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Suspensible cubic-phase CdS nanocrystal photocatalyst: Facile synthesis and highly efficient H2-evolution performance in a sulfur-rich system Huogen Yu, Wei Zhong, Xiao Huang, Ping Wang, and Jiaguo Yu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00398 • Publication Date (Web): 15 Mar 2018 Downloaded from http://pubs.acs.org on March 16, 2018

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ACS Sustainable Chemistry & Engineering

Suspensible cubic-phase CdS nanocrystal photocatalyst: Facile synthesis and highly efficient H2-evolution performance in a sulfur-rich system

Huogen Yua,b*, Wei Zhongb, Xiao Huangb, Ping Wangb, Jiaguo Yuc

a

State Key Laboratory of Silicate Materials for Architectures, Wuhan University of

Technology, 122 Luoshi Road, Wuhan, Hubei 430070, People’s Republic of China b

Department of Chemistry, School of Chemistry, Chemical Engineering and Life

Sciences, Wuhan University of Technology, 122 Luoshi Road, Wuhan, Hubei 430070, People’s Republic of China c

State Key Laboratory of Advanced Technology for Material Synthesis and

Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan, Hubei 430070, People’s Republic of China

*

E-mail:

[email protected]

(H.Yu);

Tel:

0086-27-87879468

1 ACS Paragon Plus Environment

0086-27-87756662;

Fax:

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ABSTRACT Compared with stable-phase hexagonal CdS, the metastable cubic CdS photocatalyst usually shows a lower H2-evolution performance under visible-light irradiation. Thus, the widely reported high-performance CdS photocatalysts are mainly focused on the hexagonal phase, while the cubic-phase CdS with a high H2-evolution activity has seldom been concerned. In this study, a direct precipitation method in a sulfur-rich Na2S-Na2SO3 system has been developed to prepare the suspensible cubic-phase CdS nanocrystal (c-CdS-NC) photocatalyst with a high H2-evolution activity. In this case, the resultant c-CdS-NC with a small crystal size (ca. 5 nm) and high specific surface area (>75.23 m2/g) exhibits a stable and suspensible photocatalysts due to the massive and preferential adsorption of S2-/SO32ions on the nanocrystal surface. Photocatalytic results indicated that the suspensible c-CdS-NC photocatalysts clearly exhibited an obviously higher H2-evolution performance (0.36 mmol h-1) than the traditional hexagonal CdS (0.14 mmol h-1) by a factor of 2.6 times. Based on the present results, a S2-/SO32--mediated mechanism was proposed for the enhanced H2-evolution performance of the suspensible c-CdS-NC, namely, the massive adsorbed S2- ions on the suspensible c-CdS-NC surface not only promote the rapid capture of photogenerated holes, but also can work as the effective active sites for H2-evolution reaction. The present work may provide important insights for developing high-performance photocatalytic materials. Keywords: photocatalysis; H2 evolution; cubic phase; CdS nanocrystal; preferential adsorption 2 ACS Paragon Plus Environment

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INTRODUCTION Renewable hydrogen has been considered to be a clean energy because it is possible to solve the pollution problems and CO2 emissions caused by the increasing consumption of fossil fuels.1-5 Photocatalytic technology is one of the promising routes toward the renewable clean hydrogen by photocatalytic water splitting under solar light irradiation.6-13 As a consequence, considerable research attentions have been focused on the development of advanced photocatalysts for hydrogen production.14-18 Among the numerous photocatalysts, CdS is one of the most fascinating photocatalytic materials for hydrogen evolution, owing to its relatively narrow bandgap (2.4 eV) for visible-light absorption and sufficiently negative conduction band edge (-0.51 V, vs SHE) for protons reduction.19 Unfortunately, there are several issues that still limit the H2-evolution efficiency of pure CdS photocatalyst, including the rapid recombination of photoinduced electron-hole pairs and the serious photocorrosion during photocatalytic reactions.20-23 To solve these problems, numerous efforts have been developed, such as semiconductor coupling,24-26 the deposition of cocatalyst,27-30 and quantum-sized CdS31, 32. However, the present CdS photocatalytic materials still exhibit a low photocatalytic H2-evolution performance which seriously restricts the wide practical applications. Hence, it is very necessary to further develop a facile and effective route for the preparation of highly efficient CdS photocatalytic materials. Generally, the well-known CdS photocatalyst exhibits two crystalline phases, namely hexagonal (or wurtzite) and cubic (or zinc-blende) phases.33, 34 It has been 3 ACS Paragon Plus Environment


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reported that the hexagonal CdS is a stable phase while the cubic CdS is a metastable phase.35, 36 Compared with the metastable cubic-phase CdS, many researchers have demonstrated that the hexagonal-phase CdS photocatalysts usually shows a higher photocatalytic H2-evolution performance.37-39 As a consequence, the widely reported CdS photocatalysts are mainly focused on the hexagonal phase, while the cubic-phase CdS has seldom been concerned.40, 41 In fact, compared with the hexagonal-phase CdS with a bandgap of ca. 2.42 eV,42 the cubic CdS usually exhibits a smaller bandgap (ca. 2.38 eV),35, 43 suggesting that cubic CdS can absorb more visible light with a longer wavelength. In addition, the complete transformation temperature from cubic to hexagonal phase of CdS usually should be higher than 500 oC and the internal energy difference between the two phases is very small (1.1 meV/atom), suggesting the considerable stability of cubic-phase CdS as the photocatalyst, which is similar to the well-known metastable anatase TiO2 compared with stable rutile phase.42 As a result, it is quite expected that the cubic-phase CdS can also work as a highly efficient visible-light photocatalyst for photocatalytic H2 production. In fact, in recent years, some cubic-phase CdS nanostructures such as graphene-modified CdS,14 Ni/CdS nanosheets,44 and CdS nanocubes36 have been demonstrated to be an effective visible-light

photocatalysts.

However,

compared

with

the

well-known

hexagonal-phase CdS, the reported number and related investigations about cubic-phase CdS photocatalysts are still quite insufficient due to the absence of effective methods to remarkably improve their H2-evolution performance. Moreover, the potential physicochemical reasons for the completely different photocatalytic 4 ACS Paragon Plus Environment

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performance between cubic and hexagonal CdS still remain unclear. Therefore, it is highly required and extremely important to further investigate the physicochemical properties and photocatalytic mechanism of cubic-phase CdS photocatalyst. For the hexagonal and cubic CdS photocatalysts, one of the obvious drawbacks is its photocorrosion phenomenon during photocatalytic reactions.45, 46 To prevent the serious photocorrosion of CdS, a sulfur-rich Na2S-Na2SO3 mixing solution (S2-/SO32-) has been widely used as an excellent sacrificial reagent to improve its photoinduced stability and H2-evolution activity by quickly capturing photogenerated holes on CdS surface.35 However, the real actions of S2-/SO32- reagent for the improved photocatalytic performance of CdS still have not been well revealed. In this study, in addition to the well-known sacrificial reagents by preventing the photocorrosion of CdS during photocatalytic reactions, the sulfur-rich S2-/SO32- system not only can act as a S2- precursor for the rapid synthesis of cubic-phase CdS nanocrystals, but also work as the excellent stabilizer for suspensible CdS nanocrystals. Herein, the suspensible cubic-phase CdS nanocrystal (c-CdS-NC) photocatalyst can be easily prepared by the direct addition of limited Cd2+ ions into a sulfur-rich S2-/SO32- system. It is interesting to find that massive S2-/SO32- groups can been preferentially adsorbed on the c-CdS-NC surface, resulting in the formation of a homogeneous and suspensible CdS photocatalyst. Compared with the well-known hexagonal CdS, the suspensible c-CdS-NC clearly exhibits a higher photocatalytic H2-evolution activity. A S2-/SO32--mediated mechanism is proposed for the improved H2-evolution activity of the suspensible c-CdS-NC photocatalyst. To the best of our knowledge, this is the first report about the in situ preparation of suspensible cubic-phase CdS photocatalyst with a high H2-evolution activity in the sulfur-rich S2-/SO32- system. This work not only demonstrated the importance of reaction medium for CdS photocatalyst, but also 5 ACS Paragon Plus Environment


highlights the potential applications of cubic-phase CdS materials in the energy transformation. EXPERIMENTAL SECTION Preparation of Suspensible Cubic-Phase CdS Nanocrystal Suspensible cubic-phase CdS nanocrystal (c-CdS-NC) was prepared by a facile precipitation method in a sulfur-rich Na2S-Na2SO3 system (the Na2S-Na2SO3 mixing solution is usually used as an excellent sacrificial reagent for photocatalytic H2 evolution of CdS). Typically, 11.5 mL of 0.03 mol/L Cd(NO3)2 solution was rapidly added into 80 mL of mixing solution with 0.35 mol/L Na2S and 0.25 mol/L Na2SO3, and then stirred for 10 min. In this case, the ratio of S2- to Cd2+ was controlled to be ca. 81:1. Therefore, after a complete consumption of Cd2+ ions for the CdS precipitation, there are still a lot of S2- ions in the reaction system, resulting in the formation of suspensible CdS nanocrystal photocatalyst. To characterize the morphology and microstructures of the suspensible CdS nanocrystals, the above suspension was filtered and washed with distilled water several times and then dried at 60 oC for 12 h, and the resultant sample was referred to be c-CdS-NC. For the photocatalytic H2-evolution experiment of c-CdS-NC, the above suspension solution of CdS nanocrystals (before filtration, washing and drying) was directly used for the following hydrogen-evolution experiment, and the following details are shown in section of Photocatalytic H2-Evolution Activity. To further improve the crystallization of CdS nanocrystals, the above suspension solution was directly transferred into a 150 mL Teflon-lined autoclave and then maintained at 180 oC for 12 h. After hydrothermal treatment, the resulting sample was filtered and washed with distilled water several times and then dried at 60 oC for 12 h. In this case, the resultant sample was referred to be c-CdS-180oC. 6 ACS Paragon Plus Environment

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Preparation of Traditional Hexagonal CdS Photocatalysts Traditional hexagonal CdS (h-CdS) photocatalysts were synthesized by a well-known precipitation-calcination method.19 Briefly, 100 mL of Cd(NO3)2 solution (0.1 mol/L) was added into 100 mL of Na2S solution (0.1 mol/L) under a stirring condition. After stirring for 2 h and aging for another 12 h, the product was filtered and washed with deionized water and absolute ethanol several times, and then dried at 60 °C for 12 h. In this case, the resultant sample before calcination was cubic phase and can be referred to be cubic-phase CdS aggregates (c-CdS-AG). To obtain the traditional hexagonal CdS photocatalyst (h-CdS), the above c-CdS-AG nanocrystals were calcined at 550 ºC for 4 h in a tubular furnace under nitrogen blanketing. Characterization To obtain the morphological observations, JEM-2100F transmission electron microscope (TEM, JEOL, Japan) and JSM-7500 field emission scanning electron microscope (FESEM, JEOL, Japan) were applied. X-ray diffraction (XRD) patterns were conducted on a Rigaku Ultima III X-ray Diffractometer (Japan) using Cu Kα radiation. UV−vis absorption spectra (UV-2450, Shimadzu, Japan) were acquired and BaSO4 was used as a reflectance standard. Raman and Fourier transform infrared (FTIR) spectra were obtained on an INVIA spectrophotometer (Renishaw, U.K.) and a Nexus FT-IR spectrophotometer (Thermo Nicolet, America), respectively. A nitrogen adsorption apparatus (Micromeritics, ASAP2020, USA) was used to obtain the nitrogen adsorption-desorption isotherms. X-ray photoelectron spectroscopy (XPS, a KRATOA XSAM800 XPS system) was performed and their binding energy values were referenced to the C 1s peak (284.8 eV) from the surface adventitious carbon. Photocatalytic H2-Evolution Activity



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The photocatalytic activity for H2 evolution was carried out in a 100-mL three-necked Pyrex flask, as reported in our previous studies.47,

48

In a typical

photocatalytic H2-production experiment (in addition to the c-CdS-NC nanocrystal sample), 50 mg of the photocatalyst was dispersed into 80 mL of Na2S-Na2SO3 aqueous solution, where the Na2S (0.35 M) and Na2SO3 (0.25 M) was used as sacrificial reagents. The above suspension was stirred for 10 min and then purged with nitrogen for 15 min to remove the dissolved air. Four 3-W and 420-nm LEDs lights (Shenzhen Lamplic Science Co. Ltd.) were used as the irradiation light source. During light irradiation, the above suspension was continuously stirred to keep the photocatalyst particles in suspension state. The produced hydrogen was measured by a Shimadzu GC-2014C gas chromatograph with nitrogen as a carrier gas. Photoelectrochemical Measurements The

photoelectrochemical

performance

were

conducted

on

a

CHI660E

electrochemical work station (Chenhua Instrument, Shanghai, China) in a standard three-electrode system by using Na2S(0.35 M)/Na2SO3(0.25 M) or lactic acid solution (10 vol %) as the electrolyte solution, where the photocatalyst-coated FTO, Ag/AgCl and platinum wire work as the working electrode, reference electrode and counter electrode, respectively. The light source was provided by a 3-W and 420-nm LED (Shenzhen Lamplic Science Co. Ltd.). The CdS working electrodes were prepared on a fluorine-doped tin oxide (FTO) substrate (a 1.0-cm2 active area), where the sides of FTO glass are protected by Scotch tape. Typically, 10 mg of CdS sample was dispersed into a Nafion-ethanol 8 ACS Paragon Plus Environment

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solution with 1-mL D-520 Nafion (5%, Alfa Aesar) and 1-mL anhydrous ethanol, and subsequently ultrasonicated for 30 min to obtain a suspension solution. The resulting CdS suspension was coated on the FTO surface and then dried at 40 oC for 24 h. The linear sweep voltammogram (LSV) was measured at applied bias from -1.2 to -1.7 V (in Na2S/Na2SO3) or -0.6 to -1.0 V (in lactic acid), the transient photocurrent responses (i–t curve) were measured at the open circuit voltage during repeated ON/OFF illumination cycles and EIS was determined over the frequency range of 0.001–106 Hz with an ac amplitude of 10 mV at the open circuit voltage. RESULTS AND DISCUSSION Strategy for the Facile Synthesis of Suspensible Cubic-Phase CdS Nanocrystals

Figure 1. Schematic drawing illustrating the (A) facile synthesis of suspensible cubic-phase CdS nanocrystals in a Na2S-Na2SO3 system: step (1) is the formation of cubic CdS nanocrystals; step (2) is the preferential adsorption of massive S2-/SO32- on the nanocrystal surface. (B) Traditional precipitation-calcination preparation of hexagonal CdS: step (3) is the formation of cubic CdS nanocrystal aggregates; step (4) is the phase transformation to form traditional h-CdS at 550 oC. 9 ACS Paragon Plus Environment


Figure 1 shows the schematic drawing for the synthesis of suspensible cubic-phase CdS nanocrystals (c-CdS-NC). Compared with the traditional synthesis of hexagonal-phase CdS (h-CdS) photocatalysts, the most obvious difference is that the suspensible c-CdS-NC is synthesized in a sulfur-rich Na2S-Na2SO3 system at room temperature. Herein (Figure 1A), when the limited Cd2+ ions (the atom ratio of Cd2+ to S2- is 1:81) are dropped into the sulfur-rich S2-/SO32- system, the CdS nanocrystals are formed immediately due to its very small solubility product constant (Ksp(CdS) = 3.6 *10-29) in aqueous solution.49 Owing to the rapid and complete consumption of Cd2+ ions, the produced CdS nanocrystals can be closely surrounded by the excessive S2-/SO32- ions. According to the Fajans rule for ionic adsorption (namely, the ions that form the compound will be adsorbed preferentially when two or more types of ions are available for adsorption),50-52 the excessive S2- ions can be easily and preferentially adsorbed on the c-CdS-NC surface (Figure 1A), resulting in the formation of cottony and suspensible CdS photocatalysts (shown in the photograph). According to its XRD and TEM results (Figs. 2A-C), the suspensible nanocrystals can be attributed to cubic-phase CdS (JCPDS Card No. 90-0440) with a nanocrystal size of ca. 5 nm. However, after separated from the sulfur-rich S2-/SO32- solution and washed with distilled water three times, the suspensible c-CdS-NC is easily aggregated. In this case, when those aggregated particles are further dispersed into the identical S2-/SO32- solution, no CdS nanocrystal suspension (only powdered precipitation) can be found (Figure S1A) due to the removal of adsorbed S2-/SO32ions from c-CdS-NC surface. On the basis of FESEM image (Figure S1B), those 10 ACS Paragon Plus Environment

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A Intensity (a.u.)

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


Cubic CdS JCPDS 90-0440

20

40 60 2 Theta (degree)

80

Figure 2. (A) XRD pattern and (B, C) typical TEM images of suspensible c-CdS-NC; (D, E) TEM images of traditional h-CdS.



aggregated particles show a wide particle size of 20-50 nm. According to its N2 adsorption-desorption isotherm, the specific surface area of resultant CdS nanocrystal aggregates is 75.23 m2/g (Figure S1C). In fact, for the suspensible c-CdS-NC in the sulfur-rich S2-/SO32- system, their specific surface area should be clearly higher than the present 75.23 m2/g due to their excellent dispersion and suspensible stability. Therefore, the above results strongly suggest that the preferential adsorption of excessive S2- ions on the cubic-phase CdS nanocrystal is highly desired for the formation of suspensible photocatalysts in the sulfur-rich S2-/SO32- solution. To further investigate its stability in the sulfur-rich S2-/SO32- solution, the above suspensible c-CdS-NC in the sulfur-rich S2-/SO32- solution were in situ hydrothermally treated at 180 oC for 12 h, and the resultant sample was referred to be c-CdS-180oC. It is found that after hydrothermal process, the suspensible feature of CdS nanocrystals has completely disappeared and only powdered precipitation is formed on the bottom of solution (Figure S2A), which can be attributed to the easy aggregation and rapid growth (30-100 nm, SBET = 23.62 m2/g) of the cubic CdS nanocrystals (Figure S2). The corresponding XRD result (Figure S2D) shows a stronger diffraction peaks and narrower full width at half maximum than that of c-CdS-NC, revealing the improved crystallinity and crystal growth of cubic-phase CdS nanocrystals. For comparison, the traditional hexagonal CdS photocatalysts (h-CdS) were also synthesized by a well-known precipitation-calcination method (Figure 1B). After the addition of Cd2+ into S2- solution (an atom ratio of Cd2+:S2- = 1:1), the cubic-phase 12 ACS Paragon Plus Environment

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CdS nanocrystals are also produced immediately. However, owing to the simultaneous and complete consumption of both Cd2+ and S2- ions, the resultant cubic-phase CdS sample (Figure S3C) can easily form larger aggregated particles (20-60 nm, in Figure S3A), resulting in a decreased specific surface area (62.47 m2/g, Figure S3C). In this case, the resultant cubic-phase CdS aggregate was referred to be c-CdS-AG. When the c-CdS-AG sample was filtered and then re-dispersed into the sulfur-rich S2-/SO32- system, only CdS powdered precipitate can be observed, similar to that of Figure S1A. As a consequence, after high-temperature calcination at 550 ºC for 4 h, these aggregated c-CdS-AG can transform into the well-known h-CdS photocatalysts (JCPDS Card No. 77-2306) (Figure S3D). According to the TEM, FESEM, and BET results (Figure 2 and Figure S3), the resulting h-CdS photocatalysts show a large particle size (80-200 nm) and a small specific surface area (7.6 m2/g), resulting in the rapid precipitation when dispersing them into the same S2-/SO32system, as shown in Figure 1B. It is very interesting to investigate the possible mechanism for the formation of suspensible c-CdS-NC. In general, very small nanocrystals (75.23 m2/g). It was found that the suspensible c-CdS-NC photocatalysts clearly exhibited an obviously higher H2-evolution performance (0.36 mmol h-1) than the traditional hexagonal CdS (0.14 mmol h-1) by a factor of 2.6 times. The improved H2-evolution activity of the suspensible c-CdS-NC can be ascribed to the massive adsorption of S2-/SO32- ions on the nanocrystal surface, namely, the massive adsorbed S2- ions on the suspensible c-CdS-NC surface not only promote the rapid capture of photogenerated holes, but 26 ACS Paragon Plus Environment

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also can work as the effective active sites for H2-evolution reaction. The present work may provide new insights for the rational design and controllable synthesis of highly efficient H2-evolution photocatalytic materials. This work not only demonstrated the importance of reaction medium for CdS photocatalyst, but also highlights the potential applications of cubic-phase CdS materials in the H2-evolution field.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. The photographs, FESEM images, nitrogen adsorption-desorption isotherms and XRD patterns of c-CdS-AG, c-CdS-180oC and h-CdS; the photocatalytic H2-evolution activity of c-CdS-NC and traditional h-CdS in the whole visible-light region, c-CdS-NC prepared from different cadmium sources, c-CdS-NC in S2-/SO32- solution with different concentrations, and c-CdS-NC and traditional h-CdS in lactic acid solution; photographs of suspensible c-CdS-NC and traditional h-CdS before and after photocatalytic reaction; the photocatalytic activity of ZnS-NC and traditional ZnS.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China 27 ACS Paragon Plus Environment


(21771142, 51472192, and 21477094). This work was also financially supported by the Fundamental Research Funds for the Central Universities (WUT 2017IB002).

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Synopsis: A direct precipitation method in a sulfur-rich system has been developed to prepare highly efficient CdS nanocrystal photocatalyst for sustainable H2 energy.


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Suspensible cubic-phase CdS nanocrystal ... - ACS Publications

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