Template-free synthesis of ternary sulfides submicrospheres as visible light photocatalysts by ultrasonic spray pyrolysis

Abstract

CdIn₂s₄, ZnIn₂S₄ and AgIn₅S₈ were prepared by ultrasonic spray pyrolysis method without using templates which demonstrated a general route for the synthesis of ternary sulfides submicrospheres.

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The ternary sulfides have attracted great attention as their properties can be facilely adjusted by changing the composition to allow their versatile applications, such as in optical devices,¹ sensors,² cathode materials,³ solar cell⁴ and artificial photosynthesis.⁵ Being applied as semiconductor photocatalysts, ternary sulfides have shown great promise in solar energy conversion because the semiconductor band gap and redox levels of complex metal sulfides can be controlled by the adjustment of the ratio of constituent elements, enabling the fully utilization of the solar spectrum in both UV and visible light regions. Recently, many ternary sulfides such as Zn_1−xCu_xS,⁶ Zn_xCd_1−xS⁷ and ZnIn₂S₄⁸ have been developed as photocatalysts, and a high quantum yield of >20% has been achieved for modified AgInS₂ by Kudo, when coupled with supportive water reduction catalysts.⁹

Nanostructural designs of these ternary sulfide photocatalysts have also been actively pursued due to nanostructure-enhanced physical and chemical properties that can in principle promote both surface catalysis kinetics and charge-separation. Various ternary sulfides have been synthesized via conventional approaches using surfactants as templates and structure-directing agents.¹⁰ However, the commonly used surfactants may deteriorate the properties and limit the applications of the final products due to the remnant surfactants adsorbed onto the surface of products. Additionally, the removal of the templates requires additional processing steps that can be costly, wasteful, and of environmental concern. Thus, a high-yield synthetic and template-free method that is capable of producing ternary sulfides with special nanostructures can undoubtedly benefit the design of high-performance materials in catalytic applications.

The ultrasonic spray pyrolysis (USP) approach is a convenient process allowed for continuous and large-scale production of nanomaterials as in the most well-known cases of oxide-based spherical materials such as Bi₂WO₆, InVO₄¹¹ and spherical binary metal sulfides such as MoS₂ and ZnS.¹² The USP method can also be used in the film preparation of ternary sulfides.¹³ However, to the best of our knowledge, nanostructured ternary sulfide spheres produced by USP have not been reported. Herein, we describe a simple, scalable synthetic strategy for the production of diverse ternary sulfides free from organic templates and studied their visible-light-activated photocatalytic degradation of pollutants in the air.

A productive and general methodology is developed for the preparation of submicrometer-sized ternary sulfides spheres. The soluble metal salts and excess thiourea were added into water under magnetic stirring to form a transparent colourless solution. The resulting solution was nebulized and the produced aerosol was carried into a tubular reactor with a constant air flow rate under pyrolysis conditions. The droplet in the aerosol would serve as micro-reactors and yield one spherical particle per droplet. The size of droplet in the aerosol determines the product dimension which could be adjusted through controlling the nebulizing condition.¹⁴ In the tubular reactor, when the water evaporated, the diameter of the precursor droplet decreased and the precursor concentration increased under high temperature. At the same time, the thiourea in the droplet would decompose quickly and release H₂S which would react with the metal precursor quickly and generate submicrospherical ternary sulfides. The proposed schematic diagram of this process is shown in Scheme 1.

Scheme 1 Proposed schematic diagram of the formation of ternary sulfide in the ultrasonic spray pyrolysis process.

Ternary sulfides of CdIn₂S₄, ZnIn₂S₄ and AgIn₅S₈ spheres were chosen to demonstrate the generality of using this novel approach. Fig. 1(a) shows a typical scanning electron microscopy (SEM) image of CdIn₂S₄ which clearly indicates that small crystallites aggregate to form a mesoporous spherical structure. Fig. 1(b) shows the transmission electron microscopy (TEM) image of CdIn₂S₄ spheres which also indicates these spheres with porous properties are assembled of numerous small crystallites. Fig. 1 also shows the SEM and TEM images of ZnIn₂S₄ [(c) & (d)] and AgIn₅S₈ [(e) & (f)], which both have spherical morphology composed of small crystallites. The surface of the InAg₅S₈ sample is relatively smooth and the pore size of InAg₅S₈ sphere is much smaller than that of CdIn₂S₄ and ZnIn₂S₄, which is further confirmed by the results of N₂ adsorption–desorption test (see Table 1). This may due to the high concentration of precursor in the InAg₅S₈ preparation (see Table S1†), which would lead to a spherical product containing with higher mass. Thus, the pore between the particles is small and the surface of sphere is smooth. Fig. S1† shows the overall morphology of CdIn₂S₄, ZnIn₂S₄ and AgIn₅S₈ with smaller magnifications indicating all the samples consist of polydispersive microspheres with diameters ranging from 100 nm to 1 μm. These materials can be produced continuously at a rate of several grams per hour.

Table 1 Crystalline and porous properties of the as prepared samples

Catalyst	Calcination temperature (°C)	d ^a (nm)	Pore size^b (nm)	Surface area^c (m^2 g^−1)	Pore volume^d (cm^3 g^−1)
a d values of the samples estimated by Scherrer formula. b BJH pore sizes determined from the nitrogen desorption branches. c BET surface areas determined from the nitrogen adsorption and desorption isotherm measurement. d Pore volumes is the BJH desorption cumulative pore volume of pores.
CdIn2S4	500	5.4	13.5	36.1	0.15
CdIn2S4	700	14.2	13.6	33.2	0.15
ZnIn2S4	500	10.0	12.1	61.2	0.22
ZnIn2S4	700	10.1	12.6	62.1	0.24
AgIn5S8	500	8.3	5.6	28.9	0.04
AgIn5S8	700	11.7	4.3	17.9	0.04

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Fig. 1 FSEM images and TEM images of the ternary sulfides prepared at 500 °C. (a) & (b) CdIn₂S₄; (c) & (d) ZnIn₂S₄; (e) & (f) AgIn₅S₈.

Fig. 2 shows the X-ray diffraction patterns of the as-prepared samples. The patterns of Fig. 2(a)–(c) could be indexed to cubic CdIn₂S₄ (JCPDS No. 27-60, space group: Fd3m), hexagonal ZnIn₂S₄ (JCPDS No. 72-0733, space group: P63mc) and cubic AgIn₅S₈ (JCPDS No. 25-1329, space group: Fd3m), respectively. No other impurities, such as binary sulfides, oxides or organic compounds related to reactants were detected. The nanocrystal sizes estimated from the peaks of X-ray diffraction (XRD) by the Scherrer formula are listed in Table 1 confirming the nanocrystalline nature of ternary sulfides. The high crystallinity of the spherical CdIn₂S₄, ZnIn₂S₄, and AgIn₅S₈ is also confirmed by the clear lattices observed by HRTEM images (Fig. 2(d)–(f)) of the samples prepared at 700 °C. The inter-planar distance of 0.62, 1.23 and 0.63 nm correspond to (111), (002), and (111) d-spacing of the CdIn₂S₄, ZnIn₂S₄, AgIn₅S₈ structures, respectively.

Fig. 2 XRD patterns (a–c) and HRTEM images of ternary sulfides spheres of (d) CdIn₂S₄, (e) ZnIn₂S₄ and (f) AgIn₅S₈.

It is well known that the pore structure and surface area of materials are important factors influencing the properties. Fig. S2† shows the nitrogen adsorption–desorption isotherms plots and pore-size distribution plots of CdIn₂S₄, ZnIn₂S₄ and AgIn₅S₈ samples prepared at 500 °C. Nitrogen adsorption measurements demonstrate that all the materials exhibit type IV isotherm behaviour with surface areas and pore volumes ranging from 19 to 64 m² g⁻¹ and 0.04–0.24 cm³ g⁻¹, respectively (see Table 1). The results also indicate that the crystal size increased and surface area decreased with the increasing preparation temperature for the samples of CdIn₂S₄ and AgIn₅S₈ due to the increasing of crystallinity. However, the preparation temperature did not affect the size and pore structure of ZnIn₂S₄. Similarly, this phenomenon has been reported in other studies using a hydrothermal synthetic approach.⁸

The optical properties of as prepared ternary sulfides spheres are shown in the inset part of Fig. S3.† The absorption edges of CdIn₂S₄, ZnIn₂S₄ and AgIn₅S₈ are 520, 485 and 740 nm, respectively. The results confirm that the catalysts could be activated by visible light. As CdIn₂S₄, ZnIn₂S₄ and AgIn₅S₈ are direct transition semiconductors, plots of the (αhυ)^1/2 (α is the absorption coefficient and hυ is the photon energy) vs. the energy of absorbed light can be used to interpret the band gaps of as-prepared samples.^8,15 As shown in the Fig. S3,† the band gaps obtained in such a way are approximately 2.02 eV, 2.15 eV, and 1.53 eV for the sample CdIn₂S₄, ZnIn₂S₄ and AgIn₅S₈, respectively.

The photocatalytic performances of CdIn₂S₄, ZnIn₂S₄ and AgIn₅S₈ for pollutant degradation in water or water splitting have already been demonstrated in many reports.^8,15b,16 In this study, the removal of nitric oxide (NO) at typical concentrations (500 ± 10 ppb) for indoor air quality at ambient temperature in a continuous flow (3 L min⁻¹) reactor was used to evaluate the photocatalytic performance of the prepared samples. The reaction of NO with air was negligible from the control experiments with or without light in the absence of photocatalyst. The results are shown in Fig. 3. The degradation of NO under visible light for CdIn₂S₄ and ZnIn₂S₄ were about 15 and 30%, respectively. The test also reveals that no significant decrease was observed in activity after being used repetitively for 5 times (100 min) for CdIn₂S₄ and ZnIn₂S₄. Meanwhile, for AgIn₅S₈, the photocatalytic activity was limited and decayed quickly. The difference in photocatalytic activity is possible due to two reasons. Firstly, the photocatalytic activity depends on the band gap and relative band position of ternary sulfides, which decides whether the photo-generated electrons and holes in the photocatalytic reaction have enough energy to reduce or oxidize the pollutant and produce active oxygen species such as hydroxyl radicals.¹⁷ Secondly, photocatalytic activity is significantly influenced by the surface area of ternary sulfides. It is well known that the gas solid heterogeneous photocatalysis is a surface-based process. The large surface provides more surface sites for the adsorption of reactant molecules and, ultimately, making the photocatalytic processes more efficient.¹⁸ The prepared ZnIn₂S₄ sphere has a relatively high surface area which leads to its higher photocatalytic activity. Further increasing the surface via the introduction of pore-forming agent in the preparation is in progress.

Fig. 3 Photocatalytic removal of NO for the as-prepared samples under visible light irradiation (λ > 400 nm).

In summary, we have developed a facile and highly efficient aerosol-assisted synthetic strategy for producing various ternary sulfides submicrospheres at a rate of several grams per hour. The availability of these nanometer scale ternary sulfides is expected to enable fascinating opportunities in different applications. By further controlling the synthetic method, we believe that their physicochemical properties such as BET surface and band structure could be further optimized and are promising candidates as multifunctional materials as catalysts, photocatalysts, and optoelectronic materials.

This work was supported by the Research Grants Council of Hong Kong (PolyU5204/07E), the Hong Kong Polytechnic University (GYX0L, GYF08 and GYX75), the National Natural Science Foundation of China (21103095) and the Natural Science Foundation of Fujian Province (2010J05030).

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Footnote

† Electronic supplementary information (ESI) available: Details of characterizations, photocatalytic activity measurements, the overall FSEM image, N₂ adsorption–desorption isothermal curves and UV-vis diffuse reflectance spectra of ternary sulfides sphere. See DOI: 10.1039/c2cy20053h

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