Oriented Single-Crystal-Like Porous ZnO on ITO ... - ACS Publications

Jun 8, 2017 - The ZHC crystals were obtained by simple electrochemical deposition on layer-by-layer (LbL)-coated ITO in aqueous solution and transform...
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(0001)-Oriented Single-Crystal-Like Porous ZnO on ITO Substrates via Quasi-Topotactic Transformation from (001)-Oriented Zinc Hydroxychloride Crystals Tsutomu Shinagawa,*,† Mitsuru Watanabe,† Jun-ichi Tani,† and Masaya Chigane† Electronic Materials Research Division, Osaka Municipal Technical Research Institute, Joto-ku, Osaka 536-8553, Japan S Supporting Information *

ABSTRACT: Novel ZnO nanostructures composed of ⟨0001⟩oriented porous ZnO were prepared on indium tin oxide (ITO) substrates from ⟨001⟩-oriented zinc hydroxychloride (ZHC) crystals. The ZHC crystals were obtained by simple electrochemical deposition on layer-by-layer (LbL)-coated ITO in aqueous solution and transformed into porous ZnO via calcination in air. X-ray diffraction and scanning electron microscopy were used to characterize the ZHC crystals and porous ZnO. The well-faceted ZHC crystals obtained on the LbL-coated ITO had hexagonal-plate shapes with diameters of 3−7 μm and exhibited strongly ⟨001⟩-oriented simonkolleite crystal structures. The calcination of ZHC crystals at 450−550 °C for 1 h in air led to the formation of ⟨0001⟩-oriented porous ZnO comprising nanocrystals with diameters of ∼50 nm while maintaining the original hexagonal-plate shapes. Electron backscatter diffraction analyses and epitaxial growth of ZnO nanorod array revealed that the porous ZnO hexagonal plates exhibited features of single crystals. Finally, a plausible mechanism for the topotactic-like transformation of ZHC crystals to porous ZnO is discussed on the basis of similarities in their Zn−O frameworks.

1. INTRODUCTION Oxides with porous structures have attracted increasing attention because of their specific structural features and potential applications as (photo)catalysts, gas-sensors, and electrodes for (quasi-)capacitors and batteries.1−5 Especially, single-crystal-like porous oxides comprising crystallographically oriented nanocrystal (nanoparticle) assemblies are categorized, in a broad sense, as “mesocrystals” (or mesostructured crystals),6,7 which has been developed from the concept originally proposed by Cölfen et al.8,9 Much effort has been devoted to developing synthetic methods for these hierarchical architectures and exploring their functional characteristics.6,7 To date, the main methods for the synthesis of oxide mesocrystals have been chemical-solution processes in which a biomimetic approach mediated by organic molecules is employed to assemble the nanocrystals, and the mesocrystals have been obtained as precipitates in powder form (diameters of the order of hundred nanometers to several microns). Coating mesocrystal powders on support media such as substrates, which is required for most of the applications described above, usually results in the formation of films composed of randomly oriented mesocrystals. Thus, the facile formation of oriented mesocrystal films is expected to enable possible new function that utilize the crystallographic and structural characteristics of oriented mesocrystal films. Layered metal hydroxides (LMHs) are layer-structured materials with brucite-like metal-hydroxyl host layers either © XXXX American Chemical Society

with or without charge-balancing anions in the interlayer spaces.10 LMHs composed of one type of metal are also called layered single-metal hydroxides (LSHs) or layered hydroxide salts (LHSs) and have the chemical formula MII(OH)2−mx Axm−·nH2O (M = Mg, Co, Ni, Zn, and so on).11 Although LMHs are traditionally used in anion-exchange reactions, they can also be used as synthetic precursors for porous oxides.12−14 Zinc hydroxychloride (ZHC), Zn5(OH)8Cl2·H2O, is an LSH that is thermally transformed into porous ZnO.15 In addition to hydrothermal and coprecipitation methods, which are commonly used to synthesize LMHs, ZHC has been obtained by electrochemical deposition.16,17 This versatile technique allows ZHC crystals to be grown directly on conductive substrates in aqueous solution at moderate temperatures below 100 °C and the formation of porous ZnO on the conductive substrates by calcination, which has been employed as porous electrodes in solar cells18−20 and photoelectrochemical cells.21 Thus, the electrodeposition of oriented ZHC crystals is expected to provide oriented porous ZnO (or oriented ZnO mesocrystals) on substrates. ZnO is a well-known n-type semiconductor with a wide bandgap energy of 3.3 eV and a high electron mobility, making it an attractive material with superior transparent electron transport properties.22 Thus, combining the intrinsic Received: March 20, 2017 Revised: May 31, 2017

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(visualization for electronic and structural analysis) software ver. 3.3.8.25

properties of ZnO with the extrinsic properties provided by an oriented porous structure is expected to generate novel functional characteristics. However, to the best of our knowledge, no reports have been published on the electrodeposition of oriented ZHC crystals and the orientation of the resulting porous ZnO. In this article, we report the preparation and structural characterization of ⟨0001⟩-oriented single-crystal-like porous ZnO grown on conductive indium tin oxide (ITO) substrates from precursor ⟨001⟩-oriented ZHC crystals. The orientationcontrolled electrodeposition of ZHC crystals was achieved using layer-by-layer (LbL)-coated ITO. Previously, we demonstrated that LbL layers composed of oppositely charged polymers can control the crystal orientation in the electrodeposition of ZnO.23 In the present study, we found that the LbL layers were also effective in the orientation-controlled electrodeposition of ZHC. This enabled us to conduct a detailed orientational analysis of porous ZnO resulting from the ZHC, which consisted of ZnO nanocrystals with diameters of ∼50 nm, and revealed features of single crystals (mesocrystals). Finally, a plausible mechanism for the topotactic-like thermal transformation from ZHC to ZnO is presented from the viewpoint of their Zn−O frameworks.

3. RESULTS AND DISCUSSION 3.1. Electrochemical Deposition of ⟨001⟩-Oriented Zinc Hydroxychloride. ZHC crystals, Zn5(OH)8Cl2·H2O, were electrodeposited on bare (unmodified) ITO and LbLcoated ITO substrates from aqueous solution. The LbL layer was prepared by alternately stacking PDDA (positively charged polymer) and PSSNa (negatively charged polymer) on ITO; the stacking was terminated with a PDDA layer to give the following structure: PDDA/(PSSNa/PDDA)2/ITO. The surface morphology and chemistry of the obtained LbL layer were studied by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) (Figures S1 and S2 in Supporting Information). The AFM results confirmed that LbL was coated homogeneously on ITO with a surface roughness (Ra) of 7.4 nm, slightly larger than that of bare ITO (Ra = 5.0 nm). In the XPS analyses, the presence of C, N, S, and Cl atoms derived from PDDA and PSSNa was observed for the LbL-coated ITO. Several research groups have reported the electrodeposition of ZHC using ZnCl2 or Zn(NO3)2 as a source of Zn; the deposition mechanism can be described as follows:16−19,21,26,27

2. EXPERIMENTAL SECTION Aqueous solutions for electrodeposition were prepared using reagentgrade chemicals and deionized (DI) water (>10 MΩ cm) purified by a Millipore Elix Advantage 5 system. ITO-coated glass (GEOMATEC, 5 Ω/sq.) was used as the substrate. Prior to the LbL coating or electrodeposition, the ITO substrate was treated with a UV/ozone cleaner for 12 min and then rinsed with DI water. The LbL coating of ITO was performed according to our previous study.23 The ITO substrate was immersed into a 5 w/v% solution of poly(diallyldimethylammonium chloride) (PDDA; Aldrich, MW 200,000−350,000) in DI water for 5 min and rinsed with DI water. The ITO substrate was then immersed into a 5 w/v% solution of poly(sodium p-styrenesulfonate) (PSSNa; Scientific Polymer Products Inc., MW 500,000) in DI water for 5 min and rinsed with DI water. This immersion/rinse cycle was repeated five times to terminate with a PDDA layer, i.e., PDDA/(PSSNa/PDDA)2/ITO, and the LbL-coated ITO substrate was then dried in air. The cathodic electrodeposition of ZHC (simonkolleite) on the LbL-coated ITO and bare ITO substrates was performed galvanostatically using a potentio/galvanostat (Hokuto Denko, HABF5001) in aqueous solutions containing 0.1 M (M = mol dm−3) Zn(NO3)2 and 0.2 M NaCl at a current density of 0.2 mA cm−2 and a total electric charge of 0.25 C cm−2. A Zn bar was used as a counter electrode, and the temperature of the solutions was maintained at 75 °C. After deposition, the substrates were rinsed with DI water and dried with N2 gas. The obtained ZHC was heat treated at 150, 200, 300, 450, and 550 °C for 1 h in air. The cathodic electrodeposition of ZnO nanorods on the calcined porous ZnO was performed galvanostatically from aqueous solutions (75 °C) containing 0.5 mM Zn(NO3)2, 0.1 M NaNO3, and 1.0 mM NH4NO3 at a current density of 0.2 mA cm−2 and a total electric charge of 1.0 C cm−2.24 Structural, morphological, and chemical characterizations of the obtained samples were performed with atomic force microscopy (AFM; Digital Instruments, NanoScope IIIa-Dimension3000), X-ray diffraction (XRD; RIGAKU SmartLab, Cu Kα radiation, 6 kW), fieldemission scanning electron microscopy (FESEM; JEOL JSM6700F), scanning electron microscopy with an electron backscatter diffraction analyzer (SEM-EBSD; HITACH SU6600-OXFORD NORDLY), Xray photoelectron spectroscopy (XPS; Kratos AXIS-Ultra DLD, monochromated Al Kα radiation, 72 W), and attenuated total reflection Fourier transform infrared spectroscopy with a diamond ATR crystal (ATR-FTIR; Thermo Nicolet 4700-DuraSamplIRII). The models of crystal structure for ZHC and ZnO were made in VESTA

O2 + 2H 2O + 4e− → 4OH−

(1)

NO3− + H 2O + 2e− → NO2− + 2OH−

(2)

5Zn 2 + + 8OH− + 2Cl− + H 2O → Zn5(OH)8 Cl 2·H 2O (3)

The application of a cathodic current in a substrate (the working electrode) reduces dissolved oxygen and/or nitrate ions to produce hydroxide ions in the vicinity of the substrate (eqs 1 and 2). ZHC crystals then grow on the substrate via the hydroxylation of Zn2+ while incorporating Cl− (eq 3). Note that this reaction is competitive with the precipitation of ZnO, which involves the hydroxylation of Zn2+ followed by dehydration (Zn2+ + 2OH− → Zn(OH)2 → ZnO + H2O).16 In fact, a mixture of ZnO and ZHC was obtained by potentiostatic electrodeposition from aqueous solutions containing 0.05−0.1 M Zn(NO3)2 and 0.1 M KCl at 50−70 °C.17−19,26,27 In this study, we modified the experimental conditions to obtain well-crystallized ZHC as a single product via galvanostatic electrodeposition in aqueous solution containing 0.1 M Zn(NO3)2 and 0.2 M NaCl at 75 °C. Figure 1a,b shows FESEM images of electrodeposited ZHC on a bare ITO substrate. Randomly oriented, hexagonal plate-shaped single crystals with diameters of ∼7 μm and thicknesses of ∼0.5 μm were obtained. The observed crystal growth habit is attributed to the trigonal crystal system of ZHC and the capping effect of Cl− ions on the ZHC(001) plane. The (001) plane, which corresponds to the basal face of the hexagonal plate, is stabilized by adsorbed Cl− ions, leading to preferential growth in the ⟨100⟩ direction, thereby minimizing the total surface energy.17 The number of crystal grains per unit substrate area was counted from the top-view FESEM images to be ∼300 grains per 100 × 100 μm2. Assuming that the electrodeposition of ZHC proceeds according to eqs 1−3, the deposition current efficiency of ZHC (i.e., the molar ratio of obtained deposits to the theoretical one) can be calculated from the volume of deposits and Faraday’s laws to be ∼85%. This value is B

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and c = 23.660 Å; ICDD no. 07−0155). In contrast, a strong preference for ⟨001⟩ orientation was confirmed in the ZHC electrodeposited on LbL-coated ITO (Figure 2b). In addition to strong (003) diffraction peak at 11.22°, (00l) diffraction peaks for l = 6, 9, 12, 15, and 18 were observed clearly at 22.54°, 34.10°, 46.03°, 58.51°, and 71.81°, respectively, demonstrating electrodeposition on LbL-coated ITO provided highly ⟨001⟩-oriented ZHC with high crystallinity. Although the detailed mechanism by which the crystal orientation was controlled by the LbL layers is unclear, it appears that the presence of Cl− ions as a counteranion in PDDA that is the terminated layer of the prepared LbL played an important role (see Figure S2); the Cl− ions loosely bound on the surface of ITO may act as a starting layer for the growth of ZHC crystals in the ⟨001⟩ direction. 3.2. Thermal Transformation into Porous ZnO. ZHC is known to thermally decompose to yield ZnO at temperatures above ∼400 °C.15,28−30 Although the decomposition route has not yet been completely elucidated, the reaction can be expressed formally as follows:

Figure 1. (a,c,e) Top-view and (b,d,f) cross-sectional FESEM images of zinc hydroxychloride (ZHC) electrodeposited on (a,b) bare ITO and (c−f) LbL-coated ITO substrates. (e) and (f) are zoomed-in views of (c) and (d), respectively. Inset (top right) shows the main crystal planes in trigonal ZHC.

Zn5(OH)8 Cl 2·H 2O → 5ZnO + 2HCl + 4H 2O

(4)

The decomposition involves dehydration and dehydrochlorination, and external oxygen is apparently unnecessary to form ZnO. The ZHC films electrodeposited on bare ITO and LbLcoated ITO were calcined in air at 450 °C for 1 h, and the morphologies of the resulting samples were observed by FESEM (Figure 3). In both samples, ZHC was transformed

approximately the same as that for ZnO (∼87%).24 When the LbL-coated ITO was used, the orientation of the grown ZHC changed significantly. In this case, most of the electrodeposited hexagonal plates (3−7 μm in diameter) grew parallel to the substrate and had very flat, smooth surfaces (Figure 1c−f). Figure 2 shows the θ−2θ XRD patterns of the ZHC samples depicted in Figure 1. All peaks were assigned to either

Figure 3. (a,c,e) Top-view and (b,d,f) cross-sectional FESEM images of ZnO thermally transformed from ZHC electrodeposited on (a,b) bare ITO and (c−f) LbL-coated ITO substrates. (e) and (f) are zoomed-in views of (c) and (d), respectively.

Figure 2. XRD patterns of zinc hydroxychloride (ZHC) electrodeposited on (a) bare ITO and (b) LbL-coated ITO substrates. ICDD data (no. 07−0155) for simonkolleite (Zn5(OH)8Cl2·H2O) are also presented.

into porous ZnO consisting of spherical grains with diameters of ∼50 nm, and the original hexagonal-plate morphologies of the ZHC crystals were maintained without obvious size variation. Similar morphologies have been reported regardless of the synthetic method of ZHC.15,21,31 The θ−2θ XRD patterns of the calcined samples are shown in Figure 4. All observed peaks were assigned to wurtzite ZnO (ICDD no. 36−1451) and ITO. Whereas ZnO on the bare ITO substrate showed (0002) diffraction comparable to the (101̅0) and (101̅1) diffractions, strong (0002) ZnO diffraction

simonkolleite (Zn5(OH)8Cl2·H2O) or ITO, and no other phases (e.g., ZnO) were recognized. As expected based on the FESEM images (Figure 1), ZHC electrodeposited on bare ITO showed an almost random crystal orientation (Figure 2a), and the lattice constants were calculated to be a = b = 6.341(2) Å and c = 23.70(3) Å; these lattice constants are in agreement with those of reference simonkolleite powder (a = b = 6.340 Å C

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Figure 5. EBSD analysis of a porous ZnO hexagonal plate thermally transformed from ZHC electrodeposited on LbL-coated ITO substrate. (a) SEM image of the ZnO plate and the three axes of the EBSD system. EBSD IPF maps of the ZnO plate along the (b) x-, (c) y-, and (d) z-axes.

Figure 4. XRD patterns of ZnO thermally transformed from ZHC electrodeposited on (a) bare ITO and (b) LbL-coated ITO substrates. ICDD data (no. 36−1451) for ZnO are also presented. Inset shows the main crystal planes in hexagonal ZnO.

original ZHC hexagonal plate were easily identified from the crystal shape as ⟨21̅0⟩ and ⟨100⟩ along the x- and y- axes, respectively. Since the [1̅21̅0] and [011̅0] directions in the hexagonal crystal system of ZnO are equivalent to the [21̅1̅0] and [101̅0] directions, respectively, the in-plane orientation of the calcined porous ZnO is consistent with that of the original ZHC. The EBSD IPF map for the z-axis (perpendicular to the substrate surface) confirmed that calcined ZnO had a ⟨0001⟩out-of-plane orientation (Figure 5d), which is consistent with the XRD results. These results reveal that the calcined porous ZnO hexagonal plates had single-crystal features and topotacticlike relationships with the original ZHC crystals as follows:

was observed for ZnO on the LbL-coated ITO substrate. Thus, the XRD results in Figures 2b and 4b indicate that ⟨001⟩oriented ZHC was thermally transformed into ⟨0001⟩-oriented ZnO. The lattice constants for calcined ZnO on bare ITO were a = b = 3.2499(17) and c = 5.218(6), whereas those of calcined ZnO on LbL-coated ITO were a = b = 3.2570(6) Å and c = 5.2139(3) Å; these lattice constants were slightly larger than those of the reference ZnO powder (a = b = 3.2498 Å and c = 5.2066 Å; ICDD no. 36−1451). XPS analysis revealed that small amounts of Cl were incorporated into the calcined ZnO at an atomic ratio of Cl/Zn = ∼4%; this incorporated Cl might explain the slight lattice expansion because Cl− has a larger ionic radius than O2−. For the direct electrodeposition of crystalline ZnO from an aqueous solution containing 5 mM ZnCl2 and 0.1 M KCl, little Cl (≪1%) was detected in the ZnO,32 suggesting that the transformation process via ZHC is an effective way to yield Cl-doped ZnO. 3.3. In-Plane Orientation of ⟨0001⟩-Oriented Porous ZnO. The above FESEM and XRD results show that ⟨001⟩oriented ZHC crystals with hexagonal plate-like structure (diameter = 3−7 μm) were thermally decomposed to form porous hexagonal plates consisting of ⟨0001⟩-oriented ZnO particles with diameters of ∼50 nm. To elucidate the in-plane orientations of the ZnO particles within a single hexagonal plate, we performed SEM-EBSD analysis. In-plane XRD measurements and transmission electron microscopy-electron diffraction (TEM-ED) analysis are inadequate for this task because the analysis area of the former and the later are too large and too small, respectively, with respect to the size of the porous ZnO hexagonal plate (diameter = 3−7 μm). Thus, EBSD was employed to analyze a ⟨0001⟩-oriented porous ZnO hexagonal plate with a scan step size of 200 nm. Figure 5a shows the corresponding SEM image of the ZnO plate and three axes (x, y, and z) of the SEM-EBSD system. The resulting inverse pole figure (IPF) maps for the x-, y-, and z-axes are shown in Figure 5b−d. The EBSD IPF maps for the x- and y-axes (parallel to the substrate surface) reveal that the porous ZnO had an ordered in-plane orientation all over the hexagonal plate (i.e., [1̅21̅0] and [011̅0] orientations along the x- and y-axes, respectively). The in-plane orientations of the

(001)[2 1̅ 0]ZHC||(0001)[2 1̅ 1̅ 0]ZnO

To further confirm the novel structural properties of the calcined porous ZnO, well-faceted ZnO nanorods (ZnO-NRs) were grown via electrodeposition. If the porous ZnO hexagonal plates have single crystal features via topotactic-like transformation from ZHC, ZnO-NRs should grow epitaxially on the calcined porous ZnO and exhibit an ordered in-plane orientation equivalent to the original ZHC. ZnO-NRs were electrodeposited from a solution containing 0.5 mM Zn(NO3)2, 0.1 M NaNO3, and 1.0 mM NH4NO3 at 75 °C according to our previous study.24 As shown in Figure 6, an array of ZnO-NRs with diameters of 50−100 nm and lengths of ∼150 nm was observed on both the top and lateral surfaces of the calcined porous ZnO hexagonal plate. Figure 6c,d shows magnified views of two different positions on the hexagonal plate; these positions are indicated by the white rectangles (A and B) in Figure 6c. The in-plane orientation of each ZnO-NR can be identified from its crystal shape in the top-view FESEM images (Figure 6d,e). As indicated by the red hexagonal frames in Figure 6d,e, most of the ZnO-NRs showed the same in-plane orientation as the original ZHC hexagonal plate shown in Figure 6c; this supports that the calcined porous ZnO had single crystal features via topotactic-like transformation from ZHC. 3.4. Plausible Transformation Mechanism from ZHC to ZnO. In this section, the crystal structures of ZHC and ZnO are compared to provide insight into the mechanism of the D

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Table 1. Crystal Parameters for ZHC (Simonkolleitea) and Zinc Oxideb formula crystal system lattice system space group lattice constants (Å) lattice volume (Å3) z a

simonkolleite

zinc oxide

Zn5(OH)8Cl2·H2O trigonal hexagonal R3̅m(166) a = b = 6.34, c = 23.66 α = β = 90°, γ = 120° 823.61 3

ZnO hexagonal hexagonal P63mc(186) a = b = 3.25, c = 5.21 α = β = 90°, γ = 120° 47.62 2

ICDD no. 07−0155. bICDD no. 36−1451.

sheets are composed of two units: octahedrally coordinated Zn(OH)6, Zn(Oh), with a brucite-like sheet structure; and tetrahedrally coordinated Zn(OH)3Cl, Zn(Td), located on the upper and lower sides of the Zn(Oh) sheet. Each zinc-hydroxide sheet is fixed by hydrogen bonding through Cl and interlayer water molecules (i.e., Zn−OH···Cl−Zn and Zn−OH···H2O). Thus, ZHC (simonkolleite) can be expressed as Zn(Oh)3[Zn(Td)(OH)3Cl]2(OH)2·H2O, and there are two types of OH: one coordinates to both Zn(Oh) and Zn(Td), and the other coordinates only to Zn(Oh). ZnO has a hexagonal wurtzite structure composed of only tetrahedrally O-coordinated Zn, and layers of Zn and O atoms appear alternately along the c-axis (Figure 7b). The distance between the nearest Zn layers is 2.60 Å, corresponding to the ZnO(0002) diffraction peak at 2θ = 34.42°. To compare the structures of ZHC and ZnO, their basic Zn−O frameworks were extracted and drawn along the (001) and (100) planes, as shown in Figure 8. The basic Zn−O framework extracted from the ZHC crystal lattice includes five Zn(Oh) and three Zn(Td), forming a distortional honeycomb network composed of three different Zn−O lengths (Figure 8b). However, the basic Zn−O framework extracted from the ZnO crystal lattice consists of eight equivalent Zn(Td), forming a ordered honeycomb network (Figure 8c). Although the crystal lattice of ZHC is significantly more complex than that of ZnO, each of their basic Zn−O frameworks includes a honeycomb network along the a−b plane that is stacked up along the c-axis. This three-dimensional similarity in the basic Zn−O framework may be critical in the topotactic-like transformation from ZHC to ZnO. The thermal decomposition process of ZHC powder has been analyzed previously using thermogravimetry and differential thermal analysis (TG-DTA).28,29 Yanagisawa et al.15 reported that interlayer water is first liberated during decomposition, as evidenced by an endothermic peak at 180 °C, followed by dehydroxylation from 180 to 210 °C (endothermic peak at 202 °C), and finally the release of HCl from 220 to 432 °C (endothermic peak at 432 °C). In the present study, we examined the thermal decomposition process of the ⟨001⟩-oriented ZHC samples using XRD and ATR-FTIR (Figure 9) and obtained results consistent with the literatures. In the XRD patterns (Figure 9a), compared to the as-deposited sample, heat treatment at 150 °C for 1 h decreased the ZHC(003) diffraction intensity to 1/4 of the initial value, and the peak shifted slightly toward higher angles (from 11.22° to 11.28°). These changes are attributed to the liberation of interlayer water.35 Heat treatment at 200 °C caused the intensity of the ZHC(003) diffraction to decrease to 1/100 of the initial value, and this diffraction peak nearly disappeared after heat treatment at 300 °C. However, a ZnO(0002)

Figure 6. (a,c,d,e) Top-view and (b) cross-sectional FESEM images of ZnO nanorod arrays grown on porous ZnO plates thermally transformed from ZHC electrodeposited on an LbL-coated ITO substrate; (d) and (e) are zoomed-in views of the regions A and B in (c), respectively.

topotactic-like transformation from ZHC to ZnO. The crystal structures of ZHC (simonkolleite)33 and ZnO34 are depicted in Figure 7, and their crystal parameters are summarized in Table

Figure 7. Crystal structures of (a) ZHC (simonkolleite) and (b) ZnO.

1. ZHC has a trigonal structure with a hexagonal lattice system; the long c-axis lattice length of 23.66 Å is attributed to the layered structure containing zinc-hydroxide sheets and interlayer water molecules. The distance between the nearest zinc-hydroxide sheets is 7.87 Å, which corresponds to the ZHC(003) diffraction peak at 2θ = 11.23°. The zinc-hydroxide E

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Figure 8. Crystal structures of (a) ZHC (simonkolleite) and (d) ZnO and the basic Zn−O frameworks along the (001) and (100) planes for (b) ZHC and (c) ZnO. The corresponding Zn−O bond lengths are also presented at the bottom of (b) and (c).

respectively). These XRD results suggest that the crystallization of ZnO began at ∼200 °C and was accomplished at ∼450 °C, which is consistent with the reported TG-DTA analysis.15 In the ATR-FTIR spectra (Figure 9b), the as-deposited ZHC sample showed relatively sharp peaks at 3448, 1038, 899, and 714 cm−1, attributed to OH/H2O, ZnOH, ZnOH, and OH moieties, respectively.35,36 Heat treatment at 150 °C caused splitting the former three peaks into doublet at 3564−3444, 1038−999, and 876−822 cm−1, respectively. Srivastava et al.36 reported that, whereas these doublets are a characteristic feature of dehydrated ZHC (i.e., Zn5(OH)8Cl2), probably derived from two different OH moieties; Zn(OH)2 and ZnOHCl, clear splitting in these OH moieties is inhibited by hydrogen bond with interlayer water molecules in the case of monohydrated ZHC. IR absorption peaks derived from OH moieties were still clearly observed after heat treatment at 200 °C, but the phenomena of peak splitting disappeared again, indicating that the decomposition of dehydrated ZHC occurred as consistent with the XRD results. The intensity of peaks derived from OH moieties significantly decreased by heat treatment at 300 °C, and almost disappeared at 450 °C. The thermal transformation from ZHC to ZnO involves a shrinkage in volume, resulting in the porous structure consisting of ZnO particles with diameters of ∼50 nm. The shrinkage ratio (i.e., the net volume ratio of ZnO/ZHC per Zn atom) can be estimated from the lattice volumes of ZnO and ZHC and the z factor in Table 1 to be 43.4%. In the ∼50 nmdiameter ZnO particles, there were approximately 200 Zn/O atomic layers derived from the stacking of 200 zinc-hydroxide sheets in ZHC. Since the liberation process of interlayer water molecules is a kind of “interlayer exchange reaction”, it will proceed topotactically in ZHC. 37 Regarding a thermal decomposition process in nickel hydroxide β-Ni(OH)2 with a simple layered brucite structure, Figlarz et al.38 proposed that the crystal growth of nickel oxide proceeds by an “oriented nucleation-growth type” in which the hydroxide/oxide coherence and orientation is maintained during the course of dehydroxylation−dehydration, resulting in a topotactic transformation from β-Ni(OH)2 to NiO. Thus, dehydroxylation− dehydration processes in dehydrated ZHC might also proceed through the oriented nucleation−growth mechanism with the help of the similarity in the basic Zn−O frameworks of ZHC and ZnO.

Figure 9. (a) XRD patterns and (b) ATR-FTIR spectra of ZHC electrodeposited on LbL-coated ITO substrates before and after heat treatment at 150, 200, 300, 450, and 550 °C for 1 h.

diffraction peak appeared slightly at 34.20° after heat treatment at 200 °C and increased in intensity when the temperature was increased to 450 °C. The peak position of ZnO(0002) shifted slightly toward higher angles with increasing temperature (from 34.38° at 300 °C to 34.40° and 34.42° at 450 and 550 °C, F

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4. CONCLUSIONS In this article, we have demonstrated the successful preparation of ⟨0001⟩-oriented single-crystal-like porous ZnO via quasitopotactic transformation from ⟨001⟩-oriented ZHC crystals electrodeposited on LbL-coated ITO substrates. The direct growth of ⟨001⟩-oriented ZHC crystals was achieved by simple galvanostatic electrodeposition on an LbL-coated ITO substrate in an aqueous solution (75 °C) containing Zn(NO3)2 and NaCl. The as-deposited ZHC crystals had hexagonal-plate shapes (3−7 μm in diameter) with simonkolleite Zn5(OH)8Cl2· H2O structures. The ZHC crystals were thermally transformed into ZnO at 450 °C for 1 h in air; the resulting porous ZnO consisted of nanocrystals with diameters of ∼50 nm and maintained the original hexagonal-plate shapes. XRD and SEMEBSD analyses demonstrated that the ZnO nanocrystal assemblies calcined from the ⟨001⟩-oriented ZHC crystals had single-crystal features including ⟨0001⟩ out-of-plane orientations and in-plane orientations identical with the original ZHC, indicating a topotactic relationship between the asdeposited ZHC crystals and the calcined porous ZnO. Wellfaceted ZnO nanorod arrays grew epitaxially on the calcined porous ZnO and showed the same in-plane orientations as the original ZHC crystals, also supporting the topotactic-like transformation. We also presented a plausible mechanism for the topotactic-like transformation from ZHC to ZnO based on a comparison of their basic Zn−O frameworks extracted from their crystal lattices. We found that ZHC and ZnO have a similar Zn−O honeycomb network along the a- and b-axes along with a similar stacking structure along the c-axis. Although there are several hurdles (such as dehydration and shrinkage in volume) that may cause structural collapse, these three-dimensional similarities in the Zn−O frameworks appear to make the topotactic-like transformation from ZHC to ZnO possible.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b00396. AFM images and XPS spectra of bare ITO and LbLcoated ITO substrates (PDF)

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tsutomu Shinagawa: 0000-0001-5671-1512 Present Address †

Electronic Materials Research Division, Morinomiya center, Osaka Research Institute of Industrial Science and Technology, Osaka 536-8553, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T.S. thanks Mr. M. Sato (Kyoto University) for the SEM-EBSD analyses. A part of this study was supported by Kyoto University Nanofabrication Platform in Nanotechnology Platform Project sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. G

DOI: 10.1021/acs.cgd.7b00396 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

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DOI: 10.1021/acs.cgd.7b00396 Cryst. Growth Des. XXXX, XXX, XXX−XXX