Facile fabrication and photocatalytic properties of CuxO (x = 1 and 2) nanoarrays on nanoporous copper

Materials Letters 239 (2019) 75–78

Contents lists available at ScienceDirect

Materials Letters journal homepage: www.elsevier.com/locate/mlblue

Facile fabrication and photocatalytic properties of CuxO (x = 1 and 2) nanoarrays on nanoporous copper Xiaojing Du a, Chaoqun Xia a,b,⇑, Tai Yang a, Demin Zhu a, Zhidao Yang a, Fuxing Yin a, Chunyong Liang a, Qiang Li a,c,⇑ a b c

School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China National Engineering Research Center for Equipment and Technology of Cold Strip Rolling, Yanshan University, Qinhuangdao 066004, China

a r t i c l e

i n f o

Article history: Received 11 September 2018 Received in revised form 28 November 2018 Accepted 10 December 2018 Available online 19 December 2018 Keywords: Amorphous materials Nanocomposites Functional Porous materials

a b s t r a c t Micro-nano CuxO (x = 1 and 2) composite arrays were successfully synthesized on nanoporous copper (np-Cu) via one-step anodic oxidation method. With anodic time prolonging, the surface area and the length of pine-needle CuO clusters increased. The np-Cu/CuxO supported by amorphous layer composite as photocatalyst exhibited excellent photocatalytic activity and cycling stability for the degradation of Rhodamine B. Meanwhile, compared with the difficulties of powders and nanoparticles in recycling, the flexible and free-standing composite makes it easy for recovery of heterogeneous catalysts. The photocatalytic mechanism of np-Cu/CuxO was investigated. Ó 2018 Elsevier B.V. All rights reserved.

1. Introduction Water contamination has become one of the most important issues that must be addressed [1]. Dyes originating from textile, paint, and food industries, etc., are some of the major organic pollutants. Therefore, finding accessible techniques for removing organic pollution is urgent [2]. Photocatalytic degradation is a technology that can potentially be used to address organic contamination [3]. Generally, photocatalysts mainly focus on semiconductors, especially metal oxides and metal sulfides [4]. However, as the earliest discovered photocatalyst, TiO2 with a wide energy bandgap can only be activated by 5% solar energy, which hinders its wide application [2]. Materials with a narrow bandgap attract an increasing amount of attention. In recent years, researchers investigated copper oxide, which has a narrow bandgap and is applied to photoelectrodes and photocatalysts. As nontoxic and economical transition-metal oxides, CuxO (x = 1 and 2) with narrow band gap have been applied in electrochemical storage [5], sensors [6] and photocatalytic degradation [7,8]. Until now, various CuxO nanostructures have been synthesized using ⇑ Corresponding authors at: School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China. E-mail addresses: [email protected] (C. Xia), [email protected] (Q. Li). https://doi.org/10.1016/j.matlet.2018.12.058 0167-577X/Ó 2018 Elsevier B.V. All rights reserved.

thermal oxidation and chemical reaction method [7]. Chen et al. [5] have fabricated the pine-needle like CuO arrays on Cu substrate via anodization, which can acted as anode of LIBs and exhibit super-rate capability. Zeng et al. [8] have fabricated graphene sphere decorated with CuxO, which showed excellent photocatalyic performance. Furthermore, the latest report first mentioned Cu/Cu2O/CuO heterojunction as a free-standing photocatalyst with good flexibility and easier recovery [3]. The synthesis of copper oxide on Cu substrate, foam copper, and foam nickel has been studied [9], but research on nanoporous copper is quite rare. Nanoporous copper with large internal specific surface and excellent conductivity can be acted as electron acceptors, which can reduce the recombination of photoelectron-holes pairs in the photocatalytic process [3]. Furthermore, the CuxO nanoarray in-situ grown on np-Cu possessed uniform distribution, avoiding agglomeration of nanoparticles. In this work, CuxO (x = 1 and 2) nanoarrays were successfully fabricated via one-step anodic oxidation method on the asdealloyed np-Cu layer. Meanwhile, the photocatalytic activity and stability of np-Cu/CuxO supported by amorphous layer was investigated. 2. Experimental Np-Cu was fabricated using chemical dealloying Ti50Cu50 amorphous ribbon, which was prepared as described in the reference

76

X. Du et al. / Materials Letters 239 (2019) 75–78

[10]. The anodic oxidation process was performed in direct current voltage-regulated power. The electrolyte was 0.2 M KOH solution. The anodization was carried out by applying a constant voltage of 0.6 V at 293 K. X-ray diffraction (XRD, DaVinci D8 Focus) and Transmission electron microscopy (TEM, JEM-2100F) were conducted to analyze the structures. Scanning electron microscope (SEM, JSM-7100) equipped with EDS was used to observe the morphology and the chemical composition. X-ray photoelectron spectroscopy (XPS, KAepna) was carried out to identify the surface composition. Photocatalytic activities were evaluated by the photodegradation of Rhodamine B (RhB) under ultraviolet visible (UV) light irradiation. Np-Cu/CuxO composite materials was immersed in 10 mL RhB mixed solution composed of 8 mL RhB (10 mg L 1) and 2 mL H2O2 (30 wt%). The 500 W Xe lamp was used as light source after absorption-desorption equilibrium at constant stiring speed. The photocatalytic performance was evaluated in a scanning ranging from 200 to 800 nm using UV–Vis spectrophotometer (UV-6100). 3. Results and discussion CuxO (x = 1 and 2)/np-Cu supported by an amorphous layer was fabricated through dealloying and anodization methods. Fig. 1(a) shows the as-spun Ti50Cu50 amorphous ribbon exhibiting a silver-white metallic luster. After dealloying (Fig. 1(b)), the color turned into pure copper color, indicating that the Ti dissolved, and Cu was retained to form the nanoporous structure. Subsequently, the surface color turned into gray-black, which indicated the formation of copper oxide after anodization (Fig. 1(c)). Due to the support of the amorphous layer, the samples showed good flexibility. The XRD pattern of as-spun Ti50Cu50 ribbon shows a glassy structure (Fig. 1(d)). After dealloying, the crystal peaks on the broad peak can be indexed as fcc-copper, indicating that the asdealloyed product was a nanoporous copper structure. After anodic oxidation, the extra diffraction peaks were indexed as ( 1 1 1) and (1 1 1) plane of the cupric oxide, thereby demonstrating the formation of CuO. With prolonged anodization, the peaks of CuO became sharp, indicating that the amount of CuO was enhanced. In order to further identify the anodic products, the XRD pattern of the pure

copper oxide formed on np-Cu without amorphous layer support is shown in Fig. 1(e). These crystal peaks were matched with the crystal planes of Cu, CuO, and Cu2O, thereby indicating that the components of anodic products for 20 min were CuO and Cu2O. Fig. 2(a) shows that as-dealloyed np-Cu layer is composed of homogeneously bicontinuous and nanoporous structure. For the Ti Cu binary system, the Ti was removed through selective dissolution, and Cu was retained to fabricate the nanoporous structure [10]. Fig. 2(b) shows that the nanosheets and pine-needle-like nanoclusters composite arrays were formed on np-Cu layer via anodic oxidation method. Fig. 2(b–d) shows that when the anodization time was prolonged, the entire nanosheet surface was almost covered with pine-needle-like nanocluster arrays. Furthermore, the bunched nanoclusters turned into divergence and the length of the nanoneedle grew from 1.5 lm to 3.0 lm, which is beneficial to enlarge the specific surface area and to expose more active sites. The enlarged picture in Fig. 2(e) showed that the nanoneedles with average diameters in the range 50–150 nm externally diverged and grew. Fig. 2(f) shows the EDS spectrum of as-anodized products. Ti dissolved almost completely during the dealloying process. Meanwhile, Cu and O are the main elements of anodic products, and the atomic ratios between Cu and O for pine-needle nanoclusters (area A) and nanosheets (area B) were about 2:1 and 1:1, respectively, indicating that the as-anodized products were Cu2O and CuO. As shown in Fig. 3(a), the single nanoneedle has a finer diameter of 50– 100 nm. Fig. 3(b) shows that the crystal lattices were calculated to be 2.53 Å and 2.34 Å, which agree with lattice plane of CuO ( 1 1 1) and (1 1 1), respectively [3,5]. The results of Fig. 3(c) demonstrated the nanosheets with length of 150  200 nm are Cu2O, which were located between np-Cu and CuO layer [3]. The spacing d of 2.45 and 2.10 Å were in accord with the lattice fringe of Cu2O (1 1 1) and (2 0 0), respectively. The results of TEM further indicated the as-anodized product were np-Cu/CuxO (x = 1 and 2) composite structure. Fig. 4(a) shows the XPS spectra of Cu 2p of the as-anodized products for 20 min, the two distinct peaks located at 932.4 and 952.3 eV can be attributed to Cu 2p3/2 and Cu 2p1/2, respectively{Lv, 2018 #65; Ji, 2013 #102}. The binding energies at 934.4 and 943.0 eV were close to the data of the CuO phase. The

Fig. 1. Macro photographs: (a) as-spun Ti50Cu50, (b) as-dealloyed and (c) as-anodized ribbon; (d) XRD patterns of the specimens; (e) as-anodized products of 20 min without support of amorphous layer.

X. Du et al. / Materials Letters 239 (2019) 75–78

77

Fig. 2. SEM images of (a) as-dealloyed Ti50Cu50, (b) as-anodized for 20 min, (c) 60 min and (d) 120 min; (e) enlargement of Fig (d); (f) EDS results of Fig (e).

Fig. 3. TEM and HRTEM image of CuO nanoneedle (a, b); TEM and HRTEM image of Cu2O nanosheets (c, d).

O1s spectrum for the surface of 20 min was shown in Fig. 4(b), the original O1s profile can be fitted into three peaks. The three peaks at 531.4, 530.5, and 532.2 eV came from the adsorbed oxygen, the lattice oxygen of Cu2O, and the oxygen of water vapor, respectively [11]. XPS results further indicated the coexistence of Cu2O and CuO, which agreed with the results of the XRD and EDS analyses. As shown in Fig. 4(a), at 120 min, the peaks located at 933.8 and 953.7 eV can be assigned to Cu 2p3/2 and Cu 2p1/2, respectively. Meanwhile, the existence of two shake-up satellite peaks at higher binding energies, 943.3 and 961.8 eV, can also be confirmed as the CuO phase [11]. Fig. 4(b) presents the O 1s of as-synthesized product for 120 min. The peaks located at 529.4, 530.7, and 532.2 eV can be ascribed to the lattice oxygen of CuO, adsorbed oxygen, and –OH surface of the as-anodized product. XPS analysis indicated

that the amount of CuO nanoclusters increased with prolonged anodization time. The UV–vis diffuse reflectance spectra and the band gap of npCu/CuxO composite were shown in Fig. 4 (c). It can be seen that the as-prepared composite can absorb the most visible light in the range of 300–600 nm. Meanwhile, it can also be calculated that the band gap energy (Eg) of np-Cu/CuxO composite is 1.35 eV [3]. Thus, the remarkable photocatalytic performance under visible light can attribute to the absoption wavelength and narrow band gap of np-Cu/CuxO composite [3,12]. The photocatalytic activity of the np-Cu/CuxO composites was shown in Fig. 4(d–f). As shown in Fig. 4(d), the controlled experiment proved that negligible degradation of RhB was observed in the various absences of catalyst, H2O2 addition, and light irradia-

78

X. Du et al. / Materials Letters 239 (2019) 75–78

Fig. 4. XPS spectra of Cu 2p (a) and O 1 s (b) of as-anodized products; UV–vis diffuse reflectance spectra and band gaps (c); Photodegradation curves of as-anodized proucts (d); the degeneration curves (e) and photodegradation curves over four cyces of as-anodized for 120 min (f).

tion. However, in the presence of H2O2, catalysis, and irradiation, the degradation was obvious, demonstrating that the three factors were essential for the photocatalytic performance of CuxO. Fig. 4(d) shows the degradation curves of CuxO/np-Cu composites. With prolonged anodization time, the performance of as-anodized products was enhanced. When the reaction progressed to 220 min, the degradation rates for as-anodized products of 20, 60, and 120 min were 92.4%, 94.9%, and 96.7%, respectively. Fig. 4(e) shows that the as-anodized for 120 min composite changes at different times under the action of the dye RhB degradation of absorption spectrum. The inset is the photographic images of RhB solutions under different irradiation times, showing that the suspension finally became colorless after 220 min of irradiation. Repeatability is an important issue for heterogeneous photocatalysis. As shown in Fig. 4(f), only a slight change in the photocatalytic degradation efficiency of np-Cu/CuxO was observed after four cycles, indicating the composite has excellent stability. The photocatalytic mechanism of np-Cu/CuxO can be explained by the band structure and the morphology of np-Cu/CuxO composite. The band gaps of CuO and Cu2O are 1.7 eV and 2.2 eV respectively [3], which can generate electron and holes under visible region irradiation. Furthermore, based on the different band edge position of CuO and Cu2O, the photo-generated electron from Cu2O transfer to the CB of CuO and the photo-generated holes from CuO transfer to the VB of Cu2O. The added H2O2 and conductive npCu acted as electrons acceptors facilitates the separation and transfer of photo-generated electron and holes. Meanwhile,
OH radical species generated from the reduction of H2O2 by photo-generated electrons [7], which are responsible for efficient degradation of organic dye [13]. Moreover, the composite morphology of pine-needle clusters and nanosheets formed on np-Cu layer are beneficial to the exposure of active sites, which can generate more electron-hole pairs under irradiation. With prolonged anodization time, the enhanced

photocatalytic activity was attributed to the increased number of nanoclusters and the morphology of externally diverged nanowires. 4. Conclusion In this study, the micro-nano structure of CuxO (x = 1 and 2) array consisting of pine-needle-like nanoclusters and nanosheets was successfully fabricated on np-Cu layer via in-situ anodic oxidation method. Furthermore, the photocatalytic performance of npCu/CuxO supported by amorphous layer composite was investigated. The as-anodized composite for 120 min exhibited excellent photocatalytic activity and repeatability. Declaration of interest None. Acknowledgment This work was supported by the NSFC (Grant no. 51801052). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

J-w. Ha, J. Oh, et al., J. Indus. Eng. Chem. 58 (2018) 38–44. L.V. Bora, R.K. Mewada, Renew. Sustain. Energy Rev. 76 (2017) 1393–1421. H. Li, Z. Su, S. Hu, Y. Yan, Appl. Catal. B 207 (2017) 134–142. C. Byrne, G. Subramanian, S.C. Pillai, J. Environ. Chem. Eng. 6 (3) (2018) 3531– 3555. X. Chen, N. Zhang, et al., J. Mater. Chem. 22 (2012) 15080–15084. X. Liu, W. Yang, et al., Electrochim. Acta 235 (2017) 519–526. B. Zeng, X. Chen, et al., Appl. Surf. Sci. 276 (2013) 482–486. B. Zeng, Y. Luo, et al., Ceram. Int. 40 (2014) 5055–5059. Y. Feng, X. Zheng, Nano Lett. 10 (2010) 4762–4766. Z. Dan, F. Qin, Y. Sugawara, et al., Intermetallics 29 (2012) 14–20. J. Fan, D. Tang, et al., J. Alloy. Compd. 704 (2017) 624–630. Z. Behzadifard, Z. Shaniatinia, et al., J. Mol. Liq. 262 (2018) 533–548. X. Peng, I. Izumi, Nanotechnology 22 (2011) 01570.