Controlled Growth of Sulfide on Gold Nanotriangles with Tunable

Article pubs.acs.org/JPCC

Controlled Growth of Sulfide on Gold Nanotriangles with Tunable Local Field Distribution and Enhanced Photocatalytic Activity Liang Ma,† Da-Jie Yang,† Zhi-Jun Luo,† Kai Chen,† Ying Xie,† Li Zhou,*,† and Qu-Quan Wang*,†,‡ †

Key Laboratory of Artificial Micro- and Nano-structures of the Ministry of Education and School of Physics and Technology, Wuhan University, Wuhan 430072, P. R. China ‡ The Institute for Advanced Studies (IAS), Wuhan University, Wuhan 430072, P. R. China S Supporting Information *

ABSTRACT: Metal−semiconductor heteronanostructures have attracted increasing attention due to the strong interactions between the two nanoscale-spaced components. Herein, a steerable hydrothermal method is used to control the growth of Ag2S shells onto Au nanotriangles with tunable plasmon resonance and local field distribution. Through adjusting pH value and sulfur source, three types of Au/ Ag2S heteronanostructures are obtained, including shells on the tips (Au/Ag2S (tips)), shells on the sides (Au/Ag2S (sides)), and complete shells (Au@Ag2S). The surface plasmon resonance and local field confinement are demonstrated to vary with the shell position. Furthermore, compact CdS nanoshells are coated onto the Au@Ag2S without any shape change of Au cores. By testing the photodegradation rate of Rhodamine B (RhB) under visible-light irradiation, the Au@ Ag2S@CdS hybrids exhibit enhanced photocatalytic activity compared with Au@CdS and CdS. The strong local electric field, the enhanced visible-light absorption, and the optimum band arrangement between Ag2S and CdS are thought to be the main factors. anisotropy and excellent SPR property.21,22 The synthesis of high-producting, monodispersed and stable Au NTs with controllable edge length has been achieved by Zhang’s and LizMarzán’s group through two totally different ways.23,24 Unique optical properties resulting from the in-plane and out-of-plane plasmonic modes and the large local electric field at the three tips induced by the triangle morphology make Au NTs become an ideal medium for SERS, sensing, and biological applications.25−27 However, the stability of Au NTs against oxidation is low, and the reshaping typically takes place in a few minutes after exposing at high temperature. Therefore, the synthesis of Au NT-semiconductor heteronanostructures as well as controlling the position of semiconductor on Au NTs is still a challenge. Integrating metal nanoparticles with various sulfides has been studied by many researcheres in recent years. Wang and coworkers have synthesized gold−metal sulfide (including ZnS, CdS, Ag2S, NiS, and CuS) core−shell hybrids with the help of the targeted sulfide wetting layer.28 Ouyang’s group has developed an unique cation-exchange technology to overcome the lattice mismatch between metal and sulfide.29 Thioacetamide (TAA) and sodium hydrosulfide (NaHS) are commonly used in various methods to synthesize different sulfides (ZnS,

1. INTRODUCTION Metal−semiconductor heteronanostructures with various plasmonic metal nanocrystals and low-dimension functional semiconductors have attracted increasing research interest owing to their potential applications in far-ranging fields, such as dye-sensitive solar cells (DSSCs), photocatalysis, photoelectric devices, and hydrogen production.1−4 The metal− semiconductor heteronanostructures display numerous properties far beyond those of their single component due to the strong interaction between them. Commonly, growth of semiconductor components onto metal nanocrystals could greatly improve the stability of metal nanocrystals at a high temperature, and the direct contact of metal and semiconductor could also enhance the efficiency for solar energy utilization of semiconductor at various wavelengths.5−7 In addition, the surface plasmon resonance (SPR) of metal nanoparticles could improve the properties of semiconductor, such as prolong the lifetime of electron, enhance the light harvesting, and generate the hot electron injection from metal to semiconductor.8−12 In recent years, many metal nanoparticles (including Au, Pt, and Ag) have been combined with various semiconductors, such as TiO2, ZnO, and CdS.13−20 Because the properties are highly dependent on the size, shape, and spatial distribution of each component, the synthetic strategy of metal−semiconductor heteronanostructures with controlled morphology is very important. Au nanotriangles (NTs) have attracted increased attention recently due to their extremely high © 2016 American Chemical Society

Received: September 13, 2016 Revised: November 1, 2016 Published: November 4, 2016 26996

DOI: 10.1021/acs.jpcc.6b09245 J. Phys. Chem. C 2016, 120, 26996−27002

Article

The Journal of Physical Chemistry C

Figure 1. Cross-sectional view for schematic illustration of synthesizing three types of Au/Ag2S heteronanostructures.

Ag2S, CdS, and ln2S3).30−32 TAA as a sulfur source with moderate activity is usually used at a relatively high temperature because the hydrolysis rate of TAA is greatly accelerated. NaHS is an active oxidant and exhibits high reactivity, which is dependent on the pH value of the solution.33 In present work, a controlled method is used to selectively grow Ag2S shells onto the Au NTs. Three types of Au/Ag2S heteronanostructures (including Au@Ag2S, Au/Ag2S (side), and Au/Ag2S (tips)) are obtained by flexibly adjusting the pH value of the reaction solution and changing the sulfur source. The plasmon resonance and local field confinement dependent on the shell position are analyzed. Furthermore, the compact CdS shells are further coated onto the Au@Ag2S. By testing the photodegradation rate of Rhodamine B (RhB), the Au@Ag2S@CdS hybrids exhibit the enhanced photocatalytic activity due to the strong local field and the optimum band arrangement of Ag2S− CdS as compared with Au@CdS and pure CdS.

similar to the method of Au/Ag2S (tips) except the pH value was set to 7.2. For the synthesis of Au/Ag2S (sides), 0.1 M NaOH aqueous solution was added to 1 mL of as-prepared Au/ Ag NTs to adjust the pH value to 7.2. Then, 10 uL of 0.1 M TAA was added, and the mixture was kept at 95 °C for 20 min. The final products were centrifuged, washed, and dispersed in water for further use. 2.4. Synthesis of Au@CdS and Au@Ag2S@CdS Heteronanostructures. To grow CdS shells onto Au and Au@Ag2S, 1 mL of 0.2 M CTAB, 0.1 M L-ascorbic acid, and hexamethylenetetramine were added to 5 mL of Au NTs or Au@Ag2S. Then, 10 μL of 0.1 M cadmium acetate and TAA were added. The mixture solution was kept at 80 °C for 8 h in a vacuum oven. The final products were centrifuged, washed, and dispersed in water for further use. 2.5. Photocatalytic Activity Measurement. The photocatalytic performances were measured by evaluating the photodegradation of RhB under a 300 W xenon lamp irradiation at room temperature. Ten mg of photocatalysts was added to 30 mL of 10−5 M aqueous solution of RhB. The mixture solution was kept continuously stirring during the irradiation, and the samples were taken every 10 min for further test. After removing the photocatalysts from the mixture solution by centrifugation, the concentration of RhB was calculated by measuring the extinction density of RhB at 552 nm. 2.6. Numerical Simulations. We performed the finite element method (FEM) simulations with a commercial software COMSOL Multiphysics. The refractive indices of Ag2S and water were taken from ref 34, and those of Au and Ag were taken from ref 35. The Au NT is set as an equilateral triangle with the side length of 55 nm and the thickness of 12 nm. The Ag around the Au NTs with the same thickness is set to be half ellipsoid with the major axis aligned with the Au, and the minor axis length is 7 nm. The Ag2S at the tips is a cylinder with the radius of 6 nm and the thickness of 12 nm. 2.7. Sample Characterization. The TEM and HRTEM images were obtained with a JEOL 2010 HT and a JEOL 2010 FET transmission electron microscope operated at 200 kV. Xray diffraction spectra (XRD) patterns were obtained on a Bruker D8 advance X-ray diffract meter with Cu−Kα irradiation (λ = 0.15418 nm). The absorption spectra were tested by a UV−vis−NIR spectrophotometry (Cary 5000, Varian).

2. EXPERIMENTAL SECTION 2.1. Chemicals. Chloroauric acid (HAuCl4·4H2O, 99.99%), silver nitrate (AgNO3, 99.8%), cadmium acetate (99.99%), Lascorbic acid (99.7%), thioacetamide (99%), hexamethylenetetramine (HMT, 99.99%), cyltrimethylammonium chloride (CTAC, 99.0%), cyltrimethylammonium bromide (CTAB, 99.0%), sodium iodide (NaI, 99.5%) and sodium hydrosulfide (NaHS, 99.5%) were purchased from Sinopharm Chemical Reagent (Shanghai, China). All chemicals were used as received and without further purification. All aqueous solutions were freshly prepared by using double-distilled water. 2.2. Synthesis of Au/Ag NTs. The Au NTs were prepared by a seed-mediated growth method reported previously.23 For the synthesis of Au/Ag NTs, 0.5 mL of 0.2 M CTAC aqueous solution was added to 2 mL aqueous solution of Au NTs. The mixed solution was preheated at 60 °C for 10 min. Then, 0.3 mL of 0.1 M AA, 0.2 mL of 0.2 M CTAC, and 20 uL of 0.01 M AgNO3 were added to the mixture solution one after another. The mixture solution was maintained at 60 °C for 4 h. The final products were centrifuged, washed, and dispersed in water for further use. 2.3. Selective Growth of Ag2S on Au NTs. The three types of Au/Ag2S heteronanostructures were synthesized through a one-pot sulfurization method with different sulfide sources. In brief, to synthesize Au/Ag2S (tips), 0.1 M NaOH aqueous solution was added to 1 mL of as-prepared Au/Ag NTs to adjust the pH value to ∼12. Then, 10 uL of 0.1 M NaHS was added and the mixture was left undisturbed for 10 min. For the synthesis of Au@Ag2S NTs, the process was 26997

DOI: 10.1021/acs.jpcc.6b09245 J. Phys. Chem. C 2016, 120, 26996−27002

Article

The Journal of Physical Chemistry C

Figure 2. TEM images of Au NTs (a), Au/Ag (b), Au/Ag2S (sides) (c), Au@Ag2S (d), and Au/Ag2S (tips) (e). (f) HRTEM image of a single Au/ Ag2S (sides) heteronanostructure.

Ag2S. High-yield and uniform Au NTs have side length of ∼55 nm. As shown in Figure 2b, the compact Ag shells are coated along the sides of Au NTs with the thickness of ∼6 nm. Three types of Au/Ag2S with totally different morphology are exhibited in Figure 2c−e. For the Au/Ag2S (sides) sample, the Ag2S shells are grown on the sides of Au NTs, leaving the tips outside. For the Au@Ag2S, the Au cores are completely wrapped by the Ag2S shells. The thickness of the Ag2S shells in Figure 2c,d is ∼7 nm, and the Au cores maintain the triangle shape without etching and reshaping. While in Figure 2e, the Ag2S are only grown on the tips of Au NTs. The estimated yield of Au/Ag2S (sides), Au@Ag2S, and Au/Ag2S (tips) is approximately 90, 92, and 80%, respectively. An HRTEM image of Au/Ag2S (sides) is shown in Figure 2f. The lattice plane spacing of 0.23 nm in the core region agrees well with the (111) lattice planes of the fcc gold crystal. The lattice plane distance of 0.26 nm in the side shell can be ascribed to the (121) planes of Ag2S. 3.2. Tunable Plasmon Resonance and Local Field Confinements of Three Types Au/Ag2S Heteronanostructures. The plasmon resonance properties of the three types of Au/Ag2S heteronanostructures are shown in Figure 3. The starting Au NTs have a sharp main SPR located at 634 nm and a weak shoulder around 535 nm, which can be attributed to the in-plane dipole and out-of-plane dipole resonances, respectively.23 After coating of Ag shells, the sharp SPR wavelength of Au/Ag blue-shifts to 570 nm and the shoulder blue-shifts to 505 nm owing to the negative dielectric constant of Ag (see Figure 3b). When Ag2S shells are grown, the main SPR wavelength red-shifts to 725 nm for the Au@Ag2S, 716 nm for the Au/Ag2S (sides), and 708 nm for the Au/Ag2S (tips) (see Figure 3c−e). Meanwhile, the shoulder red-shifts to 521, 532, and 550 nm for Au/Ag2S with complete, side, tips shells, respectively. All of these red-shifts are caused by the increased dielectric constant of Ag2S shells. The extinction intensity ratio of weak shoulder and main peak for each Au/Ag2S hybrid is calculated, which is found to be 1:5 for the Au@Ag2S, 1:3 for the Au/Ag2S (side), and 1:2 for the Au/Ag2S (tips), respectively. The evolution of intensity ratio indicates the absorption band could be adjusted by the position of Ag2S shells.

3. RESULTS AND DISCUSSION 3.1. Selective Growth of Ag2S on Au NTs. Figure 1 schematically shows the reaction pathways that lead to the formation of three types of Au−Ag2S heteronanostructures. The synthesis is based on a two-step seed-mediated growth. In the first step, Au/Ag NTs are prepared by introducing of AgNO3 and ascorbic acid (AA). Then, the Au/Ag NTs are used as seeds to synthesize three types of Au/Ag2S. At the presence of TAA as sulfur source, the Au NTs with Ag2S shells on the sides are obtained. When NaHS is introduced, the Ag shells are quickly sulfurized. As the pH value is adjusted to be 12 by NaOH, the Ag2S are grown onto the tips of Au NTs. Without NaOH solution to control the reaction rate, the Au NTs are completely wrapped by Ag2S shells. The growth mechanism for the synthesis of three types of Au/Ag2S is analyzed, and a similar mechanism is reported in our previous work.34 Usually, a Ag layer is employed as a “bridge” between Au and semiconductor to achieve the heterogrowth. The Ag layers could be easily deposited onto the Au NTs as the lattice constants of Ag and Au match perfectly. Moreover, the electron-negativity of Ag is similar to that of many anions, and thus the Ag layers could be modified to form corresponding compounds, such as Ag2S, AgCl, and AgBr.31,36,37 In our method, the Ag shells are grown at the sides of Au NTs, which properly leave the tips of Au NTs outside (see Figure 2b). As a mild sulfide source, TAA can vulcanize the Ag softly when the pH value of the reaction solution is adjusted to be 7.2; then, the Au/Ag2S samples with side shells are formed. NaHS is an active sulfide source and has been used to synthesize various sulfides. It could infiltrate into the metal or oxide from outside to inside.38 When the pH value is adjusted to be 7.2, the whole sulfuration reaction rate is relatively fast and the Ag layers are sulfurized to Ag2S quickly. Because of the high charge density around the tips, the Ag2S layers prefer to grow onto the tips. As the pH is increased to 12, the Ag is sulfurized tempestuously and the grown Ag2S falls off from the sides of Au (the dissociative Ag2S in Figure 2e confirms this assumption). With the appropriate reaction speed, the Ag2S shells on the tips are formed, which leads to the Au/ Ag2S with shells on the tips. Figure 2 presents the morphology and composition characterization of Au NTs, Au/Ag, and three types of Au/ 26998

DOI: 10.1021/acs.jpcc.6b09245 J. Phys. Chem. C 2016, 120, 26996−27002

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The Journal of Physical Chemistry C

Figure 3. Experimental extinction spectra of Au NTs (a), Au/Ag (b), Au@Ag2S (c) Au/Ag2S (sides) (d), and Au/Ag2S (tips) (e).

Figure 4 presents the TEM images and the local field distributions of a single sample for Au NTs, Au/Ag, and three types of Au/Ag2S heteronanostructures. The local-field distribution is calculated by the COMSOL Multiphysics software, and the perfectly matched layers are used. The simulation is based on frequency-domain calculation. We sweep the incident light wavelength to get the near- and far-field distributions. The circularly polarized light propagates perpendicularly to the plates. All of the field distribution is calculated at the in-plane dipole resonance peaks. The initial Au NT and Au/Ag shows strong local field around the three tip regions. Three types of Au/Ag2S display entirely different field distribution (see Figure 4h−j), indicating that the position of the grown Ag2S could regulate field confinement and distribution. Because there exists an intense local field around the Au NT tips, the whole Ag2S experiences the large field enhancement when the Ag2S are coated on the tips (see Figure 4h). However, for the Au/Ag2S (sides), the Ag2S are located on the side of Au NT and leave the tips outside (see Figure 4i). For the Au@Ag2S, only the Ag2S located around the tip regions experience the large field enhancement (see Figure 4j). 3.3. Growth of CdS on Au@Ag2S with Enhanced Photocatalytic Activity. The well-defined CdS nanoshells are coated on the Au@Ag2S, and the corresponding morphology and component characterization are shown in Figure 5.

Figure 5. TEM (a) and HRTEM (b) images of Au@Ag2S@CdS. (c) XRD pattern of Au@Ag2S@CdS, the standard patterns of cubic-phase Au (PDF no. 65-8061), monoclinic-phase Ag2S (PDF no. 14-0072), and hexagonal-phase CdS (PDF no. 65-3414) are also displayed.

Uniform and obvious double-shell structure is observed in the inset of Figure 5a. The thickness of CdS shells is ∼12 nm and the Au cores still maintain the consistent triangle shape. An HRTEM image in Figure 5b presents the crystalline structure of Au@Ag2S@CdS. The lattice plane distance of 0.34 nm in the outer shell can be ascribed to the (002) planes of CdS, and the

Figure 4. TEM images and calculated local field distributions for single Au NT (a,f), Au/Ag (b,g), Au/Ag2S (tips) (c,h), Au/Ag2S (sides) (d,i), and Au@Ag2S (e,j). 26999

DOI: 10.1021/acs.jpcc.6b09245 J. Phys. Chem. C 2016, 120, 26996−27002

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as the photocatalytic activity.44 On the contrary, the large local field confinement and enhancement could enhance the light absorption and photoexcited carrier concentration of semiconductor.

lattice plane distance of 0.26 nm in the middle shell can be ascribed to the (121) planes of Ag2S. Figure 5c shows the XRD pattern of Au@Ag2S@CdS. The phases of Au, Ag2S, and CdS are all observed. The photocatalytic activity of pure CdS, Au@CdS and Au@ Ag2S@CdS is compared through evaluating the photodegradation of RhB by measuring the absorption intensity of the characteristic peak at 552 nm. All of the measurements are carried out under similar conditions. As is shown in Figure 6a,

4. CONCLUSIONS We have reported a controlled hydrothermal process to selectively grow Ag2S shells on Au NTs. Three types of Au/ Ag2S nanostructures (including complete shell, shells on the tips and sides) are obtained by adjusting the pH value of reaction solution and changing the sulfur source. Through tuning the position of Ag2S shells on Au NTs, the SPR and local-field distribution are easily regulated. Furthermore, the Au@Ag2S@CdS heteronanostructures have been synthesized almost without any sharp changes of Au NT cores. Because of the strong local-field confinement of Au@Ag2S and optimum band configuration between Ag2S and CdS, the Au@Ag2S@ CdS exhibits enhanced photocatalytic activity. The plasmonenhanced metal−semiconductor material with tunable morphology and local-field distribution will be used in many applications, such as sensors, photovoltaics, nonlinear plasmonic devices, and so on.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b09245. Figure S1. TEM image of Au@CdS hetero-nanostructures. Figure S2. Calculated extinction spectra of Au, Au/ Ag, and three types of Au/Ag2S hybrids. Figure S3. Expanded XRD pattern of Au@Ag2S@CdS heteronanostructures. Figure S4. Photodegradation of RhB versus time for Au and Au@Ag2S under visible light irradiation. (PDF)

Figure 6. (a) Photodegradation of RhB versus time for pure CdS, Au@CdS, and Au@Ag2S@CdS under visible-light irradiation. (b) Schematic illustration of the charge separation at the interface of Au@ Ag2S@CdS heteronanostructure.

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AUTHOR INFORMATION

Corresponding Authors

*L.Z.: E-mail: [email protected]. Tel: (+86)027-687529893605. *Q.-Q.W: E-mail: [email protected]. Tel: (+86)02768752989-3410.

40 and 54% of the dyes are reduced by pure CdS and Au@CdS with the irradiation for 60 min. It is easy to find that there is ∼35% enhancement in the presence of Au NTs compared with CdS alone. This mechanism of plasmon-enhanced photocatalytic activity has been discussed by many previous works.39−43 We found that Au@Ag2S@CdS shows the highest photocatalytic activity among the tested samples with ∼71% of the dyes reduced in 60 min. The inner charge-transferring processes in photodegradation are further discussed and shown in Figure 6b to understand the mechanism of the enhanced photocatalytic activity. The Au@semiconductor heteronanostructure shows enhanced photocatalytic activity compared with the pure CdS sample; the Au NT acts as an electron trapper and accelerates the charge separation as well as prolongs the lifetime of photoexcited carriers in semiconductor. The Au@Ag2S@CdS shows larger photocatalytic activity than that of Au@CdS. Ag2S and CdS are narrow bandgap semiconductors whose band gaps are 0.94 and 2.4 eV, respectively. The CB level of Ag2S lies between the CB of CdS and the Fermi level of Au. In this case, the Ag2S shells act as middle medium, and it could make the electrons transfer from CdS to Au easier and faster as compared with Au@CdS. Such gradient band alignment and the stepwise electron transportation enhance the charge separation efficiency as well

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Yaoyao Ren and Jinwen Yang for the TEM measurement and also thank Dingze Lu for the XRD measurement. This work was supported in part by the National Program on Key Science Research of China (2011CB922201), NSFC (11374236 and 11674254), and the Fundamental Research Funds for the Central Universities (No. 2015202020204).

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REFERENCES

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DOI: 10.1021/acs.jpcc.6b09245 J. Phys. Chem. C 2016, 120, 26996−27002

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DOI: 10.1021/acs.jpcc.6b09245 J. Phys. Chem. C 2016, 120, 26996−27002