Novel synthesis of CuO nanofiber balls and films and their UV–visible light filteration property

Author’s Accepted Manuscript Novel synthesis of CuO nanofiber balls and films and their UV-visible light filteration property Xiulan Hu, Tianyu Zhang, Jinjie Chen, Haiguang Gao, Weifeng Cai www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(16)30018-9 http://dx.doi.org/10.1016/j.ceramint.2016.02.076 CERI12267

To appear in: Ceramics International Received date: 23 January 2016 Accepted date: 13 February 2016 Cite this article as: Xiulan Hu, Tianyu Zhang, Jinjie Chen, Haiguang Gao and Weifeng Cai, Novel synthesis of CuO nanofiber balls and films and their UVvisible light filteration property, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.02.076 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.

Novel synthesis of CuO nanofiber balls and films and their UV-visible light filteration property Xiulan Hu*, Tianyu Zhang#, Jinjie Chen, Haiguang Gao, Weifeng Cai College of Materials Science and Engineering, Nanjing Tech University, Nanjing, China. *Corresponding author at: College of Materials Science and Engineering, Nanjing Tech University, Xin-Mo-Fan Road No. 5, Nanjing, Jiangsu 210009, China. Tel./fax: +86 25 8358 7260. E-mail address: [email protected] (X. Hu). 

Tianyu Zhang and Xiulan Hu are co-first authors.

Abstract: Self-assembled cupric oxide (CuO) nanofiber balls and films were synthesized via a facile solvothermal route directly from cupric acetate monohydrate (Cu(CH3COO)2·H2O) in water and ethanol without any chemical additions or high temperature treatment. The CuO balls with size of 150 nm–1.5 µm had rough surfaces which consisted of lots of about 10 nm nanofibers in diameter. The sizes of CuO balls were controllable by changing reaction time and volume ratio of water to ethanol. CuO nanofiber films were prepared with the aid of the in situ hydrolysis of Cu(CH3COO)2·H2O coating layer on a substrate at 60 °C. CuO films showed excellent UV-visible light filteration property and could be used as a potential candidate of UV-visible light filter. Compared with traditional method to fabricate CuO films, neither precursor nor Cu substrate was needed in this study. This

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technique could be used to produce CuO films without being confined to our template and to produce CuO powders in large scale with low cost. Key words: Nanofiber balls and films; Solvothermal controllable synthesis; UV-visible light filteration property; CuO

1. Introduction As a p-type semiconductor with a narrow band gap (1.2 eV in bulk), nanoscale cupric oxide (CuO) has been attracting much attention because of its unique size, large high surface area, potential quantum size effect and so on [1-4]. Nano-structured CuO has been widely applied in catalyst for organic synthesis and decomposition [5,6], lithium ion batteries [7-9], sensors [10,11] and photocatalytic degradation of dye [12,13]. Up to now, well defined CuO nanostructures with all kinds of morphologies such as nanoplatelets [14], nanoparticles [15], nanoellipsoids [11], nanorods and nanotubes [16] were prepared by scientific researchers. Traditional synthesis methods are hydrothermal/solvothermal method [17-19], chemical precipitation method [20], solid-state thermal conversion of precursors [21], electrochemical method [22] and plasma-induced method [23], etc. Among of them, hydrothermal/solvothermal method is a very common and effective method to synthesize various CuO micro/nanostructures because it is simple to control their morphologies and sizes. Usually, alkali [24-26] or other additions [9,27,28] which can produce OH were used to control solution pH value to get homogeneous CuO nanostructures. Multiple morphologies of hierarchical CuO particles including leaf, shuttle,

flower,

dandelion

and

caddice

2

clew

were

synthesized

from

Cu(CH3COO)2·H2O by adjusting pH value (9.5–11.5) using different amount of ammonia at 90 °C for 12 h [9]. In addition, template and surfactant [29-31] were used to control morphologies and sizes of CuO. But chemical additions as the above mentioned may be toxic or environmentally harmful. So it is very essential to search for simple, efficient, green non-toxic, eco-friendly methods to prepare CuO nanostructures. CuO thin film has promising applications in photocatalytic degradation of dye [32], optoelectronic device [33] and UV light filter [34] because of its optical properties. So far, various methods were used to prepare CuO film. CuO nanowire films were prepared by electrochemical method on high-purity Cu plate with subsequently high temperature treatment (300 °C for 10 min) [32]. Also CuO nanowire films were prepared on stainless steel substrate by magnetron sputtering from high-purity Cu target and high energy consumption [35]. CuO thin film was prepared from its precursor and high temperature treatment by sol–gel technique [33]. In addition, 310 nm CuO thin film in thickness was prepared on microscopic glass substrates by successive ionic layer adsorption and reaction (SILAR) through 100 SILAR cycles with many steps and complicated processes [36]. Even though various CuO film was prepared by electrochemical method, magnetron sputtering, sol–gel technique and SILAR as the above mentioned, high temperature treatment or Cu /or stainless steel substrate or complicated processes was necessary. In this study, a facile solvothermal route was found to synthesize self-assembled CuO nanofiber balls and films directly from Cu(CH3COO)2·H2O in water and ethanol

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without any chemical additions or high temperature treatment. The effects of reaction time, temperature, volume ratio of water to ethanol, solution concentration and solvent on the formation of CuO were investigated. This technique could be used to produce CuO films without being confined to our template. And UV-visible light filteration property of CuO films were addressed. The formation mechanism of CuO was also discussed in detail.

2. Experimental section 2.1. Material synthesis CuO

powders

were

synthesized

by

a

solvothermal

method

using

Cu(CH3COO)2·H2O as raw material. In a typical procedure, 80 ml 0.02 mol/L water-ethanol mixed solution of Cu(CH3COO)2·H2O was prepared with vigorous magnetic stirring. Then as-obtained dark blue solution was transferred into a stainless steel autoclave with a 100 mL Teflon-lined chamber and kept at 60–120 °C for different time. After reaction, black precipitates were collected, centrifuged and washed with distilled water for several times, and then dried at 60 °C. A series of comparative experiments were carried by changing reaction conditions. Reaction time was fixed to 2 h, 6 h, 12 h, 24 h. Volume ratio of water to ethanol was fixed to 1:0, 3:1, 1:3, 1:9, 0:1. The effect of ethylene glycol instead of ethanol on the formation of CuO was investigated, too. CuO films were synthesized from Cu(CH3COO)2·H2O coating layer by a simple two-step approach using the same reaction solution at 60 °C. 0.002 mol/L Cu(CH3COO)2·H2O ethanol solution was prepared as coating solution. First, clean

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F–doped SnO2–coated glass (FTO, sheet resistivity: 9.5Ω·m) substrate surface was modified by spin–coating for five times to ensure uniform and completely coverage of Cu(CH3COO)2·H2O. Second, pretreated substrates were immersed into 160 ml 0.02 mol/L solution of Cu(CH3COO)2·H2O for growth of CuO nanofiber films at 60 °C for 6–24 h in a thermostatically regulated oil bath. The pretreated FTO surfaces were kept at an angle and downward facing the beaker bottom during growth process. Finally, all substrates were washed with distilled water and then air-dried at room temperature for characterization.

2.2. Material characterization The phase composition was characterized by X-ray power diffraction (XRD) using a Smart Lab diffractometer with Cu-Kα radiation (λ= 1.5418 Å). The size, morphology and microstructure were observed by a field emission scanning electron microscope (FE-SEM, Hitachi, S-4800) and a transmission electron microscopy (TEM, JEM-2100F). The specific surface area of CuO powders was measured by the Brunauer-Emmett-Teller (BET, ASAP 2020) method on a micromeritics instrument corporation sorption analyze. The UV-visible light filteration property of CuO films was detected by a UV-vis spectrophotometer (Unico, WFZ UV-4802S).

3. Results and discussion 3.1 Synthesis of self-assembled CuO nanofiber balls All of experimental results clarified that CuO were synthesized at the temperature range of 60–120 °C. In the later discussion part, we focused on the synthesis of CuO at 100 °C.

5

Figure 1 shows XRD pattern and SEM images of CuO synthesized with water to ethanol volume ratio of 3:1 at 100 °C for 12 h. All of diffraction peaks as shown in Figure 1a are in good agreement with the JCPDS card NO. 48-1548. The crystallographic phase is monoclinic CuO. All of CuO look like similar balls from low magnification SEM image in Figure 1b. The size range of CuO balls are 0.7–1.2 µm. The high magnification SEM image in Figure 1c clearly shows CuO balls are assembled with lots of about 10 nm nanofibers in diameter, look like knitting wool ball. Figure 2 shows typical TEM images of CuO. An about 900 nm CuO ball consists of lots of about 10 nm nanofibers in diameter (Figure 2a), which is consistent with SEM image in Figure 1c. The rough surface of CuO ball derived from different nanofibers in length. Figure 2b shows a surface section of CuO ball. The prickly edges can be seen distinctly. A high resolution-TEM image of a selected area in Figure 2b (marked by a dotted frame) clearly clarified CuO nanofibers have very good crystallinity. A measured spacing of crystallographic planes is 2.325 Å, which corresponds to the (111) plane of monoclinic CuO (JCPDS card NO. 48-1548). Lattice interplanar spacing is vertical to the long-axis direction, this suggests that [111] is the growth direction of CuO nanofibers. And specific BET surface area of CuO balls was about 27.27 m2/g. In order to understand formation mechanism of CuO, some comparative experiments were addressed by changing reaction conditions such as reaction time, volume ratio of water to ethanol, solvent and solution concentration.

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Figure 3 shows SEM images of CuO synthesized at 100 °C for 2 h and 6 h with water to ethanol volume ratio of 3:1. CuO balls synthesized for 2 h are 300–500 nm in diameter, smaller than that (about 750 nm in diameter) for 6 h. They are both smaller than CuO balls synthesized for 12 h in Figure 1c (about 1 µm in diameter). These results clarify CuO balls grow up with increasing reaction time. Noted that there is no big difference of sizes and morphologies between 12 h and 24 h because synthesis reaction of CuO approaches chemical equilibrium, their productivities of CuO are 68.10 % and 70.98 %, respectively. All of CuO also look like similar balls assembled with nanofibers. Figure 4 shows the effect of volume ratio of water to ethanol in mixed solvent on morphologies and sizes of self-assembled CuO nanofiber balls synthesized at 100 °C for 2 h. In the case of pure water, CuO balls have almost uniform size of 500–800 nm in diameter (Figure 4a). Figure 4b clearly shows the surface covered with thousands of nanofibers with about 10 nm in diameter. In the case of water to ethanol volume ratio of 3:1 (Figure 4c and 4d), CuO balls are 300–500 nm in size with almost uniform morphology and look like oval. In the case of water to ethanol volume ratio of 1:3 (Figure 4e and 4f), CuO balls with shapes varying from sphere to embedded ellipsoid are 200 nm–1.5 µm in size. In the case of water to ethanol volume ratio of 1:9 (Figure 4g and 4h), almost the same size CuO balls looking like shuttle are about 200 nm. If in the case of pure ethanol, no CuO particles were obtained. Thus water is necessary to synthesize CuO directly from Cu(CH3COO)2·H2O by this method. And the sizes of CuO balls are controllable by changing the volume ratio of water to

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ethanol. The productivities of CuO synthesized at 100 °C for 2 h with water to ethanol volume ratio of 3:1, 1:3, 1:9 are 29.41 %, 58.50 % and 82.21 %, respectively. Therefore in the mixture solution for short reaction time, the lower volume ratio of water to ethanol, the higher productivity of CuO, the smaller CuO balls size. Noted that more experimental results clarified CuO balls were obtained with double and triple concentrations, and there are not much different from above discussed. And the effect of ethylene glycol instead of ethanol on the synthesis of CuO was investigated. Figure 5 shows SEM images of typical CuO balls synthesized at 100 °C for different time with water to ethylene glycol volume ratio of 3:1. The sizes of CuO balls assembled with nanofibers increased more quickly from about 600 nm to 1.2 µm with prolonging reaction time up from 2 h to 12 h. If Cu(CH3COO)2·H2O with triple concentration was used at 100 °C for 12 h, almost 1.4 µm CuO balls assembled with a lot of about 50 nm nanoplates were obtained, as shown in Figure 5c. These results clarified that size and morphology of self-assembled CuO balls can be controlled in water and ethylene glycol mixture solution via changing reaction time and solution concentration.

3.2 Preparation of CuO nanofiber films and their UV-visible light filteration property Based on the above discussion of CuO balls, CuO films were successfully prepared on Cu(CH3COO)2·H2O coated FTO substrate at 60 °C for 6–24 h. Figure 6 shows the XRD pattern and SEM images of CuO films on an FTO substrate obtained

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at 60 °C for 12 h. No other phase diffraction peaks were detected excluding the peaks originating from the FTO substrate and CuO in Figure 6a. The diffraction peaks of CuO films are in good agreement with the JCPDS card NO. 48-1548. Figure 6b shows the cross-sectional view of CuO films (which were stripped from FTO substrate in the process of sample preparation). CuO films have about 780 nm of thickness and consist of nanofibers (seen roughly). From Figure 6c and 6d in top views, CuO films consist of lots of nanofiber “hemispheres”. That means CuO films were prepared directly from Cu(CH3COO)2·H2O coating layer at low temperature on a substrate. Noted that our previous experimental results clarified it is difficult to prepare CuO films on bare FTO substrate. In order to detect UV-visible light filteration property of CuO films, three samples were prepared with increasing growing time in Cu(CH3COO)2·H2O aqueous solution at 60 °C for 6, 12 and 24 h, respectively. Figure 7a shows the transmittance reduced with increasing growing time because of increased thickness of CuO films at wavelength range from 300 nm to 800 nm. Figure 7b shows UV and visible light filteration efficiency of pure FTO and three samples at 350 nm and 450 nm of wavelength, respectively. The UV light and visible light filteration efficiency of CuO films increased with prolonging growing time. Once growing time reached up to 12 h, filteration efficiency of UV light and visible light reach to about 100 %. These results clarified CuO nanofiber films have potential application not only in UV light but also visible light filter (part of the wavelength range).

3.3 Formation mechanism of self-assembled CuO nanofiber balls 9

and films On the basis of above experimental results and all analysis, it is reasonable to understand a possible formation mechanism of self-assembled CuO nanofiber balls. The whole course of reaction in all our experiments can be illustrated by the following reaction equations: Cu(CH3COO)2·H2O → Cu2+ 2CH3COO + H2O

(1)

CH3COO + H2O → CH3COOH + OH

(2)

Cu2 + 2OH → Cu(OH)2 (s)

(3)

Cu(OH)2 (s) → CuO (s) + H2O

(4)

Cu(CH3COO)2·H2O ionizes to generate Cu2, CH3COO and H2O in solution according to Eq.(1). At the primary stage of reaction process, CH3COO is hydrolyzed to form an appropriate concentration of OH according to Eq.(2). Then OH reacts with Cu2+ to generate solid-phase Cu(OH)2, which is shown in Eq.(3). CuO nanocrystal nuclei clusters are formed from Cu(OH)2 decomposion according to Eq.(4) during reaction process at 60–120 °C due to their thermodynamic instability. And then primary CuO nuclei clusters grew up to CuO nanofiber balls by oriented attachment as shown in Figure 3. Therefore CuO balls further grow up with prolonging reaction time owing to surface energy minimization by Ostwald ripening [8,37]. All above discussion can be summarized in Figure 8. Noted that, CuO films look like nanofiber “hemispheres”, it should be due to limitation of substrate in the process of growth. CuO films can only epitaxial grow along with perpendicular to substrate.

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4．Conclusions Self-assembled CuO nanofiber balls and films were synthesized via a facile solvothermal route directly from Cu(CH3COO)2·H2O in water and ethanol/or ethylene glycol mixture solution without any chemical additions or high temperature treatment. CuO balls, which assembled with a lot of about 10 nm nanofibers in diameter, were controllable in size with 150 nm–1.5 µm by changing reaction time, temperature, volume ratio of water to ethanol, solvent and solution concentration. The growth direction of CuO nanofibers was [111]. The sizes of CuO balls increased with prolonging reaction time up to almost 12 h and increasing volume ratio of water to ethanol. And CuO nanofiber films were prepared with the aid of the in situ hydrolysis of Cu(CH3COO)2·H2O coating layer on an FTO substrate at 60 °C. The formation of self-assembled CuO nanofiber balls and films derived from hydrolysis, nucleation, growth by oriented attachment and further growth by Oswald ripening. CuO films showed excellent UV-visible light filteration property and could be used as a potential candidate of UV-visible light filter. Compared with traditional method to fabricate CuO films, neither precursor nor Cu substrate was needed in this study. This technique could be utilized to fabricate CuO films without being confined to our template and to produce CuO powders in large sacle with low cost.

Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No.51372113), Program for New Century Excellent Talents in University (NCET-12-0733, China), Specially-Appointed Professors by Universities in Jiangsu

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Province (SPUJP-2012, China), the scientific research foundation for Returned Overseas Students, and Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

References [1] C. Huo, J. Ouyang, H. Yang, CuO nanoparticles encapsulated inside Al-MCM-41 mesoporous materials via direct synthetic route, Sci. Rep. 4 (2014) 3682. [2] Nittaya Tamaekong, Chaikarn Liewhiran, Sukon Phanichphant, Synthesis of thermally spherical CuO nanoparticles, Journal of Nanomaterials 2014 (2014) 1–5. [3] E. Comini, C. Baratto, G. Faglia, M. Ferroni, A. Vomiero, G. Sberveglieri, Quasi-one dimensional metal oxide semiconductors: Preparation, characterization and application as chemical sensors, Prog. Mater. Sci. 54 (2009) 1–67. [4] Q. Zhang, K. Zhang, D. Xu, G. Yang, H. Huang, F. Nie, C. Liu, S. Yang, CuO nanostructures: Synthesis, characterization, growth mechanisms, fundamental properties, and applications, Prog. Mater. Sci. 60 (2014) 208–337. [5] L. Xu, Shanthakumar Sithambaram, Y. Zhang, C. Chen, L. Jin, Raymond Joesten, Steven L. Suib, Novel urchin-like CuO synthesized by a facile reflux method with efficient olefin epoxidation catalytic performance, Chem. Mater. 21 (2009) 1253–1259. [6] Z. Zhang, D. Guo, G. Zhang, Preparation, characterization and catalytic property of CuO nano/microspheres via thermal decomposition of cathode-plasma generating Cu2(OH)3NO3 nano/microspheres, J. Colloid Interface Sci. 357 (2011) 95–100. [7] L. Wang, H. Gong, C. Wang, D. Wang, K. Tang, Y. Qian, Facile synthesis of novel tunable highly porous CuO nanorods for high rate lithium battery anodes with realized long cycle life and

1 2

high reversible capacity, Nanoscale 4 (2012) 6850–6855. [8] X. Guan, L. Li, G. Li, Z. Fu, J. Zheng, T. Yan, Hierarchical CuO hollow microspheres: Controlled synthesis for enhanced lithium storage performance, J. Alloys Compd. 509 (2011) 3367–3374. [9] J. Xiang, J. Tu, L. Zhang, Y. Zhou, X. Wang, S. Shi, Self-assembled synthesis of hierarchical nanostructured CuO with various morphologies and their application as anodes for lithium ion batteries, J. Power Sources 195 (2010) 313–319. [10] S. Weng, Y. Zheng, C. Zhao, J. Zhou, L. Lin, Z. Zheng, X. Lin, CuO nanoleaf electrode: facile preparation and nonenzymatic sensor applications, Microchimica Acta 180 (2013) 371–378. [11] X. Zhang, S. Sun, J. Lv, L. Tang, C. Kong, X. Song, Z. Yang, Nanoparticle-aggregated CuO nanoellipsoids for high-performance non-enzymatic glucose detection, J. Mater. Chem. A 2 (2014) 10073–10080. [12] L. Shi, C. Yang, X. Su, J. Wang, F. Xiao, J. Fan, C. Feng, H. Sun, Microwave-hydrothermal synthesis of CuO nanorods and their catalytic applications in sodium humate synthesis and RhB degradation, Ceram. Int. 40 (2014) 5103–5106. [13] J. Hong, J. Li, Y. Ni, Urchin-like CuO microspheres: Synthesis, characterization, and properties, J. Alloys Compd. 481 (2009) 610–615. [14] G. Zou, H. Li, D. Zhang, K. Xiong, C. Dong, Y. Qian, Well-aligned arrays of CuO nanoplatelets, J. Phys. Chem. B 110 (2006) 1632–1637. [15] L. Wang, K. Tang, M. Zhang, X. Zhang, J. Xu, Facile synthesis of CuO nanoparticles as anode for lithium ion batteries with enhanced performance, Funct. Mater. Lett. 7 (2014) 1440008. [16] Mohammad Amin Alavi, Ali Morsali, Sang Woo Joo, Bong-Ki Min, Ultrasound and

1 3

modulation assisted synthesis of {[Cu2(BDC-NH2)2(dabco)]DMF.3H2O} nanostructures; New precursor to prepare nanorods and nanotubes of copper(II) oxide, Ultrason. Sonochem. 22 (2015) 349–358. [17] Michael Rajamathi, Ram Seshadri, Oxide and chalcogenide nanoparticles from hydrothermal/solvothermal reactions, Curr. Opin. Solid State Mater. Sci. 6 (2002) 337–345. [18] Gérard Demazeau, Solvothermal reactions: an original route for the synthesis of novel materials, J. Mater. Sci. 43 (2008) 2104–2114. [19] K. Namratha, K. Byrappa, Novel solution routes of synthesis of metal oxide and hybrid metal oxide nanocrystals, Prog. Cryst. Growth Charact. Mater. 58 (2012) 14–42. [20] J. Zhu, H. Bi, Y.Wang, X.Wang, X. Yang, L. Lu, CuO nanocrystals with controllable shapes grown from solution without any surfactants, Mater. Chem. Phys. 109 (2008) 34–38. [21] Dey KK, Kumar A, Shanker R, Dhawan A, Wan M, Yadav RR, Growth morphologies, phase formation, optical & biological responses of nanostructures of CuO and their application as cooling fluid in high energy density devices, RSC Adv. 2 (2012) 1387. [22] Sunita Jadhav, Suresh Gaikwad, Madhav Nimse, Anjali Rajbhoj, Copper Oxide nanoparticles: Synthesis, characterization and their antibacterial activity, J. Cluster Sci. 22 (2011) 121–129. [23] X. Hu, X. Zhang, X. Shen, H. Li, Osamu Takai, Nagahiro Saito, Plasma-induced synthesis of CuO nanofibers and ZnO nanoflowers in water, Plasma Chem. Plasma Process. 34 (2014) 1129–1139. [24] X. Xu, H. Yang, Y. Liu, Self-assembled structures of CuO primary crystals synthesized from Cu(CH3COO)2–NaOH aqueous systems, CrystEngComm 14 (2012) 5289–5298. [25] Prakash Chand, Anurag Gaur, Ashavani Kumar, Umesh Kumar Gaur, Effect of NaOH molar

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concentration on morphology, optical and ferroelectric properties of hydrothermally grown CuO nanoplates, Mater. Sci. Semicond. Process. 38 (2015) 72–80. [26] V. Usha, S. Kalyanaraman, R. Thangavel, R. Vettumperumal, Effect of catalysts on the synthesis of CuO nanoparticles: Structural and optical properties by sol–gel method, Superlattices Microstruct. 86 (2015) 203–210. [27] J. Liu, X. Huang, Y. Li, K. M. Sulieman, X. He, F. Sun, Self-assembled CuO monocrystalline nanoarchitectures with controlled dimensionality and morphology, Cryst. Growth Des. 6 (2006) 1690–1696. [28] M. Chang, H. Liu, C. Tai, Preparation of copper oxide nanoparticles and its application in nanofluid, Powder Technol. 207 (2011) 378–386. [29] C. Wang, Y. Ye, B. Tao, B. Geng, Hydrothermal route to twinned-hemisphere-like CuO architectures with selective adsorption performance, CrystEngComm, 14 (2012) 3677. [30] Przemysław D. Gacia, Lok Kumar Shrestha, Partha Bairi, Noelia M. Sanchez-Ballester, Jonathan P. Hill, Anna Boczkowska, Hideki Abe, Katsuhiko Ariga, Low-temperature synthesis of copper oxide (CuO) nanostructures with temperature-controlled morphological variations, Ceram. Int. 41 (2015) 9426–9432. [31] Y. Zhang, S. Wang, X. Li, L. Chen, Y. Qian, Z. Zhang, CuO shuttle-like nanocrystals synthesized by oriented attachment, J. Cryst. Growth 291 (2006) 196–201. [32] Y. Wang, T. Jiang, D. Meng, H. Jin, M. Yu, Controllable fabrication of nanowire-like CuO film by anodization and its properties, Appl. Surf. Sci. 349 (2015) 636–643. [33] D. M. Jundale, P. B. Joshi, Shashwati Sen, V. B. Patil, Nanocrystalline CuO thin films: synthesis, microstructural and optoelectronic properties, J. Mater. Sci. - Mater. Electron. 23 (2012)

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1492–1499. [34] Periyayya Uthirakumar, S. Muthulingam, Rizwan Khan, Jin Hyeon Yun, Han-Su Cho, In-Hwan Lee, Surfactant-free synthesis of leaf-like hierarchical CuO nanosheets as a UV light filter, Mater. Lett. 156 (2015) 191–194. [35] B. Purusottam-Reddy, K. Sivajee-Ganesh, K. Jayanth-Babu, O. M. Hussain, C. M. Julien, Microstructure and supercapacitive properties of rf-sputtered copper oxide thin films: influence of O2/Ar ratio, Ionics 21 (2015) 2319–2328. [36] Yunus Akaltun, Effect of thickness on the structural and optical properties of CuO thin films grown by successive ionic layer adsorption and reaction, Thin Solid Films 594 (2015) 30–34. [37] Y. Qin, F. Zhang, Y. Chen, Y. Zhou, J. Li, A. Zhu, Y. Luo, Y. Tian, J. Yang, Hierarchically porous CuO hollow spheres fabricated via a one-pot template-free method for high-performance gas sensors, J. Phys. Chem. C 116 (2012) 11994–12000.

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Fig.1. XRD pattern (a) and SEM images (b, c) of self-assembled CuO nanofiber balls synthesized at 100 °C for 12 h with water to ethanol volume ratio of 3:1, (b) a low magnification image, (c) a high magnification image in Figure 1b.

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Fig.2. (a) A typical TEM image of an individual CuO nanofiber ball; (b) TEM image taken from the edge of a CuO nanofiber ball; (c) High resolution-TEM image of a selected area in Figure 2b (marked by a dotted frame).

Fig.3. SEM images of self-assembled CuO nanofiber balls synthesized at 100 °C for (a) 2 h and (b) 6 h.

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Fig. 4. SEM images of self-assembled CuO nanofiber balls synthesized at 100 °C for 2 h with different water to ethanol volume ratios: (a, b) 1:0, (c, d) 3:1, (e, f) 1:3, (g, h) 1:9.

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Fig.5. SEM images of CuO synthesized at 100 °C for different reaction time with ethylene glycol to water volume ratio of 1:3. (a) 2 h, (b) 12 h, (c) with triple raw material concentration for 12 h.

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Fig.6. XRD pattern (a) and SEM images (b, c, d) of CuO films on FTO substrate obtained at 60 °C for 12 h.

Fig.7. Transmittance (a) and filteration efficiency (b) of CuO films coated and bare FTO substrate. CuO films were obtained at 60 °C for 6 h, 12 h and 24 h, respectively.

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Fig.8. The formation mechanism of self-assembled CuO nanofiber balls.

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