on ZnS surface. In order to further analyze the interaction between ZnS, CuS and g-C3N4 in the ZnS/CuS/g-C3N4 (CZ-5CN) sample were characterized with TEM and HR-TEM. The HRTEM
analysis of CZ-5CN sample is presented by Fig. 3(l). The dispersion of CuS/ZnS nanoparticles on the surface of g-C3N4 can be seen clearly. The observed d-spacing of 0.312 nm in (002) growth direction can be ascribed to the hexagonal ZnS and the d-spacing of 0.24 nm observed in small clusters corresponds to cubic CuS in (103) growth direction. To further investigate the existence of Zn, Cu, S, C and N elements in the resultant CuS/ZnS/g-C3N4 composite photocatalyst, EDS mapping analysis was performed. Fig. 4(b–f) display the individual elemental mapping images of Zn, Cu, S, C and N, which are indicated through different colours. The obtained mapping images clearly indicates that CuS/ZnS nanoparticles are uniformly distributed over the g-C3N4 support. Optical properties of synthesized photocatalysts were analyzed by UV-Vis diffuse reflectance spectroscopy. Fig. 5(a) shows the optical absorption spectra of ZnS, CuS/ZnS, CZ-2.5CN, CZ5CN, CZ-7.5CN and CZ-10CN photocatalysis. It has clearly seen that the sharp absorption edges observed at 361, 427, 471, 480, 476 and 461 nm are corresponding ZnS, CuS/ZnS, CZ-2.5CN, CZ-5CN, CZ-7.5CN and CZ-10CN photocatalyst, respectively. The absorption edge of pure ZnS at 361 nm corresponds to its bandgap energy of 3.43 eV. The presence of small amount of CuS in the ZnS enhances the visible light absorption characteristics of ZnS owing to bulk CuS adsorption at 565 nm (Fig. 5b). Furthermore, an enhanced absorbance in the visible light region was observed when the CuS/ZnS nanocomposite was loaded on g-C3N4 (Fig. 5a). The absorption edges observed for the CuS/ZnS/g-C3N4 composites at 471 (CZ-2.5CN), 480 (CZ-5CN), 476 (CZ-7.5CN) and 461 nm (CZ-10CN). It could be clearly seen that the extend of visible light absorption increased until 5 wt % loading of g-C3N4 then it decreased significantly. Thus, the observed red shift in the absorption edge of g-C3N4 supported CuS/ZnS photocatalyst is due to the introduction of CuS and g-C3N4. [48]. The UV-Vis diffuse reflectance spectrum pure g-C3N4
shown in Fig. 5c. Tauc plot of pure g-C3N4 shows a band gap of 2.6 eV (Fig. S2). VB-XPS have been carried out to reveal the exact valance band maximum positions of ZnS, g-C3N4 and CuS [49,50]. The calculated valance band positions for ZnS, g-C3N4 and CuS are +2.54 eV, +1.57 eV and +1.70 eV, respectively (Fig. 5d). By combining the obtained valance band positions with UV-DRS results, the conduction band position minimum of ZnS, g-C3N4 and CuS were calculated as -0.89 eV, -1.03 eV and -0.40 eV, respectively. Chemical composition and the chemical oxidation states of the CZ-5CN composite were obtained from the XPS Spectra. The survey scan spectrum (Fig. 6a) for CZ-5CN exhibited the peaks of Zn 2p, Cu 2p, S 2p, C 1s and N 1s, and the weak peaks of O from the adsorbed H2O, O2 and CO2 on the surface of the nanocomposite, which is in good agreement with EDS results [51]. Fig. 6 (b-f) shows the high resolution XPS spectra of the elements Zn 2p, Cu 2p, S 2p, C 1s and N 1s. The high resolution Zn 2p spectrum (Fig. 6b) displays two peaks at the binding energy values of 1022.17 and 1045.16 eV for CZ-5CN catalyst attributes the splitting respective to Zn 2p3/2 and Zn 2p1/2. Similarly, the Cu 2p (Fig. 6c) experiences peak splitting at 932.58 and 952.45 eV respective to Cu 2p3/2 and Cu 2p1/2 [52]. Fig. 6d displays the peaks at 163.85 and 162.58 eV accountable to S2− 2p1/2 and S2− 2p3/2 as per the report of CuS/ZnS catalysis [53]. In high resolution XPS spectrum of C 1s Fig. 6(e), two deconvolution peaks at 284.2 and 288.3 eV are observed, which are assigned to graphite C−C bonds and sp2-hybridized carbon in N-containing aromatic ring (N−C=N) [39], respectively. Fig 6(f) shows the high resolution N 1s spectrum comprising of three separate binding energies. The observed peaks at 399.67, 398.30 and 397.82 eV confirms the presence of C-NH, N–(C)3 and C–N–C groups, respectively [54]. In general, surface properties such as high surface area of the heterogeneous photocatalyst significantly impact its photocatalytic activity by providing more active sites at surface for
efficient visible light adsorption [55,56]. In addition, the high porosity is expected to offer promising interconnected porous network at interiors which prompts multiple scattering and favors more interaction with reactants thereby enhancing the photocatalytic process [57]. Fig. 7 illustrates BET curves with the respective pore dimensions of as prepared ZnS, CuS/ZnS, CZ2.5CN, CZ-5CN, CZ-7.5CN and CZ-10CN. Through the IUPAC classification, type IV can be ascribed, suggesting the existence of mesoporous structure [58]. The hysteresis loop scan is classified as type H3, for the slit-like pores in line with previously reported work [59]. Inset in Fig. 7 indicates broad range of pore dimensions (2-30 nm) which further confirms the existence of mesoporous and macroporous structure for ZnS, CuS/ZnS, CZ-2.5CN, CZ-5CN, CZ-7.5CN and CZ-10CN. The surface area of 149, 108, 115, 123, 100 and 94 m2g-1 is calculated for ZnS, CuS/ZnS, CZ-2.5CN, CZ-5CN, CZ-7.5CN and CZ-10CN, respectively. It can be clearly seen that the observed pure ZnS has a more surface area and it get decreases after CuS loading on ZnS surface. Then, there is increase in specific surface area when these composite was loaded over gC3N4 and it reaches a maximum for the CZ-5CN. Further increase in the CuS/ZnS loading decrease the surface area for CZ-7.5CN and CZ-10CN composite. A higher specific surface area of 123 m2g-1 was observed for the CZ-5CN nanocomposite photocatalyst, which can provide more active sites and facilitate the charge carrier transport process compare to other g-C3N4 loaded (CZ-2.5CN, CZ-7.5CN and CZ-10CN) photocatalyst. This results in the enhancement of reactants adsorption that is beneficial for the photocatalytic activity. Generally, the electron-hole pair generated at the semiconductor photocatalyst needs to be used effectively for the reduction and oxidation reaction during the photocatalytic process. However, the recombination of these electron-hole pair will results in poor catalytic performance. Thus, the extension of charge separation that is the electron-hole pair on the photocatalyst needs to be
studied for understanding the photocatalytic process. Thus, photo-electrochemical analysis was undertaken to evaluate the excitation and transfer of photogenerated charge carriers on the photocatalysis. Fig. 8 shows the transient photocurrent response of the bare ZnS, CuS/ZnS, CZ2.5CN, CZ-5CN, CZ-7.5CN and CZ-10CN and it was analyzed for several on-off cycles under solar simulator irradiation. Among the samples, CZ-5CN catalyst exhibited a higher transient photocurrent than bare ZnS, CuS/ZnS, CZ-2.5CN, CZ-7.5CN and CZ-10CN. This may be attributed to the more charge separation in CZ-5CN catalyst that could able to produce more hydrogen. In addition, the EIS was recorded to study the charge transfer resistance and the separation efficiency between photogenerated electrons and holes during photocatalytic reactions. The Nyquist plots of ZnS, CuS/ZnS, CZ-2.5CN, CZ-5CN, CZ-7.5CN and CZ-10CN photocatalysts are shown in Fig. 9. Generally, the smaller the arc radius higher the charge carrier separation at the interface [51]. It is observed that the CZ-5CN nanocomposites has a small arc radius on the impedance plot than ZnS, CuS/ZnS, CZ-2.5CN, CZ-7.5CN and CZ-10CN, which strongly suggests that the fastest charge transfer and lower rate of photon generated electrons/holes pair recombination which will favor for enhanced photocatalytic activity [60]. The above observed result is in good agreement with transient photocurrent studies [61]. The photocatalytic hydrogen production ability of the prepared samples such as bare g-C3N4, bare ZnS, CuS/ZnS, CZ-2.5CN, CZ-5CN, CZ-7.5CN and CZ-10CN nanocomposites were studied under halogen lamp irradiation via an aqueous solution containing 0.25 M Na2S and 0.35 M Na2SO3. Among them, g-C3N4 photocatalyst showed very poor and merely negligible hydrogen production rate, which could be ascribed to the fast recombination of photo induced charge carriers. Whereas, ZnS photocatalyst exhibited weak hydrogen production of about 1218 μmol h-1g-1, which is due to the wide band gap of ZnS that results poor visible light absorption.
Whereas, the CuS/ZnS showed significantly higher hydrogen production rate (4945 μmol h-1g-1) due to the small portion of Cu2+ that taking part in the photocatalytic water splitting reaction. The observed ~4 times improvement in the photocatalytic hydrogen evolution by addition of CuS in ZnS photocatalyst is already reported in literature [42,59]. Interestingly, when the g-C3N4 support was added, the hydrogen production rate of CuS/ZnS/g-C3N4 is further increased drastically. To optimize the hydrogen production rate of the CuS/ZnS/g-C3N4 nanocomposites, the content of gC3N4 is varied from 2.5, 5, 7.5 to 10 wt%. A maximum hydrogen production rates of 9309, 9868, 9636 and 5347 μmol h-1g-1 were observed for 2.5, 5, 7.5 and 10 wt% of g-C3N4 content in the CuS/ZnS/g-C3N4 nanocomposite (Fig. 10a), respectively. Among the various concentrations of g-C3N4, the 5 wt% of g-C3N4 incorporated sample (CZ-5CN) showed a high rate of hydrogen production of 9868 μmol h-1g-1. It is noted that the obtained hydrogen production rate was 2 fold higher than that of CuS/ZnS photocatalyst. However, when increasing the g-C3N4 weight loading in the nanocomposite to 7.5 and 10 wt%, the hydrogen production rate was decreased, this may be attributed to poor light absorption by CuS/ZnS due to g-C3N4 coverage. Also, the higher concentration of g-C3N4 may introduced additional states that will acts as recombination center for the excited electron-hole pairs. However, the hydrogen evolution rate of CZ-10CN is still higher than the CuS/ZnS photocatalyst. It may due to the intact contact between CuS/ZnS and gC3N4 which is crucial for the interfacial electron transfer between the two components. Notably, the photocatalytic hydrogen production activity of the CuS/ZnS/g-C3N4 composite is higher than the various CuS/ZnS based photocatalyst reported, so far, such as CuS/ZnS [42], CuS/ZnS nanosheets [59], ZnS-ZnO-CuS-CdS [62] and ZnS-CuS-CdS [63]. To the best of our knowledge, the observed higher hydrogen evolution rate of 9868 μmol h-1g-1 for the CZ-5CN is the highest value in the literature for the CuS/ZnS/g-C3N4 nanocomposite.
To further evaluate the stability of as synthesized photocatalyst for sustainable hydrogen production activity, continuous photocatalytic experiments were conducted for CZ-5CN. The same catalyst was used for five cycles without washing or regeneration, the hydrogen production activity for all the five cycles are shown in Fig. 10b. It could be clearly seen that there is no significant loss in the hydrogen production after the four consecutive cycles. It suggests that the synthesized CZ-5CN catalyst could be utilized for large scale commercial application since it has good photocatalytic stability. To confirm the physicochemical stability of the photocatalyst, XRD was measured after the five cycles. The XRD pattern of used catalyst (Fig. S3) retains its crystallinity ever after the five cycles, it confirms that CuS/ZnS/g-C3N4 nanocomposite is promising for sustainable hydrogen production using halogen lamp illumination. Based on the above obtained results and discussions, a plausible mechanism for photocatalytic hydrogen production over CuS/ZnS/g-C3N4 nanocomposites is proposed as shown in Fig. 11. Initially, the pure ZnS cannot respond under visible light due to its wide band gap energy, it will be active only in UV region. When irradiated with the halogen lamp, the ZnS will be active due to minimal amount of UV emission. Nevertheless, loading a small portion of CuS on the ZnS surface leads to injunction of charge carriers by the formation of hetero-junction between CuS and ZnS. The formation of hetero-junction will promote the photogenerated charge separation. To improve the efficiency by extending the absorbance range of composited photocatalyst, the CuS/ZnS composites were dispersed on the g-C3N4 nanosheets which is having the band gap of 2.6 eV and suitable for the absorption of visible light. Therefore, the conduction band of g-C3N4 is higher than that of CuS/ZnS. Photo-excited electrons from the conduction band of g-C3N4 will be transferred to the surface of flower like structured ZnS catalyst. Here, it is important to note that the conduction band potential of any
semiconductor should be more positive than the normal hydrogen potential to get effective reduction of H+ ions for the hydrogen generation from water. Then, dispersion of these composite nanoparticles on the surface of g-C3N4 nanosheets results in high energy conversion. Here, the conduction band potential of ZnS was relatively positive than the water reduction potential which is suitable to convert more number of H+ ions into hydrogen. Due to interparticle charge transfer, the photogenerated electrons from g-C3N4 (visible-active) will be transferred to CuS cluster via multistep charge transfer leads to high photocatalytic performance. Therefore, the photocatalytic activity of CuS/ZnS was gradually increased by assembling heterojunction with g-C3N4 nanosheets. Notably, the above mentioned nanocomposite photocatalyst is more effective for the absorption of both visible and minimal UV light produced by halogen lamp. Thus, the photogenerated electrons and holes are efficiently separated at the heterojunctions via multi-step charge transfer. The separated electrons reduce the adsorbed H+ into H2 and the separated holes at the valance band are simultaneously utilized by the sacrificial reagents. CONCLUSIONS It is demonstrated that a novel ternary ZnS cauliflower-like CuS/ZnS/g-C3N4 composite developed using hydrothermal, cation exchange treatment and ultrasound assisted wet impregnation methods, serves well as an efficient photocatalyst for hydrogen production. A maximum rate of 9868 μmol h-1g-1 in hydrogen production was obtained at 5 wt% of g-C3N4. The achieved value is considerably ahead of CuS/ZnS by 2 fold. This could be the maximum hydrogen production rate ever reported for the g-C3N4 based nanocomposite. This fete was achieved due to the unique heterostructure as developed for CuS/ZnS and g-C3N4 which might have restricted the recombination of electron/hole pairs. Additionally, the resultant ternary
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Figure captions
Fig.1 XRD patterns of pure ZnS, CuS/ZnS, CZ-2.5CN, CZ-5CN, CZ-7.5CN and CZ-10CN nanocomposites. Fig. 2 FTIR spectra of CZ-5CN nanocomposites. Fig. 3 (a) SEM image (b & C) TEM images of different magnification (d) EDX profile of Pure ZnS; (e) SEM image (f&g) TEM images of different magnification (h) EDX profile of CuS/ZnS catalyst; (i) SEM image, (j) TEM image and (k) HR-TEM image and (m) EDX profile of CZ-5CN photocatalyst. Fig. 4 (a) Elemental mapping image of the CZ-5CN composite and (b–f) individual elemental mapping images of Zn, Cu, S, C and N, respectively. Fig.5 UV-vis absorbance spectra (recorded in DRS mode) of (a) pure ZnS, CuS/ZnS, CZ-2.5CN, CZ-5CN, CZ-7.5CN, CZ-10CN nanocomposites, (b) pure CuS (c) pure gC3N4, (d) Valence-band XPS spectra of ZnS, g-C3N4 and CuS. Fig. 6 (a) XPS spectra of CZ-5CN catalyst; and the corresponding high-resolution XPS spectra of (b) Zn 2p (c) Cu 2p (d) S 2p (e) C 1s and (f) N 1s. Fig. 7 N2 adsorption-desorption isotherms and the corresponding pore size distribution curves of (a) ZnS, (b) CuS/ZnS, (c) CZ-2.5CN, (d) CZ-5CN, (e) CZ-7.5CN and (b) CZ-10CN. Fig. 8 Transient photocurrent responses of pure ZnS, CuS/ZnS, CZ-2.5CN, CZ-5CN, CZ-7.5CN and CZ-10CN under solar simulator irradiation. Fig. 9 Nyquist plots of pure ZnS, CuS/ZnS, CZ-2.5CN, CZ-5CN, CZ-7.5CN and CZ-10CN nanocomposites. Fig.10 (a) Photocatalytic hydrogen production rate of g-C3N4, ZnS, CuS/ZnS, CZ-2.5CN, CZ-5CN, CZ-7.5CN and CZ-10CN (b) Reusability of CZ-5CN photocatalysts for five run cycles. Fig.11 Plausible mechanism for hydrogen production over CuS/ZnS/g-C3N4 photocatalyst.
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Fig. 11 Highlights Cauliflower-like CuS/ZnS/g-C3N4 composites was developed for efficient activity Metal free composites for H2 evolution under halogen lamp illumination was achieved The improved charge separation, transfer and transport results highest H2 production Best activity results are observed for CuS/ZnS catalysts supported on g-C3N4 First time we report highest H2 production activity for CuS/ZnS/g-C3N4 comoposites
Graphical abstract Cauliflower-like CuS/ZnS nanocomposites decorated g-C3N4 nanosheets as noble metal-free photocatalyst for superior photocatalytic water splitting R. Rameshbabu, P. Ravi, M. Sathish*
Cauliflower-like CuS/ZnS/g-C3N4 photocatalyst was developed using hydrothermal and ultrasound assisted wet impregnation method followed cation exchange treatment. A highest hydrogen production of 9868 μmolh-1g-1 was obtained for the above nanocomposite containing 5 wt% of g-C3N4.