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 High Performance Dye-sensitized Solar Cells Zhao-Qian Li, Li'e Mo, Wangchao Chen, Xiao-Qiang Shi, Ning Wang, Lin-Hua Hu, Tasawar Hayat, Ahmed Alsaedi, and Songyuan Dai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07321 • Publication Date (Web): 29 Aug 2017 Downloaded from http://pubs.acs.org on August 29, 2017

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ACS Applied Materials & Interfaces

Solvothermal Synthesis of Hierarchical TiO2 Microstructures with High Crystallinity and Superior Light Scattering for High Performance Dye-sensitized Solar Cells Zhao-Qian Li,1 Li-E Mo,1 Wang-Chao Chen,1 Xiao-Qiang Shi,2 Ning Wang,5 Lin-Hua Hu*1, Tasawar Hayat3,4, Ahmed Alsaedi3 and Song-Yuan Dai*2,1,3 1

Key Laboratory of Photovoltaic and Energy Conservation Materials, CAS, Institute

of Applied Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei, Anhui, 230031, P. R. China. 2

Beijing Key Laboratory of Novel Thin-Film Solar Cells, North China Electric Power

University, Beijing, 102206, P. R. China. 3

NAAM Research Group, Department of Mathematics, Faculty of Science, King

Abdulaziz University, Jeddah 21589, Saudi Arabia 4

Department of Mathematics, Quaid-I-Azam University, Islamabad 44000, Pakistan

5

High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei, 230031, P. R.

China. *

Corresponding authors:

[email protected] (+86 55165593222) [email protected] (+86 1061772268)

ABSTRACT In this paper, hierarchical TiO2 microstructures (HM-TiO2) were synthesized by a simple solvothermal method adopting tetra-n-butyl titanate (TBT) as titanium source in a mixed solvent composed of N,N-dimethylformamide (DMF) and acetic acid (HAc). Due to the high crystallinity and superior light scattering ability, the resultant HM-TiO2 are advantageous as photoanodes for dye-sensitized solar cells. When assembled to entire photovoltaic device with C101 dye as sensitizer, the pure HM-TiO2 based solar cells showed an ultrahigh photo-voltage up to 0.853 V. Finally, by employing the as-obtained HM-TiO2 as scattering layer and optimizing the 1

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architecture of dye-sensitized solar cells, both higher photo-voltage and incident photon-to-electron conversion efficiency value was 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 recent years, many efforts have been devoted to develop new and high efficiency photovoltaic devices, such as perovskite solar cells (PSCs),1-5 dye-sensitized solar cells (DSSCs),6-11 etc. Owing to the advantages of low cost, easy fabrication, colorful and screen-printing, DSSCs have attracted the worldwide focus and researches about it have achieved significant progress,12-14 However, as an important part in the DSSCs, the traditional TiO2 nanoparticles based photoanode limits the photoelectronic performance of a DSSC because of the high light transmittance and trap states resulted from the nano size of 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, etc.7,14,16-19 Because of being composed by low dimensional nanomaterials, hierarchical micro/nanomaterials often possess not only the properties of one or two-dimensional nanomaterials itself, but also some extra intriguing properties, such as, easy precipitation and separation, high surface area, fewer defects, light scattering effect, large interspace. Employing hierarchical TiO2 2


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micro/nanostructures as photoanode in DSSC has been identified to be an effective way to boost the photo-electronic conversion performance.7,14,20 For instance, to overcome the electrolyte diffusion limitations, Grätzel M. et al. designed DSSCs using mesoporous TiO2 beads as photoanode. Compared to the photoanode composed by TiO2 nanoparticles, the mesoporous TiO2 beads based photoanode showed improved diffusion property for the cobalt redox electrolyte due to the high porosity. Consequently, in combination with superior properties of high surface area and strong scattering behavior, the mesoporous TiO2 beads based DSSC achieved an efficiency of 11.4%.14 Kuang et al. fabricated 3D branched nanowire coated TiO2 hierarchical microstructures, and employed the micromaterials as photoanode. Owing to the high BET surface area, superior light scattering, and salient charge transport properties, DSSC based on this hierarchical TiO2 microstructures delivered a PCE value of 9.51%.21 Photovoltage is a crucial factor influencing the photovoltaic property of a DSSC. In recent years, efforts to improve the photovoltage mainly focused on the researches of electrolyte, such as, cobalt redox mediators,14,22 copper bipyridyl redox mediators.13,23 In addition to the electrolyte, the photoanode will also affect the photovoltage since its crystallinity and trap states have a great effect on the electron-hole recombination. Whereas, reports about improving the photovoltage by tuning the photoanode structure is still very few. Therefore, in this context, developing simple approach for the synthesis of hierarchical TiO2 microstructures with 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 of N,N-dimethylformamide (DMF) and acetic acid (HAc). Because of the high crystallinity and good light scattering effect, when applied to photoanode, the HM-TiO2 based DSSCs exhibited higher photovoltage and better light harvesting performance compared with pure TiO2 nanoparticles (NP-TiO2) based DSSCs, giving rise to an impressive PCE value up to 3



9.79%.

2. RESULTS AND DISCUSSION

c

N2 adsorption/desorption

d = 50.3 nm

30

20

10

0 30

40

50

60

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/P0)

0

3 6 9 12 15 Pore diameter (nm)

f 0.23

80

0.20

NP-TiO2 60

NP-TiO2

HM-TiO2 Absorbance

HM-TiO2

40

20

0 300

0.05

70

2θ (degree)

e

0.10

0.00

0.0

20

Pore size distribution

0.15

Pore Volume/cm3g-1nm-1

Quantity adsorbed/cm3g-1

Intensity (a.u.)

d 40

JCPDS No. 21-1272

Diffuse Reflectance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.15 0.10 0.05

400

500

600

700

0.00 400

800

450

500

550

600

650

700

Wavelength (nm)

Wavelength (nm)

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

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 4


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that the

product composes

of

well-dispersed

spherical hierarchical

TiO2

microstructures with a size of about 1 µm. With a closer observation by the high-resolution SEM image (Figure 1b), we can see 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 pure anatase TiO2 phase (JCPDS No. 21-1272) with sharp 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 high photovoltage should be achieved due to the high crystallinity of the as-obtained HM-TiO2 when applied to photovoltaic devices. Whereas, unfortunately, the high crystallinity of the HM-TiO2 resulted in much smaller surface area (25.5 m2 g-1) and 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 photoanode materials for DSSCs, we have evaluated their light scattering property by measuring diffuse reflectance of ~7µm thick ﬁlm (Figure S1a-c) and compared it to NP-TiO2 based photoanode film. As a result of the comparable size of hierarchical TiO2 microstructures to the wavelength of visible light, from Mie theory, 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, the HM-TiO2 exhibits significantly enhanced scattering, which could result in a longer path length for light utilization and consequently promote the photoelectronic conversion of the HM-TiO2 based photovoltaic devices.

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Figure 2. (a-d) SEM images of products composed of various incomplete hierarchical synthesized at 200 °C for 3 h. (e) Schematic formation of hierarchical TiO2 microstructure. To clarify the formation mechanism of the hierarchical TiO2 microstructures, we performed time-dependent experiments. Figures 2a-d present SEM images of the products synthesized after heating for 3h, one can see that the products contained various incomplete hierarchical structures composed of rhombic crystals. Thus, on the basis of our findings of the products morphologies in different reaction stage, we proposed a possible formation mechanism of the HM-TiO2. As illustrated in Figure 2e, prior to heat treatment, namely, just after stirring, the state of the 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 coordinate reagent and promote the formation of hierarchical,24 hollow TiO2.25 Therefore, here, the hierarchical structures should be induced by the coordination effect of HAc present in this system with Ti source. Besides, with the reaction time prolonging and under high reaction temperature, the Ti complex intermediates will convert into crystalline phase.24-25 Whereas, due to the high surface energy, some rhombic crystals easily dissolved and recrystallized to more stabilized crystals.26 Accompanying the 6


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dissolving-recrystallization process, the fresh formed crystals redeposited and assembled to complete hierarchical microstructures. Therefore, in this work, we observed that further increasing reaction time to 12 h, the incomplete hierarchical structures (Figure 2a-d) all transformed to complete hierarchical microstructures (Figure 1a, b).

a

b

12 10 8

NP-TiO2

6

HM-TiO2

80 70

NP-TiO2

60

HM-TiO2

50

IPCE (%)

-2

Jsc (mA cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40 30

4 20 2 0 0.0

10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 300

0.9

400

500

600

700

800

Wavelength (nm)

Voltage (V)

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.

Table 1. Photovoltaic metrics under 1 sun illumination and dye adsorption capacities for HM-TiO2 and NP-TiO2 based DSSCs. Cell

Thickness

Dye loading

Jsc (mA

Voc

FF

(10-8 mol cm-2)

cm-2)

(V)

(%)

Ƞ (%)

HM-TiO2

7.3 µm

4.59

4.04

0.853

60.76

2.09

NP-TiO2

7.1 µm

8.84

12.39

0.809

72.72

7.29

The HM-TiO2 based DSSC were characterized at one sun illumination. As a comparison, 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 NP-TiO2 based DSSC, the HM-TiO2 based DSSC shows lower photocurrent density, which must be induced by the small dye loading ability as discussed above (Figure 1f). The low photocurrent 7



density were indeed verified by the IPCE measurement in Figure 2b. Inspiringly, despite the low photocurrent, the HM-TiO2 based DSSC exhibits an ultrahigh photovoltage up to 0.853 V owing to the high crystallinity. Whereas, unfortunately, the overall power conversion efficiency is too low, only a PCE value of 2.09 % was obtained. Based on these characterization and analysis, we can conclude that, the HM-TiO2 cannot be utilized as a pure photoanode in a DSSC because of the poor dye loading property, but it possess beautiful light scattering, high crystallinity and photovoltage, therefore, it may have great potential as scattering layer in a DSSC to achieve a high photoelectronic performance.

b

1000

Rct2

Cµ1

0

NP-TiO2

-1

Fitting HM-TiO2

Cµ2

600

-2

Rct1

Rs 800

Jsc (mA cm )

a

Z''(Ω)

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Fitting

-2

HM-TiO2

-3

400

NP-TiO2

-4

200

-5 -6 0.0

0 0

400

800

1200

1600

2000

0.1

0.2

0.3

Z' (Ω)

0.4

0.5

0.6

0.7

0.8

Voltage (V)

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.

Table 2. Simulated charge transfer and recombination resistance (Rct) and chemical capacitance (Cµ) of the HM-TiO2 and NP-TiO2 based DSSCs. Cell

Rct (Ω)

Cµ (µF)

τn (ms)

HM-TiO2

1450

96.9

140.5

NP-TiO2

67.3

536

36.1

The interfacial characteristics of the DSSC system is critical for the generated photocurrent,

photovoltage

and

eventually

the

photoelectronic

conversion

performance. Thus, in the current work, to investigate the charge recombination state in the DSSCs, we conducted electrochemical impedance spectroscopy (EIS) 8


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measurements at a -0.75 V bias in the dark (Figure 4a), the fitting data results are summarized in Table 2. As shown in Figure 3a, the HM-based DSSC shows an extremely larger recombination resistance (1450 Ω) which is about 22 times larger than that of NP-TiO2 based analogue (67.3 Ω), implying an effective suppression of charge recombination at TiO2/electrolyte interface. The suppressed electron-hole recombination rate was further identified by the dark J-V characterization. From Figure 4b, it is apparently that the HM-TiO2 based DSSC showed 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 ms 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.

c 0.5

b 80

0.4

60 Absorbance

Diffuse Reflectance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40

0.3 0.2

NP-TiO2+HM-TiO2

20

0.1

NP-TiO2+HM-TiO2 NP-TiO2

0 300

400

500 600 Wavelength (nm)

700

0.0 400

800

500 600 Wavelength (nm)

9


700


d

e

17 15

80 70 60

12 -2

Jsc (mA cm )

50

IPCE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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9

NP-TiO2+HM-TiO2

6

30

NP-TiO2+HM-TiO2

20

NP-TiO2

3

40

NP-TiO2

10

0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 300

0.8

400

500

600

700

800

Wavelength (nm)

Voltage (V)

Figure 5. (a) Schematic image of the solar cell device employing HM-TiO2 as photoanode to show the superior light scattering effect of HM-TiO2. (b) Diffuse reflectance spectra of the optimized NP-TiO2+HM-TiO2. (c) Dye desorbed from anode films based on the optimized NP-TiO2+HM-TiO2 and pure NP-TiO2. (d) J-V characteristics under the illumination and (e) IPCE spectra of the optimized photo-anode film based DSSCs.

Table 3. Photovoltaic metrics measured under one sun illumination for optimized NP-TiO2+HM-TiO2 and pure NP-TiO2 based DSSCs. Cell

Thickness

Dye loading

Jsc (mA

(µm)

(10-8 mol

cm-2)

Voc (V)

FF

Ƞ (%)

(%)

cm-2) NP-TiO2+HM-TiO2

7.1+7.4

13.31

16.22

0.808

74.7

9.79

NP-TiO2

14.3

18.22

15.19

0.761

74.2

8.56

10


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200

Rs

Rct1

Rct2 NP-TiO2

150

Cµ1

Fitting NP-TiO2+HM-TiO2

Cµ2

Fitting

Z''(Ω)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

50

0 0

100

200

300

400

Z' (Ω)

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

Table 4. Simulated charge transfer and recombination resistance (Rct) and chemical capacitance (Cµ) of optimized NP-TiO2+HM-TiO2 and pure NP-TiO2 based DSSCs. Cell

Rct (Ω)

Cµ (µF)

τn (ms)

NP-TiO2+HM-TiO2

250

366.7

91.7

Pure NP-TiO2

31.8

715.5

22.8

Based on the above characterization and analysis, we can see that the as-prepared HM-TiO2 materials possessing significantly 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 composite photoanode employing NP-TiO2 and HM-TiO2 as 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 were presented in Figure 5d and summarized in Table 3. Clearly, DSSC employing the as-obtained HM-TiO2 as light scattering layer achieved not merely the enhanced photocurrent (16.22 mA cm-2), but higher photovoltage (0.808 V) compared to pure NP-TiO2 based DSSC. It is noteworthy that the photovoltage value achieved over 11



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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 NP-TiO2+HM-TiO2 based DSSC, whereas, the pure NP-TiO2 based 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 effective charge recombination suppression resulted from the high crystallinity of the HM-TiO2 (Figure 6 and Table 4).29-31 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 than that of NP-TiO2 (Figure 1f), the NP-TiO2+HM-TiO2 based DSSC inevitably exhibits a much lower IPCE value at short wavelength range. While in the visible light region from 445 to 800 nm, the HM-TiO2 based DSSC showed better IPCE value than that of NP-TiO2 based counterpart. Undoubtedly, this part of enhanced IPCE value should come from the superior light scattering property as discussed above. As a consequence, the composite photoanode consisted of HM-TiO2 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.

3. CONCLUSIONS In summary, hierarchical TiO2 microstructures (HM-TiO2) composed of highly crystallinity needle-like subunits are synthesized through a simple one-pot solvothermal method employing tetra-n-butyl titanate (TBT) as titanium source in a mixed solvent composed of N,N-dimethylformamide (DMF) and acetic acid (HAc). The as-obtained HM-TiO2 possess high crystallinity and superior light scattering 12


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effect. When applied to photoanode in DSSC, these amazing nature endows the HM-TiO2 based DSSC with both higher photovoltage (0.808 V, notably, 0.853 V for pure HM-TiO2 based DSSC) and photocurrent (16.22 mA cm-2) comparing with nanopartilces based DSSC, and consequently giving a PCE value of 9.79%. We expect the as-obtained hierarchical TiO2 materials will facilitate the development of high-performance TiO2-based solar cells and other applications, such as photocatalytic, supercapacitor.

4. EXPERIMENTAL SECTION Synthesis of hierarchical TiO2 microstructures (HM-TiO2). All chemicals from Aldrich were used as received without any further treatment. 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 minutes, 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.

Assembly of dye-sensitized solar cells (DSSCs). TiO2 films (either NP-TiO2 or HM-TiO2) were deposited on FTO glass by 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. Counter-electrode 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. Devices were assembled by a 60 µm thick Surlyn and sealed by heating. The liquid electrolyte (1 M DMII, 50 mM LiI, 30 mM I2, 0.5 M tert-butylpyridine, and 0.1 M 13



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GuNCS in a solvent mixture of 85% acetonitrile with 15% valeronitrile by volume) were injected through two holes on the counter electrode previously made by sand-blasting, and then sealed with Surlyn.

Characterization. The morphology and structure of the HM-TiO2 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 pore size distribution (BJH) were evaluated from nitrogen adsorption/desorption isotherms measured on TriStar II 3020 V1.03. UV–vis diffuse reflectance and absorption spectra were measured on a UV–vis spectrophotometer (SOLID3700, Shimadzu Co. Ltd, Japan). The thicknesses of the films were tested by a profilometer (XP-2, AMBIOS Technology, Inc., USA). 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 loaded dye solution were 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 light intensity was measured with a NREL-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.

ASSOCIATED CONTENT Supporting Information Top-view and cross-sectional images of the HM-TiO2 based and NP-TiO2+HM-TiO2 based photoanode films. This material is available free of charge via the Internet at http://pubs.acs.org. 14


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AUTHOR INFORMATION Corresponding Authors *[email protected] *[email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEGEMENTS This work was supported by the National Natural Science Foundation of China (No. 21273242 and 61404142), the National High Technology Research and Development Program of China (No. 2015AA050602), 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.

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Table of Contents

17 15 12

Voc/V

-2

Jsc (mA cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.808 -2

9

Jsc/mA cm

16.22

6

FF/% η/ %

74.7 9.79

3 0 0.0

0.2

0.4

Voltage (V)

20


0.6

0.8

Solvothermal Synthesis of Hierarchical TiO2 Microstructures with High

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