Synthesis of SnO2 hollow fiber using kapok biotemplate for application ...

Materials Letters 159 (2015) 321–324

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Synthesis of SnO2 hollow fiber using kapok biotemplate for application in dye-sensitized solar cells En Mei Jin a,1, Ju-Young Park b,1, Hal-Bon Gu c, Sang Mun Jeong a,n a

Department of Chemical Engineering, Chungbuk National University, Cheongju 361-763, Republic of Korea Green Energy Institue, Mokpo-Si 500-400, Republic of Korea c Department of Electrical Engineering, Chonnam National University, Gwangju 500-757, Republic of Korea b

art ic l e i nf o

a b s t r a c t

Article history: Received 13 January 2015 Received in revised form 3 June 2015 Accepted 5 July 2015 Available online 6 July 2015

The SnO2-hollow fibers (SnO2-HF) are prepared by a simple impregnation method using kapok as a biotemplate. This synthesis method provides a new facile route for the fabrication of hollow SnO2 fibers. For comparison, the plate shape SnO2 nanoparticles (SnO2-NP) were prepared by hydrothermal reaction. X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM) were used to investigate the crystalline structure and morphology of as-prepared SnO2-HF and SnO2-NP. SnO2-HF and SnO2-NP were applied as additives in the TiO2 photoelectrode in order to retard the electron recombination of dye-sensitized solar cells (DSSCs). The addition of SnO2 in the DSSCs led to the longest recombination time and the electrochemical properties were increased. The solar conversion efficiency of DSSC containing additional SnO2-HF in the TiO2-NP photoelectrode is 5.43% and compared with the TiO2-NP photoelectrode based DSSCs (with a η of 4.89%), the SnO2-HF based DSSCs improved approximately 11.00%. & 2015 Elsevier B.V. All rights reserved.

Keywords: TiO2–SnO2 composite Biotemplate Hydrothermal reaction Recombination Dye-sensitized solar cell

1. Introduction Since Grätzel and O'Regan first reported the prototype of dyesensitized solar cell (DSSC) in 1991 [1], DSSC have become an attractive device as the conversion of solar light into electrical energy. DSSC is consisting of a dye adsorbed nancrystalline TiO2 semiconductor photoelectrode, an I /I3 electrolyte, and a counter electrode. Recently, many research groups have paid attention to improve the recombination loss, which are mainly caused from the recombination reaction of injected electrons either with the oxidized dye molecules or with the oxidized redox couple [2–4]. In particular, the latter reaction is thought to be critical in terms of device function. A few semiconductors such as silicon have been used to obtain high solar conversion efficiency by minimizing electron recombination [5]. Various techniques such as solid-state reaction, sol–gel and hydrothermal synthesis, have been proposed to synthesize semiconductors for a direct application to photoelectrodes of DSSCs. In this work, in order to reduce recombination loss, the plate crystal and hollow fiber structures of SnO2-NP and SnO2-HF were used to additives in TiO2-NP photoelectrodes so as n

Corresponding author. Tel.: þ 82 43 261 3369. E-mail address: [email protected] (S.M. Jeong). 1 These authors contributed equally to this work.

http://dx.doi.org/10.1016/j.matlet.2015.07.023 0167-577X/& 2015 Elsevier B.V. All rights reserved.

to increase their electrochemical properties. The TiO2-NP and SnO2-NP were prepared using hydrothermal reaction. SnO2-HF was synthesized using a new method in which kapok fiber was employed as a biotemplate [6].

2. Experimental The TiO2-NP and SnO2 NP used in this work were synthesized using hydrothermal reaction and are described in the Supplementary material (S1). The SnO2-HF was prepared employing a simple and environmentally friendly method using kapok fibers (Ceiba pentandra (L.) Gaertn). A similar process was introduced in our previous work [6] and is described in the Supplementary material (S2). The assembly of the DSSCs has been performed in accordance with the work of Jin et al. [4] and the details of the process are provided in the Supplementary material (S3). In this work, three photoelectrodes have been fabricated using as-prepared materials, shown in Scheme 1. 10 wt% of either SnO2-NP or SnO2-HF was added to the TiO2 photoelectrodes. The TiO2-NP photoelectrode and the TiO2-NP photoelectrodes containing an additional 10 wt% of either SnO2-NP or SnO2-HF were subsequently indexed as T4, TSNP, and TSHF. It is understood that the large surface of the fibrous structure for the semiconductor

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E.M. Jin et al. / Materials Letters 159 (2015) 321–324

Scheme 1. Schematic diagrams of the (a) T4, (b) TSNP, and (c) TSHF photoelectrode structures.

Fig. 1. XRD patterns of JCPDS card (No. 21-1272 and 41-1445), the TiO2-NP, SnO2-NP, and SnO2-HF.

possesses an effective dye loading and effective electron pathway that improves the charge transfer in the TiO2/electrolyte interface. For this reason SnO2, a material composed of a rod and hollow fiber structure was added to the photoelectrodes and the resulting structures examined.

3. Results and discussion The XRD patterns of the TiO2-NP, SnO2-HF and SnO2-NP are shown in Fig. 1. The peaks of TiO2-NP can be indexed to the anatase phase (Card no. 21-1276 in the JCPDS database), the sharp and intense peaks at 25.3°, 37.9°, 48.1°, and 53.5° are representative of the (101), (004), (200), and (211) diffraction planes, respectively. The peaks of SnO2-HF and SnO2-NP can be assigned to the tetragonal phase (Card no. 41-1445 in the JCPDS database), the sharp and intense peaks at 26.6˚, 33.9°, 37.9°, and 51.9° are representative of the (110), (101), (200), and (211) diffraction planes, respectively. These patterns demonstrate clearly that all samples

were highly crystalline. In addition, no impurity peaks were detected. The FE-SEM images and EDX mapping image of the samples are shown in Fig. 2. FE-SEM images of TiO2-NP, SnO2-NP and SnO2-HF are displayed in Fig. 2(a), (b) and (c), respectively. From Fig. 2(a), we observe that the particle size for TiO2-NP is approximately 20 nm. The morphology of the SnO2-NP (Fig. 2(b)) is that of a plate shape, with a primary particle length ranging between 100 and 150 nm. Fig. 2(c) displays a FE-SEM image of SnO2-HF. These FESEM images show that the morphology of the prepared samples is very similar to that of the original kapok fibers. Hollow SnO2-HF with an inner diameter staring from approximately 10 μm has been obtained (see S4 in the Supplementary material). These hollow SnO2 fibers show a high catalytic activity because their hollow structure offers a large effective surface area for both adsorption and catalytic reactions [6]. Such a large surface area might improve the absorption capacity for dye molecules, the charge transfer and solar conversion efficiency (see S5 in the Supplementary material). Fig. 2(d), (e) and (f) displays FE-SEM images of the photoelectrode surfaces for T4, TSNP, and TSHF, respectively. Aggregation of TiO2-NPs on the T4 photoelectrode was observed. However, the TSNP and TSHF photoelectrodes with a smooth surface exhibited regular pore distribution and flat surfaces. In addition, more pores were distributed on the surface of TSHF. The percolation of electrons between TiO2 nanoparticles, as well as hole transport within the pore network to the cathode, has to occur with sufficient speed so as to prevent indirect electron hole recombination. The presence of these pores was caused by the increase in surface area, therefore the dye adsorption capacity of the TiO2 surface was higher and the seepage of electrolyte ability was increased. The Sn mapping analysis of the photoelectrodes shown in Fig. 2(g) and (h), indicates well dispersed SnO2-NP and SnO2-HF in the TiO2-NP photoelectrodes. In particular, the SnO2-HF in the TiO2-NP photoelectrode exists in a fibrous form even after the paste preparation process. The EIS spectra in different photoelectrode based DSSCs is displayed in Fig. 3(a) and (b). As shown in Fig. 3(a), two semicircles are observed and the medium frequency (1–103 Hz) region of the second semicircle is mainly related with the electron transport in the photoelectrode and the back reaction at the TiO2/electrolyte interface [7]. It can be seen that the second semicircle in the medium frequency range for the TSHF is smallest compared with the other cells. This result indicates that the TSHF has the fastest electron transport ability and lowest electron recombination rate. The electron recombination resistances (Rct) were found to be 19.7, 16.8, and 14.3 Ω for the T4, TSNP and TSHF photoelectrode, respectively. It is well known that Rct represents the degree of electron recombination at the interfaces of TiO2/dye/electrolyte


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Fig. 2. FE-SEM and EDS mapping images. (a), (b) and (c) are FE-SEM image of TiO2-NP, SnO2-NP, and SnO2-HF, respectively. (d), (e) and, (f) are FE-SEM image of photoelectrode surface with T4, TSNP, and TSHF, respectively. (g) and (h) are Sn mapping analysis of TSNP and TSHF photoelectrodes.

and that smaller value for Rct lead to lower electron recombination. The Bode plots of DSSCs are shown in Fig. 3(b). Electron recombination time (τr) can be determined from the position of the middle frequency peak (fmax) in the Bode plots according to the equation, τr ¼1/(2πfmax) [8]. As a result, the τr for the T4, TSNP, and TSHF photoelectrodes were 40.01, 55.10, and 76.56 ms, respectively. The photocurrent density–voltage curves of DSSCs with different photoelectrodes are shown in Fig.3(c). The solar conversion efficiency (η) of DSSC is given by the equation, η ¼ P max/Pin ¼(Isc Voc FF)/Pin, where Pmax is the maximum power, Pin is the incident light power, Isc, Voc and FF are photocurrent density, open circuit voltage and fill factor, respectively [9]. The addition of SnO2 to T4 photoelectrode leads to an increase in photocurrent density (Jsc) and solar conversion efficiency (η) while the open circuit voltage (Voc) was slightly decreased. This can be explained though consideration of the band edge diagram for the TiO2 and SnO2 mixed photoelectrodes, display in Fig. 3(d). It is well known that the band gap energy of SnO2 (3.8 eV, E vs NHE, 0.1 eV (Ec) þ3.7 eV (Ev)) is greater than that of anatase TiO2 (3.2 eV, E vs NHE, 0.4 eV (Ec) þ2.8 eV (Ev)) [10,11]. The Voc of DSSCs with TSNP and TSHF were lower than those of pure TiO2-NPs based DSSCs. The Jsc were 13.15, 15.06, and 16.01 mA cm 2. The TSHF photoelectrode based DSSCs showed a maximum power conversion efficiency of 5.43%. This observation may in part be attributed to an increase in the yield of electron injection, as already

discussed. Moreover, the photovoltaic properties of the TSHF photoelectrode are better than those of the TSNP photoelectrode. Hollow structures of the TSHF allow an effective electron pathway, which leads to increase in charge transfer at the TiO2/electrolyte interface when compared to conventional TiO2 photoelectrodes.

4. Conclusion TiO2-NP, SnO2-NP and SnO2-HF were successfully synthesized using hydrothermal reaction and a biotemplate method. The SnO2-NP and SnO2-HF were used as additives in TiO2-NP photoelectrodes in order to improve the electrochemical properties of the DSSCs, and in particular, the longer τr of the DSSCs were obtained. The longer τr value allows us to determine that the TSHF based DSSC gave the best η of 5.43%. Compared with TiO2-NP photoelectrode based DSSCs (with a value for η of 4.89%), the TiO2-NP photoelectrode based DSSCs containing SnO2-HF exhibited an improvement in η of approximately 11%. In summary, the presence of SnO2 acts as an energy barrier and increase the physical separation between injected electrons and oxidized dyes/ redox couples, there by successfully retarding recombination reactions in TiO2-NP photoelectrode based DSSCs.

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Fig. 3. (a) Nyquist and (b) bode plots, (c) photocurrent density–voltage curves of DSSCs made of T4, TSNP and TSHF photoelectrode, and (d) energy band diagram of TiO2 and SnO2 with respect to the electrochemical scale. Ev, Ec and Eg represent valence band maxima, conduction band minima and band gap, respectively.

Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MEST) (No. 2012R1A2A2A02046367).

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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.matlet.2015.07. 023.

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