Journal of Solid State Electrochemistry https://doi.org/10.1007/s10008-018-3936-9
ORIGINAL PAPER
Preparation of electrospun heterostructured hollow SnO2/CuO nanofibers and their enhanced visible light photocatalytic performance Kai Wang 1 & Weizhou Zhang 1 & Feipeng Lou 1 & Ting Wei 1 & Ziming Qian 2 & Weihong Guo 1 Received: 5 November 2017 / Revised: 26 February 2018 / Accepted: 28 February 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018
Abstract Heterostructured SnO2/CuO nanofibers with a hollow morphology were successfully fabricated by a one-step electrospinning method. The electrospun nanofibers were transformed into hollow nanostructures in the presence of camphene after a calcination process, and the obtained samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflection spectroscopy (DRS), photoluminescence spectra (PL), and photodegradation measurements. The scanning electron microscopy (SEM) images displayed a rough and hollow structure for the obtained nanofibers. X-ray photoelectron spectroscopy (XPS) and energydispersive X-ray spectroscopy (EDX) identified the molecular composition and chemical interactions of the nanofibers. Photoluminescent (PL) measurements indicated that a recombination of the photoinduced electrons and holes was further inhibited due to the hollow nanostructure. Furthermore, the photodegradation of methylene blue suggested that the heterostructured SnO2/CuO hollow nanofibers possessed higher charge separation and photodegradation abilities than those of the other samples under visible light irradiation. This work can be potentially applied to the fabrication of other inorganic oxide photocatalysts with enhanced photodegradation activity in the field of environmental remediation. Keywords Electrospinning . Heterostructure . Hollow nanostructure . Photocatalysis
Introduction In recent years, increasing attention has been paid to exploring the use of advanced materials in solving environmental pollution problems. Photocatalysts have exhibited a high potential value in green chemistry because of their wide applications in energy conversion and environmental remediation, such as exploiting clean and renewable solar energy, improving photoelectrochemical processes, and decomposing pollutants [1–4]. Owing to the superior electrochemical, sensing and optical properties, SnO 2 has
* Weihong Guo
[email protected] 1
Polymer Processing Laboratory, Key Laboratory for Preparation and Application of Ultrafine Materials of Ministry of Education, School of Material Science and Engineering, East China University of Science and Technology, Shanghai 200237, People’s Republic of China
2
Jiangsu Hengtong Power Cable Co., Ltd., Suzhou 215200, Jiangsu Province, People’s Republic of China
attracted much attention during the past few years for potential applications in environmental remediation, catalytic support materials, energy storage devices, and gas sensors [5, 6]. As a typical n-type semiconductor, SnO2 can provide photogenerated holes with high oxidizing power owing to its wide band gap energy (3.59 eV) [7, 8]. However, SnO2 can only be photoactivated under UV light irradiation, which just accounts for 4% of the solar spectrum [9]. Additionally, the wide energy gap leads to a poor quantum yield because of the rapid recombination of the photogenerated electron-hole pair, which inhibits the practical application of photocatalysts [10]. Thus, reducing the recombination rate of photogenerated charge carriers, offering a more efficient active substance, is an effective way to improve the photocatalytic activity of photocatalysts. Kadir et al. successfully synthesized granular hollow SnO2 nanofibers via an electrospinning method, and the well-aligned hollow SnO2 nanofibers containing a significant number of oxygen vacancies exhibited an enhanced gas response due to the large accessible surface area for surface reactions [11]. The deposition of noble
J Solid State Electrochem
metals such as Ag, Pt, and Au, which provides the Schottky barrier and facilitates electron capture, is also a good way to broaden the spectral response and enhance the photocatalytic efficiency of photocatalysis [12–15]. Recently, much attention has been paid to utilizing nanostructured copper oxide (CuxO) materials because of their remarkable optical, electrical, thermal, and magnetic properties. To date, nanostructured copper oxides are exceptionally versatile with unique characteristics that can be used in many applications. The most common crystal phases of CuxO include (a) copper oxide (CuO), (b) cuprous oxide (Cu2O), and (c) paramelaconite (Cu4O3) [16]. As a p-type semiconductor with narrow band gap of 1.7 eV, CuO has the advantage of being nontoxic, chemically stable, naturally abundant with excellent thermal and electrical conductivities [17]. Additionally, CuO can be used as a co-catalyst in combination with wide band gap catalysts such as ZnO and TiO2 to enhance their photocatalytic activity under visible light irradiation [18, 19]. Irie et al. proposed a mechanism for the interfacial charge transfer between TiO2 and Cu(II). The author proposed that during the photocatalytic process, the photoinduced electrons of TiO2 transfer to the surface of Cu(II) due to the negative conduction band (CB) of Cu(II), which transformed Cu(II) into Cu(I). The produced Cu(I) could reduce O2 in a multielectron process, while the remaining holes in the TiO2 valence band (VB) can effectively decompose gas pollutants exposed to the photocatalyst [20]. Malwal et al. reported that the formation of a p-n type heterojunction structure of ZnO/ CuO resulted in improved charge separation with a higher photocatalytic activity [21]. A comparison of these materials indicate that SnO2 has superior photocatalytic properties because of its similar electronic structure with that of TiO2 and broader band gap energy [22, 23]. Based on the abovementioned considerations, constructing heterostructured hollow SnO2/CuO nanofibers not only extends the photogenerated charge lifetime but also offers defects in the crystal lattice with numerous oxygen vacancies, which will greatly improve the photocatalytic activity of the nanocomposites [24, 25]. In the present work, an electrospinning technique was used to fabricate nanofibers with a larger aspect ratio, greater interconnectivity, and higher porosity [26–28]. Furthermore, the one-step electrospinning process, using a homogeneous solution of tin tetrachloride and cupric acetate, facilitated the production of heterostructured nanocomposites. To the best of our knowledge, few studies have reported the construction of hollow heterostructured nanofibers composed of SnO2 and CuO in organic contaminant photodegradation. Herein, camphene was introduced into the matrix to produce hollow heterostructured SnO2/CuO nanofibers. The photocatalytic activity of pure SnO2, CuO nanofibers, SnO2/CuO nanofibers with different SnO2/CuO rates, and SnO2/CuO hollow nanofibers were systematically investigated. The amount of camphene in the composites ranged from 0 to 15% to obtain the
different morphologies of the nanofibers. The hollow morphology of the nanostructure was confirmed by using a field emission scanning electron microscope (FESEM) and transmission electron microscopy (TEM). The recombination of photoinduced charges in the SnO2/CuO hollow nanofibers can be effectively inhibited, as detected by photoluminescence spectroscopy (PL). The photocatalytic activity of the samples was investigated by measuring the degradation of methylene blue (MB). In addition, the mechanisms of forming hollow nanostructure and the enhancement in the photocatalytic activity of SnO2/CuO hollow nanofibers are proposed.
Experiment Materials Tin tetrachloride (SnCl4·5H2O, purity of 99.0%), cupric acetate monohydrate (Cu(CH3COO)2·H2O, 99.0%), acetic acid (glacial, 99.9%), dimethylformamide (DMF, 99.5%), and camphene (C10H16, 75%) were all obtained from Shanghai Macklin Biochemical Co., Ltd. Poly(vinyl pyrrolidone) (PVP, Mw ≈ 1.3 × 106) was purchased from Shanghai Titan Technology Co., Ltd. All the chemicals were used asreceived without further modification.
Preparation of the CuO/SnO2 hollow nanofibers CuO/SnO2 hollow nanofibers were fabricated by using an electrospinning method. First, 1.0 g of tin tetrachloride was mixed with 8 mL of DMF and 2 mL of glacial acetic acid, and the mixture was stirred vigorously for more than 60 min until a transparent solution was formed. Second, 1.5 g of PVP and a certain amount of cupric acetate were added to the above solution; after 12 h of stirring, camphene was finally added to the Sn/ Cu precursor solution. The relative loading amounts of camphene to the solvent were then adjusted to be 0, 2, 5, and 10 wt% to control the morphology of the CuO/ SnO2 nanofibers. The detailed composition of the solvent was listed in Table 1. The obtained precursor was used in the electrospinning process. A typical electrospinning system contains a glass syringe with a metal spinneret (inner diameter of 0.68 mm) and a collector made of stainless steel. To obtain the CuO/ SnO2 nanofibers, the voltage and distance between a needle tip and collector were set as 15 kV and 15 cm, respectively. The feeding rate of the solutions was maintained at 0.6 mL/h. After electrospinning, the as-spun nanofibers were placed in an electric thermostatic drying oven at 60 °C for more than 6 h for the evaporation of the organic solvent. Next, the sample was placed into a furnace and annealed at 500 °C in air for 3 h at a heating rate of 2 °C/min, resulting in the formation of CuO/ SnO2 nanofibers with different morphologies, depending on the loading amount of camphene.
J Solid State Electrochem
Characterizations The morphologies of the samples were characterized with a field emission scanning electron microscope (FESEM, Hitachi S-4800) equipped with a link probe for energy dispersive X-ray (EDX) spectroscopy analysis. TEM images were obtained using a JEM-2100F TEM (JEOL Tokyo Japan) operating at 200 kV. X-ray diffraction (XRD) was carried out on a Rigaku-D/max 2550 PC (Japan) diffractometer with Cu Kα radiation (λ = 1.5406 Å). The elemental compositions and valence states were investigated via X-ray photoelectron spectroscopy (XPS, ESCA LAB 220-XL) by using an unmonochromated Al Kα (1486.6 eV) X-ray source. Ultraviolet-visible (UV-vis) diffuse reflection spectroscopy (DRS) spectra were recorded on a PerkinElmer Lambda 950 instrument using BaSO4 as the reference sample, in the range of 200–800 nm. Photoluminescence (PL) was measured at room temperature in a CEMR luminescence spectrometer with a Xenon lamp as the excitation source, and the excited wavelength was 300 nm.
Photocatalytic activity measurements The photocatalytic activities of the SnO2 nanofibers, CuO/ SnO2 nanofibers, and CuO/SnO2 hollow nanofibers for the decomposition of methylene blue (MB) solution were determined under simulated solar light irradiation with a 300-W medium Hg lamp and a ZJB 420 filter glass (cutting off light with wavelengths less than 400 nm). In a typical reaction, 0.10 g of the photocatalyst was added into a 450-mL Pexyr photoreactor containing 100 mL 3.8 × 10−5 M MB. The concentrations of the solutions were monitored colorimetrically by using a Lambda 35 UV-vis absorption spectrophotometer (PerkinElmer, USA) at 655 nm for MB. The absorbance was converted to the MB concentration using a standard curve that showed a linear relationship between the concentration and the absorbance at the corresponding wavelength. The degradation rates were calculated using the following equation: ηð%Þ ¼
C 0 −C 100% C0
ð1Þ
where C0 and C are the concentrations of the initial and remaining MB, respectively.
Results and discussion Morphology and structure of the SnO2 and CuO nanofibers with different weight ratios Figure 1a–h exhibits FESEM images of sample 1 to sample 4 before and after calcination. In Fig. 1a–d, all the samples show
a dense morphology with a uniform gray contrast because of the characteristics of the PVP [29]. The fiber diameters are intentionally controlled by adjusting the electrospinning parameters, e.g., voltage, distance, and humidity. As shown in Fig. 1, the average diameters of the samples are 430–480 nm for sample 1, 590–620 nm for sample 2, 520–540 nm for sample 3, and 420–450 nm for sample 4 before calcination, indicating the formation of uniform fiber diameters of the asspun nanofibers (NFs). After calcination at 500 °C (Fig. 1e– h), the diameters of the NFs are significantly decreased, which is due to the shrinkage effect caused by the decomposition of PVP [30]. It can be observed that the surface morphology of the NFs changes from rough to smooth with an increasing content of cupric acetate, which indicates that SnO2 and CuO have a synergistic effect on the modification of the nanofiber morphology during the calcination process. The composition and crystal structure of the NFs are characterized by XRD, as shown in Fig. 2. The XRD pattern of the pure SnO2 sample and samples 2–3 exhibits strong diffraction peaks at 2θ values of 26.5°, 33.8°, 37.8°, 51.8°, 54.6°, 61.8°, 64.6°, and 66.0°, which coincide well with the corresponding peaks of the tetragonal structure of SnO2, according to the standard data file (JCPDS 01–077-0447) [31]. The pure CuO sample displays nine reflection peaks at 2θ values of 32.3° (110), 35.5° (002), 38.5° (111), 48.6° (2(_)02), 53.2° (020), 58.2° (202), 61.5° (113), 66.1° (3(_)11), and 68.0° (220), which can be indexed as a monoclinic crystal structure of CuO (JCPDS 48-1548) [32]. No additional peaks are observed in the XRD patterns of SnO2 and CuO NFs, confirming that the obtained samples are of high purity. Notably, for CuO-decorated SnO2 NFs, the intensities of characteristic diffraction peaks at 2θ values of 26.6°, 33.8°, and 51.8° were higher than those of pure SnO2, and for SnO2decorated CuO NFs, the intensities of the CuO characteristic diffraction peaks were higher than that of pure CuO, which indicates that the formation of the SnO2/CuO heterostructure facilitates the crystallization ability of the inorganic nanocomposites. Luo et al. investigated the crystallization of Cu-Cedoped TiO2 and found that the anatase crystallization of doped TiO2 was intensively enhanced compared with that of pure TiO2 [33]. The XRD analysis in this study demonstrates the synergistic effect of the heterostructure on the crystallization of nanocomposites. Furthermore, the enhanced diffraction peaks in SnO2/CuO NFs indicate the formation of a p-n heterostructure through a one-step electrospinning method.
Morphology and structure of the SnO2/CuO hollow NFs The morphologies of the as-synthesized hollow NFs are shown in Fig. 3. It can be found that all the samples are composed of the randomly oriented NFs with a rough surface and uniform diameters along the fibers. With an increasing content
J Solid State Electrochem
Fig. 1 SEM images of a, e pure SnO2 NFs, b, f SnO2/CuO (10:1) NFs, c, g SnO2/CuO (1:10) NFs, and d, h pure CuO NFs before and after calcination, respectively
of camphene, the morphology of the NFs gradually transforms from a dense to a hollow nanostructure. In addition, the outer and inner diameters are 190 and 76 nm for sample 6 and 160 and 90 nm for sample 7, indicating that the increasing the content of camphene causes a shrinkage of the NFs and forms an ultrathin hollow nanostructure with a large length to diameter ratio. Thus, the morphology of the NFs can be adjusted by changing the content of camphene. To further investigate the chemical components and the surface morphology of the SnO2/CuO NFs, high magnification FESEM and EDS measurements are introduced in this study. In Fig. 4a, it can be clearly observed that the hollow NFs are composed of rough nanoparticles. Figure 4b shows that only elemental O, Sn, and Cu exist on the surface of the NFs, indicating that the SnO2/CuO hollow NFs are successfully obtained by one-step electrospinning method. The detailed morphologies of the SnO2/CuO NFs are further examined by TEM. As shown in Fig. 5a, b, the hollow morphology of sample 7 can be clearly observed by the relatively strong dark contrast at the edge of the nanofiber. The shell thickness of the hollow NFs is 31 nm, which agrees well with the results of the SEM measurements. Figure 5c, d shows Table 1
The formula of pure SnO2, CuO, and SnO2/CuO hollow NFs
Samples
SnO2 content (wt%)
CuO content (wt%)
Camphene content (wt%)
Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6 Sample 7
10 10 1 0 10 10 10
0 1 10 10 1 1 1
0 0 0 0 2 5 10
high-resolution images of CuO-decorated SnO2 NFs. The inter-planar distance of 0.332 nm corresponds to the (110) plane of rutile SnO2 (Fig. 5c), and the distance of 0.253 nm is related to the (002) plane of CuO (Fig. 5d). It is evident that heterostructured hollow SnO2/CuO nanocomposites were successfully synthesized in this study.
Chemical composition of the SnO2/CuO hollow nanofibers The chemical composition of the as-fabricated SnO2/ CuO hollow NFs is studied by X-ray photoelectron spectroscopy (XPS) analysis. The binding energies of 285 eV in all spectra are calibrated to the C 1s peak of the surface adventitious carbon [34]. Figure 6b–d offers detailed information regarding the chemical state of the Sn, Cu, and O elements through high-resolution XPS spectra. For the elemental Sn (Fig. 6b), two symmetric peaks are detected at 486.5 and 495.0 eV, which can be attributed to the spin-orbit doublet of Sn 3d5/2 and Sn 3d3/2 [35]. Two wrinkled peaks located at 934.1 and 954.1 eV in Fig. 6c, which correspond to the Cu 2p3/2 and Cu 2p1/2 core levels, indicate the existence of Cu 2+ in the nanocomposites [36]. The presence of shakeup satellite peaks located at 940.2, 942.8, and 962.3 eV is attributed to the formation of the Cu (3d) hole state [37]. The XPS spectrum of the O 1s peak in Fig. 6d is asymmetric and can be divided into two Gaussian components located at 530.3 and 532.8 eV, respectively. The low binding energy located at 530.3 eV is due to the O2− ion intrinsic state in the SnO2/CuO compound. The high binding energy located at 532.8 eV usually originates from chemisorbed and surface-adsorbed oxygen [38, 39].
J Solid State Electrochem
and hollow nanostructure on the photodegradation efficiency of organic pollutants. The band gap energy of the as-fabricated samples can be calculated by the Kubelka-Munk function [41]. Assuming that the SnO2/CuO NFs are indirect transition semiconductors, the relationship between the absorption coefficient α and the photon energy hυ can be written as follows: α ¼ Bi hv−E g
Fig. 2 XRD patterns of pure SnO2 NFs, SnO2/CuO (10:1) NFs, SnO2/ CuO (1:10) NFs, and pure CuO NFs
Optical absorption spectrum of the SnO2/CuO hollow nanofibers The UV-vis diffuse reflectance spectra (DRS) of the photocatalysts are illustrated in Fig. 7. The pure SnO2 NFs exhibit fundamental absorption in the UV region of the spectrum, which is due to the intrinsic absorption band derived from the band gap transition [40]. It can be observed that the SnO2/CuO NFs present significantly enhanced absorption in the visible light region, indicating the formation of a heterostructure between SnO2 and CuO that facilitates photocatalytic activity. Meanwhile, with the increasing content of camphene, the absorption intensity of visible light is further enhanced, suggesting a synergistic effect of the heterojunction
Fig. 3 SEM images of a sample 1, b sample 5, c sample 6, and d sample 7 after calcination at 500 °C
1=2
=hv
ð2Þ
where Bi is the absorption constant for the indirect transition and Eg is the material band energy. As shown in Fig. 7b, the band gaps of bare SnO2, sample 2, sample 5, sample 6, and sample 7 are 3.592, 3.494, 3.330, 3.354, and 3.348 eV, respectively. The above results indicate that the heterojunction of SnO2 and CuO provides a suitable band gap for the absorption of visible light. PL measurements are often performed to investigate the influence of charge separation and the transfer behavior of photoinduced charges [42]. Figure 8 shows the PL spectra of bare SnO2 and the other samples. It is obvious that the pure SnO2 NFs exhibit a strong intensity and broadband PL signal in the range of 350–550 nm, which can be ascribed to the fast recombination of the self-trapped electron-hole pairs [43–45]. After the combination of CuO, the PL intensity of SnO2/CuO NFs is remarkably lower than that of pure SnO2. This low PL intensity indicates that the p-n heterojunction of SnO2 and CuO provides a fast separation efficiency of the electron-hole pairs, which will be helpful to the improvement of photocatalytic activity [46]. Notably, with an increasing content of camphene, the PL intensity further decreased, which is satisfactorily consistent with the optical absorption results.
J Solid State Electrochem Fig. 4 a HRSEM image of SnO2/ CuO hollow NFs. b EDS microanalysis of SnO2/CuO hollow NFs
Photocatalytic activity The photodegradation activities of the samples are investigated by inducing an MB dye as a model reaction under visible light irradiation. For comparison, the selfdegradation of MB irradiated by visible light is also carried out under the same conditions. Figure 9a shows the degradation curves of MB for different samples. The concentration of MB is only slightly reduced during irradiation without the photocatalyst, indicating that photoinduced self-degradation can be neglected. The SnO2 NFs exhibit little photocatalytic activity under visible light irradiation due to its wide band gap energy [47]. Notably, the SnO2/CuO NFs exhibit much higher Fig. 5 TEM images of a, b SnO2/ CuO hollow NFs. c, d HRTEM of SnO2/CuO hollow NFs
photocatalytic activity than does bare SnO2 NFs, with an increasing content of camphene, and the photodegradation rate is further enhanced. The results indicate that the construction of the hollow nanostructure further improves the photocatalytic activity of the NFs. The time-dependent UV-vis spectra of MB in the presence of sample 7 are presented in Fig. 9b. The results indicate that the characteristic peak intensity decreases rapidly with an increase in the visible light irradiation time, indicating the excellent photodegradation ability of sample 7. The photodegradation activity is further investigated by introducing the photocatalytic degradation kinetics of MB as shown in Fig. 9c. The quantitative analysis is derived according to the pseudo-
J Solid State Electrochem
Fig. 6 XPS spectra of a fully scanned, b Sn 3d, c Cu 2p, and d O 1s of SnO2/CuO hollow NFs
first-order kinetics [48] as follows: −
1n C ¼ kt C0
ð3Þ
where C0 and C are the concentrations of MB at the beginning and at irradiation time t, respectively, and k is the apparent first-order rate constant (/min). The order of rate constants is summarized in the following order: black sample < sample 1 < sample 5 < sample 6 <
sample 7, which is consistent with the results of the photocatalytic degradation curves presented in Fig. 9a. The reusability of a catalyst is a vital parameter in practical applications. Thus, the stability of the catalyst is evaluated through a cycle experiment using the optimized sample 7. It can be clearly observed that the photocatalytic activity of sample 7 remains almost unchanged after 4 cycling run, suggesting the good stability and reusability of SnO 2/CuO hollow NFs in the
Fig. 7 a UV-vis absorption spectra of pure SnO2 and SnO2/CuO composites. b Calculated band gap of pure SnO2 and SnO2/CuO composites
J Solid State Electrochem
Fig. 8 PL spectra of pure SnO2 and SnO2/CuO composites
photodegradation of organic pollutants. Based on the above results, the mechanism of enhanced photocatalytic activity of SnO2/CuO hollow NFs is proposed in Fig. 10. Figure 10a shows the influence of the Kirkendall effect on the formation of hollow NFs [49]. During the step 1 period, the as-prepared NFs consisted of SnCl 4 ,
Cu(CH3COO)2, camphene, and the PVP matrix, which is due to the isotropic characteristic of the solution. Then, the introduced hydrophobic camphene is gradually converted to hydrophilic isobornyl acetate due to the interaction with the polar aprotic solvent. With an increasing annealing temperature, cavities gradually formed within the NFs, which are due to the evaporation of the isobornyl acetates. Because the diffusion coefficients of Sn and Cu ions are much higher than that of PVP, these inorganic materials gradually transfer to the outer NFs. Finally, the inner PVP matrix within the NFs completely decomposes, and the hollow nanostructure of the NFs is formed. The obtained one-dimensional hollow NFs with high specific surface area and aspect ratio provide higher charge separation efficiency, which is beneficial for photocatalytic degradation. The possible mechanism for the photodegradation of MB is proposed in Fig. 10b. Because CuO is a p-type semiconductor with a narrow band gap energy of 1.35– 1.79 eV [50], it is expected to be an ideal visible light photocatalyst. However, CuO has relatively lower photocatalytic activity because of the inappropriate potentials of the valence and conduction bands [51]. Thus, constructing p-type CuO and n-type SnO 2 heterostructures can
Fig. 9 a The photocatalytic degradation of MB curves. b Time-dependent UV-vis spectra of MB solution photodegraded by sample 7. c Kinetic linear simulation curves of MB degradation. d Recyclability of the photocatalytic degradation of MB by sample 7
J Solid State Electrochem Fig. 10 a Schematic diagram of the formation of hollow SnO2/ CuO nanofibers. b Proposed charge transfer mechanism
efficiently promote the generation, transfer, and separation process of the charge carriers [52]. When the SnO2/CuO NFs are exposed to visible light, the photoinduced electrons on the CB of the SnO2 will transfer to the CB of the CuO, and the reaction can be described by the following equation: hv
SnO2 þ CuO → e− ðCuOÞ þ hþ ðSnO2 Þ
ð4Þ
Because of the interfacial charge transmission model, the transferred photoinduced electrons from the SnO2 CB can react with Cu2+ to generate Cu+, which can transform a dissolved O2 molecule into H2O2: Cu2þ þ e− →Cuþ þ
ð5Þ þ
2Cu þ O2 þ 2H →2Cu
2þ
þ H2 O2
ð6Þ
Conclusions In summary, hollow SnO2/CuO NFs were successfully fabricated by an electrospinning method, and the hollow SnO2/ CuO NFs exhibited highest photocatalytic activity for the photodegradation of MB due to the p-n heterojunction effects, large surface areas, and more active surface reaction sites. The reasons for the different photocatalytic activity of the SnO2/ CuO NFs were fully discussed. Furthermore, the p-CuO/nSnO2 NFs could be easily recycled four times without an obvious decrease in the photocatalytic activity because of the one-dimensional morphology of the NFs. This work provides a facile and versa tile method to fabricate heterostructured hollow NFs with high photocatalytic activity, which can be used in the degradation of organic and inorganic pollutants for environmental remediation.
This is a cyclic process consuming free electrons by the interaction of Cu2+. Then, the generated hydrogen peroxides and holes will further produce hydroxyl radicals, which play an important role in decomposing organic pollutants as follows:
Acknowledgements The authors sincerely acknowledge BScientific and Technological Achievements Transformation Program of Jiangsu Province (SBA2014010034)^ and BNingbo Industrial Major Projects (201601ZD-A01026).^
⋅OH þ pollutants→CO2 þ H2 O
References
ð7Þ
The holes left on the surface of SnO2 directly participate in the degradation of organic pollutants due to their strong oxidizing ability: hþ þ organic pollutants→CO2 þ H2 O
ð8Þ
Owing to the unique physical and optical properties of the hollow nanostructure, the heterostructured hollow SnO2/CuO NFs exhibit excellent photocatalytic activity towards organic pollutions, which is in accordance with the PL results.
1.
2.
3.
4.
Kato H, Kudo A, Kobayashi H, Tsuji I (2004) Photocatalytic H2 evolution reaction from aqueous solutions over band structurecontrolled (Agln)xZn2(1-x)S2. J Am Chem Soc 126:13406–13413 Hojamberdiev M, Zhu GQ, Sujaridworakun P, Jinawath S, Liu P, Zhou JP (2012) Visible-light-driven N-F-codoped TiO2 powders derived from different ammonium oxofluorotitanate precursors. Powder Technol 218:140–148 Zhang J, Bang JH, Tang C, Kamat PV (2010) Tailored TiO2-SrTiO3 heterostructure nanotube arrays for improved photoelectrochemical performance. ACS Nano 4(1):387–395 Ebbinghaus SG, Abicht HP, Dronskowski R, Müller T, Reller A, Weidenkaff A (2009) Perovskite-related oxynitrides—recent
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5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
developments in synthesis, characterisation and investigations of physical properties. Prog Solid State Chem 37(2-3):173–205 Zhu C, Wang P, Wang L, Han L, Dong S (2011) Facile synthesis of two-dimensional graphene/SnO2/Pt ternary hybrid nanomaterials and their catalytic properties. Nano 3:4376–4382 Dong B, Hu WH, Zhang XY, Wang J, Lu SS, Li X, Shang X, Liu YR, Han GQ, Chai YM, Liu CG (2017) Facile synthesis of hollow SnO2 nanospheres uniformly coated by Ag for electro-oxidation of hydrazine. Mater Lett 189:9–12 Enesca A, Isac L, Andronic L, Perniu D, Duta A (2014) Tuning SnO2-TiO2 tandem systems for dyes mineralization. Appl Catal B Environ 147:175–184 Peng XS, Meng GW, Zhang J, Wang XF, Zhao LX, Wang YW, Zhang LD (2002) Electrochemical fabrication of ordered Ag2S nanowire arrays. Mater Res Bull 37(7):1369–1375 Hoffmann MR, Choi WY, Bahnemann DW (1994) A. General background. B. Semiconductor photocatalysis II. Mechanisms of semiconductor photocatalysis a. Basic features and characteristic times. Chem Rev 95:69–96 Gan HH, Zhang GK, Guo YD (2012) Facile in situ synthesis of the bismuth oxychloride/bismuth niobate/TiO2 composite as a high efficient and stable visible light driven photocatalyst. J Colloid Interface Sci 386(1):373–380 Kadir RA, Li Z, Sadek AZ, Rani RA, Zoolfakar AS, Field MR, Ou JZ, Chrimes AF, Kalantar-zadeh K (2014) Electrospun granular hollow SnO2 nanofibers hydrogen gas sensors operating at low temperatures. J Phys Chem C 118(6):3129–3139 Zhang YC, Yao L, Zhang G, Dionysiou DD, Li J, Du X (2014) One-step hydrothermal synthesis of high-performance visible-lightdriven SnS2/SnO2 nanoheterojunction photocatalyst for the reduction of aqueous Cr(VI). Appl Catal B Environ 144:730–738 Wang H, Kalytchuk S, Yang H, He L, Hu C, Teoh WY, Rogach AL (2014) Hierarchical growth of SnO2 nanostructured films on FTO substrates: structural defects induced by Sn(II) self-doping and their effects on optical and photoelectrochemical properties. Nano 6: 6084–6091 Gu Q, Long J, Zhuang H, Zhang C, Zhou Y, Wang X (2014) Ternary Pt/SnO(x)/TiO2 photocatalysts for hydrogen production: consequence of Pt sites for synergy of dual co-catalysts. Phys Chem Chem Phys 16(24):12521–12534 Lu G, Linsebigler A, Yates JT (1995) Photooxidation of CH3Cl on TiO2 (110): a mechanism not involving H2O. J Phys Chem 99(19): 7626–7631 Zoolfakar AS, Rani RA, Morfa AJ, O’Mullane AP, Kalantar-zadeh K (2014) Nanostructured copper oxide semiconductors: a perspective on materials, synthesis methods and applications. J Mater Chem C 2(27):5247–5270 Mageshwari K, Sathyamoorthy R, Parka J (2015) Photocatalytic activity of hierarchical CuO microspheres synthesized by facile reflux condensation method. Powder Technol 278:150–156 Qamar MT, Aslam M, Ismail IM, Salah N, Hameed A (2015) Synthesis, characterization, and sunlight mediated photocatalytic activity of CuO coated ZnO for the removal of nitrophenols. ACS Appl Mater Interfaces 7(16):8757–8769 Praveen KD, Shankar MV, Kumari MM, Sadanandam G, Srinivas B, Durgakumari V (2013) Nano-size effects on CuO/TiO2 catalysts for highly efficient H2 production under solar light irradiation. Chem Commun 49(82):9443–9445 Irie H, Kamiya K, Shibanuma T, Miura S, Tryk DA, Yokoyama T, Hashimoto K (2009) Visible light-sensitive Cu(II)-grafted TiO2 photocatalysts: activities and X-ray absorption fine structure analyses. J Phys Chem C 113(24):10761–10766 Malwal D, Gopinath P (2016) Enhanced photocatalytic activity of hierarchical three dimensional metal oxide@CuO nanostructures towards the degradation of Congo red dye under solar radiation. Cat Sci Technol 6(12):4458–4472
22.
Mourão HAJL, Junior WA, Ribeiro C (2012) Hydrothermal synthesis of Ti oxide nanostructures and TiO2:SnO2 heterostructures applied to the photodegradation of rhodamine B. Mater Chem Phys 135(2-3):524–532 23. Marzec A, Radecka M, Maziarz W, Kusior A, Pędzich Z (2016) Structural, optical and electrical properties of nanocrystalline TiO2, SnO2 and their composites obtained by the sol-gel method. J Eur Ceram Soc 36(12):2981–2989 24. Marschall R (2014) Photocatalysis: semiconductor composites: strategies for enhancing charge carrier separation to improve photocatalytic activity. Adv Funct Mater 24(17):2421–2440 25. Vadivel S, Rajarajan G (2015) Influence of Cu doping on structural, optical and photocatalytic activity of SnO2 nanostructure thin films. J Mater Sci Mater Electron 26(8):5863–5870 26. Reneker DH, Yarin AL (2008) Electrospinning jets and polymer nanofibers. Polymer 49(10):2387–2425 27. Peng S, Jin G, Li L, Li K, Srinivasan M, Ramakrishna S, Chen J (2016) Multi-functional electrospun nanofibres for advances in tissue regeneration, energy conversion & storage, and water treatment. Chem Soc Rev 45(5):1225–1241 28. Malwal D, Gopinath P (2015) Fabrication and characterization of poly(ethylene oxide) templated nickel oxide nanofibers for dye degradation. Environ Sci Nano 2(1):78–85 29. Yoon BH, Park CS, Kim HE, Koh YH (2007) In situ synthesis of porous silicon carbide (SiC) ceramics decorated with SiC nanowires. J Am Ceram Soc 90:3759–3766 30. Hwang SJ, Choi KI, Yoon JW, Kang YC, Lee JH (2015) Pure and palladium-loaded Co3O4 hollow hierarchical nanostructures with giant and ultraselective chemiresistivity to xylene and toluene. Chemistry 21(15):5872–5878 31. Unni GE, Deepak TG, Nair AS (2016) Fabrication of CdSe sensitized SnO2 nanofiber quantum dot solar cells. Mat Sci Semicond Process 41:370–377 32. Sahay R, Sundaramurthy J, Kumar SP, Thavasi V, Mhaisalkar SG, Ramakrishna S (2012) Synthesis and characterization of CuO nanofibers, and investigation for its suitability as blocking layer in ZnO NPs based dye sensitized solar cell and as photocatalyst in organic dye degradation. J Solid State Chem 186:261–267 33. Luo Y, Wang K, Chen Q, Xu Y, Xue H, Qian Q (2015) Preparation and characterization of electrospun la(1-x)Ce(x)CoO(δ): application to catalytic oxidation of benzene. J Hazard Mater 296:17–22 34. Samadi M, Pourjavadi A, Moshfegh AZ (2014) Role of CdO addition on the growth and photocatalytic activity of electrospun ZnO nanofibers: UV vs. visible light. Appl Surf Sci 298:147–154 35. Jiang Z, Zhao R, Sun B, Nie G, Ji H, Lei J, Wang C (2016) Highly sensitive acetone sensor based on Eu-doped SnO2 electrospun nanofibers. Ceram Int 42(14):15881–15888 36. Panzner G, Egert B, Schmidt HP (1985) The stability of CuO and Cu2O surfaces during argon sputtering studied by XPS and AES. Surf Sci 151(2-3):400–408 37. Senthilkumar V, Kim Y, Chandrasekaran S, Rajagopalan B, Kim E, Chung J (2015) Comparative supercapacitance performance of CuO nanostructures for energy storage device applications. RSC Adv 5(26):20545–20553 38. Nagasawa Y, Choso T, Karasuda T, Shimomura S, Ouyang F, Tabata K, Yamaguchi Y (1999) Photoemission study of the interaction of a reduced thin film SnO2 with oxygen. Surf Sci 433:226– 229 39. Popescu DA, Herrmann JM, Ensuque A, BozonVerduraz F (2001) Nanosized tin dioxide: spectroscopic (UV-VIS, NIR, EPR) and electrical conductivity studies. Phys Chem Chem Phys 3(12): 2522–2530 40. López R, Gómez R (2012) Band-gap energy estimation from diffuse reflectance measurements on sol-gel and commercial TiO2: a comparative study. J Sol-Gel Sci Technol 61(1):1–7
J Solid State Electrochem 41.
42.
43.
44.
45.
46.
Jing L, Fu H, Wang B, Wang D, Xin B, Li S, Sun JS (2006) Effects of Sn dopant on the photoinduced charge property and photocatalytic activity of TiO2 nanoparticles. Appl Catal B Environ 62:282–291 Su C, Shao C, Liu Y (2011) Electrospun nanofibers of TiO2/CdS heteroarchitectures with enhanced photocatalytic activity by visible light. J Colloid Interface Sci 359(1):220–227 Knorr FJ, Mercado CC, Mchale JL (2008) Trap-state distributions and carrier transport in pure and mixed-phase TiO2: influence of contacting solvent and interphasial electron transfer. J Phys Chem C 112(33):12786–12794 Ng J, Xu S, Zhang X, Yang HY, Sun DD (2010) Hybridized nanowires and cubes: a novel architecture of a heterojunctioned TiO2/ SrTiO3 thin film for efficient water splitting. Adv Funct Mater 20(24):4287–4294 Wang C, Shao C, Zhang X, Liu Y (2009) SnO2 nanostructures-TiO2 nanofibers heterostructures: controlled fabrication and high photocatalytic properties. Inorg Chem 48(15):7261–7268 Liu Z, Sun DD, Guo P, Leckie JO (2007) An efficient bicomponent TiO2/SnO2 nanofiber photocatalyst fabricated by electrospinning with a side-by-side dual spinneret method. Nano Lett 7(4):1081–1085
47.
48.
49.
50.
51.
52.
Xu L, Steinmiller EMP, Skrabalak SE (2014) Achieving synergy with a potential photocatalytic Z-scheme: synthesis and evaluation of nitrogen-doped TiO2/SnO2 composites. J Phys Chem C 116: 871–877 Xu SH, Fei GT, Ouyang HM, Shang GL, Gao XD, Zhang L (2017) Necklace-like NiO-CuO heterogeneous composite hollow nanostructure: preparation, formation mechanism and structure control. Sci Rep 7(1):144–157 Chen H, Leng W, Xu Y (2014) Enhanced visible-light photoactivity of CuWO4 through a surface-deposited CuO. J Phys Chem C 118(19):9982–9989 Arai T, Yanagida M, Konishi Y, Iwasaki Y, Sugihara H, Sayama K (2008) Promotion effect of CuO co-catalyst on WO3-catalyzed photodegradation of organic substances. Catal Commun 9(6): 1254–1258 Shan W, Hu Y, Zheng M, Wei C (2015) The enhanced photocatalytic activity and self-cleaning properties of mesoporous SiO2 coated Cu-Bi2O3 thin films. Dalton Trans 44(16):7428–7436 Zheng Z, Zhuge F, Wang Y, Zhang J, Gan L, Zhou X, Li H, Zhai T (2017) Decorating perovskite quantum dots in TiO2 nanotubes array for broadband response photodetector. Adv Funct Mater. https:// doi.org/10.1002/adfm.201703115