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Nanowire-based zinc-doped tin oxide microtubes for enhanced solar energy utilization efficiency
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Nanowire-based zinc-doped tin oxide microtubes for enhanced solar energy utilization efficiency ⁎
Wutao Mao1, Zhengdao Li1, Keyan Bao , Kaijun Zhang, Weibo Wang, Beibei Li College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Henan 473061, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: Zinc-doped tin oxide Nanowire-based microtubes Photocatalytic degradation of dyes Dye-sensitized solar cells
Nanowire-based Zn-doped SnO2 microtubes are synthesized via a solvothermal route for solar energy utilization. The unique architecture exhibits a hierarchical structure: the interconnected nanowires vertically stand on the surface of tube with several micrometers in length. With this structure as photoanodes for the dyesensitized solar cells (DSSCs), an overall 4.22% photoconversion efficiency is obtained, which is nearly thrice as high as that of the DSSCs constructed using a photoanode of commercial SnO2 nanoparticles. Moreover, with the Zn-doped SnO2 as the photocatalyst, it exhibits both higher photocatalytic activity and better recyclability for the degradation of dyes. These improvements are ascribed to fast electron transport, high surface area, and promoted charge separation made possible by the fancy structure of Zn doping into the SnO2 framework.
1. Introduction Tin oxide (SnO2 ), an attractive wide-band gap semiconductor [1], has been applied to the dye-sensitized solar cells (DSSCs) [2,3] and photocatalyst [4–6] due to its unique physical and chemical properties [7,8]. To utilize solar energy more efficiently, SnO2 should enhance light scattering, promote charge separation and accelerate electron transport [9]. A fair amount of theoretical and experimental work has demonstrated that SnO 2/semiconductor composite is an efficient structure to improve charge separation, such as SnO2 /TiO 2 [10], SnO 2/Al 2O 3 [11], SnO 2 /MgO [12], and SnO 2/ZnO [13]. However, the instability of these oxides needs to be resolved in the SnO 2/semiconductor composites. Thereby, Aldoped SnO2 [14], Mg-doped SnO2 [15] and Zn-doped SnO 2 [16– 18] have been studied and proved to be chemically stable ternary oxides. Among them, the ion radius of Zn2+(0.073 nm) is similar to that of Sn4+(0.071 nm). Zn2+ can be easily incorporated into the lattice of SnO 2 for the surface modification of SnO 2 to produce more oxygen vacancies, resulting in excellent photocatalytic properties [19]. Meanwhile, our previous work has also demonstrate that DSSCs based on Zn-doped SnO 2 photoanodes exhibit longer electron lifetime, higher short-circuit current (Jsc), open-circuit photovoltage (V oc) and photoconversion efficiency (ŋ) compared to undoped SnO 2 based DSSCs [20]. Therefore, Zn-doped SnO 2 is a good choice for improving the photovoltaic and photocatalytic performance of SnO 2 .
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The hierarchical hollow architecture assembled from one-dimensional (1D) nanostructured building blocks is a good structural choice to speed up electron transfer and enhance light scattering because it can take both the advantages of nanometer-sized building blocks and micrometer-sized hollow assemblies [21]. For example, in point of DSSCs, this 1D nanostructure can provide photogenerated electrons with direct electrical pathways to improve electron transport rates [22]. Moreover, the micrometer-sized aggregates can function as efficient light scatterers to enhance the light harvesting [23]. In addition, this hollow structure possesses a high surface area to adsorbed sufficient dye molecules in favor of capture of incident photons, resulting in large Jsc and high ŋ [24]; In point of the photocatalyst, the hollow structure has the larger surface, providing more surface active sites [5]. At the same time, such micro-sized photocatalyst can be recovered after use via centrifugation or filtering. Herein, we report an innovative structure of nanowire-based Zndoped SnO2 microtubes for solar energy utilization. In this hierarchical structure, the tube is several micrometers in length; the tube surface consists of vertically interconnected nanowires. With the hierarchical 3.69 at% Zn-doped SnO2 as the photoanodes material, the corresponding DSSCs give conversion efficiency up to 4.22%. With Zn-doped SnO2 materials applied in photodegradation of Rhodamine B (RhB), it shows excellent photocatalytic performance and recyclability. These improvements are ascribed to fast electron transport, high specific surface area, and promoted charge separation made possible by the fancy structure of Zn doping into the SnO2 framework.
Corresponding author. E-mail address: [email protected] (K. Bao). These authors contributed equally to this work.
http://dx.doi.org/10.1016/j.ceramint.2017.02.101 Received 11 December 2016; Received in revised form 19 February 2017; Accepted 21 February 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Mao, W., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.02.101
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Fig. 1. (a-c) SEM. (d, e) TEM and (f) HRTEM images of the Zn-doped SnO2.
2. Experimental
2.2. Preparation of the electrode
2.1. Synthesis of samples
The method for fabricating photoanode and the solution composition electrolyte have been reported in our previous paper [20]. The assembled photoanode was soaked in the ethanol solution of N719 for 20 h.
To synthesize nanowire-based Zn-doped SnO2 microtubes, 0.12 mmol of Zn(CH3COO)2·2H2O (Aladdin, 99.0%), 0.6 mmol of SnCl4·5H2O (Aladdin, 99.0%) and 7.2 mmol of NaOH (Aladdin, AR. 96%) were added to solvent which includes 15 mL H2O and 15 mL ethylenediamine (En) (Aladdin, 99.0%). After several minutes of stirring, the mixture was transferred to a 50 mL stainless Teflon-lined autoclave and reacted at 200 °C for 48 h. The precipitate was centrifuged, washed with distilled water and dried in air. The dried powder was calcined for 30 min at 500 °C. The prepared gray powder was labelled as Zn-doped SnO2. For comparison, commercial SnO2 (Aladdin, AR. 99.5%) was used as referential sample (labelled as SnO2).
2.3. Characterization The crystallinity and morphology of the samples were characterized using the transmission electron microscopy (TEM) (JEOL 3010, Japan) with an accelerating voltage of 200 kV and scanning electron microscopy (SEM) (FEI NOVA NanoSEM230, USA) with an energydispersive X-ray spectrometer (EDX) at 20 kV. The X-ray photoelectron spectroscopy (XPS) spectra were recorded using ESCALab MKII
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X-ray photoelectron spectrometer at 13 kV. The X-ray diffraction (XRD) data were obtained at room temperature by X-ray diffractometer (counting time of 10.16 s per data point, scan step of 0.05°) with Cu-Kα radiation (λ=1.54056 Å). UV–visible absorption spectrum of powder was recorded using pure BaSO4 pellet as the reference and measured by UV–vis spectrophotometer (UV-2550, Shimadzu). Brunauer-Emmett-Teller (BET) was measured on micromeritics Tristar-3000 (USA). The amount of desorbed N719 was estimated by treating dye-impregnated photoanodes into a 0.1 M NaOH solution and by the absorption spectra measurement of the solution. The thickness of film was measured using a Dektak 6 M stylus profiler. Intensity-modulated photovoltage/photocurrent spectra (IMVS/IMPS) were carried out on the electrochemical workstation (Zahner, Zennium) with a frequency response analyzer under a modulated blue light emitting diodes (457 nm) driven by a Zahner source supply (PP211). The incident-photon-to-current conversion efficiency (IPCE) signal was observed with PEC-S20 (Peccell Technology Co. Ltd.). The photocurrent–voltage characteristics (J-V) was measured using a sunlight simulator (Oriel 92251A-1000, AM 1.5 globe, 100 mW/cm2) with 0.4 cm2 active area. 2.4. Photocatalytic activity test 100 mg of sample was added into 100 mL of 10−5 mol L–1 rhodamine B (RhB) solution. The adsorption/desorption equilibrium was reached by magnetically stirred for 12 h in the dark. The sunlight simulator provided the irradiation and was placed 20 cm apart from the reactor. 3. Results and discussion 3.1. Structural properties characteristics and of samples The low magnified SEM image reveals that the Zn-doped SnO2 displays uniform tube structure with lengths in the range 6–8 µm (Fig. 1a). Highly magnified SEM image in Fig. 1b indicates that the tube is built from nanoplates of ~100 nm thickness. Fig. 1c shows that the nanowires with ~50 nm in diameter and ~600 nm in length vertically stand on the surface of tubes. These Zn-doped SnO2 tubes units possess a hexagonal orifice with the diameter of ~800 nm. The TEM images clearly show the hollow tube structure and the hexagonal orifice (Fig. 1d and e), well consistent with the SEM observation. Such a hollow 1D structure should enhance the absorption of light and facilitate electrolyte diffusion. The observed lattice spacing of ~0.335 nm of the nanowire, corresponding to the (110) planes of the tetragonal SnO2 phase (JCPDS No. 71-0652), is found in Fig. 1f. The XRD spectra of the commercial SnO2 and Zn-doped SnO2 samples showed all of the diffraction peaks correspond to the tetragonal structure of SnO2 (a=b=0.4738 nm, c=0.3187 nm, α=β=γ=90°, JCPDS Card File No. 71-0652) (Fig. 2a). The introduction of Zn caused no any extra peaks. In addition, a slight shift to smaller peak positions relative to the SnO2 indicated the successful Zn doping in the lattice of SnO2 (Fig. 2b). The lattice constants (a=b, c) have been determined according to the following formula [25]:
sin2θ =
λ 2 ⎛ h2 + k 2 l2 ⎞ ⎜ + 2⎟ 2 4 ⎝ a c ⎠
Fig. 2. (a) XRD patterns of the SnO2 and Zn-doped SnO2 samples. (b) XRD patterns of the corresponding (101) peak of two samples.
a=
λ 2 sin θ
(2)
For the (002) orientation (2θ=57.62°), the lattice constant “c” was determined by
c=
λ sin θ
(3)
The lattice constants were calculated as a=b=0.4774 nm and c=0.3196 nm for Zn-doped SnO2. The increased lattice parameters (a and c) may be attributed to the replacement of Sn4+(ionic radius of Sn4+=0.071 nm) by larger ionic radii Zn2+(0.073 nm). From comparison in XPS conducted on two samples (Fig. 3a), the Zn element signal was only presented in the spectra of the Zn-doped SnO2 power, which confirmed the doping of Zn into SnO2 (Fig. 3b). Sn 3d5/2 and Sn 3d3/2 bands with corresponding binding energies of 485.6 eV and 494.0 eV were lower 0.4 eV than those of SnO2 (486.0 for Sn 3d5/2 and 494.4 for Sn 3d3/2, which can be ascribed to oxygen deficiency (Fig. 3c)) [18]. The O 1s peak for Zn-doped SnO2 also shifted a lower value, which could be attributed to Sn–O–Zn coordination (Fig. 3d) [18]. The content of Zn is 3.69 at%, calculated by XPS curves. Furthermore,
(1)
where λ is the X-ray wavelength. For the (110) orientation (2θ=26.38°), the lattice constant “a” was obtained by
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Fig. 3. The typical XPS spectra of the SnO2 and Zn-doped SnO2. (a) the survey spectra. (b) Zn 2p. (c) Sn 3d, and (d) O 1s peaks.
(Fig. 4b). (III) Zn-doped SnO2 in the reaction solution attached onto the rough surface of the nanoplates (Fig. 4c) and the secondary nucleation process started. After 20 h, the sparse Zn-doped SnO2 nanowires started to appear (Fig. 4d); (IV) As time went on, the nanowires continued to grow in both diameter and length, and became increasingly denser (Fig. 4e). According to the solvent-coordination molecular template mechanism [26], En acted as a crucial role in the growth of nanowires. In reaction process, En molecules could be adsorbed on the specific facets of the Zn-doped SnO2 nuclei, resulting in the growth along these suppressed crystal planes. Comparatively, the microtubes with rough surface (labelled as MTs) were obtained used of pure water without ethylenediamine as the solvent, while keeping other conditions unaltered and those nanowires have not appeared (Fig. 4f).
the chemical compositions of Zn-doped SnO2 sample were analyzed using EDX (Supplementary Fig. S1). Cu, Sn, O and Zn peaks can be easily detected, indicating that Zn existed in the final product indeed. The presence of Cu mainly came from the substrates due to the SEM test. Based on the EDX analysis, the atomic ratio of Zn/Sn was almost in agreement with the result from XPS. To understand the growth mechanism of the unique Zn-doped SnO2 structure, the morphology evolution of sample with different reaction times (2, 12, 20 and 30 h) at 200 °C was investigated. The corresponding XRD patterns were shown in Supplementary Fig. S2. There was a possible four-step development mechanism: (I) In the first 2 h, under solvothermal conditions, vismirnovite ZnSn(OH)6 (a=b=c=0.772 nm, α=β=γ=90°, JCPDS Card File No. 33-1376) nuclei formed quickly by the complexation between metal ions and OH– in the NaOH and ethylenediamine (En) alkali solution, followed by the rodshaped crystals growth (Fig. 4a); (II) Under the etching effect of alkali solution, ZnSn(OH)6 micro-rod were decomposed into Sn(OH)62- and Zn(OH)42- [17], then further formed the Zn-doped SnO2 nanoplate with rough surface, and residual Zn-doped SnO2 went into the solution. With the consumption of reactant, more and more nanoplates would generate, which built nanoplate-textured micro-tubes in situ at 12 h
3.2. Photovoltaic performance of samples The Zn-doped SnO2 microtubes were utilized as the photoanode for DSSCs. Commercial SnO2 particles (100–200 nm in diameter) were used as referential sample (Supplementary Fig. S3). Comparative J-V curves of DSSCs based on two photoanodes with the same thickness (13 µm) are shown in Fig. 5a. The open-circuit voltages of the Zn-
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Fig. 4. SEM of morphological evolution of the Zn-doped SnO2. (a) 2 h. (b, c) 12 h. (d) 20 h. (e) 30 h. (f) SEM image of the MTs.
(Fig. 5b). The doping of Zn could cause the passivation of the subband-edge surface-states, resulting in the upward movement of the quasi-Fermi level of Zn-doped SnO2 and enhanced Voc [18]. One can also find that Jsc value (13.05 mA/cm2) of the Zn-doped SnO2 DSSCs was much higher than the SnO2 DSSCs (8.68 mA/cm2). To comprehend the Jsc increment of the Zn-doped SnO2-based cell, the IPCE spectra of two cells were measured, as displayed in Fig. 6a. The IPCE of Zn-doped SnO2 was obviously higher than that of SnO2 over the wavelength region of 400–800 nm. The maximum IPCE was 29% at the wavelength of 520 nm for SnO2-based DSSCs. After doping Zn into the SnO2 framework, the maximum IPCE reached up to 55%, concurring with the trend observed for Jsc in the J-V characteristics. The higher Jsc could be attributed to better dye adsorption or/and higher light-harvesting efficiency [10]. On the one hand, the Zn-doped SnO2
doped SnO2 (0.55 V) was found to be considerably higher than that of the SnO2 (0.32 V). The Voc of a DSSCs is determined by the offset between the redox potential of I-/I3- in the electrolyte and the flat-band potential (VFB) of the photoanode [27]. To ascertain the effect of the Zn doping in SnO2 on improving the Voc, the VFB of photoanodes derived from the Mott-Schottky plots by the equation 1/C2=(2/A2eεε0ND) (V– VFB–kT/e), where C denotes the space charge region capacitance, A is the electrode surface area, e denotes the electron charge, ε0 and ε are the vacuum permittivity and the dielectric constant of the semiconductor. ND, V, k and T represent the donor density, applied potential, Boltzmann constant and absolute temperature, respectively [28]. The VFB can be obtained from the intercept on the abscissa by plotting of C−2 against V. Mott-Schottky demonstrated that the VFB of SnO2 and Zn-doped SnO2 were 0.16 V and −0.06 V (vs. Ag/AgCl), respectively
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Fig. 5. (a) J–V curves and (b) Mott-Schottky plots for cells based on two electrodes. Fig. 6. (a) IPCE spectra and (b) the pH-dependent zeta-potential for two different electrodes.
photoanode (0.39×10−7 mol/cm2) doubled the dye loading amounts of SnO2 photoanode (0.21×10−7 mol/cm2) (Supplementary Fig. S4a). Whereas the BET surface area of Zn-doped SnO2 was calculated to be 45.56 m2/g, which was only higher 27.3% than SnO2 (35.78 m2/g), the increased dye adsorption could be explained by higher isoelectric point (pI). Fig. 6b showed SnO2 has pI about pH 4.1, which was consistent with the literature values (4–5). After Zn doping into the SnO2 framework, the pI increased to 4.7. Surfaces with higher pI would be in favor of the adherence of N719 with acidic carboxyl groups [4]. To investigate further, we tested the absorption spectra of the dye-loaded photoelectrodes (Fig. 7a). The strong absorption in the range of 450– 650 nm was consistent with IPCE data and demonstrated the presence of band edge of N719 [29]. Compared to pure SnO2, the intensity of the absorption peak of Zn-doped SnO2 showed drastic enhancement, which clearly revealed that the photoelectrodes prepared from Zndoped SnO2 facilitated excellent light harvesting ability. It should be emphasized that Zn-doped SnO2-based film after the dye adsorption can still present the original morphology, which indicates that the Zndoped SnO2 photoanode is chemically stable in acid environments (Supplementary Fig. S4b); On the other hand, as shown in Fig. 7b, Zndoped SnO2 film exhibited high stronger reflectance in the wavelength range from 400 nm to 800 nm, indicating that the Zn-doped SnO2 film has a higher light-scattering ability. It was believed that resonant
scattering can occur when the porous medium dimension was comparable to the wavelength of incident light [30]. Herein, the microtube with the dimension of ~800 nm can act as an efficient scattering center to improve light-harvesting efficiency [31]. It was worth noting that, the incident light can be not only reflected but also diffused throughout the microtube, so the incident photon flux could be absorbed and utilized more effectively [32]. To clarify the effect of the morphology and Zn doping in SnO2 network on the electron transport and recombination, IMPS /IMVS were further measured under different irradiation intensities from 30 to 150 W/m2. The electron transport time (τd) and recombination time (τr) can be calculated according to the expression τd=1/2πfd and τr=1/ 2πfr, where fd (fr) was the characteristic minimum frequency of the IMPS (IMVS) [33]. As shown in the Fig. 8a, Zn-doped SnO2-based DSSCs exhibited a shorter τd than that of SnO2-based DSSCs. This phenomenon could be ascribed to the fact that 1D tubes and nanowires can provide direct electrical pathways for photogenerated charges transport with less diffusive hindrance, suggesting the Zn-doped SnO2 with 1D nanowire-based microtubes were favorable for the electron transport as compared to 0D SnO2 nanoparticles [34]; As depicted in Fig. 8b, the τr of Zn-doped SnO2-based cell was longer than
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Fig. 8. (a) Transport time and (b) Lifetime based on the different photoelectrodes. Fig. 7. (a) Optical absorption spectra of two films with adsorbed dye. (b) Diffused reflection spectra of the two samples.
3.3. Photocatalytic activity of samples To develop the potential application for solar energy, their photocatalytic activity was evaluated by degrading RhB under simulated sunlight irradiation. In Fig. 9a, a blank test using Zn-doped SnO2 as the photocatalyst without irradiation showed that few RhB was degraded. Another blank test without photocatalyst demonstrated only a small quantity of RhB degradation occurred after irradiation. The RhB was degraded completely using Zn-doped SnO2 after irradiation for 150 min, whereas only 69.9% of RhB was degraded over SnO2 (Fig. 10). Fig. 9b showed that the degradation reaction follows firstorder rate law [36]. The Zn-doped SnO2 indicated the higher catalytic activity with the degradation rate constant of 0.017 min−1, about 2 times than that of SnO2 (0.008 min−1), which was a consequence of the following facts: (i) It is widely accepted that the fast recombination of the photogenerated pairs hampers the degradation of organic reagents. The Zn-doped SnO2 may improve the photocatalytic efficiency attributed to increasement of the charge separation [37]; (ii) The larger surface area and the empty tube might offer more reaction sites and possessed stronger adsorption ability for large-sized dye molecules [38]; (iii) The 1D nanowire structures benefited photogenerated electron-hole pairs to transfer from inside the photocatalyst to the
that of SnO2-based cell, indicating the obvious suppressed charge recombination at the Zn-doped SnO2/N719/electrolyte interface. It was known that the less the number of trap sites, the less the probability of charge recombination. While the trap-site density increased with the number of nanoparticles [35]. Thereby, the trapsite density of Zn-doped SnO2 microtube electrode was less than that of SnO2 nanoparticles electrode, thus reduced charge recombination in the microtube-based electrode and increased electron lifetime, which, no doubt, enable the Zn-doped SnO2 based DSSCs to achieve higher conversion efficiencies. The overall ŋ of the Zn-doped SnO2 DSSCs is 4.22%, distinctly higher than that of the SnO2 DSSCs (ŋ=1.21%) and precedent Zndoped SnO2 nanocrystals synthesized by hydrothermal method (4.18%) [16], nanoflowers (3.00%) [17], mesoporous particles (3.73%) [18], and nano-echinus (4.15%) [20]. The improvement in ŋ of the Zn-doped SnO2 can be caused by following three factor: (1) higher Voc due to negative shift in the VFB resulted from Zn doping into the SnO2 framework; (2) higher Jsc due to better dye adsorption and higher lightharvesting efficiency; (3) the lower electron recombination rate and longer electron lifetime.
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Fig. 9. (a) The degradation rate of RhB over different photocatalysts. (b) The ln(C0/C) versus time curves of photodegradation of RhB. (c) Cycling experiments of degradation over Zndoped SnO2 (•) and SnO2 (■).
unique chemical composition and structure, Zn-doped SnO2 microtubes have applications in the varied areas. With the hierarchical Zndoped SnO2 as the anode material for DSSCs, an overall 4.22% photoconversion efficiency was obtained, 249% improvement compared to the commercial SnO2 nanoparticles. With the Zn-doped SnO2 microtubes as the photocatalyst for degradation of organic dye, they showed excellent photocatalytic activity. Moreover, the micron-sized Zn-doped SnO2 powder were easy to separate from the photocatalysis reaction system and reuse in practice.
surface for degradation of RhB. In addition, our Zn-doped SnO2 can be reused with no obvious loss of photocatalytic activity (Fig. 9c) and they still preserved the original morphology after repeated photocatalytic runs (Supplementary Fig. S5a). The reuse of the hierarchical structure was facilitated compared to commercial SnO2 nanoparticles, which were difficult to recover from the reaction system by sedimentation (Supplementary Fig. S5b). 4. Conclusions In summary, nanowire-based Zn-doped SnO2 microtubes have been proposed for solar energy applications. In this hierarchical structure, one-dimensional nanowires can provide photo generated electrons with direct electrical pathways, the micrometer-sized microtubes can function as efficient light scatterers to enhance the light harvesting. Zn doping into SnO2 can increase the isoelectric point. Inspired by the
Acknowledgments This work was supported by National Natural Science Foundation of China (No. 21301101, U1404505), the Program for Innovative Talent in University of Henan Province (16HASTIT010) and Henan Province Projects (14A150047, ZX2014047).
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[34] A.-F. Kanta, A. Schrijnemakers, A. Decroly, Electrochemical characterisations of ZnO nanowires for dye-sensitised solar cells, Mater. Des. 95 (2016) 481–485. [35] Y. Rui, Y. Li, H. Wang, Q. Zhang, Photoanode based on chain-shaped anatase TiO2 nanorods for high efficiency dye-sensitized solar cells, Chem. Asian J. 7 (2012) 2313–2320. [36] H. You, R. Liu, C. Liang, S. Yang, F. Wang, X. Lu, B. Ding, Gold nanoparticle doped hollow SnO2 supersymmetric nanostructures for improved photocatalysis, J. Mater. Chem. A 1 (2013) 4097–4104. [37] J. Wang, H. Li, S. Meng, L. Zhang, X. Fu, S. Chen, One-pot hydrothermal synthesis of highly efficient SnOx/Zn2SnO4 composite photocatalyst for the degradation of methyl orange and gaseous benzene, Appl. Catal. B: Environ. 200 (2017) 19–30. [38] D. Yang, M. Wang, B. Zou, G. Zhang, Z. Lin, An external template-free route to uniform semiconducting hollow mesospheres and their use in photocatalysis, Nanoscale 7 (2015) 12990–12997.
Fig. 10. Time-dependent UV–vis absorbance spectra of the RhB solution in the presence of (a) Zn-doped SnO2 and (b) the SnO2.
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ceramint.2017.02.101. References [1] M. Akram, A.T. Saleh, W.A.W. Ibrahim, A.S. Awan, R. Hussain, Continuous microwave flow synthesis (CMFS) of nano-sized tin oxide: effect of precursor concentration, Ceram. Int. 42 (2016) 8613–8619. [2] H.J. Snaith, C. Ducati, SnO2-based dye-sensitized hybrid solar cells exhibiting near unity absorbed photon-to-electron conversion efficiency, Nano Lett. 10 (2010) 1259–1265. [3] J.W. Bae, B.R. Koo, H.R. An, H.J. Ahn, Surface modification of fluorine-doped tin oxide films using electrochemical etching for dye-sensitized solar cells, Ceram. Int. 41 (2015) 14668–14673. [4] T.B. Ivetić, N.L. Finčur, B.F. Abramović, M. Dimitrievska, G.R. Štrbac, K.O. Čajko, Environmentally friendly photoactive heterojunction zinc tin oxide nanoparticles, Ceram. Int. 42 (2016) 3575–3583. [5] L. Chen, L. Xie, M. Wang, X. Ge, Preparation of three-dimensional inverse opal SnO2/graphene composite microspheres and their enhanced photocatalytic activities, J. Mater. Chem. A 3 (2015) 2991–2998. [6] X. Fu, J. Wang, D. Huang, S. Meng, Z. Zhang, L. Li, T. Miao, S. Chen, Trace amount of SnO2-decorated ZnSn(OH)6 as highly efficient photocatalyst for decomposition of gaseous benzene: synthesis, photocatalytic activity, and the unrevealed synergistic effect between ZnSn(OH)6 and SnO2, ACS Catal. 6 (2016) 957–968. [7] M.P.S. Rana, F. Singh, S. Negi, S.K. Gautam, R.G. Singh, R.C. Ramola, Band gap engineering and low temperature transport phenomenon in highly conducting antimony doped tin oxide thin films, Ceram. Int. 42 (2016) 5932–5941. [8] P. Chesler, C. Hornoiu, S. Mihaiu, C. Munteanu, M. Gartner, Tin-Zinc oxide composite ceramics for selective CO sensing, Ceram. Int. 42 (2016) 16677–16684. [9] W. Li, J. Yang, J. Zhang, S. Gao, Y. Luo, M. Liu, Improve photovoltaic performance of titanium dioxide nanorods based dye-sensitized solar cells by Ca-doping, Mater. Res. Bull. 57 (2014) 177–183.
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Nanowire-based zinc-doped tin oxide microtubes for enhanced solar energy utilization efficiency ⁎
Wutao Mao1, Zhengdao Li1, Keyan Bao , Kaijun Zhang, Weibo Wang, Beibei Li College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Henan 473061, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: Zinc-doped tin oxide Nanowire-based microtubes Photocatalytic degradation of dyes Dye-sensitized solar cells
Nanowire-based Zn-doped SnO2 microtubes are synthesized via a solvothermal route for solar energy utilization. The unique architecture exhibits a hierarchical structure: the interconnected nanowires vertically stand on the surface of tube with several micrometers in length. With this structure as photoanodes for the dyesensitized solar cells (DSSCs), an overall 4.22% photoconversion efficiency is obtained, which is nearly thrice as high as that of the DSSCs constructed using a photoanode of commercial SnO2 nanoparticles. Moreover, with the Zn-doped SnO2 as the photocatalyst, it exhibits both higher photocatalytic activity and better recyclability for the degradation of dyes. These improvements are ascribed to fast electron transport, high surface area, and promoted charge separation made possible by the fancy structure of Zn doping into the SnO2 framework.
1. Introduction Tin oxide (SnO2 ), an attractive wide-band gap semiconductor [1], has been applied to the dye-sensitized solar cells (DSSCs) [2,3] and photocatalyst [4–6] due to its unique physical and chemical properties [7,8]. To utilize solar energy more efficiently, SnO2 should enhance light scattering, promote charge separation and accelerate electron transport [9]. A fair amount of theoretical and experimental work has demonstrated that SnO 2/semiconductor composite is an efficient structure to improve charge separation, such as SnO2 /TiO 2 [10], SnO 2/Al 2O 3 [11], SnO 2 /MgO [12], and SnO 2/ZnO [13]. However, the instability of these oxides needs to be resolved in the SnO 2/semiconductor composites. Thereby, Aldoped SnO2 [14], Mg-doped SnO2 [15] and Zn-doped SnO 2 [16– 18] have been studied and proved to be chemically stable ternary oxides. Among them, the ion radius of Zn2+(0.073 nm) is similar to that of Sn4+(0.071 nm). Zn2+ can be easily incorporated into the lattice of SnO 2 for the surface modification of SnO 2 to produce more oxygen vacancies, resulting in excellent photocatalytic properties [19]. Meanwhile, our previous work has also demonstrate that DSSCs based on Zn-doped SnO 2 photoanodes exhibit longer electron lifetime, higher short-circuit current (Jsc), open-circuit photovoltage (V oc) and photoconversion efficiency (ŋ) compared to undoped SnO 2 based DSSCs [20]. Therefore, Zn-doped SnO 2 is a good choice for improving the photovoltaic and photocatalytic performance of SnO 2 .
⁎
1
The hierarchical hollow architecture assembled from one-dimensional (1D) nanostructured building blocks is a good structural choice to speed up electron transfer and enhance light scattering because it can take both the advantages of nanometer-sized building blocks and micrometer-sized hollow assemblies [21]. For example, in point of DSSCs, this 1D nanostructure can provide photogenerated electrons with direct electrical pathways to improve electron transport rates [22]. Moreover, the micrometer-sized aggregates can function as efficient light scatterers to enhance the light harvesting [23]. In addition, this hollow structure possesses a high surface area to adsorbed sufficient dye molecules in favor of capture of incident photons, resulting in large Jsc and high ŋ [24]; In point of the photocatalyst, the hollow structure has the larger surface, providing more surface active sites [5]. At the same time, such micro-sized photocatalyst can be recovered after use via centrifugation or filtering. Herein, we report an innovative structure of nanowire-based Zndoped SnO2 microtubes for solar energy utilization. In this hierarchical structure, the tube is several micrometers in length; the tube surface consists of vertically interconnected nanowires. With the hierarchical 3.69 at% Zn-doped SnO2 as the photoanodes material, the corresponding DSSCs give conversion efficiency up to 4.22%. With Zn-doped SnO2 materials applied in photodegradation of Rhodamine B (RhB), it shows excellent photocatalytic performance and recyclability. These improvements are ascribed to fast electron transport, high specific surface area, and promoted charge separation made possible by the fancy structure of Zn doping into the SnO2 framework.
Corresponding author. E-mail address: [email protected] (K. Bao). These authors contributed equally to this work.
http://dx.doi.org/10.1016/j.ceramint.2017.02.101 Received 11 December 2016; Received in revised form 19 February 2017; Accepted 21 February 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Mao, W., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.02.101
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Fig. 1. (a-c) SEM. (d, e) TEM and (f) HRTEM images of the Zn-doped SnO2.
2. Experimental
2.2. Preparation of the electrode
2.1. Synthesis of samples
The method for fabricating photoanode and the solution composition electrolyte have been reported in our previous paper [20]. The assembled photoanode was soaked in the ethanol solution of N719 for 20 h.
To synthesize nanowire-based Zn-doped SnO2 microtubes, 0.12 mmol of Zn(CH3COO)2·2H2O (Aladdin, 99.0%), 0.6 mmol of SnCl4·5H2O (Aladdin, 99.0%) and 7.2 mmol of NaOH (Aladdin, AR. 96%) were added to solvent which includes 15 mL H2O and 15 mL ethylenediamine (En) (Aladdin, 99.0%). After several minutes of stirring, the mixture was transferred to a 50 mL stainless Teflon-lined autoclave and reacted at 200 °C for 48 h. The precipitate was centrifuged, washed with distilled water and dried in air. The dried powder was calcined for 30 min at 500 °C. The prepared gray powder was labelled as Zn-doped SnO2. For comparison, commercial SnO2 (Aladdin, AR. 99.5%) was used as referential sample (labelled as SnO2).
2.3. Characterization The crystallinity and morphology of the samples were characterized using the transmission electron microscopy (TEM) (JEOL 3010, Japan) with an accelerating voltage of 200 kV and scanning electron microscopy (SEM) (FEI NOVA NanoSEM230, USA) with an energydispersive X-ray spectrometer (EDX) at 20 kV. The X-ray photoelectron spectroscopy (XPS) spectra were recorded using ESCALab MKII
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X-ray photoelectron spectrometer at 13 kV. The X-ray diffraction (XRD) data were obtained at room temperature by X-ray diffractometer (counting time of 10.16 s per data point, scan step of 0.05°) with Cu-Kα radiation (λ=1.54056 Å). UV–visible absorption spectrum of powder was recorded using pure BaSO4 pellet as the reference and measured by UV–vis spectrophotometer (UV-2550, Shimadzu). Brunauer-Emmett-Teller (BET) was measured on micromeritics Tristar-3000 (USA). The amount of desorbed N719 was estimated by treating dye-impregnated photoanodes into a 0.1 M NaOH solution and by the absorption spectra measurement of the solution. The thickness of film was measured using a Dektak 6 M stylus profiler. Intensity-modulated photovoltage/photocurrent spectra (IMVS/IMPS) were carried out on the electrochemical workstation (Zahner, Zennium) with a frequency response analyzer under a modulated blue light emitting diodes (457 nm) driven by a Zahner source supply (PP211). The incident-photon-to-current conversion efficiency (IPCE) signal was observed with PEC-S20 (Peccell Technology Co. Ltd.). The photocurrent–voltage characteristics (J-V) was measured using a sunlight simulator (Oriel 92251A-1000, AM 1.5 globe, 100 mW/cm2) with 0.4 cm2 active area. 2.4. Photocatalytic activity test 100 mg of sample was added into 100 mL of 10−5 mol L–1 rhodamine B (RhB) solution. The adsorption/desorption equilibrium was reached by magnetically stirred for 12 h in the dark. The sunlight simulator provided the irradiation and was placed 20 cm apart from the reactor. 3. Results and discussion 3.1. Structural properties characteristics and of samples The low magnified SEM image reveals that the Zn-doped SnO2 displays uniform tube structure with lengths in the range 6–8 µm (Fig. 1a). Highly magnified SEM image in Fig. 1b indicates that the tube is built from nanoplates of ~100 nm thickness. Fig. 1c shows that the nanowires with ~50 nm in diameter and ~600 nm in length vertically stand on the surface of tubes. These Zn-doped SnO2 tubes units possess a hexagonal orifice with the diameter of ~800 nm. The TEM images clearly show the hollow tube structure and the hexagonal orifice (Fig. 1d and e), well consistent with the SEM observation. Such a hollow 1D structure should enhance the absorption of light and facilitate electrolyte diffusion. The observed lattice spacing of ~0.335 nm of the nanowire, corresponding to the (110) planes of the tetragonal SnO2 phase (JCPDS No. 71-0652), is found in Fig. 1f. The XRD spectra of the commercial SnO2 and Zn-doped SnO2 samples showed all of the diffraction peaks correspond to the tetragonal structure of SnO2 (a=b=0.4738 nm, c=0.3187 nm, α=β=γ=90°, JCPDS Card File No. 71-0652) (Fig. 2a). The introduction of Zn caused no any extra peaks. In addition, a slight shift to smaller peak positions relative to the SnO2 indicated the successful Zn doping in the lattice of SnO2 (Fig. 2b). The lattice constants (a=b, c) have been determined according to the following formula [25]:
sin2θ =
λ 2 ⎛ h2 + k 2 l2 ⎞ ⎜ + 2⎟ 2 4 ⎝ a c ⎠
Fig. 2. (a) XRD patterns of the SnO2 and Zn-doped SnO2 samples. (b) XRD patterns of the corresponding (101) peak of two samples.
a=
λ 2 sin θ
(2)
For the (002) orientation (2θ=57.62°), the lattice constant “c” was determined by
c=
λ sin θ
(3)
The lattice constants were calculated as a=b=0.4774 nm and c=0.3196 nm for Zn-doped SnO2. The increased lattice parameters (a and c) may be attributed to the replacement of Sn4+(ionic radius of Sn4+=0.071 nm) by larger ionic radii Zn2+(0.073 nm). From comparison in XPS conducted on two samples (Fig. 3a), the Zn element signal was only presented in the spectra of the Zn-doped SnO2 power, which confirmed the doping of Zn into SnO2 (Fig. 3b). Sn 3d5/2 and Sn 3d3/2 bands with corresponding binding energies of 485.6 eV and 494.0 eV were lower 0.4 eV than those of SnO2 (486.0 for Sn 3d5/2 and 494.4 for Sn 3d3/2, which can be ascribed to oxygen deficiency (Fig. 3c)) [18]. The O 1s peak for Zn-doped SnO2 also shifted a lower value, which could be attributed to Sn–O–Zn coordination (Fig. 3d) [18]. The content of Zn is 3.69 at%, calculated by XPS curves. Furthermore,
(1)
where λ is the X-ray wavelength. For the (110) orientation (2θ=26.38°), the lattice constant “a” was obtained by
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Fig. 3. The typical XPS spectra of the SnO2 and Zn-doped SnO2. (a) the survey spectra. (b) Zn 2p. (c) Sn 3d, and (d) O 1s peaks.
(Fig. 4b). (III) Zn-doped SnO2 in the reaction solution attached onto the rough surface of the nanoplates (Fig. 4c) and the secondary nucleation process started. After 20 h, the sparse Zn-doped SnO2 nanowires started to appear (Fig. 4d); (IV) As time went on, the nanowires continued to grow in both diameter and length, and became increasingly denser (Fig. 4e). According to the solvent-coordination molecular template mechanism [26], En acted as a crucial role in the growth of nanowires. In reaction process, En molecules could be adsorbed on the specific facets of the Zn-doped SnO2 nuclei, resulting in the growth along these suppressed crystal planes. Comparatively, the microtubes with rough surface (labelled as MTs) were obtained used of pure water without ethylenediamine as the solvent, while keeping other conditions unaltered and those nanowires have not appeared (Fig. 4f).
the chemical compositions of Zn-doped SnO2 sample were analyzed using EDX (Supplementary Fig. S1). Cu, Sn, O and Zn peaks can be easily detected, indicating that Zn existed in the final product indeed. The presence of Cu mainly came from the substrates due to the SEM test. Based on the EDX analysis, the atomic ratio of Zn/Sn was almost in agreement with the result from XPS. To understand the growth mechanism of the unique Zn-doped SnO2 structure, the morphology evolution of sample with different reaction times (2, 12, 20 and 30 h) at 200 °C was investigated. The corresponding XRD patterns were shown in Supplementary Fig. S2. There was a possible four-step development mechanism: (I) In the first 2 h, under solvothermal conditions, vismirnovite ZnSn(OH)6 (a=b=c=0.772 nm, α=β=γ=90°, JCPDS Card File No. 33-1376) nuclei formed quickly by the complexation between metal ions and OH– in the NaOH and ethylenediamine (En) alkali solution, followed by the rodshaped crystals growth (Fig. 4a); (II) Under the etching effect of alkali solution, ZnSn(OH)6 micro-rod were decomposed into Sn(OH)62- and Zn(OH)42- [17], then further formed the Zn-doped SnO2 nanoplate with rough surface, and residual Zn-doped SnO2 went into the solution. With the consumption of reactant, more and more nanoplates would generate, which built nanoplate-textured micro-tubes in situ at 12 h
3.2. Photovoltaic performance of samples The Zn-doped SnO2 microtubes were utilized as the photoanode for DSSCs. Commercial SnO2 particles (100–200 nm in diameter) were used as referential sample (Supplementary Fig. S3). Comparative J-V curves of DSSCs based on two photoanodes with the same thickness (13 µm) are shown in Fig. 5a. The open-circuit voltages of the Zn-
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Fig. 4. SEM of morphological evolution of the Zn-doped SnO2. (a) 2 h. (b, c) 12 h. (d) 20 h. (e) 30 h. (f) SEM image of the MTs.
(Fig. 5b). The doping of Zn could cause the passivation of the subband-edge surface-states, resulting in the upward movement of the quasi-Fermi level of Zn-doped SnO2 and enhanced Voc [18]. One can also find that Jsc value (13.05 mA/cm2) of the Zn-doped SnO2 DSSCs was much higher than the SnO2 DSSCs (8.68 mA/cm2). To comprehend the Jsc increment of the Zn-doped SnO2-based cell, the IPCE spectra of two cells were measured, as displayed in Fig. 6a. The IPCE of Zn-doped SnO2 was obviously higher than that of SnO2 over the wavelength region of 400–800 nm. The maximum IPCE was 29% at the wavelength of 520 nm for SnO2-based DSSCs. After doping Zn into the SnO2 framework, the maximum IPCE reached up to 55%, concurring with the trend observed for Jsc in the J-V characteristics. The higher Jsc could be attributed to better dye adsorption or/and higher light-harvesting efficiency [10]. On the one hand, the Zn-doped SnO2
doped SnO2 (0.55 V) was found to be considerably higher than that of the SnO2 (0.32 V). The Voc of a DSSCs is determined by the offset between the redox potential of I-/I3- in the electrolyte and the flat-band potential (VFB) of the photoanode [27]. To ascertain the effect of the Zn doping in SnO2 on improving the Voc, the VFB of photoanodes derived from the Mott-Schottky plots by the equation 1/C2=(2/A2eεε0ND) (V– VFB–kT/e), where C denotes the space charge region capacitance, A is the electrode surface area, e denotes the electron charge, ε0 and ε are the vacuum permittivity and the dielectric constant of the semiconductor. ND, V, k and T represent the donor density, applied potential, Boltzmann constant and absolute temperature, respectively [28]. The VFB can be obtained from the intercept on the abscissa by plotting of C−2 against V. Mott-Schottky demonstrated that the VFB of SnO2 and Zn-doped SnO2 were 0.16 V and −0.06 V (vs. Ag/AgCl), respectively
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Fig. 5. (a) J–V curves and (b) Mott-Schottky plots for cells based on two electrodes. Fig. 6. (a) IPCE spectra and (b) the pH-dependent zeta-potential for two different electrodes.
photoanode (0.39×10−7 mol/cm2) doubled the dye loading amounts of SnO2 photoanode (0.21×10−7 mol/cm2) (Supplementary Fig. S4a). Whereas the BET surface area of Zn-doped SnO2 was calculated to be 45.56 m2/g, which was only higher 27.3% than SnO2 (35.78 m2/g), the increased dye adsorption could be explained by higher isoelectric point (pI). Fig. 6b showed SnO2 has pI about pH 4.1, which was consistent with the literature values (4–5). After Zn doping into the SnO2 framework, the pI increased to 4.7. Surfaces with higher pI would be in favor of the adherence of N719 with acidic carboxyl groups [4]. To investigate further, we tested the absorption spectra of the dye-loaded photoelectrodes (Fig. 7a). The strong absorption in the range of 450– 650 nm was consistent with IPCE data and demonstrated the presence of band edge of N719 [29]. Compared to pure SnO2, the intensity of the absorption peak of Zn-doped SnO2 showed drastic enhancement, which clearly revealed that the photoelectrodes prepared from Zndoped SnO2 facilitated excellent light harvesting ability. It should be emphasized that Zn-doped SnO2-based film after the dye adsorption can still present the original morphology, which indicates that the Zndoped SnO2 photoanode is chemically stable in acid environments (Supplementary Fig. S4b); On the other hand, as shown in Fig. 7b, Zndoped SnO2 film exhibited high stronger reflectance in the wavelength range from 400 nm to 800 nm, indicating that the Zn-doped SnO2 film has a higher light-scattering ability. It was believed that resonant
scattering can occur when the porous medium dimension was comparable to the wavelength of incident light [30]. Herein, the microtube with the dimension of ~800 nm can act as an efficient scattering center to improve light-harvesting efficiency [31]. It was worth noting that, the incident light can be not only reflected but also diffused throughout the microtube, so the incident photon flux could be absorbed and utilized more effectively [32]. To clarify the effect of the morphology and Zn doping in SnO2 network on the electron transport and recombination, IMPS /IMVS were further measured under different irradiation intensities from 30 to 150 W/m2. The electron transport time (τd) and recombination time (τr) can be calculated according to the expression τd=1/2πfd and τr=1/ 2πfr, where fd (fr) was the characteristic minimum frequency of the IMPS (IMVS) [33]. As shown in the Fig. 8a, Zn-doped SnO2-based DSSCs exhibited a shorter τd than that of SnO2-based DSSCs. This phenomenon could be ascribed to the fact that 1D tubes and nanowires can provide direct electrical pathways for photogenerated charges transport with less diffusive hindrance, suggesting the Zn-doped SnO2 with 1D nanowire-based microtubes were favorable for the electron transport as compared to 0D SnO2 nanoparticles [34]; As depicted in Fig. 8b, the τr of Zn-doped SnO2-based cell was longer than
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Fig. 8. (a) Transport time and (b) Lifetime based on the different photoelectrodes. Fig. 7. (a) Optical absorption spectra of two films with adsorbed dye. (b) Diffused reflection spectra of the two samples.
3.3. Photocatalytic activity of samples To develop the potential application for solar energy, their photocatalytic activity was evaluated by degrading RhB under simulated sunlight irradiation. In Fig. 9a, a blank test using Zn-doped SnO2 as the photocatalyst without irradiation showed that few RhB was degraded. Another blank test without photocatalyst demonstrated only a small quantity of RhB degradation occurred after irradiation. The RhB was degraded completely using Zn-doped SnO2 after irradiation for 150 min, whereas only 69.9% of RhB was degraded over SnO2 (Fig. 10). Fig. 9b showed that the degradation reaction follows firstorder rate law [36]. The Zn-doped SnO2 indicated the higher catalytic activity with the degradation rate constant of 0.017 min−1, about 2 times than that of SnO2 (0.008 min−1), which was a consequence of the following facts: (i) It is widely accepted that the fast recombination of the photogenerated pairs hampers the degradation of organic reagents. The Zn-doped SnO2 may improve the photocatalytic efficiency attributed to increasement of the charge separation [37]; (ii) The larger surface area and the empty tube might offer more reaction sites and possessed stronger adsorption ability for large-sized dye molecules [38]; (iii) The 1D nanowire structures benefited photogenerated electron-hole pairs to transfer from inside the photocatalyst to the
that of SnO2-based cell, indicating the obvious suppressed charge recombination at the Zn-doped SnO2/N719/electrolyte interface. It was known that the less the number of trap sites, the less the probability of charge recombination. While the trap-site density increased with the number of nanoparticles [35]. Thereby, the trapsite density of Zn-doped SnO2 microtube electrode was less than that of SnO2 nanoparticles electrode, thus reduced charge recombination in the microtube-based electrode and increased electron lifetime, which, no doubt, enable the Zn-doped SnO2 based DSSCs to achieve higher conversion efficiencies. The overall ŋ of the Zn-doped SnO2 DSSCs is 4.22%, distinctly higher than that of the SnO2 DSSCs (ŋ=1.21%) and precedent Zndoped SnO2 nanocrystals synthesized by hydrothermal method (4.18%) [16], nanoflowers (3.00%) [17], mesoporous particles (3.73%) [18], and nano-echinus (4.15%) [20]. The improvement in ŋ of the Zn-doped SnO2 can be caused by following three factor: (1) higher Voc due to negative shift in the VFB resulted from Zn doping into the SnO2 framework; (2) higher Jsc due to better dye adsorption and higher lightharvesting efficiency; (3) the lower electron recombination rate and longer electron lifetime.
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Fig. 9. (a) The degradation rate of RhB over different photocatalysts. (b) The ln(C0/C) versus time curves of photodegradation of RhB. (c) Cycling experiments of degradation over Zndoped SnO2 (•) and SnO2 (■).
unique chemical composition and structure, Zn-doped SnO2 microtubes have applications in the varied areas. With the hierarchical Zndoped SnO2 as the anode material for DSSCs, an overall 4.22% photoconversion efficiency was obtained, 249% improvement compared to the commercial SnO2 nanoparticles. With the Zn-doped SnO2 microtubes as the photocatalyst for degradation of organic dye, they showed excellent photocatalytic activity. Moreover, the micron-sized Zn-doped SnO2 powder were easy to separate from the photocatalysis reaction system and reuse in practice.
surface for degradation of RhB. In addition, our Zn-doped SnO2 can be reused with no obvious loss of photocatalytic activity (Fig. 9c) and they still preserved the original morphology after repeated photocatalytic runs (Supplementary Fig. S5a). The reuse of the hierarchical structure was facilitated compared to commercial SnO2 nanoparticles, which were difficult to recover from the reaction system by sedimentation (Supplementary Fig. S5b). 4. Conclusions In summary, nanowire-based Zn-doped SnO2 microtubes have been proposed for solar energy applications. In this hierarchical structure, one-dimensional nanowires can provide photo generated electrons with direct electrical pathways, the micrometer-sized microtubes can function as efficient light scatterers to enhance the light harvesting. Zn doping into SnO2 can increase the isoelectric point. Inspired by the
Acknowledgments This work was supported by National Natural Science Foundation of China (No. 21301101, U1404505), the Program for Innovative Talent in University of Henan Province (16HASTIT010) and Henan Province Projects (14A150047, ZX2014047).
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Fig. 10. Time-dependent UV–vis absorbance spectra of the RhB solution in the presence of (a) Zn-doped SnO2 and (b) the SnO2.
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