J. Phys. Chem. C 2008, 112, 8881–8889
8881
Studying the Reactions of CdTe Nanostructures and Thin CdTe Films with Ag+ and AuCl4Tamar Danieli,† Nikolai Gaponik,‡ Alexander Eychmu¨ller,‡ and Daniel Mandler*,‡ Department of Inorganic and Analytical Chemistry, The Hebrew UniVersity of Jerusalem, Jerusalem 919904, Israel and Physikalische Chemie, TU Dresden, Bergstr. 66b, 01062 Dresden, Germany ReceiVed: January 30, 2008; ReVised Manuscript ReceiVed: March 12, 2008
The reactions between Ag+ and AuCl4- ions with three different CdTe systems, that is, thermally evaporated thin CdTe films, CdTe nanoparticles (NPs) stabilized by thioglycolic acid (TGA), and 20 layers of these CdTe NPs embedded in poly(diallyldimethylammonium) chloride (CdTe-20lbl), were studied. We found that AuCl4- oxidized the CdTe, in all investigated systems, to form metallic gold. However, the kinetics of the reaction was substantially sluggish for the thin CdTe films than with the CdTe NPs systems. On the other hand, the reaction with Ag+ was rather complex, and our findings alluded to different reaction for each system. Although it is possible that a Ag-SR bond was formed between Ag+ and the thiol group of the TGA-stabilized NPs, cation exchange between Ag+ and Cd2+ is evidenced when these CdTe NPs were embedded in a polymer (CdTe-20lbl). Furthermore, Ag+ reacted with thin CdTe films to form a precipitate consisting of silver. In addition, we investigated the reactions between locally generated Ag+ and AuCl4- and CdTe surfaces using scanning electrochemical microscopy (SECM). A flux of silver and gold chloride ions was electrochemically generated from their respective microelectrodes upon applying a positive potential close to thin CdTe films and CdTe-20lbl. The flux of AuCl4- resulted in the local formation of Au NPs on the CdTe surface, whreas local quenching of CdTe-20lbl was achieved by the flux of Ag+ ions. Finally, the current transients recorded during the SECM experiments provided invaluable information regarding the reactions at the CdTe surface below the microelectrodes. Introduction CdTe nanoparticles (NPs) exhibit interesting physical properties such as large excitation Bohr radius, narrow emission band, and high photoluminescence.1 These properties make them attractive in a variety of applications, such as light emitting diodes, silver ions detectors, biological labels, and solar cells.1–5 Thin CdTe films play a significant role as IR and γ detectors; in solar cells, X-ray imaging in dentistry, and mammography; and are used as substrates for the epitaxial growth of Hg-based semiconductors.6–11 Several studies aimed at implanting gold or silver ions or metals into CdTe and other semiconductor surfaces for two main reasons: (i) creating an ohmic contact10,12,13 and (ii) as an anchor point for attaching other species, such as organic molecules.14 Different mechanisms were proposed for the reaction between these metal ions and CdTe or CdSe;4,10,12,14–19 for example, Wang et al.4 and Gheorghita et al.,10 who studied the reaction between silver ions and thin CdTe films, suggested that some of the Cd ions were replaced by silver ions, thus changing the electrical and optical properties of the semiconductor. On the other hand, Dong et al.16 examined the reaction between Ag+ and CdTe NPs and claimed that silver ions formed bonds with Te and sulfide groups (end group of the stabilizer) on the NPs surface. Son et al.15 reported cation exchange between Ag+ and Cd2+ of CdTe tetrapods and CdSe NPs, a reaction that did not proceed for micrometer size CdSe powder. Leung et al.20 have also examined the ion exchange between Ag+ and Cd2+ on a * Correspondence author address phone: +972 2 6585831; fax: +972 2 6585319; e-mail:
[email protected]. † The Hebrew University of Jerusalem. ‡ Physikalische Chemie.
CdSe surface. The reaction between gold ions and CdTe12,17,21 resulted in either the deposition of metallic gold and loss of CdTe from the surface or the formation of a Au-Te bond. While CdTe films are homogeneous and made of continuous crystalline structure, which provides them bulk physical properties, such as conductivity, CdTe NPs are separated by an organic stabilizer bearing negatively charged groups. In light of this, it is interesting to investigate the reactions of silver and gold ions with different CdTe systems, that is, compare between NPs and thin CdTe films. Full comparison of such systems requires, in principle, high lateral resolution. Yet, the reactions between silver and gold ions with CdTe have never been studied locally. We and others have shown22–27 that it was possible to electrochemically generate a flux of Ag+ or AuCl4- using scanning electrochemical microscopy (SECM). This scanning probe microscopy technique, which has been thoroughly described,28–30 enables us, in a simple way, to carry out chemical and electrochemical processes locally on a variety of interfaces. The generation of a flux of metal ions, for example, Ag+, can easily be accomplished by applying a positive potential to a metallic tip that is embedded in an insulating sheath. The potential and time of the positive pulse control the flux of ions that is generated. Evidently, bringing a surface into the diffusion layer of the electrogenerated ions can result in a local chemical reaction on the surface. Moreover, the faradic current at the tip is sensitive to such a reaction and to the diffusion of the species, which allows monitoring both the distance between the tip and the surface as well as the interfacial reaction. The size of the tip dictates the resolution of the surface process. SECM has been applied for modifying and studying semiconductors.22,24,31–34 The etching of GaAs and other III-V
10.1021/jp800877a CCC: $40.75 2008 American Chemical Society Published on Web 05/27/2008
8882 J. Phys. Chem. C, Vol. 112, No. 24, 2008 semiconductors has been reported.31,32 The etching mechanism of Si was studied by the local electrogeneration of bromine in fluoride solutions.33 On the other hand, a few groups have reported the local deposition of metals by SECM.22–27,35 Au was deposited on Si and indium tin oxide by anodically dissolving an Au microelectrode.22,24 The deposition of Co was reported using a similar approach.35 Ag deposition on pyrite was investigated by elctrooxidation of an Ag microelectrode.25 The same approach was used for studying Ag+ binding by a Langmuir phospholipid monolayer, whereby the association of Ag+ ions by the layer affected the flux of silver ions at the microelectrode.27 In this paper, three different systems of CdTe were examined: (i) CdTe NPs in an aqueous solution, (ii) CdTe NPs embedded in a polymer by the layer-by-layer deposition, and (iii) thermally evaporated thin CdTe films. We found that although AuCl4oxidized the CdTe, forming metallic gold on the semiconductor surface, silver ions react differently when the CdTe structure is of a continues film, as compared to nanoparticles. The mechanism of the silver reaction could not be conclusively determined. Some of the results suggest ion exchange between Ag+ and Cd2+ for CdTe NPs embedded in a polymer, whereas Ag+ ions seem to bind with the sulfur groups of the CdTe NPs organic stabilizer. Ag+ reacted with thin CdTe films to form a precipitate consisting of silver. Finally, SECM was successfully used to locally modify thin CdTe films and nanoparticles embedded in a polymer. Gold nanoparticles were locally formed, whereas silver deposits were detected on the surface. Experimental Section Instrumentation: A bipotentiostat (CHI-750B, CH Instruments Inc., TX, USA) with a preamplifier was used for controlling the potential in all SECM experiments as well as for conducting chronoamperometric measurements and for cyclic voltammetry (CV). Electrochemical experiments were carried out in a three-electrode cell using a Ag/AgCl (KCl saturated) or a Ag/AgCl wire [0.20 V vs normal hydrogen electrode (NHE)] as a reference electrode and a graphite or Pt wire as a counter electrode. SECM measurements were performed using a homemade SECM apparatus, based on stepper motors (MPAPP, NewPort, USA) with 0.060 µm resolution. The actuators were placed in an X-Z configuration, controlled by NewPort motion controller (MM2500). In-house software (LabView, National Instrument, TX, USA) for positioning the microelectrode was used. Characterization of the different samples was carried out using high resolution scanning electron microscopy (HR-SEM, Sirion, FEI company, Fl, USA), X-ray photoelectron and Auger spectroscopy (XPS, AXIS ULTRA, Kratos Analytical Ltd., Manchester, UK), and high resolution transmission electron microscopy (HR-TEM, Tecnai F20 G2, Tokyo, Japan). Optical and fluorescent microscopy (λ ) 365 nm) was performed with an Olympus BX6000 microscope (Tokyo, Japan). Solutions of nanoparticles were also inspected under UV light at λ ) 365 nm (ENF-260CIF, Spectroline, New-York, USA). Chemicals and Reagents: Fluorescent CdTe nanoparticles stabilized by thioglycolic acid (TGA) in aqueous solution and embedded in a poly(diallyldimethylammonium) chloride (PDDA) film were prepared as described elsewhere.3 Twenty alternate layers (termed CdTe-20lbl) of CdTe nanoparticles and PDDA were deposited on glass by the layer-by-layer deposition method.36,37 Thin CdTe films thermally deposited on glass-BK7 were generously provided by Ophir Optronics Ltd. (Jerusalem,
Danieli et al. Israel). Ag 0.025 mm (99.9%) and Au 0.025 (99.95%) mm wires (Alfa-Aesar, Lancashire, UK), silver epoxy (EPO-TEK H2OS, Epoxy technology inc., Billerica, U.S.A), and glass capillaries (disposable micropipette with ring mark, Blaubrand, intraMark, Wertheim, Germany) were used for preparing microelectrodes. All chemicals used were of analytical grade. Solutions were prepared with deionized water 18 MΩ/cm (EasyPure UV, Barnstead). All chemicals apart from HCl (J.T. Baker) and KNO3 (Merck, Frankfurt, Germany) were purchased from Sigma-Aldrich (St. Louis, USA). Procedures: Ru(NH3)6(NO3)3 Preparation. Ru(NH3)6Cl3 was dissolved in water, to which a saturated solution of KNO3 was added until a white precipitate of Ru(NH3)6(NO3)3 was obtained. The precipitate was collected by a centrifuge and was washed with saturated KNO3 solution to remove excess chloride. Microelectrode Preparation. Glass capillaries were pullheated with a micropipette puller (P-97, Sutter Instruments Co., CA, USA) and then sealed with a flame. Either 0.025 mm Ag or Au wires were inserted inside the pulled capillaries and were sealed under vacuum with a micropipette puller (PP-830, Narishige group, Tokyo, Japan). The sealed capillaries were polished with 400, 600, and 1200 grit emery paper (Buhler, Lake Bluff, USA) to form a smooth metal disk insulated in a glass sheath. The ratio between the diameter of the insulating sheath and the metal disk was approximately 10. Further polishing of the electrode to a mirror-like finish was accomplished with 9, 3, and 0.3 µm alumina paper (Fibramet discs, Buhler). Silver epoxy was used to make electric contact to copper wires. Previous to all experiments, the microelectrodes were polished with 0.05 µm alumina powder. The Au and Ag microelectrodes were electrochemically characterized by inspecting their CV in 2 mM Ru(NH3)6Cl3 (0.1 M KCl) and 2 mM Ru(NH3)6(NO3)3 (0.1 M KNO3), respectively. Reaction of CdTe with Metal Ions. Samples of thin CdTe films were washed with water and ethanol prior to immersing them in either 1 mM AgNO3 (3 min) or 0.1 mM HAuCl4 (150 s), followed by careful washing with H2O and drying under N2. The CdTe-20lbl were immersed in either 1 mM AgNO3 or 1 mM HAuCl4 (30 s), washed with H2O, and carefully dried under N2. CdTe nanoparticles in aqueous solutions were mixed with small amounts of either 1 mM HAuCl4 or 1 mM AgNO3. The solutions were inspected under UV light. For HR-TEM measurements, a few drops of 1 mM HAuCl4 and 1 mM AgNO3 solutions were mixed with CdTe nanoparticles solutions and were dropped on a TEM grid. SECM Experiments. SECM experiments were conducted by approaching a small piece of either a thin film of CdTe or 20 layers of CdTe/PDDA. The samples were immersed in 2 mM HCl/20 mM KCl or 10 mM KNO3 with a 25 µm diameter Au or Ag microelectrode, respectively. Approaching was accomplished by monitoring the cathodic current of oxygen reduction.25,38 Prior to each experiment, CV was used to determine the oxidation potentials (at which we obtained a current of ca. 10 nA) of gold and silver microelectrodes. As soon as the tip current decreased to ca. 10-20% it was stopped, and the potential was stepped (pulse varied between 1-5 s) to drive the oxidation of the tip. Results and Discussion The reaction of silver and gold ions with three different CdTe systems is presented here. Thermally evaporated thin CdTe films represent a continuous crystalline structure with semiconductor properties. On the other hand, the ions of the coinage metals also reacted with CdTe nanoparticles stabilized by thioglycolic
Reactions of CdTe with Ag+ and AuCl4-
J. Phys. Chem. C, Vol. 112, No. 24, 2008 8883 TABLE 1: Diffraction Ring Diameters As Calculated by HR-TEM Measurementsa ring No. 1 2 3 4 5 6 7
Figure 1. Photo taken under UV light (λ ) 365 nm) of (a) CdTe NPs solution, after the addition of a few drops of (b) 1 mM Ag+ solution and (c) 1 mM AuCl4- solution.
Figure 2. HR-TEM images of (a) CdTe NPs, (b) after mixing with diluted HAuCl4 solution (the dark spots are gold particles), and (c) after mixing with diluted AgNO3 solution.
acid (CdTe NPs) in aqueous solutions as well as with these NPs embedded (by the layer-by-layer deposition method) in a PDDA film. Twenty layers of the CdTe/PDDA (termed CdTe20lbl) were always used of this latter system. The results are divided, just for the sake of simplicity, into two sections according to the coinage metal. Reaction of AuCl4- with CdTe. Figure 1 shows the change of the fluorescence of the CdTe NPs upon the addition of AuCl4-. The change is instantaneous and clearly indicates that a fast reaction between the gold ions and the NPs occurs. A precipitate was formed. CdTe NPs are negatively charged, due to the carboxylic acids of the thioglycolic stabilizer.3 The negative charge of AuCl4- refutes the possibility of precipitation as a result of ion coupling. Further investigation of the reaction was carried out with HRTEM. The HR-TEM image obtained for CdTe NPs is presented in Figure 2a and the diffraction image and profile are shown in Figure 3a. The diffraction pattern consists of rings, which is typical to diffraction of small NPs. The diameters of the rings
ring diameter (nm), sample a and c
ring diameter (nm), sample b
0.3813 0.2304 0.1936
0.3786 0.2895 0.2471 0.2300 0.1745 0.1463 0.1159
a Sample a, CdTe NPs; sample b, after immersion in diluted AuCl4- solution; and sample c, after immersion in diluted Ag+ solution.
(Table 1) are characteristic of a face centered cubic zinc blende structure.14 The obtained rings are fairly wide, which points to the small dimensions of the NPs (1-3 nm in diameter). Mixing of the CdTe NPs solution with diluted HAuCl4 solution led to Figure 2b, where the diffraction pattern and profile are shown in Figure 3b. The darker spots in Figure 2b are associated with gold NPs, whereas the brighter spots are CdTe NPs. This assignment is primarily based on a couple of the new rings obtained in the diffraction pattern39 (0.2300 and 0.1463 nm diameters) as well as the stronger interaction of the heavier element, namely, Au, with the electron beam resulting in a darker image. The diameter values suggest that we obtained primarily gold and CdTe NPs as a result of a chemical reaction. A conceivable explanation is that gold chloride ions reacted with CdTe to form metallic gold (eq 1). 2+ 2AuCl+ 3Te 4 + 3CdTe f 2Au + 8Cl + 3Cd
(1)
The dramatic change in the diffraction pattern (Figure 3) reveals the formation of Au. Previous studies described the mechanism of reaction between AuCl4- and CdTe as a process where Cd2+ diffused into the solution and an Au-Te bond was formed.17,40 Other reports could not confirm such process and supported the formation of elemental gold.12,41 We compared our TEM diffraction results with those reported for Au-Te42 and found no similarity between the diameters. Therefore, we conclude that only metallic gold was formed. The next step was to examine the same reaction but with thin CdTe films. The latter has a continuous crystalline structure that is likely to confine the reaction onto the surface. HR-SEM
Figure 3. Diffraction patterns and profiles of the HR-TEM images of Figure 2. The diffraction ring diameters are given in Table 1.
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Figure 4. HR-SEM images of (a) thin CdTe films, (b) after 2.5 min immersion in diluted AuCl4- solution, and (c) after 3 min immersion in diluted Ag+ solution. The respective energy dispersive spectra are shown below each image (note: the gray line in spectrum c is the EDS analysis of one of the bright particles).
TABLE 2: XPS Results of the Atomic Percent of the Different Elements (Only Relevant Ones), and Te 3d5/2 and 3d3/2 Binding Energies for Each Samplea compound type CdTe Te0 Ag2Te TeO2 Te Oxide
binding energies (eV) for Te 3d5/2, Te 3d3/2 sample a
sample b
sample c
572.165, 582.542 572.886, 583.276 571.318, 581.709 575.841, 586.218 576.189, 586.579 574.800, 585.190
% element Cd 6.76 Te 1.81 Au Ag
1.08 0.98 2.69
Cd/Te
2.14
3.73
4.10 1.92 5.12 1.10
a
Sample a, CdTe NPs; sample b, after immersion for 30 sec in AuCl4- solution; and sample c, after immersion for 30 sec in Ag+ solution.
Figure 5. XPS spectra of Te 3d (a) CdTe-20lbl multilayers, (b) after 30 s immersion in 1 mM HAuCl4 solution, and (c) 1 mM AgNO3. The percentage of each element and the binding energy peak areas are summarized in Table 2.
images of thin CdTe films before and after immersing it into a diluted (1µM) HAuCl4 solution for 2.5 min are shown in Figure 4a-b. The energy dispersive spectra (EDS) are shown also. Whereas the thin CdTe films are composed of ca. 100-200 nm grains, clear gold NPs (with an average diameter of approximately 20-30 nm) are observed on the CdTe surface. EDS verified the presence of gold. It is worth mentioning that EDS analysis is performed on the uppermost 1 µm, which explains the minor change in the intensities of the Cd and Te signals. The third system that we examined involved CdTe NPs embedded in polymeric monolayers. These systems have been the subject of intensive research.43–50 For example, CdTe nanoparticles have been incorporated into conducting polymers, such as polypyrrole.49,51,52 Such systems are used for assembling a variety of light emitting devices. CdTe NPs were also used for luminescent labeling of capsules for drug delivery, which
consisted of different polymers, for example, poly(styrene sulfonate) and poly(allylamine hydrochloride).53 The CdTe NPs embedded in polymeric multilayers were made by layer-bylayer deposition, which is based on the electrostatic attraction of the negatively charged thioglycolic acid and the positively charged PDDA polymer. These assemblies represent an intermediate state between the dissolved NPs and of a continuous crystalline phase. The reaction between CdTe-20lbl and metal ions, such as AuCl4-, must be quite complex due to the possible hindrance of the metal ions diffusion inside the film. On the other hand, reaction between the metal ions and the NPs inside the film should be feasible and cannot be excluded. Unfortunately, we found that HR-SEM was not adequate for studying the CdTe-20lbl system due to charging (we could not coat the samples by a thin Au/Pd film, and coating with carbon was not successful). Hence, XPS was used for characterization, which allowed us to also evaluate the degree of oxidation of each element. Figure 5a-b shows the Te 3d XPS spectra of the CdTe-20lbl before and after 30 s immersion in 1 mM HAuCl4 solution. The atomic percent of the various elements is presented in Table 2, as well as the binding energies and the energy level peak areas. Two doublets are detected (I and II), which indicate two forms of Te. In principle, a change in the biding energy between the signals of a doublet indicates the formation of a different Te compound. Doublet I in (Figure 5a) corresponds to Te2- in CdTe,54 whereas the second doublet (II) corresponds to Te4+
Reactions of CdTe with Ag+ and AuCl4in the oxide TeO2.54–56 A careful inspection of Figure 5b reveals a slight change in the peak energy of ca. 0.8 eV to higher values. Such shift to higher energies is characteristic of Te oxidation. The first doublet (I) is associated with elemental Te,54 whereas the second doublet (II) can be attributed to an oxide form of Te. These findings support the reduction of AuCl4- by CdTe to form Au and elemental Te (eq 1), as was also previously reported1221 in a study of thin CdTe films. The XPS spectra reveal significant changes in the ratio between the elements before and after exposure to an AuCl4- solution. The initial Cd/Te ratio is 3.73, which is substantially higher than that expected in CdTe. This high ratio is attributed to the structure of the CdTe NPs, which contains a CdTe core surrounded by a Cd-SR shell.3 Nevertheless, the high ratio might also be due to the small particle size, which intensifies the ratio on the surface. The ratio decreases dramatically, to ca. 1.1, as a result of the reaction with gold ions. This implies that the inner structure of the CdTe NPs changed and that a significant amount of cadmium was washed away, presumably upon the oxidation of Te2-. The elemental ratio between Au/Te is 2.7, which only indicates of an excess of Au on the surface. Reaction of Ag+ with CdTe. Silver ions are positively charged and are a weaker oxidant as compared with AuCl4-. Yet, an instantaneous reaction occurred upon addition of a few drops of 1 mM AgNO3 into the CdTe NPs solution. The fluorescence was immediately quenched (Figure 1b), and precipitate was obtained. Figure 2c shows the HR-TEM image of CdTe NPs after exposure to diluted AgNO3 solution. Small nanoparticles that are covered by carbon contamination are hardly seen. We recall that the only source for carbon is the stabilizer (thioglycolic acid), which indicates the organic shell of the CdTe NPs was at least partially detached by the silver ions. The latter can bind to the thiol groups16 of the glycolic acid to form an Ag-SR salt. Whether or not there had been a reaction involving the CdTe NPs is clarified by the diffraction pattern (Figure 3c) of the reacted solution. Comparison with the diffraction of NPs with no added Ag+ ions (Figure 3a) provides an almost identical pattern. From this observation we conclude that the crystal structure of CdTe remained untouched; hence, no reaction took place between Ag+ and CdTe. Our results are in agreement with the study by Dong et al.,16 who reported that there was no reaction between Ag+ and CdTe NPs and, moreover, that Ag+ formed a bond with the thiol group. On the other hand, Son et al.15 examined the reaction between Ag+ and CdTe tetrapods and reported the ion exchange reaction between Ag+ and Cd2+. The next system involved the reaction of Ag+ with thermally evaporated thin CdTe films. The HR-SEM image of a sample that was immersed in 1 mM AgNO3 solution for 3 min is presented in Figure 4c. The EDS analysis is also shown. The SEM image does not show a significant difference as compared with the image of unreacted CdTe films. There are some scattered bright particles, which were analyzed by EDS (Figure 4c). This analysis confirmed that the crystals contain Ag. Cd and Te were also found, although in smaller amounts as compared with the analysis of the entire surface (Figure 4c black line). Taking into account that the e-beam deeply penetrates, ca. 1 µm into the CdTe films, clearly indicates that if a reaction between Ag+ and CdTe took place it was only superficial. Finally, it should be noted that SEM is not an ideal tool for investigating this system because the contrast between Cd and Ag is very poor. Because there are no thiols involved in this system (as was the case with the dissolved NPs), the observed change in the
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Figure 6. Cyclic voltammetry of Au and Ag microelectrodes in 2 mM HCl/20 mM KCl and 10 mM KNO3 solutions, respectively. The inset is the plot of the potential vs the natural logarithm of the current.
SEM image and its EDS analysis suggests that silver ions reacted with CdTe either via ion exchange (to form Ag2Te) or by Te2- oxidation. Another possibility is that silver hydroxide was formed and precipitated on the surface. EDS analysis cannot provide conclusive evidence for the true nature of the reaction, and it is plausible that more than one reaction occurred. We can conclude that Ag+ interacted with both forms of CdTe systems, that is, NPs and thin films, although it appears to be by different paths. To investigate the nature of the reaction of silver ions with CdTe-20lbl, XPS was performed after the sample was immersed in 1 mM AgNO3 solution for 30 s. The XPS spectra of Te are shown in Figure 5. The atom ratio between Cd/Te is summarized in Table 2. It is clear that the Cd/Te ratio decreased as a result of immersion into the silver ions solution. An ion exchange reaction between Ag+ and Cd2+, resulting in the formation of Ag2Te, can explain this change. Figure 5 and Table 2 show that the binding energy of Te shifts toward lower values upon immersion in Ag+ ions. This excludes the oxidation of CdTe by Ag+, similarly to the reaction with AuCl4-, as this would have shifted the Te signal to higher binding energy. Moreover, there is at least one reported study that associates the first Te doublet (I) and the peak obtained for Ag 3d5/2 (572.8 eV) with Ag2Te.57 The second doublet (II) must be associated with an oxidized form of Te; however, it is slightly shifted toward lower binding energies as compared with the sample before immersion. We cannot relate this doublet to a specific species. Local Modification of CdTe Surfaces with the SECM Technique. Our findings clearly show that gold and silver ions react with CdTe by different paths. So far, the reactions were carried out on the entire surface or solution. Scanning electrochemical microscopy (SECM) allows driving chemical and electrochemical processes locally at interfaces. Because we have already shown in previous studies the ability to generate a flux of silver and gold ions by SECM,22–24,27 the next logical step was to use this unique technique for localizing the reaction between the metallic ions and CdTe. Furthermore, the SECM can be used, vide infra, for studying these processes. The electrochemical generation of a flux of metal ions was achieved by anodically pulsing either a gold or silver microelectrode for a given time. Specifically, whereas Ag+ flux was generated in 0.01 M KNO3, a flux of AuCl4- species was produced in 0.002 M HCl and 0.02 M KCl solution. The chloride ions are essential for dissolving the gold microelectrode at positive potentials. Prior to approaching a CdTe surface, that is, thin film or CdTe-20lbl, the microelectrodes were electrochemically inspected for the reversibility of metal dissolution.25 Figure 6 shows the CV of Au and Ag microelectrodes. The
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Figure 7. Normalized transient current vs the reciprocal square root of time (t-1/2) measured in the solution (far from a surface). Potential of the Au and Ag microelectrodes was 1.0 and 0.2 V, respectively. All other parameters are as in Figure 6.
anodic currents are due to their dissolution, which is thermodynamically reversible, as confirmed by plotting the potential (E) versus the logarithm of the current (I), where n is the number of electrons per species oxidized.25
E∝
0.059 ln i n
(2)
A slope of 45 and 30 mV is obtained for Au and Ag, respectively. While we obtained the theoretical value for the dissolution of silver, the value of the slope for Au dissolution is substantially higher than theory predicts. Evidently, the process is more complex, as it involves chloride ions. Yet, we do not have a simple explanation for this reproducible discrepancy besides having a contribution of water oxidation, which commences at very close potentials. A second test of the performance of the microelectrodes involved the chronoamperometry far from the surface. The theory of the current-time transient of a microelectrode is well-developed.28,58,59 In essence, the current should decay as a function of t-1/2 and attain a steady state current (i∞) equal to 4nFDCr, where F is the Faraday constant, D is the diffusion coefficient of the species, C is their concentration, and r is the radius of the microelectrode. Figure 7 shows the plot of the normalized current, which is the actual current divided by I∞, versus t-1/2. From the slope and the extrapolated intercept, the diffusion coefficients of AuCl4- and Ag+ can be derived and are 3.5 × 10-6 and 5.5 × 10-6 cm2 s-1, respectively. These values are in good agreement with previously reported diffusion coefficients of the species.25,60 The gold and silver microelectrodes were brought close to the CdTe surfaces by following the negative feedback current of oxygen reduction.25,38 In most cases three consecutive pulses were applied at the same location before the tip was moved across the surface to a new location and pulsed again. Optical microscopy of thin CdTe films surface and CdTe20lbl multilayers, after local modification with AuCl4-, is shown in Figures 8a and 8d, respectively. Three spots can clearly be seen where the SECM tip was pulsed. Each of the spots was obtained after three consecutive 2 s pulses. An uneven distribution of the product on the surface is evident in both pictures, and a higher distribution is found at the periphery. Applying a positive potential to the gold tip in chloride solutions generates a flux of AuCl4-. Therefore, this flux strongly depends on the radial diffusion of chloride ions to the tip. This explains the higher product concentration at the periphery where the flux of AuCl4- is maximal. It should be noted that the tip-surface distance has an effect on the size of the spots that are formed. Increasing the tip-surface distance increases the diameter of
Figure 8. Results of SECM experiments with an Au microelectrode in 2 mM HCl/20 mM KCl solution. Each spot was obtained after three consecutive 2 s pulses (E ) 1.0 V). (a) Optical microscope and (b) HR-SEM images of thin CdTe films. The calculated diameter was 33.2 µm. (c) EDS analysis of thin CdTe films ( outside (black) and inside (red) a spot); (d) optical microscope and; (e) HR-SEM images of CdTe20lbl multilayers. Because of poor contrast, the spots are circled with a doted line. The calculated spot diameters were 34.7µm (inner circle) and 48.7 µm (outer circle).
the spot due to the radial diffusion, provided that the surface is still within the diffusion layer of the electrogenerated species. Figures 8b and 8e show HR-SEM images of the same spots. The contrast of the images, in particular that of the multilayers, is very low in spite of the fact that an ultrathin sputtered layer of a conducting material was deposited prior to inspection. The difference between the precipitation on thin CdTe films and CdTe-20lbl multilayers is evident from both (optical and electron) microscopy images. The major difference is the size of the patterns. The diameter of two spots is marked in the HRSEM images and is equal to 33.2 and 48.7 µm for the thin CdTe film and the CdTe-20lbl multilayers, respectively. In addition, the precipitate on the CdTe-20lbl multilayers forms a ring structure, which is associated with the pulses, namely, each pulse generates a single larger ring. Finally, the deposits formed on CdTe-20lbl surface are not as dense as on thin CdTe films.
Reactions of CdTe with Ag+ and AuCl4-
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Figure 9. Results of SECM experiments with an Ag microelectrode in 10 mM KNO3 solution (Etip ) 0.2 V). (a) Optical microscopy picture of thin CdTe films. Each spot is a result of three consecutive 2 s pulses; (b) EDS analysis of thin CdTe films surface outside (gray) and inside (black) a spot; (c) a picture taken with a microscope under UV light (λ ) 365 nm). The spots are a result of 9, 3, 1, and 1 s pulses (left to right); (d) EDS analysis inside (black) and outside (gray) of the spot obtained for 9 s pulse.
These differences can be explained by the significant lower density of CdTe in CdTe-20lbl multilayers than in thin CdTe films. The flux of AuCl4- that reaches the CdTe-20lbl surface reacts fast with all CdTe nanoparticles located below the tip. Moreover, the amount of generated AuCl4- is stoichiometrically larger than that of CdTe, causing the excess of AuCl4- to diffuse laterally and to form a ring type deposit that is larger in diameter than the SECM tip. The next pulse generates AuCl4- ions that need to diffuse further from the center in order to react with CdTe that did not yet react. Nevertheless, the diameter of the deposits in both systems was larger than the diameter of the microelectrode (25 µm). This can be explained for the patterns on thin CdTe films by the sluggish kinetics of the reaction between AuCl4- and by the surface, due to its high density. Yet, the high density of the thin CdTe film results in a denser deposit and a smaller diameter. Figure 8c shows EDS analysis of an area inside and outside of a pattern made on a thin CdTe film. Gold was detected inside the reaction region and not outside; moreover, the amounts of cadmium and tellurium are lower inside the reaction region. These findings support our previous assumption that AuCl4- is reduced by CdTe to metallic gold as described in eq 1. An optical micrograph of a thin CdTe film after local modification with Ag+ is presented in Figure 9a. Figure 9c shows a fluorescence optical micrograph of a CdTe-20lbl surface after a similar experiment, using a UV light cut off at 365 nm. The deposits were formed upon applying a positive potential for different durations (see figure caption). It can be seen that, as the potential pulse is made longer, the fluorescence decreases. Careful examination of the size of the spots reveals that they grow with pulse length. This can be explained by the effect of a larger flux of oxidants generated at the tip and by the relatively low amount of active sites on the surface. Disk patterns with diameters similar to that of the SECM tip can be seen in both pictures. It should be mentioned that fluorescence optical microscopy could not be applied in the experiments involving AuCl4- ions because the chloride solution (2 mM HCl/20 mM KCl) irreversibly quenched the fluorescence. On the other hand, a clear local quenching of the fluorescence on the CdTe-20lbl surface is detected as a result of the reaction with Ag+. EDS analysis of the modified surfaces (Figure 9b and 9d) detected a small amount of silver, just barely above the level
of detection, with minute changes in the amounts of Cd and Te. These results are in agreement with our previous findings that only a superficial reaction occurs between Ag+ and CdTe. Hence, so far none of our results can decisively confirm a reaction between Ag+ and CdTe. The local deposits by SECM must be associated with the flux of Ag+; however, we cannot exclude the formation of, for example, AgOH or other silver salts, which do not necessarily involve CdTe. The current transients of a SECM tip measured within the diffusion layer from a surface provide significant information about the reactions taking place on the surface below the tip.28 Therefore, we analyzed the transients of the Au and Ag microelectrodes pulsed close to CdTe (Figure 10). The transients far from a surface, as well as close to an insulating nonreacting glass, were also recorded (Figure 10) for comparison. Figure 10a shows the first pulse of an Au microelectrode recorded over thin CdTe film and the CdTe-20lbl multilayers. The transient above glass decays faster to a lower value as compared with that recorded far from the surface. Both transients are in good agreement with the theoretical curves generated by Unwin.28 If a fast reaction between the electrogenerated species and the surface takes place, then the current is expected to be larger than that of an insulator, providing that the dissolution of the Au tip is reversible.58 Under these conditions, every AuCl4- ion that is reduced by the surface triggers the generation of another ion at the tip in order to allow the systems to reach equilibrium (between the Au microelectrode and the gold ions in the solution). It can be seen that the transient recorded over a thin CdTe film overlaps that of glass, implying that the reaction of AuCl4- with this surface is very slow. The fact that gold was observed on the surface (Figure 8a-c) will be explained below. In contrast, the transient measured above the CdTe-20lbl multilayers is significantly different. The fast initial decay is followed by an increase of the current that ends with a slow decay. This behavior clearly indicates that the electrogenerated AuCl4- ions reacted fast with the CdTe-20lbl surface. The difference in the kinetics of the reaction between AuCl4- and both CdTe surfaces can be attributed to their different nanoscopic structure. While the thin CdTe film is a densely packed crystal, the CdTe-20lbl multilayers do not limit the diffusion of the AuCl4- ions across the film. Moreover, the surface area
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Figure 10. Chronoamperometry of Au microelectrode in 2 mM HCl/20 mM KCl solution (Etip ) 1.0 V); (a) during 2 s pulses above different surfaces (it should be noted that the lines for glass surface and thin CdTe films are overlapping); (b) comparison of three consecutive pulses above different CdTe surfaces. (c) Chronoamperometry of Ag microelectrode in 10 mM KNO3 solution during 2 s pulses (Etip ) 0.2 V) above different surfaces.
of the CdTe nanoparticles is orders of magnitude larger than that of the thin film. Furthermore, the transient over a CdTe20lbl surface is a result of a nucleation and growth mechanism.35 The increase of the current is due to the formation of nucleation sites for further deposition of gold. Once these sites overlap the reaction on the surface will slow down and the current will decay. Modeling of the SECM transient is complicated and therefore has not been attempted. Such nucleation is not observed for the thin CdTe films system, or it is conceivable that the number of nucleation sites is too low to affect the transient. Further insight to the mechanism of deposition was obtained by consecutively pulsing the microelectrodes at the same location. Figure 10b shows three consecutive pulses recorded over thin CdTe film and CdTe-20lbl multilayers. It can be seen that the current of each consecutive pulse above the thin CdTe film increases. This is evident of the increase number of nucleation sites generated by each pulse. Therefore, the reduction of AuCl4- and the deposition of gold is more facile at the second pulse as compared with the first. Interestingly, the third pulse over the thin CdTe film begins to resemble the first transient over a CdTe-20lbl surface, implying that sufficient nucleation sites were formed after three pulses. The consecutive transients recorded over CdTe-20lbl multilayers show somewhat different behavior. Although the current of the second pulse is higher than that of the first pulse, the current of the third pulse is lower than that of the second. This can be accounted for by the thickness of the layer or, in other words, by the insufficient amount of CdTe nanoparticles. Clearly, all the CdTe below the tip reacts with the AuCl4-, and therefore the second and third transients are affected by the lateral diffusion of the AuCl4- ions and their reaction with the layer at larger distances. This is in excellent agreement with our microscopy images (Figure 8d). The decay of the three pulses reaches the same current and is controlled by the lateral diffusion of AuCl4-. The transients recorded with an Ag microelectrode are shown in Figure 10c. The first pulses (Figure 10c) obtained close to a
thin CdTe film and CdTe-20lbl surface are between the transients recorded close to glass and far from a surface. This suggests of a moderate interaction between the electrogenerated Ag+ ions and the CdTe surface. There is no clear difference between the two surfaces, which implies that there is no fast and significant reaction between silver ions and CdTe. Furthermore, the consecutive pulses do not show any significant difference and therefore are not shown. Yet, the microscopy images (Figure 9) clearly showed the quenching of the local fluorescence of the CdTe-20lbl multilayers that alludes to the interaction and deposition or adsorption of silver ions on this surface. Hence, the nature of the reaction between Ag+ and CdTe could not be conclusively determined using the SECM. Conclusions The reactions between different CdTe systems and AuCl4or Ag+ ions were investigated and compared. SECM was used for studying these reactions locally by generating a flux of the ions close to the CdTe surfaces. We found that CdTe reacts differently with AuCl4- and Ag+ ions. AuCl4- ions oxidize the CdTe to form metallic gold, whether or not the samples were thin CdTe films or NPs, either in an aqueous solution or embedded in PDDA. The reactions by SECM resulted in local deposition of Au on the surfaces. Comparison of SECM results obtained for CdTe-20lbl multilayers and thin CdTe films demonstrates the different behavior of NPs and continuous crystalline systems. While the kinetics of the reaction between anodically generated AuCl4- with thin CdTe films was sluggish, that with CdTe-20lbl was very facile. This was supported by SECM current transients showing both nucleation and growth stages only in the case of CdTe-20lbl multilayers. The fast kinetics between AuCl4- and the CdTe-20lbl multilayers also resulted in depletion of the CdTe below the tip and therefore a significant lateral diffusion of AuCl4- ions. The excess of Au deposition observed at the periphery was due to the radial diffusion of chloride ions. The reaction between silver ions and CdTe systems was found to be more complex and could not be conclusively determined.
Reactions of CdTe with Ag+ and AuCl4Different path reactions are suggested and depend on the individual system. Ag+ ions presumably react with the thiol groups of the NPs stabilizer, whereas silver deposits as either elemental silver or as silver salt as a result of reaction with the thin CdTe films. Finally, the reaction between Ag+ and the multilayers of CdTe NPs involves cation exchange between the silver and cadmium ions. The difference between the reaction of AuCl4- and Ag+ with CdTe might be due to the thermodynamic driving force, which is higher for the gold chloride ions and, on the other hand, the higher affinity of Ag+ to the thiol groups of the thioglycolic acid used as the NPs stabilizer. Finally, SECM is again revealed as a useful tool not only for the local modification of CdTe surfaces (fluorescence quenching and semiconductor decomposition) but also as a means of studying surface reactions. Acknowledgment. This study was supported by the GermanIsraeli Project (DIP) program and the Israel Science Foundation (contract 485-06). Ophir Optronics (Jerusalem, Israel) is acknowledged for providing thin CdTe samples. Dr A. Shavel and Dr. V. Lesnyak are acknowledged for the assistance in synthesis and assembly of CdTe nanoparticles. The Harvey M. Krueger Family Center for Nanoscience and Nanotechnology of the Hebrew University is acknowledged. References and Notes (1) Wu, F. X.; Lewis, J. W.; Kliger, D. S.; Zhang, J. Z. J. Chem. Phys. 2003, 118, 12–16. (2) Rogach, A. L.; Franzl, T.; Klar, T. A.; Feldmann, J.; Gaponik, N.; Lesnyak, V.; Shavel, A.; Eychmuller, A.; Rakovich, Y. P.; Donegan, J. F. J. Phys. Chem. C 2007, 111, 14628–14637. (3) Gaponik, N.; Talapin, D. V.; Rogach, A. L.; Hoppe, K.; Shevchenko, E. V.; Kornowski, A.; Eychmuller, A.; Weller, H. J. Phys. Chem. B 2002, 106, 7177–7185. (4) Wang, J. H.; Wang, H. Q.; Zhang, H. L.; Li, X. Q.; Hua, X. F.; Cao, Y. C.; Huang, Z. L.; Zhao, Y. D. Anal. Bioanal. Chem. 2007, 388, 969–974. (5) Wang, D. Y.; Rogach, A. L.; Caruso, F. Nano Lett. 2002, 2, 857– 861. (6) Kozyrev, S. P.; Vodopyanov, L. K. Semicond. Sci. Technol. 1999, 14, 660–665. (7) Mandal, K. C.; Kang, S. H.; Choi, M.; Wei, J.; Zheng, L.; Zhang, H.; Jellison, G. E.; Groza, M.; Burger, A. J. Electron. Mater. 2007, 36, 1013–1020. (8) Schulz-Drost, C.; Sgobba, V.; Guldi, D. M. J. Phys. Chem. C 2007, 111, 9694–9703. (9) Chu, T. L.; Chu, S. S.; Ferekides, C.; Britt, J.; Wu, C. Q. J. Appl. Phys. 1992, 71, 3870–3876. (10) Gheorghita, L.; Cocivera, M.; Nelson, A. J.; Swartzlander, A. B. J. Electrochem. Soc. 1994, 141, 529–535. (11) Rams, J.; Sochinskii, N. V.; Munoz, V.; Cabrera, J. M. Appl. Phys. A: Mater. Sci. Process. 2000, 71, 277–279. (12) Mancini, A. M.; Quirini, A.; Vasanelli, L.; Perillo, E.; Rosato, E.; Spadaccini, G.; Barbarino, E. J. Appl. Phys. 1982, 53, 5785–5788. (13) Cordes, H.; Schmidfetzer, R. Semicond. Sci. Technol. 1995, 10, 77–86. (14) Bendor, L.; Yellin, N.; Shaham, H. Mater. Res. Bull. 1983, 18, 1229–1233. (15) Son, D. H.; Hughes, S. M.; Yin, Y. D.; Alivisatos, A. P. Science 2004, 306, 1009–1012. (16) Dong, C. Q.; Qian, H. F.; Fang, N. H.; Ren, J. C. J. Phys. Chem. B 2006, 110, 11069–11075. (17) Musa, A.; Ponpon, J. P.; Grob, J. J.; Hageali, M.; Stuck, R.; Siffert, P. J. Appl. Phys. 1983, 54, 3260–3268. (18) Akutagawa, W.; Turnbull, D.; Chu, W. K.; Mayer, J. W. J. Phys. Chem. Solids 1975, 36, 521–528. (19) Wolf, H.; Wagner, F.; Wichert, T.; Isolde Collaboration, Phys. B 2003, 340, 275–279. (20) Leung, L. K.; Komplin, N. J.; Ellis, A. B.; Tabatabaie, N. J. Phys. Chem. 1991, 95, 5918–5924. (21) Mancini, A. M.; Quirini, A.; Vasanelli, L.; Perillo, E.; Rosato, E.; Spadaccini, G. Thin Solid Films 1982, 89, 407–412.
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