Ultrarapid Sonochemical Synthesis of ZnO Hierarchical Structures

Article pubs.acs.org/cm

Ultrarapid Sonochemical Synthesis of ZnO Hierarchical Structures: From Fundamental Research to High Efficiencies up to 6.42% for Quasi-Solid Dye-Sensitized Solar Cells Yantao Shi,†,‡ Chao Zhu,‡ Lin Wang,‡ Chunyu Zhao,† Wei Li,‡ Kwok Kwong Fung,‡ Tingli Ma,*,† Anders Hagfeldt,§ and Ning Wang*,‡ †

State Key laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian, 116024, China Department of Physics and the William Mong Institute of Nano Science and Technology, The Hong Kong University of Science and Technology, Hong Kong, China § Department of Physical and Analytical Chemistry, Uppsala University, Box 259, SE-751 05 Uppsala, Sweden ‡

S Supporting Information *

ABSTRACT: Zinc oxide (ZnO) hierarchical structures (HSs) have recently demonstrated notable photochemical and photovoltaic performances attributed to their nano/micro combined architectures. In this study, ZnO HSs were synthesized at room temperature using ultrarapid sonochemistry. This novel approach can effectively overcome deficiencies in the synthesis via traditional direct precipitation by promoting nucleation and accelerating diffusion. Only 15 min was needed to complete the formation of highly crystallized and uniformed HSs consisting of interconnected monocrystalline nanosheets using sonochemistry. The formation of HSs through in situ observations was interpreted using a new mechanism based on oriented attachment and reconstruction. In the nonequilibrium synthesis system, thicker, porous, and coarse crystallized ZnO sheets were first constructed via oriented attachment of small-sized nanocrystals. After reconstruction, untrathin, integrated, and monocrystalline nanosheets were obtained. According to the two-dimensional nanosheets to three-dimensional HSs, the formation was much more sophisticated because repeated and parallel heterogeneous oriented attachments with reconstructions dominated the final morphologies of the HSs. The relationships between synthetic conditions and HSs structures were established. Based on the photoanodes in dye-sensitized solar cells (DSCs), the performances of these differently structured HSs were tested. HSs with densely assembled nanosheets exhibited better performances in photoelectric conversions. Systematic investigations were also carried out by selecting two representative HSs to demonstrate the critical factors governing the optical and electrical properties of photoanodes. Finally, under AM 1.5 and 100mW cm−2 light irradiation, high photoelectric conversion efficiencies of up to 6.42% were achieved. These results established a new record for quasi-solid ZnO-based DSCs. KEYWORDS: sonochemical synthesis, ZnO hierarchical structure, photovoltaic, quasi-solid, dye-sensitized solar cell

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(DSCs).16,17 However, the massive interparticle boundaries in these aggregates may hinder the transport of photogenerated electrons.18,19 Thus, novel HSs, in which electrons can travel freely, are highly desired. ZnO, one of the most commonly used photoanode material, has a similar band structure with TiO2, but its electron mobility is much higher. ZnO is much easier to crystallize and construct into various structures from zero dimensional (0D, nanoparticles) to one-dimensional (1D, nanowires or nanorods), two-dimensional (2D, nanosheets or nanoribbons), and three-dimensional (3D, HSs). Among the recently reported ZnO HSs, those composed

INTRODUCTION

Hierarchical structures (HSs) or superstructures, in which substructures (e.g., nanoparticles, nanorods, or nanoplates) are involved, have been used intensively because of their ability to demonstrate dual and multiple functions and remarkable performances in photochemical,1−4 photovoltaic,5−9 and other device applications.10,11 The nanosized particles of these spherically shaped nanocrystalline aggregate HSs can provide a large surface area to ensure sufficient sites for photochemical reactions, whereas their microsized dimensions can reflect the incident photons to prolong their traveling distance to enhance light-capture.12−15 Previous studies have indicated that nanocrystal aggregates of oxide semiconductors such as titanium dioxide (TiO2) and zinc oxide (ZnO) are effective in improving the photovoltaic performances of dye-sensitized solar cells © 2013 American Chemical Society

Received: January 18, 2013 Revised: February 28, 2013 Published: March 1, 2013 1000

dx.doi.org/10.1021/cm400220q | Chem. Mater. 2013, 25, 1000−1012

Chemistry of Materials

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into 1 L of NaOH at a given rate. Once this process was finished, ultrasound irradiation and mechanical stirring stopped 30 s later until the white suspension became homogeneous. One hour later, the assynthesized white precipitates were washed with absolute alcohol, filtered out, and finally dried in an oven at 100 °C. Fabrication of ZnO Photoanodes. First, ZnO nanoparticles (10% in total weight and 20 nm in size) were blended with ZnO HSs to increase the mechanical strength of our photoanodes. Then, ZnO paste was prepared by adding 1 g of blended ZnO powder into 3 g of a mixture of absolute alcohol and deionized water (volume ratio = 2:1). After continuous shaking and ultrasonic dispersion, the ZnO paste was dropped on the FTO glass and then doctor-blade technique was used to fabricate photoanode film with the thickness being controlled by adhesive tape. After drying in air, the photoanode film was heated at 200 °C for 90 min in an oven and then sensitized with a mixed solution (acetonitrile/tert-butyl alcohol, 1:1 in volume) containing 0.3 mM N719 at 45 °C for 2 h. Solar Cell Fabrication. The PEO-based polymer gel electrolyte that consists of 0.1 M LiI, 0.1 M I2, 0.6 M DMPII, 0.45 M NMBI, MePN, and PEO was prepared according to our published research work.31 The counter electrodes were fabricated according to previous work from other group.32 When fabricating the DSC device, first, the polymer gel electrolyte was heated up to 90 °C to decrease its viscosity and then coated on the dye-sensitized ZnO photoanode film. Subsequently, a chemically platinized counter electrode was covered on the ZnO photoanode film with their space being controlled by a 50 μm thicked adhesive tape. Finally, the DSC devices were baked at 90 °C for 20 min to ensure the penetration of PEO gel electrolyte into the porous photoanode. In addition, for each sample, we fabricated three DSC devices, taking the average as the final result in the manuscript. Characterization. The structure of ZnO HSs was characterized by scanning electron microscopy (SEM, RAITH, e-LiNE) and high resolution transmission electron microscopy (HRTEM, JEOL, JEM2010F). Specific surface areas of the samples were characterized by Coulter SA 3100 surface area analyzer. Porosity was calculated through measuring the weight and volume of the photoanode film. Photovoltaic performance of the DSCs was measured by a KEITHLEY 2420 source meter under AM 1.5, 100 mW cm2 irradiation (Xenon lamp solar simulator, Newport, Class A) with light intensity being calibrated with a standard crystalline silicon solar cell. The total active area of our DSC device was 0.25 cm2. A UV light spectrophotometer (Lambda 20) was used to measure dye-loading and diffuse-reflectance spectra. Dyeloading tests were carried out by first desorbing dye molecules with 0.05 M NaOH aqueous solution, and then calculating from UV−vis spectra (Lambda 20). Electrical properties of the photoanode film were characterized by measuring the electrochemical impedance spectra (EIS, CHI 660D) of the DSCs in dark.

of nanosheets have exhibited superior optical and electrical performances. Compared with ZnO nanocrystal aggregates, nanosheet-based HSs are more advantageous in electron transport.20,21 In our previous study, the electron diffusion coefficients (Dn) of the photoanodes fabricated with nanosheetbased HSs in DSCs were 1 order to 2 orders higher than that of photoanodes fabricated with nanoparticles.22−24 Moreover, ZnO HSs with similar architectures also demonstrated excellent performances in the fields of photocatalysts,25,26 gas sensors,27 and so on. Therefore, this specially structured HSs should be further studied for their high potential in photochemical or photovoltaic applications. Nanosheet-based HSs can be synthesized through the following approaches: hydrothermal,20,28 chemical bath deposition (CBD),21,29 electrodeposition,30 direct precipitation,22,24 and so on.25 Among these methods, one-step direct precipitation is the most simple approach. However, the structure and homogeneity of HSs fabricated by direct precipitation are difficult to control. In this study, nanosheet-based HSs were prepared by introducing ultrasound irradiation into the direct precipitation process.23 The results showed that both the structure and the subsequent performances of the HSs in quasi-solid DSCs had been largely improved with the assistance of ultrasound. Thus, sonochemistry can be regarded as another convenient approach to fabricate ZnO HSs. The advantages of sonochemistry in the synthesis and formation processes of HSs should be elucidated, and the relationships between the structures of HSs and their synthetic conditions should be established to systematically understand this novel approach. The results of this study will also help reveal the critical factors governing the photovoltaic performances of DSCs. As it is revealed in this work, sonochemistry is a powerful and effective approach for room-temperature synthesis of wellconstructed ZnO HSs with multiple functions. Compared with direct precipitation, sonochemistry is efficient in accelerating nucleation, promoting diffusion, and shaping the structures of HSs. The ZnO HSs, composed of interconnected and monocrystalline nanosheets, were constructed within only 15 min in an aqueous solution system without any templates. In situ observations were carried out and a new mechanism to interpret the formation processes was proposed to better understand the construction process of these HSs. A series of HSs with various structures were controllably prepared and tested as photoanodes in quasi-solid DSCs under different synthetic conditions. HSs with densely assembled nanosheets performed much better than the others. The optical and electrical properties of two representatives were systematically compared. The experimental data and mechanism analysis revealed the critical factors dominating the performances of the DSCs. Finally, under AM 1.5 and 100 mW cm2 light irradiation, high photoelectric conversion efficiencies of up to 6.42% were achieved for quasisolid DSCs.

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RESULTS AND DISCUSSION In Situ Observation of the Formation Processes. Several studies carried out in situ observations to reveal the formation mechanisms of the individual nanosheets.33,34 Recent studies attempted to explain these highly complicated formation processes in response to the advent and widespread applications of nanosheet-based HSs.35,36 However, to date, the mechanism through which a large number of 2D nanosheets are assembled into one 3D HS remains unclear. In the current study, scanning electron microscopy (SEM) was used in combination with high resolution transmission electron microscopy (HRTEM) to elucidate the formation processes of the HS. For in situ observations, the intermediate products at different stages were selected, and the formation processes were quickly terminated by diluting them in a viscous solvent (PEG, Mw = 400 g mol−1) at 0 °C. In this nonequilibrium synthesis system, the formation of ZnO is initiated by mixing 1 L of Zn(NO3)2 (0.1 M) solution with 1 L of NaOH (0.5 M), which can be expressed as follows:

EXPERIMENTAL SECTION

Materials and Chemicals. FTO glass (15 Ω/sq, 2.2 mm in thickness), Zinc nitrate hexahydrate (Zn(NO3)2, 99%), Sodium hydroxide (NaOH, reagent grade), Polyethylene glycol (PEG, Mw = 400 g mol−1), Poly(ethylene oxide) (PEO, Mw = 2 × 106 g mol−1), 1, 2dimethyl-3-propyl imidazolium iodide (DMPII, 99%), Lithium iodide (LiI, 99%), Iodine (I2, analytically pure), 3-methoxypropionitrile (MePN, 99%), N-methyl -benzimidazole (NMBI, 99%), Ruthenizer 535-bisTBA (N719). All chemicals were used without any purification. Preparation of HSs. Under ultrasound irradiation (Brasonic 3510EMT, 100W) and mechanical stirring, 1 L Zn(NO3)2 solution was added 1001



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Figure 1. SEM (the first and second rows show the low and high magnifications, respectively) and HRTEM (the third row) images of the samples taken out at different reaction stages. Each column corresponds to one sample: (a), (b), and (c) 3 min; (d), (e), and (f) 6 min; (g), (h), and (i) 8 min; (j), (k), and (l) 15 min. All scale bars in the inset of the first row are 1 μm.

Scheme 1. Schematic Diagram of Formation of ZnO Nanosheets and Hierarchical Structure via Oriented Attachment of SmallSized Nanocrystals and Reconstruction

Zn 2 + + 4OH− → Zn(OH)4 2 − → ZnO + H 2O + 2OH−

than the imtermediates of the prepared HSs. Xia et al. emphasized in their work that small nanocrystals are prerequisite for the formation of 2D nanosheets in a solution system. A few primary architectures of HSs can be observed with the increase of the reaction time to 6 min, as shown in Figure 1d. In addition, only a few nanosheets were constructed within such a short period for each immature HS. Figure 1e clearly shows the morphology of the porous nanosheets, whereas Figure 1f illustrates the worse crystallized structure of the nanosheets, which probably resulted in the wide distribution of the micropores and lattice dislocations at this stage. The exposed crystal planes are denoted as (11−20) of the Wurtzite-structured ZnO, as shown in Figure 1f. More HSs with larger sizes formed

The molar ratio of OH− to Zn2+ is 5:1. The excess OH− anions in the solution favors the dehydration of Zn(OH)42−, thereby promoting nucleation and crystallization. Formation processes of the nanosheet-based HSs are illustrated in Figure 1. The SEM images with low and high magnifications are exhibited for each sample. The morphologies of the sample at the early stage (t = 3 min) are all small-sized ZnO nanoparticles with a diameter of approximately 15 nm, as shown in Figures 1a and 1b. HRTEM images further confirmed that these nanoparticles are polycrystalline composed of randomly assembled nanocrystals, as shown in Figure 1c. These nanocrystal aggregates resulted from the sampling method, rather 1002



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and growth. Chen. et al. observed several spherical aggregates of nanocrystals during the initial stages, and nanosheet-based HSs were finally formed based on these solid “cores”.36 However, in this study, no solid aggregates were formed from the beginning to the end of the synthesis. In our previous study, the inner parts of the prepared HSs were highly porous.23 In this study, the proposed mechanism can be considered as the consequence of a series of oriented attachments demonstrated in Scheme 1. After the primary attachment of nanocrystals into one nanosheet, the secondary continues, then multiple attachments follow until the final assembling process is finished. The overall assembling processes from 2D to 3D can be understood in terms of heterogeneous oriented attachments, a new concept similar with heterogeneous nucleation,40 but not exactly identical. The former is always used to interpret the nucleation and subsequent growth behaviors, whereas the latter emphasizes nanocrystal attachments and reconstructions. According to Scheme 1, the orientations of the nanosheets on HSs is strictly confined by the crystal structure of ZnO. In addition, the angles between some nanosheets on the currently and previously prepared HSs were irregular to some extent. Therefore, the heterogeneous attachments are not strictly confined by crystal orientations, and the spatial arrangement of the nanosheets on HSs is both regular and irregular. Advantages of Sonochemistry in Synthesis of ZnO HSs. This section demonstrates the advantages of sonochemistry in the synthesis of ZnO HSs by preparing four samples under different conditions (Table 1). Sample A was synthesized

after 2 min (t = 8 min), as shown in Figure 1g. At this stage, the morphology of the HS has varied significantly. The interconnected nanosheets were densely assembled, as shown in Figure 1h. Compared with the observation on the second stage (t = 6 min), the thicknesses of these nanosheets were largely decreased. The reduction in thickness in the formation processes of HSs was calculated by Weller et al. before,33 whereas it is the first time finding and reporting this interesting behavior. The crystallization of the nanosheets improved significantly, in which even a spot of lattice defects were still detectable, as shown in Figure 1. No other product was formed, except for the enlarged HSs when the reaction time was prolonged to 15 min, as shown in Figure 1j. This result indicates that the nanocrystals detected in the last three stages were totally assembled into nanosheets. Figure 1k demostrates the details of the nanosheets on HS. The evolution of their morphology and thickness are obvious. As shown in Figure 1k, these ultrathin nanosheets are bendable, smooth, and highly interconnected. The HRTEM image further reveals that the nanosheets on HSs at this stage have a perfect monocrystalline structure, which signifies the end of the assembly process, as shown in Figure 1l. Although ZnO HSs can be prepared by various strategies, ZnO HSs preparation via ultrarapid synthesis has never been reported. Generally, the oriented attachment of small-sized nanocrystals dominates the formation of 2D nanosheets.35,37,38 According to previous studies, the driving forces to assemble nanocrystals into nanosheets are anisotropic hydrophobic attraction and electrostatic interactions derived from dipole moments or surface charges.37,38 In our synthesis, no additional organic guiding molecule was needed to offer hydrophobic groups that drive this oriented assembling process. Therefore, electrostatic interaction is the main driving force that governs the assembly process. Scheme 1 was provided in this section to better understand the formation mechanism. Wurtzite ZnO has a typical anisotropic structure, especially along c and -c axes, and a totally different charge distributions on the (0001) and (000−1) planes. Thus, the electrostatic interactions were strong enough to assemble these nanocrystals orientedly along [0001]. For the nanocrytals, structure matching is another prerequisite for the formation of nanosheets. Tang et al.38 emphasized that no steric constraints should be present between the nanocrystals. Banfield et al.39 stated that the matching of crystal plane would ensure the structural continuity across the interface. Hence, crystal plane coupling along [10−10] can be regarded as another driving force for oriented attachment. During the attachment process, randomly oriented nanocrystals initially aggregate, and then rotate to ensure that their orientations are parallel in 3D. At this stage, these nanosheets are “coarse” because of worse crystallization. Subsequently, the nanosheets will experience reconstruction, an important process that has never been mentioned in previous studies related to the formation of nanosheet-based HSs. In the current study, reconstruction can be understood as redissolution, diffusion, and accommodation of the atoms.33 Internanocrystal boundaries can be eliminated, and porous polycrystalline nanosheets will fuse into monocrystalline with a smooth surface after continuous reconstruction, as shown in Figures 1j and 1l. Finally, the thickness of the nanosheet will be largely decreased. The construction mechanism from 2D nanosheets to 3D HS is also important for the formation of ZnO nanosheet-based HSs. Previous studies simply ascribed the formation of ZnO nanosheet-based HSs to these consecutive processes composed of aggregation, oriented attachment, heterogeneous nucleation,

Table 1. Synthetic Parameters of the Four Samples in Figure 2 sample

COH− (M)

CZn2+ (M)

sonochemistry

A B C D

0.50 0.50 0.50 0.50

0.10 0.10 0.20 0.20

No Yes No Yes

without ultrasonic assistance, whereas its reactant concentrations were kept similar to that of sample B, which was used for in situ observation in the previous section. Without ultrasonic irradiation, the intermediate products of this sample at t = 15 min were removed. Figure 2a shows an immaturate HS in sample A, indicating that the construction was in process. Several HSs can be observed within such a large scope with a smaller magnification, as shown in Figure 2b. This finding indicates a relatively slow formation process when using direct precipitation. In this study, the different formation speeds between ultrasonicassisted and direct precipitation is probably derived from initial nucleation process. Generally, ultrasound irradiation can generate tremendous amounts of energy through the effect of acoustic cavitation in the solution.41 Thus, the nucleation process can be largely accelerated with the assistance of ultrasound, which in turn facilitate the formation process of HSs. However, a longer time (ca. 40 min) was needed to complete the formation of HSs without ultrasound irradiation, as shown in Figure 2c. In addition, HSs in sample A were nonuniform, with sizes spanning from 3 to 13 μm. Some flattened rods stretching out of the HSs in sample A and other structures resembling overlapped leaves were also observed, as shown in Figure 2c and Supporting Information, Figure S1. The synthesis of nanostructures through direct precipitation is still challenging because it always suffers from local inhomogeneous diffusion, nucleation, and growth. Hence, different structures can be obtained in sample A. On the 1003



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attachment using the external energy provided by ultrasound. Furthermore, although we do not have direct evidence, ultrasonic irradiation is believed to also be favorable for reconstruction through accelerating ZnO redissolution. The synthesis of sample D matches the concept of “Atom Economy”,42 whereby more ZnO HSs with high qualities can be obtained without increasing the amount of NaOH. In general, sonochemistry has been proven an effective approach for ultrarapid and controllable synthesis of nanostructures. Synthesis of ZnO HSs under Different Conditions. The structures of the products are very sensitive to synthetic conditions, such as reactant concentration, temperature, shapeguiding materials, precipitation speed, and mechanical stirring. In this study, we focus on reactant concentration, synthetic temperature, and precursor addition rate. Three groups of samples were prepared and named as GC-X, GT-X, and GP-X, where the subscripts “C, T, P” represent the concentration, temperature, and precursor addition rate, respectively. Four samples were prepared for each group. The detailed parameters of the synthetic conditions of the prepared samples are listed in Table 2. Table 2. Synthetic Parameters of the Three Groups of Samples Illustrated in Figure 3

Figure 2. SEM images of ZnO samples obtained at different conditions. (a) and (b). In-situ observation of sample A at t = 15 min; (c) sample A; (d) sample B; (e) sample C; (f) sample D.

contrary, HSs in sample B synthesized by sonochemistry are highly uniform and have largely reduced dimensions ranging from approximately 2 to 5 μm. This effect can be attributed to the strong blast and high-speed microjet produced by acoustic cavitation that served as powerful assistance for effective ionic diffusion and nanocrystal dispersal. The excess OH− in the solution is one important driving force for ZnO formation. The products synthesized through direct and ultrasonic-assisted precipitation were compared by keeping the concentration of OH− unchanged while doubling that of Zn2+. A specially constructed architecture was obtained through direct precipitation and was designated as sample C, in which the substructures are not 2D nanosheets, as shown in Figure 2e. The HS in sample C had a larger size of approximately 6 μm. Interestingly, sample D, prepared with the same concentrations, but through ultrasonic-assisted precipitation, exhibited the desired nanosheet-based HSs with their sizes confined to ∼ 2 μm, as shown in Figure 2f. This finding again confirms the uniqueness and importance of sonochemistry. The driving force for dehydration of Zn(OH)42− into ZnO is weakened in a condition with high concentration of Zn2+ but lacking excessive OH−. Thus, a slow and inhomogeneous nucleation, as well as random aggregation are thought to be responsible for the failure of nanosheet-based HSs formation by direct precipitation. The slow dehydration process resulted in the massive accumulation of Zn(OH)2 flocculation to OH−. Thus, the surface charges on the early formed ZnO nanocrystals were not enough for effective electrostatic interaction and oriented attachment. For the synthesis of sample D, the advantages of sonochemistry mentioned previously can be exerted effectively. The dehydration process of Zn(OH)42− will be significantly accelerated and redundant OH− will be released promptly for use in oriented

a

sample

T (°C)

COH− (M)

CZn2+ (M)

SP (mL s−1)a

GC-1 GC-2 GC-3 GC-4

22 ± 1 22 ± 1 22 ± 1 22 ± 1

0.25 0.50 2.50 0.50

0.05 0.10 0.50 0.05

25.0 25.0 25.0 25.0

GT-1 GT-2 GT-3 GT-4

8±1 45 ± 1 55 ± 1 72 ± 1

0.50 0.50 0.50 0.50

0.10 0.10 0.10 0.10

25.0 25.0 25.0 25.0

GP-1 GP-2 GP-3 GP-4

22 ± 1 22 ± 1 22 ± 1 22 ± 1

0.50 0.50 0.50 0.50

0.10 0.10 0.20 0.20

142.8 3.3 8.8 3.3

SP is the precursor addition rate.

The first three samples in group 1 were prepared at a fixed temperature (∼ 22 °C), whereas the concentrations of NaOH and Zn(NO3)2 solutions increased proportionally (5:1) from GC-1 to GC-3. The SEM images clearly demonstrated the structural evolution of the HSs in these three samples synthesized with different reactant concentrations, as shown in Figure 3. From GC-1 to GC-3, the sizes of the HSs decreased from ∼4 μm to ∼1 μm. The assembling densities and orientations of the nanosheets on the HSs also varied significantly. HSs in sample GC-1 appeared as spheres, with their densely assembled, porous, and spokewise nanosheets stretching from the center to the outside (Supporting Information, Figure S2). For GC-2, the HSs were cylindrical because most of their nanosheets are assembled along a central axis. As shown in the SEM image of GC-3, the nanosheets became smoother and sparsely arranged on the HSs with a much higher concentration. For the synthesis of sample GC-4, the concentration of Zn2+ was reduced by half compared with GC-2. The HSs sizes of GC-4 and assembling density of their nanosheets both decrease and some parallel nanosheets are obviously assembled on the “flat trunks” along specific orientations, as shown in Figure 3. This evidence can well 1004



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Figure 3. SEM images of three groups of ZnO samples, GC-X, GT-X, and GP-X, for which the investigated issues are influences of reactant concentration, synthetic temperature, and precursor addition rate on the structures of products.

at high ionic concentration. This mechanism can also be used to explain the different size distributions of HSs in GC-1 and GC-3. Figure 3 illustrates the influence of the synthetic temperature on the morphologies of ZnO HSs. In group 2, sample GP-1 was prepared at 8 °C. The nanosheets on HSs were arranged in a highly oriented manner along a central axis. This cylindrical shape was demonstrated by Gc-2. Gc-2 can be also cited as a reference because other conditions were the same as those for GT-X, except for temperature (∼ 22 °C). In GT-1, the nanosheets were too thin that they easily deform even under the irradiation of the electon beam used for the SEM observations. The synthetic temperature for GT-2 was 45 °C. The products were spheric HSs with randomly arranged nanosheets. The increased thickness of the nanosheets is another important structural variation (Supporting Information, Figure S3). GT-3 was synthesized at 55 °C; the characteristic of the nanosheets on the HSs was diluted significantly, except for the increased thickness (Supporting Information, Figure S4). GT-4, the last member of this group, was prepared at 72 °C. The architectures of its HSs were totally different from those of the others. The substructures in GT-4 were semispindles rather than nanosheets. The evolution of the substructures from ultrathin nanosheets to semispindles with increasing synthetic temperature was observed. The prerequisite for the formation of 2D nanosheets is the generation of nanocrystals with extremely small sizes. Smaller nanocrystals are more desirable in the formation of nanosheets. The size of the nanocrystal has a strong relationship with the thickness of the final nanosheet. Generally, favorable thermodynamic conditions can offer additional energy for the dehydration of Zn(OH)42− into ZnO, which in turn accelerates the nucleation process and facilitates the growth of nuclei into nanocrystals. At lower temperature (8 °C), the sizes of the asgenerated nanocrystals were extremely small (Supporting

support our conjecture on the formation mechanism of HSs, by which the heterogeneous attachments of nanocrystals into nanosheets proceed orientedly. Hence, the ionic concentration is very important in shaping and aligning these nanosheets on HSs, including their integrity (or porosity), spatial orientation, and assembling density. The effects of reactant concentrations on the formation of ZnO nanosheets can be interpreted in terms of kinetics and thermodynamics. Higher reactant concentrations can promote the reactions for rapid synthesis, and favor nucleation and reconstruction by providing a driving force to overcome the energetic barrier. In the proposed synthesis of ZnO HSs, the crystallization of the nanosheets was improved from porous nanosheets on GC-1 to integrated ones on GC-3 or GC-4 with increasing reactant concentrations. The assembling density and orientation of the nanosheets are also crucial for synthesis. As mentioned previously, the assembly of nanosheets into 3D HSs is based on the mechanism of heterogeneous oriented attachments. Based on the irregular assembly on GC-1 to regular assembly on GC-3 or GC-4, the higher ionic concentration in the aqueous solution can avoid the randomly heterogeneous oriented attachments. According to the theory of “heterogeneous nucleation”, we propose that the roughness of the nanosheets has a critical role because the coarse surface during the reconstruction process is more favorable to subsequent heterogeneous oriented attachments. For GC-1, the reconstruction of the nanosheets from porous polycrystalline to smooth monocrystalline needs a longer time than that of others. Parallel with this “long period”, the heterogeneous oriented attachments proceed easily based on the early assembled, immature, and coarse nanosheets. For GC-3 and GC-4, the reconstruction process of the early nanosheets is too rapid that only a few nanosheets can be formed through heterogeneous attachments 1005



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Figure 4. (a), (b), and (c) J−V curves of the quasi-solid DSC devices based on GC-X, GT-X, and GP-X, respectively; (d) Distribution graph showing the PCEs of all DSC devices based on different samples in our experiment.

nanosheets on the HSs of Gc-2 are more regular because they are aligned along a central axis. Another important effect of the SP on the structure of HSs is illustrated by comparing the SEM images of Gp-3 and Gp-4. The Zn2+ concentration for these two samples are twice that of Gp-1, as shown in Table 2. The nanosheet-based HSs cannot be obtained with this concentration through direct precipitation. The SP for Gp-3 is 8.8 mL s−1, HSs obviously have a hollow structure, and the nanosheets are highly porous (Supporting Information, Figure S7). HSs with relatively small sizes and smooth nanosheets can be obtained by decreasing the SP to 3.3 mL s−1. The key effects of excess OH− ions, such as promoting nucleation, facilitating oriented attachment, and reconstruction, in the proposed synthesis should be discussed again to understand the influence of SP on the morphology of HSs. Massive OH− ions are occupied in the forms of Zn(OH)2 or Zn(OH)42− by fast addition of precursor in sample Gp-1. Hence, the driving force provided by surface charges is limited for oriented attachment. Massive nanocrystals will densely and randomly aggregate together into large-sized HSs. For Gp-2, the desired OH− can be released from Zn(OH)2 or Zn(OH)42− promptly for the subsequent requirements in constructing nanosheet-based HSs when lowering the SP, thereby effectively

Information, Figure S5). Thus, the thickness of the nanosheet on HSs for GT-1 was thinner than that of the others. At higher temperatures, such as 55 °C or higher, the sizes of the nanocrystals were larger. Thus, thicker nanosheets can be assembled for GT-2 and GT-3. Small sized nanocrystals cannot possibly form at extremely high synthetic temperature. Thus, 2D nanosheets were left for GT-4. TEM images (Supporting Information, Figure S5) of the intermediate product from sample GT-4 at the primary stage (t = 1 min) were provided to prove our hypothesis. Within such a short time, the overall formation processes for GT-4 were finished. Previous studies have fabricated similar ZnO HSs through hydrothermal approaches; however, their synthesis require organic capping materials (templates or shape-guiding materials).20,28 Therefore, high synthetic temperature without organic additives is unfavorable for the fabrication of nanosheet-based structures. The precursor addition rate (SP) for group 3 also has an important role on the final structures of the HSs, as shown in Figure 3. The SP values for Gp-1 and Gp-2 were 142.8 mL s−1 and 3.3 mL s−1, respectively, which differ greatly. The SEM images reveal that HSs in Gp-1 obviously have much larger sizes compared with those in Gp-2, as shown in Figure 3. SP can also affect the arrangement of the nanosheets. The orientations of the 1006



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Table 3. Detailed Parameters of GC-X, GT-X and GP-X Based DSC Devices photovoltaic performance sample

specific area (m2 g−2)

film thickness (μm)

dye-loading (10−7 mol cm−2)

Voc (V)

Jsc (mA cm−2)

FF (%)

η (%)

GC-1 GC-2 GC-3 GC-4

22.95 26.60 22.40 24.41

22.5 20.1 20.7 23.7

1.34 1.21 0.86 0.94

0.690 0.710 0.736 0.717

12.16 11.99 5.69 8.96

68.0 69.1 68.0 69.6

5.70 5.88 2.85 4.47

GT-1 GT-2 GT-3 GT-4

16.45 17.13 18.54 12.64

18.7 23.3 24.2 19.4

0.87 0.98 0.91 0.76

0.646 0.727 0.727 0.725

7.73 8.17 7.20 6.54

72.2 70.7 68.0 69.2

3.60 4.20 3.50 3.28

GP-1 GP-2 GP-3 GP-4

22.40 25.34 24.33 25.40

26.5 24.7 23.1 23.6

1.23 1.38 1.37 1.45

0.697 0.692 0.692 0.695

9.30 12.26 11.21 12.21

72.3 68.7 72.1 68.9

4.69 5.82 5.59 5.85

All photoanode films were fabricated using 150 μm thicked adhesive tape as the spacer; the film thicknesses were determined by SEM imaging the cross-section of the photoanodes; η is the photoelectric conversion efficiency, PCE for short. a

loading and subsequent poor photovoltaic performances. For GT-4, the semispindle based HSs, the short-circuit current (Jsc), and PCE of the DSC are only 6.54 mA cm−2 and 3.28%. The samples in group GP-X demonstrate considerable photovoltaic performances in DSCs. For the large-sized HSs in GP-1 synthesized with ultrasound irradiation for only 7 s, the corresponding DSC device exhibits a PCE of 4.67%. In such a too densely assembled HS, the pore filling of the polymer gel electrolyte is likely insufficient. Structural improvements from GP-1 to GP-2 can elicit an increased PCE of 5.82%. For GP-3 to GP-4, their structures vary significantly, but only slight changes in PCE are observed for their DSC devices, from 5.59% to 5.85%. We graphically illustrated the PCEs of the samples in Figure 4d to easily compare their photovoltaic performances. Five samples yielded PCEs above 5.50%, and they all share one characteristic, which is a high assembling density of the nanosheets on HSs. GC3 has a comparable specific area to that of highly prepared samples such as GC-1, GP-1, and so on. However, the number of nanosheets on each HS is extremely limited, and its performance in DSC is far below that of others. The same conclusion is also available to interpret the lower PCE of GC-4 based DSC devices. Recent studies have reported the synthesis and performance of the ZnO nanosheet-based HSs in DSCs. Although the electrolyte used in this study is a highly viscous polymer gel electrolyte, the PCEs of the prepared ZnO nanosheet-based HSs are much higher than those reported previously. This study systematically compared and illustrated the photovoltaic performances of a series of differently structured ZnO HSs. The results of this study will guide future designs and syntheses of novel HSs to realize further photovoltaic improvements. Finally, the key factors controlling the photovoltaic performances of the prepared samples in DSCs should be illustrated in detail. Systematic Comparisons between Two Representatives. GC-2 and GC-3, which have comparable specific areas but notable differences in arrangements of nanosheets and photovoltaic performances, were selected as the representatives for systematic comparisons and further investigations. The contents cover the optical and electrical characterizations, as well as mechanism explanations. On the basis of these two samples, we fabricated two photoanode films with thicknesses of 33.2 and 32.8 μm for subsequent characterizations and discussions. Figure 5 exhibits the SEM images of the cross sections of the films

avoiding the random aggregation behaviors of nanocrystals. Therefore, prolonging the precursor addition time (decreasing SP), with the assistance of ultrasound irradiation, can maintain the concentration of OH− to drive the formation of HSs. This mechanism can also be used to explain the morphology differences between Gp-3 and Gp-4. Soulantica et al. have recently indicated that nanocrystal polymorphism can be obviously affected by SP. Their findings are similar to the results of our study.43 We attempted to prepare nanosheet-based HSs via direct precipitation using the same reactant concentrations with Gp-3 or Gp-4. However, no nanosheet-based HSs were obtained by altering SP in the synthesis (Supporting Information, Figure S8). Photovoltaic Performance of the HSs in Quasi-Solid Dye-Sensitized Solar Cells. We fabricated the HSs into photoanodes for quasi-solid DSCs to compare the performances in photovoltaic devices. Figures 4a to 4c demonstrate the J−V curves of each group. Table 3 summarizes the specific areas, film thicknesses, and photovoltaic parameters. The samples in the first group, GC-1 and GC-2, achieved high photoelectric conversion efficiencies (PCEs) of 5.70% and 5.88%, respectively, as shown in Figure 4a. However, without any obvious reduction in specific area, the DSC device based on GC-3 exhibited a low PCE of only 2.85%. The GC-3 based photoanode did not adsorb enough dye molecules compared with GC-1 or GC-2, as shown in Table 3. Similarly, GC-4 has a comparable specific area with the other three members in this group, but the PCE of its DSC device is still lower. Therefore, specific area is not the only factor governing the dye-loading of the photoanode and its subsequent photovoltaic performance. Further studies are needed to illustrate the inherent mechanism. Compared with GC-1, the samples synthesized at higher reactant concentrations demonstrate higher open circuit voltages (Voc) in their corresponding DSC devices, which may be related to the band structure of the samples or charge recombination at the interface of ZnO/ electrolyte. Compared with group GC-X, the overall data of GT-X based DSC devices do not exhibit further photovoltaic improvements. Instead, this group presents much lower PCEs, whether for low temperature or high temperature synthesized samples. Table 3 shows that the specific areas of the HSs in this group are relatively deficient. Thus, it is easier to understand their insufficient dye1007



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Figure 5. SEM images of the cross sections of the GC-2 (a) and GC-3 (b) based photoanode films on FTO glass.

fabricated on FTO glass. The photovoltaic performances of quasi-solid DSCs based on these two samples are illustrated in Figure 6. The detailed parameters are summarized in Table 4. Figure 7. Reflectance spectra of the films with different structures and thicknesses.

GC-3 based. We infer that the broad interspaces among these large-sized HSs (see Figure 5a) cause the loss of incident photons. No obvious channels for the loss of photons was observed in the GC-3 based photoanode, instead, the randomly distributed nanosheets can be used as “mirrors” to reflect the incident light many times to largely prolong the traveling distance of the photons. Moreover, unlike the NP-based spectrum, where a sharp decrease appears with the increase of wavelength, the spectra of the GC-2 or GC-3 based films indicate that these HSs can effectively scatter the sunlight in the longwave range. This advantage of the HSs can well match the future demands of the DSCs based on infrared or near-infrared dyes. Under working conditions, a series of complex, continuous physical, and chemical processes, including (i) injection of the photogenerated electrons into the conduction band of the semiconductors, (ii) transport of the electrons across the porous film, (iii) subsequent collection by transparent conductive oxide (TCO) glass, (iv) electron transfer at the semiconductor/dye/ electrolyte and Pt/electrolyte interfaces, and (v) shuttles of the redox couples in the electrolyte, occur inside DSCs. In this section, electrical properties, such as charge transport, recombination, and chemical capacitance, were characterized and analyzed through the well-established technique of electrochemical impedance spectra (EIS).44 The mechanism of our investigation is based on the widely used diffusion-recombination model.45 In the dark, the electrons are injected from the TCO into the ZnO conduction band upon forward bias. The ZnO network across the film is then charged by electron accumulation. Finally, some of these injected electrons are lost by the charge recombination with I3− ions in the electrolyte.46 In this study, we will systematically discuss the charge recombination, electron transport, chemical capacitance, and so on through fitting our EIS data. Under higher bias, the electron transport behavior will be overlapped by the charge recombination process in EIS. Thus, we carried out the measurements with lower bias (−0.60 V to −0.50 V).47,48 Figure 8a shows the typical Nyquist plot from the EIS characterization of the GC-2 based DSC device at a bias of −0.50 V. This plot is composed of a straight line and a semicircle at higher and lower frequencies, respectively. Upon small perturbation from external AC, the response of the impedance Z for the photoanode can be expressed as eq 145

Figure 6. J−V curves of the quasi-solid DSC devices based on GC-2 and GC-3, as mentioned in the beginning of this section, their film thicknesses are 33.2 and 32.8 μm, respectively.

Table 4. Photovoltaic Parameters Derived from the J−V Curves in Figure 6 sample

Voc (V)

Jsc (mA cm−2)

FF (%)

η (%)

GC-2 GC-3

0.697 0.726

13.43 7.05

68.58 68.48

6.42 3.50

Notably, a high PCE of 6.42% was achieved by sample GC-2. By contrast, the PCE of GC-3 based DSCs was only 3.50%. This low PCE could be mainly ascribed to the small photocurrent of 7.05 mA cm−2, as shown in Table 4. Comparing the Voc of these two DSCs, 0.697 V for GC-2 and 0.726 V for GC-3, the 29 mV variation is also evident. Below discussions focus on analyzing the results of optical and electrical characterizations. We first tested the dye uptake of the photoanodes. The results showed that the dye-loading amount for these two films with similar thicknesses were 1.40 × 10−7 mol cm−2 and 1.04 × 10−7 mol cm−2, indicating that GC-2 based film obviously prevail over GC-3 based film in terms of optical density. Aside from the dyeloading amount, light-scattering should also be considered for optical estimation. Figure 7 shows the reflectance spectra of the films with different configurations and thicknesses, wherein the reflectance spectra of nanoparticle (NP, 20 nm in size)-based film is cited as a reference. It is found that the light-scattering ability of the GC-2 based photoanode is not as good as that of the 1008



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Figure 8. (a) Typical Nyquist plot and the corresponding fitting line for GC-2 based DSC devices under −0.50 V, the inset is the enlarged part at high frequencies showing the electron transport behavior; (b) Nyquist plots of GC-2 and GC-3 based DSC devices under −0.50 V, the inset is the enlarged part at high frequencies; (c) resistance of charge recombination (Rrec) at the ZnO/electrolyte interface and electron transport (Rtr) across the film, (d) chemical capacitance (Cμ), (e) electron diffusion coefficient (Dn) and (f) diffusion length (Ln) of the photoanode based on GC-2 and GC-3, respectively.

where Rrec, Rtr, ωk, and ωd represent the charge-recombination resistance, resistance of the electron transport in ZnO, rate constant for recombination, and characteristic angular frequency for electron diffusion in a finite layer, respectively. The shape of

⎛ R trR rec ⎞1/2 Z=⎜ ⎟ coth[(ω k /ωd)1/2 (1 + iω/ω k )1/2 ] ⎝ 1 + iω/ω k ⎠ (1) 1009



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their band structure. Frank et al. indicated that the network geometry of the porous films in DSCs has significant effects on the electron transport.51 In our previous study, we have proven that the HSs composed of interconnected monocrystalline nanosheets have an advantage over nanoparticles on electron transport. GC-2 and GC-3 based HSs have different dimensions and assembling densities of the nanosheets. We infer that the boundaries among these HSs are the main obstructions for rapid diffusion of electrons across the photoanode film. Compared with the GC-3 based photoanode, much fewer boundaries can be observed between adjacent HSs within a given thickness for GC-2 based photoanode, as shown in Figure 5. This explanation is also tenable from the perspective of porosity. Frank et al. proposed through simulations of the random-walk behavior of electrons in the porous film that the coordination numbers for each particle with neighbors in high porous films are too deficient that the poor contact of the network will slow down the electron transport.52 The results show that porosities are 59% and 73% for GC-2 and GC-3 based photoanodes, respectively. Hence, the traveling distance of the electrons in GC-3 based photoanode will be prolonged before collection by TCO substrate. Electron diffusion in the porous film can be interpreted in terms of a multiple trapping model.53 Although the nanosheets in the HSs of these two samples are both monocrystalline, their band structures and trap distributions are more or less different because of the different synthetic conditions. The behavior of electron transport from the viewpoint of band structure was analyzed. Equation 6 can be used to interpret the exponential distributions of Rtr as a function of bias V and the relationships between some important parameters.46

the Nyquist plot confirm that photogenerated electrons can be collected by the TCO substrate efficiently, as shown in Figure 8a. In this case, eq 1 switches its expression to eqs 2 and 3 Z=

R rec 1 R tr + 3 1 + iω/ω k

Z′ = R tr(iω/ωd)1/2

(2) (3)

at lower and higher frequencies, which correspond to the resistance of the charge recombination and electron transport, respectively. Theoretically, a straight line with a slope of 1 and a semicircle with Rrec as its diameter can be obtained in the Nyquist plot. However, distortions are observed at the beginning and the end of the plot, as shown in Figure 8a. This result is attributed to the impedances of the charge-transfer at the Pt/electrolyte, TCO/ZnO interfaces, and diffusion of the I−/I3− redox couple in the electrolyte. On the basis of the equivalent circuit from the diffusion-recombination model,49 we obtained the desired parameters through fitting the Nyquist plot illustrated in Figure 8a. Figure 8b demonstrates the Nyquist plots obtained under −0.50 V for these two different DSC devices. Compared with the GC-2 based DSCs, both Rrec and Rtr of the GC-3 based DSCs are obviously larger. Using eq 4, tn = 1/ω k = 1/2πf

(4)

the characteristic frequency f (defined as the peak of semicircle) of the charge recombination can be used to calculate the lifetime tn of the injected electrons in the conduction band of ZnO. Marked with blue squares, the characteristic frequencies for GC-2 and GC-3 based DSC devices are 1.778 and 0.681 Hz, respectively. This result suggests that tn in the GC-3 based photoanode is longer than that in the GC-2 based one. Therefore, the structures of HSs and their synthetic conditions have notable effects on the electrical performances of the DSC devices. For further investigations, we carried out EIS measurements at different biases, and then obtained a series of Nyquist plots. Figure 8c shows the fitted results of Rrec for these two representatives. With increasing bias, Rrec decrease exponentially. This behavior can be interpreted using eq 5:50 ⎡ β ⎤ R rec = R 0 exp⎢ − V⎥ ⎣ kBT ⎦

⎡ e ⎛ Eredox − Ecb ⎞⎤ ⎜V + ⎟⎥ R tr = R 0′ exp⎢ − ⎠⎦ e ⎣ kBT ⎝

(6)

In this equation, R0′ is a constant (different from R0 in eq 5), e is the elemental charge, Eredox is the energy of redox couple, and Ecb is the ZnO conduction band. At a given value of Rtr, the shift from GC-2 to GC-3 is about 24 mV, as shown in Figure 8c. According to eq 6, this variation can be regarded as the shift of Ecb. This result is in good accordance with that of the aforementioned 29 mV variation of the Voc for the DSCs based on GC-2 to GC-3. Intrinsically the shift of Ecb can directly affect electron density in the conduction band of ZnO.48 The higher the electron density is, the faster the electrons will diffuse. Therefore, the GC-3 has a higher conduction band compared with GC-2, indicating that synthetic conditions can affect both morphologies and internal band structures of the prepared HSs. In addition to Rrec and Rtr, other useful data can also be obtained by the following equations.44

(5)

where R0 is a constant, β is the transfer coefficient, kB is the Boltzmann constant, T is the absolute temperature, and V is the bias applied in the EIS measurements. Under the same bias, the Rrec of GC-3 based DSCs are higher than that of GC-2 based one, as shown in Figure 8c. However, the slopes of these two lines show that the charge coefficients of β for these two samples are nearly the same. The difference of Rrec for GC-2 and GC-3 can be attributed to the micro/nano structures of the photoanodes. As mentioned previously, the film thicknesses for these two photoanodes are almost the same, but the HSs in GC-2 has densely assembled nanosheets. The effective surface area in the photoanode is larger than that of its opponent GC-3. Therefore, the sites for charge-transfer at the ZnO/electrolyte interface in the photoanode based on GC-2 are more sufficient. For the electron transport in these two ZnO photoanode, the GC-2 based one exhibits lower Rtr values. Therefore, the structure of HSs in GC-2 based photoanode is more advantageous in electron transport. The subsequent discussions on this issue will be concentrated on two factors, the network in photoanode and

Cμ =

tn R rec

(7)

Dn =

L2 R trCμ

(8)

Ln = L

R rec R tr

(9)

where Cμ is the chemical capacitance, Dn is the diffusion coefficient, and L and Ln are the film thickness and diffusion length, respectively. As shown in Figure 8d, the distribution of 1010



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the Cμ, which reflects the distribution of trap states in the gap, is also exponential in light of eq 10:50 Cμ =

⎡ α ⎤ e2 exp⎢ (E Fn − Ecb)⎥ kBT ⎣ kBT ⎦

ogies and band structure on the optical and electrical properties have been well addressed; and (v) high photovoltaic performance was achieved.

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CONCLUSIONS We first reported sonochemistry as an ultrarapid, controllable, and facile approach for room-temperature and mass-production of ZnO nanosheet-based HSs with multiple functions. Ultrasound irradiation was proven effective in accelerating nucleation and promoting ionic and mass diffusion. In combination with in situ observations, we proposed mechanisms to state the formation processes, in which heterogeneous oriented attachment and reconstruction of polycrystalline into monocrystalline were involved. Various HSs with different structures were then prepared, and the relationships between micro/nano structures and synthetic conditions were established. Several HSs in the photoanodes in quasi-solid DSCs with densely assembled nanosheets demonstrated high photovoltaic performances. The large surface area of these HSs made them easily accessible for incident photons, dye molecules, and gel electrolyte. Furthermore, the interconnected nanosheets provided an ideal network for fast electron transport. We used the EIS technique to systematically analyze the quasi-solid DSCs based on differently structured HSs. The result revealed that both the band structure and the morphologies of the HSs had obvious effects on the electrical properties. Larger porosity was unfavorable to the electron transport. Finally, under 100 mW cm−2, AM 1.5 light irradiation, a record PCE of 6.42% was achieved.

(10)

where EFn is the Fermi level and the constant α governs the slope of the line. In detail, α = T/T0, T0 is the characteristic temperature describing the depth of the trap states.54 Therefore, the distribution of the trap states for these two samples with different structures and synthetic conditions is obviously different. Further studies are needed to investigate this phenomenon in detail. After calculations using eq 8, Dn is plotted in Figure 8e as a function of bias. Dn increases exponentially. Figure 8e proves the better performance of GC-2 in electron diffusion. However, the negative effect from slower electron transport can be offset because of the longer lifetime of electrons. Thus, GC-3 prevails over GC-2 on Ln (see Figure 8f). Finally, the morphologies and the band structures of the ZnO HSs that have been widely synthesized, used, and investigated should be considered for the analysis of electrical properties. After detailed discussions of our experimental data, some important conclusions can be summarized briefly. Of all the factors, assembling density of the nanosheets on HSs is the most important one to determine the final photovoltaic performance. As the building blocks for the photoanode, HSs with smaller sizes can scatter the incident light more efficiently while those with larger sizes and densely assembled nanosheets can transport electrons more freely. However, because of the large surface area in the photoanode composed of larger HSs, charge recombination has slightly deteriorated. For our HSs, in spite of a thick photoanode (nearly 33.2 μm), electrons can also be collected efficiently. Finally, it is thought that the performance of the DSC could be further improved through increasing the assembling density of the nanosheets on HSs. Anta et al. recently reviewed the progress of the ZnO-based DSCs.55 Fujihara (2008) and Vomiero (2011) presented the top two PCE records for pure ZnO-based DSCs, which were 6.5% and 7.5%.56,17 The photoanodes of these two highly efficient DSCs were fabricated with commercial powder (nanoparticles plus a scattering layer) and HSs (nanocrystals aggregates plus a buffer layer). According to previous study, all ZnO-based DSCs with PCEs above 5% used liquid electrolytes as hole conductors. In this study, the hole conductor for the prepared ZnO-based DSCs is a highly viscous polymer gel electrolyte, which exhibits almost no fluidity even when heated at 100 °C. Therefore, in terms of preventing volatilization and leakage, the gel electrolyte will be more beneficial for practical fabrication and application of DSCs. Moreover, unlike the densely packed aggregates of nanocrystals, the open structures of the prepared nanosheetbased HSs make ZnO surface more accessible for interactions with incident photons, dye molecules, and polymer gel electrolytes. To date, the PCE data in this study is the one with the highest record for the ZnO-based quasi-solid DSCs. Several studies have reported the synthesis and application of nanosheet-based HSs in DSCs. In this study, (i) the approach is quite facile because high quality HSs can be rapidly and controllably synthesized without any organic templates or hightemperature treatments; (ii) from in situ observation to mechanism interpretation, the formation processes of the prepared HSs has been well discussed; (iii) the relationships between structures and synthetic conditions have been well established and discussed; (iv) the influences of the morphol-

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ASSOCIATED CONTENT

S Supporting Information *

SEM image of the HSs synthesized through direct precipitation (Figure S1); SEM image of the HSs in GC-1 synthesized with lower reactants concentrations (Figures S2); SEM image of the HSs in GT-2 synthesized at 45 °C (Figures S3); SEM image of the HSs in GT-3 synthesized at 55 °C (Figures S4); TEM image of the nanocrystals taken out at the early stage (t = 1 min) in the synthesis of GT-1 at 8 °C (Figures S5); TEM images of the products taken out at the early stage (t = 1 min) in the synthesis of GT-4 at 72 °C (Figures S6); SEM image of the porous nanosheets of GP-3 (Figure S7); SEM image of the HSs synthesized without ultrasonic-assistance while other conditions were kept the same as that in the synthesis of GP-4 (Figure S8). This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (N.W.), [email protected] (T.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support from National Natural Science Foundation of China (Grant No. 51273032), National High Technology Research and Development Program for Advanced Materials of China (Grant No. 2009AA03Z220), Research Grants Council of Hong Kong (project numbers: HKUST/SRFI, RPC10S C04, 603408, 604009), Doctoral Fund of Ministry of Education o f China (Grant No. 20110041110003), Open Project Program of the State Key Laboratory of Physical Chemistry of Solid Surfaces from Xiamen 1011



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University (201210). The technical support of the RaithHKUST Nanotechnology Laboratory for the electron-beam lithography facility at MCPF (project number: SEG_HKUST08) is acknowledged.

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