Article pubs.acs.org/JPCC
Controlling the Crystallinity of Electrochemically Deposited CdS Nanowires B. S. Simpkins,*,† T. Brintlinger,‡ R. M. Stroud,‡ S. Sherrill,§ S. B. Lee,§,⊥ and P. E. Pehrsson† †
Chemistry Division and ‡Materials Science and Technology Division, Naval Research Laboratory § Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States ⊥ Graduate School of Nanoscience and Technology (WCU), Korea Advance Institute of Science and Technology, Daejeon 305-701, Korea ABSTRACT: This report describes two distinct growth regimes occurring during the templated electrochemical deposition of CdS nanowires. The abundance of Cd2+ ions at the growth front dictates crystal nucleation and growth processes. At high Cd2+ concentration, randomly oriented polycrystalline material is formed, whereas local depletion of Cd2+ results in highly oriented crystals with the [002] crystalline direction aligned with the nanowire axis. Single nanowires exhibiting both regions were produced for direct imaging of the microstructure associated with these regimes. The current dependence of the time needed to locally deplete Cd2+ allowed for the extraction of a diffusion coefficient for Cd2+ of 1.23 × 10−6 cm2/s, which reflects reduced diffusion in nanoscale channels. Knowledge of these growth regimes and the parameters governing them allows for the growth of materials with targeted structural properties. Specifically, single-crystal materials can be preferentially formed by depositing at relatively high growth currents or by reducing the initial concentration of the Cd precursor in solution.
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INTRODUCTION Semiconducting nanowires (NWs)1 have been applied to biomedicine,2 nanoelectronics,3,4 nanooptics,5,6 and nanoscale microscopy 7 and provide unique testbeds to address fundamental scientific questions regarding surface effects,8,9 carrier quantization,10 nanoscale mechanics,11 and thermal energy transfer.12 The II−VI semiconductor cadmium sulfide (CdS) is well-suited for nonlinear optics,13 photovoltaics (PV),14,15 and photoelectrochemical (PEC)16 applications, owing to a visible band gap (∼490 nm) that aligns well for water splitting, and can be made using a variety of techniques including chemical bath deposition,17,18 vapor−liquid−solid growth,19,20 and electrochemical deposition (ECD).21,22 PV cells composed of vertical NW arrays take advantage of orthogonalization of the light absorption and carrier collection processes,23 and PEC processes in NW arrays benefit from higher surface area for catalyst attachment.24 Increased light absorption due to light trapping25 in such arrays improves performance in both PV and PEC processes. It is in this context that we investigate production of vertical arrays of CdS through templated electrochemical deposition, where arrays are a natural byproduct of the template geometry and growth rates are amenable to studies of electrochemical processes occurring during growth. Although ECD has been applied to thin films21,23 and templated deposition,26−28 little attention has been paid to the influence of growth rate on the growth mechanism and the resulting microstructure. We describe the electrochemical environment at the growth front in two distinct growth regimes and analyze the structural character and crystallinity of materials grown in these two © 2013 American Chemical Society
regimes, thus providing a clear correlation between growth conditions, the electrochemical environment, and material structure. These goals are achieved by (i) galvanostatically depositing CdS into anodized alumina membranes over a range of deposition currents, (ii) evaluating the microstructure of the deposited materials through X-ray diffraction (XRD) and transmission electron microscopy (TEM), and (iii) evaluating chronopotentiometry and cyclic voltammetry response to extract information about the electrochemical activity at the growth front. Knowledge of these processes gives guidance for the production of single-crystal CdS NW arrays.
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METHODS Electrochemical deposition (ECD) and acquisition of cyclic voltammograms (CVs) were controlled by a Gamry Reference 600 potentiostat. All CV scans were started at 0 V, scanned in the negative direction, then scanned in the positive, and finally scanned back to 0 V. The templated deposition was carried out in two different types of anodized aluminum oxide (AAO) membranes. Commercially available (Whatman Anodisc, pore diameter ≈ 230 nm) and in-house fabricated membranes29 (pore diameter ≈ 80 nm) were first metalized (10/300 nm Cr/ Au) and then mounted into a Teflon three-electrode deposition cell with the membrane acting as the working electrode against a carbon counter electrode. For Au deposition, an SCE reference was used, while a Pt wire served as a pseudoreference Received: March 18, 2013 Revised: April 25, 2013 Published: May 1, 2013 11843
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during CdS deposition. After sample mounting, a blocking layer of ∼5 μm of Au was electrochemically deposited (Technic, Inc. Orotemp24 Au deposition solution with 0.5% tritonX405 surfactant) to ensure complete blocking of the pores. Deposition of CdS was then carried out roughly following ref 21 using 55 mM CdCl2, 190 mM elemental S, and 0.5 vol % tritonX405 in dimethyl sulfoxide (DMSO). Each deposition was carried out at a fixed constant current (galvanostatic) at 87 °C under currents ranging from −25 to −250 μA (−56 to −560 μA/cm2). After growth, membranes were rinsed in hot DMSO, acetone, and isopropyl alcohol and then blown dry with N2. Membranes were cleaved for cross-sectional scanning electron imaging on a LEO field emission SEM. Portions of membranes were mounted on slides with epoxy and then soaked in 1 M NaOH to remove the alumina template, yielding arrays of undressed vertical NWs suitable for XRD studies carried out on a Rigaku Smartlab diffractometer. Aliquots of NWs in solution were pipetted onto lacey-carbon-coated grids for TEM analyses. The TEM studies were carried out with a JEOL 2200FS operating at 200 kV.
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RESULTS This study focuses on the templated ECD of CdS into AAO membranes. Figure 1 summarizes this process with (a) a crosssectional SEM image of a commercial AAO membrane (pore density ≈ 109 cm−2; diameter ≈ 230 nm) partially filled with Au/CdS, (b) top-view image of undressed Au/CdS NWs prepared for XRD, and (c) a summary of the measured CdS growth rates as a function of growth current. Electron scattering contrast between the Au and CdS layers segments (Figure 1a) allowed measurement of the CdS wire length and calculation of the linear and volumetric growth rates. A linear fit to these rates resulted in a slope of 7.3 × 103 μm/h/μA. Deposition in this system has been described22 as following either (i) reduction of Cd2+ on the growth surface followed by reaction with S or (ii) reduction of S to S2− at the growth front followed by reaction with Cd2+. Either mechanism would result in two electron charges passed per Cd + S unit deposited. A well-behaved ECD in which all passed current corresponds to deposition of Cd2+ ions would result in a growth rate R = (iMCdS)/(ρnAF). This is essentially Faraday’s equation, where i is the current, MCdS and ρ are the molecular mass and density of CdS, n is the number of charges per deposited ion, A is the deposition area, and F is Faraday’s constant. This expression predicts a slope of 2.81 × 104 μm/h/μA for our system (solid line in Figure 1c), which is ∼3.8 times that of the extracted fit, giving a deposition efficiency of 26%. This reduced efficiency may be due to oxidative reactions at the anode or dissociation of molecular S into atomic species during deposition.30 The crystal phase and crystallographic texturing were evaluated using XRD of undressed NWs. Data acquired in a standard θ−2θ configuration are shown in Figure 2a and exhibit diffraction peaks at 26.7 and 28.5° corresponding to the (002) and (101) planes of hexagonal CdS (ICSD 154187). The two peaks found at 38.° and 44.3° are associated with the (111) and (200) planes of the electrochemically deposited Au layer (ICSD 52700). All samples exhibited a CdS (002) peak except for that grown at the lowest growth current due to insufficient material present. Although the (002) hexagonal peak is positioned very near the (111) cubic peak located at 26.5° (ICSD 29278), the presence of the hexagonal CdS (101) peak, which is weak but visible, supports assignment of the hexagonal phase. Further support of this assignment is provided by the
Figure 1. (a) Cross-sectional view of typical Au/CdS NW growth. (b) Top view of Au/CdS NWs after template removal. NWs in (a) and (b) are ∼230 nm in diameter. (c) Plot of the linear growth rate of CdS versus the current along with the linear fit and theoretical prediction based on Faraday’s law.
pole figure analysis discussed later. The peak intensity ratios in Figure 2a are informative. Powder diffraction patterns for hexCdS should produce a peak intensity ratio of I(002)/I(101) ≈ 0.45. The observed ratio is >25, indicating strong crystalline texturing of the material with a preference for the (002) planes to be oriented parallel to the substrate (i.e., the [002] direction is parallel to the growth direction and NW axis). These two planes are illustrated in Figure 2b. The (002) plane is the basal plane of the hexagonal prism, and the (101) plane is a pyramidal plane whose normal is tilted 61.9° from the basal normal. The degree of texturing can be estimated through acquisition of an X-ray pole figure.31,32 For our in-plane pole figure measurements, the source and detector are placed in position to detect diffraction from the planes of interest ((002) planes in the case of Figure 3a). The plane formed by the source, detector, and beam path is then tilted about the in-plane 11844
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increased numbers of (002) planes perpendicular to that direction. Figures 3a and b are (002) and (101) pole figures for CdS NWs grown at relatively high galvanostatic currents. Figure 3a shows diffraction intensity highly localized in the center of the pole figure, indicating that the [002] direction lies predominantly normal to the sample surface and along the NW growth direction. The corresponding (101) pole figure for the same sample, Figure 3b, shows a ring of intensity oriented 60.8° tilted from normal, in reasonable agreement with the predicted orientation of 62°. The presence of (101) diffraction localized at this orientation confirms the presence of the hexagonal phase rather than cubic. Growth carried out at low current, Figure 3c, shows (002) diffraction intensity that is diffused over the entire orientation space and quite weak (note that the intensity is scaled 30 times that of Figure 3a). This indicates little to no texturing. The degree of orientation for the samples was compared by fitting a Gaussian peak to the pole figure intensity distribution and plotting this peak width as a function of growth current in Figure 3d. Lower peak widths correspond to more effective alignment of the [002] direction along the NW axis. The results reveal a near step function for material alignment, with a high degree of alignment found for samples grown above 75 μA. Additionally, there is an anticorrelation between the peak width and the fraction of growth time after τ, as discussed below. These results clearly identify the grown material as hexagonal CdS and give guidance for the effective growth of highly ordered and crystallographically textured CdS NWs. Next, we will discuss the electrochemical processes governing this growth.
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Figure 2. (a) XRD results showing only peaks indexed to hexagonal CdS and Au. Peak intensity ratios indicate significant texturing of the [002] direction along the NW axis. (b) Schematic of the hex-CdS structure with relevant planes pictured.
DISCUSSION The growth of CdS proceeds through two distinct regimes, as revealed in the chronopotentiogram shown in Figure 4a. As growth is initiated at t = 0, the applied voltage quickly drops from its open-circuit value near 0 to ∼−0.6 V in order to drive the required galvanostatic current (−100 μA in the case of the data shown in Figure 4a). Deposition occurs for some time under a moderately varying voltage until t = τ. At τ, a change in the system causes the need for greater voltage to maintain the current set point. Growth proceeds at a roughly constant voltage thereafter. Growth in these two regimes (pre-τ and post-τ) commences through different processes and produces structurally distinct materials. The transition time τ is a function of the deposition current and can be interpreted as a result of ion depletion at the growth front, which occurs under galvanostatic conditions.33 As Cd2+ ions are deposited, their local concentration decreases, and at time τ, the concentration of Cd2+ at the growth front reaches zero, causing a transition to a diffusion-limited growth mode. This behavior is described by the Sand equation34
Figure 3. (a) The (002) and (b) (101) in-plane pole figures for the sample grown at −100 μA. The fit to (b) confirms (101) planes tilted 60.8° from normal, in close agreement with the expected value of 62°. (c) The (002) in-plane pole figure for a sample grown at −50 μA, demonstrating very little texturing. (d) Plots of the fwhm for a Gaussian fit to (002) pole figures (filled circles), demonstrating a correlation between the reduced fwhm (highly textured) and the fraction of growth time spent after t (open squares).
nFAD01/2π 1/2 iτ1/2 = C0 2
(1)
Here, i is the driving current, C0 the original concentration of the ion being depleted, n the ion charge state, A the working electrode area, and D0 the ion diffusion coefficient. For a wellbehaved system, the term on the left is not independent on i or C0. This is not the case for our system. However, accounting for currents associated with charging35,36 of the zone near the growth front leads to qτ = aC0τ1/2 + b, where qτ is the total charged that has passed from t = 0 to τ, a = nFAD01/2π1/2/2, and b is a correction accounting for charging of the double
projection of the beam direction. At each tilt position, the sample is rotated 360° about the sample normal, generating a three-dimensional data set of (002) diffraction intensity. Regions of high intensity in the pole figure correspond to 11845
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Figure 4. (a) Typical chronopotentiogram of galvanostatic growth of CdS (data corresponds to growth at −100 μA). (inset) Plot of qτ, the charge passed from t = 0 to τ, as a function of growth current. (b) The linear response of qτ verses t1/2 allows extraction of the Cd2+ diffusion constant.
Figure 5. Direct TEM imaging of a microstructure generated in the pre-τ and post-τ growth regimes. (a) Chronopotentiogram showing a previously discussed transition with τ ≈ 1600 s, verifying that both growth modes are represented in the Au−CdS−Au NW pictured in (b). High-resolution TEMs of the left and right of (b) are shown in (c) and (d), respectively. SAED patterns taken with the aperture in the location of the black circles in (b) are shown in (e) and (f). Results show a polycrystalline and highly ordered single plane material in pre-τ and post-τ growth regimes, respectively. The inset to (a) is schematic of crystallites with the [002] direction indicated by arrows, and it illustrates the transition from random to oriented growth.
layer. Therefore, a linear response of qτ versus τ1/2 supports this mechanism and allows for extraction of the diffusion constant because all other parameters are known. Accordingly, τ values were extracted, and a plot of qτ versus τ1/2 is shown in Figure 4b. These data behave linearly and yield D0 = 1.23 × 10−6 cm2/ s. This value is approximately a factor of 2 less than the 2.64 × 10−6 cm2/s measured by Baranski37 for Cd2+ diffusion in DMSO and is likely due to reduced diffusion in our nanoscale pores.38 With the concept of pre-τ and post-τ growth regimes in mind and τ values extracted for the growths, the fraction of total growth time occurring after τ can be computed and is plotted on the second set of axes in Figure 3d. Anticorrelation was found between the fraction of growth time after τ and the FWHM of the pole figure intensity, suggesting that growth occurring after τ produces highly textured material oriented along the [002] direction and growth before τ produces randomly oriented polycrystalline material. This hypothesis will be examined below. Direct structural imaging of material grown in the pre-τ and post-τ regimes is seen in the TEM data in Figure 5. Here, deposition was carried out in a membrane with 80 nm pores in order to produce electron-transparent NWs suitable for TEM
imaging and selected-area electron diffraction (SAED). The CdS segment is 0.8 μm long and is capped by a long (1.5 μm) Au segment at the bottom and a short (0.2 μm) Au segment at the top, allowing one to distinguish CdS material grown at the beginning of the deposition (pre-τ) from that grown at the end (post-τ). The chronopotentiogram for this growth (Figure 5a) shows the same general features observed for deposition into larger-pored membranes with τ ≈ 1600 s. The NW pictured in Figure 5b shows the longer Au segment on the left, indicating that growth proceeded from left to right in this image. The two SAED patterns of Figure 5c and d correspond to material grown pre-τ and post-τ, respectively, and are taken with apertures at locations indicated by black circles in Figure 5b. The SAED pattern corresponding to pre-τ growth, Figure 5c, exhibits spotty rings indexed to hexagonal CdS, indicating polycrystalline material. Diffraction from the post-τ growth regime, Figure 5d, shows very strong spots corresponding to 11846
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(002) planes lying along the axial direction of the NW along with weaker (101) spots and none of the rings associated with polycrystalline material. Spots from the capping gold also appear for gold (111) and (002) planes. The SAED patterns indicate that post-τ material is composed of highly textured material with the [001] direction lying along the NW axis. Similarly, the HRTEM images of Figure 5e and f correspond to pre-τ and post-τ growth, respectively. There are multiple sets of lattice fringes observed in the pre-τ image associated with multiple crystallites, while the post-τ region exhibits a single set of lattice fringes extending over large areas. Although the SAED and HRTEM results suggest a transition from polycrystalline to single-crystalline material, the post-τ region does exhibit angular spread (“arcing”) of the (002) SAED spots and diffraction contrast in the bright-field TEM images, indicating that this highly oriented material (
