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ZIF-8 composite for enhanced photocatalytic reduction of CO2
Applied Surface Science 498 (2019) 143899
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Photostable core-shell CdS/ZIF-8 composite for enhanced photocatalytic reduction of CO2
T
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Ying Liua, Liu Denga, , Jianping Shenga, Feiying Tanga, Ke Zenga, Liqiang Wanga, Kaixin Lianga, ⁎⁎ Hang Hua, You-Nian Liua,b, a b
College of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan 410083, PR China State Key Laboratory of Powder metallurgy, Central South University, Changsha, Hunan 410083, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: CdS MOFs Photocatalysis CO2 reduction Photostability
Photocatalytic conversion of CO2 is recognized as a promising method to reduce CO2 emission and produce available chemical energy simultaneously. CdS-based photocatalysts have been intensively studied for CO2 reduction, due to their excellent optical response and suitable band structures. However, the photocorrosion of CdS seriously restricts their application. To reduce the photocorrosion, CdS/ZIF-8 composites with core-shell structures are prepared. The experimental results show that the modification of ZIF-8 shell can not only improve the photostability of CdS, but also enhance its CO2 adsorption capacity without impairing its light harvesting ability. CdS/ZIF-8 catalyst shows higher efficiency and better selectivity than that of unmodified CdS in photocatalytic reduction of CO2 to CO, which is attributed to its higher CO2 adsorption capability. Moreover, CdS/ ZIF-8 composites exhibit high photostability.
1. Introduction Photocatalytic CO2 reduction is recognized as an emerging method to alleviate series of global environmental problems caused by excessive CO2 emission and produce available chemical energy simultaneously [1–7]. Up to date, a large number of semiconductors, such as TiO2 [8], Ga2O3 [9], Zn2GeO4 [10], CdS [11] and ZnIn2S4 [12], have been explored as photocatalysts for CO2 reduction. However the low photocatalytic conversion efficiency, weak visible-light absorption, high recombination rate of photogenerated electron and hole, strongly limit their real applications [13,14]. CdS, a typical transition metal sulfidebased photocatalyst, has been employed as a photocatalyst for CO2 reduction because of its high visible-light absorption (with a band gap of 2.4 eV), suitable valence band (VB), conduction band (CB) position and high electron mobility [15,16]. However, like many other semiconductor photocatalysts, CdS suffers from low CO2 conversion efficiency. In addition, CdS is unstable under light irradiation because the photogenerated hole can easily react with the sulfide ions of CdS, resulting in the formation of sulfur and the mass loss of photocatalysts [17–19]. Consequently, the specific engineering for CdS is required to obtain the highly efficient catalyst for CO2 reduction. It has been proven that the stability and catalytic performance of
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nanocatalyst can be improved by surface modification [20–22]. Metal–organic frameworks (MOFs), which consist of metal clusters and organic ligands, have attracted great attention in adsorption and catalysis [23–31]. Some MOFs have been used for CO2 capture and storage due to their unique porous structures and high specific surface area [32]. The combination of CdS and MOFs could make it a potentially highlyefficient catalyst for photocatalytic CO2 reduction. Furthermore, the directly in-situ growth of MOFs shell with CdS core could be a promising strategy for enhancing the stability and catalytic activity of CdS catalyst. Herein, CdS/ZIF-8 composites with core-shell structures are prepared by growing ZIF-8 nanocrystals on the surface of CdS nanoparticles. The core-shell structure can effectively reduce the photocorrosion of CdS. The modification of ZIF-8 shell can not only improve the photostability of CdS, but also enhance its CO2 adsorption capacity without impairing its light harvesting ability. CdS/ZIF-8 appears a 22.0% increase and better selectivity in photocatalytic reduction of CO2 to CO compared with CdS, which is attributed to its higher CO2 adsorption capability. Moreover, the catalyst exhibits high photostability.
Corresponding author. Correspondence to: College of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan 410083, PR China. E-mail addresses: [email protected] (L. Deng), [email protected] (Y.-N. Liu).
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https://doi.org/10.1016/j.apsusc.2019.143899 Received 7 April 2019; Received in revised form 3 August 2019; Accepted 7 September 2019 Available online 07 September 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 498 (2019) 143899
Y. Liu, et al.
2. Experimental section
Kα radiation as the X-ray source. UV–vis diffuse reflectance spectra (DRS) of the samples were recorded via a Shimadzu UV-2550 spectrophotometers (Japan). The nitrogen adsorption-desorption isotherms were measured using a Micromeritics ASAP 2020 equipment (USA), and the carbon dioxide adsorption-desorption isotherms were determined at 25 °C by the same equipment. All the materials were degassed at 90 °C before the measurements.
2.1. Materials All reagents were of analytical grade and used without further purification. Thiourea, Cd(NO3)2·4H2O, PVP, Zn(NO3)2·6H2O, 2-methylimidazole, 2,2′-bipyridine, triethanolamine (TEOA) and CoCl2 were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Milli-Q water was used through the whole experiments.
2.6. Photocatalytic reduction of CO2
2.2. Preparation of CdS nanoparticles
Photocatalytic reduction of CO2 was carried out by using a gasclosed quartz reactor (250 mL) with an optically-clear quartz cover [36]. At first, 40 mg catalyst, 200 mg 2,2′-bipyridine, 10 mL TEOA and 10 μmol CoCl2 were added into 60 mL component solvent (acetonitrile/ H2O = 2:1) and dispersed uniformly. Then, the mixture was transferred into the quartz reactor and stirred with a magnetic stirrer. Before illumination, the reaction system was bubbled with high-purity CO2 in the dark for 15 min, followed by being tightly closed at 0.1 MPa CO2 pressure. The reactor was irradiated by a 300 W Xe lamp equipped with a 420 nm cut-off filter through the top quartz window. The light intensity was kept at 500 mW/cm2 and the temperature of the reaction system was maintained at 25 °C by a circulating water bath. In the process of photocatalytic reaction, gaseous products were analyzed on an Agilent 7890B gas chromatograph equipped with a methanizer, a TCD and an FID at set intervals. The calibration curves of CO and H2 were obtained by injecting different amount of pure gases into the reaction system. The cycle stability performance was evaluated by recovering the photocatalysts and repeating the photocatalytic reaction using a fresh solution.
CdS nanoparticles were prepared according to a previously reported method. [17] Briefly, thiourea (0.53 g,7.0 mmol), Cd(NO3)2·4H2O (2.16 g, 7.0 mmol) and PVP (0.78 g, 7.0 mmol) were dissolved in ethylene glycol (70 mL) to form a clear solution. Then, the mixture was transferred to a Teflon-lined stainless steel autoclave with 100 mL inner volume and maintained at 120 °C for 12 h. After the autoclave was cooled to room temperature, 50 mL methanol was added into the obtained solution to precipitate CdS nanoparticles. The yellow products were collected by centrifugation at 10000 rpm for 10 min, washed with methanol and water alternatively for three times, and then freeze-dried under vacuum for 12 h. 2.3. Fabrication of CdS/ZIF-8 composite CdS/ZIF-8 composite was fabricated by in-situ heterogeneous deposition. In detail, 32 mg CdS nanoparticles were added into 10 mL methanol and sonicated for few minutes to ensure good dispersion. Next, 0.2974 g Zn(NO3)2·6H2O was dissolved in 50 mL methanol (20 mM), then mixed with the CdS suspension (10 mL, 3.2 mg·mL−1) and stirred at room temperature for 1 h. After that, 2-methylimidazole (0.3284 g) was dispersed into 50 mL methanol, and the obtained solution was poured into the former mixture under stirring, followed by being stirred for another 3 h to make sure ZIF-8 grew on CdS nanoparticles uniformly and sufficiently. The precipitate was collected by centrifugation at 1500 rpm for 10 min. The upper suspension layer was light milky white because there were a few uncombined ZIF-8 crystals generated inevitably. Finally, the products were washed with methanol and water, then freeze-dried under vacuum for 12 h. In addition, the CdS/ZIF-8 composites with different ZIF-8 shell thickness were prepared by altering the concentration of Zn2+. When the concentrations of Zn2+ were fixed to 40 and 80 mM, the obtained composites were noted as CdS/ZIF-8-40 and CdS/ZIF-8-80, respectively.
Photocurrent measurements were performed on an RST electrochemical workstation using a typical three-electrode system. An Ag/ AgCl electrode and a Pt plate served as the reference electrode and the counter electrode respectively. The working electrode was made by dipcoating samples onto fluorine‑tin oxide (FTO) glasses. During this process, the FTO glass was covered partly by medical tape with a 1.4 cm2 exposed area. 5 mg samples were dispersed into 0.5 mL ethanol uniformly, and then 20 μl suspension was spread onto the exposed area of the FTO glass for three times, followed by being dried in the air for 12 h. The three-electrode system was placed in a 0.5 M Na2SO4 aqueous solution, and a 300 W Xe lamp equipped with a 420 nm cut-off filter was used as the light source.
2.4. Fabrication of ZIF-8 nanocrystals
3. Results and discussion
ZIF-8 nanocrystals were fabricated according to a previous report with a slight modification [33]. In brief, Zn(NO3)2·6H2O (0.5948 g) was dissolved in 50 mL methanol (40 mM), and 2-methylimidazole (0.6568 g) was dissolved in 50 mL methanol (160 mM). Subsequently, the latter solution was added into the former under magnetic stirring for 3 h. The milky white suspension was centrifuged at 10000 rpm for 10 min. The precipitate was collected, washed with methanol and water, then freeze-dried under vacuum for 12 h.
The crystal structure of as-prepared samples is analyzed by powder X-ray diffraction (XRD). As shown in Fig. 1, all of the characteristic diffraction peaks can be assigned to hexagonal phase CdS (JCPDS 41–1049). Meanwhile, the pattern of the pure ZIF-8 nanocrystals is in a good agreement with that of the literature [32]. As expected, it can be seen from Fig. 1 that the peaks in the XRD pattern of CdS/ZIF-8 composite are the combination of those of CdS and ZIF-8. The peaks at 7.4° (011), 12.8° (112) and 18.1° (222) are attributed to ZIF-8 and the peaks at 24.9° (100), 26.5° (002), 28.2° (101), 43.8° (110), 47.8° (103) and 51.9° (112) correspond to CdS (JCPDS 41–1049). However, the peak intensities ascribed to ZIF-8 are relatively weak, mainly because the mass fraction of ZIF-8 in the CdS/ZIF-8 composite is much smaller than that of CdS. The morphology and structure of pure CdS nanoparticles and CdS/ ZIF-8 composite are measured by SEM and TEM. Fig. 2a reveals that CdS nanoparticles are uniform spherical particles with diameters of 280–360 nm and rough surface. After ZIF-8 nanocrystals grew on CdS, the spherical morphology of the nanoparticles is well maintained and the average diameter increases slightly (see Fig. 2b). Moreover, the
2.7. Photocurrent measurement
2.5. Characterizations Powder X-ray diffraction (XRD) patterns were recorded on a D/Max 2550 X-ray diffractometer (Rigaku, Japan) using a Cu Kα radiation source. Scanning electron microscopy (SEM) images were obtained by FEI HELIOS Nano Lab 600i (USA). Transmission electron microscopy (TEM) imaging and energy-dispersive X-ray (EDX) elemental mapping were performed on FEI Titan G2 60–300 with spherical aberration correction (USA). X-ray photoelectron spectroscopy (XPS) data were gained from ESCALAB 250Xi (Thermo Fisher Scientific, USA) with Al 2
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Fig. 1. XRD pattern of fabricated CdS/ZIF-8 composite.
surface of CdS/ZIF-8 composite is much glossier and smoother than that of CdS nanosphere, suggesting the successful growth of ZIF-8 nanocrystals on the surface of CdS nanoparticles. Meanwhile, from Fig. 2c & d, we can easily figure out that CdS nanoparticles are coated by ZIF-8 crystals uniformly and closely, resulting in a typical core-shell structure. It is noteworthy that the pure CdS nanoparticles are negatively charged (see Fig. S2). Thus, in the fabrication process of CdS/ZIF-8 composite, zinc ions were first mixed with CdS suspension to ensure good adsorption, which could further promote the uniform and close growth of ZIF-8 nanocrystals on the surface of CdS. The higeher magnification TEM images are illustrated in Fig. 2e & f, they reveal that the thickness of the ZIF-8 shell is about 30 nm. Fig. 2g shows the elemental mapping images of a single CdS/ZIF-8 nanoparticle, which indicates that the Cd and S elements are mainly distributed in the center of the nanoparticle and the Zn is uniformly distributed over the entire nanoparticle, further verifying the core-shell structure of CdS/ZIF-8 composite. The surface chemical states of the samples are analyzed by XPS. Fig. 3a shows the full XPS survey spectra of CdS/ZIF-8 composite, suggesting the existence of S, C, Cd, N, O, and Zn elements in the composite. Although most of the CdS/ZIF-8 nanoparticles have compact core-shell structre, there are inevitably some exposed CdS nanoparticles, leading to the appearance of S and Cd signals in the XPS spectra. The XPS survey spectra of CdS, CdS/ZIF-8 and ZIF-8 are displayed in Fig. S3, which shows that the peaks of CdS/ZIF-8 composite are the combination of those of CdS and ZIF-8. The high-resolution XPS spectra of Zn 2p shows two peaks centered at binding energies of 1021.9 and 1044.9 eV, which are assigned to Zn 2p3/2 and Zn 2p1/2 respectively. There are two separated peaks located at binding energies of 404.9 and 411.8 eV, corresponding to Cd 3d5/2 and Cd 3d3/2 respectively. The appearance of these two peaks are attributed to the spin orbit splitting of Cd 3d orbital, and the spin orbit separation is 6.9 eV, demonstrating the chemical state of Cd is Cd2+. The S 2p spectra can be fitted to two peaks at binding energies of 161.2 and 162.4 eV, which are ascribed to S 2p1/2 and S 2p3/2 respectively, indicating the existence of S2− state [34,35]. The specific surface area of three samples is investigated by N2 adsorption-desorption measurements. Fig. 4 shows the measuring results of pure CdS nanoparticles and CdS/ZIF-8 composite while Fig. S4 shows the measuring result of ZIF-8 nanocrystals. The nitrogen adsorption-desorption isotherms of the three samples are type IV according to the BDDT (Brunauer-Deming-Demin-Teller) classification. In this situation, the isotherms possess hysteresis loop as a result of
Fig. 2. SEM images of pure CdS nanoparticles (a) and CdS/ZIF-8 composite (b); TEM images of pure CdS nanoparticles (c and e) and CdS/ZIF-8 composite (d and f); HAADF-STEM image of CdS/ZIF-8 composite and the corresponding elemental mapping images of S, Cd and Zn.
capillary condensation at high relative pressures. Moreover, the hysteresis loops of all the three samples are H3 type according to the IUPAC classification, indicating the presence of slit-shaped pores due to the aggregation of nanoparticles [36]. ZIF-8 is a typical MOFs material with high porosity, low density and high specific surface area. It can be seen from Fig. S4 that the as-synthesized ZIF-8 crystals show extremely high nitrogen adsorption capacity with a BET surface area of 1240.3 m2 g−1. As shown in Fig. 4, the nitrogen adsorption of CdS/ZIF8 composite is much higher compared with pure CdS in the full range mainly because of the high porosity of deposited ZIF-8 crystals. By contrast, the BET surface area of pure CdS is only 13.6 m2 g−1, which is much smaller than that of CdS/ZIF-8 composite (74.0 m2 g−1). Fig. 5 shows the CO2 adsorption isotherms of pure CdS and CdS/ZIF8 composite. The CO2 adsorption on both pure CdS and CdS/ZIF-8 composite has linear relationship with pressure in the range of 0–0.1 MPa, indicating the physical adsorption of CO2 onto the photocatalysts. At 298 K and 0.1 MPa, the maximum CO2 adsorption for CdS and CdS/ZIF-8 is 0.87 mL/g and 3.08 mL/g respectively. CdS/ZIF-8 composites show an approximately 3.5 times increase in CO2 3
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Fig. 3. XPS survey spectra of CdS/ZIF-8 composite and the corresponding high-resolution XPS spectra of Zn 2p, Cd 3d, and S 2p.
Fig. 5. CO2 adsorption isotherms of pure CdS and CdS/ZIF-8 composite.
Fig. 4. N2 adsorption/desorption isotherms of pure CdS and CdS/ZIF-8 composite.
with pure CdS nanoparticles, the UV–vis diffuse reflectance spectra of CdS/ZIF-8 composite only change a little, which means the light harvesting ability of CdS/ZIF-8 composite only has a very slight decrease though the spherical CdS is wholly wrapped by ZIF-8 crystals. The band gaps of pure CdS and CdS/ZIF-8 composite are calculated to be 2.42 and 2.41 eV respectively, according to Tauc plots (Fig. 6b). The deposition of ZIF-8 crystals results in little influence on the band structure of CdS nanoparticles. Three-electrode system was used to measure the transient photocurrent responses of CdS and CdS/ZIF-8 composite. The working electrode was made by dip-coating samples onto fluorine‑tin oxide (FTO) glasses. 0.5 M Na2SO4 aqueous solution served as the electrolyte and a 300 W Xe lamp equipped with a 420 nm cut-off filter
adsorption than that of CdS because of the high CO2 adsorption capability of ZIF-8 (see Fig. S5). The high CO2 adsorption is benificial for the photocatalytic CO2 reduction. The optical absorption of pure CdS, CdS/ZIF-8 composite and ZIF-8 crystals are investigated by UV–vis diffuse reflectance spectra (see Fig. 6a). Pure CdS nanoparticles have strong absorption in the UV–vis region and mainly absorb light with wavelength shorter than 550 nm, which is consistent with the previous litreature reports [37]. However, ZIF-8 crystals are white powder with the main light absorption in the ultraviolet region and nearly no visible light absorption. Compared 4
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Fig. 6. (a) UV–vis diffuse reflectance spectra of pure CdS (red), CdS/ZIF-8 composite (black) and ZIF-8 crystals (blue); (b) Transformed Kubelka-Munk function versus the irradiated light energy for CdS (red) and CdS/ZIF-8 composite (black); Transient photocurrent responses of CdS (c) and CdS/ZIF-8 composite (d) under visible light irradiation (λ > 420 nm).
demonstrating that CdS/ZIF-8 has better selectivity towards CO relative to CdS. Fig. 7c shows the CO and H2 evolution rate on CdS and CdS/ZIF8 composite. The CO production rates on CdS and CdS/ZIF-8 composite are 26.33 μmol h−1 and 32.13 μmol h−1, respectively. In addition, the H2 evolution rates on CdS and CdS/ZIF-8 composite are 5.77 μmol h−1 and 7.09 μmol h−1, respectively. The CO selectivity of CdS/ZIF-8 is around 83.96%, higher than that of CdS. The control experiment showed that ZIF-8 exhibited no catalytic activity. When using N2 as the gas feedstock, only H2 was generated, confirming that the photogenerated CO comes from the reduced CO2. The photostability of CdS/ ZIF-8 composite is further checked by recovering the photocatalysts and repeating the photocatalytic reaction using a fresh solution for 75 min. As shown in Fig. 7d, no obvious decrease of CO evolution is observed after 4 cycles. The CO production of CdS/ZIF-8 composite for the fourth cycle remains at 90% of the initial reaction. The TEM image of the used CdS/ZIF-8 composite is shown in Fig. S8, indicating excellent structure stability of CdS/ZIF-8 NPs. The cycle stability experiment accompanied with the photocurrent measurement reflects the good photostability of CdS/ZIF-8 composite. Compared with previous papers in Table S1, our proposed CO2RR photocatalyst exhibits excellent CO2 reduction performance. To test the impact of the thickness on the photocatalytic performance, samples with various thickness were prepared. It was reported that the thickness of ZIF-8 shell can be controlled by adjusting the concentration of Zn2+, because Zn2+ can accelerate the growth rate of ZIF-8 nanocrystals [22]. Therefore, the CdS/ZIF-8 composites with different ZIF-8 shell thickness were prepared by altering the concentration of Zn2+. When the concentration of Zn2+ was fixed at 40 mM, the thickness of ZIF-8 shell of the as-obtained composite (CdS/ ZIF-8-40) is about 60 nm, much thicker than that of CdS/ZIF-8 (the Zn2+ concentration was 20 nm). Moreover, when the concentration of Zn2+ was increased to 80 mM, the composite (CdS/ZIF-8-80) possesses multiple-core-shell structures (see Fig. S9). Then the CO2 photo-reduction performance of the CdS/ZIF-8, CdS/ZIF-8-40 and CdS/ZIF-8-80 composites was investigated. As shown in Fig. 8, the CO evolution on CdS/ZIF-8-40 and CdS/ZIF-8-80 is 13 μmol and 9 μmol, respectively. By
was used as the light source. As shown in Fig. 6c & d, both CdS and CdS/ZIF-8 composite exhibit photocurrent responses under several onoff cycles of visible light illumination. Interestingly, we found that the initial photocurrent intensity of CdS and CdS/ZIF-8 composite are almost the same, but the photocurrent stability of CdS/ZIF-8 is much better than that of CdS. The photocurrent response of CdS is initially around 2 μA, and then decreases continuously under the irradiation of light, consistent with literature data [38–40], which attributes the photocurrent decrease of CdS to photocorrosion. By contrast, the photocurrent response of CdS/ZIF-8 appears stable at 2 μA, mainly because the core-shell structure of CdS/ZIF-8 can efficiently protect CdS from photocorrosion by stabilizing sulfide and cadmium ions inside the core. As a consequence, CdS/ZIF-8 possesses better photostability compared with CdS. The above characterization results indicate that the modification of ZIF-8 can certainly improve the CO2 adsorption capability and photocurrent stability of CdS without impairing its light harvesting ability, which tends to bring advantages to the photocatalytic reduction of CO2. The photocatalytic reaction was carried out in a component solvent system (acetonitrile/H2O). TEOA and Co(bpy)32+ were added as a sacrificial agent and an co-catalyst, respectively. The evolution amounts of CO and H2 are calculated according to the calibration curves (see Fig. S6 & 7). Fig. 7a shows the CO evolution over CdS and CdS/ZIF-8 composite under visible light illumination. At the beginning of the light irradiation, there is nearly no CO detected in both CdS and CdS/ZIF-8 photocatalytic system, and that period of CdS/ZIF-8 is even longer than that of CdS. Whether the catalytic active sites are on CdS or on ZIF-8, there is a porous shell between photogenerated electron and most CO2 molecules, which may inevitably increase the initial reaction time. When CO can be detected, the CO evolution of CdS and CdS/ZIF-8 both increase linearly with the irradiation time. Moreover, the accumulated CO production of CdS/ZIF-8 exceeds that of CdS at 100 min. The improved CO evolution rate of CdS/ZIF-8 can be attributed to its higher CO2 adsorption capability, which can increase the CO2 concentration around the catalysts. Different from the CO production, the H2 evolution of CdS/ZIF-8 is generally lower than that of CdS (see Fig. 7 b),
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Fig. 7. Time-yield plots of CO (a) and H2 (b) over CdS and CdS/ZIF-8 composite under visible light illumination; (c) gas evolution rate and CO/H2 ratio on CdS and CdS/ZIF-8 composite; (d) recycling tests of the photocatalytic CO evolution over CdS/ZIF-8 composite.
reduce the adsorbed CO2 to form CO, while the abundant holes on the VB of CdS are able to oxidize surface-adsorbed TEOA to yield TEOA+, as shown in Fig. 9. The core-shell structure of CdS/ZIF-8 can efficiently protect CdS from photocorrosion by stabilizing sulfide and cadmium ions inside the core.
4. Conclusions In summary, CdS/ZIF-8 composites with core-shell structures are prepared and applied in photocatalytic reduction of CO2. CdS/ZIF-8 shows higher efficiency and better selectivity in photocatalytic reduction of CO2 to CO compared with CdS, which is attributed to its higher CO2 adsorption capability. The modification of ZIF-8 shell can improve the CO2 adsorption capacity and photostability of CdS without impairing its light harvesting ability. The photostability of CdS/ZIF-8 is verified by photocurrent measurement and cycle stability experiment. There is no obvious decrease in the performance of photocatalytic CO2 reduction after 4 cycles for CdS/ZIF-8 composites. This work provides a way to construct core-shell semiconductor/MOFs composite with semiconductor as the core and MOFs as the shell, which can combine the advantages of both and improve the optical stability of semiconductors for many applications, such as catalysis, energy conversion and devices.
Fig. 8. CO evolution on CdS/ZIF-8 composite with different ZIF-8 shell thickness.
contrast, the CO evolution on CdS/ZIF-8 is around 16 μmol. The photoreduction activities of these samples decrease with the increase of ZIF-8 shell thickness, which could be attributed to the stronger barriers between CO2 and photogenerated electrons caused by thick ZIF-8 shells. Mott-Schottky (M-S) plots were performed to investigate the flat band potential as well as Fermi energy level of semiconductor. As shown in Fig. S10, the observed positive slopes on the M-S plots exhibit the typical n-type characteristics for CdS and CdS/ZIF-8. Based on the intersection points, the flat band potentials of CdS and CdS/ZIF-8 are nearly identical (approximately −0.7 V vs. Ag/AgCl), implying that the ZIF-8 shell has no effect on the semiconductor properties of CdS. In general, the flat band potential of n-type semiconductor (intercept value at the x-axis) can be used to estimate the conduction band (CB) of semiconductor. Therefore, the CB edge position of CdS can be estimated to be −0.78 V (vs. Ag/AgCl). Besides, the band baps determined from Tauc plot are 2.38 eV for CdS. Since the CB potential of CdS (−0.7 V vs. Ag/AgCl) is more negative compared with CO2/CO (−0.12 V vs. Ag/ AgCl), the photoinduced electrons on the CB potentials of CdS could
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21636010 and 21878342).
Declaration of competing interest The authors declare that there is no conflict of interest. 6
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Fig. 9. Schematic illustration of the photocatalytic CO2 reduction process over CdS/ZIF-8 composites under visible light irradiation.
Appendix A. Supplementary figures
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Photostable core-shell CdS/ZIF-8 composite for enhanced photocatalytic reduction of CO2
T
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Ying Liua, Liu Denga, , Jianping Shenga, Feiying Tanga, Ke Zenga, Liqiang Wanga, Kaixin Lianga, ⁎⁎ Hang Hua, You-Nian Liua,b, a b
College of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan 410083, PR China State Key Laboratory of Powder metallurgy, Central South University, Changsha, Hunan 410083, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: CdS MOFs Photocatalysis CO2 reduction Photostability
Photocatalytic conversion of CO2 is recognized as a promising method to reduce CO2 emission and produce available chemical energy simultaneously. CdS-based photocatalysts have been intensively studied for CO2 reduction, due to their excellent optical response and suitable band structures. However, the photocorrosion of CdS seriously restricts their application. To reduce the photocorrosion, CdS/ZIF-8 composites with core-shell structures are prepared. The experimental results show that the modification of ZIF-8 shell can not only improve the photostability of CdS, but also enhance its CO2 adsorption capacity without impairing its light harvesting ability. CdS/ZIF-8 catalyst shows higher efficiency and better selectivity than that of unmodified CdS in photocatalytic reduction of CO2 to CO, which is attributed to its higher CO2 adsorption capability. Moreover, CdS/ ZIF-8 composites exhibit high photostability.
1. Introduction Photocatalytic CO2 reduction is recognized as an emerging method to alleviate series of global environmental problems caused by excessive CO2 emission and produce available chemical energy simultaneously [1–7]. Up to date, a large number of semiconductors, such as TiO2 [8], Ga2O3 [9], Zn2GeO4 [10], CdS [11] and ZnIn2S4 [12], have been explored as photocatalysts for CO2 reduction. However the low photocatalytic conversion efficiency, weak visible-light absorption, high recombination rate of photogenerated electron and hole, strongly limit their real applications [13,14]. CdS, a typical transition metal sulfidebased photocatalyst, has been employed as a photocatalyst for CO2 reduction because of its high visible-light absorption (with a band gap of 2.4 eV), suitable valence band (VB), conduction band (CB) position and high electron mobility [15,16]. However, like many other semiconductor photocatalysts, CdS suffers from low CO2 conversion efficiency. In addition, CdS is unstable under light irradiation because the photogenerated hole can easily react with the sulfide ions of CdS, resulting in the formation of sulfur and the mass loss of photocatalysts [17–19]. Consequently, the specific engineering for CdS is required to obtain the highly efficient catalyst for CO2 reduction. It has been proven that the stability and catalytic performance of
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nanocatalyst can be improved by surface modification [20–22]. Metal–organic frameworks (MOFs), which consist of metal clusters and organic ligands, have attracted great attention in adsorption and catalysis [23–31]. Some MOFs have been used for CO2 capture and storage due to their unique porous structures and high specific surface area [32]. The combination of CdS and MOFs could make it a potentially highlyefficient catalyst for photocatalytic CO2 reduction. Furthermore, the directly in-situ growth of MOFs shell with CdS core could be a promising strategy for enhancing the stability and catalytic activity of CdS catalyst. Herein, CdS/ZIF-8 composites with core-shell structures are prepared by growing ZIF-8 nanocrystals on the surface of CdS nanoparticles. The core-shell structure can effectively reduce the photocorrosion of CdS. The modification of ZIF-8 shell can not only improve the photostability of CdS, but also enhance its CO2 adsorption capacity without impairing its light harvesting ability. CdS/ZIF-8 appears a 22.0% increase and better selectivity in photocatalytic reduction of CO2 to CO compared with CdS, which is attributed to its higher CO2 adsorption capability. Moreover, the catalyst exhibits high photostability.
Corresponding author. Correspondence to: College of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan 410083, PR China. E-mail addresses: [email protected] (L. Deng), [email protected] (Y.-N. Liu).
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https://doi.org/10.1016/j.apsusc.2019.143899 Received 7 April 2019; Received in revised form 3 August 2019; Accepted 7 September 2019 Available online 07 September 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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2. Experimental section
Kα radiation as the X-ray source. UV–vis diffuse reflectance spectra (DRS) of the samples were recorded via a Shimadzu UV-2550 spectrophotometers (Japan). The nitrogen adsorption-desorption isotherms were measured using a Micromeritics ASAP 2020 equipment (USA), and the carbon dioxide adsorption-desorption isotherms were determined at 25 °C by the same equipment. All the materials were degassed at 90 °C before the measurements.
2.1. Materials All reagents were of analytical grade and used without further purification. Thiourea, Cd(NO3)2·4H2O, PVP, Zn(NO3)2·6H2O, 2-methylimidazole, 2,2′-bipyridine, triethanolamine (TEOA) and CoCl2 were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Milli-Q water was used through the whole experiments.
2.6. Photocatalytic reduction of CO2
2.2. Preparation of CdS nanoparticles
Photocatalytic reduction of CO2 was carried out by using a gasclosed quartz reactor (250 mL) with an optically-clear quartz cover [36]. At first, 40 mg catalyst, 200 mg 2,2′-bipyridine, 10 mL TEOA and 10 μmol CoCl2 were added into 60 mL component solvent (acetonitrile/ H2O = 2:1) and dispersed uniformly. Then, the mixture was transferred into the quartz reactor and stirred with a magnetic stirrer. Before illumination, the reaction system was bubbled with high-purity CO2 in the dark for 15 min, followed by being tightly closed at 0.1 MPa CO2 pressure. The reactor was irradiated by a 300 W Xe lamp equipped with a 420 nm cut-off filter through the top quartz window. The light intensity was kept at 500 mW/cm2 and the temperature of the reaction system was maintained at 25 °C by a circulating water bath. In the process of photocatalytic reaction, gaseous products were analyzed on an Agilent 7890B gas chromatograph equipped with a methanizer, a TCD and an FID at set intervals. The calibration curves of CO and H2 were obtained by injecting different amount of pure gases into the reaction system. The cycle stability performance was evaluated by recovering the photocatalysts and repeating the photocatalytic reaction using a fresh solution.
CdS nanoparticles were prepared according to a previously reported method. [17] Briefly, thiourea (0.53 g,7.0 mmol), Cd(NO3)2·4H2O (2.16 g, 7.0 mmol) and PVP (0.78 g, 7.0 mmol) were dissolved in ethylene glycol (70 mL) to form a clear solution. Then, the mixture was transferred to a Teflon-lined stainless steel autoclave with 100 mL inner volume and maintained at 120 °C for 12 h. After the autoclave was cooled to room temperature, 50 mL methanol was added into the obtained solution to precipitate CdS nanoparticles. The yellow products were collected by centrifugation at 10000 rpm for 10 min, washed with methanol and water alternatively for three times, and then freeze-dried under vacuum for 12 h. 2.3. Fabrication of CdS/ZIF-8 composite CdS/ZIF-8 composite was fabricated by in-situ heterogeneous deposition. In detail, 32 mg CdS nanoparticles were added into 10 mL methanol and sonicated for few minutes to ensure good dispersion. Next, 0.2974 g Zn(NO3)2·6H2O was dissolved in 50 mL methanol (20 mM), then mixed with the CdS suspension (10 mL, 3.2 mg·mL−1) and stirred at room temperature for 1 h. After that, 2-methylimidazole (0.3284 g) was dispersed into 50 mL methanol, and the obtained solution was poured into the former mixture under stirring, followed by being stirred for another 3 h to make sure ZIF-8 grew on CdS nanoparticles uniformly and sufficiently. The precipitate was collected by centrifugation at 1500 rpm for 10 min. The upper suspension layer was light milky white because there were a few uncombined ZIF-8 crystals generated inevitably. Finally, the products were washed with methanol and water, then freeze-dried under vacuum for 12 h. In addition, the CdS/ZIF-8 composites with different ZIF-8 shell thickness were prepared by altering the concentration of Zn2+. When the concentrations of Zn2+ were fixed to 40 and 80 mM, the obtained composites were noted as CdS/ZIF-8-40 and CdS/ZIF-8-80, respectively.
Photocurrent measurements were performed on an RST electrochemical workstation using a typical three-electrode system. An Ag/ AgCl electrode and a Pt plate served as the reference electrode and the counter electrode respectively. The working electrode was made by dipcoating samples onto fluorine‑tin oxide (FTO) glasses. During this process, the FTO glass was covered partly by medical tape with a 1.4 cm2 exposed area. 5 mg samples were dispersed into 0.5 mL ethanol uniformly, and then 20 μl suspension was spread onto the exposed area of the FTO glass for three times, followed by being dried in the air for 12 h. The three-electrode system was placed in a 0.5 M Na2SO4 aqueous solution, and a 300 W Xe lamp equipped with a 420 nm cut-off filter was used as the light source.
2.4. Fabrication of ZIF-8 nanocrystals
3. Results and discussion
ZIF-8 nanocrystals were fabricated according to a previous report with a slight modification [33]. In brief, Zn(NO3)2·6H2O (0.5948 g) was dissolved in 50 mL methanol (40 mM), and 2-methylimidazole (0.6568 g) was dissolved in 50 mL methanol (160 mM). Subsequently, the latter solution was added into the former under magnetic stirring for 3 h. The milky white suspension was centrifuged at 10000 rpm for 10 min. The precipitate was collected, washed with methanol and water, then freeze-dried under vacuum for 12 h.
The crystal structure of as-prepared samples is analyzed by powder X-ray diffraction (XRD). As shown in Fig. 1, all of the characteristic diffraction peaks can be assigned to hexagonal phase CdS (JCPDS 41–1049). Meanwhile, the pattern of the pure ZIF-8 nanocrystals is in a good agreement with that of the literature [32]. As expected, it can be seen from Fig. 1 that the peaks in the XRD pattern of CdS/ZIF-8 composite are the combination of those of CdS and ZIF-8. The peaks at 7.4° (011), 12.8° (112) and 18.1° (222) are attributed to ZIF-8 and the peaks at 24.9° (100), 26.5° (002), 28.2° (101), 43.8° (110), 47.8° (103) and 51.9° (112) correspond to CdS (JCPDS 41–1049). However, the peak intensities ascribed to ZIF-8 are relatively weak, mainly because the mass fraction of ZIF-8 in the CdS/ZIF-8 composite is much smaller than that of CdS. The morphology and structure of pure CdS nanoparticles and CdS/ ZIF-8 composite are measured by SEM and TEM. Fig. 2a reveals that CdS nanoparticles are uniform spherical particles with diameters of 280–360 nm and rough surface. After ZIF-8 nanocrystals grew on CdS, the spherical morphology of the nanoparticles is well maintained and the average diameter increases slightly (see Fig. 2b). Moreover, the
2.7. Photocurrent measurement
2.5. Characterizations Powder X-ray diffraction (XRD) patterns were recorded on a D/Max 2550 X-ray diffractometer (Rigaku, Japan) using a Cu Kα radiation source. Scanning electron microscopy (SEM) images were obtained by FEI HELIOS Nano Lab 600i (USA). Transmission electron microscopy (TEM) imaging and energy-dispersive X-ray (EDX) elemental mapping were performed on FEI Titan G2 60–300 with spherical aberration correction (USA). X-ray photoelectron spectroscopy (XPS) data were gained from ESCALAB 250Xi (Thermo Fisher Scientific, USA) with Al 2
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Fig. 1. XRD pattern of fabricated CdS/ZIF-8 composite.
surface of CdS/ZIF-8 composite is much glossier and smoother than that of CdS nanosphere, suggesting the successful growth of ZIF-8 nanocrystals on the surface of CdS nanoparticles. Meanwhile, from Fig. 2c & d, we can easily figure out that CdS nanoparticles are coated by ZIF-8 crystals uniformly and closely, resulting in a typical core-shell structure. It is noteworthy that the pure CdS nanoparticles are negatively charged (see Fig. S2). Thus, in the fabrication process of CdS/ZIF-8 composite, zinc ions were first mixed with CdS suspension to ensure good adsorption, which could further promote the uniform and close growth of ZIF-8 nanocrystals on the surface of CdS. The higeher magnification TEM images are illustrated in Fig. 2e & f, they reveal that the thickness of the ZIF-8 shell is about 30 nm. Fig. 2g shows the elemental mapping images of a single CdS/ZIF-8 nanoparticle, which indicates that the Cd and S elements are mainly distributed in the center of the nanoparticle and the Zn is uniformly distributed over the entire nanoparticle, further verifying the core-shell structure of CdS/ZIF-8 composite. The surface chemical states of the samples are analyzed by XPS. Fig. 3a shows the full XPS survey spectra of CdS/ZIF-8 composite, suggesting the existence of S, C, Cd, N, O, and Zn elements in the composite. Although most of the CdS/ZIF-8 nanoparticles have compact core-shell structre, there are inevitably some exposed CdS nanoparticles, leading to the appearance of S and Cd signals in the XPS spectra. The XPS survey spectra of CdS, CdS/ZIF-8 and ZIF-8 are displayed in Fig. S3, which shows that the peaks of CdS/ZIF-8 composite are the combination of those of CdS and ZIF-8. The high-resolution XPS spectra of Zn 2p shows two peaks centered at binding energies of 1021.9 and 1044.9 eV, which are assigned to Zn 2p3/2 and Zn 2p1/2 respectively. There are two separated peaks located at binding energies of 404.9 and 411.8 eV, corresponding to Cd 3d5/2 and Cd 3d3/2 respectively. The appearance of these two peaks are attributed to the spin orbit splitting of Cd 3d orbital, and the spin orbit separation is 6.9 eV, demonstrating the chemical state of Cd is Cd2+. The S 2p spectra can be fitted to two peaks at binding energies of 161.2 and 162.4 eV, which are ascribed to S 2p1/2 and S 2p3/2 respectively, indicating the existence of S2− state [34,35]. The specific surface area of three samples is investigated by N2 adsorption-desorption measurements. Fig. 4 shows the measuring results of pure CdS nanoparticles and CdS/ZIF-8 composite while Fig. S4 shows the measuring result of ZIF-8 nanocrystals. The nitrogen adsorption-desorption isotherms of the three samples are type IV according to the BDDT (Brunauer-Deming-Demin-Teller) classification. In this situation, the isotherms possess hysteresis loop as a result of
Fig. 2. SEM images of pure CdS nanoparticles (a) and CdS/ZIF-8 composite (b); TEM images of pure CdS nanoparticles (c and e) and CdS/ZIF-8 composite (d and f); HAADF-STEM image of CdS/ZIF-8 composite and the corresponding elemental mapping images of S, Cd and Zn.
capillary condensation at high relative pressures. Moreover, the hysteresis loops of all the three samples are H3 type according to the IUPAC classification, indicating the presence of slit-shaped pores due to the aggregation of nanoparticles [36]. ZIF-8 is a typical MOFs material with high porosity, low density and high specific surface area. It can be seen from Fig. S4 that the as-synthesized ZIF-8 crystals show extremely high nitrogen adsorption capacity with a BET surface area of 1240.3 m2 g−1. As shown in Fig. 4, the nitrogen adsorption of CdS/ZIF8 composite is much higher compared with pure CdS in the full range mainly because of the high porosity of deposited ZIF-8 crystals. By contrast, the BET surface area of pure CdS is only 13.6 m2 g−1, which is much smaller than that of CdS/ZIF-8 composite (74.0 m2 g−1). Fig. 5 shows the CO2 adsorption isotherms of pure CdS and CdS/ZIF8 composite. The CO2 adsorption on both pure CdS and CdS/ZIF-8 composite has linear relationship with pressure in the range of 0–0.1 MPa, indicating the physical adsorption of CO2 onto the photocatalysts. At 298 K and 0.1 MPa, the maximum CO2 adsorption for CdS and CdS/ZIF-8 is 0.87 mL/g and 3.08 mL/g respectively. CdS/ZIF-8 composites show an approximately 3.5 times increase in CO2 3
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Fig. 3. XPS survey spectra of CdS/ZIF-8 composite and the corresponding high-resolution XPS spectra of Zn 2p, Cd 3d, and S 2p.
Fig. 5. CO2 adsorption isotherms of pure CdS and CdS/ZIF-8 composite.
Fig. 4. N2 adsorption/desorption isotherms of pure CdS and CdS/ZIF-8 composite.
with pure CdS nanoparticles, the UV–vis diffuse reflectance spectra of CdS/ZIF-8 composite only change a little, which means the light harvesting ability of CdS/ZIF-8 composite only has a very slight decrease though the spherical CdS is wholly wrapped by ZIF-8 crystals. The band gaps of pure CdS and CdS/ZIF-8 composite are calculated to be 2.42 and 2.41 eV respectively, according to Tauc plots (Fig. 6b). The deposition of ZIF-8 crystals results in little influence on the band structure of CdS nanoparticles. Three-electrode system was used to measure the transient photocurrent responses of CdS and CdS/ZIF-8 composite. The working electrode was made by dip-coating samples onto fluorine‑tin oxide (FTO) glasses. 0.5 M Na2SO4 aqueous solution served as the electrolyte and a 300 W Xe lamp equipped with a 420 nm cut-off filter
adsorption than that of CdS because of the high CO2 adsorption capability of ZIF-8 (see Fig. S5). The high CO2 adsorption is benificial for the photocatalytic CO2 reduction. The optical absorption of pure CdS, CdS/ZIF-8 composite and ZIF-8 crystals are investigated by UV–vis diffuse reflectance spectra (see Fig. 6a). Pure CdS nanoparticles have strong absorption in the UV–vis region and mainly absorb light with wavelength shorter than 550 nm, which is consistent with the previous litreature reports [37]. However, ZIF-8 crystals are white powder with the main light absorption in the ultraviolet region and nearly no visible light absorption. Compared 4
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Fig. 6. (a) UV–vis diffuse reflectance spectra of pure CdS (red), CdS/ZIF-8 composite (black) and ZIF-8 crystals (blue); (b) Transformed Kubelka-Munk function versus the irradiated light energy for CdS (red) and CdS/ZIF-8 composite (black); Transient photocurrent responses of CdS (c) and CdS/ZIF-8 composite (d) under visible light irradiation (λ > 420 nm).
demonstrating that CdS/ZIF-8 has better selectivity towards CO relative to CdS. Fig. 7c shows the CO and H2 evolution rate on CdS and CdS/ZIF8 composite. The CO production rates on CdS and CdS/ZIF-8 composite are 26.33 μmol h−1 and 32.13 μmol h−1, respectively. In addition, the H2 evolution rates on CdS and CdS/ZIF-8 composite are 5.77 μmol h−1 and 7.09 μmol h−1, respectively. The CO selectivity of CdS/ZIF-8 is around 83.96%, higher than that of CdS. The control experiment showed that ZIF-8 exhibited no catalytic activity. When using N2 as the gas feedstock, only H2 was generated, confirming that the photogenerated CO comes from the reduced CO2. The photostability of CdS/ ZIF-8 composite is further checked by recovering the photocatalysts and repeating the photocatalytic reaction using a fresh solution for 75 min. As shown in Fig. 7d, no obvious decrease of CO evolution is observed after 4 cycles. The CO production of CdS/ZIF-8 composite for the fourth cycle remains at 90% of the initial reaction. The TEM image of the used CdS/ZIF-8 composite is shown in Fig. S8, indicating excellent structure stability of CdS/ZIF-8 NPs. The cycle stability experiment accompanied with the photocurrent measurement reflects the good photostability of CdS/ZIF-8 composite. Compared with previous papers in Table S1, our proposed CO2RR photocatalyst exhibits excellent CO2 reduction performance. To test the impact of the thickness on the photocatalytic performance, samples with various thickness were prepared. It was reported that the thickness of ZIF-8 shell can be controlled by adjusting the concentration of Zn2+, because Zn2+ can accelerate the growth rate of ZIF-8 nanocrystals [22]. Therefore, the CdS/ZIF-8 composites with different ZIF-8 shell thickness were prepared by altering the concentration of Zn2+. When the concentration of Zn2+ was fixed at 40 mM, the thickness of ZIF-8 shell of the as-obtained composite (CdS/ ZIF-8-40) is about 60 nm, much thicker than that of CdS/ZIF-8 (the Zn2+ concentration was 20 nm). Moreover, when the concentration of Zn2+ was increased to 80 mM, the composite (CdS/ZIF-8-80) possesses multiple-core-shell structures (see Fig. S9). Then the CO2 photo-reduction performance of the CdS/ZIF-8, CdS/ZIF-8-40 and CdS/ZIF-8-80 composites was investigated. As shown in Fig. 8, the CO evolution on CdS/ZIF-8-40 and CdS/ZIF-8-80 is 13 μmol and 9 μmol, respectively. By
was used as the light source. As shown in Fig. 6c & d, both CdS and CdS/ZIF-8 composite exhibit photocurrent responses under several onoff cycles of visible light illumination. Interestingly, we found that the initial photocurrent intensity of CdS and CdS/ZIF-8 composite are almost the same, but the photocurrent stability of CdS/ZIF-8 is much better than that of CdS. The photocurrent response of CdS is initially around 2 μA, and then decreases continuously under the irradiation of light, consistent with literature data [38–40], which attributes the photocurrent decrease of CdS to photocorrosion. By contrast, the photocurrent response of CdS/ZIF-8 appears stable at 2 μA, mainly because the core-shell structure of CdS/ZIF-8 can efficiently protect CdS from photocorrosion by stabilizing sulfide and cadmium ions inside the core. As a consequence, CdS/ZIF-8 possesses better photostability compared with CdS. The above characterization results indicate that the modification of ZIF-8 can certainly improve the CO2 adsorption capability and photocurrent stability of CdS without impairing its light harvesting ability, which tends to bring advantages to the photocatalytic reduction of CO2. The photocatalytic reaction was carried out in a component solvent system (acetonitrile/H2O). TEOA and Co(bpy)32+ were added as a sacrificial agent and an co-catalyst, respectively. The evolution amounts of CO and H2 are calculated according to the calibration curves (see Fig. S6 & 7). Fig. 7a shows the CO evolution over CdS and CdS/ZIF-8 composite under visible light illumination. At the beginning of the light irradiation, there is nearly no CO detected in both CdS and CdS/ZIF-8 photocatalytic system, and that period of CdS/ZIF-8 is even longer than that of CdS. Whether the catalytic active sites are on CdS or on ZIF-8, there is a porous shell between photogenerated electron and most CO2 molecules, which may inevitably increase the initial reaction time. When CO can be detected, the CO evolution of CdS and CdS/ZIF-8 both increase linearly with the irradiation time. Moreover, the accumulated CO production of CdS/ZIF-8 exceeds that of CdS at 100 min. The improved CO evolution rate of CdS/ZIF-8 can be attributed to its higher CO2 adsorption capability, which can increase the CO2 concentration around the catalysts. Different from the CO production, the H2 evolution of CdS/ZIF-8 is generally lower than that of CdS (see Fig. 7 b),
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Fig. 7. Time-yield plots of CO (a) and H2 (b) over CdS and CdS/ZIF-8 composite under visible light illumination; (c) gas evolution rate and CO/H2 ratio on CdS and CdS/ZIF-8 composite; (d) recycling tests of the photocatalytic CO evolution over CdS/ZIF-8 composite.
reduce the adsorbed CO2 to form CO, while the abundant holes on the VB of CdS are able to oxidize surface-adsorbed TEOA to yield TEOA+, as shown in Fig. 9. The core-shell structure of CdS/ZIF-8 can efficiently protect CdS from photocorrosion by stabilizing sulfide and cadmium ions inside the core.
4. Conclusions In summary, CdS/ZIF-8 composites with core-shell structures are prepared and applied in photocatalytic reduction of CO2. CdS/ZIF-8 shows higher efficiency and better selectivity in photocatalytic reduction of CO2 to CO compared with CdS, which is attributed to its higher CO2 adsorption capability. The modification of ZIF-8 shell can improve the CO2 adsorption capacity and photostability of CdS without impairing its light harvesting ability. The photostability of CdS/ZIF-8 is verified by photocurrent measurement and cycle stability experiment. There is no obvious decrease in the performance of photocatalytic CO2 reduction after 4 cycles for CdS/ZIF-8 composites. This work provides a way to construct core-shell semiconductor/MOFs composite with semiconductor as the core and MOFs as the shell, which can combine the advantages of both and improve the optical stability of semiconductors for many applications, such as catalysis, energy conversion and devices.
Fig. 8. CO evolution on CdS/ZIF-8 composite with different ZIF-8 shell thickness.
contrast, the CO evolution on CdS/ZIF-8 is around 16 μmol. The photoreduction activities of these samples decrease with the increase of ZIF-8 shell thickness, which could be attributed to the stronger barriers between CO2 and photogenerated electrons caused by thick ZIF-8 shells. Mott-Schottky (M-S) plots were performed to investigate the flat band potential as well as Fermi energy level of semiconductor. As shown in Fig. S10, the observed positive slopes on the M-S plots exhibit the typical n-type characteristics for CdS and CdS/ZIF-8. Based on the intersection points, the flat band potentials of CdS and CdS/ZIF-8 are nearly identical (approximately −0.7 V vs. Ag/AgCl), implying that the ZIF-8 shell has no effect on the semiconductor properties of CdS. In general, the flat band potential of n-type semiconductor (intercept value at the x-axis) can be used to estimate the conduction band (CB) of semiconductor. Therefore, the CB edge position of CdS can be estimated to be −0.78 V (vs. Ag/AgCl). Besides, the band baps determined from Tauc plot are 2.38 eV for CdS. Since the CB potential of CdS (−0.7 V vs. Ag/AgCl) is more negative compared with CO2/CO (−0.12 V vs. Ag/ AgCl), the photoinduced electrons on the CB potentials of CdS could
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21636010 and 21878342).
Declaration of competing interest The authors declare that there is no conflict of interest. 6
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Fig. 9. Schematic illustration of the photocatalytic CO2 reduction process over CdS/ZIF-8 composites under visible light irradiation.
Appendix A. Supplementary figures
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