Crystalline TiO2 Nanorod Aggregates - Wiley Online Library

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Crystalline TiO2 Nanorod Aggregates: Template-Free Fabrication and Efficient Light Harvesting in Dye-Sensitized Solar Cell Applications Xiao Li Zhang,* Yang Chen, Alexander M. Cant, Fuzhi Huang, Yi-Bing Cheng, and Rose Amal* Among the emerging technologies that utilize clean, free, and largely abundant energy from sunlight, dye-sensitized solar cells (DSC) represent a low-cost, low-tech manufacture process with competitive energy and cost payback periods. At the center of DSC, a mesoscopic semiconductor film, typically TiO2, plays an important role in device performance measured by the power conversion efficiency (PCE). Aiming to achieve a high PCE, a variety of concepts have so far been investigated for prompting charge collection and light harvesting. This includes incorporating one-dimensional nanostructures[1–6] or conductive network such as graphene[7–9] and embedding reflection or scattering centers.[10,11] The above-mentioned concepts, however, not only showed very limited improvement on PCE but also accompanied multistep construction process with considerable cost increases or stability issues in real life application. In contrast, the concept of utilizing hierarchical porous structures that were prepared with either hard or soft templates[12–16] has exhibited great advantages in PCE improvement. Hierarchical porous structures provide unique combination of a large specific surface area for dye adsorption, a crystalline framework for rapid charge transport and minimized recombination, as well as an anticipated light-trapping capability for efficient light harvesting. The fabrication of hierarchical morphology has so far mainly relied on utilizing template-directing agents to determine the morphology of the target materials by forming either inorganic–organic or inorganic–inorganic composites.[12–16] This includes block copolymers, surfactants, and porous inorganics, such as carbon and SiO2. An additional template removal process by calcination or dissolution to obtain the mesoporous morphology is generally required.[12–16] Meanwhile, considerable concerns were also raised from the incomplete removal of

Dr. X. L. Zhang, A. M. Cant, Prof. R. Amal ARC Centre of Excellence for Functional Nanomaterials School of Chemical Engineering The University of New South Wales Sydney, 2052, Australia E-mail: [email protected]; [email protected] Y. Chen, Dr. F. Huang, Prof. Y.-B. Cheng Department of Materials Engineering Monash University Melbourne, 3800, Australia

DOI: 10.1002/ppsc.201300132

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the template due to the strong bond between template agents and the surface of the target materials.[17] In this work, we demonstrate a much simpler, cost friendly synthetic approach, a template-free fabrication for spherical mesoporous TiO2 aggregates. The template-free fabrication process enabled a simultaneous crystalline development throughout the surface and the interior of the aggregates by preventing the template agent/molecular barrier on nuclei surface. The resulted highly crystallized nanorods with welldeveloped inter-particle connections paid great tributes to the efficient charge transport of photon-generated electrons. When used as a photoanode material, this resulted in a high overall PCE of 9.1% measured by using a mask (excess 10.4% not using a mask) without any further device optimization such as post-treatment in TiCl4 solution. This was well beyond the PCE of 8.3% reached by using template-synthesized mesoporous TiO2 in our previous work[18] and also far greater than the PCE of 5.6% from DSCs based on Degussa P25 TiO2 nanoparticles under the same device preparation method. Characterizations of the crystalline TiO2 nanorod aggregates and photoanode films, including surface area and pore distribution analysis, crystalline structure–size and morphology analysis, diffuse reflectance, dye adsorption, electron diffusion length, and coefficient measurements, were fully exploited for a comprehensive understanding of the factors that contribute to the high PCE performance. As shown in Figure 1, electron microscope observations reveal overall size distribution and morphology of the samples—precursor spheres (sample P), hydrothermally treated TiO2 nanoparticle aggregates obtained after hydrothermal treatment (sample H), and crystalline TiO2 nanorod aggregates prepared through hydrothermal treatment in presence of ammonia (sample HA). All three samples display polydisperse spherical morphologies with a diameter range from 400 to 800 nm. The sample P, in Figure 1a, has smooth appearances with no recognizable coarse feature on the surfaces. The corresponding X-ray powder diffraction (XRD) analysis, in Figure 2a, suggests sample P is amorphous. Hydrothermally treating sample P at 150 °C for 15 h obtained sample H, as displayed in Figure 1b,c, exhibiting relatively rough surfaces containing aggregations of numerous small crystallized spherical nanoparticles with a size range identified by scanning electron microscopy (SEM) to be approximately 10–15 nm. The crystalline development was further confirmed by XRD analysis. Significantly increased intensity and better resolution of the diffraction peaks for sample H witness the crystalline growth and can be indexed to anatase TiO2 (JCPDS card number 21–1272). With additional ammonia in the hydrothermal

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Part. Part. Syst. Charact. 2013, 30, 754–758



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is likely to allow the additional ammonia to accelerate the crystalline development simultaneously throughout surface and interior of the aggregates and avoid the influential adsorption for template agent/molecular on the surface of TiO2 precursor/nuclei. This led to well-crystallized nanoparticles, better interconnection between nanoparticles and extended pore diameter of sample HA. A worth noting pore broadening on sample HA is observed in BET analysis as shown in Figure 2b inset and listed in Table 1. For sample HA, the average pore size distribution extended to 16.6 nm and the pore volume increase to 0.53 cm3 g–1, when compared with those of sample H with values of 10.0 nm and 0.41 cm3 g–1, respectively. The high pore volume and comparable surface area of HA would tolerate volumetric dye molecular to reach interior of the aggregates and enrich Figure 1. SEM and TEM images of the spherical TiO2 nanoparticle aggregates: a) precursor dye uptake for the same amount of photospheres (sample P), b,c) hydrothermal-treated aggregates (sample H), and d–f) crystal- anode materials. This would also reduce semiline nanorod aggregates (sample HA) prepared with 0.5 mL ammonia during hydrothermal conductor light adsorption as less photoanode treatment. material is required for the device, contributing to efficient light harvesting. Additionally, when compared with sample H, sample HA also presents favorable thermal stability for photoanode application, exhibiting an almost unchanged specific surface area of excess 100 m2 g–1, steady pore size of 16.6 nm, and pore volume of 0.491 cm3 g–1 after a calcination process at 500 °C for 30 min. Photoanode materials with great lighttrapping capacity are preferable for efficient light harvesting, which can be accomplished by enhancing the scattering effect of the material. The optical properties and sensiFigure 2. a) Corresponding XRD patterns and b) nitrogen sorption isotherms of the precursor tizer uploading capacities of the photoanodes spheres (sample P), hydrothermal treated TiO2 aggregates (sample H), and crystalline TiO2 3 −1 nanorod aggregates (sample HA). The isotherm of sample HA is shifted by 120 cm g . Inset is prepared from Degussa P25 TiO2 and crystalthe corresponding pore diameter distribution. The curve of sample HA is shifted by 0.1 cm3 g−1. line TiO2 nanorod aggregates (sample HA) are revealed in Figure S2 (Supporting Information). Comparison in Figure S2a (Supprocess, sample HA presents the roughest surfaces in the three porting Information), HA gives higher diffuse reflectance for samples. Extended rod-shape nanoparticles up to 40–50 nm in wavelength ranged from 370–400 nm to 450–800 nm compared longitude and 10–20 nm in diameter can be clearly observed in Figure 1d–f, indicating a rapid crystalline enlargement in the Table 1. Physical properties of the spherical titanium oxide aggregates. presence of ammonia. An increase of approximately 25% in the intensity of main XRD peaks found in the sample HA evidences the better crystalline structure development. Samples PDa) SBETa) Vspa) 2 g−1] 3 g−1] [nm] [m [cm Development of the mesoscopic structured aggregates in terms of specific surface area, pore size distribution, and pore BC AC BC AC BC AC volume during hydrothermal process, as well as thermal staP 7.8 – 33.1 – 0.053 – bility of the aggregates, was determined by nitrogen gas sorpH 127.3 100.6 10.9 11.4 0.407 0.353 tion using the Barrett–Joyner–Halenda (BJH) method, as HA 106.2 100.9 16.6 16.6 0.529 0.491 shown in Figure 2b and summarized in Table 1. Sample H and a)SBET = BET specific surface area obtained from N2 adsorption data in the P/Po HA provide typical IV type isotherms and H1 type hysteresis range from 0.0666 to 0.2664. PD = average pore size determined by using the BJH loops, standing on well-developed mesoporous structures after model from the N2 adsorption data. Vsp = single-point pore volume calculated hydrothermal treatment with specific surface areas of 127.3 and from the adsorption isotherm at P/Po = 0.98. BC/AC = before (BC)/after calcina106.2 m2 g–1, respectively. The template-free synthetic process tion (AC) at 500 °C for 30 min.




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measured with a 6 × 6 mm2 mask (or over 10.4% measured lacking a mask, Figure S4, Supporting Information) was reached by utilizing 18 μm thick HA films under simulated 1.5 AM 1000 W m–2 sunlight with commercialized N719 (Dyesol). By contrast, with a similar film thickness, P25 photoanode merely gives an efficiency of 5.4%. The slightly reduced PCE compared with that (5.6%) of 12 μm thick films of P25 displays an upper limit performance of the P25 DSCs, suggesting the occurrence of a possible charge transport issue when film thickness exceeds 12 μm. Whereas with HA, increasing the film thickness brought a significant enhancement of photocurrent density from 12.8 to 14.9 mA cm–2 and stable performances of open circuit voltage beyond 800 mV with fill factor in excess of 0.76. This led to a greatly enriched overall PCE from 7.7 to 9.1%, exhibiting an intensively improved efficiency of over 65% by simply replacing the P25 nanoparticle with the Figure 3. a) I–V curves of the sensitized TiO2 photoanodes prepared from P25 nanoparticles highly crystalline TiO2 nanoparticle aggreand sample HA with varying film thickness. b) Corresponding IPCE curves and c) normalized IPCE curves of the sensitized TiO2 photoanodes, 18 μm, prepared from P25 nanoparticles and gates as photoanode material without any further optimization. sample HA. HA photoanode displays a superior IPCE throughout the entire illumination range, with a maximum of 76.6% at 550 nm tailing off up to with that of Degussa P25 attributed to the scattering effect from 800 nm, while the P25-based photoanode gives a maximum the unique aggregate morphology of HA. Subsequently, dyeIPCE of only 64.5% at 520 nm, as shown in Figure 3b. A uploading capacities were measured by detaching dye molecular noteworthy enhancement in light harvest in 550–800 nm from sensitized photoanode films based on P25 and HA as disarea can be easily observed from normalized IPCE spectrum played in Figure S1b (Supporting Information). Despite having in Figure 3c. This indicates the grand scale light-harvesting similar film thickness, the HA photoanode has an approxicapacity of HA for lower energy state photons (red region mately 38% greater dye uptake than the P25-based film, resulted of light) due to its strong light-trapping capability from the from the difference between the specific surface areas of higher diffusion reflectance capacity. The improved phoDegussa P25, approximately 60 cm3 g–1 (Figure S3 and Table S1, tovoltaic performance observed from the HA photoanodes Supporting Information), and HA, greater than 100 cm3 g–1. is mainly attributed to the enhanced IPCE over the whole To determine the device performance, DSCs based on range of the illuminated light, standing on the enlarged dyeDegussa P25 and HA TiO2 were subjected to photocurrent uploading capacity and efficient light harvesting when comdensity–voltage (J–V) characteristics and the incident photopared with P25 nanoparticle. to-electron conversion efficiency (IPCE) measurement, as To recognize the influential factors in reaching a higher PCE shown in Figure 3 and summarized Table 2. It is worth noting observed in sensitized HA photoanodes, the dynamics of electhat without any optimization, an overall PCE exceeding 9.1% tron transport and recombination of the devices were probed by using the electrochemical impedance spectroscopy technique in Table 2. Photovoltaic performances of the corresponding sensitized the dark, where the devices behave as leaking capacitors. Under TiO2 photoanodes at AM 1.5 simulated light as in Figure 3a, presenting forward external bias, the electronic processes in the devices average values out of 10 cells for each sample.Photovoltaic perforinclude electron injection from FTO to TiO2, charging photomances of the corresponding sensitized TiO2 photoanodes at AM 1.5 anode film by electron propagation, and electron consumption simulated light as in Figure a, presenting average values out of 10 cells for each sample. in reducing I3− in the electrolyte that is described by a transmission line model.[19,20] Sample name FF Efficiency Voc Jsc Figure 4a shows electron diffusion length (Ln) and diffu[%] [mV] [mA cm–2] sion coefficient (Dn) of sensitized photoanodes in the presence 815 9.5 0.73 5.6 P25–12 μm of P25 and crystalline TiO2 nanorod aggregates (HA) as function of externally applied biases. For the entire range of applied 802 9.2 0.73 5.4 P25–18 μm biases evaluated, the values of Ln for HA photoanode are over 805 12.8 0.74 7.7 HA–12 μm twice greater than those of the P25-based DSC, accordant with 800 14.9 0.76 9.1 HA–18 μm the lower position of the conduction band. The HA photoanode

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without any further device optimization while photoanode based on P25 merely gives a maximum efficiency of 5.6% through the same preparation process. Impedance analysis shows the photoanode film based on crystalline TiO2 nanorod aggregates has a superior electron transport property built on the combination of a greatly reduced electron transport resistance (Rt), a significantly improved diffusion length (Ln), and diffusion coefficient (Dn).

Experimental Section Experimental Details are given in the Supporting Information.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements

Figure 4. a) Electron diffusion length (Ln) and diffusion coefficient (Dn), and b) electron transfer resistance (Rt) and recombination resistance (Rct) of sensitized photoanodes prepared from P25 and HA as a function of externally applied biases in the dark.

also has an over fourfold increase in the values of electron diffusion coefficient (Dn) compared with P25 DSC under the same applied biases. This is likely due to the better inter-particle connections in the HA films leading to a shorter electron transport path than in the P25 films. Figure 4b shows that Rt and Rct are exponentially dependent with bias as commonly experienced in DSCs.[19–21] The observed Rt