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A facile method for the synthesis of quaternary Ag–In–Zn–S alloyed nanorods† Xiaosheng Tang,*a Zhigang Zang,*a Zhiqiang Zu,a Weiwei Chen,a Yan Liu,a Genquan Han,a Xiaohua Lei,a Xianmin Liu,a Xiaoqin Du,a Weimin Chen,a Yu Wangb and Junmin Xuec
Received 12th June 2014 Accepted 4th August 2014
Ag–In–Zn–S nanorods with tunable photoluminescence were formed by a convenient synthetic approach, and the nanorods demonstrated a relatively long fluorescence lifetime of 1.248 ms. In addition, Ag–In–Zn–S
DOI: 10.1039/c4nr03231d
nanorods of nail shape and rod-particle dimers were successfully produced by adjusting the reaction
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parameters.
1. Introduction Semiconductor nanocrystals have attracted considerable attention as a result of their unique electronic and optical properties, allowing their use in the development of light-emitting diodes (LEDs),1–4 solar cells, and biolabels.5–7 In recent decades II–VI semiconductor nanoparticles have been extensively studied for uorescent labeling in biological research as a result of their excellent optical properties.8–10 However, a large number of conventional II–VI semiconductor nanoparticles contain environmentally unfriendly elements such as Hg, Cd and Pb, and this has limited their application.11–15 There is therefore a need for alternative luminescent semiconductor nanocrystals that avoid the use of toxic heavy metals. Ternary I–III–VI2 semiconductor materials, such as AgInS2,16–22 CuInS214,23–25 AgGaS226 and Cu(InxGa1x)S2,27,28 have been widely investigated for their potential use in photovoltaic solar cells, LEDs, non-linear optical devices, and for photocatalytic hydrogen evolution under visible light irradiation. Recently, a number of approaches have been employed to prepare I–III–VI2 nanocrystals using solvothermal methods, decomposition of singlesource methods, and hot injection techniques.16–22 In our previous research a hot injection method was successfully used for the synthesis of CuInS–ZnS and AgInS–ZnS alloy nanoparticles with bright and tunable photoluminescence.29,30 However, most of the research was focused on the synthesis of
the nanoparticles, and the mechanism of the crystallographic growth was not fully explored. The bandgap of semiconductor nanoparticles can be adjusted by varying their size, based on the quantum connement effect and their chemical composition.11,28 In addition, the shape of the nanoparticles can inuence their photoluminescence.28,31–34 For instance, it has been reported that nanorods and nanowires have different optical properties from those of nanoparticles.31–34 There is therefore still a need for a simple way of tuning the shape of semiconductor nanoparticles. Despite the efforts mentioned above to synthesize AgInS2 nanocrystals for strong photoluminescence, further studies are required to develop a convenient synthesis for the preparation of color-tunable Ag–In–Zn–S nanorods. In the present study we have demonstrated a simple route for the synthesis of Ag–In–Zn–S nanorods with tunable emission. In this, Ag–In–Zn–S nanorods with different aspect ratios have been obtained by changing the ratio of the precursors, for example the emission of the resulting Ag–In–Zn–S nanorods can be tuned by adjusting the proportion of Zn. The Ag–In–Zn–S nanorods obtained show adjustable photoluminescence spectra between 550 and 650 nm. Furthermore, the Ag–In–Zn–S nanorods have a much longer lifetime than that of the corresponding nanoparticles, which might avoid disturbing the autouorescence of biological tissues, and suggests its potential application in time-resolved uoroimmunoassay.
2.
Experimental
a
Key Laboratory of Optoelectronic Technology and Systems of the Education Ministry of China, College of Optoelectronic Engineering, Chongqing University, Chongqing 400044, China. E-mail: [email protected]; [email protected]
b
College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China
c Department of Materials Science and Engineering, Faculty of Engineering, National University of Singapore, Singapore
† Electronic supplementary 10.1039/c4nr03231d
information
(ESI)
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available.
See
DOI:
2.1
Materials
Silver nitrate (AgNO3, $98%), silver acetate (AgAc, $98), oleic acid (99.0%), zinc acetate (Zn(Ac)2, $90%), trioctylphosphine (TOP, 90%), technical grade 1-octadecene (ODE, 90%), indium(III) acetate (In(Ac)3, 99.99%), oleylamine (OA, 70%), 1dodecanethiol (DDT, $98%), sulfur (S, 90%) and technical grade zinc stearate, were purchased from Sigma-Aldrich.
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Toluene (99.99%), ethanol (99.9%) and chloroform (99.99%) were obtained from Fisher Scientic. All chemicals were used as received.
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2.2
Preparation of Ag–In–Zn–S nanorods
2.2.1 Synthesis of Ag–In–Zn–S nanorods and dimers. 0.1 mM AgAc, 0.1 mM In(Ac)3 0.1 mM Zn(Ac)2, 5 mM (1.2 mL) DDT, 3 mM (610.5 mg) oleic acid and 4 mL of ODE were placed in a 100 mL three-necked ask and sonicated for 30 min. The reaction mixture was then heated for 30 min at 180 C under nitrogen with magnetic stirring, and 0.3 mM sulfur dissolved in oleylamine at 180 C injected. The temperature was then increased to 210 C. The nanorods obtained were rst washed using absolute ethanol and toluene to remove unreacted precursors, and the washing process was repeated three times. The puried nanorods were then dispersed in toluene or chloroform for storage. 2.2.2 One-pot synthesis of Ag–In–Zn–S nail-like nanorods. Ag–In–Zn–S nail-like nanorods were prepared by thermal decomposition of a mixture of 0.1 mM AgNO3, 0.1 mM In(Ac)3 and 5 mM (1.2 mL) DDT in a 100 mL three-necked ask with 4 mL of ODE, 0.2 mM (56.4 mg) oleic acid, and about 46.5 mL of TOP. The reaction mixture was heated at 60 C for 60 min under nitrogen with magnetic stirring. 0.1 mM zinc stearate and 0.3 mM sulfur powder were dissolved in a 4/1 mixture of oleylamine and ODE at 120 C, and 10 drops of the solution were added to the three-necked ask and allowed to react for 5 min; addition of zinc stearate and sulfur solution was repeated until both of the two precursors were exhausted, and the temperature was then increased to 210 C for 2 h. The nanorods were nally washed and stored as described above. 2.2.3 Characterization. X-ray diffractometry (XRD) was carried out using an Advanced Diffractometer System (D8 Advanced Diffractometer System, Bruker, Karlsruhe, Germany). Transmission electron microscopy (TEM) images were obtained using a JEOL 3010 microscope at an acceleration voltage of 200 kV. Samples were prepared by dipping carbon-coated copper grids into a solution of the sample followed by drying at room temperature. Photoluminescence (PL) spectra were measured in open 1 cm path-length sample containers using a PerkinElmer LS 55 spectrouorimeter.
3.
Results and discussion
Fig. 1 illustrates the XRD pattern of the Ag–In–Zn–S straight nanorods collected. Peaks were observed at 2q of 28.4 , 47.6 and 56.1 , corresponding to the (002), (110) and (112) indices, respectively, of the hexagonal InAgZn2S4 crystal structure (JCPDF 25-0383).16,30 Its composition was conrmed by energydispersive X-ray spectroscopy. As shown in ESI Fig. S1,† signals corresponding to silver, indium, zinc and sulphur were detected. The morphology of the Ag–In–Zn–S samples was measured by TEM, and in Fig. 2A it can be seen that high quality Ag–In–Zn–S nanorods were obtained. The nanorods had a narrow size distribution, with an average width of 5 nm and
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Fig. 1
XRD pattern of Ag–In–Zn–S straight nanorods.
length between 60 and 100 nm. Fig. 2B shows the HRTEM image of part of a single Ag–In–Zn–S nanorod. The lattice of the single Ag–In–Zn–S nanorods may be clearly observed, which indicates a high degree of crystallinity in the nanorod structure. Additionally, the two-dimensional fast Fourier transform (2DFFT) of the relevant images was conducted (Fig. 2C). The distance between adjacent planes in the marked area was 0.29 nm, corresponding to (010) planes of the hexagonal structure, which is consistent with the results obtained by XRD.34,35 The optical behavior of the Ag–In–Zn–S nanorods obtained was characterized. The emission peaks of the nanorods were centered at 550, 602 and 650 nm and their corresponding photoluminescence quantum yields were 38, 42 and 44% (ESI Fig. 2S†). The tunable photoluminescence spectra were adjusted by the ratio of Zn (ESI Table S1†), and veried in the AgInS–ZnS alloy nanoparticle system.29 The corresponding photographs of the Ag–In–Zn–S nanorods under excitation at 365 nm are shown in Fig. 2D (inset), in which the colors of the Ag–In–Zn–S nanorods are tuned from yellow to red. In order to study the mechanism of formation of the Ag–In–Zn–S nanorods in detail, further analysis was carried out, as illustrated in Scheme 1. Fig. 3A shows the samples prepared over 5 min. It can be seen that the size of the Ag–In–Zn–S nanoparticles was 4 nm and the size distribution was quite narrow. A TEM image at higher magnication is shown in Fig. 3B, the nanoparticles being distributed in hexagonal order. The HRTEM shown in Fig. 3C further conrmed the structure of the nanoparticles obtained, and it can be seen the lattice distance was 0.29 nm, which is consistent with the (010) planes of Ag–In–Zn–S.34,35 The EDX spectrum in Fig. 3D conrms the elemental composition of the nanoparticles to be Ag, In, Zn and S, which is consistent with ESI Fig. S1.† A similar system of Cu–Zn–In–S spherical nanoparticles of size about 3 nm could be achieved by sampling, but this did not identify Cu–Zn–In–S nanorods.4 As our process became further developed, however, further oleic acid was introduced into the reaction, resulting in a rod structure. Aer 2 h reaction, Ag–In–Zn–S nanorods were intermittently obtained. Fig. 4A shows the TEM image of the Ag–In–Zn–S
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(A) TEM images of the Ag–In–Zn–S straight nanorods (inset: histograms showing the diameter and length distribution of the nanorods); (B) HRTEM image of a typical straight nanorod; (C) 2D-FFT images of the nail-like nanorods in the red area marked in (B); and (D) PL spectra of Ag–In–Zn–S nanorods with different emissions (inset: photographs of the Ag–In–Zn–S nanorods dispersed in toluene under excitation at 365 nm).
Fig. 2
Scheme 1
Schematic illustration showing the growth of the Ag–In–Zn–S nanorods.
intermittent nanorods, and it was observed that the Ag–In–Zn–S nanorods had achieved a rod shape, but the rod structure was not entirely straight. Under more careful observation, it was seen that the intermittent Ag–In–Zn–S nanorods were formed from a number of nanoparticles arranged along one axis. A HRTEM image is given in Fig. 4B, and the magnied TEM image illustrates the intermittent structure more clearly (ESI Fig. S3†). From this it can be seen that the lattice directions of the yellow circles were obviously not along a single axis, further indicating that the intermittent Ag–In–Zn–S nanorods had grown from a number of smaller Ag–In–Zn–S nanoparticles. Fig. 4C gives a TEM of the Ag–In–Zn–S nanorod formation aer 2.5 h, from which it can be seen that the Ag–In–Zn–S nanorods were 2.4 nm in width and 36 nm in length. In addition, it is clear that nearly all the nanorods were curved, which was quite different from the intermittent rod shape. HRTEM images of the curved Ag–In–Zn–S nanorods are shown in Fig. 4D, and it
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will be observed that the rod structures were not straight; the lattice spacing was 0.29 nm, which corresponded to the (101) planes of the hexagonal Ag–In–Zn–S structure. Aer 3 h reaction a mixture of Ag–In–Zn–S straight and curved nanorods was achieved. Fig. 5A shows a TEM image of the mixture, and it was observed that many of the straight Ag–In–Zn–S nanorods (red rectangle) had evolved from the curved rods (yellow circles) as the reaction progressed. Fig. 5B shows a high magnication TEM image, clearly showing a mixture of straight and curved Ag–In–Zn–S nanorods, but the curved rods are in the majority. Aer 4 h reaction nearly all the Ag–In–Zn–S nanorods were straight (Fig. 2A). In our earlier reported results, we used hot injection to obtain AgInS2–ZnS heterodimers with tunable photoluminescence.27–30 In the present study, we changed some of the precursors and obtained unique nanorods, one portion being Ag–In–Zn–S and the other having a higher silver content. The
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(A) TEM images of the mixture of straight and curved Ag–In–Zn–S nanorods; (B) TEM image at higher magnification of the mixture of straight and curved Ag–In–Zn–S nanorods.
Fig. 5
(A) TEM images of the Ag–In–Zn–S nanoparticles; (B) Higher magnification TEM image of nanoparticles; (C) TEM image of nanoparticles at higher resolution; and (D) EDX spectra of the Ag–In–Zn–S nanoparticles.
Fig. 3
(A) TEM images of the Ag–In–Zn–S dimer nanoparticles; (B) TEM images of the Ag–In–Zn–S dimer nanorods (inset: histograms showing the diameter and length distribution of the nanorods); (C) HRTEM image of typical dimers; and (D) a 2D-FFT image of the Ag–In–Zn–S nanorod dimer.
Fig. 6
(A) TEM image of the Ag–In–Zn–S intermittent nanorods after reaction for 2 h; (B) HRTEM image of Ag–In–Zn–S nanorods; (C) TEM image of the Ag–In–Zn–S nanorods (inset: histograms showing the diameter and length distribution of the nanorods); and (D) HRTEM image of a number of typical nanorods.
Fig. 4
TEM results and 2D-FFT images are shown in Fig. 6, from which it can be seen that nearly all the dimer structures have a darker tip, which may be attributed to silver (Fig. 6B). However, it was difficult to verify the composition of such a small area. From the lattice distance and 2D-FFT images of the dimer structure, it may be that one part represents the (101) planes of Ag–In–Zn–S,
11806 | Nanoscale, 2014, 6, 11803–11809
which gives a similar image to the straight Ag–In–Zn–S nanorods. Regarding the other part, the Ag ions could easily form a complex with the surfactant, and the decomposition temperature would then be decreased.34,35 This results in the Ag complex initially decomposing to form a dimer structure. When the ultrasonic time was increased to 2 h the dimer structure became shorter, as seen in Fig. 6A. Nail-like nanorods were synthesized using a drop-injection method. TEM was further used to investigate the detailed structure of the “nail-like” Ag–In–Zn–S nanorods, shown in Fig. 7A. It can be seen that nearly all the nanorods had a naillike aspect, with a thick end and a narrower sharp end (hence our denition, “nail-like”). Furthermore, the nail-like Ag–In–Zn–S nanorods could be well dispersed, implying a highly nanocrystalline character.
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Energy-dispersive X-ray spectroscopy was used to conrm the elementary composition of the Ag–In–Zn–S nail-like nanorods. As shown in ESI Fig. S4,† four signals were found in the spectrum, corresponding to silver, indium, zinc and sulfur. In addition, the X-ray photoelectron spectroscopy (XPS) of the Ag–In–Zn–S nail-like nanorods, shown in ESI Fig. S5,† is consistent with the EDX results. The Ag–In–Zn–S nanorods had diameter 6.0 nm and length 45 nm, further conrmed by high-resolution TEM (Fig. 7B). Fig. 7C shows a typical HRTEM image of the nail-like Ag–In–Zn–S nanorod, and the twodimensional fast Fourier transform (2D-FFT) of the relevant images is given in Fig. 7D–F). As shown in Fig. 2C, the distance between two adjacent planes in the area labeled “a” was 0.19 nm, corresponding to the (110) planes of a close-packed hexagonal structure, and was consistent with the results obtained from the XRD patterns (ESI Fig. S6†). The corresponding FFT image of the nanorods labeled “a” indicated that the [110] direction was aligned with the nanorod axis. Furthermore, the portion labeled “b” in Fig. 2C was measured as 0.19 nm and 0.29 nm, which corresponded to the (110) and
(A) TEM images of the Ag–In–Zn–S nail-like nanorods (inset: histograms showing the diameter and length distribution of the nanorods); (B) HRTEM image of a variety of typical nail-like nanorods; (C), (D) and (E) are 2D-FFT images of the nail-like nanorods of the areas in (C) labeled (a), (b) and (c), respectively.
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(101) planes. The FFT image of “b” in Fig. 7C is also seen in Fig. 7E, and further conrms the preferential growth along the [110] direction. Fig. 3F shows the FFT image of part “c” in Fig. 7C, which displays the (010) planes. The key to the formation of nail-like Ag–In–Zn–S nanorods was drop-by-drop addition of the zinc and sulfur precursors. From the results we reported previously, the radius of the zinc ion is almost the same as that of indium, and a silver ion could diffuse into the AgInS2 nanoparticles at elevated temperature. In our drop-by-drop method, the zinc and sulfur precursors were added more slowly, which was quite different from the allat-once injection method. This allowed the zinc and sulfur to be located at specic positions in the AgInS2 nanoparticles, leading to the formation of nail-like nanorods. Similar results have been reported for Ag2S and Cu2S, used as catalysts in the growth of AgInZn7S9 nanowires.17 In order to further study the effect of Ag–In–Zn–S nanorod PL in the nanocrystals, PL relaxations of the Ag–In–Zn–S straight nanorods were chosen for characterization, as shown in Fig. 8A, which shows representative PL decay curves for straight Ag–In–Zn–S nanorods (emission at 620 nm) at different emission wavelengths. In general, the PL relaxations of the samples were not single-exponential. Aer tting the relaxation plot using the biexponential equation (eqn (1)), the PL relaxation can be separated into two decay components:18,35–39 I(t) ¼ A1 exp(t/s1) + A2 exp(t/s2), (1) where s1 and s2 represent the decay time of the PL emission and A1 and A2 are the amplitudes of the decay components. From
Fig. 7
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(A) Photoluminescence relaxation of the Ag–In–Zn–S straight nanorods; and (B) a table showing the fitting parameters of the relaxation plots. The parameters are derived from the equation: I(t) ¼ A1 exp(t/s1) + A2 exp(t/s2).
Fig. 8
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Fig. 8B it can be seen the lifetime of the straight Ag–In–Zn–S nanorods was as long as 1.248 ms, which is completely different from the results previously reported.35–39 In binary semiconductor nanoparticles, electrons and holes usually combine quickly, and PL lifetimes are only of the order of tens of nanoseconds, due to the direct band gap and the surface states. In the Ag–In–Zn–S nanorods, excitons easily form intrinsic point defects, due to the existence of three types of cations of different size. Due to the large number of intrinsic defects in the Ag–In–Zn–S rod structure, the ratio between radiative recombination induced by surface and intrinsic states decreased, which even became lower than for nanoparticles.35–39 The lifetime of the straight Ag–In–Zn–S nanorods achieved 1.248 ms, which is a signicant improvement compared to the nanoparticles. In this sense, the time-resolved measurements indicated two PL radiative mechanisms in the nanorods, the PL decay time increasing with the PL wavelength. The long-lived excitons of the straight Ag–In–Zn–S nanorods may provide potential applications as photocatalysts and in photovoltaics.38
4. Conclusions We have developed a convenient approach for synthesizing straight Ag–In–Zn–S nanorods 5 nm in width and 100 nm in length. The growth mechanism of the Ag–In–Zn–S straight nanorods is discussed in detail. The photoluminescence of the straight Ag–In–Zn–S nanorods can be tuned from 550 nm to 650 nm by adjusting their composition. The straight Ag–In–Zn–S nanorods demonstrated a long lifetime of 1.248 ms, which may offer potential applications in photocatalysis and photovoltaics. On the other hand, nail-like Ag–In–Zn–S nanorods and rod– particle dimer structures were achieved by adjusting the reaction parameters, and the emissions of the nail-like Ag–In–Zn–S nanorods and rod–particle dimer structures could be tuned by modifying their composition.
Acknowledgements This work was supported by the Hundred Young Talents Plan at Chongqing University (no. 0210001104430 and no. 0210001104431).
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A facile method for the synthesis of quaternary Ag–In–Zn–S alloyed nanorods† Xiaosheng Tang,*a Zhigang Zang,*a Zhiqiang Zu,a Weiwei Chen,a Yan Liu,a Genquan Han,a Xiaohua Lei,a Xianmin Liu,a Xiaoqin Du,a Weimin Chen,a Yu Wangb and Junmin Xuec
Received 12th June 2014 Accepted 4th August 2014
Ag–In–Zn–S nanorods with tunable photoluminescence were formed by a convenient synthetic approach, and the nanorods demonstrated a relatively long fluorescence lifetime of 1.248 ms. In addition, Ag–In–Zn–S
DOI: 10.1039/c4nr03231d
nanorods of nail shape and rod-particle dimers were successfully produced by adjusting the reaction
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parameters.
1. Introduction Semiconductor nanocrystals have attracted considerable attention as a result of their unique electronic and optical properties, allowing their use in the development of light-emitting diodes (LEDs),1–4 solar cells, and biolabels.5–7 In recent decades II–VI semiconductor nanoparticles have been extensively studied for uorescent labeling in biological research as a result of their excellent optical properties.8–10 However, a large number of conventional II–VI semiconductor nanoparticles contain environmentally unfriendly elements such as Hg, Cd and Pb, and this has limited their application.11–15 There is therefore a need for alternative luminescent semiconductor nanocrystals that avoid the use of toxic heavy metals. Ternary I–III–VI2 semiconductor materials, such as AgInS2,16–22 CuInS214,23–25 AgGaS226 and Cu(InxGa1x)S2,27,28 have been widely investigated for their potential use in photovoltaic solar cells, LEDs, non-linear optical devices, and for photocatalytic hydrogen evolution under visible light irradiation. Recently, a number of approaches have been employed to prepare I–III–VI2 nanocrystals using solvothermal methods, decomposition of singlesource methods, and hot injection techniques.16–22 In our previous research a hot injection method was successfully used for the synthesis of CuInS–ZnS and AgInS–ZnS alloy nanoparticles with bright and tunable photoluminescence.29,30 However, most of the research was focused on the synthesis of
the nanoparticles, and the mechanism of the crystallographic growth was not fully explored. The bandgap of semiconductor nanoparticles can be adjusted by varying their size, based on the quantum connement effect and their chemical composition.11,28 In addition, the shape of the nanoparticles can inuence their photoluminescence.28,31–34 For instance, it has been reported that nanorods and nanowires have different optical properties from those of nanoparticles.31–34 There is therefore still a need for a simple way of tuning the shape of semiconductor nanoparticles. Despite the efforts mentioned above to synthesize AgInS2 nanocrystals for strong photoluminescence, further studies are required to develop a convenient synthesis for the preparation of color-tunable Ag–In–Zn–S nanorods. In the present study we have demonstrated a simple route for the synthesis of Ag–In–Zn–S nanorods with tunable emission. In this, Ag–In–Zn–S nanorods with different aspect ratios have been obtained by changing the ratio of the precursors, for example the emission of the resulting Ag–In–Zn–S nanorods can be tuned by adjusting the proportion of Zn. The Ag–In–Zn–S nanorods obtained show adjustable photoluminescence spectra between 550 and 650 nm. Furthermore, the Ag–In–Zn–S nanorods have a much longer lifetime than that of the corresponding nanoparticles, which might avoid disturbing the autouorescence of biological tissues, and suggests its potential application in time-resolved uoroimmunoassay.
2.
Experimental
a
Key Laboratory of Optoelectronic Technology and Systems of the Education Ministry of China, College of Optoelectronic Engineering, Chongqing University, Chongqing 400044, China. E-mail: [email protected]; [email protected]
b
College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China
c Department of Materials Science and Engineering, Faculty of Engineering, National University of Singapore, Singapore
† Electronic supplementary 10.1039/c4nr03231d
information
(ESI)
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available.
See
DOI:
2.1
Materials
Silver nitrate (AgNO3, $98%), silver acetate (AgAc, $98), oleic acid (99.0%), zinc acetate (Zn(Ac)2, $90%), trioctylphosphine (TOP, 90%), technical grade 1-octadecene (ODE, 90%), indium(III) acetate (In(Ac)3, 99.99%), oleylamine (OA, 70%), 1dodecanethiol (DDT, $98%), sulfur (S, 90%) and technical grade zinc stearate, were purchased from Sigma-Aldrich.
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Toluene (99.99%), ethanol (99.9%) and chloroform (99.99%) were obtained from Fisher Scientic. All chemicals were used as received.
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2.2
Preparation of Ag–In–Zn–S nanorods
2.2.1 Synthesis of Ag–In–Zn–S nanorods and dimers. 0.1 mM AgAc, 0.1 mM In(Ac)3 0.1 mM Zn(Ac)2, 5 mM (1.2 mL) DDT, 3 mM (610.5 mg) oleic acid and 4 mL of ODE were placed in a 100 mL three-necked ask and sonicated for 30 min. The reaction mixture was then heated for 30 min at 180 C under nitrogen with magnetic stirring, and 0.3 mM sulfur dissolved in oleylamine at 180 C injected. The temperature was then increased to 210 C. The nanorods obtained were rst washed using absolute ethanol and toluene to remove unreacted precursors, and the washing process was repeated three times. The puried nanorods were then dispersed in toluene or chloroform for storage. 2.2.2 One-pot synthesis of Ag–In–Zn–S nail-like nanorods. Ag–In–Zn–S nail-like nanorods were prepared by thermal decomposition of a mixture of 0.1 mM AgNO3, 0.1 mM In(Ac)3 and 5 mM (1.2 mL) DDT in a 100 mL three-necked ask with 4 mL of ODE, 0.2 mM (56.4 mg) oleic acid, and about 46.5 mL of TOP. The reaction mixture was heated at 60 C for 60 min under nitrogen with magnetic stirring. 0.1 mM zinc stearate and 0.3 mM sulfur powder were dissolved in a 4/1 mixture of oleylamine and ODE at 120 C, and 10 drops of the solution were added to the three-necked ask and allowed to react for 5 min; addition of zinc stearate and sulfur solution was repeated until both of the two precursors were exhausted, and the temperature was then increased to 210 C for 2 h. The nanorods were nally washed and stored as described above. 2.2.3 Characterization. X-ray diffractometry (XRD) was carried out using an Advanced Diffractometer System (D8 Advanced Diffractometer System, Bruker, Karlsruhe, Germany). Transmission electron microscopy (TEM) images were obtained using a JEOL 3010 microscope at an acceleration voltage of 200 kV. Samples were prepared by dipping carbon-coated copper grids into a solution of the sample followed by drying at room temperature. Photoluminescence (PL) spectra were measured in open 1 cm path-length sample containers using a PerkinElmer LS 55 spectrouorimeter.
3.
Results and discussion
Fig. 1 illustrates the XRD pattern of the Ag–In–Zn–S straight nanorods collected. Peaks were observed at 2q of 28.4 , 47.6 and 56.1 , corresponding to the (002), (110) and (112) indices, respectively, of the hexagonal InAgZn2S4 crystal structure (JCPDF 25-0383).16,30 Its composition was conrmed by energydispersive X-ray spectroscopy. As shown in ESI Fig. S1,† signals corresponding to silver, indium, zinc and sulphur were detected. The morphology of the Ag–In–Zn–S samples was measured by TEM, and in Fig. 2A it can be seen that high quality Ag–In–Zn–S nanorods were obtained. The nanorods had a narrow size distribution, with an average width of 5 nm and
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Fig. 1
XRD pattern of Ag–In–Zn–S straight nanorods.
length between 60 and 100 nm. Fig. 2B shows the HRTEM image of part of a single Ag–In–Zn–S nanorod. The lattice of the single Ag–In–Zn–S nanorods may be clearly observed, which indicates a high degree of crystallinity in the nanorod structure. Additionally, the two-dimensional fast Fourier transform (2DFFT) of the relevant images was conducted (Fig. 2C). The distance between adjacent planes in the marked area was 0.29 nm, corresponding to (010) planes of the hexagonal structure, which is consistent with the results obtained by XRD.34,35 The optical behavior of the Ag–In–Zn–S nanorods obtained was characterized. The emission peaks of the nanorods were centered at 550, 602 and 650 nm and their corresponding photoluminescence quantum yields were 38, 42 and 44% (ESI Fig. 2S†). The tunable photoluminescence spectra were adjusted by the ratio of Zn (ESI Table S1†), and veried in the AgInS–ZnS alloy nanoparticle system.29 The corresponding photographs of the Ag–In–Zn–S nanorods under excitation at 365 nm are shown in Fig. 2D (inset), in which the colors of the Ag–In–Zn–S nanorods are tuned from yellow to red. In order to study the mechanism of formation of the Ag–In–Zn–S nanorods in detail, further analysis was carried out, as illustrated in Scheme 1. Fig. 3A shows the samples prepared over 5 min. It can be seen that the size of the Ag–In–Zn–S nanoparticles was 4 nm and the size distribution was quite narrow. A TEM image at higher magnication is shown in Fig. 3B, the nanoparticles being distributed in hexagonal order. The HRTEM shown in Fig. 3C further conrmed the structure of the nanoparticles obtained, and it can be seen the lattice distance was 0.29 nm, which is consistent with the (010) planes of Ag–In–Zn–S.34,35 The EDX spectrum in Fig. 3D conrms the elemental composition of the nanoparticles to be Ag, In, Zn and S, which is consistent with ESI Fig. S1.† A similar system of Cu–Zn–In–S spherical nanoparticles of size about 3 nm could be achieved by sampling, but this did not identify Cu–Zn–In–S nanorods.4 As our process became further developed, however, further oleic acid was introduced into the reaction, resulting in a rod structure. Aer 2 h reaction, Ag–In–Zn–S nanorods were intermittently obtained. Fig. 4A shows the TEM image of the Ag–In–Zn–S
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(A) TEM images of the Ag–In–Zn–S straight nanorods (inset: histograms showing the diameter and length distribution of the nanorods); (B) HRTEM image of a typical straight nanorod; (C) 2D-FFT images of the nail-like nanorods in the red area marked in (B); and (D) PL spectra of Ag–In–Zn–S nanorods with different emissions (inset: photographs of the Ag–In–Zn–S nanorods dispersed in toluene under excitation at 365 nm).
Fig. 2
Scheme 1
Schematic illustration showing the growth of the Ag–In–Zn–S nanorods.
intermittent nanorods, and it was observed that the Ag–In–Zn–S nanorods had achieved a rod shape, but the rod structure was not entirely straight. Under more careful observation, it was seen that the intermittent Ag–In–Zn–S nanorods were formed from a number of nanoparticles arranged along one axis. A HRTEM image is given in Fig. 4B, and the magnied TEM image illustrates the intermittent structure more clearly (ESI Fig. S3†). From this it can be seen that the lattice directions of the yellow circles were obviously not along a single axis, further indicating that the intermittent Ag–In–Zn–S nanorods had grown from a number of smaller Ag–In–Zn–S nanoparticles. Fig. 4C gives a TEM of the Ag–In–Zn–S nanorod formation aer 2.5 h, from which it can be seen that the Ag–In–Zn–S nanorods were 2.4 nm in width and 36 nm in length. In addition, it is clear that nearly all the nanorods were curved, which was quite different from the intermittent rod shape. HRTEM images of the curved Ag–In–Zn–S nanorods are shown in Fig. 4D, and it
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will be observed that the rod structures were not straight; the lattice spacing was 0.29 nm, which corresponded to the (101) planes of the hexagonal Ag–In–Zn–S structure. Aer 3 h reaction a mixture of Ag–In–Zn–S straight and curved nanorods was achieved. Fig. 5A shows a TEM image of the mixture, and it was observed that many of the straight Ag–In–Zn–S nanorods (red rectangle) had evolved from the curved rods (yellow circles) as the reaction progressed. Fig. 5B shows a high magnication TEM image, clearly showing a mixture of straight and curved Ag–In–Zn–S nanorods, but the curved rods are in the majority. Aer 4 h reaction nearly all the Ag–In–Zn–S nanorods were straight (Fig. 2A). In our earlier reported results, we used hot injection to obtain AgInS2–ZnS heterodimers with tunable photoluminescence.27–30 In the present study, we changed some of the precursors and obtained unique nanorods, one portion being Ag–In–Zn–S and the other having a higher silver content. The
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(A) TEM images of the mixture of straight and curved Ag–In–Zn–S nanorods; (B) TEM image at higher magnification of the mixture of straight and curved Ag–In–Zn–S nanorods.
Fig. 5
(A) TEM images of the Ag–In–Zn–S nanoparticles; (B) Higher magnification TEM image of nanoparticles; (C) TEM image of nanoparticles at higher resolution; and (D) EDX spectra of the Ag–In–Zn–S nanoparticles.
Fig. 3
(A) TEM images of the Ag–In–Zn–S dimer nanoparticles; (B) TEM images of the Ag–In–Zn–S dimer nanorods (inset: histograms showing the diameter and length distribution of the nanorods); (C) HRTEM image of typical dimers; and (D) a 2D-FFT image of the Ag–In–Zn–S nanorod dimer.
Fig. 6
(A) TEM image of the Ag–In–Zn–S intermittent nanorods after reaction for 2 h; (B) HRTEM image of Ag–In–Zn–S nanorods; (C) TEM image of the Ag–In–Zn–S nanorods (inset: histograms showing the diameter and length distribution of the nanorods); and (D) HRTEM image of a number of typical nanorods.
Fig. 4
TEM results and 2D-FFT images are shown in Fig. 6, from which it can be seen that nearly all the dimer structures have a darker tip, which may be attributed to silver (Fig. 6B). However, it was difficult to verify the composition of such a small area. From the lattice distance and 2D-FFT images of the dimer structure, it may be that one part represents the (101) planes of Ag–In–Zn–S,
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which gives a similar image to the straight Ag–In–Zn–S nanorods. Regarding the other part, the Ag ions could easily form a complex with the surfactant, and the decomposition temperature would then be decreased.34,35 This results in the Ag complex initially decomposing to form a dimer structure. When the ultrasonic time was increased to 2 h the dimer structure became shorter, as seen in Fig. 6A. Nail-like nanorods were synthesized using a drop-injection method. TEM was further used to investigate the detailed structure of the “nail-like” Ag–In–Zn–S nanorods, shown in Fig. 7A. It can be seen that nearly all the nanorods had a naillike aspect, with a thick end and a narrower sharp end (hence our denition, “nail-like”). Furthermore, the nail-like Ag–In–Zn–S nanorods could be well dispersed, implying a highly nanocrystalline character.
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Energy-dispersive X-ray spectroscopy was used to conrm the elementary composition of the Ag–In–Zn–S nail-like nanorods. As shown in ESI Fig. S4,† four signals were found in the spectrum, corresponding to silver, indium, zinc and sulfur. In addition, the X-ray photoelectron spectroscopy (XPS) of the Ag–In–Zn–S nail-like nanorods, shown in ESI Fig. S5,† is consistent with the EDX results. The Ag–In–Zn–S nanorods had diameter 6.0 nm and length 45 nm, further conrmed by high-resolution TEM (Fig. 7B). Fig. 7C shows a typical HRTEM image of the nail-like Ag–In–Zn–S nanorod, and the twodimensional fast Fourier transform (2D-FFT) of the relevant images is given in Fig. 7D–F). As shown in Fig. 2C, the distance between two adjacent planes in the area labeled “a” was 0.19 nm, corresponding to the (110) planes of a close-packed hexagonal structure, and was consistent with the results obtained from the XRD patterns (ESI Fig. S6†). The corresponding FFT image of the nanorods labeled “a” indicated that the [110] direction was aligned with the nanorod axis. Furthermore, the portion labeled “b” in Fig. 2C was measured as 0.19 nm and 0.29 nm, which corresponded to the (110) and
(A) TEM images of the Ag–In–Zn–S nail-like nanorods (inset: histograms showing the diameter and length distribution of the nanorods); (B) HRTEM image of a variety of typical nail-like nanorods; (C), (D) and (E) are 2D-FFT images of the nail-like nanorods of the areas in (C) labeled (a), (b) and (c), respectively.
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(101) planes. The FFT image of “b” in Fig. 7C is also seen in Fig. 7E, and further conrms the preferential growth along the [110] direction. Fig. 3F shows the FFT image of part “c” in Fig. 7C, which displays the (010) planes. The key to the formation of nail-like Ag–In–Zn–S nanorods was drop-by-drop addition of the zinc and sulfur precursors. From the results we reported previously, the radius of the zinc ion is almost the same as that of indium, and a silver ion could diffuse into the AgInS2 nanoparticles at elevated temperature. In our drop-by-drop method, the zinc and sulfur precursors were added more slowly, which was quite different from the allat-once injection method. This allowed the zinc and sulfur to be located at specic positions in the AgInS2 nanoparticles, leading to the formation of nail-like nanorods. Similar results have been reported for Ag2S and Cu2S, used as catalysts in the growth of AgInZn7S9 nanowires.17 In order to further study the effect of Ag–In–Zn–S nanorod PL in the nanocrystals, PL relaxations of the Ag–In–Zn–S straight nanorods were chosen for characterization, as shown in Fig. 8A, which shows representative PL decay curves for straight Ag–In–Zn–S nanorods (emission at 620 nm) at different emission wavelengths. In general, the PL relaxations of the samples were not single-exponential. Aer tting the relaxation plot using the biexponential equation (eqn (1)), the PL relaxation can be separated into two decay components:18,35–39 I(t) ¼ A1 exp(t/s1) + A2 exp(t/s2), (1) where s1 and s2 represent the decay time of the PL emission and A1 and A2 are the amplitudes of the decay components. From
Fig. 7
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(A) Photoluminescence relaxation of the Ag–In–Zn–S straight nanorods; and (B) a table showing the fitting parameters of the relaxation plots. The parameters are derived from the equation: I(t) ¼ A1 exp(t/s1) + A2 exp(t/s2).
Fig. 8
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Fig. 8B it can be seen the lifetime of the straight Ag–In–Zn–S nanorods was as long as 1.248 ms, which is completely different from the results previously reported.35–39 In binary semiconductor nanoparticles, electrons and holes usually combine quickly, and PL lifetimes are only of the order of tens of nanoseconds, due to the direct band gap and the surface states. In the Ag–In–Zn–S nanorods, excitons easily form intrinsic point defects, due to the existence of three types of cations of different size. Due to the large number of intrinsic defects in the Ag–In–Zn–S rod structure, the ratio between radiative recombination induced by surface and intrinsic states decreased, which even became lower than for nanoparticles.35–39 The lifetime of the straight Ag–In–Zn–S nanorods achieved 1.248 ms, which is a signicant improvement compared to the nanoparticles. In this sense, the time-resolved measurements indicated two PL radiative mechanisms in the nanorods, the PL decay time increasing with the PL wavelength. The long-lived excitons of the straight Ag–In–Zn–S nanorods may provide potential applications as photocatalysts and in photovoltaics.38
4. Conclusions We have developed a convenient approach for synthesizing straight Ag–In–Zn–S nanorods 5 nm in width and 100 nm in length. The growth mechanism of the Ag–In–Zn–S straight nanorods is discussed in detail. The photoluminescence of the straight Ag–In–Zn–S nanorods can be tuned from 550 nm to 650 nm by adjusting their composition. The straight Ag–In–Zn–S nanorods demonstrated a long lifetime of 1.248 ms, which may offer potential applications in photocatalysis and photovoltaics. On the other hand, nail-like Ag–In–Zn–S nanorods and rod– particle dimer structures were achieved by adjusting the reaction parameters, and the emissions of the nail-like Ag–In–Zn–S nanorods and rod–particle dimer structures could be tuned by modifying their composition.
Acknowledgements This work was supported by the Hundred Young Talents Plan at Chongqing University (no. 0210001104430 and no. 0210001104431).
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