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Letter
Au Nanowires Striped Cu3P Platelet Photoelectrocatalysts Anirban Dutta, Aneeya Kumar Samantara, Samrat Das Adhikari, Bikash Kumar Jena, and Narayan Pradhan J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b00341 • Publication Date (Web): 03 Mar 2016 Downloaded from http://pubs.acs.org on March 5, 2016
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The Journal of Physical Chemistry Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
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Figure 1. (a) Schematic presentation of stripy pattern growth of Au nanowires on Cu3P platelets. (b) Atomic model presenting Au wire growth on Cu3P platelet.
Figure 2. (a−c) TEM images and (d, e) HAADF-STEM images showing Au nanowires stripped on Cu3P platelets. (f−h) Elemental mapping results of Au, Cu, and P from the HAADF-STEM image e. This has been carried out in Mo microscopic grid. Scale bar, 50 nm. 76 77 78 79 80 81 82 f1
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For the stripy growth, Cu3P platelets were first prepared following our previously reported synthetic protocol44 where PH3 was used as phosphide source. Before these platelets were harvested, a solution of Au3+ precursor was injected to the reaction flask at 200 °C. Within seconds, a stripy growth of Au nanowires on Cu 3P platelets was obtained. Schematic presentation of the formation protocol of these heterostructures is presented in Figure 1a, and the synthetic procedure is provided in the Supporting Information. The large area interface between Au nanowires on a Cu3P platelet along with their crystallographic orientations are shown in Figure 1b. Figure 2 presents microscopic images of these intriguing nanostructure shapes. Transmission electron microscopy (TEM) images of Au−Cu3P heterostructures are shown in Figure 2a−c, and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images are shown in Figure 2d,e (and Figure S1a). A HAADF-STEM image showing typical high dense nanowires grown on Cu3P is shown in Figure S1b, and a magnified image is presented in Figure S1c. These images suggest that nanowires were nucleated from the edges and continued to grow inside the platelets. The diameter of these nanowires was found to be ∼3.5 nm, and these were grown in both sides of each platelet. To confirm Au nanowires on Cu3P platelets, elemental mapping was carried out in a dark field image. Figure 2f−h
shows the mapping results obtained from image Figure 2e, and these confirmed the presence of Au, Cu, and P throughout the platelets. To further supporting the characterization of the heterostructures, powder X-ray diffraction was carried out, and the pattern is presented in Figure S2. From the peak positions, presence of both Cu3P and Au were confirmed. The phase of Cu3P remained hexagonal as reported previously,44 and Au was observed to be cubic. Furthermore, high-resolution TEM (HRTEM) imaging of the heterostructured platelets was carried out to understand the epitaxial relation between Au and Cu3P. The image is presented in Figure 3a, and the selected area fast Fourier transform (FFT) pattern is shown in Figure 3b. From the analysis, it was observed that the (002) plane of cubic Au overlaps with the (030) plane of hexagonal Cu3P. An atomic model showing similar alignment is shown in Figure 1b. However, a tilted view along a different viewing direction is presented in Figure 3c for clarity. This arrangement also indicates that the {002} plane of Au aligns with the {030} of Cu3P, and in both cases the d-spacing is 0.20 nm. Powder X-ray diffraction patterns also show both these peaks are in the same position (Figure S2). Hence, the lattice mismatch along these two sets of planes is almost zero, and perhaps this drives the fast growth of Au on Cu3P. For understanding the origin of this rare stripy-like Au growth on the Cu3P platelets, intermediate samples were B
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heterostructures obtained in the presence of excess TOP. Hence, this suggests that TOP, which has a strong binding ability on the semiconductor surfaces, has a definitive role in controlling the growth of Au. However, because a limited amount of TOP is required for such patterning growth, it can be assumed here that these strong binding ligands create an obstacle for the flat growth. Hence, the wires are grown in zigzag or stripy pattern on the surface of the semiconductor platelets. A schematic model showing ligand obstacles and movement of nanowire growth is shown in Figure 5.
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Figure 3. (a) HRTEM image of a Au nanowire striped Cu3P platelet. (b) Selected area FFT pattern from the area marked in panel a. (c) Atomic model showing attachments of Au on Cu3P platelets. Au is viewed along [220] and Cu3P along [001]. 126 f4
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collected and analyzed. The TEM images of the sample collected within 5 s of Au injection are shown in Figure 4 (Figure S3), and these confirmed that the stripy growth nucleated at the edges of the platelets and gradually proceeded toward the center. To further understand the growth mechanism of such heterostructures and the role of phosphine, several controlled experiments were performed. Rather than introducing Au3+ directly to the as-synthesized Cu3P platelets, when it was added to a purified and redispersed system, no stripy growths were observed. Even purging PH3 gas into the same reaction did not initiate the nanowire growth. Hence, all this information suggests that for stripy growth, PH3-mediated synthesis of Cu3P and subsequent injection of Au3+ precursor are required. This further indicates that PH3 gas adsorbed on the surface of Cu3P during the synthesis helped the reduction of Au3+ on the platelets. Apart from this PH3 adsorption, to understand the role of any other ligands in the stripy growth, we have investigated the reaction system carefully. Here, oleylamine was used as solvent and trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP) were used for controlling the growth of Cu3P. No change in stripy growth was observed upon increasing the amount of TOPO after the formation of Cu3P. However, an increase in the amount of TOP twisted the growth pattern; instead of nanowires, dots were randomly decorated on the platelets. Figure S4 shows the TEM images of the Au−Cu3P
Figure 5. Schematic presentation of stripy pattern growth of Au nanowires. The growth is shown in the presence of TOP. Stripes of Au nanowires are shown in golden color, and ligands are marked in green standing bars.
Both Au and Cu3P have visible absorption, and the optical properties of the Au nanowire-striped Cu3P heterostructures were further investigated. Figure 6a shows the absorption spectra for Cu3P disks and also Au−Cu3P nanostructures. For the case of Cu3P, both bandedge and surface plasmon peaks were present; but for the Au nanowire-striped Cu3P, a broad peak was observed. This might be due to dominated surface plasmon of the dense Au nanowires. These heterostructures were further explored for photoelectrochemical sensing of NADH using cyclic voltammetry. For the experiment, purified nanostructures were first ligand exchanged by treating with 1,2ethandithiol, embedded with nafion, and then drop casted on a glassy carbon electrode. For comparison, Cu3P platelets without Au were also measured. Details of the experimental procedures are provided in the Supporting Information. Panels b and c of Figure 6 present the cyclic voltammograms of NADH oxidation in the presence and absence of light for Cu3P and Au−Cu3P modified electrodes, respectively. It was observed that, in the absence of light, the analyte NADH was oxidized at a higher potential for both Cu3P and Au−Cu3P. However, in the presence of light, the overpotential is
Figure 4. TEM images of the samples collected within 5 s of reaction at different resolution. C
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Figure 6. (a) Absorption spectra of Cu3P and Au nanowire-striped Cu3P heterostructures. (b, c) Cyclic voltamograms of oxidation of NADH (0.5 mM) in presence and absence of light using Cu3P and Au−Cu3P modified photoanodes, respectively. (d) Photoresponse study of Au−Cu3P in absence and presence of NADH (0.5 mM). (e) Amperometric current density versus time plot for the detection of NADH by the Au−Cu3P modified electrode; 5 μM NADH was added to 0.1 M PBS at a regular interval of time marked by the arrows. Polarization potential: 0.54 V (vs Ag/ AgCl). (f) Corresponding calibration plot. 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217
substantially reduced for both the cases. Importantly, these heterostructures (Au−Cu3P) oxidized the NADH at 140 mV lower overpotential with higher current density compared to only Cu3P in the presence of light. These results suggest that the Au−Cu3P shows the better catalytic activity toward the oxidation of NADH in the presence of light. This photoelectrocatalytic response was further verified by the AC impedance measurement to deduce the charge-transfer ability of the catalyst. The Nyquist plot for Cu3P and Au−Cu3P in the presence of light is presented in Figure S5. The lower chargetransfer resistance of Au−Cu3P indicates it has an enhanced photoelectrocatalytic performance compared to Cu3P. This enhanced activity was further tested for sensing NADH. This has been carried out toward the photocurrent measurement under the on−off illumination mode of light at the oxidation peak potential of NADH (0.54 V vs Ag/AgCl). Figure 6d shows the photoresponse activity without NADH and with NADH. Intriguingly, the Au−Cu3P delivered a more than 30fold enhancement in current density in the presence of NADH compared to its absence. We further employed the chronoamperometric measurement for evaluating the performance of Au−Cu3P toward the detection of NADH. Because the real application of any transducer requires a constant potential amperometric measurement, we have performed the amperometric analysis with a constant potential of 0.54 V (vs Ag/AgCl). The data was recorded at a regular interval of time (Figure 6e). A rapid increase in the current was observed on subsequent addition of NADH to the supporting electrolyte. The measured current shows a linear response (R2 = 0.999) to the increase of concentration of NADH (Figure 6f). The above observation demonstrates the excellent photoresponse of the Au−Cu3P modified electrode toward NADH. Therefore, these heterostructures can find promising applications as highly active photosensors of interest.
Because Au is a metal and has plasmon, coupling with the semiconductor Cu3P facilitates the suppression of the exciton recombination.2,4,8,45,46 As a result, the lifetime of the electron in the excited state increases. Therefore, also the hole resides for more time in the conduction band of Cu3P. This helps NADH collect these holes and aids in oxidization. A similar mechanism has already been established for nonhetero nanostructures.47−51 In conclusion, a very unique stripy patterned growth of thin Au nanowires on Cu3P platelets is reported. While metal− semiconductor heterostructures are mostly restricted to a minimized interface area, these Au−Cu3P nanostructures are observed with a wide area epitaxial interface. The role of adsorbed phosphine for triggering such rare growth is also discussed. Finally, as these heterostructures have absorption in the visible window, they are explored as photoanode materials for catalytic oxidation of NADH, and their high photosensitivity is reported. While heterostructures are studied extensively for photocatalytic electron-transfer reactions, these results suggest that on proper combination of materials, the heterostructures can also act as a good photoanode for triggering various holetransfer-induced chemical reactions.
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ASSOCIATED CONTENT
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b00341. Experimental section and supporting figures (DOC)
AUTHOR INFORMATION
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*N.P.: e-mail,
[email protected] *B.K.J.: e-mail,
[email protected]. D
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS DST of India (SR/NM/NS-1383/2014(G)) is acknowledged for funding.
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