From Copper Nanocrystalline to CuO Nanoneedle Array: Synthesis

5050

J. Phys. Chem. C 2007, 111, 5050-5056

From Copper Nanocrystalline to CuO Nanoneedle Array: Synthesis, Growth Mechanism, and Properties Yueli Liu,†,‡ Lei Liao,† Jinchai Li,† and Chunxu Pan*,† Department of Physics and Key Laboratory of Acoustic and Photonic Materials and DeVices of Ministry of Education, Wuhan UniVersity, Wuhan 430072, China, and School of Materials Science and Engineering, Wuhan UniVersity of Technology, Wuhan 430070, China ReceiVed: December 30, 2006; In Final Form: January 30, 2007

In the present work, a novel method for synthesizing a one-dimensional CuO nanoneedle array was introduced; that is, first, a pure copper nanocrystalline layer was plated by using a periodic reverse pulse plating process, and then the CuO nanoneedle array grew upon the layer through thermal oxidation in the air. The nanocrystalline layer and nanoneedles were characterized by using scanning electron microscopy, regular and high-resolution transmission electron microscopies, X-ray diffraction, X-ray photoelectron spectroscopy, and a thermal analysis system (thermogravimetry and differential scanning calorimetry). The results showed that the oxidation temperature and the Cu2O intermediate phase played key roles for growing CuO nanoneedles. A self-catalyzed based-up diffusion model was proposed for interpreting the nanoneedles’ growth mechanism. The field emission property of the nanoneedles showed that the electron emission turn-on field is about 0.5 V µm-1 at current density 10 µA cm-2, and the maximum current density is 2.5 mA cm-2 with a linear F-N relationship. The full Sun efficiency (η) of the CuO nanoneedle used as a cathode in dye-sensitized solar cells is 1.12%, which shows it to be a potential material in solar cells.

Introduction

TABLE 1: Bath Compositions and Pulse Plating Conditions

As a p-type semiconductor with a narrow band gap (1.2 eV),1 cupric oxide is widely used as a gas sensor,2 as magnetic storage media,3 in solar-energy transformation,4 in electronics,5 as a semiconductor,6 and so forth. Since nanoscale materials exhibit novel physical and chemical properties, the research on cupric oxide nanomaterials has been investigated a great deal. Up to now, there have been many methods for synthesizing onedimensional (1-D) cupric oxide nanomaterials, such as thermal decomposition of the CuC2O4 precursor,7 the wet chemical route,8 and thermal oxidation.9-13 Recently, CuO nanowires have been obtained by thermally oxidizing a Cu grid in the air9 and Cu foils or substrates.10-13 In general, the acceptable growth mechanisms for 1-D CuO nanomaterials are the V-L-S (vapor-liquid-solid)14-15 and V-S (vapor-solid) models,16-17 which indicate that, with high Cu and CuO vapor pressures at a suitable temperature, a catalyst metal becomes a nanoscale liquid droplet and absorbs Cu and CuO reactants from the vapor phase for forming a eutectic alloy droplet. This droplet acts as a nucleation site for growing an individual nanowire and confines its diameter when the liquid droplet is supersaturated with the growth material. As a result, a eutectic catalyst particle can be observed at the tip of the nanowires. However, in the V-S model, no flux material is necessary during growth. CuO is considered to be a functional material such as the field emitter and cathode material in dye-sensitized solar cells.18-22 Recently, nanosized CuO particles, films, and nanofiber arrays were shown to exhibit novel field emission (FE)

composition or parameter

data

cupric sulfate (CuSO4‚5H2O) boric acid (H3BO3) saccharin (C7H5NO3S) sodium fluoride (NaF) PH value on-time (ton) off-time (toff) positive pulse work time negative pulse work time average voltage of positive electrode average voltage of negative electrode bath temperature plating time

210 g/L 45 g/L 5 g/L 0.5 g/L 1.0 2.5 ms 4.0 ms 8T 2T 10 V 5V 25 °C 5 min

properties.18 Furthermore, due to the comparable work functions between CuO (Φµ ) 5.3 eV) and Pt (Φµ ) 5.65 eV), CuO nanorod arrays possess the potential to be used as cathodes to replace Pt in dye-sensitized solar cells,23 and this may also be desirable for economic reasons. It is well-known that pulse plating is an effective electrodeposition technique for synthesizing nanocrystalline layers with a grain size in the range of 6-100 nm. 24-26 In the present work, we propose a process for growing CuO nanoneedle arrays from the copper nanocrystalline layer by using thermal oxidation in the air, which can not only accelerate the growth rate but also control the diameters. It is expected that the present CuO nanoneedles exhibit better field emission properties and wide potential applications in dye-sensitized solar cells. Experimental Section

* To whom correspondence should be addressed. Tel.: +86-27 62367023. Fax: +86-27 68752569. E-mail: [email protected]. † Wuhan University. ‡ Wuhan University of Technology.

Synthesis. A numerical control double-pulse (square-wave pulse) plating electric source (GKDM 30-15, Xin Du, China) was used for preparing the copper nanocrystalline layer. The

10.1021/jp069043d CCC: $37.00 © 2007 American Chemical Society Published on Web 03/09/2007

From Cu Nanocrystalline to CuO Nanoneedle Array

J. Phys. Chem. C, Vol. 111, No. 13, 2007 5051

Figure 1. Copper-plated nanocrystalline layer on a copper substrate at the output frequency 100 Hz and duty cycle 10%: (a) SEM morphology and (b) XRD pattern.

anode was a standard copper electrode with a purity > 99% and sized 50 × 50 mm2. The cathode was a commercial pure copper (Cu) substrate of size 30 × 30 mm2, which was mechanically polished for certain roughnesses by using different grades of abrasive papers or given a mirror finish before plating. The bath compositions and plating conditions are listed in Table 1. The pH value of the electrolyte was adjusted by adding sulfuric acid (H2SO4) and potassium hydroxide (KOH). Specimens of the plated Cu nanocrystalline layer were peeled off and cut into small pieces, then washed in an aqueous solution of 1.5 M HCl for 1 min, rinsed in deionized water, and at last dried in nitrogen. The specimens were oxidized in a thermogravimetric (TG) and differential scanning calorimetry (DSC) analyzer with the process, that is, first heated at a rate of 5 °C to the oxidation temperatures of 700, 800, 900, and 1000 °C and held for 1 h in the air and then cooled to room temperature. Characterization. The specimens for transmission electron microscopic (TEM) observations were prepared by scraping the black-color synthesized materials from the surface of the substrate and then sonicating them in ethanol. A droplet was dispersed on a TEM microgrid. The morphologies and microstructures of the plated layers and CuO nanoneedles were characterized using scanning electron microscopy (SEM) (SIRION, FEI, The Netherlands), TEM (JEM2010, JEOL, Japan), high-resolution transmission electron microscopy (HRTEM) (JEM2010FEF, JEOL, Japan), and X-ray diffraction (XRD) (D8 Advanced XRD, Bruker AXS, Germany). X-ray photoelectron spectroscopy (XPS) measurement was performed in the Escalabmk-II XPS apparatus (VG Scientific, England) with the Al target. The emission angle between the photoelectron spectra and the sample surface was 45°, and the calibration of the binding energy of the electron spectrometer was made by using the maximum of the adventitious C1s signal at 284.6 eV. The

Figure 2. Representative XPS spectra of the plated Cu nanocrystalline layers: (a) survey spectrum, (b) Cu 2p spectrum (A, 2p peak of Cu0 phase; B, 2p peak of Cu2+ phase), and (c) O 1s spectrum (A, 1s peak of O2- phase; B, 1s peak of O0 phase).

TG and DSC analysis was performed in the thermal analysis system (STA 409PC/4/H Luxx, NETZCSH, Germany), and the accuracy is 2 µg and 10 µV in the TG and DSC analyzer, respectively. The apparatus for field emission characterization consisted of two electrodes, a sample cathode and a copper circular anode with diameter of 2 mm; the gap between the cathode and anode had been adjusted with a micrometer caliper. As a result, the gap between these two electrodes could be controlled from 10 to 500 µm accurately; in the present experiments, the space between them was 100 µm. The voltage of the two electrodes could be varied from 10 V to 3 kV, and the emission current was measured by a picoammeter (Keithley 2000). The pressure in the chamber was under 1 × 10-6 Pa.

5052 J. Phys. Chem. C, Vol. 111, No. 13, 2007

Liu et al.

Figure 3. As-grown CuO nanoneedles after thermal oxidation in 700 °C: (a) SEM morphology and (b) XRD pattern.

The present dye-sensitized solar cell was constructed and assembled according to the process described in ref 27. It has an ITO/TiO2/dye/E/CuO/Cu structure with a 1-D CuO nanoneedle film/Cu substrate as the cathode. The current (I)-voltage (V) profiles of the solar cell were measured under 1 Sun illumination (100 mW cm-2). Results and Discussion Figure 1 shows the SEM morphology and XRD pattern of the plated nanocrystalline copper layer with the following plating conditions: output frequency ) 100 Hz and duty cycle ) 10%. The nanosized particles or grains are in the range of 3545 nm calculated from the XRD patterns according to the Scherrer formula. Figure 2 illustrates the further XPS measurements for the plated layer. The survey spectrum reveals the existence of the Cu and O elements. The Cu 2p spectrum shows that the plated Cu nanocrystalline layer consists of two kinds of phases; that is, a 2p3/2 peak of a Cu substance formation is located at 932.5 eV in portion A, while the Cu2+ 2p3/2 peak occurs at 933.4 eV in portion B, and the relative ratio of the quality content between the Cu and Cu2+ phases is about 3.15:1. It revealed that the Cu nanocrystalline layer was naturally oxidized in the air, which was also proved by the O 1s spectrum shown in Figure 2c; that is, the 1s peak of the O2- phase is located at 531.3 eV in part A, and the absorbed O substance material is observed at 532.9 eV in part B. The difference between these XRD and XPS results is due to the difference in their experimental mechanisms. XPS measurement mostly shows the surface information with a depth of 3-5 nm, while XRD generally detects the bulk material. In the present case, the XRD results indicate the main phase pure Cu of the plated layer.

Figure 4. Representative XPS spectra of the as-grown CuO nanoneedles: (a) survey spectrum, (b) Cu 2p spectrum, and (c) O 1s spectrum (A, 1s peak of O2- phase; B, 1s peak of O0 phase).

However, the layer surface may have been slightly oxidized due to exposure in the atmosphere, which is measured by XPS. When the plated layers were thermally treated under the temperature 700 °C, well-aligned and vertically grown CuO nanoneedles were obtained within the host materials with a length over 10 µm and a root diameter of 30-50 nm, as shown in Figure 3a. Figure 3b illustrates the XRD profiles. Obviously, after oxidation, the peaks in the plating layer disappeared and the peaks of the CuO and Cu2O appeared, which confirms that the pure Cu nanocrystalline layer has been transformed into the CuO and Cu2O phases, which is in accord with the other research results.12 It seems that the Cu2O phase is the intermediate formation during the synthesis process of CuO materials. Figure 4 illustrates the XPS profiles of the CuO nanoneedles. The survey spectrum shows the presence of Cu and O elements.



Figure 5. HRTEM image of CuO nanoneedles (FFT transformation and magnified image inserted).

Comparing to Figure 2a, the present Cu 2p spectrum illustrates the Cu2+ phase with a 2p3/2 peak at 933.84 eV, and no Cu+ phase is observed as described by the XRD result in Figure 3b. The present O 1s spectrum at 531.8 eV in part A belongs to an O2- phase in Figure 4c, and the 1s peak at 532.9 eV in part B is due to the absorption of the O atom in the host material. Therefore, the Cu+ phase only exists in the oxidation layer between the CuO nanoneedles and the Cu nanocrystalline layer, which also suggests that the Cu2O acts as an intermediate material during CuO nanoneedle growth.12 Figure 5 shows the HRTEM microstructures of CuO nanoneedles, that is, monocline structure, and straight lattice fringes normal to the nanoneedle axis with a CuO {001} interplanar spacing of 0.253 nm, which is also co-incident with the XRD result in Figure 3b. When the plated layers were thermally treated under variant temperatures 700, 800, 900, and 1000 °C, it was shown that the temperatures between 700and 800 °C provided a desired

Figure 7. Thermal analysis measurement of the plated Cu nanocrystalline at different oxidation temperatures: (a) DSC curve and (b) TG curve.

condition for growing well-aligned and high-density CuO nanoneedles, as shown in Figure 6. However, when the temperature increased above 900 °C, the oxidation was nanosized clusters with few nanoneedles. The DSC and TG analyses were measured as a function of oxidation time at different oxidation temperatures, as shown in

Figure 6. SEM morphologies of CuO nanoneedles synthesized under variant temperatures: (a) 700, (b) 800, (c) 900, and (d) 1000 °C.


Liu et al.

Figure 8. XRD patterns of the samples prepared at different temperatures: (a) 800, (b) 900, and (c) 1000 °C (the sample at the 700 °C is shown in Figure 3b).

Figure 7. In general, the Cu oxidation process includes two steps:

Figure 9. EBSD patterns of a single CuO nanoneedle: (a) nanocrystalline particles adjacent to nanoneedle, (b) root portion, and (c) top portion.

12

4Cu + O2 f 2Cu2O

(1)

2CuO + O2 f 4CuO

(2)

Figure 7a illustrates a peak at 430 °C which is related to Cu2OCuO phase transformation, due to the insufficient Cu amount for the reaction in eq 1 in a film specimen. Therefore, the reaction in eq 2 is enhanced and forms the higher-valence CuO phase.12 These results are also co-incident with the above XRD and XPS results. In addition, the oxidation rate obeyed the parabolic law with the temperature variation, as shown in Figure 7a, and it increased below 683 °C and decreased above

683 °C and vanished at 900 °C. It was also confirmed in the TG analysis, as shown in Figure 7b; that is, the mass loss existed when the Cu2O-CuO phases formed around 430 °C. And with the oxidization temperature increased from 700 to 1000 °C, the mass loss also increased from 1.71% to 4.76%, due to the CuOCu2O and Cu2O-Cu phase equilibriums at high temperatures in the Cu-O phase diagram.12,28 This confirmed the CuO nanoneedles’ decomposition and cluster formation when the oxidation temperature was over 900 °C due to the nanosized phenomenon. The phase transformation at variant temperatures was also confirmed by the XRD pattern, as shown in Figure 8 (the XRD



Figure 11. Dye-sensitized solar cells assembled by full Sun efficiency of CuO nanoneedles. Figure 10. Field emission data showing a plot of the field-emission current vs electric field of CuO nanoneedles (F-N property plot inserted).

pattern of the sample prepared at the 700 °C temperature is shown in Figure 3b). When the temperature was below 800 °C, the phases of the as-grown product were mostly CuO and Cu2O, as shown in Figures 3b and 8a, and the content of the Cu2O phase at 800 °C was larger than that at 700 °C. However, if the temperature exceeded 900 °C, the phase equilibriums of Cu2OCu appeared and were further enhanced at 1000 °C. In a word, the phase transformations at different temperatures were consistent with the DSC and TG results, as shown in Figure 7. Obviously, the growth mechanism of the present CuO nanoneedles is not related to the regular V-L-S14-15 and V-S models,16-17 because there are no spherical particles observed at the tip of the nanoneedles and no trace of the nanoneedles found outside of the substrate layers. Therefore, a base-up selfdiffusion model is proposed; that is, the growing process of the CuO nanoneedles is controlled by the diffusion of the copper ions from the substrate, which is caused by the local electrical field set up by the oxygen ions at the solid/gas interface.29 And when the temperature increases to a certain value, the nanosized copper becomes active and starts to react with the oxygen, which leads to the precipitation of a Cu2O phase. Then, the intermediate Cu2O phase acts as a catalyst site inside which the Cu and O atoms diffuse and forms the CuO nanoneedles. When the temperature is over certain value, the CuO phase transforms into a Cu2O or Cu phase; then, the nanoneedles will be destroyed and form clusters. Therefore, the Cu2O phase plays a key role in the formation of CuO nanoneedles in this process. The growth orientation of a single CuO nanoneedle is evaluated by using an electron back-scattered diffraction (EBSD) technique attached to the SEM. Figure 9 shows the Kikuchi line patterns of the nanocrystalline particles adjacent to the nanoneedle, root, and top portions of a single nanoneedle. It is found that the nanocrystalline “catalyst” and the nanoneedle

exhibit similar orientation relationships, which experimentally approves the growth mechanism as discussed above. The field emission property may be a potential application of CuO nanoneedles. Figure 10 illustrates the plot of the emission current densities versus electric field (J-E) for the CuO nanoneedles. It is found that the electron emission turnon field (Eto) is about 0.5 V µm-1, when Eto is defined as the macroscopic fields required to produce a current density of 10 µA cm-2. In addition, the present maximum current density is 2.5 mA cm-2 when the electron emission field is 27 V µm-1. An emission current density of 1 mA cm-2 is achieved at 21 V µm-1, which represents the minimum emission current required to produce a luminance of 300 cdm-2 from a variable-gain amplifier field emission display with a typical high-voltage phosphor screen efficacy of 9l mW-1.30 Compared with the results listed in Table 2,18-21 the present turn-on voltage (Eto) is quite low and the FE current density is still quite high. This is because that the as-synthesized CuO nanoneedles possessed smooth facets and small radii of curvature at the tips, which provides stable and efficient field emission properties. In general, the FE current depends on the work function and geometry of the sample.31 And the work function can be obtained from the Fowler-Nordheim (F-N) equation in which the FE current from a metal or semiconductor is attributed to the tunneling of electrons from the material into a vacuum under the influence of an electric field.32,33 The emission current can be expressed in terms of the experimental parameters according to the following equation:

ln

()

I 1 ) (-6.8 × 107 RRtipΦ3/2) + offset 2 V V

where I is the current density, V is the applied field, Rtip is the tip radius of curvature, R is a modifying factor, and Φ is the work function. RRtipΦ3/2 can be estimated from the slope of the F-N plot of ln(I/V2) against 1/V. In the present work, the F-N plot shows a linear relationship with a two-stage slope, as shown in Figure 10. The R value

TABLE 2: FE Properties of Variant 1-D CuO Nanomaterials from Different Processes

method self-catalytic growth18 V-L mode20 gas-solid reaction19 aqueous solution21 present result

type nanofibers nanorods nanoparticles nanowire nanofibrils nanobelts nanoneedles

(3)

Eto (V µm-1) 6-7 3.3 6 11 0.5

current density (mA cm-2)

work function (eV)

9.0 0.2 8 × 10-4 0.45 3.0∼9.5 1.2 2.5

0.56-2.62 0.30-1.39 2.5-2.8 4.1-4.3 none 0.52-2.28 0.21-0.98


Liu et al.

between 1 and 10 has been used in a pioneer study.18,22 When an R of 1-10 is used, with Rtip) 30 nm for our calculations, the work functions (Φ) of the aligned CuO nanoneedles are in ranges of 0.52-2.28 eV and 0.21-0.98 eV for slopes (i) and (ii), respectively. The value is close to the results of 0.32.62 eV as reported by Chen et al. in Table 2,18 which implies that the field emission from the CuO nanoneedle followed the F-N theory and the emitted current is generated by quantum tunneling.34 Similar to Yang et al.’s research on the dye-sensitized solar cell,36 the present work reveals that the full Sun efficiency (η) is 1.12% with a fill factor of 0.37 when using CuO nanoneedles as the cathode, as shown in Figure 11. For comparison, the full Sun efficiency (η) with the regular solar cells using a Pt cathode (ITO/TiO2/dye/E/Pt/ITO) under the same illumination conditions is illustrated in the upper plot in Figure 11. In addition, the present full Sun efficiency (η) is much higher than that of Yang’s 0.12-0.29%23 and also higher than the value of 0.8% using Cu2O film.22 Generally, the power conversion efficiency η of a solar cell is given as

P ) FF × |Jsc| × Voc

(4)

Where FF is the fill factor, |Jsc| is the absolute value of the current density at short circuit, Voc is the photo voltage at open circuit, and Pin is the incident light power density. In principle, the maximum |Jsc| is determined by how well the absorption window of the dye overlaps the solar spectrum.23 In the present process, the 1-D CuO nanoneedles are tightly adhered to the Cu substrate and the needlelike tip provides a higher surface area than that of Yang et al.’s work.23 Consequently, the present nanoneedles provided an enhanced electron diffusion length and dye adsorption.23 However, this is an initial result, and in fact, the full Sun efficiency can be greatly improved by reducing the resistance between the nanoneedles and substrates through avoiding oxide layer formation during thermal oxidization. Conclusions In conclusion, the plated copper nanocrystalline layer provides a desired catalytic action for growing dense, well-aligned, and single-crystalline CuO nanoneedles based upon a self-catalyzed base-up diffusion process. It is found that the oxidation temperature plays a key role for the growth and morphology of 1-D CuO nanoneedles. The DSC, TG, and XRD measurements confirm that Cu2O is an intermediate phase that acts as a catalyst site for synthesizing CuO nanoneedles. Physical properties show that the present CuO nanoneedles possess a good field emission property and a high full Sun efficiency for potential application as the cathode in dye-sensitized solar cells. Acknowledgment. We would like to thank Dr. Peng Liu and Academician Shoushan Fan (Department of Physics, Tsinghua University) for their kind and excellent technical assistance on filed emission experiments. This work was supported by a Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China (FANEDD; No. 200233).

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From Copper Nanocrystalline to CuO Nanoneedle Array: Synthesis

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