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Novel Preparation and Photoelectrochemical Properties of g-CuI Semiconductor Nanocrystallites on Screen-Printed Carbon Electrodes Cheng-Teng Hsu, Hsieh-Hsun Chung, Annamalai Senthil Kumar, Jyh-Myng Zen* Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan *e-mail: [email protected] Received: April 10, 2005 Accepted: June 13, 2005 Abstract Cuprous iodide (g-CuI) is an important semiconductor material having a bang gap of 3.1 eV often used for visible light assisted photoelectrochemical and solar energy conservation systems. We report the first and unique preparation of fine and precisely controlled g-CuI semiconductor nanocrystallites on the surface of a screen-printed carbon electrode using a photoelectrochemical copper nanoparticle deposition method with tris(hydroxymethyl)aminomethane (Tris) buffer solution as a control medium. Tris buffer helps to split Cu I2 O and CuIIO oxidation states through specific complexation mechanism and in turn to selective iodination of Cu I2 O to the formation of g-CuI on the electrode. Stable and linear photoelectrochemical response was further demonstrated against variable light intensity up to 400 Klux using the g-CuI modified system. Keywords: Semiconductor nanocrystallites, Copper(I) iodide, Photoelectrochemical response, Screen-printed electrode DOI: 10.1002/elan.200503320

1. Introduction Nano-crystalline semiconductor materials exhibit a wide range of chemical and physical properties that are finding application in devices such as solar cells, photocatalytic coatings, electrochromic windows, and supercapacitors [1 – 8]. Among these, g-CuI is a convenient p-type semiconductor material used to construct a fully solid-state dyesensitized photovoltaic cell because of its optical transparency and hole conductivity [9 – 12]. Various copper forms, such as Cu metal, Cuþ2 ion, organometallic Cuþ2, Cu2O or CuO under different bath conditions have been reported for the chemical preparation of CuI through iodination [13 – 16]. Nevertheless technological applications often require the immobilization of the semiconductor nanocrystallites on the surfaces of solid supports. Instead of the transferal suspension to a surface, the direct synthesis of semiconductor nanocrystallites on a surface of interest appears to be a better approach. Penner?s group developed a hybrid electrochemical/chemical method for the synthesis of CuI selectively on a defect mode-freshly cleaved highly oriented pyrolytic graphite (HOPG) surface with three discrete steps: (1) electrodeposition of Cu nanoparticles, (2) oxidation of Cu0 ! Cu I2 O, and (3) displacement of oxygen in Cu I2 O by iodide [17, 18]. Selective generation and stabilization of the Cuþ1 form and the ca. 1 nm HOPG flasks on electrode surface structure were concluded as key factors for the chemical formation of CuI. Such a hybrid procedure Electroanalysis 17, 2005, No. 20, 1822 – 1827

resulted in b-CuI semiconductor nanocrystallites. Note that CuI is water insoluble solid with three crystalline phases (a, b, and g) and both the cubic a-phase and the hexagonal bphase are ionic conductor [19 – 25]. Moreover, specific tuning the copper surface to Cuþ1 state is difficult since the oxidation state is relatively unstable over the Cuþ2 state. We report here a novel and simple route, in which Cuþ2 chelating tris(hydroxymethyl)aminomethane (Tris) compound is used as an indicator, to generate the Cu2O form for the preparation of pure and fine g-CuI semiconductor nanocrystallites on disposable screen-printed carbon electrodes (SPE/Cu2O). The Tris medium is crucial as it can help to effectively split the Cuþ1 and Cuþ2 states and thus makes the precisely control of Cu I2 O possible. Controlled displacement of oxygen in the Cu2O by iodide was then performed to prepare the g-CuI. The g-CuI semiconductor nanocrystallites prepared in this work shows excellent stability and photoelectrochemical response.

2. Experimental 2.1. Reagents Tris(hydroxymethyl)aminomethane was bought from Aldrich. All the other compounds used in this work were of ACS-certified reagent grade. A 200-ppm Cuþ2 solution in 0.1 M HNO3 was used for the platting experiments. AqueE 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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ous solutions were prepared using deionized water. Unless otherwise mentioned, a pH 8 Tris buffer solution of I ¼ 0.1 M was used as the base electrolyte.

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experiments were performed at room temperature (25  2 8C).

3. Results and Discussion 2.2. Apparatus Cyclic voltammetric (CV) and chronoamperometric (i-t) experiments were carried out with a CHI 627 electrochemical workstation (CH Instruments, Austin, TX, USA) with 10 mL of base electrolyte. The three-electrode system consists of a Cun/SPE working electrode (geometric area ¼ 0.2 cm2), an Ag/AgCl reference electrode, and a platinum (geometric area ¼ 0.07 cm2) auxiliary electrode. The disposable SPEs were purchased from Zensor R&D (Taichung, Taiwan). A stencil having a structure of continuous electrodes was used in printing the conducting carbon on a flexible polypropylene film (50  70 mm). A silver layer (not in the working portion) was first printed before coating the carbon ink (Acheron, Japan) to make the SPE effectively conductive. Then, the unit was cured in an oven at 100 8C for 30 min. After drying, an insulating layer was finally printed over the SPE leaving a working area of 0.196 cm2 with a conductive track radius of 2.5 mm. The measured average resistance was 200  2.10 W/cm. Prior to the experiments the SPE was thoroughly washed with copious amount of water. The light intensity was measured by a TES-1335 light meter (Taiwan). The thin film of g-CuI was characterized by using a reflectance mode solid-state UV-visible absorption spectrophotometer (Labguide, Model 2000, Taiwan). A JEOL JSM-6700F electron microscope operating at 2 and 5 kV was used for SEM imaging and EDX spectroscopy. XRD data was obtained from a Shimadzu XRD-6000 instrument.

2.3. Procedure The Cun/SPE was prepared by electrochemical plating on a SPE in a 200 mg/L Cu(NO3)2 aqueous solution at  0.7 V vs. Ag/AgCl for 300 s. The optimized plating time was chosen based on experiments with various plating times (100 – 400 s) in the presence of 100 or 200 ppm Cuþ2 solution under photo-irradiation (Perkin Elmer Xenon fiber optic light source) [26]. The Cun/SPE was washed thoroughly with deionized water before subsequent static experiments in pH 8 Tris buffer solution. The surface concentration of Cu (GCu(II) ¼ Q/nFA) on the Cun/SPE can be calculated by integration of the respective cathodic peak area (i.e., Q) on the cyclic voltammogram obtained at a scan rate (v) of 2 mV/s (where the peak is relatively defined) in pH 8 Tris buffer solution. A value of GCu(II) ¼ 13 nmol/cm2 was calculated for the Cun/SPE at pH 8 PBS. The Cun/SPE was then held at  0.05 V until the current became constant in a pH 8 Tris buffer solution, followed by reaction with 0.1 M KI for 100 s. In the photodetector application, the photocurrent was measured in a pH 8 Tris buffer solution in the presence of 10% KI at a biased potential (Ebias) of  0.2 V. All Electroanalysis 17, 2005, No. 20, 1822 – 1827

Scheme 1 illustrates the procedure employed for the g-CuI semiconductor nanocrystallites preparation as per the following steps: (1) electrochemical deposition of copper (200 ppm Cuþ2 in 0.1 M HNO3) at  0.7 V vs. Ag/AgCl under concurrent photoirradiation to form the Cun/SPE enriched with Cu2O, (2) electrochemical tuning of Cun/SPE to ca. 100% Cu2O on the surface with Tris buffer solution, and finally (3) displacement of the oxygen in Cu2O by iodide in an aqueous KI solution to obtain semiconducting CuI. We recently reported this photoelectrochemical deposition method to prepare fine copper nanoparticles on a rough and unordered SPE surface [26, 27]. The Cu particle size can be easily controlled by adjusting the light intensity, e.g., ca. 500 nm (without illumination), ca. 400 nm (illumination at 49 Klux), and ca. 100 nm (illumination at 399 Klux), respectively. The ca. 100 nm samples (designated as Cun/ SPE100-nm) were uniformly taken here for further investigation. Note that such a procedure yields an enriched Cu I2 O behavior with better stability than those of conventional copper electrodes. The chemical displacement of oxygen in Cu2O from the Cun/SPE100-nm with KI can yield fine g-CuI semiconductor nanocrystallites (designated as g-CuI/SPE) on a particle-by-particle basis. These semiconductor nanocrystallites accomplish narrow size dispersion with good photoresponse as will be discussed later. The success of the proposed preparation route lies in the appropriate control of the reaction matrix. More precisely, the formation of g-CuI relies on the chemical reaction between Cu2O and an unlikely chemical reaction with CuO. Figure 1A shows the CV response of the Cun/SPE100-nm in pH 8, 0.1 M PBS at a slow scan rate of 5 mV/s. As can be seen, a well-defined and enhanced Cu I2 O cathodic peak current response at  0.1 V overlapping with that of Cu0 at  0.05 V was observed. In the anodic side, a relatively smaller shoulder like peak behavior was noticed containing the overlapped oxidation signals from both Cu0 ! Cu I2 O and Cu I2 O ! CuIIO. It is thus almost impossible to 100% generate the Cu2O species under such oxidation conditions.

Scheme 1. Preparation of g-CuI on a disposable screen-printed carbon electrode support

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Fig. 1. Cyclic voltammetric responses of the Cun/SPE100-nm in pH 8 PBS (A) and Tris buffer solution (B) at a scan rate of 5 mV/s. Insert: SEM picture corresponding the surface morphology of the Cun/SPE100-nm system prepared under photoelectrochemical deposition method.

In classical systems, the chemical iodination reaction may proceed with a Cu2O þ CuO mixture and thus always results in different morphology [13 – 16]. Figure 1B shows the typical CV response of the Cun/ SPE100-nm in the presence of KI at a scan rate of 5 mV/s in pH 8, 0.1 M Tris buffer solution. As can be seen, welldefined and separated anodic peaks corresponding to Cu2O ( 0.1 V) and CuO (0.1 V), respectively, were observed. The highly repeatable CV response in the potential window of  0.4 to  0.05 V indicates good reproducibility and stability of Cu2O in this matrix. A very fast foul-off behavior, however, was observed once the potential was scanned to the region where the CuO started to occur (i.e.,  0.4 to 0.3 V). It is well known that Cuþ2 can interact strongly with Tris under conditions commonly employed in biochemistry and the complexation reaction often ends with polymeric like products [28]. Different potential segment analysis clearly confirms that the complexation between Cuþ2 and Tris buffer solution can indeed deactivate the Cun/SPE. The foul-off behavior of the electrode surface after three consecutive scans is attributed to the formation of polymeric products between CuO and Tris buffer. In other words, it is possible to assure the existence of the Cu2O species simply by maintaining a suitable potential range of 0.0 to  0.2 V vs. Ag/AgCl. Tris buffer can thus act like a warning system to prevent CuO from interfering with the g-CuI formation. Overall, in Tris buffer solution, the anodic Cu2O and CuO responses can split apart by ca. 200 mV. The foul-off behavior of the Cun/SPE100-nm starts to occur as the Cuþ2 is electrogenerated on the working surface through the formation of a strong chelating complex with Tris. The g-CuI/SPE film prepared in this work shows specific diffraction pattern at spectroscopy (XRD). As shown in Figure 2, detailed comparison in the range of 358 to 508 of

Fig. 2. X-ray diffractograms of (A) g-CuI/SPE and (B) bare SPE. Electroanalysis 17, 2005, No. 20, 1822 – 1827

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Fig. 3. Scanning electron microscopic pictures of (a) g-CuI/SPE (  10000) and (b) g-CuI/SPE (  30000); EDX of (c) Cu/SPE and (d) g-CuI/SPE.

the three diffraction patterns (25.528, 42.508, and 47.508 corresponding to the (111), (200), and (311) reflections of the CuI nanocrystals [19 – 22]. The peak at 25.528 is slightly deviated from the characteristic peak for CuI at 25.78. This deviation may be due to diffusion of iodine atoms form the unit cell. However, this peak indicates that the films are highly (111) oriented and composed of a polycrystalline gphase of CuI. Surface morphology by scanning electron microscopy (SEM) did not show agglomerates and/or groves at the g-CuI/SPE films even at the maximum magnification of the SEM (Fig. 3). Same phenomenon was also reported for the thin films of CuI prepared by pulse laser deposition technique [14]. The formation of ultrafine CuI grains may be one of the reasons. CuI grains with a cubic shape, however, were indeed obtained from a normal-sized Cu/SPE (i.e., Cu/SPE prepared without illumination). As shown in the SEM pictures, fine and uniform crystallites with a particle size of ca. 500 nm without any surface grain were obtained. Parallel energy dispersive X-ray analysis (EDAX) experiment confirmed the Cu and I molecules on the surface. The fact that the produced CuI will be tunable according to the experimental conditions is of great importance in practical applications. Figure 4 shows the solid-state UV-visible spectrum of the g-CuI/SPE. As can be seen, the onset of the absorption peak at ca. 420 nm with a steep slope clearly indicates the Electroanalysis 17, 2005, No. 20, 1822 – 1827

formation of high uniformity in particle size distribution of nanoparticles on the surface. The abrupt change in intensity at 400 nm in the absorption spectrum originates from the excitation of electrons from the valence band to the conduction band. The direct band gap energy of the CuI films was evaluated as 3.1 eV from the TUPAC plot (not shown in the text). Blue shift in the onset of optical absorption of the CuI films to that of polycrystalline powder of CuI and the band gap value observed agrees well with the previously reported CuI thin films prepared by pulse laser deposition technique [13 – 16]. The fact that the produced CuI particle size can be tunable based on the experimental conditions is of great importance in practical applications. To demonstrate the application of the proposed system in photodetection, photoelectrochemical experiments were performed in pH 8, 0.1 M Tris buffer solution containing 10% KI at a suitable biased potential (Ebias). Figure 5 shows typical photoresponses of the g-CuI/SPE under different Ebias (0 to  400 mV vs. Ag/AgCl). As can be seen, complicated photoresponses due to chemical complications were noticed when Ebias was set as either 0 or  400 mV. At an Ebias of  300 mV, relatively higher photocurrent response was observed except with poor stability. The most stable photoresponses was observed at an Ebias of  200 mV; in consideration with practical utility for long runs, Ebias ¼  200 mV was chosen as an optimum for further experi-

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Fig. 4. Solid-state absorption spectra of the g-CuI/SPE.

Fig. 6. Typical photoresponses of the g-CuI/SPE under irradiation at a biased potential of  0.2 V vs. Ag/AgCl in pH 8, 0.1 M Tris þ 10% KI solution. Insert Figure is a plot of the observed photocurrent versus photo-intensity.

Fig. 7. Practical utility of the g-CuI/SPE to sense variation in the indoor light by opening/closing window blind.

Fig. 5. Typical photocurrent against time response of the g-CuI/ SPE at different Ebias in pH 8, 0.1 M Tris þ 10% KI solution under a constant light intensity of 200 Klux.

ments. To test its application as a photodetector, various photointensities were individually exposed to the g-CuI/ SPE, which was held at the optimum condition. As can be seen in Figure 6, the light source was switched on resulting in Electroanalysis 17, 2005, No. 20, 1822 – 1827

an increased photocurrent. The photocurrent response returned to baseline when the light source was switched off. Increases in measured current resulting from increases in photointensity were observed and a linear response between photointensity and measured current is obvious according to the Figure plot inset. The calibration response is linear in the window of 13 – 400 Klux with a photocurrent sensitivity and regression coefficient of 40.55 nA/Klux and 0.996, respectively. The RSD values of intra (n ¼ 15) and inter (n ¼ 3) g-CuI/SPE responses were all < 5%. To further extend into practical applications, the proposed system was demonstrated to sense the variation of indoor sunlight intensity. As can be seen in Figure 7, the working system is highly sensitive to sense the indoor light by closing/opening the window blind. Overall, the spectral response of g-CuI/ SPE in the visible region is the main merit of the present

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system for the possible application of photoelectrochemical cells and to other photosensing experiments. The work is in progress.

4. Conclusions We have presented a new approach for preparing g-CuI semiconductor nanocrystallites on a disposable SPE. The procedure is fast, inexpensive, and suitable to be integrated into large area device fabrication techniques. The fine g-CuI films have a highly crystalline structure composed of narrow band-gap semiconductor nanocrystals, as determined by several analytical methods. Furthermore, we demonstrated the potential application of the g-CuI/SPE in photodetection. Further investigations are currently underway to expand the scope of this system into other possible applications.

5. Acknowledgements We would like to thank the financial support from National Science Council of Taiwan. Helpful discussions from Dr. Hong-Yi Tang and Dr. Eric D. Conte are appreciated.

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