CdS

Phosphine-Free, Low-Temperature Synthesis of TetrapodShaped CdS and Its Hybrid with Au Nanoparticles Yaping Du, Bo Chen, Zongyou Yin, Zhengqing Liu, and Hua Zhang*

Tetrapod-shaped CdS colloidal nanocrystals are synthesized using a facile, phosphinefree synthesis approach at low temperature. The arm length and diameter of CdS tetrapods can be easily tuned by using different source of sulphureous precursors, i.e., sulfur powder, thioacetamide, and sodium diethyldithiocarbamate. Moreover, the growth of Au nanoparticles onto CdS to form metal–semiconductor hybrid nanocrystals is also demonstrated. The tetrapod-shaped CdS nanocrystals exhibit strong arm-diameter-dependent absorption and photoluminescence characteristics. Importantly, the as-obtained CdS tetrapods exhibit promising photocatalytic activity for the water-splitting reaction in photoelectrochemical cells.

1. Introduction Tetrapod-shaped II–VI semiconductor colloidal nanocrystals have attracted much attention due to their interesting optical, electronic and mechanical properties,[1,2] and their promising potential applications in solar cells, transistors, field emitters, etc.[3–5] As one of the important wide-band gap II-VI semiconductor nanomaterials, cadmium sulfide (CdS) nanocrystals are of particular interest.[6–8] Although previous reports have focused on the synthesis and property study of

Prof. Y. P. Du, Z. Q. Liu Frontier Institute of Chemistry Frontier Institute of Science and Technology jointly with College of Science Xi’an Jiaotong University 99 Yanxiang Road Yanta District, Xi’an Shaanxi Province 710054, China B. Chen, Dr. Z. Y. Yin, Prof. H. Zhang School of Materials Science and Engineering Nanyang Technological University 50 Nanyang Avenue 639798, Singapore E-mail: [email protected] B. Chen 3 Environmental Chemistry and Materials Group Nanyang Environment and Water Research Institute Interdisciplinary Graduate School Nanyang Technological University 50 Nanyang Avenue, Singapore 639798, Singapore DOI: 10.1002/smll.201402756 small 2014, DOI: 10.1002/smll.201402756

the tetrapod-shaped CdS colloidal nanocrystal,[5,9–12] there are two obvious shortcomings in the current solution-based chemical syntheses. On one hand, most of the used methods involve the use of alkylphosphines, such as trioctylphosphine and tributylphosphine, which react with cadmium and sulfur precursors at high temperature (typically above 250 °C), thus making the whole process non-environmentally benign and noneconomic. On the other hand, most of the synthetic ways used for synthesis of high quality tetrapod-shaped CdS nanocrystals are based on the “seed-growth” approach. In this case, the nuclei are co-injected with the precursors required for growth of the tetrapod-shaped CdS into surfactants that impel the formation of tetrapod shape, and thus the synthesis process is not facile. Therefore, the development of a relative facile and green synthetic route towards tetrapod-shaped CdS nanocrystals that overcomes the aforementioned drawbacks still remains challenge. Herein, for the first time, we report a facile, phosphinefree synthesis approach for preparation of tetrapod-shaped CdS colloidal nanocrystals using CdCl2 and sulphureous precursors in a mixture of alkylamine surfactants at a relative low reaction temperature (90 °C). The arm length and diameter of CdS tetrapods can be tuned by using different sources of sulphureous precursors, i.e. sulfur (S) powder, thioacetamide (TAA), and sodium diethyldithiocarbamate (Na(DDTC)). In addition, the growth of Au nanoparticles (NPs) on the obtained CdS to form metal-semiconductor hybrid nanocrystals is also investigated. Importantly, the tetrapod-shaped CdS nanocrystals show the arm-diameterdependent absorption and photoluminescence characteristics.

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As a proof of concept, the CdS tetrapods are used for water splitting in photoelectrochemical cells, which exhibit promising photocatalytic activity.

2. Results and Discussion The transmission electron microscopy (TEM) images in Figure 1a–c demonstrate the shape of synthesized CdS nanocrystals is tetrapod. Importantly, the sulphureous source plays an important role in formation of tetrapod-shaped CdS nanocrystals with different dimensions of arm length (L) and diameter (D). For example, when the volume of solvent was fixed, if 1.5 mmol of CdCl2 and 4.5 mmol of S powder in oleylamine/octylamine (1:1, v/v) were used for reaction at 90 °C for 16 h, the CdS tetrapod with average L of 21.8 ± 3.9 nm and D of 10.0 ± 1.8 nm, referred to as “CS1”, was obtained (Figure 1a). If S power was replaced with TAA, the L and D values of obtained CdS tetrapod, referred to as “CS2”, are 84.1 ± 9.5 nm and 11.8 ± 0.9 nm, respectively (Figure 1b). Moreover, the replacement of S power with Na(DDTC) resulted in the CdS tetrapod with L of 20.9 ± 2.9 nm

and D of 6.8 ± 1.6 nm, referred to as “CS3” (Figure 1c). The photos in inset of Figure 1a-c show the as-obtained tetrapod-shaped CdS nanocrystals are all readily dispersed and highly stable in organic solvents (e.g. toluene) for more than 6 months, implying the surface of nanocrystals was well capped by alkylamine ligands, which was also confirmed by the FTIR spectra (Figure S1).[13] Figure 1d shows the powder X-ray diffraction (XRD) patterns of the tetrapod-shaped CdS nanocrystals with different arm length and diameter. All of the diffraction peaks from these three samples can be indexed to wutzite type CdS with lattice constant of a = b = 4.121 Å, c = 6.682 Å (hexagonal, JCPDS: 80−0006). Notably, the (002) diffraction peak is sharp in the XRD pattern, suggesting the preferred growth direction of CdS nanocrystals. Figure 2 shows the high-resolution transmission electron microscopy (HRTEM) images of the individual CdS tetrapod with different arm length and diameter. Interplanar spacings of 0.34, 0.33, and 0.34 nm were observed in three arms of the tetrapods, respectively. The lattice spacings were ascribed to the (002) plane of wurtzite CdS, indicating all the arms grow along the directions.

Figure 1. TEM images of tetrapod-shaped CdS nanocrystals with different arm length (L) and diameter (D). L and D are 21.8 ± 3.9 nm and 10.0 ± 1.8 nm (a), 84.1 ± 9.5 nm and 11.8 ± 0.9 nm (b), 20.9 ± 2.9 and 6.8 ± 1.6 nm (c), respectively. Inset: (a-c) top right: photographs of tetrapod-shaped CdS nanocrystals dispersed in toluene; (a) bottom right: structure of a tetrapod-shaped CdS nanocrystal. (d) XRD patterns of tetrapod-shaped CdS nanocrystals.

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Phosphine-Free, Low-Temperature Tetrapod-Shaped CdS and Its AuNP Hybrid

Figure 2. HRTEM images of individual tetrapod-shaped CdS nanocrystals with different arm length (L) and diameter (D), and magnified images of the lattice fringe of the three arms: (a) CS1; (b) CS2; (c) CS3.

The ultraviolet and visible (UV-vis) absorption spectra of three samples (CS1, CS2, CS3) show an excitonic peak (obvious onset of UV-vis) at around 448, 478, 479 nm (Figure 3a), which is blue-shifted from the bulk band gap value (CdS, Eg = 2.4 eV, 517 nm) due to the quantum small 2014, DOI: 10.1002/smll.201402756

confinement in the as-prepared CdS nanocrystals.[6,8,14] Interestingly, tetrapods with comparable arm length but different diameter (e.g. CS1 and CS3) show remarkable differences in their band gap energy, whereas the absorption spectra of tetrapods with comparable arm diameter but different arm length (e.g. CS1 and CS2) are almost identical (Figure 3a), indicating that most of the confinement energy is along the diameter direction of the arm.[4] Their corresponding photoluminescence (PL) spectra show typical narrow direct electron-hole excitonic band-to-band emission bands centered at 497, 497, and 486 nm for the CS1, CS2, CS3 tetrapods, respectively (Figure 3b). A broad and weak emission is observed at around 550–700 nm, which corresponds to the trap-related electron-hole recombination and stems from the surface state emission of the material.[15–17] As a proof-of-concept application, these three samples, i.e. CS1, CS2, CS3, were used as active photoanode materials in photoelectrochemical cells (PECs). As schematically illustrated in Figure 4a, the electron-hole pairs generated when the active layer (CdS tetrapods) of photoanode was irradiated. The electrons were immediately transferred from the CdS tetrapods (working electrode) via the fluorine-doped tin oxide (FTO) on glass substrate towards the Pt counter electrode, where the H+ from water was reduced to generate H2. On the other hand, the remained holes on the surface of CdS tetrapods will oxidize OH− from water to generate O2. In a typical experiment, the working electrode was prepared by spin-coating the CdS tetrapod solution onto FTO substrate. The detailed experiment procedures are described in the Experimental Section.[18,19] Figure 4b and c display the typical SEM images of FTO substrate before and after spin-coating of CS2 sample. The FTO substrate is very rough, where crystalline particles with size range of 100–500 nm were observed (Figure 4b). As shown in Figure 4c, the CdS tetrapods have been coated on FTO, confirmed by energy dispersive X-ray spectroscopy (EDS) spectra (Figure 4d). As compared to the FTO before spin-coating of CS2 sample, the EDS spectrum of CS2-coated FTO, referred to as FTO/CS2, gives two obvious new peaks around 2.3 and 3.2 keV, which can be assigned to S and Cd, respectively (Figure 4d). Figure 4e compares the linear sweep voltammograms of CdS tetrapodbased PECs, in which the photocurrent of 8.9, 31.8 and 22.1 µA cm−2 was obtained in the PECs based on CS1, CS2 and CS3, respectively, at the potential of 0.5 V under same light illumination. As a reference, the FTO showed the negligible photocurrent (0.5 µA cm−2) under the same condition. CS2, which has the longest arm length (84.1 ± 9.5 nm), showed better performance than the other two CdS tetrapods. The higher photoresponse from CS2 might be associated to the longer arm length of the tetrapod, which brings with favorable fewer branch connection points/junctions, which otherwise probably block the transportation of photoexcited electrons/holes by trapping them. As for better performance of CS3 than that of CS1, the higher absorption energy in CS3 results in higherenergy carriers generation after photoexcitation.[3,4] This increases water splitting probability before excited carriers decay, thus enabling a higher performance for CS3.[18] Their corresponding ON/OFF photocurrent is shown in Figure 4f, where the photocurrent under the illumination significantly

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Figure 3. (a) UV-vis and (b) PL spectra of tetrapod–shaped CdS nanocrystals with different arm diameter and length.

increased to a higher level (ON state), as compared to the current under dark (OFF state). Consistent with the curves of photocurrent versus potential in Figure 4e, the ON/OFF ratio of CS2-based PEC is about 2.2 and 1.5 times higher than that of CS1- and CS3-based devices, respectively. The construction of heterostructured hybrid metal-semiconductor nanocrystals through wet-chemistry method has been paid great attention, since these hybrid nanocrystals possess the unique optical, electrical, magnetic, and catalytic properties that extend beyond those of single-component counterparts.[20–34] As well known, a common strategy for preparation of such nano-architectures

Figure 4. (a) Schematic representation of PEC with CdS tetrapods on FTO and commercial Pt wire as photoanode and photocathode, respectively. e−: electron, h+: hole. (b) SEM images of FTO substrate. Inset: high-resolution SEM image. (c) SEM images of FTO coated with CS2. Inset: highresolution SEM image. (d) EDS spectra of FTO and FTO/CS2. (e) Linear sweep voltammograms for PECs with different working electrodes, i.e. FTO/CS1, FTO/CS2 and FTO/CS3, with glass/FTO as reference. (f) Amperometric I-t cycles with potential of 0.5 V for PEC with different working electrodes, i.e. FTO/CS1, FTO/CS2 and FTO/CS3.

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Phosphine-Free, Low-Temperature Tetrapod-Shaped CdS and Its AuNP Hybrid

Figure 5. (a) TEM image, (b) HRTEM image and (c) HAADF-STEM image of CSA1 hybrid nanocrystals. Inset in (a): photograph of CSA1 dispersed in toluene. (d) XRD patterns of hybrid nanocrystals of CSA1, CSA2, and CSA3.

is to synthesize the semiconductor nanoparticles and then to proceed with heterogeneous nucleation and growth of the metal counterparts via a deposition process.[20,23,24,31] In this work, we also investigated the Au nanocrystals grown on the CdS tetrapods to form the Au-CdS hybrid nanocrystals. We referred to hybrid nanocrystals as CSA1, CSA2 and CSA3 by using CS1, CS2 and CS3, respectively, to grow Au nanoparticles (NPs). Figure 5a shows the TEM images of as-obtained CSA1 hybrid nanocrystals synthesized from the seed growth method, where some tetrapod-shaped CdS nanocrystals were dis-assembled into nanorods after ∼4 nm Au NPs were grown on their surface. Further characterization was shown in the HRTEM image (Figure 5b), proving that both CdS nanorods and Au NPs are single crystalline. The lattice spacings measured from the HRTEM in Figure 5b match very well with the corresponding bulk values for the (002) planes of hexagonal CdS and the (111) planes of face-centered cubic Au. Figure 5c shows the highangle annular dark field-scanning TEM (HAADF-STEM) image of Au-CdS nanorods with small Au NPs attached to CdS nanorods. The lighter and darker areas represent the Au NPs and CdS, respectively. Figure 5d shows the XRD pattern of as-obtained hybrid nanocrystals. The CdS component remains the wurtzite structure after Au NPs are grown small 2014, DOI: 10.1002/smll.201402756

onto the nanorods. The additional peaks appeared in the XRD spectra are corresponding to Au, and no other species such as Au2S exist after the reaction. By using the same seed growth method, Au NPs were also deposited onto CS2 and CS3 to form the corresponding CSA2 and CSA3 hybrid nanocrystals, respectively (Figure 6). As indicated in Figure 3a, the tetrapod-shaped CdS nanocrystals exhibit a strong peak in the absorbance at 479, 478, 448 nm corresponding to the first exciton transition. The absorbance peaks of the three Au-CdS hybrid nanocrystals, i.e.CSA1, CSA2 and CSA3, located at 469, 467, 435 nm, respectively (Figure 7a). Compared with the pure CdS nanocrystals, the exciton peak has clearly shifted to higher energies, and this blue-shift phenomenon could be due to the overlap of the electronic states of the different components of the hybrid nanocrystals which modifies the surface plasmon resonance.[35–37] The photoluminescence (PL) spectra of hybrid nanocrystals of CSA1, CSA2, and CSA3 are shown in Figure 7b. The PL peaks at 486 and 497 nm of pure CdS nanocrystals (Figure 3b) are quenched after growth of Au NPs onto the CdS nanorods, possibly owing to the intimate contact of the CdS with Au, facilitating the transfer of photo-generated electrons from the semiconductor to the metallic component.[36,38–40]

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Figure 6. (a) TEM and (b) HAADF-STEM images of CSA2 hybrid nanocrystals in which the size of Au NPs is ∼8 nm. Inset in (a): photograph of CSA2 dispersed in toluene. (c) TEM and (d) HAADF-STEM images of CSA3 hybrid nanocrystals in which the size of Au NPs is ∼3 nm. Inset in (c): photograph of CSA3 dispersed in toluene.

3. Conclusions For the first time, we report a facile, phosphine-free synthesis approach for preparation of tetrapod-shaped CdS nanocrystals at low temperature. The arm length and

diameter of CdS tetrapods can be simply tuned by changing the sulphureous precursors. The tetrapod-shaped CdS nanocrystals exhibited the strong arm-diameter-dependent absorption and photoluminescence characteristics. Moreover, the growth of Au nanoparticles onto CdS to form hybrid nanocrystals was also demonstrated. As a proof-of-concept application, the synthesized CdS tetrapods were used for water splitting in PEC. This work opens the way to develop a cost-effective and eco-friendly colloidal strategy for the synthesis of tetrapod complex nanostructures, which might have promising applications in clean energy and optoelectronics.

4. Experimental Section

Figure 7. (a) UV−vis absorption and (b) PL spectra of the hybrid nanocrystals of CSA1, CSA2, and CSA3.

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Chemicals: Cadmium chloride (CdCl2, 99.995%, Sigama-Aldrich), sulfur powder (S, Sigama-Aldrich), thioacetamide (TAA, SigamaAldrich), sodium diethyldithiocarbamate

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Phosphine-Free, Low-Temperature Tetrapod-Shaped CdS and Its AuNP Hybrid

(Na(DDTC), Sigama-Aldrich). Oleylamine (OM; >80%, SigamaAldrich), Octylamine (OTA; 99%, Sigama-Aldrich), ethanol (AR) and toluene (AR) were used as received without further purification. Synthesis of Tetrapod-Shaped CdS Nanocrystals: A typical experimental procedure is described as follows: 1.5 mmol of CdCl2 was added into the mixture of 5 mL of OM and 5 mL of OTA in a three-necked flask (100 mL) at room temperature. The obtained slurry was heated to 100 °C to remove water and oxygen with vigorous magnetic stirring under vacuum for ∼30 min in a temperaturecontrolled electromantle. Then the solution was reacted at 120 °C for 2 h to form an optically transparent solution. After that, the dispersion of sulfurous source (S, TAA, or Na(DDTC)), formed by ultrasonication of 4.5 mmol of sulfurous source powder in a mixture of 2.5 mL of OTA and 2.5 mL of OM at room temperature, was quickly injected into the aforementioned optically transparent solution at 90 °C. The resulting mixture was kept at 90 °C for 16 h to give a yellow solution. After the solution was cooled down to room temperature, the CdS nanocrystals were precipitated by adding excess absolute ethanol into it. The obtained CdS nanocrystals were easily redispersed in various apolar organic solvents (e.g. toluene). Growth of Au Nanoparticles on CdS Nanocrystals: Briefly, 50 mg of tetrapod-shaped CdS powder and 5 mL of oleylamine were degassed in vacuum at 100 °C for 10 min (the solution became yellow and clear) and subsequently cooled down to 50 °C. After that, 10 mg of HAuCl4 dispersed in oleylamine were injected into the aforementioned solution, and the resulting mixture was kept at 120 °C for 30 min. Then, the obtained hybrid nanostructures were precipitated by adding excess absolute ethanol into the reacted solution. The obtained hybrid nanostructures were dispersed in toluene for further characterization. Fabrication of photoelectrochemical cell (PEC) Devices: The fabrication processes are similar to those in our previous reports.[18,19] Typically, FTO glass substrates (1 cm × 2 cm) were cleaned with acetone and water, and then dried with nitrogen blow. To prepare the PEC device, the as-prepared CdS tetrapods were first dispersed in 1 mL toluene with concentration of 1 mg mL−1 followed by sonication. After that, the solution was deposited onto the surface of FTO substrate by spin-coating (acceleration rate: 1,000 rpm sec−1; rotation speed: 3,000 rpm), which was used as the photoanode layer. Finally, the obtained CdS/FTO sample was annealed in Ar at 350 °C for 1 h before it was used for water splitting experiments. Characterization. The powder X-ray diffraction (XRD) patterns of dried powders were recorded on Bruker D8 diffractometer (German) with a slit of 1/2° at a scanning rate of 2° min−1, using Cu Kα radiation (λ = 1.5406 Å). The lattice parameters were calculated with the least-squares method. Samples for transmission electron microscopy (TEM) analysis with a JEOL 2100F (Japan) operated at 200 kV were prepared by drying a drop of nanocrystal dispersion in toluene on amorphous carbon-coated copper grids. Samples for field emission scanning electron microscopy (FESEM, JSM-7600) coupled with energy dispersive X-ray spectroscopy (EDS) analysis were prepared by drying a drop of nanocrystal dispersion in toluene on silicon substrate. The fluorescence spectra of nanocrystals were obtained on a Shimadzu/RF-5301PC fluorescence spectrometer (Japan) at room temperature. The UV-vis absorption spectra of nanocrystals were obtained on a Shimadzu/ UV-1800 UV-vis spectrometer (Japan). The Fourier Transform Infrared (FTIR) absorption spectra of nanocrystals were carried on Perkin Elmer/Spectrum GX FTIR Spectrometer (USA). small 2014, DOI: 10.1002/smll.201402756

Water Splitting Experiments: All the water splitting experiments were carried out in the ambient conditions. To measure the photocurrent, an electrochemical workstation (CHI600C) was used with a three-electrode configuration, i.e. the prepared CdS/FTO, Ag/AgCl and Pt-wire used as the working, reference and counter electrodes, respectively. The 150 W Halogen lamp light source (DOLAN JENNER Model 150 Illuminator) was used to provide the illumination on samples. The light intensity is 400 mW cm−2 measured by Newport Power Meter (Model 1918-R) at the wavelength of 500 nm. The buffer solution was 0.1 mmol mL−1 KH2PO4 aqueous solution with pH = 7.0.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements Y. P. Du and B. Chen contributed equally to this work. We gratefully acknowledge the financial aid from the start-up founding from Xi'an Jiaotong University and the NSFC (Grant No. 21371140). This work was supported by MOE under AcRF Tier 2 (ARC 26/13, No. MOE2013-T2–1–034), AcRF Tier 1 (RG 61/12, RGT18/13 and RG5/13), and Start-Up Grant (M4080865.070.706022) in Singapore. This Research is also conducted by NTU-HUJ-BGU Nanomaterials for Energy and Water Management Programme under the Campus for Research Excellence and Technological Enterprise (CREATE), that is supported by the National Research Foundation, Prime Minister’s Office, Singapore.

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small 2014, DOI: 10.1002/smll.201402756

Phosphine-free, low-temperature synthesis of tetrapod-shaped CdS and its hybrid with Au nanoparticles.

Tetrapod-shaped CdS colloidal nanocrystals are synthesized using a facile, phosphine-free synthesis approach at low temperature. The arm length and di...
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