Synthesis of Bi2S3–Au Dumbbell Heteronanostructures with

Article pubs.acs.org/Langmuir

Synthesis of Bi2S3−Au Dumbbell Heteronanostructures with Enhanced Photocatalytic and Photoresponse Properties Baoying Li,† Yihe Zhang,*,† Ruifeng Du,† Lin Gan,*,‡ and Xuelian Yu*,† †

Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, 100083 Beijing, P. R. China ‡ School of Materials Science and Engineering, State Key Laboratory of Materials Processing and Die and Mold Technology, Huazhong University of Science and Technology, 430074 Wuhan, P. R. China S Supporting Information *

ABSTRACT: In this article, novel types of Bi2S3−Au heterostructures are fabricated through rationally controlling the growth atmosphere. Under argon, Au nanoparticles are preferentially deposited onto the tips of Bi2S3 nanorods to form Bi2S3−Au dumbbell heterostructures. In contrast, because of the etching effect by amine, Au nanoparticles are randomly anchored onto the surface of nanorods to form Bi2S3−Au nanocorns in the presence of oxygen. Furthermore, the size of gold nanoparticles can be controlled through adjusting the concentration of reaction precursors. Bi2S3−Au dumbbells show superior activity for the photodegradation of organic pollutants and an enhanced photoresponse compared to the Bi2S3−Au nanocorns. The significantly improved photocatalytic performance of Bi2S3−Au dumbbells is ascribed to the more efficient charge separation compared to that of Bi2S3−Au nanocorns. These heterostructures composed of environmentally friendly elements are expected to be promising for applications in the field of clean energy.

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INTRODUCTION Colloidal semiconductor nanocrystals have evoked great interest because of their size-dependent optical and electronic properties. Recently, considerable effort has been focused on the synthesis of heterostructured nanomaterials, especially metal−semiconductor heterostructures. Because of their unique interfacial electronic characteristics, as-synthesized nanoarchitectures usually exhibit superior performance for applications in photocatalytic water splitting, pollutant degradation, and photovoltaics.1−10 Currently, the biggest challenge is to reveal the interfacial mechanism, which is crucial for developing highperformance metal−semiconductor heterostructures. For example, artificial Z-scheme photocatalysts have attracted tremendous attention because of the analogous concept of mimicking the natural photosynthesis process.11 Among different methods for hybrid nanostructures, the heterogeneous seeded nucleation method is the primary route. It depends sensitively on the characteristics of the materials, including the crystal structures and the growth conditions, to suppress undesired homogeneous nucleation. Until now, it has been an open question as to fabricating heterostructures with modulated structures and investigating the impact of interfacial structures. Since the report of growing gold nanocrystals on the tips of CdSe nanorods in 2004,12 the colloidal method has attracted increasing attention. This strategy has been successfully used for controllably loading metal nanoparticles (Au, Ag, Pt, and Pd to Co, Fe, and Ni) onto various of semiconductors, such as CdS, PbS, and InAs.13−20 This type of heteronanostructure © 2016 American Chemical Society

combines materials with distinctly different physical and chemical properties to yield a unique hybrid nanosystem with multifunctional capabilities and tunable or enhanced properties that may not be attainable otherwise. Besides, the different geometries of these heterostructures often have different optical properties because of the different degrees of overlap of the electronic structure.21 However, because of the toxicity of cadmium- and lead-based semiconductor, there is a major restriction for many applications. Research is still continuing in searching for lessor nontoxic systems that will show effective interference of their electronic states for efficient carrier transportation.22−25 Recently, we have reported the synthesis of a Cu2ZnSnS4based heteronanostructure, and it demonstrated enhanced photocatalytic hydrogen production and pollutant degradation properties owing to the improved electron−hole separation ability.26−28 Similar to Cu2ZnSnS4, bismuth sulfide (Bi2S3) also belongs to the class of nontoxic semiconductor materials.29 Besides, it is an important material for optoelectronics because of its high absorption coefficiency (104 cm−1) as well as its optimal band gap (1.3−1.6 eV). So far, many efforts have addressed their application to optoelectronic devices, photodetectors, and solar cells.30−32 The heterostructure with Bi2S3 nanorods randomly Received: August 30, 2016 Revised: October 13, 2016 Published: October 18, 2016 11639

DOI: 10.1021/acs.langmuir.6b03213 Langmuir 2016, 32, 11639−11645

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The photocatalytic activities of Bi2S3 and Bi2S3−Au heterostructures were evaluated by the degradation of methyl blue (MB, 2 × 10−5 mol/ L). A 500 W xenon lamp coupled with a 420 nm cutoff filter was used to provide visible light irradiation. Twenty-five milligrams of nanocrystals was dispersed in a 50 mL dye solution with a concentration of 50 ppm. Before illumination, the solution was stirred in the dark for 1 h to achieve adsorption−desorption equilibrium. Thereafter, the light was turned on. Three milliliters of the suspension was taken out at certain intervals and separated by centrifugation. The concentration of MB was analyzed by a Cary 5000 UV−vis spectrophotometer, thorough recording the absorbance of the characteristic peak at 664 nm. Photocurrent and Photoresponse Measurement. For photocurrent measurement, a device was made using the electrode pattern (10 nm Ti/40 nm Au) via a standard photolithograph technique on a clean 300 nm SiO2/Si substrate. The samples with the same concentration were dispersed between electrodes within a 5 μm channel by the spin-coating method. After that, the device was annealed under an argon atmosphere at 300 °C for 1 h. The optoelectronic measurement was carried out on the platform of a Keithley 4200 and probe station (Lake Shore, CRX-6.5K) at room temperature. A wide-band white lamp source (LDLS, eq 1500) was employed for various wavelengths of incident light.

decorated with Au nanoparticles on has shown enhanced nonlinear optical properties.33 However, Bi2S3−Au heterojunctions with modulated structures are rarely reported. Because of the anisotropy of one-dimensional Bi2S3 nanorods, controlling the spatial distribution of Au nanoparticles should contribute to the improved photocatalytic and photoelectronic properties. In this work, a simple colloidal method was used to controllably load Au nanoparticles on environmentally friendly Bi2S3 nanorods. We demonstrated that the growth sites of gold on Bi2S3 could be well controlled by the preparation atmosphere. When the reaction was carried out in the presence of argon, Bi2S3−Au dumbbell nanostructures with gold nanoparticles at both ends of the nanorod were fabricated. In contrast, the air atmosphere facilitated the nucleation of Au nanoparticles on the lateral sides of Bi2S3, leading to the formation of Bi2S3−Au nanocorns. Furthermore, the photocatalytic and photoresponsive properties of these two types of heterostructures were comprehensively investigated. It was found that Bi2S3−Au dumbbells exhibited superior photoactivities under visible light irradiation, which were more than 18 and 4 times higher than those of pure Bi2S3 nanorods and Bi2S3−Au nanocorns, respectively, suggesting a promising future in optoelectronic applications.

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RESULTS AND DISCUSSION In Figure 1a, a typical TEM image shows a uniform size of the synthesized Bi2S3 nanorods, with a diameter of 4 nm and a

EXPERIMENTAL SECTION

Preparation. All reactants were acquired from Sigma-Aldrich and were used without additional purification. A facial colloidal method was used to synthesize Bi2S3 nanorods.29 In a typical experiment, 0.3 g of bismuth acetate and 15 mL of oleic acid were mixed with 60 mL of 1-octadecene in a three-necked flask. The solution was heated to 90 °C under vacuum and maintained at this temperature for 30 min to remove water and other low-boiling-point impurities. Afterward, an argon atmosphere was introduced, and the temperature was increased to 160 °C. Sulfur solution, prepared by dissolving 0.03 g of elemental sulfur into 2 mL of oleylamine, was injected into the reaction solution using a syringe. The reaction lasted for 5 min, and the received samples were thoroughly purified by multiple precipitation and redispersion steps using 2-propanol and chloroform. To fabricate Bi2S3−Au dumbbell heteronanostructures, different amounts of AuCl3 (5−20 mg), didecyl dimethylammonium bromide (DDAB, 40 mg), and dodecylamine (DDA, 140 mg) were dissolved in toluene (4.0 mL), which was sonicated for 30 min under an Arsaturated medium. Under the flow of argon, the mixture was added dropwise to the Bi2S3 dispersion in toluene (20 mL) for 5 min at room temperature. The product was then precipitated by adding methanol with centrifugation and finally dispersed in toluene. For comparison, Bi2S3−Au nanocorns were also fabricated through similar processes under an air atmosphere. Characterization. Powder X-ray diffraction (XRD) patterns were obtained with Cu Kα1 (λ = 1.5406 Å) radiation in reflection geometry on a Bruker D8 operating at 40 kV and 40 mA. X-ray photoelectron spectroscopy (XPS) was carried out using the XPS spectrometers (ESCALab220I-XL). The morphological, chemical, and structural characterizations of the nanoparticles were carried out with a highresolution transmission electron microscope (HR-TEM, JEOL-2010). The composition of heterostructures was determined by ICP-MS (X Series 2, Thermo Scientific USA). Photocatalytic Activity. Prior to the reaction, nanocrystals were transferred from toluene to aqueous media via ligand exchange with mercaptopropionic acid (MPA). Typically, nanocrystals dispersed in hexane (∼50 mg in 5 mL) were mixed with 5 mL of MPA and 5 mL of cyclohexanone. The mixture was sonicated for 30 min. Subsequently, nanocrystals were centrifuged, and the precipitate was successively washed with 10 mL of cyclohexanone, chloroform, and ethanol. Finally, Bi2S3 nanocrystals and Bi2S3−Au heterostructures were collected for further use.

Figure 1. (a) TEM image of as-prepared Bi2S3 nanorods, (b) TEM image of Bi2S3−Au dumbbell heterostructures, and (c, d) TEM images of Bi2S3−Au nanocorns with different sizes of Au nanoparticles.

length of up to 50 nm. Figure 1b shows the TEM image of Audecorated Bi2S3 nanorods obtained under the bubbling of Ar gas. Obviously, Au nanoparticles selectively grow onto the tips of the nanorod, forming Bi2S3−Au dumbbell heteronanostructures. The diameter of Au nanoparticles is around 3.5 ± 0.6 nm. The energy-dispersive X-ray (EDX) analysis further identified the coexistence of Bi2S3 and Au in the heteronanostructure. By controlling the concentration of Au precursor, the size of Au nanoparticles on the tips can be 11640

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preferential growth of Bi2S3 nanorods. Thus, these tips are more reactive, which facilitates the preferential adsorption of Au precursors on the tips of the nanorods and the formation of dumbbell heterostructures under an atmosphere of argon. However, it has been reported that a toluene solution of DDA and DDAB can attack the surface of chalcogenide nanocrystals on the sidewall.35−37 Under an air atmosphere, the presence of dissolved oxygen can significantly accelerate the etching rate. Thus, these defect sites caused by the etching process will promote the growth of Au nanoparticles on the lateral sides of Bi2S3 nanorods, forming Bi2S3−Au nanocorns. In contrast, Au nanoparticles are mainly grown on the tips of nanorods under anaerobic conditions to form Bi2S3−Au dumbbells since the etching effect is much slower in the absence of air. The growth mechanism of different types of Bi2S3−Au heterostructures is illustrated in Scheme 1. To investigate the effect of structural modulation of Bi2S3− Au heterostructures on the photoactivities, heterostructures with the same size of Au nanoparticles (3.5 nm) were used as photocatalysts for pollutant removal in wastewater. Figure 4 presents the photocatalytic degradation of MB over Bi2S3−Au dumbbell-shaped and nanocorn-shaped heterostructures under visible light irradiation. On the basis of the variation of MB concentration (C/C0) with irradiation time (Figure 5a), 32% of dye molecules can be eliminated by Bi2S3 nanorods within 80 min. The decoration of Bi2S3 with Au nanoparticles obviously improved the photocatalytic removal ratio of MB. Within the same time intervals, Bi2S3−Au nanocorns can reduce 66% of dye molecules, whereas 90% is removed by Bi2S3−Au dumbbells under similar reaction conditions. The photodegradation kinetics are fit by the equation ln(C0/C) = kt, where k is the apparent rate constant (Figure 4b). All samples obey a pseudo-first-order reaction. Bi2S3−Au dumbbells show the highest average apparent rate constant of 0.0978 min−1, which is about 18-fold higher than that of pure Bi2S3 nanorods (0.0054 min−1). The average apparent rate constant for Bi2S3− Au nanocorns is 0.0263 min−1. As a result, Bi2S3−Au dumbbells indeed demonstrate superior photocatalytic performance compared to that of Bi2S3−Au nanocorns. In addition to the photocatalytic activity, we have also studied the photocurrent and photoresponse properties of these heterostructures. For the measurement, a thin film was made by spin-casting a solution of the nanocrystals in toluene on the insulating substrate with a gold electrode (details in Figure S2). The measured current versus voltage plots for Bi2S3−Au dumbbells and Bi2S3−Au nanocorns under the irradiation of light of different wavelengths are shown in Figure S3a,b, respectively. And the highest value of the photocurrent for both samples was achieved at a wavelength of 400 nm, coming from the coupling absorption of Au and Bi2S3 (Figure S4). Figure 5a,c shows typical I−V curves of Bi2S3−Au dumbbells and Bi2S3−Au nanocorns measured in the dark and under 400 nm light illumination, respectively. For both cases, irradiation results in a higher current flow than in the case of dark. We know that Au, either in dumbbells or nanocorns, shows some extent of metallic character, which aids the transportation of the carriers. The actual Au loading in Bi2S3−Au nanocorns is higher than that in Bi2S3−Au dumbbells from the ICP analysis, so a hike in the dark current is observed in nanocorns. Figure 5b,d shows the time response of the devices to the pulsed incident 400 nm light created by a manual chopper. The calculated photocurrent gain (Iphoton/Idark) of the Bi2S3−Au dumbbells is observed to be around 4, which is much

rationally controlled in the dumbbell heterostructures, from 3.5 nm (Figure 1b) to 5.0 nm (Figure S1). When the experiments were carried out under an air atmosphere, different growth processes were observed. As shown in Figure 1c, the presence of air leads to the uniform distribution of Au nanoparticles both on the tips and on the lateral surface of Bi2S3, forming corn-shaped Bi2S3−Au heterostructures. The concentration of Au precursor has a significant effect on the size of Au nanoparticles. When the amount of AuCl3 increased to 10 mg, Au nanoparticles with an average diameter of 3.5 ± 0.3 nm were observed in Figure 1d. The structures of the as-synthesized Bi2S3 and Bi2S3−Au heterostructures are further characterized by XRD, as shown in Figure 2. The main diffraction peaks at 25.03, 28.79, 31.86,

Figure 2. XRD patterns of the Bi2S3 and Bi2S3−Au heterostructures. The standard patterns of orthorhombic Bi2S3 (JCPDS 17-0320) and cubic Au (JCPDS04-0784) are shown for comparison.

35.60, 46.53, and 52.66° could be indexed to the (310), (211), (301), (240), (431), and (351) planes of orthorhombic Bi2S3 (JCPDS 17-0320). In addition, characteristic peaks at 37.8° can also be observed, which are assigned to the (111) plane of cubic Au in the Bi2S3−Au heterostructures. It demonstrates the successful formation of Bi2S3−Au heterostructures. XPS analysis is useful for investigating the surface chemical compositions and the oxidation states of the composite. The XPS spectra of Bi2S3−Au heterostructures are shown in Figure 3. The typical XPS survey spectrum (Figure 3a) shows that the sample is mainly composed of Bi, S, and Au except for C and O elements derived from the surface modification and environment. The high-resolution spectrum of Bi 4d is shown in Figure 3b. The sharp peaks located at around 465 and 441.3 eV are assigned to the 4d3/2 and 4d5/2 peaks of Bi(III).34 Meanwhile, Figure 3c shows the S 2s peak at a binding energy of 225.4 eV, confirming the formation of Bi2S3. Au 4f7/2 and 4f5/2 doublets with binding energies of 84 and 87.7 eV are observed in Figure 3d, which unambiguously suggests that Au in the heterostructures is in the metallic form. All of these results confirm that Bi2 S 3−Au heterostructures have been successfully prepared. On the basis of the above results, the formation of Bi2S3−Au dumbbell heterostructures is assigned to the preferential adsorption of the Au complex onto the tips of nanorods. The imperfect passivation of the ligands on these facets leads to the 11641

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Figure 3. XPS spectra of Bi2S3−Au heterostructures. (a) Survey spectrum, (b) Bi 4d, (c) S 2s, and (d) Au 4f.

3 s. These observations suggest that the obtained Bi2S3−Au dumbbells provide more excellent charge dissociation and transportation benefiting from their intriguing pattern compared to Bi2S3−Au nanocorns. Thus, Bi2S3−Au dumbbells can be used as a better photodetecting device than Bi2S3−Au nanocorns. On the basis of the above results, we discuss the band structure of the Bi2S3−Au hybrids in detail, which is schematically displayed in Scheme 2. According to previous reports, the Fermi level of Au is lower than the conduction band of Bi2S3. Consequently, under visible light irradiation, the migration of electrons from Bi2S3 to the Au nanoparticles is thermodynamically favorable in both of the Bi2S3−Au heterostructures, which enhances the separation of photoinduced holes and electrons compared to the pure Bi2S3 nanorods. As follows, the photoelectrons will trap the adsorbed

Scheme 1. Scheme of the Growth Process of AuNanoparticle-Loaded Bi2S3 Nanorods

higher than that of Bi2S3−Au nanocorns (∼2) at 1 V bias. By examining their response time, it is observed that the response time of the Bi2S3−Au dumbbells photodetector is 0.3 s, whereas the response time of the Bi2S3−Au nanocorns photodetector is

Figure 4. Visible-light-driven photodegradation of MB over Bi2S3, Bi2S3−Au dumbbells, and Bi2S3−Au nanocorns. 11642

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Figure 5. (a, c) Current versus voltage plots of Bi2S3−Au dumbbells and Bi2S3−Au nanocorns. (b, d) Current against time for on−off cycles for Bi2S3−Au dumbbells and Bi2S3−Au nanocorns, respectively.

nanorods. This offers evidence that the junctions between Bi2S3 and Au are electronically perfect, leading to effective electron transfer to the metals. The different degree of suppression also proves the more efficient charge separation in Bi2S3−Au dumbbells compared to that in Bi2S3−Au nanocorns. As a result, the Bi2S3−Au heterostructure with dumbbell morphology is much more advantageous in accelerating the rate of reduction of MB and the photoresponse activity than are the Bi2S3−Au nanocorns.

Scheme 2. Schematic Representation of Conditions for the Favorable Photoreduction of MB

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CONCLUSIONS We have first reported the preparation of Bi2S3−Au dumbbell heterostructures via a simple colloidal method. We find that the distribution of Au nanocrystals on Bi2S3 is strongly dependent on the growth atmosphere. When air is excluded during the reaction, Au nanoparticles mainly grow on the tips of Bi2S3 nanorods, resulting in Bi2S3−Au dumbbell heterostructures. By contrast, Bi2S3−Au nanocorns are the dominant morphology when the reaction is carried out in air. The reason is ascribed to the lateral etching effect of Bi2S3 nanorods by amine molecules, which provides more nucleation sites for Au deposition. Furthermore, Bi2S3−Au dumbbell heterostructures demonstrate photocatalytic and photoresponse properties that are superior to those of Bi2S3−Au nanocorns because of the promoted separation and transportation of charge carriers. Our research provides adequate pathways to tailor metal−semiconductor heterostructures for promising applications in optoelectronics and photocatalysis.

O2 on the surface of samples to produce superoxide anion radicals, and the photoholes left in the VB of Bi2S3 will react with OH− to generate hydroxy radicals. These radicals are powerful oxidants and can decompose the organic chemicals more efficiently. However, too many Au nanoparticles on the surface of the Bi2S3 nanorod in the Bi2S3−Au nanocorns system can also be the recombination centers of photoinduced charges rather than the electron acceptors.38,39 The actual Au loading of the heterostructure was determined to be about 1 and 1.5 wt % for Bi2S3−Au dumbbells and Bi2S3−Au nanocorns, respectively, by inductively coupled plasma−mass spectrometry (ICP−MS). Therefore, it may cover the active sites of Bi2S3, which will decrease the number of photogenerated carriers. Photoluminescence (PL) measurements were also used to confirm the effective charge separation at the interface. As illustrated in Figure S5, the dominant emission peak for Bi2S3 is located at 1.4 eV, which originates from the band-edge emission, and the photoluminescence spectra of heterostructures exhibit the quenching effect of the emissions originating from the Bi2S3

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b03213. 11643

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TEM of Bi2S3−Au nanocrystals, schematic diagram of the device for photoresponse measurement and I−V characteristics of the Bi2S3−Au heterostructures under illumination at different wavelengths. UV−vis and PL spectra of Bi2S3−Au heterostructures. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Fundamental Research Funds for the Central Universities (2652015086).

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DOI: 10.1021/acs.langmuir.6b03213 Langmuir 2016, 32, 11639−11645

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DOI: 10.1021/acs.langmuir.6b03213 Langmuir 2016, 32, 11639−11645