Au2 S x /CdS Nanorods by Cation Exchange: Mechanistic Insights into the Competition Between Cation-Exchange and Metal Ion Reduction

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1 Heterostructures Au2 S x /CdS Nanorods by Cation Exchange: Mechanistic Insights into the Competition Between Cation-Exchange and Metal Ion Reduction Subhajit Kundu, Paromita Kundu, Gustaaf Van Tendeloo, and N. Ravishankar * Semiconductor-based heterostructures find applications in photovoltaics, hydrogen generation, catalysis, labeling and sensing. [1 11 ] While several routes are available for the synthesis of such heterostructures, cation exchange is one of the most favored methods. [12 15 ] The primary advantage of cation exchange is that the morphology of the parent phase may be retained leading to possibility of synthesis of anisotropic shapes that do not conform to the point group of the material. Additionally, kinetically controlled conditions extend its utility to create materials with metastable composition and phases. [12,16 ] Preservation of anionic framework during cation exchange [17,18 ] enables synthesis of multicomponent heterostructures with enhanced level of control over morphology and chemistry. [19 ] Site specificity of the exchange reaction gives rise to several interesting possibilities of nanostructuring including Ag 2 S bar-coding in CdS, [20 ] formation of Cu 2 S on the tip of CdS nanorods [21,22 ] and multipodal structures. [23 ] In spite of its significant advantages, one of the primary limitations of cation exchange is that the metals with higher electron affinity tend to undergo reduction rather than cation exchange, thus making the process very system specific. [20 ] This competition between cation exchange and reduction is not well understood. Using CdS/Au as a model system, we show that cation exchange is feasible even in the case of metals with higher electron affinity like Au. We use thermodynamic arguments to show that the process depends on a delicate balance between the formation energy of the product sulfide and the by-products of cation exchange as compared to the free energy for the reduction of the metal. While thermodynamics can predict that the sulfide can indeed form, the experimental realization depends on the kinetics of the process. By employing advanced transmission S. Kundu, Prof. N. Ravishankar Materials Research Centre Indian Institute of Science C.V. Raman Avenue Bangalore , India nravi@mrc.iisc.ernet.in Dr. P. Kundu, Prof. G. V. Tendeloo Electron Microscopy for Materials Research (EMAT) University of Antwerp Groenenborgerlaan 171, 2020, Antwerp, Belgium electron microscopy (TEM) techniques, we confirm that cation exchange does indeed take place in the CdS/Au system leading to the formation of Au 2 S x (x = 1 or 3). The stability of the product, however, depends on the external driving force available. Under conditions where the system is able to overcome the kinetic barrier (under electron beam irradiation, for instance), we observe the conversion of the metastable Au 2 S x to stable Au. The observations and analysis have interesting and important implications for the design and choice of systems for the formation of multifunctional nanoscale heterostructures using cation exchange. CdS nanorods used for the study were prepared by a modified literature method. [24] The as-grown nanorods have a uniform morphology and are wurtzite in phase with an average diameter of 10 nm and a length of 100 nm. To study the feasibility of cation exchange, different amounts of these CdS nanorods were dispersed in 20 ml of de-ionized water and 10 ml (1.8 m m ) HAuCl4 solution was added. An immediate change of color of the CdS powder from bright yellow to brownish yellow was observed with progressive darkening of the yellow color over an hour. To investigate the change, UV visible absorbance spectra ( Figure 1a) of the centrifuged supernatant of reaction were acquired after an hour that shows a significant drop in the concentration of HAuCl 4. For continuous monitoring of the change in the supernatant, the consumption of HAuCl 4 was studied as a function of time (inset). The decay kinetics of the 284 nm peak that corresponds to the ligand-to-metal charge transfer peak of HAuCl 4 was monitored [25,26] which indicates a significant reduction in the rate of decay with time. To facilitate a faster completion of reaction, the product was irradiated with a microwave at 800 W for 3 min that results in a fall in the HAuCl 4 concentration below the detectable limit as indicated by the absorbance plot. Figure 1 b shows the SEM-EDS data of bare CdS and the resulting product after reaction with HAuCl 4 showing the S, Cd and Au atomic percent in each case. The S:Cd ratio in as-synthesized CdS is nearly 1:1 indicating stochiometric CdS nanorods. The S:Cd ratio is higher after reaction with HAuCl 4 but the S:(Cd+Au) ratio is maintained at 1:1. This observation indicates that Au is incorporated in the CdS lattice replacing Cd indicating a possible cation exchange reaction. Energy-dispersive X-ray analysis in the SEM of the supernatant on drying shows the Cd:Cl:S atomic percent ratio to be 30:62:8. The Cd:Cl ratio indicates the formation of wileyonlinelibrary.com 1

2 communications S. Kundu et al. Figure 1. a) UV visible absorbance spectra of supernatant, after addition of HAuCl 4 (HA) to an aqueous dispersion of CdS nanorods, recorded before and after irradiation with microwave (MW). It shows a sharp decrease in concentration of HA in the supernatant on mixing which further drops after MW irradiation. Inset shows the kinetics of the decay in concentration of HA. b) SEM-EDS of the hybrid samples with different loadings of gold, indicating a possible cation exchange. c) XRD data of the CdS-Au 2 S x showing peaks for cubic Au 2 S phase. d) XPS confirming the presence of +1 and +3 oxidation state of gold along with Au 0. CdCl 2 further supporting the replacement of Cd in the CdS. Expulsion of a small amount of S from the CdS lattice is also seen. In addition to the Na 2 S solution, the colorless supernatant yields a yellow CdS precipitate indicating the removal of the Cd 2+ ions from the rods, further supporting the hypothesis that Cd ions in the nanorods are exchanged by Au ions. XRD data of the samples after reaction show the presence of peaks corresponding to the wurtzite phase of CdS. Additional peaks due to the formation of Au 2 S are not clearly seen (Figure 1 c) in samples with low loading of Au ( 10%). However at higher loading ( 22% and 39%) of Au the peak due to Au 2 S [27 ] becomes more prominent confirming cation-exchange. To confirm the formation of sulfide of Au, core-level X-ray photoelectron spectroscopy (XPS) was carried out. Au4f spectra (Figure 1 d) show the presence of Au 1+ [28 ] and Au 3+ in addition to Au 0. The absence of a significant peak in the survey around 200 ev due to Cl2p eliminates any possibility of chloride of Au. EDS observations clearly indicate a 1:1 replacement of Cd ions by Au. Based on the charge balance, it is clear that 2 Cd 2+ ions are replaced by one Au 1+ and one Au 3+ ion leading to both site and charge balance. Based on the XPS analysis, we observe that the ratio of (Au 0 + Au 1+ ):Au 3+ is 1:1 indicating that some of the Au 1+ ions on the surface are converted to the Au 0 state. Au2 S 3 formation is not detected in XRD due to its amorphous nature as reported earlier. [29] Cd3d spectra of CdS (Supporting Information, Figure S1e) shows the presence of S 2 and OH type species. An additional peak around 409 ev and 415 ev appears (Supporting Information, Figure S1f) for the hybrid sample which may be due to the emergence of dangling bonds of Cd as has been reported previously. [30] This is possibly due to the presence of an excess amount of Au 3+ at the surface. Two Au 3+ ions can bond with three S 2 ions replacing two Cd 2+ ions while the third Cd 2+ bond remains unsatisfied. This is also consistent with the fact that some of Au 1+ ions on the surface are converted to Au 0. The morphology of the nanorods remains intact after cation exchange as shown by the bright-field TEM image ( Figure 2 a); it also shows the presence of Au nanoparticles of 2 3 nm on the surface. However on careful observation, we find that more of these particles form under the electron 2

3 Mechanistic Insights into the Competition Between Cation-Exchange and Metal Ion Reduction Figure 2. a) BF-TEM image of a low-loading CdS-Au 2 S x sample. b) HRSTEM image shows a faceted Au particle, bound by {111} and {001} planes, on CdS rod in [002] growth direction. c) HAADF-STEM image and elemental maps of ultra-high loading sample reveals the presence of Au 2 S x region along with CdS and Au regions. d) Same region after exposing to electron beam shows the transformation of Au 2 S x to Au as marked by the red circles. beam. In case of low loading, the Au 2 S x formation is mostly at the surface of the CdS rods and the nucleation burst takes place due to reduction under the electron beam, which results in faceted fine particles of Au as is evident from the highresolution high angle annular dark field (HAADF) image acquired in scanning transmission (STEM) mode (Figure 2 b). Such beam effect is highly accentuated(supporting Information, Movie 1,2) in the case of higher loading where the metastable Au 2 S x formation is across the depth of the CdS rods (HAADF micrograph in Figure 2 c,d). The possible reason for non-uniform distribution, size and shape of the Au particles could be due to the diffusion of Au 0 from the bulk of the rod to the surface leading to uncontrolled growth of the formed nuclei as well as migration and aggregation of the juxtaposed particles on the surface. To further confirm the presence and distribution of Au 2 S/Au 2 S 3 phase in the CdS nanorods energy dispersive X-ray (EDS) mapping in the STEM mode has been carried out on the ultra-high loading sample as given in Figure 2 c. It is evident that S is distributed uniformly throughout the rod; Cd is concentrated mostly on the places of lighter contrast in correspondence to the HAADF image and Au is specifically present in the brighter contrast region. Therefore this implies that under high percentage replacement the anionic sites remain conserved and only the cation gets replaced, as reported earlier. [17 ] The effect of the electron beam has also been studied for these samples; the same specimen region was exposed for a few minutes under a focused electron beam. Figure 2 d shows the low magnification HAADF-STEM image and the elemental maps acquired immediately after exposing which confirms the growth of the Au particle at the tip of the rod as marked in the figure. A similar transformation has been shown at different times in Figure 3. Using high resolution HAADF-STEM, heavier and lighter atomic columns can be clearly distinguished which makes the images directly interpretable which is otherwise difficult by HRTEM. [31,32 ] Therefore, we exploit this mode of electron microscopy to reveal the phase distribution. The HAADF-STEM image in Figure 4 a clearly shows three different regions with different contrast as bound by the dotted lines. The HRSTEM image in Figure 4 b confirms that the lighter contrast region corresponds to CdS and the brighter contrast region corresponds to the cubic Au 2 S phase imaged along the [110] zone. It shows that a portion of rod remains 3

4 communications S. Kundu et al. Figure 3. Transformation of the CdS-Au 2 S x sample under converged electron beam. intact as CdS with [002] growth direction and a part is converted to cubic Au 2 S, grown in the [110] direction along the rod. The Au 2 S phase gets reduced to Au under focused electron beam exposure as evident from the HAADF-STEM image (Supporting Information, Figure S2a,b). The free energy of formation of gold sulfides by cation exchange reaction as a function of temperature is shown in Figure 5 a (details provided in Supporting Information). The Gibbs free energy change of formation (ΔG) for both Au 2 S/ Au 2 S 3 is positive which indicates that the formation of these sulfides is not thermodynamically feasible. However, due to a highly favorable formation energy of the by-product CdCl 2 on cation exchange, the overall reaction is favorable in this case. According to calculations, the formation of Au 2 S 3 is thermodynamically more favorable than the formation of Au 2 S. To compare this with the competing reaction of metal ion reduction, we also evaluated the free energy change for the formation of Au with water acting as the reducing agent in the medium by a method reported in literature. [33 35 ] We note that this free energy change is negative indicating that water can indeed reduce Au salt to metal at room temperature. However, our control Figure 4. a) HAADF-STEM image clearly shows the presence of CdS, Au 2 S and Au phase as indicated by regions of different contrast bound by blue dotted lines. b) HRSTEM of a selected portion (marked in a ) showing a significant conversion of hexagonal CdS to cubic Au 2 S x, as imaged in [ ] zone orientation. c) A perfect crystal lattice of simple cubic Au 2 S consisting of few unit cells where Au forms a fcc lattice configuration. d) View of the lattice in [ ] zone orientation matching with the Au atom positions in the HRSTEM image. 4

5 Mechanistic Insights into the Competition Between Cation-Exchange and Metal Ion Reduction Figure 5. a) Plot of Δ G as a function of temperature for cation-exchange (c.e.) and reduction (red.). Plots for individual formation of Au 2 S and Au 2 S 3 (half-filled circles) are also shown. b) Schematic of the thermodynamic and kinetic scenario indicating that cation-exchange is more favorable than reduction under the experimental conditions. experiments indicate that there could be significant kinetic barriers and thus higher temperatures and/or a lowering of nucleation barrier by lowering the interfacial energy for heterogeneous nucleation may be needed for the formation of Au [36 ] (also see Supporting Information). Our calculations show that the formation of Au 2 S 3 /Au 2 S is much more favorable than the formation of Au by reduction. As noted above, although thermodynamic calculations indicate the possibilities of formation, the actual formation of the product under experimental conditions is limited by the kinetic barriers for nucleation. It is very likely that the barrier for cation exchange where the product phase forms coherently in the parent is likely to have a lower interfacial energy and barrier for nucleation as compared to the formation of a metal with a heterointerface. Once formed, the Au 2 S/Au 2 S 3 phase is possibly stabilized by the kinetic barrier for the decomposition reaction. Under ambient conditions and even in the presence of a strong reducing agent, the sulfide phase is stable and intact in bulk of the rod as the reduction (if any) takes place only at the surface of the nanorods in contact with the reducing agent. However, the electron beam can interact intensely with a significant depth of the material and thus we observe that Au 2 S/Au 2 S 3 decomposes in situ during microscopy to give more stable elemental form of the constituent elements. The reaction conditions significantly affect the competition between reduction and exchange. For instance, it has been reported earlier [37 39 ] that for the same system viz., AuCl 3 and CdS, cation exchange is not favorable when the reaction was carried out in an organic medium in the presence of surfactants. However, both our experiments and thermodynamic calculations clearly show that cation exchange does indeed take place in an aqueous medium. The temperature dependence of Δ G predicts that the increment in driving force per unit change in temperature is more for reduction than for cation exchange. But the Δ G value for exchange is much more than that for reduction in the temperature range of interest indicating that still cation exchange is dominant. The EDS data of a CdS-Au 2 S x sample before and after microwave (MW) show a sharp increase in the Au content from 2 to 4 atomic percent indicating that the MW has a significant role in driving the reaction to completion in a short period of time. More importantly XPS shows that the Au 0 :Au 0/1+/3+ peak area ratio increases slightly on doing MW as predicted by the thermodynamic calculations. A change in concentration of HAuCl 4 has a negligible change on the driving force of both reduction and exchange (Supporting Information, Figure S4). Thus, we conclude that in the temperature and concentration range of interest cation exchange is favored with a relatively weak dependence on reaction parameters unlike reduction. In summary, we have demonstrated that Au, inspite of having a high electron affinity, may undergo cation-exchange with Cd ions in CdS. The formation of a Au 2 S phase in an exchange reaction with CdS, has been pointed out clearly for the first time using electron microscopy. Thermodynamic calculations on the possibility of cation exchange consolidate the mechanistic understanding. This predictability is directly applicable to other system combinations and provides insights on the competition between cation exchange and reduction process. Experimental Section Synthesis of CdS Nanorods : Synthesis of CdS nanorods have been carried out by solvothermal reaction of a mixture of Cd(NO 3 ) 2 (247 mg), thiourea (122 mg), deionized water (20 ml) and ethylenediamine (20 ml) at 160 C for 8 h. Cleaning was done by repeated sonication and centrifugation with ethanol and water. The product was dried and characterized before further use. Synthesis of Au 2 S x -CdS: For synthesis of Au 2 S x -CdS hybrid, CdS (20 mg) was dispersed in deionized water (20 ml). HAuCl 4 solution (4 mg in 10 ml deionized water) was added to vigorously stirring CdS colloid. The mixture was stirred for 5 min and then irradiated with microwave at 800 W for 3 min. The product was cleaned by repeated sonication and centrifugation with water and dried for characterization. The loading(till 10%) was varied by varying the initial amount of CdS keeping the HAuCl 4 concentration same. However for synthesizing ultra-high loading sample(22%, 39%, and 52% Au) CdS amount was kept fixed while the concentration of HAuCl 4 solution was varied. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements N.R. acknowledges Department of Science and Technology (DST) India for financial support. P.K. and G.V.T. acknowledge ERC grant 5

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