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Hierarchically structured nanocrystalline photoanode: Self-assembled bi-functional TiO2 towards enhanced photovoltaic performance
Nano Energy (2014) 8, 247–254
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journal homepage: www.elsevier.com/locate/nanoenergy
RAPID COMMUNICATION
Hierarchically structured nanocrystalline photoanode: Self-assembled bi-functional TiO2 towards enhanced photovoltaic performance Zheng-Ying Gu, Xiang-Dong Gaon, Xiao-Min Li, Yong-Qing Wu, Yu-Di Huang, Song-Wang Yang, Yan Liu State Key Lab of High performance Ceramics and Superfine Structure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, No. 1295, Dingxi Road, Shanghai 200050, People Republic of China Received 7 March 2014; received in revised form 23 May 2014; accepted 10 June 2014 Available online 25 June 2014
KEYWORDS
Abstract
Hierarchical photoanode; TiO2 nanosheets; Mesoporous aggregates; Dye-sensitized solar cells
We report a simple strategy towards nanocrystalline photoanodes with structural hierarchies (hnelectrode) and simultaneously enhanced dye-loading capacity and electron conductivity. Totally built up by TiO2 nanocrystallite (5–20 nm), the hn-electrode comprised two major parts: the nanocrystalline matrix, and the bi-functional TiO2 mesostructure (BF-TiO2). BF-TiO2 was obtained from common sol–gel, ambient-drying and annealing processes, modified with very high H2O/Ti ratio (48.9:1) and hexamethylenetetramine additive. Two morphologies were found in BF-TiO2: high-surface-area mesoporous nanograin aggregates, and conductive nanosheets composed of both anatase TiO2 and oxygen-deficient Ti6O11, corresponding to its two functions. The preferential capping of amino groups on high-energy facets of TiO2 and the self-assembly process of nanograins along high-energy facets promoted the formation of nanosheets. Hn-electrodes with BF-TiO2 level of 5–40 % were prepared, which exhibited the maximum improvement of 110% in the dye loading capacity, much lower recombination frequency (8.1 Hz vs. 31.5 Hz), and much longer electron lifetime than the disordered counterpart. The optimal hn-electrode (20% BF-TiO2, 16 μm) with and without the scattering layer exhibited the conversion efficiency of 8.03% and 7.99%, 29% higher than the control nanocrystalline electrode (6.24%). The work provides a versatile route towards functional-integrating hierarchical materials, and may find vast applications in relevant areas such as lithium batteries, supercapacitors and photocatalysts. & 2014 Elsevier Ltd. All rights reserved.
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Corresponding author. Tel.: +86 21 52412441. E-mail address: [email protected] (X.-D. Gao).
http://dx.doi.org/10.1016/j.nanoen.2014.06.012 2211-2855/& 2014 Elsevier Ltd. All rights reserved.
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Introduction Multi-length scale or hierarchical materials mimicking the geometry of natural products such as flowers, trees and popcorn-balls are underpinning revolutionary advances in dye-sensitized solar cell (DSSC) [1–4] – a strong competitor in new type photovoltaic cells due to its high-efficiency at an affordable price [5–7]. Being used as the photoanode – a nanoporous electrode bearing several key functions in a sandwich-structured DSSC, the hierarchical structures can display great advantages over the primitive nanocrystalline electrode (nc-electrode), which has long been criticized for its disordered microstructure and inferior electron transport [8– 9]. Versatile hierarchical photoanodes based on 1D or quasi-1D nanostructures (nanowire, nanotube, nanosheet, and nanosphere) have been developed to enhance the dye-loading capacity, electron transport, or light scattering [2–5,10–13], which advance our knowledge on the hierarchical materials significantly. However, hierarchical photoanodes face big challenges in propelling their efficiencies equal to or higher than that of nc-electrode. The coexistence of the trans-scale morphologies, the poor compatibilities among different building blocks, and the complicated preparation process will inevitably induce severe problems including the high porosity/cavity, the low gross dye adsorption, or the bad electron transport across interfaces, thus resulting in the imbalance between two major functions of the photoanode (i.e., the dye adsorption, and the electron transport) [14–16]. However, all these problems are not present in the oldest nc-electrode, which consisted of only one building unit – nanocrystallite. This striking contrast triggers us to construct hierarchical morphologies using nanocrystallite and propose the concept of “hierarchical nanocrystalline electrode” (hn-electrode). The hn-electrode uses nanocrystallite as the only building unit at nano-scale and consists of at least two morphologies at submicron/micron scale, including those either with high surface area (e.g. 4100 m2/g) or with ordered nanostructures. While the small size of nanocrystallite (5–20 nm) can effectively inhibit the microstructure deterioration at the interfaces originating from the trans-scale hybridization, the submicron/micron components can enhance the major functions of the photoanode and realize the efficiency improvement. In this work, a special bi-functional TiO2 (BF-TiO2) nanostructure possessing two morphologies (mesoporous aggregates (MA) and conductive nanosheets) was prepared, and the hn-electrode was constructed by blending the BF-TiO2 into the nanocrystalline matrix, with MA to improve the dye adsorption and nanosheets to enhance the electron transport (Figure 1). We demonstrated its potential in integrating the advantages of two morphologies easily and establishing the new function balance at a level higher than the nc-electrode, thus realizing the substantial increase in the light-harvesting capacity of DSSC.
Experimental Preparation of BF-TiO2 Sol–gel and ambient-drying processes were used. TiO2 sol (13 ml, molar ratio of tetrabutyl titanate (TBT): H2O: HCl: ethanol = 1: 48.9: 15.1: 8.9) was modified with 0.2 g
Figure 1 Schematic of “hn-electrode” using bi-functional TiO2 (BF-TiO2) nanostructures.
hexamethylenetetramine (HMTA). After gelling, the gel was aging at 60 1C for 6 h, and then was crashed into small particles and immersed in ethanol (2 times) and cyclohexane (2 times) in sequence with each step lasting for 12 h. Then the powders were filtered and dried in air at 110 1C for 2 h, followed by sintering at 500 1C for 1 h. The annealed powders were milled for 2 h in a ball-miller with ethanol as the dispersant.
Preparation of electrode films Nc-electrodes were prepared by the established procedure (weight ratio of TiO2:ethyl cellulose:terpineol = 18:9:73) [17]. Hn-electrodes were prepared by blending BF-TiO2 powders into the pure TiO2 slurry, with BF-TiO2/TiO2 weight ratio being 5, 10, 20, 30, and 40%. The films were deposited on FTO substrate via the doctor-blade method. The scattering layer was deposited on the top of nc-electrode or hnelectrode using the mixed slurry with the equal weight of large TiO2 (200 nm) and hydrothermal TiO2 nanoparticles. All the electrodes had an area of 0.8 0.8 cm2.
Sensitization of electrodes and assembly of the cell N-719 dye solution in acetonitrile–tertbutyl alcohol (1:1 by volume) (0.0005 mol l 1) and the electrolyte containing 0.6 mol l 1 BMII, 0.03 mol l 1 I2, 0.10 mol l 1 GuSCN and 0.5 mol l 1 4-tertbutylpyridine in acetonitrile–valeronitrile (85:15, by volume) solvent were used. Detailed chemicals and procedure were given elsewhere [17].
Characterization JEOL JSM-6700F scanning electron microscope, and JEOL 2010 transmission electron microscope were used to obtain SEM and TEM images. Micromeritics ASAP 2010 analyzer was used to measure the N2 adsorption–desorption isotherms. Optical transmittance and absorbance spectra were recorded from Shimadzu UV-3101PC UV–vis–NIR spectrophotometer (reference: FTO glass or KOH aqueous solution). An electrochemical work station (CHI660B) and a solar simulator (Oriel, 91160, 300 W) were used to measure J–V and dark-current curves, EIS and OCVD spectra (light intensity: 0.1 W cm ; AC bias for EIS: 0.01 V; frequency: 0.1–105 Hz). A metal mask (0.07 cm2) was used to define the projected light area for all J–V and EIS measurements.
Hierarchically structured nanocrystalline photoanode
Results and discussion The preparation of BF-TiO2 was a first step towards the hnelectrode, including two steps: (1) Sol–gel and ambientdrying processes, which were traditionally used to prepare xerogel – a porous material with high surface area [18]; (2) Annealing, which transformed the disordered xerogel into mesoporous aggregates (MA) and nanosheets (Figure 2a). Two modifications on the precursor guaranteed the high surface area and the formation of nanosheets in BFTiO2: (a) Increase of the H2O/Ti ratio from stoichiometric 4:1 to 48.9:1, to speed up the gelling process (the gel time decreased from 4 h to 10–15 s), and enhance the crystallinity of TiO2 grains. (b) Addition of hexamethylenetetramine (HMTA), which decomposed upon heating in step 1 and released OH and NH4+ ions [19], with the former
249 neutralizing the acidic precursor and preventing the peptization of TiO2 gel, and the latter promoting the formation of nanosheets. While NH4+ ions can adsorb on the surface of negatively charged TiO2 grains [20,21], we controlled NH4+ to adsorb preferentially on the high-energy facets (e.g. (103), (001), (100) facet) by maintaining its low concentration [21]. When the adsorbed NH4+ ions were removed by annealing, the exposed high-energy facets tended to fuse together driven by the minimum total potential energy theory, leaving the stable low-energy facet (e.g. (101) facet) exposed. When numerous TiO2 grains were assembled in this way, the nanosheets were formed (Figure 2a). This mechanism was supported by the detection of N–H and C–N bonds in FTIR spectrum of as-prepared BF-TiO2 (Figure 3c), the formation of oxygen-defective Ti6O11 phase in nanosheets (resulted from the reducing action of NH4+ at
Figure 2 (a) Schematic formation of mesoporous aggregates (MA) and nanosheets in BF-TiO2; (b) SEM and (c) TEM image of asprepared sample; inset of (c) is the corresponding SAED pattern. (d) SEM and (e) TEM image of annealed sample; inset of (d) is magnified view of MA. (f) High-resolution TEM image of nanosheets.
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Figure 3 (a) X-ray diffraction patterns, (b) N2 adsorption–desorption isotherms, and (c) Fourier transform infrared spectra of as-prepared and annealed BF-TiO2.
high temperature) [22], and the exposure of low-energy facet for most TiO2 and Ti6O11 grains (Figure 2f). As-prepared BF-TiO2 showed one morphology – a nanoporous structure built up by TiO2 gains (Figure 2b), with the particle size of 4–7 nm (Figure 2c), average pore size of 6.1 nm, and BET surface area of 397 m2/g. The sample showed good crystallinity due to the use of high H2O/Ti ratio precursor, which was advantageous over amorphous TiO2 aerogels in retaining the nanopores in the annealing process [23]. The annealed BF-TiO2 exhibited two morphologies (Figure 2d), mesoporous nanograin aggregates and irregular nanosheets, both in the submicron size. TEM analysis (Figure 2e) indicated that the TiO2 grains composing MA included both large (10–20 nm) and small grains (5– 10 nm), exhibiting obvious porous feature, while the nanosheets contained only small grains (5–10 nm), showing no sign of nanopores. High resolution TEM images showed one phase (anatase TiO2) in MA structures, and two phases (anatase TiO2 and Ti6O11 (JCPDS: 500788)) in nanosheets (Figure 2f), which was also consistent with XRD analysis (Figure 3a). The exposure of low-energy facets ((101) for TiO2 and (101̄ ) for Ti6O11) proved that the self-assembly of TiO2 grains occurred on the high-energy facets, which promoted the formation of sheet-like structures. The BET surface area of the annealed BF-TiO2, though obviously decreased, was maintained at 170 m2/g (Figure 3b), much higher than nc-electrode and previous reports [24,25]. The average pore size of annealed BF-TiO2 increased to 12.1 nm from the collapse of smaller pores, which was more favorable for the dye's adsorption and the infiltration of electrolyte [26]. The hn-electrodes were fabricated via the doctor-blade method by incorporating BF-TiO2 into the hydrothermal TiO2 slurry. Their surface and cross-section morphologies were similar to the nc-electrode despite a few submicron cracks and humps. Specially, TiO2 nanoparticles in the slurry was difficult to penetrate into the pore of the BF-TiO2 and fill the internal pores, due to the similar size of the pores in BFTiO2 (12.1 nm) and TiO2 nanoparticles (10–20 nm). But TiO2 nanoparticles could accumulate on the surface of the BF-TiO2 and fill part of the larger pores on the surface, thus forming an intimate interfacial contact between BF-TiO2 and bulk film matrix, which was beneficial to the electron transport. Hn-electrodes with varied BF-TiO2 content (BTC, 5–40% by weight) were prepared, and their optical transmittance spectra and the absorption spectra of dye molecules
Figure 4 (a) Optical transmittance spectra of nc-electrode and hn-electrode films; (b) absorbance spectra of desorbed N719 from the electrode (normalized by film thickness). (Film thickness: nc-electrode: 4.8 μm; hn-electrode with 10% BFTiO2: 5.8 μm, 20% BF-TiO2: 5.2 μm, 30% BF-TiO2: 5.3 μm, and 40% BF-TiO2: 4.9 μm.)
desorbed from the electrodes were shown in Figure 4. With the increase of BTC from 5% to 40%, the transmittance of the hn-electrode in 350–800 nm wavelength band decreased gradually, indicating the increase of the scattering effects resulting from submicron nanosheets and cracks in the bulk film, which were similar to previous studies observed in submicron SiO2/TiO2 core/shell particles and submicron
Hierarchically structured nanocrystalline photoanode cavities [27,28]. But the reduction amplitude (e.g. from 71% to 38% at 550 nm for 40% BTC sample) was moderate compared with previous results based on micron-sized aerogel or other hybrid photoanode system [29,30]. In contrast, the dye-loading capability of the hn-electrode increased significantly due to the high surface area of BFTiO2, and a roughly uniform increasing trend in the dyeloading quantity was observed with the BTC going up, with the maximum improvement of 110% for the 40% BTC sample (Figure 4b and Table 1). A possible shortage for such increase in dye loading is the cost increase because the dye is usually quite expensive. But it may be more meaningful for the DSSCs using inexpensive sensitizer such as the porphyrin [6] and perovskite quantum dot [7]. In addition, the increase of dye loading has other benefits, including the thinner electrode to produce equivalent photoexcited electrons, the reduced diffusion length of electrons to the bottom electrode, and more design spaces to enhance the light scattering in the electrode. Table 1 showed the photovoltaic characteristics of ncelectrodes and hn-electrodes with varied BF-TiO2 content (BTC, 5–40% by weight) and typical J–V curves were given in Figure 5. The cells based on one-layer hn-electrode exhibited higher efficiency (η=6.02–6.68%) than the nc-electrode (η=5.73%), and the highest efficiency of 6.68% was achieved in 20% BTC cell, amounting to an improvement of 17%. In view of the slight reduction of the fill factor (from 0.73 to 0.68) and basically similar Voc value, the major contribution to the efficiency improvement came from the increase in Jsc, i.e., from the increase in the dye adsorption. For the 20% BTC cell, the increase in the dye adsorption was 43%, which improved Jsc by 27% and boosted the cell efficiency to 6.68%. The negative part of Figure 5 was originated from the dark current densities. Three hn-electrodes with the same 20% BTC exhibited very similar dark current–V curve, slightly higher than the ncelectrode, which may be related to the higher exposed TiO2 surface, the presence of more surface defects, and the lessconductive framework of the mesoporous part in BF-TiO2. The addition of BF-TiO2 into the nanocrystalline system produced two effects. (1) The positive effect came mainly from the increase of dye loading, the scattering, and the longer electron life, which propelled Jsc. (2) The negative effect came from the increased resistance to the electron transport in the electrode, due to the less conductive mesoporous TiO2
Table 1
251 framework in BF-TiO2, which reduced FF. At the lower BF-TiO2 content (e.g., 5–20%), the conducting path built up by nanocrystallite matrix and nanosheets in BF-TiO2 worked well, and the positive factors played a predominant role, thus improving Jsc. At higher BF-TiO2 level (30–40%), nearly 50–60% of the volume in the electrode was occupied by BF-TiO2 due to its low density (0.8 g/cm3). The huge resistance of the mesoporous part in BF-TiO2 to the electron transport cannot be neglected anymore, which resulted in the reduction in Jsc and FF. Different from the variation of Jsc and FF, the Voc value was mainly related to the conduction band edge of BF-TiO2, which was basically similar to that of nanocrytsallite matrix due to their similar anatase phase and similar particle size. So no improvement in Voc was observed. The electron transport and the electron–hole recombination process in hn-electrodes were analyzed by the electrochemical impedance spectra (EIS) and the open-circuit voltage decay (OCVD) curves. All hn-electrodes exhibited lower recombination frequency (8.1–21.2 Hz) than the nc-electrode (31.5 Hz) (Figure 6), indicating the higher electron life (0.124–0.057 s vs. 0.0317 s) [31]. The resistance at TiO2/dye/electrolyte interface was decreased from 36.9 Ω to 23.15 Ω with the increase of BTC from 5% to 20% and then increased to 76.4 Ω and 100.7 Ω with BTC increasing to 30% and 40% respectively, and this trend was consistent with the variations of Jsc of the
Figure 5 Typical J–V curves for nc-electrode and hn-electrode based cells. (Mask area: 0.07 cm2; Light intensity: 0.1 W cm 2)
Photovoltaic properties of DSSCs based on nc-electrodes and hn-electrodes.
Sample series
Description
Thickness (μm)
Jsc (mA cm
nc-Electrode-1 hn-Electrode-1
Without BF-TiO2 BF-TiO2: 5%an BF-TiO2: 10% BF-TiO2: 20% BF-TiO2: 30% BF-TiO2: 40% With scattering layer With scattering layer No scattering layer
4.8 5.1 5.8 5.2 5.3 4.9 15.7 14.9 16.8
9.32 9.97 11.05 11.87 9.97 10.65 10.29 13.16 13.26
nc-Electrode-2 hn-Electrode-2 a
((in weight percentage)).
2
)
Voc (V)
FF
η (%)
Dye adsorption ( 108 mol cm 2)
0.748 0.742 0.746 0.717 0.751 0.721 0.738 0.741 0.737
0.73 0.72 0.69 0.70 0.68 0.68 0.68 0.68 0.68
5.73 6.02 6.62 6.68 6.28 6.22 6.24 8.03 7.99
6.60 – 7.19 7.71 8.24 10.40 – – –
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Figure 6 Nyquist plots of typical DSSCs based on nc-electrode and hn-electrodes. (ωmax: nc-electrode: 31.5 Hz; hn-electrode with 5% BTC: 17.4 Hz; 10% BTC: 21.2 Hz; 20% BTC: 17.4 Hz; 30% BTC: 9.8 Hz; 40% BTC: 8.1 Hz.)
Figure 7 Electron life derived from OCVD curves for ncelectrode and hn-electrode based cells.
cell with the BF-TiO2 content. Meanwhile, all hn-electrodes exhibited much longer electron life (Figure 7), indicating the inhibited electron-I3 ions recombination in the hn-electrodes [32]. In special, at BTCZ20%, the electron life was as long as several seconds (at Voco0.55 V), much higher than the electrodes using nanowire, or hierarchical structures [33,34]. This high electron life came from mainly two aspects: the highly conductive nanosheets, which effectively increased the diffusion length of electrons, and the impeding function of the smaller pores in BF-TiO2 to the electrolyte diffusion, mainly the I3 ions. Previous studies proved that the pores smaller than 5– 7 nm had significant steric hindrance on the electrolyte, resulting in much lower effective diffusion coefficient [35]. There were reasonably a lot of nanopores less than 7 nm in BFTiO2 while its average pore size was 12 nm. Therefore, once the I3 ions in the smaller nanopores were depleted via the recombination with electrons in TiO2 skeleton, the remaining electrons would survive because of the much slowed supplementary rate of I3 ions via the diffusion process. It is this slow diffusion rate of I3 ions in smaller pores that resulted in the higher electron lifetime of the photoanodes with high BF-TiO2
Z.-Y. Gu et al. content. Similar results were also reported in aerogel blocking layer where the slow diffusion rate in smaller nanopores allowed the effective electron transport from mesoporous layer to FTO substrate [36]. Further studies are required to confirm the detailed influences of this phenomena on the photon-toelectron conversion behavior. To further demonstrate the capability of hn-electrode in enhancing the performance of DSSC, we fixed the BTC at optimal 20% and prepared two photoanodes with increased film thickness ( 16 μm), with one composed of 2 hierarchical layers and 1 normal scattering layer, and the other composed of 3 hierarchical layers. Two hn-electrode cells gave very similar J–V curve and exhibited the total conversion efficiency of 8.03% and 7.99%, amounting to 29% improvement compared with the nc-electrode with one scattering layer at similar thickness (6.24%, the best result on the current platform) (Figure 5). Figure 8 presented IPCE spectra of typical nc-electrodes and hn-electrodes. In the wavelength band of 300–700 nm, all of hn-electrodes exhibited higher photoelectric response than nc-electrode with similar thickness, consistent with the photovoltaic performance of the cell. In addition, at 380–570 nm, several small bumps were detected in two thicker hn-electrodes (3 layer and 2 layer with scattering layer samples), indicating the scattering effect from submicron-sized nanosheets in BF-TiO2. Apart from demonstrating a novel photoanode in DSSC, our work provides a new design principle for hierarchical materials. With nanocrystallite as the building block, a 2-step route can be generalized, (1) self-assembly of functional components with different morphologies; and (2) integration of the components into the building-block matrix. Versatile facet-capping, self-assembly or semiconductor doping strategies can be used to tune the morphology, the physical property, and the spatial configuration of components, thus achieving substantial hierarchical nanostructures, which may integrate multiple otherwisecontradictory properties (e.g. high-surface-area with high conductivity) more easily, propelling forcefully their photoelectrical, photocatalytic or sensing applications.
Figure 8 IPCE curves of nc-electrodes and hn-electrodes with different film thicknesses and cell structures.
Hierarchically structured nanocrystalline photoanode
Conclusion In summary, the concept of “hierarchical nanocrystalline electrode” was proposed and demonstrated by integrating the BF-TiO2 with high-surface-area and conductive nanosheets into the nc-electrode matrix. The incorporation of submicron BFTiO2 particles improved the dye-loading capacity, the electron transport properties at TiO2/dye/electrolyte interface and the electron life significantly. The highest conversion efficiency of 8.03% and the maximum efficiency improvement of 29% were obtained in the 20% BF-TiO2 hn-electrode. Our work opens new vistas to optimize the microstructure and functions of the photoanode in DSSC, and possesses marginal potential in the development of function-integrating hierarchical materials used in areas such as lithium batteries, supercapacitors and photocatalysts.
Acknowledgment We thank the financial support from the 973-Project (2009CB623304), and the Basic Research Program (51072214, 51002174) of National Natural Science Foundation of China.
Appendix A.
Supporting information
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2014.06.012.
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Zheng-Ying Gu received his B.S. degree in Materials from Fudan University in Shanghai of China in 2012. Currently she is a Master student in Shanghai Institute of Ceramics, Chinese Academy of Sciences, under the supervision of Prof. Xiang-Dong Gao. Her research mainly focus on porous nanostructured materials used in dye-sensitized solar cells.
Xiang-Dong Gao received the Ph.D. degree in Materials from Tongji University in Shanghai, China in 2002. Currently he is a professor in Shanghai Institute of Ceramics, Chinese Academy of Sciences. He has authored and co-authored over 100 publications and 3 book chapters in the area of semiconductor nanostructures. His current interests include microstructure and proprty modulation of porous semiconductor nanostructures and new-type photovoltaic devices.
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Xiao-Min Li received the B.S. degree from Northeast University in China in 1982, and the Ph.D. degree in Physics from Osaka University in Japan in 1994. Currently he is a senior professor in Shanghai Institute of Ceramics, Chinese Academy of Sciences. He has authored and co-authored over 200 publications and 30 patents in the area of semiconductor films and nanostructures. His current interests include functionalized thin films, low-dimensional nanostructures and dye-sensitized solar cells. Yong-Qing Wu received the B.S. degree from Ceramic Institute of Jingdezhen in 2000 and the Ph.D. degree in Physic from Shanghai Institute of Optics and Fine Mechanics Chinese Academy of Sciences in China in 2008. Currently he is an assistant professor in Shanghai Institute of Ceramics, Chinese Academy of Sciences. His interests include nanostructured carbon/semiconductors and photovoltaic applications.
Song-Wang Yang received the Ph.D. degree in Materials from Shanghai Institute of Ceramics, Chinese Academy of Sciences in China in 2006. Now he is an associate professor in Shanghai Institute of Ceramics, Chinese Academy of Sciences and focuses on the preparation and performance studies of low-dimensional nanomaterials and their application in new energy areas.
Yan Liu received the B.S. degree from Shanghai Jiaotong University in 1991, and the Ph.D. degree from Shanghai Institute of Ceramics, Chinese Academy of Sciences in China in 2004. Currently he is a professor and vice director of Shanghai Institute of Ceramics, Chinese Academy of Sciences. His interests include space materials science and engineering, and the crystal design and growth in extreme situations.
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RAPID COMMUNICATION
Hierarchically structured nanocrystalline photoanode: Self-assembled bi-functional TiO2 towards enhanced photovoltaic performance Zheng-Ying Gu, Xiang-Dong Gaon, Xiao-Min Li, Yong-Qing Wu, Yu-Di Huang, Song-Wang Yang, Yan Liu State Key Lab of High performance Ceramics and Superfine Structure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, No. 1295, Dingxi Road, Shanghai 200050, People Republic of China Received 7 March 2014; received in revised form 23 May 2014; accepted 10 June 2014 Available online 25 June 2014
KEYWORDS
Abstract
Hierarchical photoanode; TiO2 nanosheets; Mesoporous aggregates; Dye-sensitized solar cells
We report a simple strategy towards nanocrystalline photoanodes with structural hierarchies (hnelectrode) and simultaneously enhanced dye-loading capacity and electron conductivity. Totally built up by TiO2 nanocrystallite (5–20 nm), the hn-electrode comprised two major parts: the nanocrystalline matrix, and the bi-functional TiO2 mesostructure (BF-TiO2). BF-TiO2 was obtained from common sol–gel, ambient-drying and annealing processes, modified with very high H2O/Ti ratio (48.9:1) and hexamethylenetetramine additive. Two morphologies were found in BF-TiO2: high-surface-area mesoporous nanograin aggregates, and conductive nanosheets composed of both anatase TiO2 and oxygen-deficient Ti6O11, corresponding to its two functions. The preferential capping of amino groups on high-energy facets of TiO2 and the self-assembly process of nanograins along high-energy facets promoted the formation of nanosheets. Hn-electrodes with BF-TiO2 level of 5–40 % were prepared, which exhibited the maximum improvement of 110% in the dye loading capacity, much lower recombination frequency (8.1 Hz vs. 31.5 Hz), and much longer electron lifetime than the disordered counterpart. The optimal hn-electrode (20% BF-TiO2, 16 μm) with and without the scattering layer exhibited the conversion efficiency of 8.03% and 7.99%, 29% higher than the control nanocrystalline electrode (6.24%). The work provides a versatile route towards functional-integrating hierarchical materials, and may find vast applications in relevant areas such as lithium batteries, supercapacitors and photocatalysts. & 2014 Elsevier Ltd. All rights reserved.
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Corresponding author. Tel.: +86 21 52412441. E-mail address: [email protected] (X.-D. Gao).
http://dx.doi.org/10.1016/j.nanoen.2014.06.012 2211-2855/& 2014 Elsevier Ltd. All rights reserved.
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Introduction Multi-length scale or hierarchical materials mimicking the geometry of natural products such as flowers, trees and popcorn-balls are underpinning revolutionary advances in dye-sensitized solar cell (DSSC) [1–4] – a strong competitor in new type photovoltaic cells due to its high-efficiency at an affordable price [5–7]. Being used as the photoanode – a nanoporous electrode bearing several key functions in a sandwich-structured DSSC, the hierarchical structures can display great advantages over the primitive nanocrystalline electrode (nc-electrode), which has long been criticized for its disordered microstructure and inferior electron transport [8– 9]. Versatile hierarchical photoanodes based on 1D or quasi-1D nanostructures (nanowire, nanotube, nanosheet, and nanosphere) have been developed to enhance the dye-loading capacity, electron transport, or light scattering [2–5,10–13], which advance our knowledge on the hierarchical materials significantly. However, hierarchical photoanodes face big challenges in propelling their efficiencies equal to or higher than that of nc-electrode. The coexistence of the trans-scale morphologies, the poor compatibilities among different building blocks, and the complicated preparation process will inevitably induce severe problems including the high porosity/cavity, the low gross dye adsorption, or the bad electron transport across interfaces, thus resulting in the imbalance between two major functions of the photoanode (i.e., the dye adsorption, and the electron transport) [14–16]. However, all these problems are not present in the oldest nc-electrode, which consisted of only one building unit – nanocrystallite. This striking contrast triggers us to construct hierarchical morphologies using nanocrystallite and propose the concept of “hierarchical nanocrystalline electrode” (hn-electrode). The hn-electrode uses nanocrystallite as the only building unit at nano-scale and consists of at least two morphologies at submicron/micron scale, including those either with high surface area (e.g. 4100 m2/g) or with ordered nanostructures. While the small size of nanocrystallite (5–20 nm) can effectively inhibit the microstructure deterioration at the interfaces originating from the trans-scale hybridization, the submicron/micron components can enhance the major functions of the photoanode and realize the efficiency improvement. In this work, a special bi-functional TiO2 (BF-TiO2) nanostructure possessing two morphologies (mesoporous aggregates (MA) and conductive nanosheets) was prepared, and the hn-electrode was constructed by blending the BF-TiO2 into the nanocrystalline matrix, with MA to improve the dye adsorption and nanosheets to enhance the electron transport (Figure 1). We demonstrated its potential in integrating the advantages of two morphologies easily and establishing the new function balance at a level higher than the nc-electrode, thus realizing the substantial increase in the light-harvesting capacity of DSSC.
Experimental Preparation of BF-TiO2 Sol–gel and ambient-drying processes were used. TiO2 sol (13 ml, molar ratio of tetrabutyl titanate (TBT): H2O: HCl: ethanol = 1: 48.9: 15.1: 8.9) was modified with 0.2 g
Figure 1 Schematic of “hn-electrode” using bi-functional TiO2 (BF-TiO2) nanostructures.
hexamethylenetetramine (HMTA). After gelling, the gel was aging at 60 1C for 6 h, and then was crashed into small particles and immersed in ethanol (2 times) and cyclohexane (2 times) in sequence with each step lasting for 12 h. Then the powders were filtered and dried in air at 110 1C for 2 h, followed by sintering at 500 1C for 1 h. The annealed powders were milled for 2 h in a ball-miller with ethanol as the dispersant.
Preparation of electrode films Nc-electrodes were prepared by the established procedure (weight ratio of TiO2:ethyl cellulose:terpineol = 18:9:73) [17]. Hn-electrodes were prepared by blending BF-TiO2 powders into the pure TiO2 slurry, with BF-TiO2/TiO2 weight ratio being 5, 10, 20, 30, and 40%. The films were deposited on FTO substrate via the doctor-blade method. The scattering layer was deposited on the top of nc-electrode or hnelectrode using the mixed slurry with the equal weight of large TiO2 (200 nm) and hydrothermal TiO2 nanoparticles. All the electrodes had an area of 0.8 0.8 cm2.
Sensitization of electrodes and assembly of the cell N-719 dye solution in acetonitrile–tertbutyl alcohol (1:1 by volume) (0.0005 mol l 1) and the electrolyte containing 0.6 mol l 1 BMII, 0.03 mol l 1 I2, 0.10 mol l 1 GuSCN and 0.5 mol l 1 4-tertbutylpyridine in acetonitrile–valeronitrile (85:15, by volume) solvent were used. Detailed chemicals and procedure were given elsewhere [17].
Characterization JEOL JSM-6700F scanning electron microscope, and JEOL 2010 transmission electron microscope were used to obtain SEM and TEM images. Micromeritics ASAP 2010 analyzer was used to measure the N2 adsorption–desorption isotherms. Optical transmittance and absorbance spectra were recorded from Shimadzu UV-3101PC UV–vis–NIR spectrophotometer (reference: FTO glass or KOH aqueous solution). An electrochemical work station (CHI660B) and a solar simulator (Oriel, 91160, 300 W) were used to measure J–V and dark-current curves, EIS and OCVD spectra (light intensity: 0.1 W cm ; AC bias for EIS: 0.01 V; frequency: 0.1–105 Hz). A metal mask (0.07 cm2) was used to define the projected light area for all J–V and EIS measurements.
Hierarchically structured nanocrystalline photoanode
Results and discussion The preparation of BF-TiO2 was a first step towards the hnelectrode, including two steps: (1) Sol–gel and ambientdrying processes, which were traditionally used to prepare xerogel – a porous material with high surface area [18]; (2) Annealing, which transformed the disordered xerogel into mesoporous aggregates (MA) and nanosheets (Figure 2a). Two modifications on the precursor guaranteed the high surface area and the formation of nanosheets in BFTiO2: (a) Increase of the H2O/Ti ratio from stoichiometric 4:1 to 48.9:1, to speed up the gelling process (the gel time decreased from 4 h to 10–15 s), and enhance the crystallinity of TiO2 grains. (b) Addition of hexamethylenetetramine (HMTA), which decomposed upon heating in step 1 and released OH and NH4+ ions [19], with the former
249 neutralizing the acidic precursor and preventing the peptization of TiO2 gel, and the latter promoting the formation of nanosheets. While NH4+ ions can adsorb on the surface of negatively charged TiO2 grains [20,21], we controlled NH4+ to adsorb preferentially on the high-energy facets (e.g. (103), (001), (100) facet) by maintaining its low concentration [21]. When the adsorbed NH4+ ions were removed by annealing, the exposed high-energy facets tended to fuse together driven by the minimum total potential energy theory, leaving the stable low-energy facet (e.g. (101) facet) exposed. When numerous TiO2 grains were assembled in this way, the nanosheets were formed (Figure 2a). This mechanism was supported by the detection of N–H and C–N bonds in FTIR spectrum of as-prepared BF-TiO2 (Figure 3c), the formation of oxygen-defective Ti6O11 phase in nanosheets (resulted from the reducing action of NH4+ at
Figure 2 (a) Schematic formation of mesoporous aggregates (MA) and nanosheets in BF-TiO2; (b) SEM and (c) TEM image of asprepared sample; inset of (c) is the corresponding SAED pattern. (d) SEM and (e) TEM image of annealed sample; inset of (d) is magnified view of MA. (f) High-resolution TEM image of nanosheets.
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Figure 3 (a) X-ray diffraction patterns, (b) N2 adsorption–desorption isotherms, and (c) Fourier transform infrared spectra of as-prepared and annealed BF-TiO2.
high temperature) [22], and the exposure of low-energy facet for most TiO2 and Ti6O11 grains (Figure 2f). As-prepared BF-TiO2 showed one morphology – a nanoporous structure built up by TiO2 gains (Figure 2b), with the particle size of 4–7 nm (Figure 2c), average pore size of 6.1 nm, and BET surface area of 397 m2/g. The sample showed good crystallinity due to the use of high H2O/Ti ratio precursor, which was advantageous over amorphous TiO2 aerogels in retaining the nanopores in the annealing process [23]. The annealed BF-TiO2 exhibited two morphologies (Figure 2d), mesoporous nanograin aggregates and irregular nanosheets, both in the submicron size. TEM analysis (Figure 2e) indicated that the TiO2 grains composing MA included both large (10–20 nm) and small grains (5– 10 nm), exhibiting obvious porous feature, while the nanosheets contained only small grains (5–10 nm), showing no sign of nanopores. High resolution TEM images showed one phase (anatase TiO2) in MA structures, and two phases (anatase TiO2 and Ti6O11 (JCPDS: 500788)) in nanosheets (Figure 2f), which was also consistent with XRD analysis (Figure 3a). The exposure of low-energy facets ((101) for TiO2 and (101̄ ) for Ti6O11) proved that the self-assembly of TiO2 grains occurred on the high-energy facets, which promoted the formation of sheet-like structures. The BET surface area of the annealed BF-TiO2, though obviously decreased, was maintained at 170 m2/g (Figure 3b), much higher than nc-electrode and previous reports [24,25]. The average pore size of annealed BF-TiO2 increased to 12.1 nm from the collapse of smaller pores, which was more favorable for the dye's adsorption and the infiltration of electrolyte [26]. The hn-electrodes were fabricated via the doctor-blade method by incorporating BF-TiO2 into the hydrothermal TiO2 slurry. Their surface and cross-section morphologies were similar to the nc-electrode despite a few submicron cracks and humps. Specially, TiO2 nanoparticles in the slurry was difficult to penetrate into the pore of the BF-TiO2 and fill the internal pores, due to the similar size of the pores in BFTiO2 (12.1 nm) and TiO2 nanoparticles (10–20 nm). But TiO2 nanoparticles could accumulate on the surface of the BF-TiO2 and fill part of the larger pores on the surface, thus forming an intimate interfacial contact between BF-TiO2 and bulk film matrix, which was beneficial to the electron transport. Hn-electrodes with varied BF-TiO2 content (BTC, 5–40% by weight) were prepared, and their optical transmittance spectra and the absorption spectra of dye molecules
Figure 4 (a) Optical transmittance spectra of nc-electrode and hn-electrode films; (b) absorbance spectra of desorbed N719 from the electrode (normalized by film thickness). (Film thickness: nc-electrode: 4.8 μm; hn-electrode with 10% BFTiO2: 5.8 μm, 20% BF-TiO2: 5.2 μm, 30% BF-TiO2: 5.3 μm, and 40% BF-TiO2: 4.9 μm.)
desorbed from the electrodes were shown in Figure 4. With the increase of BTC from 5% to 40%, the transmittance of the hn-electrode in 350–800 nm wavelength band decreased gradually, indicating the increase of the scattering effects resulting from submicron nanosheets and cracks in the bulk film, which were similar to previous studies observed in submicron SiO2/TiO2 core/shell particles and submicron
Hierarchically structured nanocrystalline photoanode cavities [27,28]. But the reduction amplitude (e.g. from 71% to 38% at 550 nm for 40% BTC sample) was moderate compared with previous results based on micron-sized aerogel or other hybrid photoanode system [29,30]. In contrast, the dye-loading capability of the hn-electrode increased significantly due to the high surface area of BFTiO2, and a roughly uniform increasing trend in the dyeloading quantity was observed with the BTC going up, with the maximum improvement of 110% for the 40% BTC sample (Figure 4b and Table 1). A possible shortage for such increase in dye loading is the cost increase because the dye is usually quite expensive. But it may be more meaningful for the DSSCs using inexpensive sensitizer such as the porphyrin [6] and perovskite quantum dot [7]. In addition, the increase of dye loading has other benefits, including the thinner electrode to produce equivalent photoexcited electrons, the reduced diffusion length of electrons to the bottom electrode, and more design spaces to enhance the light scattering in the electrode. Table 1 showed the photovoltaic characteristics of ncelectrodes and hn-electrodes with varied BF-TiO2 content (BTC, 5–40% by weight) and typical J–V curves were given in Figure 5. The cells based on one-layer hn-electrode exhibited higher efficiency (η=6.02–6.68%) than the nc-electrode (η=5.73%), and the highest efficiency of 6.68% was achieved in 20% BTC cell, amounting to an improvement of 17%. In view of the slight reduction of the fill factor (from 0.73 to 0.68) and basically similar Voc value, the major contribution to the efficiency improvement came from the increase in Jsc, i.e., from the increase in the dye adsorption. For the 20% BTC cell, the increase in the dye adsorption was 43%, which improved Jsc by 27% and boosted the cell efficiency to 6.68%. The negative part of Figure 5 was originated from the dark current densities. Three hn-electrodes with the same 20% BTC exhibited very similar dark current–V curve, slightly higher than the ncelectrode, which may be related to the higher exposed TiO2 surface, the presence of more surface defects, and the lessconductive framework of the mesoporous part in BF-TiO2. The addition of BF-TiO2 into the nanocrystalline system produced two effects. (1) The positive effect came mainly from the increase of dye loading, the scattering, and the longer electron life, which propelled Jsc. (2) The negative effect came from the increased resistance to the electron transport in the electrode, due to the less conductive mesoporous TiO2
Table 1
251 framework in BF-TiO2, which reduced FF. At the lower BF-TiO2 content (e.g., 5–20%), the conducting path built up by nanocrystallite matrix and nanosheets in BF-TiO2 worked well, and the positive factors played a predominant role, thus improving Jsc. At higher BF-TiO2 level (30–40%), nearly 50–60% of the volume in the electrode was occupied by BF-TiO2 due to its low density (0.8 g/cm3). The huge resistance of the mesoporous part in BF-TiO2 to the electron transport cannot be neglected anymore, which resulted in the reduction in Jsc and FF. Different from the variation of Jsc and FF, the Voc value was mainly related to the conduction band edge of BF-TiO2, which was basically similar to that of nanocrytsallite matrix due to their similar anatase phase and similar particle size. So no improvement in Voc was observed. The electron transport and the electron–hole recombination process in hn-electrodes were analyzed by the electrochemical impedance spectra (EIS) and the open-circuit voltage decay (OCVD) curves. All hn-electrodes exhibited lower recombination frequency (8.1–21.2 Hz) than the nc-electrode (31.5 Hz) (Figure 6), indicating the higher electron life (0.124–0.057 s vs. 0.0317 s) [31]. The resistance at TiO2/dye/electrolyte interface was decreased from 36.9 Ω to 23.15 Ω with the increase of BTC from 5% to 20% and then increased to 76.4 Ω and 100.7 Ω with BTC increasing to 30% and 40% respectively, and this trend was consistent with the variations of Jsc of the
Figure 5 Typical J–V curves for nc-electrode and hn-electrode based cells. (Mask area: 0.07 cm2; Light intensity: 0.1 W cm 2)
Photovoltaic properties of DSSCs based on nc-electrodes and hn-electrodes.
Sample series
Description
Thickness (μm)
Jsc (mA cm
nc-Electrode-1 hn-Electrode-1
Without BF-TiO2 BF-TiO2: 5%an BF-TiO2: 10% BF-TiO2: 20% BF-TiO2: 30% BF-TiO2: 40% With scattering layer With scattering layer No scattering layer
4.8 5.1 5.8 5.2 5.3 4.9 15.7 14.9 16.8
9.32 9.97 11.05 11.87 9.97 10.65 10.29 13.16 13.26
nc-Electrode-2 hn-Electrode-2 a
((in weight percentage)).
2
)
Voc (V)
FF
η (%)
Dye adsorption ( 108 mol cm 2)
0.748 0.742 0.746 0.717 0.751 0.721 0.738 0.741 0.737
0.73 0.72 0.69 0.70 0.68 0.68 0.68 0.68 0.68
5.73 6.02 6.62 6.68 6.28 6.22 6.24 8.03 7.99
6.60 – 7.19 7.71 8.24 10.40 – – –
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Figure 6 Nyquist plots of typical DSSCs based on nc-electrode and hn-electrodes. (ωmax: nc-electrode: 31.5 Hz; hn-electrode with 5% BTC: 17.4 Hz; 10% BTC: 21.2 Hz; 20% BTC: 17.4 Hz; 30% BTC: 9.8 Hz; 40% BTC: 8.1 Hz.)
Figure 7 Electron life derived from OCVD curves for ncelectrode and hn-electrode based cells.
cell with the BF-TiO2 content. Meanwhile, all hn-electrodes exhibited much longer electron life (Figure 7), indicating the inhibited electron-I3 ions recombination in the hn-electrodes [32]. In special, at BTCZ20%, the electron life was as long as several seconds (at Voco0.55 V), much higher than the electrodes using nanowire, or hierarchical structures [33,34]. This high electron life came from mainly two aspects: the highly conductive nanosheets, which effectively increased the diffusion length of electrons, and the impeding function of the smaller pores in BF-TiO2 to the electrolyte diffusion, mainly the I3 ions. Previous studies proved that the pores smaller than 5– 7 nm had significant steric hindrance on the electrolyte, resulting in much lower effective diffusion coefficient [35]. There were reasonably a lot of nanopores less than 7 nm in BFTiO2 while its average pore size was 12 nm. Therefore, once the I3 ions in the smaller nanopores were depleted via the recombination with electrons in TiO2 skeleton, the remaining electrons would survive because of the much slowed supplementary rate of I3 ions via the diffusion process. It is this slow diffusion rate of I3 ions in smaller pores that resulted in the higher electron lifetime of the photoanodes with high BF-TiO2
Z.-Y. Gu et al. content. Similar results were also reported in aerogel blocking layer where the slow diffusion rate in smaller nanopores allowed the effective electron transport from mesoporous layer to FTO substrate [36]. Further studies are required to confirm the detailed influences of this phenomena on the photon-toelectron conversion behavior. To further demonstrate the capability of hn-electrode in enhancing the performance of DSSC, we fixed the BTC at optimal 20% and prepared two photoanodes with increased film thickness ( 16 μm), with one composed of 2 hierarchical layers and 1 normal scattering layer, and the other composed of 3 hierarchical layers. Two hn-electrode cells gave very similar J–V curve and exhibited the total conversion efficiency of 8.03% and 7.99%, amounting to 29% improvement compared with the nc-electrode with one scattering layer at similar thickness (6.24%, the best result on the current platform) (Figure 5). Figure 8 presented IPCE spectra of typical nc-electrodes and hn-electrodes. In the wavelength band of 300–700 nm, all of hn-electrodes exhibited higher photoelectric response than nc-electrode with similar thickness, consistent with the photovoltaic performance of the cell. In addition, at 380–570 nm, several small bumps were detected in two thicker hn-electrodes (3 layer and 2 layer with scattering layer samples), indicating the scattering effect from submicron-sized nanosheets in BF-TiO2. Apart from demonstrating a novel photoanode in DSSC, our work provides a new design principle for hierarchical materials. With nanocrystallite as the building block, a 2-step route can be generalized, (1) self-assembly of functional components with different morphologies; and (2) integration of the components into the building-block matrix. Versatile facet-capping, self-assembly or semiconductor doping strategies can be used to tune the morphology, the physical property, and the spatial configuration of components, thus achieving substantial hierarchical nanostructures, which may integrate multiple otherwisecontradictory properties (e.g. high-surface-area with high conductivity) more easily, propelling forcefully their photoelectrical, photocatalytic or sensing applications.
Figure 8 IPCE curves of nc-electrodes and hn-electrodes with different film thicknesses and cell structures.
Hierarchically structured nanocrystalline photoanode
Conclusion In summary, the concept of “hierarchical nanocrystalline electrode” was proposed and demonstrated by integrating the BF-TiO2 with high-surface-area and conductive nanosheets into the nc-electrode matrix. The incorporation of submicron BFTiO2 particles improved the dye-loading capacity, the electron transport properties at TiO2/dye/electrolyte interface and the electron life significantly. The highest conversion efficiency of 8.03% and the maximum efficiency improvement of 29% were obtained in the 20% BF-TiO2 hn-electrode. Our work opens new vistas to optimize the microstructure and functions of the photoanode in DSSC, and possesses marginal potential in the development of function-integrating hierarchical materials used in areas such as lithium batteries, supercapacitors and photocatalysts.
Acknowledgment We thank the financial support from the 973-Project (2009CB623304), and the Basic Research Program (51072214, 51002174) of National Natural Science Foundation of China.
Appendix A.
Supporting information
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2014.06.012.
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Zheng-Ying Gu received his B.S. degree in Materials from Fudan University in Shanghai of China in 2012. Currently she is a Master student in Shanghai Institute of Ceramics, Chinese Academy of Sciences, under the supervision of Prof. Xiang-Dong Gao. Her research mainly focus on porous nanostructured materials used in dye-sensitized solar cells.
Xiang-Dong Gao received the Ph.D. degree in Materials from Tongji University in Shanghai, China in 2002. Currently he is a professor in Shanghai Institute of Ceramics, Chinese Academy of Sciences. He has authored and co-authored over 100 publications and 3 book chapters in the area of semiconductor nanostructures. His current interests include microstructure and proprty modulation of porous semiconductor nanostructures and new-type photovoltaic devices.
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Xiao-Min Li received the B.S. degree from Northeast University in China in 1982, and the Ph.D. degree in Physics from Osaka University in Japan in 1994. Currently he is a senior professor in Shanghai Institute of Ceramics, Chinese Academy of Sciences. He has authored and co-authored over 200 publications and 30 patents in the area of semiconductor films and nanostructures. His current interests include functionalized thin films, low-dimensional nanostructures and dye-sensitized solar cells. Yong-Qing Wu received the B.S. degree from Ceramic Institute of Jingdezhen in 2000 and the Ph.D. degree in Physic from Shanghai Institute of Optics and Fine Mechanics Chinese Academy of Sciences in China in 2008. Currently he is an assistant professor in Shanghai Institute of Ceramics, Chinese Academy of Sciences. His interests include nanostructured carbon/semiconductors and photovoltaic applications.
Song-Wang Yang received the Ph.D. degree in Materials from Shanghai Institute of Ceramics, Chinese Academy of Sciences in China in 2006. Now he is an associate professor in Shanghai Institute of Ceramics, Chinese Academy of Sciences and focuses on the preparation and performance studies of low-dimensional nanomaterials and their application in new energy areas.
Yan Liu received the B.S. degree from Shanghai Jiaotong University in 1991, and the Ph.D. degree from Shanghai Institute of Ceramics, Chinese Academy of Sciences in China in 2004. Currently he is a professor and vice director of Shanghai Institute of Ceramics, Chinese Academy of Sciences. His interests include space materials science and engineering, and the crystal design and growth in extreme situations.