Electrochemically Controllable Growth and Tunable Optical Properties

Electrochemically Controllable Growth and Tunable Optical Properties of Zn1-xCdxO Alloy Nanostructures Gao-Ren Li,* Qiong Bu, Fu-Lin Zheng, Cheng-Yong Su, and Ye-Xiang Tong* MOE Laboratory of Bioinorganic and Synthetic Chemistry/School of Chemistry and Chemical Engineering/Institute of Optoelectronic and Functional Composite Materials, Sun Yat-Sen UniVersity, Guangzhou 510275, China

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 3 1538–1545

ReceiVed May 14, 2008; ReVised Manuscript ReceiVed December 21, 2008

ABSTRACT: Here we explore an electrochemical route to prepare ternary Zn1-xCdxO (0 < x < 0.15) rod-like nanoparticle aggregates in a solution of ZnCl2 + CdCl2 + KCl at a temperature of 75 °C. The longest rod-like nanoparticle aggregates of 10-15 µm can be achieved. X-ray energy dispersive spectroscopy (EDS) results demonstrated that Cd, Zn, and O elements existed in the deposits, and ternary Zn1-xCdxO compounds were obtained. high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) analyses confirmed that these nanoparticles in Zn1-xCdxO rod-like nanoparticle aggregates were singlecrystalline. The Zn1-xCdxO nanoparticle aggregate structures can lead to a blueshift of the UV emission peak compared with outof-nanoparticles and nanorods. In addition, the luminescent experiment showed obvious a redshift in the ultraviolet emission with Cd substitution increasing in the Zn1-xCdxO rod-like nanoparticle aggregates. The visible emission band was not observed in photoluminescence spectra of Zn1-xCdxO rod-like nanoparticle aggregates, suggesting almost no point defect exists. The ZnxCd1-xO nanowire clusters can be routinely obtained when the electrochemical deposition was carried out in solution of Zn(NO3)2 + Cd(NO3)2 + citric acid. In addition, we also can successfully prepare ZnxCd1-xO nanobar and nanoparticle clusters when citric acid was replaced by tartaric acid and NaNO3, respectively.

1. Introduction Among various metal oxides, ZnO is one of the most attractive functional semiconductor materials with a wide band gap of 3.37 eV because of its excellent chemical and thermal stability and its specific optoelectronic and electrical properties.1 In order to design ZnO-based devices, a crucial step is the realization of band gap engineering to create barrier layers and quantum wells in device heterostructures. CdO is an n-type degenerate semiconductor with a direct band gap of 2.3 eV, and it is a promising candidate for optoelectronics applications and other applications, including photodiodes, phototransistors, solar cells, transparent electrodes, and gas sensors.2 By alloying with CdO, the obtained ternary Zn1-xCdxO will allow a continuous expansion from 3.37 eV (band gap of ZnO) to a narrower band gap, that is, into the visible spectral range. The ternary Zn1-xCdxO alloys have been a focus of extensive research because they are regarded as an ideal candidate for ZnO-based devices.3-7 The shape and size of nanostructures are two crucial factors in determining the physical and chemical properties of nanomaterials, and thus, the control of shape and size is of great interest.8 During the past decade, one-dimensional (1D) nanostructured materials, such as nanorods, nanowires, and nanotubes have sparked a worldwide interest because of their unique optical, electronic, and mechanical properties and their potential applications in nanodevices and functional materials.9 Recently, nanoscience development has been beyond the simple pursuit for single nanoparticles, and many efforts have been focused on complex nanostructures. The hierarchical assembly of nanoscale of building blocks, such as nanocrystals, nanoplatelets, nanobelts, and nanowires, into ordered superstructures or complex architectures is a crucial process to realize functional nanosystems and will offer new opportunities to investigate their novel collective physical and chemical properties.10 However, * To whom correspondence should be addressed. E-mail: ligaoren@ mail.sysu.edu.cn (G.-R.L.).

Figure 1. Cyclic voltammograms of redox processes in (1) the solution of 0.1 M ZnCl2 + 0.03 M CdCl2 + 0.1 M KCl (no oxygen); (2) the oxygen saturated solution of 0.1 M ZnCl2 + 0.03 M CdCl2 + 0.1 M KCl; (3) the oxygen saturated solution of 0.05 M ZnCl2 + 0.01 M CdCl2 + 0.1 M KCl at 90 °C.

the assembly of nanoparticles into 1D chains or aggregates is largely unexplored.11 Compared with randomly dispersed nanoparticles, the hierarchical nanoparticle chains or aggregates can show distinct optical,12 magnetic,13 and electrical properties.14 At present, it is still a challenge to find a simple and universal strategy with a high degree of control for fabricating the nanoaggregates or clusters. The assembly of nanoscale of building blocks, such as nanowires, nanobars, and nanoparticles into complex nanoaggregates or clusters will offer new opportunities to investigate their novel collective physical and chemical properties. To the best of our knowledge, this is the first report on high-density ternary metal oxide nanoaggregates or clusters. For the synthesis of Zn1-xCdxO alloy, an inherent problem is the small thermodynamic solubility of CdO in ZnO. The electrochemical deposition technique possibly is a good candidate to solve this problem, and it has many other advantages, such as simple, quick, cost-effective, and large scaleup production. The prepared 1D rod-like Zn1-xCdxO nanoparticle aggregates are freestanding, which is favorable for subsequent

10.1021/cg800496d CCC: $40.75  2009 American Chemical Society Published on Web 02/04/2009

Properties of Zn1-xCdxO Alloy Nanostructures

Crystal Growth & Design, Vol. 9, No. 3, 2009 1539

Figure 2. SEM images of hierarchical ZnxCd1-xO nanoparticle aggregates prepared in a solution of 0.1 M ZnCl2 + 0.03 M CdCl2 + 0.1 M KCl at 90 °C with a current density of 0.1 mA/cm2. (a) ×30000; (b) ×90000. (c) The typical HRTEM image and SAED pattern (inset) of ZnxCd1-xO nanoparticles in the hierarchical nanoparticle aggregates.

Figure 3. EDS spectrum of the hierarchical ZnxCd1-xO nanoparticle aggregates.

mounting or handling. In addition, ZnxCd1-xO nanowire, nanobar, and nanoparticle clusters can also be successfully synthesized.

2. Experimental Procedures The electrochemical deposition of Zn1-xCdxO was carried out in an aqueous bath composed of dissolved oxygen (bubbled through the electrolyte), 0.1 M ZnCl2, 0.03 M CdCl2, and 0.1 M KCl. Before electrodeposition, the Cu substrate was cleaned ultrasonically in 0.1 M HCl, distilled water, and acetone and then rinsed in distilled water again. All the electrochemical deposition experiments were carried out in a configured glass cell at 90 °C, in which a Cu plate (99.99 wt%, 1.0 cm2), a graphite rod (spectral grade, 1.5 cm2), and a saturated calomel electrode (SCE) served as the working electrode, counter electrode, and reference electrode, respectively. The cyclic voltammetry and electrochemical deposition experiments were carried out with Chi750 electrochemical workstation and HDV-7C transistor potentiostatic apparatus, respectively. After electrodeposition, the deposits were

Figure 4. XRD pattern of the hierarchical ZnxCd1-xO nanoparticle aggregates. (Red lines are the peak positions for ZnO). removed from the electrolyte, and rinsed first with acetone and then deionized water. The surface morphology and the phase identification of the deposited Zn1-xCdxO were characterized by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM, JEM-2010HR), and power X-ray diffractometry (D/ MAX 2200 VPC with Cu KR radiation), respectively. X-ray energy dispersive spectroscopy (EDS) was used to determine the elements of O, Zn, and Cd in deposits. The photoluminescence (PL) measurements were carried out on a fluorescence spectrophotometer at room temperature, and the excited wavelength was 325 nm.

3. Results and Discussion The cyclic voltammogram (CV) of redox processes on Cu substrates in a solution of 0.1 M ZnCl2 + 0.03 M CdCl2 + 0.1 M KCl is shown in Figure 1(1) (no oxygen), and two cathodic

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Figure 5. SEM images of (a) the hierarchical ZnxCd1-xO nanoparticle aggregates prepared in a solution of 0.1 M ZnCl2 + 0.03 M CdCl2 + 0.1 M KCl with a current density of 0.5 mA/cm2; (b) out-of-order Zn1-xCdxO nanoparticles prepared with a current density of 1.0 mA/cm2. (c) ZnxCd1-xO nanorods prepared in a solution of 0.1 M ZnCl2 + 0.03 M CdCl2 + 0.1 M KCl with a current density of 0.05 mA/cm2 at 90 °C.

Figure 6. Room temperature PL spectra of hierarchical ZnO nanoparticle aggregates (a) and hierarchical ZnxCd1-xO nanoparticle aggregates with different Cd substitutions (b-d): x ) 0.071 ( 0.004, 0.113 ( 0.006, and 0.149 ( 0.007.

waves (b) and (c) were observed. The cathodic wave (b) corresponds to the electrochemical reduction of Cd2+ to Cd (eq 1).

Cd2+ + 2e f Cd

(1)

Zn2+ + 2e f Zn

(2) -

O2+ 2H2O + 4e f 4OH

(3)

The cathodic wave (c) corresponds to the electrochemical reduction of Zn2+ to Zn (eq 2). When the cyclic voltammogram was measured in the oxygen saturated above deposition solution, the cathodic wave (a) appeared beside the cathodic waves (b) and (c). The cathodic wave (a) was assigned to the electrochemical reduction of O2 and the electrogeneration of base

Figure 7. The PL maxima of ZnxCd1-xO as a function of composition x determined from PL data.

(eq 3). The peak potentials of cathodic waves (b) and (c) in Figure 1(2) both shift negative compared with those in Figure 1(1), suggesting the concentrations of Zn2+ and Cd2+ are lower. This can be attributed to the reactions between Zn2+, Cd2+, and OH- ions (eq 4). However, when cyclic voltammetry was carried out in the oxygen saturated solution with a lower concentration of ZnCl2 and CdCl2, namely, 0.05 M ZnCl2 + 0.01 M CdCl2 + 0.1 M KCl, the current densities of cathodic waves of Cd2+ and Zn2+ were smaller as shown in Figure 1(3), which suggests that the electroreduction rates of Cd2+ and Zn2+ are slower. The formation process of Zn1-xCdxO deposits can be explained as follows according to the above cyclic voltammograms. First, the dissolved O2 in the electrolyte is electroreduced to form the base via reaction 3. Then the formation of Zn1-xCdxO alloys will happen via eq 4.


(1 - x)Zn2+ + xCd2++ 2OH- f (1 - x)Zn(OH)2 · xCd(OH)2 f Zn1-xCdxO + H2O


(4)

The electrodeposition of Zn1-xCdxO was carried out in the oxygen saturated solution of 0.1 M ZnCl2 + 0.03 M CdCl2 + 0.1 M KCl at 75 °C, and the current density was chosen in the region of 0.1-0.5 mA/cm2. Figure 2a shows a typical lowmagnification SEM image of Zn1-xCdxO deposits prepared with a current density of 0.1 mA/cm2, and it can be clearly seen that the Zn1-xCdxO deposits are entirely composed of linear rodlike nanoparticle aggregates. The enlarged image shows that the sizes of these nanoparticles are in the range of 50-75 nm as shown in Figure 2b. The lengths of these rod-like nanoparticle aggregates are very long, and they can be achieved about 10-15 µm. The high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) pattern of Zn1-xCdxO nanoparticles in the aggregates were measured, and the typical HRTEM image and SAED pattern are shown in Figure 2c, revealing that Zn1-xCdxO nanoparticles in the aggregates exhibited a single-crystal structure. EDS measurements of Zn1-xCdxO rod-like nanoparticle aggregates were carried out at a number of locations throughout the specimens, and the representative EDS spectrum is shown in Figure 3. The composition analysis showed that the Cd concentration in the Zn1-xCdxO nanoparticle aggregate was x ) 0.071 ( 0.004. The copper peaks from the substrate. The representative X-ray diffraction spectrum of prepared rodlike Zn1-xCdxO nanoparticle aggregates is shown in Figure 4. The stick spectrum in the bottom graph of Figure 4 is the XRD peaks of the ZnO (wurtzite) source powders which have been assigned according to the standard JCPDS data. The XRD spectrum of Zn1-xCdxO nanoparticle aggregates shows three peaks corresponding to (100), (002), and (101), respectively, and they all can be indexed as the ZnO wurtzite structures (hexagonal phase, space group P63mc) according to JCPDS card (No. 36-1451). No peaks corresponding to CdO are observed, which indicates the absence of cubic CdO, and consequently all the cadmium is incorporated into the ZnO lattice. Although the crystal structure of ZnO was not altered, it should be noted that a lower angle shift was observed in the XRD spectrum in Figure 4 for Zn1-xCdxO deposits compared with that of pure ZnO, indicating Cd has entered into the ZnO lattice. As the radius of Cd is bigger than that of Zn, so the cell volume of the ZnO is increased when Cd enters into ZnO lattices, which will cause shifts of ZnO (100), (002), and (101) peaks to lower angles. In addition, the peaks marked with Cu (111) and (200) correspond to Cu substrate. For the formation of Zn1-xCdxO, the thermodynamic solubility limit of Cd in ZnO is about 2.0 at% in bulk.4 However, under our experimental growth conditions the maximum Cd incorporation is moved toward larger concentrations, achieving a maximum Cd content of 15.0 at%. (The cubic CdO inclusions will be found in deposits for higher Cd contents). The reasons for high content of Cd in the rod-like Zn1-xCdxO nanoparticle aggregates can be explained as follows. It is well-known that nanostructures can dramatically facilitate the dissolution of the solid elements in alloy or compounds.6 The sizes of nanoparticles in the prepared rod-like Zn1-xCdxO nanoparticle aggregates is only about 50-75 nm, which favors the lattice to become elastically soft to a great extent. The lattice distortion in Zn1-xCdxO nanoparticle aggregates will be easily relaxed compared with that in bulk materials. Therefore, CdO solubility in the obtained Zn1-xCdxO nanoparticle aggregates will be obviously enhanced. The formation of rod-like Zn1-xCdxO nanoparticle aggregates correlated with the spontaneous oscil-

Figure 8. Room temperature PL spectra of ZnxCd1-xO (x ) 0.071 ( 0.004) hierarchical nanoparticle aggregates (a); out-of-order nanoparticles (b); nanorods (c).

lation of electrode potential occurring during electrodeposition, and it can be understood as follows. Under the electric field Zn2+, Cd2+, and NO3- ions in deposition solution are first driven to the cathode, and then the NO3- ions will be electroreduced to produce OH- ions, which will lead to the formation of Zn1-xCdxO deposits. As we all know, Zn2+, Cd2+, and NO3ions are gradually consumed during the formation of Zn1-xCdxO, and at the same time the transfers of Zn2+, Cd2+, and NO3ions are confined by ultrathin geometry of the electrolyte film. Therefore, the concentrations of Zn2+, Cd2+, and NO3- ions will decrease in front of the growing interface, and the Laplacian fields (both the concentration field and the electric field) will spend some time to compensate for this reduction.15 In the meantime, the electrode potentials of NO3- ions will drop, and the formation of Zn1-xCdxO will largely decrease or stop when the equilibrium electrode potentials of NO3- ions are lower than the actual electrode potentials. However, we should notice that after a short time the concentrations of Zn2+, Cd2+, and NO3- ions in front of the growing interface will change greatly again via the transfers of Zn2+, Cd2+, and NO3- ions in the deposition solution, and the electrode potentials of NO3- ions will increase gradually. The formation rate of Zn1-xCdxO will increase again. Therefore, the periodic Zn1-xCdxO nanoparticles will form because of the electrode potential oscillations with the deposition time increasing, which finally leads to the formation of rod-like nanoparticle aggregates. The sizes of nanoparticles or densities of rod-like nanoparticle aggregates can be controlled by changing the synthetic parameters. It is obvious that the density of rod-like nanoparticle aggregates in Figure 5a is much higher than that in Figure 2b when the current density is increased to 0.5 mA/cm2. The outof-order nanoparticles were obtained when the current density was increased to 1.0 mA/cm2 or larger as shown in Figure 5b. However, when the current density was decreased to 0.05 mA/ cm2 or smaller, Zn1-xCdxO nanorods were obtained as shown in Figure 5c. In order to successfully obtain the Zn1-xCdxO rodlike nanoparticle aggregates, the current density should be kept in the range of 0.1-0.8 mA/cm2. The effects of two main electrodeposition parameters on the compositions of Zn1-xCdxO were also studied: (1) relative concentrations of Zn2+ and Cd2+ in solution and (2) current densities. For the Zn2+/Cd2+ ratio of 10:3 in solution, Cd concentrations in Zn1-xCdxO rod-like nanoparticle aggregates were approximately x ) 0.071 ( 0.004 when electrochemical deposition was carried out with a current density of 0.1 mA/cm2 and 0.5 mA/cm2 for 60 min, respectively. However, when the current density was 0.5 mA/cm2, Cd


Li et al.

Figure 9. SEM images of the prepared samples in solution of 0.1 mol/L Zn(NO3)2 + 0.03 mol/L Cd(NO3)2 + 0.01 mol/L citric acid at 85 °C with a deposition potential of -1.5 V. (a) ×1500; (b) ×17000. (c) TEM, SEAD (upper inset), and HRTEM (lower inset); (d) EDS pattern.

concentrations in Zn1-xCdxO rod-like nanoparticle aggregates was roughly x ) 0.049 ( 0.002 and 0.082 ( 0.004 for Zn2+/ Cd2+ ratios of 5:1 and 2:1, respectively. Therefore, the composition of Zn1-xCdxO rod-like nanoparticle aggregates was sensitive to the concentration ratio of Zn and Cd salts, and it is hardly affected by the current density. Therefore, by simply tuning the compositions of deposition solution or the ratios of ZnCl2 to CdCl2, we are able to obtain different Cd contents in Zn1-xCdxO rod-like nanoparticle aggregates for the desired applications in the novel memory and optical device. The PL spectra of as-prepared Zn1-xCdxO rod-like nanoparticle aggregates with different Cd substitutions are shown in Figure 6, and there are obvious differences among these PL spectra. Before doping, a UV emission peak centering at about 377 nm (3.29 eV) was observed for ZnO rod-like nanoparticle aggregates as shown in Figure 6a. However, after doping Cd concentration with x ) 0.071 ( 0.004, an emission peak centered at about 385 nm (3.22 eV) dominates in the PL spectra as shown in Figure 6b. The emission band was redshifted and broadened compared with that of undoped ZnO rod-like nanoparticle aggregates. The redshift of the UV emission peak was attributed to the narrowing of Eg, which was reduced from 3.29 to 3.22 eV. The broadening of the emission peak can be attributed to the bound states related to the crystallographic defects, Cd content fluctuation, and band gap modification.7,16 When Cd substitution was increased from x ) 0.071 ( 0.004 to 0.149 ( 0.007, the peak positions in PL spectra of Zn1-xCdxO rod-like nanoparticle aggregates were shifted from 385 to 408 nm, reflecting the change in the exciton energy. In addition, it

can be clearly seen that the full width at half-maximums of the peaks of Zn1-xCdxO samples are slightly broadened with Cd substitution increasing. So alloyed Zn1-xCdxO with tunable bandgap emissions were successfully realized by changing the Cd content. The PL maxima for the Zn1-xCdxO rod-like nanoparticle aggregates are plotted in Figure 7 as a function of composition x. It can be clearly seen that the variation of the energy gaps with composition x almost exhibits linear dependence. For the Zn1-xCdxO system, the energy gap of the Zn1-xCdxO alloy can be expressed as a function of the Cd content (x) as6

Eg(x) ) 3.3 - 1.22x + 1.26x2

(5)

where Eg(x) is the energy gap of Zn1-xCdxO. So here the energy gaps are represented as a quadratic function of composition x. According to eq 5, predetermined bandgap emissions, that is, energy gaps, can be obtained by synthesizing alloyed Zn1-xCdxO with the corresponding composition. When the Cd concentration was x ) 0.071 ( 0.004, 0.113 ( 0.006, and 0.149 ( 0.007, we obtained a band gap of about 3.208, 3.172, and 3.145 eV, respectively. These calculated Eg values were bigger than those obtained in the corresponding PL spectra at room temperature. The difference may be attributed to the special Zn1-xCdxO nanoparticle aggregate structures. The optical properties of 1D rod-like Zn1-xCdxO nanoparticle aggregates are different from those of out-of-order nanoparticles and nanorods as shown in Figure 8. Figure 8b shows the PL spectrum of the out-of-order Zn1-xCdxO (x ) 0.071 ( 0.004)



Figure 10. SEM images of the prepared Zn1-xCdxO nanobar clusters in solution of 0.1 mol/L Zn(NO3)2 + 0.03 mol/L Cd(NO3)2 + 0.01 mol/L tartaric acid at 85 °C with deposition potential of -1.5 V. (a) ×6000; (b) ×50000; (c) TEM and SEAD (the inset); (d) EDS pattern.

nanoparticles, and the UV emission peak is at about 398 nm. Figure 8c shows the PL spectrum of Zn1-xCdxO (x ) 0.071 ( 0.004) nanorods. Besides the UV emission peak centering at about 390 nm, there is a visible emission band centering at about 486 nm, which has been suggested mainly due to the present of deep-level defects in Zn1-xCdxO nanorods. The PL spectrum of rod-like Zn1-xCdxO (x ) 0.071 ( 0.004) nanoparticle aggregates is shown in Figure 8a, and the UV emission peak appears at about 385 nm. Therefore, the nanoparticle aggregate structures of Zn1-xCdxO lead to obvious blueshift of UV emission peak compared with out-of-order nanoparticles and nanorods. In addition, the visible emission band was not observed in the PL spectrum of rod-like Zn1-xCdxO nanoparticle aggregates, suggesting no deep-level defects exist. Therefore, constructing novel nanostructures and doping different Cd contents, which will probably provide a valuable route to realize the resulting ZnO-based optoelectronic nanodevices, can effectively control the optical properties of Zn1-xCdxO. Our synthetic parameters allow further structural manipulation for Zn1-xCdxO alloy nanostructures. The ZnxCd1-xO nanowire clusters are routinely obtained when electrochemical deposition was carried out in solution of 0.1 mol/L Zn(NO3)2 + 0.03 mol/L Cd(NO3)2 + 0.1 mol/L citric acid at -1.5 V at 85 °C for 60 min. The SEM images of the prepared samples are shown in Figure 9a,b. Figure 9a shows the obtained sample is composed of many clusters at a low magnification, and it can be clearly seen that these clusters are composed of many nanowires at a higher magnification as shown in Figure 9b. These obtained nanowires are interlaced. The average diameters of these

nanowires are about 100 nm, and their lengths are very long and can be obtained to about 20 µm. Figure 9c shows transmission electron microscopy (TEM) images of the partial ZnxCd1-xO nanowire, which shows the diameter of nanowire is about 100 nm. The high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) pattern of Zn1-xCdxO nanowires were also measured, and the typical HRTEM image and SAED pattern are shown in the inset in Figure 9c. The SAED pattern reveals the singlecrystal wurtzite structure of the nanowire with a [0001] growth direction along the wire axis. The HRTEM image indicates a lattice spacing of 0.26 nm, corresponding to the (0001) planar spacing of ZnO in the wurtzite phase. EDS measurements of Zn1-xCdxO nanowires were carried out at a number of locations throughout the films, and the representative EDS spectrum was shown in Figure 9d. The composition analysis showed that Cd concentration in Zn1-xCdxO nanoparticle aggregates was x ) 0.051 ( 0.003. The copper peaks come from the substrate. Some typical different Zn1-xCdxO crystals were obtained when citric acid was replaced by tartaric acid. Figure 10a,b displays the ZnxCd1-xO nanobar clusters deposited in solution of 0.1 mol/L Zn(NO3)2 + 0.03 mol/L Cd(NO3)2 + 0.01 mol/L tartaric acid with deposition potential of -1.5 V. The diameters of these nanobars are about 100-150 nm, and their lengths are about 400-500 nm. Figure 10c shows the TEM image of the ZnxCd1-xO nanobars, which also shows the diameters of nanobars are about 100-150 nm and the lengths are about 400 nm. The typical SAED pattern is shown in the inset in Figure 10c, and it reveals the single-crystal wurtzite structure of the


Li et al.

Figure 11. SEM images of the prepared Zn1-xCdxO nanoparticle clusters in a solution of 0.1 mol/L Zn(NO3)2 + 0.03 mol/L Cd(NO3)2 + 0.1 mol/L NaNO3 at 85 °C with a deposition potential of -1.5 V. (a) ×1500; (b) ×17000; (c) TEM and SEAD (the inset); (d) EDS pattern.

Figure 12. PL spectra of the prepared Zn1-xCdxO (x ) 0.092 ( 0.005) nanoparticle clusters (1), nanobar clusters (2), and nanowire clusters (3) at room temperature.

nanowire with a [0001] growth direction along the bar axis. EDS measurements of Zn1-xCdxO nanowires were carried out at a number of locations throughout the films, and the representative EDS spectrum is shown in Figure 10d. The composition analysis showed that Cd concentration in Zn1-xCdxO nanowire aggregate was x ) 0.092 ( 0.005. The copper peaks come from the substrate. When citric acid was replaced by NaNO3, namely, the electrochemical deposition was carried out in a solution of 0.1 mol/L Zn(NO3)2 + 0.03 mol/L Cd(NO3)2 + 0.01 M NaNO3 with a deposition potential of -1.50 V, the Zn1-xCdxO nanoparticle clusters were prepared as shown in Figure 11a,b. It is can be clearly seen at a low magnification that the obtained

ZnxCd1-xO deposits are composed of many clusters in Figure 11a, and Figure 11b shows these clusters are composed of many nanoparticles. The sizes of the nanoparticles are about 150-200 nm. TEM image of the ZnxCd1-xO nanoparticles were shown in Figure 11c, which also shows the sizes of nanoparticles are about 150-200 nm. The typical SAED pattern was shown in the inset in Figure 11c, and it also reveals the single-crystal wurtzite structure. EDS measurements of Zn1-xCdxO nanoparticles were carried out at a number of locations throughout the films, and the representative EDS spectrum was shown in Figure 11d. The composition analysis showed that Cd concentration in Zn1-xCdxO nanowire aggregate was x ) 0.122 ( 0.006. The copper peaks come from the substrate. The optical properties of Zn1-xCdxO (x ) 0.092 ( 0.005) nanowire, nanobar, and nanoparticle clusters are investigated, and their PL spectra are shown in Figure 12. The PL spectrum of Zn1-xCdxO nanoparticle clusters is shown in Figure 12(1), and the UV emission peak appears at about 395 nm. Figure 12(2) shows the PL spectrum of Zn1-xCdxO nanobar clusters, and the UV emission peak appears at about 398 nm. Figure 12(3) shows the PL spectrum of Zn1-xCdxO nanowire clusters, and the UV emission peak appears at about 400 nm. Therefore, the nanowire aggregate structures of Zn1-xCdxO lead to an obvious redshift of the UV emission peak compared with nanoparticle and nanobar clusters. In addition, the PL intensity of the UV emission peaks of Zn1-xCdxO nanowire clusters is also higher than those of nanobar and nanoparticle clusters. Since the composition of these nanocrystals is the same, the red-shift of the PL spectra should be due to the structure effect.


These results suggest that the surface states of the alloyed nanocrystals also have an important effect on their PL properties. Therefore, constructing novel nanostructures and doping different Cd contents, which will probably provide a valuable route to realize the resulting ZnO-based optoelectronic nanodevices, can effectively control the optical properties of Zn1-xCdxO.

4. Conclusions The electrodeposition is a versatile and facile pathway to fabricate different nanostructures with novel architectures because of the abundance of controllable parameters. Here we report an electrodeposition route for the preparation of rod-like Zn1-xCdxO (0 < x < 0.15) alloy nanoparticle aggregates via electrochemical oscillation process in the solution of ZnCl2 at a temperature of 75 °C. This technique is simple, quick, and cost-effective. The results of HRTEM, SAED, and XRD showed that the as-synthesized Zn1-xCdxO nanoparticles in the aggregates were a pure ZnO wurtzite structure with a single-crystal structure. The nanoparticle aggregates of Zn1-xCdxO led to the blueshift of the UV emission peak in the PL spectra compared with out-of-order nanoparticles and nanorods. In addition, with the Cd content increasing in Zn1-xCdxO nanoparticle aggregates, an obvious redshift and broadening was observed in the ultraviolet emission. The effective control of optical properties of Zn1-xCdxO by constructing novel nanostructures and doping different Cd contents will probably provide a valuable route to realize the resulting ZnO-based optoelectronic nanodevices. In addition, we also can successfully prepare ZnxCd1-xO nanowire, nanobar and nanoparticle aggegrates by changing the electrodeposition parameters. The unique synthetic mechanism of this approach is expected to generate other metal oxide nanoaggregates or clusters of technological importance with well-controlled architectures. Acknowledgment. This work was supported by the Natural Science Foundations of China (Grant Nos. 20603048 and 20573136), the Natural Science Foundations of Guangdong Province (Grant Nos. 06300070, 06023099, and 04205405), and the Foundations of Potentially Important Natural Science Research and Young Teacher Starting-up Research of Sun YatSen University.


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CG800496D

Electrochemically Controllable Growth and Tunable Optical Properties

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