Morphology-controllable polycrystalline TiO2 nanorod arrays for efficient charge collection in dye-sensitized solar cells

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Morphology-controllable polycrystalline TiO2 nanorod arrays for efficient charge collection in dye-sensitized solar cells Peng Zhong, Xiaohua Ma, Xinpeng Chen, Rong Zhong, Xuehong Liu, Dongjie Ma, Maolin Zhang, Zhimin Li

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Received date: 7 April 2015 Revised date: 2 June 2015 Accepted date: 3 June 2015 Cite this article as: Peng Zhong, Xiaohua Ma, Xinpeng Chen, Rong Zhong, Xuehong Liu, Dongjie Ma, Maolin Zhang, Zhimin Li, Morphology-controllable polycrystalline TiO2 nanorod arrays for efficient charge collection in dyesensitized solar cells, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2015.06.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Morphology-controllable polycrystalline TiO2 nanorod arrays for efficient charge collection in dye-sensitized solar cells

Peng Zhongab, Xiaohua Maab*, Xinpeng Chena, Rong Zhonga, Xuehong Liua, Dongjie Maa, Maolin Zhanga, Zhimin Lia a

School of Advanced Materials and Nanotechnology, Xidian University, 266 Xinglong Section of Xifeng Road, Xi’an 710126, Shaanxi, People’s Republic of China b

Key Lab of Wide Band-Gap Semiconductor Materials and Devices, Xidian University, Xi’an 710071, Shaanxi, People’s Republic of China

*

Corresponding authors:

Email address:

[email protected]

Tel.: +86-29-81891149; Fax: +86-29-81891149

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Abstract In this work, rutile TiO2 nanorod arrays (NRAs) are prepared on TiO2 seeding/FTO substrates by a hydrothermal process. The synthetic recipe is systematically studied to probe the morphology-synthesis interactions. Results indicate that increasing the seeding thickness greatly improves the uniformity of the top NRAs, and the initial growth for nanorods can be classified into three steps, i.e., nucleation, plant-shaped growth and array-appearance formation. The structural study suggests that each TiO2 nanorod is in fact closely-stacked by many tiny secondary nanorods parallel to each other, which could be opened by a chemical etching process. Upon etching, the performance of dye-sensitized solar cells (DSSCs) based on TiO2 NRAs is substantially improved. The device efficiency achieves a highest value of 4.46 % using a 14 µm NRAs etched for 3 h. The charge dynamics of DSSCs was further investigated

by

intensity-modulated

photocurrent/photovoltage

spectroscopy,

electrochemical

impedance spectroscopy and open-circuit voltage decay. Results show that the electron collection efficiency in the TiO2 NRAs is greatly enhanced after etching, mainly due to the retarded charge recombination, which is probably induced by the morphology change of the etched NRAs. The present study reveals some important issues for growth of rutile TiO2 NRAs on seeded FTO substrates, introduces a novel etching way for improving charge collection in semiconductor NRAs, and would advance the applications of TiO2 NRAs in various areas, such as solar cells, sensors and batteries. Keywords: TiO2 nanorod array; sol-gel thin film; hydrothermal reaction; solar cell; electron collection efficiency; recombination

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1. Introduction Recently, TiO2 has been intensively studied in various fields such as solar cells [1-10], photocatalysis [11-18], lithium batteries [19-22] and sensors [23, 24], because they are photoelectrochemically active, highly stable and environmentally benign etc. By different preparation ways, TiO2 could be tailored into nanostructures with diverse geometries, including nanotubes [25], nanowires [26], nano-flowers [27], nano-tapes [28] and so on, which would further modify the properties of TiO2 [29]. For device applications, vertically-aligned one-dimensional (1-D) TiO2 nanostructure arrays could be fabricated on a substrate via designed “bottom-up” or “top-down” approaches [3, 7, 29-35]. These ordered TiO2 nanostructure arrays provide direct pathways for charge transport, and meanwhile they preserve large specific surface areas as well as facilitate the filling of other guest materials. For example, hexagonal-arranged TiO2 nanopore arrays with tunable pore diameters could be fabricated on arbitrary substrates by soft template imprinting using viscous TiO2 sols as resists [30, 31]. Highly-ordered TiO2 nanotube arrays with their length in a large range could be prepared on a titanium sheet by electrochemical anodization [3, 7]. Currently, oriented rutile TiO2 nanorod arrays (NRAs) derived by wet chemical routes have attracted more and more attentions [36-48], mainly due to the intrinsic electronic properties of their single-crystalline nanostructures [49]. In 2008, Feng et al., [36] for the first time prepared densely-packed rutile TiO2 NRAs up to 5 µm directly on a FTO substrate via a low-temperature (at 180 o

C) solvothermal reaction. Subsequently in 2009, Liu et al., [38] replaced the organic solvent (toluene)

by water, and thus make the hydrothermal reaction environmental-friendly; they systematically studied the effects of a series of factors including growth temperature, growth time, initial reactant concentration, acidity and additives on the morphological properties of the TiO2 NRAs. In the pioneering reports, bare FTO substrates were chosen to deposit nanorods. However, in many recent applications (e.g., solar cells), a compact TiO2 thin film is always needed underneath the nanorods [39-41, 48, 50, 51], which is used to block holes and thus avoid short circuit, especially in a solid-state device. In addition, this TiO2 thin film could also act as a seed layer to improve the nanorod growth. For instance, Wang et al., [40] prepared a dense TiO2 film with a thickness of 150 nm by spray pyrolysis, which was employed as the seeding for rutile TiO2 NRAs and the hole-blocking layer in solid-state dye-sensitized solar cells (DSSCs). However, until now there is not a systematic

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investigation concerning on the nucleation and growth of rutile TiO2 NRAs on top of a TiO2 seed layer/FTO substrate, which needs to be fully studied. The DSSC is regarded as one of the most potential candidates to replace the traditional Si photovoltaic (PV) [1, 52], due to its low cost, easy manufacturing and high efficiency. Generally, the photoanode of a DSSC consists of a mesoporous film based on interconnected TiO2 nanoparticles, which is sensitized by a monolayer of dye molecules. Upon illumination, the photo-excited dye molecule injects an electron into the TiO2 conduction band. Then, electrons diffuse through the TiO2 network, and are collected by the transparent electrode. In the meantime, the excited dye molecule is electrochemically regenerated by an electrolyte based on an iodide/triiodide redox couple. One factor limiting the performance of DSSCs is the electron transport pattern in the TiO2 nanoparticle film [53, 54]. The crystallite boundaries between nanoparticles and the trap states on the surfaces of nanoparticles serve as electron trapping sites. As a result, the electron transport is hindered, and the chances for charge recombination are remarkably increased. Incorporation of hydrothermal rutile TiO2 NRAs into DSSCs is a feasible way to enhance the electron transport and collection [4, 28, 36, 38, 40, 44, 47, 50, 51, 55, 56], which would thus improve the performance of PV devices. However, to the best knowledge of us, there are few experimental studies concentrating on the charge dynamics in this system [37, 49]. For example, Henry et al., [49] investigated the electron transport in the ordered rutile TiO2 NRAs and the random TiO2 mesoporous film by terahertz spectroscopy, and the remarkably enhanced electron mobility for NRAs (1 cm2/(Vs)) as compared to that for nanoparticles (10-2 cm2/(Vs)) was contributed to the local field screening, besides the well-known effects of direct walk of electrons through the single-crystalline nanorods and few charge trapping at defect. Feng et al., [37] discovered that the electron transport in the oriented rutile TiO2 NRAs is greatly improved as compared to that in the interconnected TiO2 nanoparticle film by using the IMPS/IMVS technique, and they further found that the density of sub-band-gap defect states in the NRAs based DSSC is significantly lower than that in the nanoparticle based DSSC. In fact, the above two studies only compare the electron transport in rutile TiO2 NRAs and in TiO2 nanoparticle film, and are shortage of exploring the relation between the morphology of NRAs and the charge dynamic characteristics in DSSCs. In addition, an in-depth study is called for to further indentify the electronic features in rutile TiO2 NRAs synthesized from solutions by using a series of comprehensive techniques.

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In this work, a dense TiO2 thin film is prepared on a FTO substrate by using a sol-gel method, which acts as the seed layer for growing rutile TiO2 NRAs in a hydrothermal bath. The effects of growth recipe on the morphological property of the TiO2 NRAs are systematically studied, including the seeding thickness, the initial reactant content, the growth duration and the growth temperature. Besides, the structural, componential and optical characteristics of the TiO2 nanorods are characterized carefully. The effects of seeding thickness and chemical etching on PV performances of the DSSCs based on rutile TiO2 NRAs are systematically investigated. Besides, a comprehensive study is also conducted to probe the charge dynamic properties of DSSCs assembled with rutile TiO2 NRAs etched for different durations.

2. Experimental 2.1. Preparation of TiO2 seed layers FTO substrates (7 Ω/□, Nippon Sheet Glass, Japan) were ultrasonically cleaned in deionized (DI) water, acetone and ethanol for 10 min, respectively, followed by N2 blow drying. The TiO2 seed films were prepared by a sol-gel process [57]. Briefly, Butyl titanate (TEOT) as the initial reactant was first dissolved in ethanol, which was denoted as Solution A. Then ethanol, DI water and HCl were mixed to form Solution B. Subsequently the two solutions were mixed together by dropping wise Solution B into Solution A, followed by stirring for 24 h at room temperature. The final composition of the TiO2 sol in a molar ratio was TEOT : H2O : HCl = 1 : 1.2: 0.15 and ethanol = 20, 30 and 50 M, respectively. The TiO2 thin films with varied thicknesses were prepared by spin-coating the TiO2 sols with different ethanol contents onto FTO substrates at 3000 rpm for 30 s, and immediately heat treated at 200 oC for 30 min, followed by calcinations at 500 oC for 1 h. In order to measure the thicknesses of the TiO2 thin films, bare glass substrates were also employed. 2.2. Growth of rutile TiO2 NRAs 15 ml DI water was first mixed with 15 ml HCl (36.5 ~ 38 wt%) in a beaker. After stirring for 5 min, 0.25 ~ 0.5 g TEOT was added slowly, followed by stirring for another 10 min. Then the mixture solution was moved to a 40 ml Teflon-lined stainless steel autoclave. The seeded FTO substrates were placed against the Teflon-liner wall at about 70o with the conducting side facing down. The hydrothermal reaction was conducted at 160 ~ 200 oC for 0 ~ 7 h in an oven. After synthesis, the autoclave was immediately cooled by flowing cold water. The collected samples were rinsed by DI

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water extensively and dried by N2 flow. For growth of 14 µm-long TiO2 NRAs, 1.5 ml TiCl4 were chosen as the initial reactant instead of TEOT to speed up the reaction; and the seeding was prepared by soaking bare FTO substrates into 0.2 M TiCl4 aqueous solution, followed by calcinations at 550 oC for 1 h, in order to improve adhesion between the substrates and the long nanorods. While other parameters remain as above, the whole hydrothermal reaction lasts at 150 oC for 6 h for the recipe of using TiCl4 as the starting precursor. The etching reaction for the as-prepared TiO2 NRAs was conducted in a bath of 10 ml DI water and 20 ml HCl (36.5 ~ 38 wt%), hydrothermally treated at 150 o

C for various durations.

2.3. Fabrication of DSSC devices The TiO2 NRAs/seeded FTO samples were annealed at 400 oC for 1 h to improve the crystallinity before device encapsulation. The fabrication of DSSCs were carried out based on a modified method we reported previously [58]. In detail, for absorbing dye molecules the samples were first immersed into 0.3 mM ethanol solution of ruthenium complex Ru[LL’-(NCS)2] (N719) at room temperature for 24 h. FTO substrates DC-sputtered with Pt thin films were employed as the counter electrodes. Then the dye-sensitized TiO2 NRAs on a seeded FTO substrate and a counter electrode were sandwiched by using a 25µm-thick hot-melt polymer (Solaronix SX1170-25). The electrolyte was commercially available (Wuhan Geo Hi-Tech Co., Ltd., China), of which the composition mainly consisted of 1-propyl-3-methyl-imidazolium iodide (PMII), I2, LiI, Guanidine thiocyanate (GuNCS) and 4-tert-butylpyridine in acetonitrile. 2.4. Characterization and measurements The thicknesses and morphology of the TiO2 seed layers were characterized by an Ellipsometer (M-2000UI) and an Atomic Force Microscope (AFM, Veeco Nanoscope Ⅲa), respectively. The morphology of the as-prepared TiO2 NRAs was observed by Field-Emission Scanning Electron Microscopy (FESEM, Zeiss Ultra 55 and FEI Quanta 250 FEG) equipped with a Energy Dispersive Spectrometer (EDS). The crystalline properties of all samples were characterized by X-ray Diffraction (XRD, Rigaku D/max 2400) and Transmission Electron Microscopy (TEM, JEOL. JSM-2100). The compositions of the as-synthesized TiO2 NRAs were determined by X-ray Photoelectron Spectroscopy (XPS, Thermo Fisher Scientific, K-Alpha). The diffuse reflectance spectra and the photoluminescence (PL) spectra were recorded by using a JASCO-570 UV-vis-NIR spectroscope and a Hitachi F-7000 fluorescence spectrophotometer. The PV performance of DSSCs was measured by recording the

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current density-voltage (J-V) curves of the devices with an active area of 0.19 cm-2, under an illumination of a AM 1.5G solar simulator (100 mWcm-2, Zolix, SS150). The IPCE spectra as a function of wavelength from 400 nm to 800 nm were obtained by using a Zahner CIMPS-

Ⅱ

photoelectrochemical system. The intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS) measurements were carried out on an electrochemical workstation (Zahner, Zennium) with a frequency response analyzer under modulated blue light emitting diodes (LEDs, λmax=480 nm) driven by a source supply (Zahner, PP211). The LEDs can provide both dc and ac components of illumination. A small sinusoidal intensity modulation (

±5 %) was superimposed onto the much greater

dc illumination intensity in a frequency range of 0.1 Hz to 10 kHz, for both of the IMPS and IMVS measurements. The incident light intensity was controlled from 35 to 140 mWcm−2. The Electrochemical impedance spectroscopy (EIS) was performed on the Zennium electrochemical workstation in a frequency range between 10 mHz and 1 MHz, with a magnitude of the alternative signal of 10 mV. The devices were forward biased under AM 1.5G illumination during the EIS measurements. The open-circuit voltage decay (OCVD) measurements were conducted by recording the decay of the open-circuit voltage within 120 s after removing the AM 1.5G illumination.

3. Results and discussion 3.1. Morphology 3.1.1. Effect of seeding thickness The thickness of TiO2 thin films derived from a sol-gel method decreases with the increase of ethanol contents. By using an ellipsometer, their thicknesses at the ethanol contents of 20 M, 30 M and 50 M are measured to be 95 nm, 68 nm and 40 nm, respectively. Fig.1(a) shows the XRD patterns of the TiO2 thin films. It can be seen that all samples are anatase phase of TiO2, in terms of the JCPDS Patterns No. 21-1272. When the film thickness is 40 nm, there is only one diffraction peak located at 25.32o, corresponding to the [101] crystal plane of anatase phase. With the thickness increased to 68 nm, the intensity of the [101] peak becomes stronger, and another two small peaks assigned to [004] and [200] crystal planes appear. With further increasing the thickness to 95 nm, the intensities of all the above-appeared diffraction peaks become stronger. Besides, there are another two peaks assigned to [105] and [211] crystal planes of anatase phase. The crystallite sizes along the [101] orientation are evaluated to be 14.9 nm, 18.9 nm and 16.1 nm, for the films with their thicknesses of 40 nm, 68 nm and 95 nm, respectively, according to Scherer’s Formula: D = Kλ / Bcosθ , where K is a dimensionless constant, λ is the wavelength of the used X-ray radiation, θ is the diffraction angle, and B is the half-maximum full width of the diffraction peak. Fig.1(b-d) show the AFM views of the morphology of the TiO2 thin films with different

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thicknesses. It can be observed from Fig.1(b) that at 40 nm the film is smooth and closely packed with uniform grains with their sizes of tens of nanometers. However, with increasing the thickness, some grains in the TiO2 thin films apparently become larger as marked with circles in Fig.1(c,d), and the density of large grains in the 95 nm-thick film is higher than that in the 68 nm-thick film. Considering the crystallite sizes in the TiO2 thin film are less than 20 nm as determined by XRD, these identified grains with their sizes in the range of 200 ~ 300 nm might be formed by nanocrystal clusters. Besides, the TiO2 thin films become uneven with a high thickness, because the root-mean-square (RMS) roughness continuously increases from 0.356 nm (at 40 nm) to 0.558 nm (at 68 nm) and 0.578 nm (at 95 nm), respectively. Thus, it can be concluded from the XRD and AFM results that increasing the thickness of TiO2 thin films would induce exposure of more crystal planes of anatase phase, increase sizes of the grains dispersed in the thin film and raise surface roughness, while the nanocrystallite sizes are not obviously influenced. Fig.1(e-l) show the SEM views of the TiO2 NRAs deposited on bare and seeded FTO substrates, and the insets show the corresponding photographs. It can be observed from Fig.1(e) that the coverage of TiO2 NRAs on a bare FTO substrate is not full, with large-area domains of bare FTO exposed. In the nanorod areas, there are many oval pits with their sizes of ~ 20 µm, of which some small holes with diameters of ~ 2 µm exist at the bottom. In the inset of Fig.1(e), there are many irregular holes distributed in the film, indicating that TiO2 NRAs cannot be deposited at these places. As shown in the closer view (Fig.1(f)), the TiO2 nanorods are square-shaped, with their sizes in the sub-micron range, and they tend to aggregate on bare FTO substrates. Besides, the rod tips are not smooth with a number of second-order nanostructures on their tops. When using a 40 nm-thick TiO2 seed layer, it can be seen from Fig.1(g) that the pits are much deeper than those on bare FTO substrates, and the visual irregular holes on the surface are replaced by some pin-holes (inset of Fig.1(g)). As shown in Fig.1(h), the TiO2 NRAs based film is dense, suggesting that aggregation is still obvious using a 40 nm seeding. With increasing the thickness to 68 nm as shown in Fig.1(i,j), the morphology does not show apparent change as compared to that at 40 nm, but the pin-holes gradually disappeared at present conditions (inset of Fig.1(i)). However, with further increasing the thickness up to 95 nm, it can be observed from Fig.1(k,l) that all TiO2 nanorods stand freely in a large area without aggregation and fusion. As shown in the inset of Fig.1(k), the pin-holes observed on thinner seeding vanish completely, leading to a smooth film. Thus, the uniformity of the top TiO2 NRA film is greatly improved with increasing the

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underneath seeding thickness. In our study, it is difficult to obtain rutile TiO2 NRAs uniformly in a large area if on a bare FTO substrate, which is in contradiction with Liu B’ previous report [38]. This discrepancy might be contributed to the difference of used FTO substrates. Liu et al., chose a FTO with 15 Ω/□ resistivity, while a 7 Ω/□ one was employed in this work. Similar phenomenon has also been discovered by Berhe et al., [42] but they obtained large crystal grains instead of NRAs on a 7 Ω/□ FTO substrate. Generally, commercial FTO substrates reduce their resistivity by increasing the film thickness, and meanwhile their surface roughness is decreased to minimize haze. Although FTO substrates have been verified as useful seed layers [38], their surface roughness might play an important role for depositing TiO2 NRAs. Thus, the relatively-smooth surface of FTO used in this work might be responsible for the uneven morphology of the top TiO2 NRAs as observed in Fig.1(e,f). Based on above SEM observations, the anatase TiO2 thin film should be a better seed layer than the 7 Ω/□ FTO for epitaxial nucleation and growth of rutile TiO2 nanorods. If a dense and thin film (e.g. 40 nm-thick) composed of closely-packed small grains was employed, spacing for neighboring nuclei is close, and thus the nanorods have a tendency of growing at a low angle with respect to the substrate normal. Since all surrounding facets for tetragonal nanorods are [110] crystal planes, the side faces of neighboring nanorods tend to get close naturally. As a consequence, the fusion for nanorods is severe when their sizes gradually become large. Finally, the nanorod bundling would induce a discontinuous morphology for the TiO2 NRAs based film due to tensile stress. By using a relatively-thick and coarse seed layer (e.g. 95 nm), the nanocrystals based grains disperse uniformly in the TiO2 thin film, with enough inter-grain spacing for subsequent nucleation and growth of the top TiO2 NRAs. Since the growth of nanorods were not obviously constrained by neighboring ones, free-standing rutile TiO2 NRAs can be obtained through increasing the seeding thickness to 95 nm. Therefore, it is inferred that the seeding thickness effect on the nanorod morphology might be closely related to the inter-grain spacing, surface roughness, grain size and crystalline orientation of the TiO2 thin films. 3.1.2. Effect of growth duration Fig.2(a-j) show the SEM views of the surface and cross-sectional (inset) morphologies of the rutile TiO2 NRAs on a seeding FTO substrate at different growth stages. It can be observed from Fig.2(a) that at 30 min, there are many interconnected sub-micron domains consisting of densely-packed grains with uniform sizes of ~ 30 nm, which should be the epitaxial rutile TiO2 at the

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starting stage of the hydrothermal reaction [59]. With increasing the reaction time to 35 min as shown in Fig.2(b), it is interesting to find that the grain sizes become smaller (~ 25 nm) than those at 30 min, and the inter-grain distances are much longer. Besides, the deposited grains at 35 min are no longer round as those at 30 min; instead, their shapes become irregular and the surfaces are coarser. The morphology change for the TiO2 grains might be due to the acid etching during the nucleation process [60], because at this moment the growth rate is probably slower than the etching rate. With prolonging the growth time to 40 min, it can be seen from Fig.2(c) that elongated grains (~ 30 × 100 nm) have been formed at the places where the TiO2 grains deposited, indicating that the initial growth of nanorods begins at this moment. At a longer time of 45 min (Fig.2(d)), the elongated TiO2 grains keep on growing along and normal to axis with their sizes increased to ~ 35 × 150 nm. It can also be discovered that the TiO2 nanorods do not grow along the substrate normal direction in a free-standing way; instead, they bundle like a plant with their tips facing towards different directions, which originates from the root nucleation spot. As a result, a relatively-loose seeding is required to prepare sparse TiO2 NRAs, because of the initial bundling behavior. With further increasing the growth time to 50 min (Fig.2(e) and its inset), the TiO2 nanorods displays array-like appearance with their length of ~ 220 nm and the diameter of ~ 40 nm. It is inferred that the vertical array morphology at 50 min is more likely contributed to the TiO2 nanorods close to the substrate normal, while growth of nanorods initially towards other directions might be gradually ceased due to being blocked by nearby ones. At 60 min as shown in Fig.2(f) and its inset, the diameter and the length of nanorods are both increased to 60 nm and 550 nm, respectively. It can be observed from the cross-sectional view (inset of Fig.2(f)) that some broken nanostructures exist at the bottom of the array, which might be related to the nanorods having titled growth angles with respect to the substrate normal. It is important to reveal the starting growth process for hydrothermal rutile TiO2 NRAs, in order to adjust the synthetic recipe for rationally controlling their morphology. Unfortunately there are few reports concerning on this topic previously. It is discovered in our work that there are three steps for initially depositing TiO2 nanorods, that is, nucleation (< 35min), plant-shaped growth (40 ~ 45 min) and array-appearance formation (50 ~ 60 min). The nucleation follow an epitaxial mechanism from the bottom anatase TiO2 grains, and the nuclei experience a modification process by the acid etching. Then the plant-like shapes are observed at the initial stage for nanorod growth. Finally the nanorod arrays are

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formed upon the growth competition of individual nanorods in the plant-shaped bundling. After 1 h, the nanorod growth steps into a high-speed stage. As shown in Fig.2(g) and its inset, upon 2 h reaction the length of the TiO2 NRAs is increased up to 2.3 µm, and the diameter is widened to be ~ 150 nm, indicating that anisotropic growth happens for the TiO2 nanorods along the axis direction. It can be further observed from Fig.2(h) and its inset that at 4 h the length of NRAs is increased to 7 µm, and the nanorod fusion is more obvious than that at 2 h. However, after prolonging the growth duration to 6 h (Fig.2(i) and its inset), the nanorod length is shortened (3.8 µm) as compared to that at 4 h, and the secondary nanostructures can be resolved on top of the nanorod tips. With further increasing the time to 7 h, the TiO2 NRAs peel off the substrate (inset of Fig.2(j)), indicating that the boundaries between the nanorods and the seed layer are destroyed. Besides, the top secondary nanostructures are sculpted more clearly for the 7 h sample (Fig.2(j)). The exposure of second nanostructures and shortening of nanorod lengths should be caused by an acid etching reaction as follows: hydration

TiO2 → Ti ( (1)

Ti (

Ⅳ ) complex

Ⅳ ) complex + HCl → Ti (Ⅳ ) + H O 2

(2) The hydrothermal reaction of nanorod growth is continuously accompanied by the above etching process. When the growth duration is below 4 h, the growth rate is faster than the etching rate, indicating that the TiO2 NRAs follow an antistrophic growth. Above 4 h, because the precursor is gradually consumed, the remaining strong acid might act as etching reactant to partially dissolve the as-synthesized TiO2 NRAs. Fig.2(k,l) shows the correlation curves between the length and the diameter of the TiO2 NRAs and the growth durations, respectively. It can be calculated that the TiO2 NRAs grow along the substrate normal direction nearly at a constant speed of ~ 35 nm/min in the range of 50 min ~ 4 h, and achieve a maximum length of ~ 7 µm at 4 h, and then the nanorod length reduces after 4 h. Besides, the nanorod diameter keeps on increasing below 4 h, and decreases after 4 h due to acid etching. 3.1.3. Effects of TEOT content and growth temperature Fig.S1 shows the SEM views of the TiO2 NRAs synthesized with different TEOT contents. With various TEOT contents, the TiO2 NRAs based film show completely different morphologies. As shown

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in Fig.S1(a), with 0.25 g TEOT, most nanorods have tiny tips with their average diameter smaller than 100 nm, and there is only a small portion of nanorods with larger sizes, which should be due to fusion of neighboring small ones. From the cross-sectional view (Fig.S1(e)), it can be seen that the profile of NRAs is not clear, which might be due to the fact that the tiny nanorods are too slender to stand freely. With increasing the TEOT content to 0.35 g, the TiO2 nanorods apparently have larger diameter (~ 150 nm) and display clearer array appearance (Fig.S1(b,f)). However, as shown in Fig.S1(f), the length of NRAs at 0.35 g TEOT is decreased to 3.1 µm from 4 µm at 0.25 g TEOT, which might be caused by the nanorod aggregation. With further increasing the TEOT contents up to 0.425 g and 0.5 g, it is discovered that the sizes of the NRAs keep on increasing as shown in Fig.S1(c,d,g,h). The length of NRAs achieves the maximum of 7.6 µm with a TEOT content of 0.5 g. Fig.S2 provides the quantitative relations between the sizes of the TiO2 nanorods and the TEOT contents. Several points can be concluded: (Ⅲ) the sizes of NRAs generally increase with the initial TEOT contents; (Ⅲ) the nanorod density on a seeded FTO substrate is higher than that on a bare FTO substrate at similar synthetic conditions [38]; (Ⅲ) the TiO2 nanorods probably follow a growth mechanism of stacking small units (e.g., [Ti(OH)2Cl2(OH)2]0) at the step edges of inner nanorods [61], because the tip diameters of the nanorods gradually become larger by adding TEOT in our experiments. Fig.S3 displays the SEM views of the TiO2 NRAs prepared at different growth temperatures. With increasing the temperature, the coverage of TiO2 NRAs is greatly improved as observed from Fig.S3(a-c). Until 200 oC, a full coverage of NRAs cannot be achieved on the seeded FTO substrates for 1 h growth. As shown in Fig.3(d-f), both of the diameter and the length of the TiO2 nanorods increase with the growth temperature.

3.2. Structures, components and optical properties Fig.3(a) shows the XRD pattern of the as-prepared TiO2 NRAs on a seeded FTO substrate. It can be observed that beside the FTO signals, the remaining two diffraction peaks are assigned to (002) and (101) crystal planes of the tetragonal rutile phase (P42/mnm), according to JCPDS card No. 88-1175. The intensity of the (002) peak located at ~ 62.8o is substantially enhanced as compared to powder diffraction pattern, while other signals corresponding to rutile phase are mostly absent, which suggest that the TiO2 NRAs might grow along the preferred (001) direction with the growth axis perpendicular

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to substrates and the nanorods are probably single crystalline. In addition, no anatase phase of TiO2 is detected probably due to the fact that the seed film is too thin. Fig.3(b) displays the low-magnitude TEM view for the as-prepared TiO2 NRAs, which are scraped from a seeded FTO substrate and ultrasonically dispersed in ethanol. The obtained NRAs form a plant-shaped bundle, with its bottom dense and upper portion sparse, and the growth directions of individual nanorods are not parallel to each other. As a result, the nanorod growth might originate from the root nucleation spots, and follow a certain competition mechanism for the outmost nanorods, which are in good agreement with the SEM observations. As shown in the medium-magnitude TEM view (Fig.3(c)), it is surprising to discover that one individual nanorod with a diameter of ~ 80 nm is actually composed by dozens of closely-packed tiny ones with each diameter in the range of 5 ~ 10 nm, and the intervals between two tiny nanorods are clearly identified and paralleled to each other. This finding suggests that the nanorod growth might follow a process of stacking smaller building blocks [38]. Fig.3(d) shows the HRTEM and SAED (inset) views of the as-synthesized TiO2 nanorods. The SAED pattern clearly confirms that the TiO2 nanorods are single crystalline. It can be seen from the HRTEM result that the lattice fringes can be identified along the entire nanorod length. The inter-planar spacing perpendicular to the direction of the nanorod length can be measured to be 0.321nm, which corresponds to the (110) crystal planes of rutile phase of TiO2. Therefore, it can be concluded from the TEM results, the rutile TiO2 nanorods should be strictly classified as polycrystalline structures, parallel stacked with tiny single-crystalline nanorods, which is different from some previous reports [36, 38]. Fig.3(e,f) show the high-resolution XPS patterns of the Ti2p and O1s peaks for the as-prepared TiO2 NRAs, respectively. As shown in Fig.3(e), the two bands located at 458.2 eV and 463.9 eV are assigned to Ti2p3/2 and Ti2p1/2 spin-orbital splitting photoelectrons, and the distance between two peaks is about 5.7 eV indicating that the element titanium exists in the form of Ti4+. As shown in Fig.3(f), the O1s peak centered at around 530.3 eV is attributed to the binding energy of O2- in the TiO2 lattice, which is due to the Ti-O band. The chemical stoichiometry of the as-synthesized TiO2 NRAs is examined by EDX (Fig.S4), and the atomic ratio of Ti and O is determined to be ~ 1 : 2. Besides, a very small amount of Cl and Sn has been detected, which might come from the absorbed chloride on the surface of as-prepared TiO2 NRAs [38] and from the FTO substrate, respectively. Fig.3(g) shows the optical absorption spectra of the TiO2 NRAs on a seeded FTO substrate by an integrating sphere based reflective mode. Only UV light below 400 nm can be absorbed by the sample.

13

The band gap is extrapolated to be ~ 3.09 eV (Fig.3(h)), a value slightly larger than that of rutile TiO2 (3.0 eV) reported in literatures [62], which might be due to the jointed effect of the bottom substrate and rutile TiO2. Fig.3(i) shows the PL spectra of the as-prepared TiO2 NRAs on a seeded FTO substrate. Upon excitation at 325 nm, photoemissions at different wavelengths are observed. The broad band centered at 3.12 eV is attributed to the near-band-edge UV emission of TiO2, that is, the indirect transition from edge to center of brillouin zone [63, 64]. The visible emissions located at 2.75 eV, 2.64 eV and at lower energies are probably due to electron transition trapped at the oxygen vacancy level, which is lower than conduction band edge [63, 64]. Since the intensity of UV emission is much higher than those of visible emissions, it can be inferred the as-prepared TiO2 nanorods have good crystal structures with few defects, which is consistent with the structural analysis.

3.3. Photovoltaic properties 3.3.1. Seeding effect Fig.4(a) shows the J-V curves of the TiO2 NRAs based DSSCs as a function of the seeding thickness. The detailed PV parameters are also provided in Table 1. Without TiCl4 treatment, both of the Voc and FF increase with the thickness of seed layers, probably due to the fact that thick seed layers could avoid pin-holes on the film, leading to reduced recombination. But the Jsc decreases with the thickness, which might result from the retarded charge transport across thick TiO2 films. As a consequence, the highest PCE (1.84 %) is achieved using a 40 nm-thick seed layer. Upon TiCl4 treatment, it is found that the PCEs of all devices have improvements. But the PCE enhancements show great discrepancies when using seed layers with different thicknesses. At 40 nm, the Voc, Jsc and FF do not show obvious change, resulting in only a slight PCE improvement from 1.84 % to 1.93 %. However, the device PCEs sharply increase after TiCl4 treatment from 1.71 % to 2.21% at 20 nm and from 1.53 % to 2.07 % at 68 nm, respectively. It is further observed that the detailed PV parameters contribute unequally to the enhancement of device performance in the two cases. In detail, at 20 nm the PCE increase is mainly ascribed to the Voc (from 0.65 V to 0.79 V) and FF (from 55 % to 66 %) improvements, while at 68 nm the PCE increase is mainly contributed by the Jsc improvement (from 3.71 mAcm-2 to 5.21 mAcm-2). Thus, with TiCl4 treatment, the device PCE achieves the lowest value at 40 nm. It is inferred that the enhancement mechanisms for DSSC PCEs upon TiCl4 treatment are totally

14

discrepant when using seed layers with different thicknesses. Since on a very thin seeding (20 nm) the TiO2 nanorods tend to aggregate into dense bundles as discussed, the TiCl4 treatment could not lead to obvious increase of surface areas and Jsc. But at present conditions, the TiCl4 treatment would greatly reduce back recombination by covering pin-holes of the underneath seed layers, resulting in substantially improved Voc, FF and thus the PCE. However, with a relatively-thick seeding (68 nm), the PCE enhancement is mainly contributed by the Jsc improvement, because a thick seed layer would greatly improve the uniformity of free-standing TiO2 NRAs, which provide more active sites for TiCl4 modification and thus increase surface areas. Therefore, the TiO2 seeding thickness not only influences the morphology of the top TiO2 NRAs, but also has great impact on the electrical and PV properties for DSSCs. 3.3.2. Etching effect Since an extensive growth (over 4 h) could induce exposure of secondary nanostructures for rutile TiO2 NRAs, it is expected that after removing the Ti precursor from the hydrothermal bath, the reaction would happen totally towards the etching direction as shown in Equation (1,2). As a result, the etching reaction would be speeded up, so as to enable nanorods opened into tiny ones as fully as possible. In this work, an etching process in a mixture bath (H2O:HCl=1:2, V/V) is imposed on the as-synthesized TiO2 NRAs. As shown in Fig.S5, the polycrystalline TiO2 nanorods stacked by tiny single-crystalline ones could be opened in a large area after 5 h etching. It can be observed from the inset of Fig.S5 that the nano-array structure still remains even after such etching in a strong acid bath. Fig.S6 shows the photographs of the TiO2 NRAs based films as a function of the etching time before and after dye loading. It can be observed from Fig.S6(a) that the transparency of the TiO2 NRAs is gradually reduced upon etching, which is probably due to the light scattering effect. From Fig.S6(b), the dye-loading substantially improves after acid etching, indicating that the etching could greatly increase surface areas of the TiO2 NRAs based film, which is in consistence with previous reports [50, 56]. Fig.4(b,c) show the J-V curves of the DSSCs assembled with TiO2 NRAs of 7 µm and 14 µm as a function of the etching time, respectively. The detailed PV parameters are also displayed in Table 1. For the 7 µm sample, although the device PCE first have a small decrease from 2.43 % to 2.11 % after etching for 1 h, the efficiency continues to be improved to 2.84 % and 3.29 % for the 7µm-etching-3h and 7µm-etching-5h samples, respectively. In order to achieve a higher PCE, the 14 µm TiO2 samples are also employed to be incorporated into DSSCs. As shown in Fig.4(c) and Table 1, it is found that the

15

PCE first increases greatly from 1.56 % (14µm-etching-0h) to 2.68 % (14µm-etching-1h), and then keeps on a sharp improvement to a highest value of 4.46 % (14µm-etching-3h), and finally the PCE shows slight decrease to 4.16 % and 4.08 % for the 14µm-etching-5h and 14µm-etching-7h samples, respectively. On the whole, the DSSC performance shows obvious enhancement upon the acid etching, which is mainly attributed to the Jsc and FF increase. The Jsc enhancements for the etched devices might be mainly due to the increased surface areas of TiO2 nanorods (i.e. improved dye-loading amount) as observed in Fig.S5 and Fig.S6 and as reported previously [56]. As shown in Fig.4(d), the variations of IPCE for the 14 µm devices are in consistence with those of Jsc. The small decrease of Jsc and IPCE for the 14µm-etching-5h and 14µm-etching-7h samples is probably due to reduced surface areas caused by over-etching. FF is directly influenced by the series and shunt resistances of PV devices. The FF enhancements should be closely related to the charge transport and recombination in the etched TiO2 NRAs, because other parameters influencing inner resistances of solar cells remain the same, such as the nernst diffusion of I−3 in the electrolyte, the sheet resistance of FTO and the charge transfer resistance between the electrolyte and the counter electrodes. A series of characterizations are employed to study the charge dynamics of DSSCs assembled with 14 µm TiO2 NRAs etched for different durations, including IMPS/IMVS, EIS and OCVD. Fig.S7 shows the complex-plane IMPS and IMVS plots for DSSCs at short circuit and at open circuit, respectively. The electron transport time (τd) and the electron lifetime (τr) present the time constants of injected electrons diffusing in the TiO2 photoanode and recombining with the electrolytes, respectively, which can be determined according to the equations as follows [65]:

τd = 1/ 2πf d , τ r = 1/ 2πf r , where fd

and fr are the characteristic frequency minimum of the imaginary component of the IMPS and IMVS semicircles, respectively. It can be observed from Fig.S7(a) that there are distinct two semicircles in the IMPS plot for the device without etching (0 h), indicating that two electrons transport mechanisms might exist [66]. The high-frequency semicircle (fd=49.79 Hz) might correspond to the trap-free mode transit for photo-generated electrons, while the low-frequency one (fd=11.26 Hz) is probably due to the trap-limited mode transit. The trap-free mode represents that a fraction of electrons transport in a relatively-short time, and the trap-limited mode means that the residual electrons transport in a relatively-long time. After etching for 1 h or a longer duration, only one semicircle remains, suggesting that the trap-free mode disappears. Fig.5(a) show the correlation plots between τd and light intensity as a function of the etching time. It should be stressed here that electron transport time of trap-free mode part is not taken into account for τd at 0 h. On the whole, the τd at various light intensities has an obvious trend of increase upon acid etching, which indicates that etching prolongs the electron transport time in the TiO2 NRAs. It is inferred that the τd increase after etching is closely related to the disappearance of the trap-free mode for electron transport, which might be due to the fact that the etching partially destroys the crystalline structures of the TiO2 nanorods, and introduces defects in some extent hampering the electron transport. Fig.5(b) displays the correlation plots between τr and light intensity as a function of the etching time. The τr of all the etched samples shows a remarkable 16

increase as compared to the un-etched one, indicating that the etching process could substantially prolong the electron lifetime. As observed in Fig.S5, after being etched by strong acid, polycrystalline TiO2 nanorods are opened into their secondary nanostructures, and are even converted into nanotubes surrounded by tiny single-crystalline nanorods at many places marked with yellow circles, which was also reported by Pan et al. [39]. The observed τr increase might be due to the intrinsic electronic property of the nanotube geometry. In fact, an internal radical electric field can be generated within the nanotube walls. The potential difference within the nanotube walls (U) can be calculated by the 2

following equation [67]: U = eNd / 2εε 0 , where e is the elementary charge, N is the ionized donor density, d is the half wall thickness, ε is the dielectric constant of TiO2, and ε0 is the permittivity of free space. A potential difference in the range of 30 ~ 80 mV can be estimated if the thickness value of tube walls is regarded between 50 nm to 100 nm, which could provide adequate driving force to enable electrons far away from wall surfaces, and thus prevent back recombination. In addition, in a nanoporous structure, the screening effect of applied field could reduce the charge mobility, which probably influences the electron transport in the etched TiO2 NRAs as well [49]. Taking the TiO2 film thickness (d) into account, the electron diffusion coefficient (Dn) and electron effective diffusion length (Ln) can be calculated by the following expressions [54, 68]:

D n = d 2 / 4τ d , Ln = Dn τd . Dn is another index reflecting the charge transport feature in the TiO2 NRAs. As shown in Fig.5(c), the Dn values of the etched samples are much lower than that of the un-etched sample, implying that the transit velocity of electrons is reduced due to etching, which is in consistence with the τd results. The chemical etching used in this study might damage the crystalline structures of nanorods and introduce trapping sites, which would interrupt the electron diffusion. Ln represents the distance of electron transit in the TiO2 film before recombination with species in the electrolyte or with oxidized states of dye molecules, which reflect a comprehensive effect of electron transport and recombination [58]. Fig.5(d) shows the correlation curves between Ln and light intensity as a function of the etching time. It can be observed that the Ln values of the un-etched samples are in the range of 20 ~ 22 µm. It is surprising to find that the Ln values are increased to over 24 µm for all the etched samples, and especially for the 1 h sample, the values are even improved to ~ 30 µm. The electron collection efficiency (ηcc) is another index evaluating electron transport and recombination in a combined way, which is given by the following equation [69]:

ηcc = 1 − τ d / τ r . As shown Fig.5(e),

the ηcc values are remarkably increased from 88 ~ 90 % (un-etched sample) up to over 92 % for the etched samples (~ 95 % for the 1 h sample), indicating that the etching process could greatly improve the electron collection for the TiO2 NRAs in the working DSSCs. In fact, the enhanced electron collection for the etched TiO2 NRAs should be mainly contributed to the greatly reduced recombination, since the electron diffusion is in fact limited at present conditions. EIS and OCVD are further carried out to investigate the charge recombination properties of DSSCs. Fig.5(f) shows the EIS Nyquist plots of the DSSCs assembled with TiO2 NRAs as a function of the etching time, and the inset shows the used equivalent circuit. There are two semicircles in the Nyquist plots, in which the small one in the high-frequency range (> 1 kHz) represents charge transfer at the interface of counter electrode/electrolyte, and the large one in the medium-frequency area (1 ~ 100 Hz) corresponds to recombination at the TiO2/dye/electrolyte interface [70]. The fitted parameters as displayed in Table S1. The electron lifetime (τ) can be calculated according to the expression:

17

τ = R1 × CPE1 , where R1 is the recombination resistance, and CPE1 is the chemical capacitance. It can be observed from Table S1 that the τ value is greatly increased by etching the NRAs, e.g., from ~ 20 ms for the un-etched sample to ~ 50 ms for the 3 h etched one, which supports the IMVS results. The OCVD technique could also be used to study the recombination kinetics of DSSCs by monitoring the Voc decay rate, which is proportional to the recombination rate. As shown in Fig.S8(a), the Voc decay time is in a following sequence: nanoparticle < nanorod < etched nanorod. The longer Voc decay time means the slower recombination rate, which would thus generate a longer electron lifetime (τn) as −1

shown in Fig.S8(b), according to the equation [3]:

 k T   dV  τ n =  − B  oc  , where kB is the  e  dt 

Boltzmann constant, T is the temperature, and e is the positive elementary charge.

4. Conclusion In summary, polycrystalline TiO2 NRAs composed of parallel tiny single-crystalline nanorods are hydrothermally synthesized on a TiO2 seed layer/FTO substrate. The effects of the preparation parameters on the morphology of NRAs are systematically studied. It is found that the seeding thickness obviously influences the morphology of the NRAs, and by probing the early stage, the initial growth for nanorods can be classified into three steps: nucleation, plant-shaped growth and array-appearance formation. By an acid etching process, the TiO2 nanorods can be opened into their secondary nanostructures, which greatly reduce recombination and thus enhance the electron collection efficiency in NRAs based photoanode, examined by IMPS/IMVS, EIS and OCVD. A maximum PCE of 4.46 % can be obtained for the DSSC assembled with 14 µm TiO2 NRAs etched for 3 h. The present work deepens the understandings of the growth process and the electronic properties of rutile TiO2 NRAs with controllable morphology, and will promote their applications in a diversity of areas.

Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities under Grant No. JB141401, and the Specialized Research Fund for the Doctoral Program of Higher Education under Grant No. 20130203120016. Some authors (X. P. Chen, R. Zhong and X. H. Liu) also thank the financial support from National University Student Innovation Program for undergraduates.

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Table captions Table 1

Detailed PV parameters of DSSCs assembled with TiO2 NRAs

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Figure captions Fig.1 (a) XRD patterns and (b-d) AFM views of the surface morphology of the TiO 2 thin films with different thicknesses; SEM views and photographs (inset) of the surface morphology of the TiO 2 NRAs synthesized on a TiO2 seed layer/FTO substrate with the seeding thickness of (e,f) 0 nm, (g,h) 40 nm, (i,j) 68 nm, and (k,l) 95 nm Fig.2 SEM views of the surface and cross-sectional (inset) morphology of the TiO 2 NRAs synthesized on a TiO2 seed layer/FTO substrate with a growth duration of (a) 30 min, (b) 35 min, (c) 40 min, (d) 45 min, (e) 50 min, (f) 60min, (g) 2 h, (h) 4 h, (i) 6 h and (j) 7 h; inset of Fig.2(j) shows the photographs of the surface of the TiO2 NRAs with a growth duration of 7 h; correlation plots between the length (k), diameter (l) of the TiO2 nanorods and the growth time Fig.3 (a) XRD pattern of the TiO 2 NRAs synthesized on a TiO 2 seed layer/FTO substrate; (b-d) TEM views of the TiO2 nanorods with different resolutions, and inset of (d) shows the SAED pattern of the TiO2 nanorods; high-resolution XPS pattern of the (e) Ti2p and (f) O1s peaks for the TiO 2 NRAs; (g) UV-vis absorption spectra of the TiO2 NRAs on a TiO2 seed layer/FTO substrate; (h) transformed Kubelka-Munk function vs energy of the excitation source derived from UV-vis spectra; (i) PL spectra of the TiO2 NRAs on a TiO2 seed layer/FTO substrate, excited by 325 nm UV light Fig.4 (a) J-V curves of DSSCs based on TiO2 NRAs on a TiO2 seed layer/FTO substrate with different seeding thicknesses with/without TiCl4 treatment; (b) J-V curves of DSSCs assembled with 7 µm TiO2 NRAs as a function of the etching time; (c) J-V curves of DSSCs assembled with 14 µm TiO2 NRAs as a function of the etching time; (d) IPCE spectra of DSSCs assembled with 14 µm TiO2 NRAs as a function of the etching time Fig.5 Correlation plots between (a) electron transport time, (b) electron lifetime, (c) electron diffusion coefficient, (d) electron effective diffusion length, (e) electron collection efficiency and light intensity as a function of the etching time; (f) EIS Nyquist plots of DSSCs as a function of the etching time

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Samples

Voc / V

Jsc / mAcm-2

FF / %

PCE / %

20nm 40nm 68nm 20nm-TiCl4 40nm-TiCl4 68nm-TiCl4 7µm-etching-0h 7µm-etching-1h 7µm-etching-3h 7µm-etching-5h 14µm-etching-0h 14µm-etching-1h 14µm-etching-3h 14µm-etching-5h 14µm-etching-7h

0.65 0.68 0.70 0.79 0.67 0.69 0.75 0.74 0.73 0.72 0.70 0.69 0.68 0.67 0.66

4.84 4.75 3.71 4.26 4.93 5.21 5.49 5.06 5.74 6.89 3.90 6.53 10.21 9.74 9.53

55 57 59 66 59 58 59 57 68 67 58 60 65 64 65

1.71 1.84 1.53 2.21 1.93 2.07 2.43 2.11 2.84 3.29 1.56 2.68 4.46 4.16 4.08

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Peng Zhong received his Ph.D. in Electronic Science and Technology from Xi’an Jiaotong University in 2012. He worked at Nanyang Technological University of Singapore from 2011.02 to 2011.08 as a visiting scholar. He is now a lecturer in School of Advanced Materials and Nanotechnology at Xidian University, and is also with the Key Lab of Wide Band-Gap Semiconductor Materials and Devices. His current research interest focuses on new-type solar cell materials, PV device physics, advanced nanofabrication technology and electrochemistry.

Xiaohua Ma obtained his Ph.D. in Microelectronics and Solid-State Electronics from Xidian University in 2007 with Prof. Yue Hao. He then worked at School of Technical Physics of Xidian University. Now he is a full professor at School of Advanced Materials and Nanotechnology, and is also with the Key Lab of Wide Band-Gap Semiconductor Materials and Devices. His current research interest lies in semiconductor devices and reliability, wide-band-gap semiconductor materials, solid-state devices and microwave integrated-circuit design, new energy materials and devices.

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Xinpeng Chen is a third-year undergraduate student in Materials Science and Engineering of School of Advanced Materials and Nanotechnology at Xidian University. His current research interest lies in dye-sensitized solar cells and integrated circuit.

Rong Zhong is a third-year undergraduate student in Materials Science and Engineering of School of Advanced Materials and Nanotechnology at Xidian University. Her current research interest lies in dye-sensitized solar cells based on vertical ZnO nanorod arrays.

Xuehong Liu is a third-year undergraduate student in Electronic Science and Technology of School of Physics and Optoelectronic Engineering at Xidian University. Her current research interest lies in synthesizing ZnO nanorod arrays from solutions.

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Dongjie Ma is a fourth-year undergraduate student in Materials Science and Engineering of School of Advanced Materials and Nanotechnology at Xidian University. His current research interest lies in dye-sensitized solar cells based on rutile TiO2 nanorod arrays.

Maolin Zhang received his Ph.D. in Electronic Science and Technology from Xi’an Jiaotong University in 2011, and then worked at Xidian University. He is now an associate professor in School of Advanced Materials and Nanotechnology at Xidian University. His current research interest is new energy materials and devices, sensitive functional materials and devices and stealth composites.

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Zhimin Li received his Ph.D. in Materials Science and Engineering from Northwestern Polytechnical University in 2008. He worked at National Institute for Material Science (NIMS) of Japan from 2010.07 to 2011.07 as a postdoctoral research fellow. He is now an associate professor in School of Advanced Materials and Nanotechnology at Xidian University. His current research interest is new absorbing materials, and new energy materials and devices.

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Research Highlights

1. Increasing seeding thickness greatly improves uniformity of rutile TiO2 NRAs 2. Nucleation, plant-shaped growth and array-appearance formation are revealed 3. Each nanorod is closely-packed by many parallel tiny single-crystalline nanorods 4. Chemical etching substantially enhance electron collection in NRA-based DSSCs 5. A highest PCE of 4.46 % is achieved for DSSC with 14 µm NRAs etched for 3 h

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Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

*Graphical Abstract