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XPS studies and photocurrent applications of alkali-metals-doped ZnO nanoparticles under visible illumination conditions
Author’s Accepted Manuscript XPS studies and photocurrent applications of alkali-metals-doped ZnO nanoparticles under visible illumination conditions Abdolhossein Saa´edi, Ramin Yousefi, Farid Jamali-Sheini, Ali Khorsand Zak, Mohsen Cheraghizade, M.R. Mahmoudiand, Mohammad Amin Baghchesara, Abbas Shirmardi Dezaki
PII: DOI: Reference:
www.elsevier.com/locate/physe
S1386-9477(15)30310-6 http://dx.doi.org/10.1016/j.physe.2015.12.002 PHYSE12227
To appear in: Physica E: Low-dimensional Systems and Nanostructures Received date: 10 November 2015 Revised date: 2 December 2015 Accepted date: 10 December 2015 Cite this article as: Abdolhossein Saa´edi, Ramin Yousefi, Farid Jamali-Sheini, Ali Khorsand Zak, Mohsen Cheraghizade, M.R. Mahmoudiand, Mohammad Amin Baghchesara and Abbas Shirmardi Dezaki, XPS studies and photocurrent applications of alkali-metals-doped ZnO nanoparticles under visible illumination conditions, Physica E: Low-dimensional Systems and Nanostructures, http://dx.doi.org/10.1016/j.physe.2015.12.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
XPS studies and photocurrent applications of alkali-metals-doped ZnO nanoparticles under visible illumination conditions
Abdolhossein Saa´edi1, Ramin Yousefi2*, Farid Jamali-Sheini3, Ali Khorsand Zak4, Mohsen Cheraghizade1, M. R. Mahmoudiand5, Mohammad Amin Baghchesara6, Abbas Shirmardi Dezaki7 1
2
Young Researchers and Elite Club, Ahvaz Branch Islamic Azad University, Ahvaz, Iran
Department of Physics, Masjed-Soleiman Branch, Islamic Azad University (I.A.U), MasjedSoleiman, Iran 3
Department of Physics, Ahvaz Branch, Islamic Azad University, Ahvaz, Iran 4
5
6
7
Nanotechnology Laboratory, Esfarayen University, North Khorasan, Iran
Department of Chemistry, Shahid Sherafat, University of Farhangian, 15916 Tehran, Iran
Department of Metallurgy and Materials Engineering, Masjed-Soleiman Branch, Islamic Azad University (I.A.U), Masjed-Soleiman, Iran
Department of Chemistry, Masjed-Soleiman Branch, Islamic Azad University (I.A.U), MasjedSoleiman, Iran
*Corresponding author: E-mail: [email protected], [email protected] Tel: +989166224993, Fax: +986143260093
Keywords: ZnO nanoparticles; Alkali-metals-doped ZnO; Photocurrent application; XPS spectrometer
1
Abstract: The present work is a study about a relationship between X-ray photoelectron spectrometer (XPS) results and photocurrent intensity of alkali-metals-elements doped ZnO nanoparticles, which is carried out under visible illumination conditions. The nanoparticles were synthesized by a simple sol–gel method. Structure and morphology studies of the NPs were carried out by X-ray diffraction analysis (XRD) and transmission electron microscopy (TEM). The effect of doping on the optical band-gap was investigated by using UV-visible spectrometer. The absorption peak of the doped ZnO NPs was red-shifted with respect to that of the undoped ZnO NPs. After that, the photocurrent application of the products was examined under a white light source at 2 V bias. The photocurrent results showed that, the current intensity of the ZnO NPs was increased by doping materials. However, K-doped ZnO NPs showed the highest photocurrent intensity. Finally, a discussion was carried out about the obtained photocurrent results by the O-1s spectra of the XPS of the samples. Our results suggest that the alkali-metalsdoped ZnO NPs exhibit considerable promise for highly sensitive visible-light photodetectors. Keywords: ZnO nanoparticles, Alkali-metals-doped ZnO, Photocurrent application, XPS spectrometer.
2
1- Introduction It is known that, photocurrent applications of semiconductor materials are the base of optoelectronic devices. Among different optical materials that have photocurrent applications, ZnO is the most important optical material that showed excellent photocurrent [1]. It is a semiconductor with a direct band-gap; it has piezoelectric and pyroelectric properties, it can be grown with different shapes, synthesize of ZnO nanostructures is easy and coast effective, and it is an environment- and human body-friendly material. It is also a promising material for ultraviolet nano-optoelectronic devices and lasers operating at room temperature due to its wide band gap. For these unique properties, it has been one of the hot topics of the 21st century so far. Therefore, researchers have studied about different applications of ZnO nanostructures in these years. ZnO can be used in many applications, such as photocatalyst applications [2], transparent conductive coatings [3], electrodes for dye-sensitized solar cells [4], gas sensors [5], and field emission materials [6]. In nanometer size range, the ZnO nanostructures are expected to possess interesting physical properties and pronounced coupling quite different from their bulk counterpart [7-10]. One of the best ways to improve ZnO nanostructure properties is doping. Therefore, to obtain better crystallization quality, optical, electrical, and ferromagnetic properties, researchers have been studied effect of dopant materials on ZnO properties. The most efforts have been to achieve n-type ZnO nanostructures with different applications [11-14]. However, the key challenge that needs to be overcome for the realization of most ZnO based applications is the fabrication of p-type material. P-type ZnO may be achieved by the substitution of alkali metals elements on Zn-site [15–17] and group-V elements on O-site [18– 21], respectively. Although significant progress has been made recently full control over the 3
materials conductivity type is still to be obtained and hence a comprehensive investigation of the fundamental properties of acceptors in ZnO is needed. Alkali metals elements are better dopants materials in comparison to group-V elements in terms of the shallowness of the accepter level [21]. Therefore, the investigation of different effects of alkali metals elements as dopants on structure and optical properties of ZnO nanostructures can lead to obtain different applications. Several groups and researchers have tried to study effect of alkali metals on different properties of ZnO nanostructures [15-19]. We believe that although the researches on the effect of alkali metals elements on optical and electrical of ZnO has been performed, the effects of alkali metals elements on photocurrent applications of ZnO nanostructures have not been reported to date. Such knowledge can enhance development of optoelectronic devices in the future. For these reasons, the undoped and alkalimetals-doped ZnO-NPs were prepared by a sol–gel method in the present work. We report a comparative study of the effects of alkali metals elements on the optical properties and photocurrent applications of the ZnO nanoparticles. 2- Experimental To begin the synthesis of alkali-metals-doped ZnO-NPs, analytical grade zinc nitrate hexahydrate (Zn(NO3)2 + 6H2O), lithium nitrate (LiNO3, >99%), sodium nitrate (NaNO3, >99%), potassium nitrate (KNO3, >99%), gelatin (type B from bovine skin), and distilled water were used as starting materials. All of the materials used were purchased from Sigma-Aldrich. The precursors were measured as Zn0.97X0.03O, (X = Li, Na, and K) to obtain 5 g of final product. The detailed of the NPs synthesize have been reported in our previous work [20]. The resulting powders were characterized by several tools to check their qualities. X-ray diffraction (XRD) (Philips, X’Pert, CuKa) was used to evaluate the phase characteristics of the
4
samples. Transmission electron microscopy (TEM, Hitachi H-7100) were employed to characterize morphology and structural properties of the NPs. Elemental analyses of the products were conducted using an X-ray photoelectron spectroscopy (XPS; VG-Microtech ESCA-2000). The XPS spectra were recorded using Mg-Kα radiation (1256.6 eV). The optical properties of the ZnO-NPs were characterized at room temperature using a UV–vis (PerkinElmer, Inc). Finally, the photocurrent behaviors of the NPs, which were under a white light source and chopped light illumination, were characterized by a DC circuit. For fabricated a photocurrent device, a mixture of 0.01 g NPs and 0.6 mL chitosan were used. This mixture was ultrasonicated in an ultrasonic bath and then dropped on a glass substrate between two silver electrodes. Then the device was used in a DC circuit, which was connected to an oscilloscope. Photocurrent intensities were measured under a 2 V bias potential. A 20 W compact fluorescent lamp (CFL) was employed as an excitation source. 3- Results and discussion The TEM images of the undoped and doped ZnO-NPs are shown in Fig. 1. The undoped and Li-doped ZnO NPs are spherical and uniform size approximately with an average diameter around 45 nm (Figs. 1(a) and 1(b), respectively). On the other hand, the Na- and K-doped ZnO NPs are non-uniform in sizes and shape with smaller average diameter (Figs. 1(c) and 1(d), respectively). This non-uniform shapes for the Na- and K-doped ZnO NPs could be due to a big difference between ionic radii of doping materials and ionic radius of Zn2+. In fact, Na+ and K+ ions play as passivation roles that cause to decrease the size of the nanoparticles and as well as obtained non-uniform shapes. This passivation role causes that the NPs cannot grow very fast.
5
The XRD patterns of doped and undoped ZnO NPs are indicated in Fig. 2. There are no other peaks related to any impurity or other mixed compounds. The crystalline sizes of the ZnONPs were measured by means of an X-ray line-broadening method, using the Scherrer equation: D = (kλ/βhklcosθ)
(1)
where D is the crystalline size in nanometers, λ is the wavelength of the radiation (1.54056 Å for Cu Kα radiation), k is a constant equal to 0.94, βhkl is the peak width at half-maximum intensity, and θ is the peak position. The (101) plane was chosen to calculate the crystalline size (the inset of Fig. 2). It can be seen that, the (101) peak of Na- and K-doped ZnO NPs is wider than the (101) peak of the undoped and Li-doped. Therefore, the obtained results from the XRD results are also indicated that, the crystalline size of the undoped and Li-doped ZnO NPs is the same, while; the crystalline size of the Na- and K-doped ZnO NPs is smaller. The XPS analysis was used for further evaluation of their purity and the composition of the nanoparticles and the spectra are sown in Fig. 3. It can be seen that, the spin–orbit splitting of 23 eV for Zn-2p3/2 and Zn-2p1/2 confirms that the Zn atoms are in a completely oxidized state in all samples (Fig. 3(a)). The peak exhibited at around 532 eV is attributed to the oxidized metal ions in the nanoparticles, namely O–Zn in the ZnO lattice (Fig. 3(b)). The photoemission peak located at around 56 eV for Li-1s in Fig. 3 (c), at around 1072 eV for Na-1s in Fig. 3(d), and at around 291 eV for K-2p in Fig. 3(e) shows the seepaging of alkali metals impurities in the atomic structure of ZnO. Therefore, the XPS results confirm that the alkali metals element substituting for Zn. The UV–vis absorption spectra of the ZnO NPs at room temperature are shown in Fig. 4. The spectra reveal a characteristic absorption peak for the undoped at 365 nm (3.39 eV), for the Li-doped at 370 nm (3.35 eV), for the Na-doped at 380 nm (3.26 eV), and for the K-doped at 390
6
nm (3.18 eV), which can be assigned to the intrinsic band-gap absorption of ZnO. Compared with the undoped ZnO NPs, the results of the doped ZnO NPs show an obvious red-shift in the UV emission. This red-shift in the UV emission could be a result of obtaining p-type ZnO NPs [21-22]. Steady state and transient photocurrent measurements were performed the undoped and doped ZnO NPs. Here, the ZnO NPs were exposed to a white fluorescent lamp with a chopper. The schematic of fabricated device is shown in Fig. 5. It is well known, when the device was illuminated, that the photogenerated charges produced under the effect of the applied electric field lead to a photocurrent that was added to the bias current, effectively increasing the conductivity of the device. The photocurrent responses of the ZnO NPs devices were studied at bias voltages of 2 voltages and the results are shown in Fig. 6(a). As can be seen, the photocurrent intensity increases from undoped to K-doped ZnO NPs. This higher photocurrent of doped ZnO NPs in comparison to the undoped ZnO NPs could be caused by the large carrier concentration of doped ZnO NPs. In addition, the rise and fall times of the K-doped ZnO NPs are the smallest and these times are the biggest for the Li-doped ZnO NPs. This faster and higher response of the K-doped ZnO NPs could be due to the smaller size that causes to create larger surface area of the K-doped ZnO NPs in comparison with that of the other NPs. A large surface area causes strong surface adsorption and higher photocurrent. According to the Uv-vis results, the band gap of the K-doped ZnO NPs is narrower than the band gap of the other NPs. Therefore, the K-doped ZnO NPs have wider window to catch more photons from the white light source. The variation of photocurrent with the input light intensity is shown in Fig. 6 (b) on a log-log scale. Power law dependence can fit the relation of IPhoto vs. P quite well for all intensities [2325]:
7
IPhoto=APr
(2)
where A is a constant for a certain wavelength, and exponent r determines the response of photocurrent to light intensity. Fitting eqn (2) to the curves yields different r for different samples. It can be seen, the exponent is non-unity for the all samples and it decrease from the undoped to K-doped ZnO NPs. The non-unity exponent was also observed for ZnO nanowires and CdS nanoribbons [25-27], which is a result of the complex process of electron–hole generation, trapping, and recombination within the semiconductor. Therefore, the smallest exponent for the K-doped ZnO NPs causes to create the highest photocurrent and fastest response in comparison to the other NPs. Huang et al. performed a systematic study based on ab initio total energy calculation about diffusion Li, Na, and K in ZnO. They found, the doping behaviors of K are completely different in comparison to Li and Na [16]. For example, they found that, K interstitial has a relatively smaller energy barrier than Li and Na. In addition, they understood that, the diffusion of the Li interstitial is isotropic, whereas diffusion of K interstitials is highly anisotropic. Therefore, such differences between group I elements could be caused to result different photocurrent behaviors. However, more effort needs to confirm this conclusion. For more understanding of the obtained photocurrent results, the O-1s spectra of the XPS results were analyzed with more details. The asymmetric peak was deconvoluted into two components with binding energies of 531.57 and 533.25 eV. The 531.57 eV peak is attributed to the O2- ions on the wurtzite structure of the hexagonal Zn2+ ion array, which are surrounded by zinc atoms with the full supplement of nearest-neighbor O2- ions (Fig. 7). Therefore, the 531.57 eV peaks of the O-1s spectra could be attributed to the Zn–O bonds (OL). On the other hand, the higher binding energy component at 533.25 eV is assigned to the oxygen vacancies within the ZnO matrix (OV) [28]. It can be seen that, the proportion of OV in the non-lattice oxygen of
8
alkali-metals-doped ZnO NPs is higher than proportion of OV in the non-lattice oxygen of undoped ZnO NPs and it increases from Li-doped to K-doped ZnO NPs. It is known, there is a direct relationship between O vacancy and visible absorption and emission of ZnO [11, 29-30]. Therefore, it can be concluded that, another reason for obtaining higher photocurrent intensity of K-doped ZnO NPs in comparison with photocurrent intensity of the other ZnO NPs by a white light source could be due to higher oxygen vacancy in the K-doped ZnO structure. 4- Conclusion Photocurrent studies under a visible light source of the undoped and alkali-metals-elements doped ZnO NPs showed that, the K-doped ZnO NPs had higher current intensity and faster response than other NPs. It was understood that, there were some reasons to obtain the highest photocurrent intensity for the K-doped ZnO NPs compared to the other samples such as smaller average particle size, smaller band gap, and more oxygen vacancy resulting more absorbed incident light. In fact, the XPS results of O-1s spectra indicated that K-doped ZnO NPs had more oxygen vacancy in comparison to the other samples. The findings suggest the great application potential of alkali-metals-doped ZnO NPs devices for high-sensitivity and high-speed nanoscale photodetectors and optoelectronic switches.
Acknowledgment R. Yousefi and F. Jamali-Sheini gratefully acknowledge Islamic Azad University MasjedSoleiman and Ahwaz Branches for their financial supports in this research work.
References [1] M. Azarang, A. Shuhaimi, R. Yousefi, M. Sookhakian, J. Appl. Phys. 116 (2014) 084307. 9
[2] M. Azarang, A. Shuhaimi, R. Yousefi, M. Sookhakian, , RSC Advances 5 (2015) 21888. [3] D.R. Sahu, Jow-Lay Huang, Solar Energy Materials & Solar Cells 93 (2009) 1923. [4] A. Qurashi, M. F Hossain, M. Faiz, N. Tabet, M. W Alam, N. K Reddy, J. Alloy. Compd. 503 (2010) L40.
[5] L. Wang, Y. Kang, X. Liu, S. Zhang, W. Huang, S. Wang, Sensors and Actuators B 162 (2012) 237–243. [6] F. Jamali-Sheini, R. Yousefi, Dilip S. Joag, Mahendra A. More, Vaccum. 101 (2014) 233. [7] M. Law, J. Goldberger, P. Yang, Annu Rev Mate Res 34 (2004) 83.
[8] Y. Sun, J. Yang, L. Yang, M. Gao, X. Shan, Z. Zhang, M. Wei, Y. Liu, L. Fei, H. Song, Journal of Luminescence 134 (2013) 35.
[9] Y.Y. Lai, Y.P. Lan, T.C. Lu, Light: Science & Applications 2 (2013) e76.
[10] P. K. Shrestha, Y.T. Chun, D. Chu, Light: Science & Applications 4 (2015) e259 [11] R. Yousefi, CrystEngComm, 17 (2015) 2698. [12] R. Yousefi, F. Jamali-Sheini, M. Cheraghizade, S. Khosravi-Gandomani, A. Sa΄aedi, N.M. Huang, W.J. Basirun, M. Azarang, Mater Sci Semi Process. 32 (2015) 152. [13] R. Yousefi, M. R. Muhamad, J Solid State Chem 183 (2010) 1733. [14] S. Ghandomani, R. Yousefi, F. Jamli-Sheini, N. Ming Huang, Ceram Inter 40 (2014) 7957. [15] K. Jayanthi, S. Chawla, K.N. Sood, M. Chhibara, S. Singh, Appl. Surf. Sci. 255 (2009) 5869. [16] G. Y. Huang, C. Y. Wang, and J. T. Wang, J. Phys.: Condens. Matter. 21 (2009) 345802. [17] E.C. Lee, K.J. Chang, Phys Rev B 70 (2004) 115210. [18] B. Xiao, Z. Ye, Y. Zhang, Y. Zeng, L. Zhu, B. Zhao, Appl. Surf. Sci. 253 (2006) 895.
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[19] J. G. Lu, Y. Z. Zhang, Z. Z. Ye, L.P. Zhu, L. Wang, B. H. Zhao, Q. L. Appl. Phys. Lett. 88 (2006) 222114. [20] R. Yousefi, A. Khorsand-Zak, F. Jamali-Sheini, Ceramics International.39 (2013) 1371. [21] M. A. Thomas, J.B. Cui, The J. Phys. Chem. Lett. 1 (2010) 1090. [22] S. Shet, K. S. Ahn, Y. Yan, T. Deutsch, K. M. Chrustowski, J. Turner, M. Al-Jassim, N. Ravindra, J. Appl. Phys. 103 (2008) 073504. [23] P. K. C. Pillai, Niloufer Shroff, N. Nanda Kumar, and A. K. Tripathi, Phys Rev B, 32 (1985) 8228. [24] R. K. Gupta, M. Cavas, Ahmed A. Al-Ghamdi, Z. H. Gafer, F. El-Tantawy, F. Yakuphanoglu, Sol Energ 92 (2013) 1. [25] J. Pan, J. Li, Z. Yan, B. Zhou, H. Wua and Xiang Xiong, Nanoscale 5 (2013) 3022. [26] H. Kind, H. Yan, B. Messer, M. Law, and P. Yang, Adv. Mater. 14 (2002) 158. [27] T. Gao, Q. H. Li and T. H. Wang, Appl. Phys. Lett. 86 (2005) 173105. [28] J.H. Zheng, Q. Jiang, J.S. Lian, Appl. Surf. Sci., 257 (2011) 5083. [29] X-G. Han, H-Z He, Q. Kuang, X. Zhou, X-H Zhang, T Xu, Z-X. Xie, L-S. Zheng, J. Phys. Chem. C, 113 (2009) 584. [30] H. Tian, J. Xu, Y. Tian, P. Deng, H. Wen, CrystEngComm, 15 (2013) 5345.
Figure captions Figure 1. TEM images of (a) undoped, (b) Li-doped, (c) Na-doped, and (d) K-doped ZnO NPs. Figure 2. XRD patterns of the undoped and doped ZnO NPs. The inset shows (101) peaks of samples. Figure 3. XPS spectrum of (a) Zn-2p, (b) O-1s, (c) Li-1s, (d) Na-1s, and (e) K-2p.
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Figure 4. UV-vis spectra of the undoped and doped ZnO NPs. Figure 5. Schematic of the device that is used to measure photocurrent of the samples. Figure 6. (a) Photocurrent response of the undoped and doped ZnO NPs. (b) Variation of photocurrent as a function of incident power intensity. Figure 7. High resolution XPS spectra of O-1s: (a) undoped ZnO NPs, (b) Li-doped ZnO NPs, (c) Na-doped ZnO NPs, and (d) K-doped ZnO NPs.
Fig. 1
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Fig. 2
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Fig. 3
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Fig. 4
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Fig. 5
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Fig. 6
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Fig. 7
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Highlights Photocurrent applications of the alkali-metals-doped ZnO NPs were investigated. K-doped ZnO NPs showed a faster response and higher photocurrent in comparison to the other NPs. Doped ZnO NPs devices can be used as high-sensitivity and high-speed nanoscale photodetectors.
19
PII: DOI: Reference:
www.elsevier.com/locate/physe
S1386-9477(15)30310-6 http://dx.doi.org/10.1016/j.physe.2015.12.002 PHYSE12227
To appear in: Physica E: Low-dimensional Systems and Nanostructures Received date: 10 November 2015 Revised date: 2 December 2015 Accepted date: 10 December 2015 Cite this article as: Abdolhossein Saa´edi, Ramin Yousefi, Farid Jamali-Sheini, Ali Khorsand Zak, Mohsen Cheraghizade, M.R. Mahmoudiand, Mohammad Amin Baghchesara and Abbas Shirmardi Dezaki, XPS studies and photocurrent applications of alkali-metals-doped ZnO nanoparticles under visible illumination conditions, Physica E: Low-dimensional Systems and Nanostructures, http://dx.doi.org/10.1016/j.physe.2015.12.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
XPS studies and photocurrent applications of alkali-metals-doped ZnO nanoparticles under visible illumination conditions
Abdolhossein Saa´edi1, Ramin Yousefi2*, Farid Jamali-Sheini3, Ali Khorsand Zak4, Mohsen Cheraghizade1, M. R. Mahmoudiand5, Mohammad Amin Baghchesara6, Abbas Shirmardi Dezaki7 1
2
Young Researchers and Elite Club, Ahvaz Branch Islamic Azad University, Ahvaz, Iran
Department of Physics, Masjed-Soleiman Branch, Islamic Azad University (I.A.U), MasjedSoleiman, Iran 3
Department of Physics, Ahvaz Branch, Islamic Azad University, Ahvaz, Iran 4
5
6
7
Nanotechnology Laboratory, Esfarayen University, North Khorasan, Iran
Department of Chemistry, Shahid Sherafat, University of Farhangian, 15916 Tehran, Iran
Department of Metallurgy and Materials Engineering, Masjed-Soleiman Branch, Islamic Azad University (I.A.U), Masjed-Soleiman, Iran
Department of Chemistry, Masjed-Soleiman Branch, Islamic Azad University (I.A.U), MasjedSoleiman, Iran
*Corresponding author: E-mail: [email protected], [email protected] Tel: +989166224993, Fax: +986143260093
Keywords: ZnO nanoparticles; Alkali-metals-doped ZnO; Photocurrent application; XPS spectrometer
1
Abstract: The present work is a study about a relationship between X-ray photoelectron spectrometer (XPS) results and photocurrent intensity of alkali-metals-elements doped ZnO nanoparticles, which is carried out under visible illumination conditions. The nanoparticles were synthesized by a simple sol–gel method. Structure and morphology studies of the NPs were carried out by X-ray diffraction analysis (XRD) and transmission electron microscopy (TEM). The effect of doping on the optical band-gap was investigated by using UV-visible spectrometer. The absorption peak of the doped ZnO NPs was red-shifted with respect to that of the undoped ZnO NPs. After that, the photocurrent application of the products was examined under a white light source at 2 V bias. The photocurrent results showed that, the current intensity of the ZnO NPs was increased by doping materials. However, K-doped ZnO NPs showed the highest photocurrent intensity. Finally, a discussion was carried out about the obtained photocurrent results by the O-1s spectra of the XPS of the samples. Our results suggest that the alkali-metalsdoped ZnO NPs exhibit considerable promise for highly sensitive visible-light photodetectors. Keywords: ZnO nanoparticles, Alkali-metals-doped ZnO, Photocurrent application, XPS spectrometer.
2
1- Introduction It is known that, photocurrent applications of semiconductor materials are the base of optoelectronic devices. Among different optical materials that have photocurrent applications, ZnO is the most important optical material that showed excellent photocurrent [1]. It is a semiconductor with a direct band-gap; it has piezoelectric and pyroelectric properties, it can be grown with different shapes, synthesize of ZnO nanostructures is easy and coast effective, and it is an environment- and human body-friendly material. It is also a promising material for ultraviolet nano-optoelectronic devices and lasers operating at room temperature due to its wide band gap. For these unique properties, it has been one of the hot topics of the 21st century so far. Therefore, researchers have studied about different applications of ZnO nanostructures in these years. ZnO can be used in many applications, such as photocatalyst applications [2], transparent conductive coatings [3], electrodes for dye-sensitized solar cells [4], gas sensors [5], and field emission materials [6]. In nanometer size range, the ZnO nanostructures are expected to possess interesting physical properties and pronounced coupling quite different from their bulk counterpart [7-10]. One of the best ways to improve ZnO nanostructure properties is doping. Therefore, to obtain better crystallization quality, optical, electrical, and ferromagnetic properties, researchers have been studied effect of dopant materials on ZnO properties. The most efforts have been to achieve n-type ZnO nanostructures with different applications [11-14]. However, the key challenge that needs to be overcome for the realization of most ZnO based applications is the fabrication of p-type material. P-type ZnO may be achieved by the substitution of alkali metals elements on Zn-site [15–17] and group-V elements on O-site [18– 21], respectively. Although significant progress has been made recently full control over the 3
materials conductivity type is still to be obtained and hence a comprehensive investigation of the fundamental properties of acceptors in ZnO is needed. Alkali metals elements are better dopants materials in comparison to group-V elements in terms of the shallowness of the accepter level [21]. Therefore, the investigation of different effects of alkali metals elements as dopants on structure and optical properties of ZnO nanostructures can lead to obtain different applications. Several groups and researchers have tried to study effect of alkali metals on different properties of ZnO nanostructures [15-19]. We believe that although the researches on the effect of alkali metals elements on optical and electrical of ZnO has been performed, the effects of alkali metals elements on photocurrent applications of ZnO nanostructures have not been reported to date. Such knowledge can enhance development of optoelectronic devices in the future. For these reasons, the undoped and alkalimetals-doped ZnO-NPs were prepared by a sol–gel method in the present work. We report a comparative study of the effects of alkali metals elements on the optical properties and photocurrent applications of the ZnO nanoparticles. 2- Experimental To begin the synthesis of alkali-metals-doped ZnO-NPs, analytical grade zinc nitrate hexahydrate (Zn(NO3)2 + 6H2O), lithium nitrate (LiNO3, >99%), sodium nitrate (NaNO3, >99%), potassium nitrate (KNO3, >99%), gelatin (type B from bovine skin), and distilled water were used as starting materials. All of the materials used were purchased from Sigma-Aldrich. The precursors were measured as Zn0.97X0.03O, (X = Li, Na, and K) to obtain 5 g of final product. The detailed of the NPs synthesize have been reported in our previous work [20]. The resulting powders were characterized by several tools to check their qualities. X-ray diffraction (XRD) (Philips, X’Pert, CuKa) was used to evaluate the phase characteristics of the
4
samples. Transmission electron microscopy (TEM, Hitachi H-7100) were employed to characterize morphology and structural properties of the NPs. Elemental analyses of the products were conducted using an X-ray photoelectron spectroscopy (XPS; VG-Microtech ESCA-2000). The XPS spectra were recorded using Mg-Kα radiation (1256.6 eV). The optical properties of the ZnO-NPs were characterized at room temperature using a UV–vis (PerkinElmer, Inc). Finally, the photocurrent behaviors of the NPs, which were under a white light source and chopped light illumination, were characterized by a DC circuit. For fabricated a photocurrent device, a mixture of 0.01 g NPs and 0.6 mL chitosan were used. This mixture was ultrasonicated in an ultrasonic bath and then dropped on a glass substrate between two silver electrodes. Then the device was used in a DC circuit, which was connected to an oscilloscope. Photocurrent intensities were measured under a 2 V bias potential. A 20 W compact fluorescent lamp (CFL) was employed as an excitation source. 3- Results and discussion The TEM images of the undoped and doped ZnO-NPs are shown in Fig. 1. The undoped and Li-doped ZnO NPs are spherical and uniform size approximately with an average diameter around 45 nm (Figs. 1(a) and 1(b), respectively). On the other hand, the Na- and K-doped ZnO NPs are non-uniform in sizes and shape with smaller average diameter (Figs. 1(c) and 1(d), respectively). This non-uniform shapes for the Na- and K-doped ZnO NPs could be due to a big difference between ionic radii of doping materials and ionic radius of Zn2+. In fact, Na+ and K+ ions play as passivation roles that cause to decrease the size of the nanoparticles and as well as obtained non-uniform shapes. This passivation role causes that the NPs cannot grow very fast.
5
The XRD patterns of doped and undoped ZnO NPs are indicated in Fig. 2. There are no other peaks related to any impurity or other mixed compounds. The crystalline sizes of the ZnONPs were measured by means of an X-ray line-broadening method, using the Scherrer equation: D = (kλ/βhklcosθ)
(1)
where D is the crystalline size in nanometers, λ is the wavelength of the radiation (1.54056 Å for Cu Kα radiation), k is a constant equal to 0.94, βhkl is the peak width at half-maximum intensity, and θ is the peak position. The (101) plane was chosen to calculate the crystalline size (the inset of Fig. 2). It can be seen that, the (101) peak of Na- and K-doped ZnO NPs is wider than the (101) peak of the undoped and Li-doped. Therefore, the obtained results from the XRD results are also indicated that, the crystalline size of the undoped and Li-doped ZnO NPs is the same, while; the crystalline size of the Na- and K-doped ZnO NPs is smaller. The XPS analysis was used for further evaluation of their purity and the composition of the nanoparticles and the spectra are sown in Fig. 3. It can be seen that, the spin–orbit splitting of 23 eV for Zn-2p3/2 and Zn-2p1/2 confirms that the Zn atoms are in a completely oxidized state in all samples (Fig. 3(a)). The peak exhibited at around 532 eV is attributed to the oxidized metal ions in the nanoparticles, namely O–Zn in the ZnO lattice (Fig. 3(b)). The photoemission peak located at around 56 eV for Li-1s in Fig. 3 (c), at around 1072 eV for Na-1s in Fig. 3(d), and at around 291 eV for K-2p in Fig. 3(e) shows the seepaging of alkali metals impurities in the atomic structure of ZnO. Therefore, the XPS results confirm that the alkali metals element substituting for Zn. The UV–vis absorption spectra of the ZnO NPs at room temperature are shown in Fig. 4. The spectra reveal a characteristic absorption peak for the undoped at 365 nm (3.39 eV), for the Li-doped at 370 nm (3.35 eV), for the Na-doped at 380 nm (3.26 eV), and for the K-doped at 390
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nm (3.18 eV), which can be assigned to the intrinsic band-gap absorption of ZnO. Compared with the undoped ZnO NPs, the results of the doped ZnO NPs show an obvious red-shift in the UV emission. This red-shift in the UV emission could be a result of obtaining p-type ZnO NPs [21-22]. Steady state and transient photocurrent measurements were performed the undoped and doped ZnO NPs. Here, the ZnO NPs were exposed to a white fluorescent lamp with a chopper. The schematic of fabricated device is shown in Fig. 5. It is well known, when the device was illuminated, that the photogenerated charges produced under the effect of the applied electric field lead to a photocurrent that was added to the bias current, effectively increasing the conductivity of the device. The photocurrent responses of the ZnO NPs devices were studied at bias voltages of 2 voltages and the results are shown in Fig. 6(a). As can be seen, the photocurrent intensity increases from undoped to K-doped ZnO NPs. This higher photocurrent of doped ZnO NPs in comparison to the undoped ZnO NPs could be caused by the large carrier concentration of doped ZnO NPs. In addition, the rise and fall times of the K-doped ZnO NPs are the smallest and these times are the biggest for the Li-doped ZnO NPs. This faster and higher response of the K-doped ZnO NPs could be due to the smaller size that causes to create larger surface area of the K-doped ZnO NPs in comparison with that of the other NPs. A large surface area causes strong surface adsorption and higher photocurrent. According to the Uv-vis results, the band gap of the K-doped ZnO NPs is narrower than the band gap of the other NPs. Therefore, the K-doped ZnO NPs have wider window to catch more photons from the white light source. The variation of photocurrent with the input light intensity is shown in Fig. 6 (b) on a log-log scale. Power law dependence can fit the relation of IPhoto vs. P quite well for all intensities [2325]:
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IPhoto=APr
(2)
where A is a constant for a certain wavelength, and exponent r determines the response of photocurrent to light intensity. Fitting eqn (2) to the curves yields different r for different samples. It can be seen, the exponent is non-unity for the all samples and it decrease from the undoped to K-doped ZnO NPs. The non-unity exponent was also observed for ZnO nanowires and CdS nanoribbons [25-27], which is a result of the complex process of electron–hole generation, trapping, and recombination within the semiconductor. Therefore, the smallest exponent for the K-doped ZnO NPs causes to create the highest photocurrent and fastest response in comparison to the other NPs. Huang et al. performed a systematic study based on ab initio total energy calculation about diffusion Li, Na, and K in ZnO. They found, the doping behaviors of K are completely different in comparison to Li and Na [16]. For example, they found that, K interstitial has a relatively smaller energy barrier than Li and Na. In addition, they understood that, the diffusion of the Li interstitial is isotropic, whereas diffusion of K interstitials is highly anisotropic. Therefore, such differences between group I elements could be caused to result different photocurrent behaviors. However, more effort needs to confirm this conclusion. For more understanding of the obtained photocurrent results, the O-1s spectra of the XPS results were analyzed with more details. The asymmetric peak was deconvoluted into two components with binding energies of 531.57 and 533.25 eV. The 531.57 eV peak is attributed to the O2- ions on the wurtzite structure of the hexagonal Zn2+ ion array, which are surrounded by zinc atoms with the full supplement of nearest-neighbor O2- ions (Fig. 7). Therefore, the 531.57 eV peaks of the O-1s spectra could be attributed to the Zn–O bonds (OL). On the other hand, the higher binding energy component at 533.25 eV is assigned to the oxygen vacancies within the ZnO matrix (OV) [28]. It can be seen that, the proportion of OV in the non-lattice oxygen of
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alkali-metals-doped ZnO NPs is higher than proportion of OV in the non-lattice oxygen of undoped ZnO NPs and it increases from Li-doped to K-doped ZnO NPs. It is known, there is a direct relationship between O vacancy and visible absorption and emission of ZnO [11, 29-30]. Therefore, it can be concluded that, another reason for obtaining higher photocurrent intensity of K-doped ZnO NPs in comparison with photocurrent intensity of the other ZnO NPs by a white light source could be due to higher oxygen vacancy in the K-doped ZnO structure. 4- Conclusion Photocurrent studies under a visible light source of the undoped and alkali-metals-elements doped ZnO NPs showed that, the K-doped ZnO NPs had higher current intensity and faster response than other NPs. It was understood that, there were some reasons to obtain the highest photocurrent intensity for the K-doped ZnO NPs compared to the other samples such as smaller average particle size, smaller band gap, and more oxygen vacancy resulting more absorbed incident light. In fact, the XPS results of O-1s spectra indicated that K-doped ZnO NPs had more oxygen vacancy in comparison to the other samples. The findings suggest the great application potential of alkali-metals-doped ZnO NPs devices for high-sensitivity and high-speed nanoscale photodetectors and optoelectronic switches.
Acknowledgment R. Yousefi and F. Jamali-Sheini gratefully acknowledge Islamic Azad University MasjedSoleiman and Ahwaz Branches for their financial supports in this research work.
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Figure captions Figure 1. TEM images of (a) undoped, (b) Li-doped, (c) Na-doped, and (d) K-doped ZnO NPs. Figure 2. XRD patterns of the undoped and doped ZnO NPs. The inset shows (101) peaks of samples. Figure 3. XPS spectrum of (a) Zn-2p, (b) O-1s, (c) Li-1s, (d) Na-1s, and (e) K-2p.
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Figure 4. UV-vis spectra of the undoped and doped ZnO NPs. Figure 5. Schematic of the device that is used to measure photocurrent of the samples. Figure 6. (a) Photocurrent response of the undoped and doped ZnO NPs. (b) Variation of photocurrent as a function of incident power intensity. Figure 7. High resolution XPS spectra of O-1s: (a) undoped ZnO NPs, (b) Li-doped ZnO NPs, (c) Na-doped ZnO NPs, and (d) K-doped ZnO NPs.
Fig. 1
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Fig. 2
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Fig. 3
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Fig. 4
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Fig. 5
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Fig. 6
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Fig. 7
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Highlights Photocurrent applications of the alkali-metals-doped ZnO NPs were investigated. K-doped ZnO NPs showed a faster response and higher photocurrent in comparison to the other NPs. Doped ZnO NPs devices can be used as high-sensitivity and high-speed nanoscale photodetectors.
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