Hollow cadmium sulfide tubes with novel morphologies for enhanced stability of the photocatalytic hydrogen evolution

Applied Surface Science 495 (2019) 143642

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Hollow cadmium sulfide tubes with novel morphologies for enhanced stability of the photocatalytic hydrogen evolution Guanqiong Li, Tong Xu , Ruifang He, Chunping Li, Jie Bai ⁎

T

⁎

Chemical Engineering College, Inner Mongolia University of Technology, Hohhot 010051, People's Republic of China Inner Mongolia Key Laboratory of Industrial Catalysis, Hohhot 010051, People's Republic of China

ARTICLE INFO

ABSTRACT

Keywords: Hollow CdS tubes H2 production Photocatalytic activities

Photocatalytic water splitting is a promising method to obtain clean hydrogen energy, and the corresponding catalyst has received extensive attention. Among them, cadmium sulfide (CdS) has a relatively narrow band gap (2.4 eV) and a high conduction band position. It can be considered to be a good visible light photocatalyst in response to visible light. However, its application is limited because the existence of rapid photo-electron-hole recombination and photo-corrosion. In this work, a series of hollow CdS tubes have been successfully prepared with the novel morphology via simple one-step solvo-thermal method. The as-prepared catalysts displayed great photocatalytic activity for photocatalytic split water product H2 under light (λ ≥ 350 nm). Due to the presence of the solvent carbon layer, the photo-corrosion of CdS was prevented, so it had good cycle performance. Meanwhile, the appropriate mole of cadmium sulfur dosage was explored and found that 3 mmol was the most suitable. The CdS-3 sample performed the optimal hydrogen (H2) evolution rate (312.6 μmol g−1 h−1) within 4 h. In addition, based on various characterization results of the photocatalyst, the novel structure and probable catalytic mechanism were further explored.

1. Introduction Environmental deterioration and energy shortage have become the two main problems of present society, which caused seriously effects on the survival of mankind. Researchers have taken many approaches to address these two challenges, including developing new technology and saving energy developing new technology and saving energy [1–5]. Photocatalysis was regarded as a green technology, which could transform solar energy into clean, pollution-free, sustainable chemical energy with great potential for development. Therefore, more and more researchers began to pay close attention on this technology because it had a wide range of applications, such as sewage treatment, photolysis of H2O, reduction of CO2 and air pollutant purification [6–13]. Since 1972, Japan's Fujishu and Honda found that titanium dioxide (TiO2) had a great photocatalytic properties under ultraviolet light. Owing to its good stability, non-toxic, low price and easy circulation, TiO2 had been widely used in photocatalysis research. However, TiO2 has its own disadvantages which limit its applications in photocatalysis. The band gap energy of pure TiO2 (Eg = 3.0–3.2 eV) is too high to respond the natural light, on the other hand, the recombination rate of photo-generated electrons and holes of the TiO2 photocatalyst is high, leading to a low quantum efficiency and the low photocatalytic activity of TiO2 ⁎

[14–16]. Hence, it is a critical needed to find a semiconductor photocatalyst which has a strong response to visible light. In recent years, many researchers begin to focus on metal sulfide photocatalysts. Among most the metal sulfide semiconductors, cadmium sulfide (CdS) has been studied due to its narrow band gap (Eg = 2.4 eV), strong visible-light response and high photocatalytic activity [17–23]. For instance, Yu [24] fabricated CdS photocatalysts with varied morphologies through the solvothermal method, including nanowires, nanoparticles, hollow spheres and urchin-like shapes, and improved their photocatalytic properties through changing the morphology. Meng [25] et al. first prepared a metal-organic framework (MIL-101) as a carrier and then used a double solvent method to immobilize CdS quantum dots (QD) in the pores of MIL-101 to increase the hydrogen production without any the noble metal cocatalyst. Shi [26] and his co-workers prepared CdS QDs, and then the chemical properties of CdS QDs were adjusted via Se doping. Because the introduction of Se element, the Fermi level of the CdS raised to a higher site, resulting in effectively suppressed recombination of electron and hole pairs which extremely improved the catalytic behaviors of CdS. Although CdS is a kind of visible light response catalysts which has a suitable band structure and band gap, the CdS powder exhibits some shortcomings, such as rapid photo-electron-hole recombination, easy

Corresponding authors at: Chemical Engineering College, Inner Mongolia University of Technology, Hohhot 010051, People's Republic of China. E-mail addresses: [email protected] (T. Xu), [email protected] (J. Bai).

https://doi.org/10.1016/j.apsusc.2019.143642 Received 11 May 2019; Received in revised form 29 July 2019; Accepted 10 August 2019 Available online 10 August 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. The XRD patterns of pure CdS powder and CdS-X samples with different contents of CdS.

agglomeration and difficult recovery, which seriously hindered its practical application on the large scale [27–29]. Meanwhile, CdS will undergo the photocorrosion under prolonged irradiation (Eq. (1-1)).

CdS + 2 h+

Cd2+ + S

The remaining reagents such as Cd(NO3)2·4H2O and thioacetamide (TAA) were bought separately from the Sinopharm Chemical Reagent Co., Ltd. and Shanghai Aladdin Bio-Chem Technology Co., Ltd. Chemical reagents mentioned above were all the analytical grade and had not been further treated.

(1-1)

Based on the above disadvantages of CdS powder, and combined with previous reports, the photocatalyst with a hollow sphere or tubular structure can achieve multiple reflections of incident light to enhance the photocatalytic activity [30]. Inspired by the above-mentioned results, in this work, novel CdS hollow tubes were successfully fabricated through electro-spinning technology and a simple one-step hydrothermal means. Compared to CdS powder, the obtained samples had larger specific surface area and more active sites and these increased the utilization of light and improved the photocatalytic activity of CdS. Furthermore, the hollow tube structure was divided into two layers inside and outside, and the carbon layer of the outer layer played a role of protection and cross-linking. Due to the presence of the carbon layer, the novel hollow tube had great chemical stability. The various characterization results demonstrated the crystal phase, morphology details, surface elemental compositions and the chemical state of CdS hollow tubes. The obtained samples were tested via hydrogen production experiment without noble metal as cocatalyst. And compared with the pure CdS powder, CdS hollow tubes displayed great photocatalytic behaviors and chemical stability. At the same time, a possible photocatalytic mechanism was presented.

2.2. Preparation of photocatalysts The PAN nanofibers were prepared according to the previous reports [31], where 1 g PAN was dispersed in 9 g DMF under quickly stirred for 24 h at 40 °C to form a uniform solution. Then, the PAN nanofibers were synthesized through electrostatic spinning technology under the voltage of 16 kV with a feed speed of 2.0 mL/h. The distance between the needle and the aluminum plate was fixed at 18 cm. Hollow tubes loaded with different CdS contents were prepared by a simple one-step solvothermal method. Typically, 1 mmol Cd (NO3)2·4H2O and 1 mmol TAA were added to 30 mL of ethanol solution in order, wherein the volume ratio of ethanol to water was 1:2. Then, the obtained solution was rapidly stirred for the purpose of forming a uniform solution. Thereafter, 0.05 g of PAN nanofibers was added to the solution under a slow stirring for 30 min. The final mixtures were transferred into a 40 mL Teflon lined stainless autoclave and held temperature at 180 °C for 5 h. After cooling to room temperature, the obtained samples were cleaned for several times with ethanol and deionized water and dried at 65 °C overnight in a vacuum oven. The obtained samples were marked as CdS-X. where “X” was 1, 2, 3, 4, 5 mmol, which represented the Cd(NO3)2·4H2O and TAA dosages during the hydrothermal process. In this work, pure CdS powder was synthesized through the same method without PAN.

2. Experimental section 2.1. Materials Polyacrylonitrile (PAN, Mw = 80,000) was purchased from Kunshan Hongyu Plastics Co., Ltd. N,N‑Dimethylformamide (DMF, AR) was derived from the Tianjin Guangfu Technology Development Co.

2.3. Characterization The morphologies and structures of samples were performed via a

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Fig. 2. The FE-SEM of (a) PAN film, (b) CdS-3, (c) CdS powder and (d) CdS-3 sample after five cycle.

FE-SEM (Phenom LE, Phenom-world B. V. Netherlands), and product details were observed by Transmission electron microscopy (TEM, Tecnai F20 S-TWIN microscope). The phase compositions of samples were certified through X-ray diffraction (XRD, Rigaku Ultima IV, Japan) in which samples were noted by a Rigaku Ultima IV X-ray diffractometer with Cu Kα radiation. The element content was recorded through the inductively coupled plasma-optical emission spectroscopy (ICP-OES 730, Agilent, USA). The surface elemental compositions and the chemical state of samples was characterized by an X-ray photoelectron spectroscopy (XPS, Escalab 250xi, ThermoFisher Scientific USA).

under vacuum status. Then, the solution was irradiated by a 300 W Xenon lamp (PLS-SXE 300, Beijing Perfectlight Technology Co., Ltd). The amount of generated hydrogen gas was measured by an online gas chromatograph (GC7920-7F2A, CEAULIGHT, Beijing, TCD, N2 as carrier). After the reaction was end, the sample was easily separated from the reaction system, because the sample was membranous structure on the macro level. Then, the obtained product was washed three times with deionized water and dried at 65 °C overnight for the next cyclic experiment.

2.4. Photocatalytic experiment

3.1. Characterization of CdS-X composite samples

The photocatalytic H2 production reaction was proceeded in a 100 mL front-irradiated heat-resistant quartz reactor (CEL-PAEM-D6, CEAULIGHT, Beijing). The 5 mg of catalysts were put in a 50 mL uniform solution which contained 10% wt. lactic acid as the sacrificial agent. Before the test started, the reaction system was degassed with a pump for 40 min to get rid of air and make sure the reaction system

To confirm the existence and crystalline nature of CdS, the XRD profiles of PAN and CdS-X (X = 1–5) samples were obtained [32]. As could be seen from Fig. S1, a weak diffraction peak was found at 2θ ≈ 17.5° from PAN curve, which corresponding to the (100) diffraction of the PAN. And the wide diffraction peaks at 25.0° agreed with amorphous carbon [33]. As showed in Fig. 1, CdS-X exhibited four main

3. Results and discussion

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Fig. 3. (a) TEM and (b–c) the high-resolution TEM image for CdS-3; mapping images of (d) C, (e) N, (f) O, (g)Cd, and (h) S.

peaks at 2θ ≈ 26.51°, 30.81°, 43.96° and 52.13°, which corresponded to the crystal planes (111), (200), (220) and (311) of the pure CdS powder (JCPDS No. 10-0454) [34]. After the hydrothermal reaction, only CdS-1 could be found a weak diffraction peak at 2θ ≈ 17.5° because there was the residual of PAN. As the contents of CdS continuously increased, the diffraction peaks of CdS became more and more sharp, and the peak of PAN disappeared, indicating that the crystallinity of CdS was increased and the dissolution of PAN. Interestingly, XRD results showed that CdS3 was unique and its peak intensity was higher than CdS-4 and CdS-5. Therefore, the average particle size of CdS-X sample was calculated by Scherrer formula (D = (Κ × λ) / (β × cosθ)), and found that the particle size of CdS-3 sample was smallest [34]. For the CdS-1 and CdS-2 samples, their crystallinity was low. But for the CdS-4 and CdS-5 samples, their sizes were relatively large. However the CdS-3 sample not only had high crystallinity but also small particle size. So this fact explained why the peak intensity of CdS-3 sample was higher than that of other samples. In addition, ICP-OES test was conducted of the CdS-3 composite catalyst and the result showed that the Cd element content was 61.6 wt%. To investigate the morphologic details of the hollow tubes, PAN electrospinning fibers, CdS-X (X = 1–5) samples and the pure CdS powder were observed by a FE-SEM [35]. As could be seen from Fig. 2a, PAN fibers had the smooth surface and their mean diameter was approximate 300 nm. During the hydrothermal process, CdS was gradually produced, and PAN was gradually dissolved to form the hollow tubes. Fig. S2(a–c) showed as the loading of CdS was improved, the hollow tubes still keep integral structures. When the loading of CdS increased continuously, excessive CdS would agglomerate into big spherical particles and corrode the hollow tubes into small segments in Fig. S2(d–e). Typically, the CdS-3 hollow tubular structure that the inner diameter size was similar to the mean diameter of PAN fibers (Fig. 2b), which revealed the formation of tubular structure was due to

the dissolution of PAN fibers during the hydrothermal process. Moreover, compared to the CdS powder (Fig. 2c), CdS nanoparticles loaded on CdS-3 had smaller size, which proved that the presence of PAN could effectively prevent the aggregation of CdS nanoparticles. From Fig. 2d, it can be observed that after five cycles of hydrogen production experiment of CdS-3 sample, CdS particles appeared to agglomeration but the hollow structure did not collapse. To observe the further morphology details of the CdS-3 sample, TEM and HRTEM images were obtained as shown in Fig. 3. It could be observed from Fig. 3a and b that a hollow nanotube with double layered structure was encapsulated by a thin carbon film of which the thickness was approximately 2–5 nm. Since TAA could be hydrolyzed into S2− and CH3CONH2 (alkaline substance) at high temperature, S2− reacted with Cd2+ and generated the CdS loadings on the surface of PAN fibers. PAN was a polymer matrix with high carbon content, and will be hydrolyze to amino group under alkaline conditions and formed a hollow structure. During the natural cooling process, the carbon in the solution slowly deposited outside the tube wall to form a thin carbon layer [36]. The lattice spacing of nanoparticles on the surfaces was 0.35 nm, which corresponding to the typical (100) plane of hawleyite CdS (Fig. 3c), indicating the CdS crystal loading particles on the surface of tubes. C, N, O, Cd and S elements distributed on the surfaces of tubes were detected as the elemental mapping images showed in Fig. 3d–h, these five elements were uniformly distributed on the tube surfaces, and the distribution of C element explain the CdS hollow tubes were covered by a solvent carbon layer. Compared to CdS powder, the CdS loaded on hollow tubes dispersed uniformly, sizes were small and more active sites were exposed. The carbon layer outside of the tube wall played an important role which prevented photo-corrosion of CdS and protected active substances. Furthermore, light could be reflected multiple times on the inner wall of the tubes, improving the utilization of light and catalytic performance.

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Therefore, the relationship of the struction and its activity had synergistic effects. It was well known that the activity of photocatalysts was related to its optical properties. So, the light adsorption of CdS-X samples, CdS powder and PAN were analyzed by an UV–vis spectra. As shown in Fig. 4a, pure CdS powders exhibited super absorption intensity in the visible light region from 400 nm to 700 nm and a broad absorption property which its absorption edge near 610 nm [36,37]. For CdS-X samples, as the loading of CdS was improved, the absorption strength and optical response range were enhanced in a way. But the CdS-X samples displayed a slight blue shift under the same conditions, due to the existence of a small amount of PAN. Nevertheless the UV–vis spectrum of CdS-5 and CdS powder had a similar photoabsorption range, indicating that PAN was dissolved completely. Based on the previous reports, the band gap of semiconductors could be calculated by the following equation:

h =

(h

Eg)1/2

(3-1)

where α represents absorption coefficient, h was Planck's constant, ν was light frequency, Eg was band gap and A was constant [38]. Therefore, as shown in Fig. 4b and c, the band gap (Eg) of the CdS-3 sample and pure CdS powders were 2.18 eV and 2.08 eV, respectively. Meanwhile Eg of other samples were 2.18, 2.19, 2.16 and 2.08 eV for CdS-1, CdS-2, CdS-4, and CdS-5, respectively. Although the Eg of CdS-3 sample was not the smallest, it was still lower than the conventional CdS powder (Eg = 2.4 eV). This showed that the photocatalytic performance of the hollow tubular structure was still improved. The surface elemental compositions and the chemical state of the CdS-3 sample were further verified by an XPS measurements. It could be observed in Fig. 5a that the CdS-3 sample was composed by the elements Cd, S, C, N and a bit of O and further certified that there were no else elements in the photocatalyst. While the presence of a small amount of O might be attributed to the oxygen gas (O2) on the sample surface. The Cd 3d spectrum had two peaks which located at 411.6 eV and 404.7 eV (Fig. 5b) belonging to the binding energy of Cd2+ [39]. The S 2p of the CdS-3 sample could be divided into two peaks, as displayed in Fig. 5c. The contribution at 162.4 eV and 161.2 eV were ascribed to the S2− 2p of CdS [40]. For C 1s peak could be divided into three peaks at 285.3 eV, 286.4 eV and 288.2 eV (Fig. 5d). The former peak was assigned to carbon atoms because the amorphous carbons were adsorbed on the sample surface. The binding energies at 286.4 eV and 288.2 eV were assigned to CeO and C]O bond, respectively [18,41,42]. The main N 1s peak at 399.4 eV (Fig. 5e) was identified as the CeN]C (sp2 bond N), which kept with previous literature [43]. A high binding energy at 404.8 eV with a strong peak was corresponded to π excitation [44]. The final XPS results further proved that CdS was successfully fabricated. Fig. 6 showed the PL emission spectra of the CdS-X samples and pure CdS powder with a fixed excitation wavelength of 380 nm. It was well known that there was a close link between the PL and recombination of photo-generated e−-h+ pairs of photocatalysts [45]. Generally speaking, the low PL intensity indicated that the photo-generated electrons separation and transport capacity of catalyst was strong, corresponding to better photocatalytic performance. It could be found that the pure CdS powders displayed the highest PL intensity emission peak centering at 518 nm. Compared with the other samples, the PL spectra of CdS-3 sample were significantly lowest demonstrating it had the best photocatalytic activity in theory. The results also proved that the excessive loadings of CdS nanoparticles were not conducive to the separation of photogenerated electrons and holes. This phenomenon matched the photocatalytic performance of the samples.

Fig. 4. (a) UV–vis absorption spectra of pure CdS and CdS-X (X = 1–5) samples; (b,c) calculated band gaps of CdS-3 and pure CdS powder.

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Fig. 5. The XPS spectra of CdS-3 sample. (a) the full XPS survey spectrum; (b) Cd 3d; (c) S 2p; (d) C 1s and (e) N 1s.

In order to confirm the superiority of CdS-X photocatalyst, series of hydrogen production tests were proceeded under the light irradiation (λ ≥ 350 nm) using the solution containing 10 vol% lactic acid solution as a sacrificial agent at room temperature (Fig. 7a). It should be noted

that no precious metals were added as cocatalysts. The CdS-X (X = 1,2,3,4,5) corresponding rates of H2 production were 53.9, 158.3, 312.6, 98.7 and 54.9 μmol g−1 h−1, respectively. Compared to the other samples, the CdS-3 sample exhibited the highest H2 evolution rate

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Fig. 6. The PL emission spectra of CdS-X (X = 1–5).

within 4 h. And based on the result of the ICP-OES test, the TON and TOF of CdS-3 were calculated as 0.61 and 0.2, respectively. While the CdS contents further increased, the hydrogen production properties declined significantly. This was probably because excessive CdS would agglomerate into big spherical particles and corrode the PAN fibers into small segments, which was consistent with the FE-SEM results. The hydrogen production results indicated that the content of CdS had a great influence on the photocatalytic activity. Additionally, several related literatures were compared in the Table. 1. From the results of hydrogen production data, it could be seen that changing the morphology of CdS was more conducive to the improvement of its performance. As is well-known, chemical stability is a key evaluation factor for photocatalysts. In order to evaluate the chemical stability of samples, a series of repeated tests were conducted. The cyclic results of CdS-3 sample were displayed in Fig. 7b. After five continuous cycles experiments (20 h), the H2 production effect was relatively stable, which proved the great stability of the CdS-3 sample. It was attributed to the special structure of the photocatalyst. In other words, solvent carbon wrapping around the outer layer of the CdS hollow tubes played a protective role to prevent photo-corrosion of CdS. The H2 generation rate of pure CdS powder was 195.3 μmol g−1 h−1, unfortunately, in the second cycle test, the H2 evolution rate was only 115.9 μmol g−1 h−1, which was 40% lower than the first time. The cycle stability of CdS powder was poor due to the existence of its own photocorrosion. Moreover, the XRD results of the CdS-3 samples before and after the reaction were shown in Fig. 8. It could be seen that their peak positions did not change significantly and the peak intensity decreased slightly, demonstrating the catalyst stability of the CdS-3 sample, and verified its prospects for future development (see Scheme 1).

3.2. Photocatalytic mechanism In view of the above characterization results, a possible mechanism for photocatalytic hydrogen evolution by CdS hollow tubes was presented showed in Scheme 2. According to the UV–vis data, the Eg of CdS was 2.18 eV. Meanwhile, based on the XPS result, the EVB of CdS was 1.57 eV (Fig. S3), ECB could be calculated by the following equation:

Eg = EVB –ECB

(3-2)

where the ECB of CdS was −0.61 eV. Under light (λ ≥ 350 nm) excitation, the photo-exited electron-hole pairs (e−-h+) of the CdS hollow tubes were formed. Subsequently, the photo-exited e− on CB of the CdS would be transferred to the VB. And the carbon layer acted as a good conductor playing a role of electron acceptor and transporter, which could achieve the separation of e−-h+ and then decompose the H+ in aqueous solution into H2. The h+ on the VB of CdS was oxidized powerfully the lactic acid into H+ and other small compounds. And the carbon layer acted as a conductor not only prevented photo-corrosion of CdS but also accelerated charge transport. 4. Conclusions In summary, a novel structure of CdS hollow tubes was fabricated via electrospinning technology and simple one-step solvothermal technology. Furthermore, when the CdS-3 hollow tubes were applied to photolysis water into H2, it showed the highest H2 evolution rate of 312.6 μmol g−1 h−1 within 4 h. Photocorrosion of CdS was prevented due to the presence of solvent carbon layer so it had great cycling property when it was used for cycling tests. Based on a series of characterization results of catalysts, a possible photocatalytic mechanism was presented. Therefore, the hollow CdS tubes had broad application prospects and practical application value.

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Fig. 7. (a) Comparison of photocatalytic H2 production activities of different samples and (b) cycling runs five times for the H2 production over CdS-3 sample.

Table 1 Comparison of hydrogen evolution rate of CdS with different modification methods. Photocatalyst

Co-catalyst

Sacrificial reagent

Activity μmol g−1 h−1

Reference

CdS CdS CdS CdS CdS

Carbon layer Carbon dots C60–1 wt% Pt SiO2 Co3O4

312.6 51 1730 157 236.3

This work Zhu et al. [46] Cai et al. [47] [48] Zhao et al. [49]

CdS nanorods

Co(OH)2

10 vol% lactic acid – 10 vol% lactic acid 50 vol% ethanol 0.5 M Na2S 0.5 M Na2SO3 25 vol% ethanol

61

Zhang et al. [50]

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Fig. 8. XRD patterns before and after cyclic reaction of CdS-3 sample.

Scheme 1. The main experimental procedures of CdS hollow tubes samples.

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Scheme 2. Possible photocatalytic mechanism of the CdS-3 sample under light irradiation (λ ≥ 350 nm).

Acknowledgements

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