Solvothermal Synthesis of Hierarchical TiO2 Microstructures with High

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Solvothermal Synthesis of Hierarchical TiO2 Microstructures with High Crystallinity and Superior Light Scattering for HighPerformance Dye-Sensitized Solar Cells Zhao-Qian Li,† Li-E Mo,† Wang-Chao Chen,† Xiao-Qiang Shi,§ Ning Wang,‡ Lin-Hua Hu,*,† Tasawar Hayat,∥,⊥ Ahmed Alsaedi,∥ and Song-Yuan Dai*,†,§,∥ †

Key Laboratory of Photovoltaic and Energy Conservation Materials, Institute of Applied Technology, Hefei Institutes of Physical Science and ‡High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei, Anhui 230031, P. R. China § Beijing Key Laboratory of Novel Thin-Film Solar Cells, North China Electric Power University, Beijing 102206, P. R. China ∥ NAAM Research Group, Department of Mathematics, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia ⊥ Department of Mathematics, Quaid-I-Azam University, Islamabad 44000, Pakistan S Supporting Information *

ABSTRACT: In this article, hierarchical TiO2 microstructures (HM-TiO2) were synthesized by a simple solvothermal method adopting tetra-n-butyl titanate as the titanium source in a mixed solvent composed of N,N-dimethylformamide and acetic acid. Due to the high crystallinity and superior lightscattering ability, the resultant HM-TiO2 are advantageous as photoanodes for dye-sensitized solar cells. When assembled to the entire photovoltaic device with C101 dye as a sensitizer, the pure HM-TiO2-based solar cells showed an ultrahigh photovoltage up to 0.853 V. Finally, by employing the as-obtained HM-TiO2 as the scattering layer and optimizing the architecture of dye-sensitized solar cells, both higher photovoltage and incident photon-to-electron conversion efficiency value were harvested with respect to TiO2 nanoparticles-based dye-sensitized solar cells, resulting in a high power conversion efficiency of 9.79%. This work provides a promising strategy to develop photoanode materials with outstanding photoelectric conversion performance. KEYWORDS: hierarchical microstructures, TiO2, solar cells, acetic acid, solvothermal, light scattering

1. INTRODUCTION With the ongoing exhaustion of fossil fuels and the increasing negative global environmental impact, deliberate actions aiming to develop more sustainable and renewable energy resources are needed. Undoubtedly, utilizing solar energy to generate power is an indispensable and efficient way. Therefore, in the recent years, many efforts have been devoted to develop new and high-efficiency photovoltaic devices, such as perovskite solar cells,1−5 dye-sensitized solar cells (DSSCs),6−11 and so on. Owing to the advantages of low cost, easy fabrication, and colorful and screen printing, DSSCs have attracted the worldwide focus and researches have achieved significant progress.12−14 However, as an important part in the DSSCs, the traditional TiO2 nanoparticles (NP-TiO2)-based photoanode limits the photoelectronic performance of a DSSC because of the high light transmittance and trap states resulting from the nanosize of the TiO2 nanoparticles.7,15 To overcome this limitation, hierarchical micro/nanomaterials comprising one- or two-dimensional nanostructures have been regarded as promising candidates, including spherical, hollow, core−shell architectures constructed by nanoparticles, nanowires, nanorods, nanosheets, and so on.7,14,16−19 Because © 2017 American Chemical Society

hierarchical micro/nanomaterials are composed of low-dimensional nanomaterials, they often possess not only the properties of one- or two-dimensional nanomaterials but also some extra intriguing properties, such as easy precipitation and separation, high surface area, fewer defects, light-scattering effect, and large interspace. Employing hierarchical TiO2 micro/nanostructures as the photoanode in DSSC has been identified to be an effective way to boost the photoelectronic conversion performance.7,14,20 For instance, to overcome the electrolyte diffusion limitations, Grätzel et al. designed DSSCs using mesoporous TiO2 beads as the photoanode. Compared with the photoanode composed of TiO2 nanoparticles, the mesoporous TiO2 beadsbased photoanode showed improved diffusion property for the cobalt redox electrolyte due to the high porosity. Consequently, in combination with the superior properties of a high surface area and a strong scattering behavior, the mesoporous TiO2 beads-based DSSC achieved an efficiency of 11.4%.14 Kuang et al. fabricated the three-dimensional branched nanowire-coated Received: May 24, 2017 Accepted: August 29, 2017 Published: August 29, 2017 32026

DOI: 10.1021/acsami.7b07321 ACS Appl. Mater. Interfaces 2017, 9, 32026−32033

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a, b) SEM images, (c) X-ray diffraction (XRD) pattern, and (d) N2 adsorption and desorption isotherms and the corresponding pore size distribution plots of HM-TiO2. (e) Diffuse reflectance spectra and (f) dye desorbed from the anode films based on HM-TiO2 and NP-TiO2.

photovoltage mainly focused on the researches of electrolyte, such as cobalt redox mediators14,22 and copper bipyridyl redox mediators.13,23 In addition to the electrolyte, the photoanode will also affect the photovoltage because its crystallinity and trap states have a great effect on the electron−hole recombination. However, reports about improving the photovoltage by tuning the photoanode structure are still scarce. Therefore, in this context, developing a simple approach for the synthesis of

TiO2 hierarchical microstructures and employed the micromaterials as photoanode. Owing to the high Brunauer− Emmett−Teller (BET) surface area, superior light scattering, and salient charge transport properties, the DSSCs based on this hierarchical TiO2 microstructures delivered a power conversion efficiency (PCE) value of 9.51%.21 Photovoltage is a crucial factor influencing the photovoltaic property of a DSSC. In the recent years, efforts to improve the 32027


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Table 1. Photovoltaic Metrics under 1 sun Illumination and Dye Adsorption Capacities for HM-TiO2- and NP-TiO2-Based DSSCs cell

thickness (μm)

dye loading (10−8 mol cm−2)

Jsc (mA cm−2)

Voc (V)

FF (%)

η (%)

HM-TiO2 NP-TiO2

7.3 7.1

4.59 8.84

4.04 12.39

0.853 0.809

60.76 72.72

2.09 7.29

Figure 2. (a−d) SEM images of the products composed of various incomplete hierarchical synthesized at 200 °C for 3 h. (e) Schematic formation of hierarchical TiO2 microstructure.

diffraction peaks, implying a high crystallinity. The average grain size was calculated to be ∼50.3 nm according to the Scherrer equation. In a DSSC, the crystallinity of TiO2 determines the trap states distribution of the photoanode, which will greatly affect the electron−hole recombination rate and therefore the photoelectronic conversion performance. Thus, here, we can speculate that a reduced electron−hole recombination rate and a high photovoltage should be achieved due to the high crystallinity of the as-obtained HM-TiO2 when applied to photovoltaic devices. Unfortunately, the high crystallinity of HM-TiO2 resulted in much smaller surface area (25.5 m2 g−1) and an average pore size distribution (8.5 nm) (Figure 1d), which naturally lead to the low dye adsorption ability of the HM-TiO2-based photoanode (Figure 1f and Table 1). To demonstrate the advantages of these hierarchical TiO2 microstructures as the photoanode materials for DSSCs, we evaluated their light-scattering property by measuring the diffuse reflectance of a ∼7 μm thick film (Figure S1a−c) and compared it with that of the NP-TiO2-based photoanode film. As a result of the comparable size of the hierarchical TiO2 microstructures to the wavelength of visible light, from Mie theory, a strong light-scattering effect throughout the visible region is expected. As shown in Figure 1e, it is apparent that, in the visible light region, contrary to NP-TiO2, HM-TiO2 exhibits a significantly enhanced scattering, which could result in a longer path length for light utilization and consequently

hierarchical TiO2 microstructures with a high crystallinity is highly desirable and technologically important. Inspired by the successful applications of hierarchical TiO2 materials and to further flourish the performance of photovoltaic devices, in this work, hierarchical TiO2 microstructures (HM-TiO2) were synthesized by means of a solvothermal approach in a mixed solvent comprising N,N-dimethylformamide (DMF) and acetic acid (HAc). Because of the high crystallinity and good light-scattering effect, when applied as a photoanode, the HM-TiO2-based DSSCs exhibited a higher photovoltage and a better light-harvesting performance compared with pure TiO2 nanoparticles (NP-TiO2) based DSSCs, giving rise to an impressive PCE value of up to 9.79%.

2. RESULTS AND DISCUSSION The synthesis of HM-TiO2 is simple and straightforward via a modified solvothermal approach reported previously. The morphology of the as-prepared TiO2 samples was characterized by the scanning electron microscopy (SEM). From Figure 1a, it is clear that the product composes of well-dispersed spherical hierarchical TiO2 microstructures with a size of about 1 μm. A closer observation with a high-resolution SEM image (Figure 1b) shows that the hierarchical structures are assembled by needle-like subunits pointing radially outward. Powder X-ray diffraction (XRD) analysis (Figure 1c) confirms that the as-prepared hierarchical material is a pure anatase TiO2 phase (JCPDS No. 21-1272) with sharp 32028


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Figure 3. Current density−voltage (J−V) characteristics under the illumination (a) and incident photon-to-electron conversion efficiencies (IPCE) (b) of HM-TiO2- and NP-TiO2-based DSSCs.

Figure 4. (a) Nyquist plots measured at a −0.75 V bias and (b) J−V characteristics of HM-TiO2- and NP-TiO2-based DSSCs in the dark.

assembled to complete the hierarchical microstructures. Therefore, in this work, we observed that by further increasing the reaction time to 12 h, the incomplete hierarchical structures (Figure 2a−d) are all transformed to complete hierarchical microstructures (Figure 1a,b). The HM-TiO2-based DSSC were characterized at 1 sun illumination. As a comparison, the NP-TiO2-based DSSC was also fabricated. Figure 3a shows their photocurrent density− voltage curves and Table 1 summarizes their performance. Clearly, from Figure 3a, we can see that, compared with NPTiO2-based DSSC, the HM-TiO2-based DSSC shows a lower photocurrent density, which must be induced by the small dyeloading ability, as discussed above (Figure 1f). The low photocurrent density was indeed verified by the incident photon-to-electron conversion efficiencies (IPCE) measurement in Figure 2b. Inspiringly, despite the low photocurrent, the HM-TiO2-based DSSC exhibits an ultrahigh photovoltage of up to 0.853 V owing to a high crystallinity. Unfortunately, the overall power conversion efficiency is too low; only a PCE value of 2.09% was obtained. On the basis of these characterization and analysis, we can conclude that HM-TiO2 cannot be utilized as a pure photoanode in a DSSC because of the poor dye-loading property, but it possess a good lightscattering ability, a high crystallinity, and a high photovoltage; therefore, it may have a great potential as the scattering layer in a DSSC to achieve a high photoelectronic performance.

promote the photoelectronic conversion of HM-TiO2-based photovoltaic devices. To clarify the formation mechanism of the hierarchical TiO2 microstructures, we performed time-dependent experiments. Figure 2a−d presents the SEM images of the products synthesized after heating for 3 h; one can see that the products contain various incomplete hierarchical structures composed of rhombic crystals. Thus, on the basis of our findings of the products morphologies in different reaction stages, we proposed a possible formation mechanism of HM-TiO2. As illustrated in Figure 2e, prior to the heat treatment, namely, just after stirring, the state of TiO2 precursor was white precipitate composed of NP-TiO2. After heating at 200 °C for 3 h, the precursor gradually crystallized to incomplete hierarchical structures constructed by rhombic crystals. Previous works have reported that acetic acid (HAc) can react as a coordinate reagent and promote the formation of a hierarchical,24 hollow TiO2.25 Therefore, here, the hierarchical structures should be induced by the coordination effect of HAc present in this system with a Ti source. Besides, with prolongation in the reaction time and under high reaction temperature, the Ti complex intermediates will convert into crystalline phase.24,25 However, due to the high surface energy, some of the rhombic crystals easily dissolved and recrystallized into more stable crystals.26 Accompanying the dissolution−recrystallization process, the freshly formed crystals are redeposited and 32029


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ACS Applied Materials & Interfaces The interfacial characteristics of the DSSC system are critical for the generated photocurrent, photovoltage, and eventually photoelectronic conversion performance. Thus, in the current work, to investigate the charge recombination state in the DSSCs, we conducted electrochemical impedance spectroscopy (EIS) measurements at a −0.75 V bias in the dark (Figure 4a), the fitting data results are summarized in Table 2. As shown in

To demonstrate the superior light-scattering property, the IPCE measurements were conducted. As shown in Figure 5c, due to the poorer dye adsorption ability of HM-TiO2 compared with NP-TiO2 (Figure 1f), the NP-TiO2 + HM-TiO2-based DSSC inevitably exhibits a much lower IPCE value at the short wavelength range. However, in the visible light region from 445 to 800 nm, the HM-TiO2-based DSSC showed a better IPCE value than the NP-TiO2-based counterpart. Undoubtedly, this part of enhanced IPCE value should be coming from the superior light-scattering property, as discussed above. As a consequence, the composite photoanode consisted of HMTiO2- and NP-TiO2-harvested higher photocurrent and eventually the higher PCE value. Therefore, according to the as-obtained data and analysis, we can see that, notwithstanding the small BET surface area, the superior light-scattering property endows the HM-TiO2 materials with a promising photoanode in DSSC.

Table 2. Simulated Charge Transfer and Recombination Resistance (Rct) and Chemical Capacitance (Cμ) of the HMTiO2- and NP-TiO2-Based DSSCs cell

Rct (Ω)

Cμ (μF)

τn (ms)

HM-TiO2 NP-TiO2

1450 67.3

96.9 536

140.5 36.1

Figure 3a, the HM-based DSSC shows an extremely larger recombination resistance (1450 Ω), which is about 22 times larger than that of the NP-TiO2-based analogue (67.3 Ω), implying an effective suppression of the charge recombination at TiO2/electrolyte interface. The suppressed electron−hole recombination rate was further identified by the dark J−V characterization. As apparent from Figure 4b, the HM-TiO2based DSSC shows a much smaller current value at the same forward bias voltage value. Therefore, the high photovoltage is obtained in the HM-TiO2-based DSSC as a matter of course. According to the fitting data Rct and Cμ, the electron lifetime (τn(EIS) = Rct × Cμ)27,28 values are calculated to be 140.5 and 36.1 ms for HM-TiO2- and NP-TiO2-based DSSCs, respectively. Clearly, the HM-TiO2-based DSSC also showed a longer electron lifetime, which will favor the generated electron collection. On the basis of the above characterization and analysis, we can see that the as-prepared HM-TiO2 materials possessing significant light-scattering property and high crystallinity is undoubtedly a promising scattering layer in the DSSC. Hence, to efficiently utilize these advantages of HM-TiO2 and the high dye-loading property of nanoparticles, we fabricated a composite photoanode employing NP-TiO2 and HM-TiO2 as the bottom layer and scattering layer (Figure S1d), respectively, and assembled it to an integrated DSSC. Meanwhile, for comparison, the DSSC assembled by pure NP-TiO2 was also fabricated. The J−V characteristics are presented in Figure 5d and summarized in Table 3. Clearly, the DSSC employing the as-obtained HM-TiO2 as the light-scattering layer achieved not merely an enhanced photocurrent (16.22 mA cm−2) but also a higher photovoltage (0.808 V) compared with pure NP-TiO2based DSSC. It is worth noting that the photovoltage value achieved over 0.808 V even at a large thickness of ∼14 μm, which is higher than that (0.70−0.78 V) of typical DSSCs. As a result, a high PCE value of 9.79% was obtained for the NPTiO2 + HM-TiO2-based DSSC, whereas the pure NP-TiO2based DSSC only achieved a PCE value of 8.56% even if it has a better dye-loading ability (Figure 5c). Importantly, the average PCE value for 10 devices is more than 9.62%, which shows the good reproducibility of the NP-TiO2 + HM-TiO2-based DSSC (Figure S2). The enhanced photoelectronic conversion efficiency of NP-TiO2 + HM-TiO2-based DSSC can be ascribed to the improved light-utilizing efficiency induced by the superior light-scattering effect (Figure 5a,b), and the effective charge recombination suppression resulted from the high crystallinity of HM-TiO2 (Figure 6 and Table 4).29−31

3. CONCLUSIONS In summary, the hierarchical TiO2 microstructures (HM-TiO2) composed of highly crystalline needle-like subunits are synthesized through a simple one-pot solvothermal method employing tetra-n-butyl titanate (TBT) as the titanium source in a mixed solvent composed of N,N-dimethylformamide (DMF) and acetic acid (HAc). The as-obtained HM-TiO2 possess a high crystallinity and a superior light-scattering effect. When applied to the photoanode in DSSC, these amazing properties endow the HM-TiO2-based DSSC with both higher photovoltage (0.808 V, notably, 0.853 V for pure HM-TiO2based DSSC) and higher photocurrent (16.22 mA cm−2) compared with nanoparticles-based DSSC, consequently giving a PCE value of 9.79%. We expect the as-obtained hierarchical TiO2 materials to facilitate the development of high-performance TiO2-based solar cells and other applications, such as photocatalytic and supercapacitor. 4. EXPERIMENTAL SECTION 4.1. Synthesis of Hierarchical TiO2 Microstructures (HMTiO2). All of the chemicals from Aldrich were used as received without any further treatment. The hierarchical TiO2 materials were synthesized via a modified solvothermal approach building on the previous reported method.26 Typically, 2 mL of tetra-n-butyl titanate (TBT) was added into the solution containing N,N-dimethylformamide (DMF, 25 mL) and acetic acid (HAc, 25 mL) under vigorous stirring. Then, after continuous stirring for 5 min, the resulting mixture was sealed within a Teflon-lined stainless-steel autoclave (100 mL) and heated at 200 °C for 12 h. The white product was separated by centrifugation, washed with ethanol three times, and then dried at 60 °C overnight. 4.2. Assembly of Dye-Sensitized Solar Cells (DSSCs). TiO2 films (either NP-TiO2 or HM-TiO2) were deposited on the FTO glass by the screen-printing method (34T mesh size screen), followed by a sintering process at 510 °C for 30 min to remove the organic compounds. After calcination, the films were immersed into the C101 dye solution (300 μM cheno-3a,7a-dihydroxy-5b-cholic acid + 300 μM C101 complex in a mixture of tert-butanol and acetonitrile solvent (1:1 by volume)). Following the immersion process, the sensitized films were rinsed with acetonitrile and dried in air. Counterelectrode was prepared by spreading out a drop of 5 mM H2PtCl6 isopropyl alcohol solution onto the TEC15 TCO glass and heated at 450 °C for 30 min in air. The devices were assembled by a 60 μm thick Surlyn and sealed by heating. The liquid electrolyte (1 M 1,3-dimethylimidazolium iodide, 50 mM lithium iodide, 30 mM I2, 0.5 M tert-butylpyridine, and 0.1 M guanidine thiocyanate in a solvent mixture of 85% acetonitrile with 15% valeronitrile by volume) were injected through two holes on 32030


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Figure 5. (a) Schematic image of the solar cell device employing HM-TiO2 as the photoanode to show the superior light-scattering effect of HMTiO2. (b) Diffuse reflectance spectra of the optimized NP-TiO2 + HM-TiO2. (c) Dye desorbed from the anode films based on the optimized NPTiO2 + HM-TiO2 and pure NP-TiO2. (d) J−V characteristics under the illumination and (e) IPCE spectra of the optimized photoanode film-based DSSCs. the counterelectrode previously made by sand-blasting and then sealed with Surlyn. 4.3. Characterization. The morphology and structure of HMTiO2 was investigated using scanning electron microscopy (FEI XL-30 SFEG coupled to a TLD) and X-ray diffraction (XRD, Bruker-AXS

D5005). The BET surface area and the pore size distribution (Barrett−Joyner−Halenda) were evaluated from nitrogen adsorption/ desorption isotherms measured on TriStar II 3020 V1.03. The UV−vis diffuse reflectance and absorption spectra were measured on a UV−vis spectrophotometer (SOLID3700; Shimadzu Co. Ltd, Japan). The 32031


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Table 3. Photovoltaic Metrics Measured under 1 sun Illumination for Optimized NP-TiO2 + HM-TiO2-Based and Pure NPTiO2-Based DSSCs cell

thickness (μm)

dye loading (10−8 mol cm−2)

Jsc (mA cm−2)

Voc (V)

FF (%)

η (%)

NP-TiO2 + HM-TiO2 NP-TiO2

7.1 + 7.4 14.3

13.31 18.22

16.22 15.19

0.808 0.761

74.7 74.2

9.79 8.56

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +86 55165593222 (L.-H.H.). *E-mail: [email protected]. Tel: +86 1061772268 (S.-Y.D.). ORCID

Song-Yuan Dai: 0000-0001-5710-9208 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21273242 and 61404142); the National High Technology Research and Development Program of China (No. 2015AA050602); and the External Cooperation Program of BIC, Chinese Academy of Sciences under Grant No. GJHZ1607. A portion of this work was performed on the Steady High Magnetic Field Facilities, High Magnetic Field Laboratory, Chinese Academy of Sciences.

Figure 6. Nyquist plots measured at −0.75 V forward bias of optimized NP-TiO2 + HM-TiO2-based and pure NP-TiO2-based DSSCs in the dark.

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Table 4. Simulated Charge Transfer and Recombination Resistance (Rct) and Chemical Capacitance (Cμ) of Optimized NP-TiO2 + HM-TiO2-Based and Pure NP-TiO2Based DSSCs cell

Rct (Ω)

Cμ (μF)

τn (ms)

NP-TiO2 + HM-TiO2 pure NP-TiO2

250 31.8

366.7 715.5

91.7 22.8

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thicknesses of the films were tested by a profilometer (XP-2, AMBIOS Technology, Inc.). Dye desorption measurements were carried out by detaching the C101 dye from the photoanode using a solution containing 0.5 mL of tetrabutylammonium hydroxide (10 wt % in water) and 10 mL of N,N-dimethylformamide (DMF). The concentration of the loaded dye solution was analyzed from the UV−vis absorption spectrum of the detached dye solution (ε = 1.75 × 104 M−1 cm−1 for C101 at 541 nm). The current density−voltage measurements were tested with a simulated AM 1.5 illumination (Newport solar simulator), and the light intensity was measured with a National Renewable Energy Laboratory-calibrated Si solar cell. The cells were covered by a mask (5 × 5 mm2). Photon-to-electron conversion efficiency (IPCE) spectra were performed as a function of the wavelength from 300 to 800 nm. The electrochemical impedance spectra (EIS) were recorded on an electrochemical workstation (Autolab 320) with a frequency range from 10 mHz to 1000 kHz.

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REFERENCES

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b07321. Top-view and cross-sectional images of the HM-TiO2based and NP-TiO2 + HM-TiO2-based photoanode films (PDF) 32032


Research Article

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Solvothermal Synthesis of Hierarchical TiO2 Microstructures with High

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