Nanowires: Structural, Optical, and Electrochemical Properties

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Paraotwayite-type α‑Ni(OH)2 Nanowires: Structural, Optical, and Electrochemical Properties Tao Gao*,† and Bjørn Petter Jelle‡,§ †

Department of Architectural Design, History and Technology, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway ‡ Department of Civil and Transport Engineering, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway § SINTEF Building and Infrastructure, Department of Materials and Structures, NO-7465 Trondheim, Norway ABSTRACT: Paraotwayite-type nickel hydroxide [Ni(OH)2] nanowires with typical diameters of 20−30 nm and lengths up to several micrometers were prepared via a simple hydrothermal synthesis. The as-prepared nanowires had a mean composition of Ni(OH)1.64(SO4)0.18·0.3H2O and crystallized in a layered monoclinic structure (unit cell parameters: a = 0.78867(1) nm, b = 0.29661(3) nm, c = 1.35164(2) nm, and β = 91.1°), which is isostructural to α-Ni(OH)2 and has sulfate anions and water molecules sandwiched in the two-dimensional Ni(OH)2 principle layers. The as-prepared Paraotwayite-type α-Ni(OH)2 nanowires showed an indirect-allowed electron transition with a band gap energy Eg of about 3.8 eV, whereas the corresponding NiO nanowires obtained from a topotactic transformation of Paraotwayite-type α-Ni(OH)2 nanowire precursors exhibited a direct-allowed electron transition with a similar band gap energy of Eg ∼ 3.8 eV. Both Paraotwayite-type α-Ni(OH)2 nanowires and NiO nanowires performed similar electrochemical redox reactions in aqueous alkaline solutions, whereas the OH− ion insertion/ extraction reactions were found to be greatly enhanced in Paraotwayite-type α-Ni(OH)2 nanowires due to their intrinsic layered structure. The as-prepared Paraotwayite-type α-Ni(OH)2 nanowires exhibited an anodic electrochromism related to the redox couple of Ni2+ → Ni3+, which corresponds to the coloration from light green to dark brown. Paraotwayite-type α-Ni(OH)2 nanowires are an interesting material system for photoelectrochemical devices and energy-storage applications.

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research and development of α-Ni(OH)2 materials for high performance electrochemical devices are of great interest. Significant research efforts have recently been dedicated to nanostructured Ni(OH)2 with various sizes and morphologies, such as nanosheets,12−14 nanorods,15,16 nanowires,17−19 nanobelts,7,18,20 nanotubes,21,22 and nanoflowers.23,24 Thanks to their small sizes and consequently large surface areas, Ni(OH)2 nanomaterials usually enable a direct and sufficient contact between electrolyte and electrodes, thereby representing a promising material system for achieving faster kinetics for highperformance electrochemical devices.25 It is worthwhile to note that, due to their intrinsic layered structure, Ni(OH)2 tends to crystallize into 2D lamellar materials, such as nanosheets or thin flakes;12−14,24 for the synthesis of one-dimensional (1D) Ni(OH)2 nanomaterials with small featured sizes and large aspect ratios,15−22 nonconventional synthetic methods are usually required. For example, Matsui et al. reported an anodic alumina template-assistant approach for the synthesis of βNi(OH)2 nanorods,15 where the growth of Ni(OH)2 was confined within the 1D nanopores of the anodic alumina films. A similar method was also used by Cai et al. for the preparation

INTRODUCTION

Nickel hydroxide [Ni(OH)2] represents a technologically important material with distinctive structural and physical properties and wide applications in catalysts,1 batteries,2 and electrochromic devices.3,4 Ni(OH)2 has also been used as a solid-state precursor for metallic Ni as well as NiO.5 As a typical layered material, Ni(OH)2 crystallizes in two different crystallographic polymorphs, α- and β-Ni(OH)2. It is known that βNi(OH)2 is isostructural with brucite and consists of closely stacked two-dimensional (2D) Ni(OH)2 principle layers, whereas α-Ni(OH)2 is isostructural with hydrotalcite and consists of hydroxyl-deficient Ni(OH)2−x host layers and interlayer species, such as anions and water molecules.6 A general formula of α-Ni(OH)2 can be given as Ni(OH)2−x (An−)x/n·yH2O, where x = 0.2−0.4, y = 0.6−1, and A = chloride, sulfate, nitrate, carbonate, or other anions.7 The large interlayer spacing of α-Ni(OH)2 enables high mobility of the interlayer ions and little structural rearrangement during the electrochemical cycles.8 Moreover, structural and physical properties of α-Ni(OH)2 materials can be conveniently modified by, among others, doping with other divalent or trivalent elements.9−11 Taking into account that α-Ni(OH)2 exhibits a superior theoretical specific capacity of 433 mAh/g, compared to 289 mAh/g of β-Ni(OH)2,7 it is not surprising that the © 2013 American Chemical Society

Received: May 24, 2013 Revised: June 27, 2013 Published: July 3, 2013 17294

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of β-Ni(OH)2 nanotubes21 and by Wang et al. for the preparation of α-Ni(OH)2 nanowires.19 Other template-free methods, such as hydrothermal16 or solvothermal synthesis,17,22 where the nucleation and growth of materials can be performed at relatively high temperatures and high pressures, have also been developed to prepare 1D Ni(OH)2 nanomaterials. Usually, the products of the hydrothermal or solvothermal synthesis are dependent on the reaction medium, temperature, pH value, reaction time, etc.22,26−30 It is important to point out that the hydrothermal or solvothermal approaches are very promising for a large-scale production of 1D Ni(OH)2 nanomaterials at relatively simple and mild conditions, which represents an important factor for their practical applications. It has been extensively studied that, Paraotwayite, a natural αNi(OH)2 mineral with a chemical composition of Ni(OH)2−x (SO4,CO3)0.5x (x ≈ 0.6),31 can readily be prepared in forms of nanowires or nanobelts with large aspect ratios under mild hydrothermal conditions;7,20,27−29,32−35 the composition of these synthesized Paraotwayite-type α-Ni(OH)2 nanomaterials usually corresponds to Ni(OH)2−x(SO4)0.5x·yH2O (x ≈ 0.6, 0 < y < 1). However, compared to the great success in material synthesis, physical properties of these 1D Paraotwayite-type αNi(OH)2 nanomaterials have not been widely discussed. Previously, Zhou et al. reported that Ni(OH)1.4(SO4)0.3 nanobelts could be used as efficient photocatalysts for the degradation of methyl orange in aqueous solutions,32 though the corresponding photocatalytic mechanism remained. By using a two-step hydrothermal synthesis, Yang et al. prepared the core−shell type Ni(OH)1.4(SO4)0.3/C nanobelts,33 which could be converted to NiO/C core−shell nanobelts with interesting catalytic properties during the electrooxidation of glucose, although the electrochemical properties of the Ni(OH)1.4(SO4)0.3 nanobelts were not reported. Obviously, an improved understanding of the physical properties of 1D Paraotwayite-type α-Ni(OH)2 nanomaterials is interesting and important for their applications in photoelectrochemical devices. It is also worth pointing out that, in addition to the possible size effect that is typical for nanomaterials with small featured sizes and large surface areas, the presence of intercalated sulfate anions in Paraotwayite-type α-Ni(OH)2 may bring new physical properties when compared with other α-Ni(OH)2 nanomaterials.19 In this work, by using a simple hydrothermal reaction of NiSO4 and NaOH at 160 °C, we have successfully achieved a mass production of single-crystalline Paraotwayite-type αNi(OH)2 nanowires with typical diameters of 20−30 nm and lengths up to several micrometers. Structural and chemical analyses indicate that the as-prepared nanowires have a mean composition of Ni(OH)1.64(SO4)0.18·0.3H2O and crystallize in a layered monoclinic structure (unit cell parameters: a = 0.78867(1) nm, b = 0.29661(3) nm, c = 1.35164(2) nm, and β = 91.1°). NiO nanowires are also prepared from a topotactic transformation of the Paraotwayite-type α-Ni(OH)2 nanowire precursors. Thanks to their intrinsic layered structure, the asprepared Paraotwayite-type α-Ni(OH)2 nanowires show enhanced electrochemical performance compared to the corresponding NiO nanowires, such as fast kinetics and high energy density. The as-prepared Paraotwayite-type α-Ni(OH)2 nanowires exhibit also an interesting anodic electrochromism and can change color from light green to dark brown upon OH− ion insertion. Details will be reported in the following sections of this paper.

Article

EXPERIMENTAL PROCEDURES

Chemicals. All chemicals (reagent grade) used in this work were purchased from Sigma-Aldrich Co. and used without further purification. Distilled water and ethanol (96%) were used throughout the experiment. Synthesis of Paraotwayite-type α-Ni(OH)2 Nanowires. The materials were synthesized by reacting freshly precipitated nickel hydroxide with high concentrations of nickel sulfate under hydrothermal conditions.18 For a typical synthesis, 2.5 g of NiSO4·6H2O was dissolved into 40 mL of water under constant stirring at 500 rpm, and to this solution was added 0.15 g of NaOH (note that a white precipitation forms immediately upon mixing). The mixture was kept stirring for 10 min until a uniform slurry was formed, which was subsequently transferred into a Teflon-lined stainless steel autoclave, sealed, and maintained at 160 °C for 24 h without shaking or stirring during the reaction. After the autoclave had been cooled down to room temperature by tap water, the green-colored precipitates were filtered, washed three times with distilled water, and finally dried at 80 °C overnight. Preparation of Paraotwayite-type α-Ni(OH)2 Nanowire Electrodes. Electrochemical electrodes were prepared by spin-coating of a nanowire suspension on indium tin oxide (ITO) glass substrates at 2000 rpm. The nanowire suspension was prepared by adding 0.04 g of the as-prepared Paraotwayitetype α-Ni(OH)2 nanowires into 20 mL of ethanol under ultrasonic dispersion. Before the spin-coating process, the ITO glass substrates (25 mm × 25 mm × 1 mm; surface resistance: 30−60 Ω/square) were cleaned sequentially in ultrasonic baths of ethanol and acetone, each for about 10 min. The spincoating process was repeated several times to form a uniform nanowire coating on the ITO glass substrates. Finally, the coated ITO glass substrates were dried at 80 °C overnight before further characterization. Optical Property Measurements. Optical absorption/ transmission spectra of the as-prepared materials were measured on a PerkinElmer Lambda 1050 UV/vis/NIR spectrophotometer within the ultraviolet−visible-near-infrared spectral region. Samples for optical absorption measurement were prepared by ultrasonically dispersing a small amount of asprepared Paraotwayite-type α-Ni(OH)2 nanowires in water to form a dilute suspension with a concentration of about 5 × 10−4 g/L. For comparison purposes, samples were also prepared by spin-coating the nanowire suspension on float glass substrates; spectra were then collected in the transmission mode. Characterization. The crystal structure of the as-synthesized materials was determined by XRD (Bruker AXS D8 Advance diffractometer with Cu Kα 1 radiation). The morphology and chemical composition of the as-prepared materials were investigated by field-emission scanning electron microscopy (SEM, Zeiss Supra 55VP) and transmission electron microscopy (TEM, JEOL JEM-2010) equipped with energy-dispersive X-ray spectrometers (EDS). Thermogravimetric analysis (TGA) was performed on a Netzsch STA 449C thermo-microbalance in a nitrogen atmosphere at a heating rate of 10 °C min−1. Attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectra were recorded on a Nicolet 8700 FTIR Spectrometer (Thermo Scientific). Electrochemical properties were characterized on an Autolab electrochemical workstation (PGSTAT302N). A three-electrode electrochemical cell was prepared, where Pt wire, a nanowire-ITO electrode, 17295

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a Ag/AgCl (3.0 M KCl) electrode, and 1 M NaOH aqueous solution acted as counter electrode, working electrode, reference electrode, and electrolyte, respectively. All measurements were performed at room temperature.

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RESULTS AND DISCUSSION Structural Features. Figure 1 shows a typical XRD pattern of the as-synthesized product. The positions of the XRD peaks

Figure 1. XRD pattern of the as-synthesized material from the hydrothermal reaction of NiSO4 and NaOH at 160 °C. The bars at the bottom represent the calculated diffraction pattern from JCPDS 411424 (wavelength: 1.5406 Å).

do not match the two known Ni(OH)2 polymorphs, that is, αand β-Ni(OH)2. Instead, the diffraction pattern can be wellindexed on the basis of a nickel sulfate hydroxide, Paraotwayitetype α-Ni(OH)2, with monoclinic unit cell of dimensions of a = 0.789 nm, b = 0.296 nm, c = 1.363 nm, and β = 91.1° (JCPDS 41-1424).31 Some of the high-angle reflections are found to be slightly offset, indicating a certain degree of structural modifications in the as-prepared materials. The calculated lattice parameters for the as-synthesized Paraotwayite-type αNi(OH)2 are a = 0.78867(1) nm, b = 0.29661(3) nm, c = 1.35164(2) nm, and β = 91.1°; however, detailed structural information, such as atomic sites of the as-synthesized Paraotwayite-type α-Ni(OH)2 materials, may require structural refinement of high-quality XRD data, which is difficult at this stage. It is also worthwhile to note that the width of (h0l) reflections is larger than the width of the (010) and (212̅) reflections, indicating that the as-synthesized materials probably have an anisotropic morphology. Because the b axis is very short compared to the a and c axes, an enhanced growth kinetics along [010] can be expected. The anisotropic morphology has been confirmed by the subsequent microstructure characterizations. Figure 2a shows the general morphology of the assynthesized Paraotwayite-type α-Ni(OH)2 crystallites. It is evident that a large quantity of fiber-like nanostructures has been produced. The fibrous morphology of the product is indicative of an anisotropic growth behavior of the Paraotwayite-type α-Ni(OH)2, in agreement with the XRD pattern analysis (Figure 1). Detailed SEM analyses reveal that the Paraotwayite-type α-Ni(OH)2 is pure nanowires without any secondary phases, such as nanoparticles or nanosheets, indicating the advantage of the solution-based synthetic method used in this work. Figure 2b shows an EDS spectrum of the Paraotwayite-type α-Ni(OH)2 nanowires, revealing clearly that

Figure 2. (a) SEM image and (d) EDS pattern of the as-synthesized Paraotwayite-type α-Ni(OH)2 nanowires.

the as-prepared materials are composed of three elements, namely, O, Ni, and S (note that H is not detectable by EDS). The average atomic ratio of O:Ni:S is about 69.3:26.0:4.7, within the experimental error range, which gives a mean chemical composition of Ni(OH)1.64(SO4)0.18·0.3H2O for the as-synthesized nanowires. It is worthwhile to note that the sulfate content obtained in this work is lower than that reported previously, Ni(OH)1.4(SO4)0.3 (JCPDS 41-1424),31 which may account for the slight mismatches observed for high-angle XRD reflections (Figure 1). Yang et al. reported also Paraotwayitetype α-Ni(OH)2 nanowires with different stoichiometries, such as Ni(OH)1.62(SO4)0.19·0.24H2O or Ni(OH)1.68(SO4)0.16· 0.19H2O.7,34 These experimental results indicate that the crystallization of Paraotwayite-type α-Ni(OH)2 nanomaterials is different from that of the bulk materials and involves a certain degree of local structural modifications. More details about microstructures of the as-prepared nanowires were obtained from TEM images, as shown in Figure 3. Again, the TEM analysis reveals that the assynthesized materials are pure nanowires without any secondary phases presented. Moreover, the Paraotwayite-type α-Ni(OH)2 nanowires are structurally uniform and have a fairly narrow diameter distribution of about 20−30 nm. Figure 3c displays a typical ED pattern taken from an individual wire; the single set of diffraction spots indicates clearly its singlecrystalline nature. The growth direction of the nanowires is found to be along ⟨010⟩, that is, the b axis. This finding is also 17296

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Figure 3. (a, b) TEM images of the as-synthesized Paraotwayite-type α-Ni(OH)2 nanowires. (c) The corresponding ED pattern from an individual fiber; the analyzed area is marked by the circle in panel (b).

Figure 5. TGA curve of the as-prepared Paraotwayite-type α-Ni(OH)2 nanowires.

consistent with the XRD data (Figure 1), where reflections with large “k” components in the Miller indices have usually a narrow peak width in comparison with the (h0l) reflections. It is important to point out that the as-synthesized Paraotwayite-type α-Ni(OH)2 nanowires are very sensitive to the electron beam irradiation under high-vacuum conditions, as reported previously.29 For example, after being illuminated by the electron beam for several minutes, the nanowires rapidly degrade and a copious amount of holes with diameters of several nanometers are formed within the nanowires (Figure 4a). Figure 4b shows an ED pattern of a nanowire, which is

the free water molecules that are absorbed on the surface or trapped in the interlayer region; from 200 °C to about 400 °C, the second weight loss is about 14.0%, which is mainly due to the dehydroxylation of the Ni(OH)2 principle layers; and the third weight loss, from 400 °C to about 800 °C, is about 14.5%, which can be attributed to the decomposition of the sulfate anions from the interlayer space. The involved dehydration and decomposition processes can be understood as follows: 30 − 200 °C

Ni(OH)1.64 (SO4 )0.18 ·0.3H 2O ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Ni(OH)1.64 (SO4 )0.18 + 0.3H 2O

(1) 200 − 400 °C

Ni(OH)1.64 (SO4 )0.18 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 0.18NiSO4 + 0.82NiO + 0.82H 2O

(2) 400 − 800 °C

0.18NiSO4 + 0.82NiO ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ NiO + 0.18SO3

(3)

The theoretical weight loss calculated for eqs 1−3 is about 4.8, 14.2, and 16.2%, respectively, in harmony with the experimental values. It indicates that the composition Ni(OH)1.64(SO4)0.18· 0.3H2O can be used as a good stoichiometry to represent the Paraotwayite-type α-Ni(OH)2 nanowires obtained in this work. However, it should be pointed out that a complete decomposition of sulfate anions according to eq 3 would give a theoretical weight loss of ∼16.2%, which is higher than the experimental value, ∼14.5%. It implies that the decomposition of sulfate anions is not completed even at 800 °C. This has been observed by the subsequent chemical analyses. The dehydration and decomposition reactions of the assynthesized Paraotwayite-type α-Ni(OH)2 nanowires at elevated temperatures can also be understood by XRD patterns and the corresponding SEM images (also with EDS data) reported in Figures 6 and 7, respectively. As shown in Figure 6, pattern b, after being annealed at 300 °C, a certain degree of degradation of the original monoclinic phase can be clearly noticed, though the main phase is still preserved. At 500 °C, the monoclinic structure has transformed into cubic NiO (space group Fm3̅m; lattice parameter: a = 0.41407 nm). No NiSO4 has been detected by XRD (eq 2 and Figure 6, pattern c); therefore, it may exist as an amorphous structure. The corresponding SEM images shown in Figure 7a,b indicate that the dehydration reactions (see eqs 1 and 2) do not change the fibrous morphology of the materials. However, as the

Figure 4. (a) TEM image of an individual Paraotwayite-type αNi(OH)2 nanofiber after being irradiated by the electron beam for 5 min. (b) The corresponding ED pattern.

taken after the nanowire has been irradiated by the electron beam for about 5 min. The ED pattern can be indexed on the basis of a cubic phased NiO (space group: Fm3̅m (225); lattice parameter: a = 0.41407 nm). It indicates that the Paraotwayitetype α-Ni(OH)2 can transform into NiO by electron-beam irradiation under high-vacuum conditions due to the rapid dehydration/decomposition of Ni(OH)2.6,36 Figure 5 reports the thermogravimetric analysis (TGA) results of the as-synthesized Paraotwayite-type α-Ni(OH)2 nanowires, revealing clearly the characteristic stepwise weight loss due to the dehydration and decomposition reactions.6,36 The total weight loss up to 800 °C is about 33.5%, which consists of three distinguished contributions: the first weight loss (∼4%) from 30 °C to about 200 °C can be attributed to 17297

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may bond to either the hydroxyl groups from the Ni(OH)2 principle layers or the sulfate anions through hydrogen bonds. These distinctive bonding structures were discussed in this work by vibrational spectroscopy, FTIR. Figure 8a reports the FTIR spectrum of the as-prepared Paraotwayite-type α-Ni(OH)2 nanowires, showing character-

Figure 6. XRD patterns of (a) as-prepared Paraotwayite-type αNi(OH)2 nanowires and those being annealed in air at (b) 300, (c) 500, and (d) 800 °C for 2 h.

Figure 8. FTIR spectrum of (a) the as-synthesized Paraotwayite-type α-Ni(OH)2 nanowires and (b) NiO nanowires. The noise around 2000 cm−1 is due to the ATR diamond crystal.

Figure 7. SEM images of the as-prepared Paraotwayite-type αNi(OH)2 nanowires after an annealing treatment in air at (a) 300, (b) 500, and (c) 800 °C for 2 h. Panel (d) shows the corresponding EDS pattern of the materials annealed at 800 °C (c), revealing the residual of sulfur (∼1.3% by atomic ratio).

istic vibrational features related to OH−, H2O, SO42−, and Ni-O species.28,32,35 The absorption bands at about 3430 and 1630 cm−1 can be attributed to hydrogen-bonded OH stretching and bending vibrations, respectively, indicating the presence of water molecules in the structure. Moreover, there are sharp and intensive O−H vibrations at about 3640 cm−1, which could be related to the free O−H groups in the as-prepared Paraotwayite-type α-Ni(OH)2 nanowires.37 These free O−H groups are related to the Ni(OH)2 principle layers and accommodate in the interlayer region. The intercalated sulfate anions give also characteristic vibrations from 650−1200 cm−1. For example, the two strong absorptions around 1100 and 710 cm−1 can be attributed to HSO4− and SO42− vibrations, respectively.28 It indicates that the interlayer water molecules are most likely also bonded with sulfate anions. Ni−O vibrations result in typical absorption bands at low wavenumbers, such as at 408, 451, and 600 cm−1, in agreement with the previous reports.28,32,35,38 The intrinsic vibrational features of the as-prepared Paraotwayite-type α-Ni(OH)2 nanowires are helpful for characterizations of other layered-structured Ni(OH)2 materials. As discussed above, the annealing treatment of the asprepared Paraotwayite-type α-Ni(OH)2 nanowires at 500 °C results in an isomorphous transformation to NiO nanowires

annealing temperature increases to 800 °C, on the one hand, the crystallinity of NiO products has been improved (Figure 6, pattern d); on the other hand, NiO nanoparticles instead of nanowires are produced (Figure 7c). These results indicate a potential method for the preparation of NiO nanowires/ nanoparticles by annealing the as-synthesized Paraotwayite-type α-Ni(OH)2 nanowires at suitable temperatures.20,29,33 However, the existence of sulfur residuals in the obtained NiO materials, as evidenced by the chemical analysis (Figure 7d), must be taken into account when analyzing the property of NiO nanowires/nanoparticles. Vibrational Properties. Details of the crystal structure of Paraotwayite-type α-Ni(OH)2 are not well-understood so far.31,37 Previously, Zhang et al. reported a preliminary structural model for Paraotwayite-type α-Ni(OH)2,29 which is constructed by intercalating sulfate anions and water molecules into the interlayer region of a basic β-Ni(OH)2 structure. Such a layered configuration may involve some complicated bonding structures within the interlayer region. For example, the intercalated sulfate anions may bond to the Ni(OH)2 principle layers through hydroxyl deficiency, whereas the interlayer water 17298

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(Figure 6, pattern c, and Figure 7b).20,29 During the dehydration and decomposition processes, intensive local structural evolutions in the interlayer region of Paraotwayitetype α-Ni(OH)2 nanowires can be expected. Figure 8b shows the FTIR spectrum of the resulting NiO nanowires. It can be seen clearly that the sharp and intensive O−H vibrations at about 3640 cm−1 have disappeared, which is related to the dehydroxylation of the Ni(OH)2 → NiO transformation. Moreover, a significant modification on vibrational features of the intercalated sulfate anions can be observed, which corresponds to the decomposition of the sulfate anions. However, the presence of sulfur-related residuals is also noticeable, as evidenced by the absorptions at about 1112 and 1058 cm−1, which may be attributed to S−O symmetric stretching vibrations.39 In this regard, the obtained NiO nanowires from the annealing treatment of Paraotwayite-type α-Ni(OH)2 nanowire precursors are actually S-doped, which shall be taken into account when discussing their related physical properties.33 However, details of these sulfur-related residuals remain at this stage. Optical Properties. Understanding the optical properties of the as-prepared Paraotwayite-type α-Ni(OH)2 nanowires is obviously important. Taking into account that the band gap energy (Eg) is one of the most important parameters of semiconductors, we determine the Eg of the as-prepared Paraotwayite-type α-Ni(OH)2 nanowires on the basis of their UV−visible optical absorption spectrum, in which the energy dependence of the absorption coefficient α in the region near the absorption edge can be expressed as40 α∝

plot of (αhv)1/2 versus hv, which indicates a band gap energy of around 3.8 eV for the as-synthesized Paraotwayite-type αNi(OH)2 nanowires; moreover, no linear relation was observed for the same data when (αhv)2 versus hv was plotted. It reveals that the as-prepared nanowires are semiconducting with indirect-allowed transitions. Figure 10 shows the absorption spectrum of NiO nanowires obtained by annealing the as-prepared Paraotwayite-type α-

Figure 10. UV−visible-near-infrared absorption spectrum for the obtained NiO nanowires by annealing the as-synthesized Paraotwayitetype α-Ni(OH)2 nanowires at 500 °C. Inset is the (αhv)2 vs hv curve.

Ni(OH)2 nanowires at 500 °C (see also Figure 6, pattern c, and Figure 7b). The optical transitions at 402 and 670 nm observed for the hydroxide precursors (Figure 9) have almost disappeared, although a broad absorption ranging from 450 to 600 nm is still noticeable. The band gap energy of the obtained NiO nanowires was also determined, and a directallowed electron transition with an Eg ∼ 3.8 eV was obtained (inset of Figure 10). In contrast, the indirect-allowed transition model, which has been reported previously for some NiO nanomaterials,41 resulted in a smaller Eg value of about 3.0 eV in this work, compared to 3.8−4.3 eV that are typical for NiO materials.42 In this regard, we suggest that the direct-allowed electron transition with an Eg ∼ 3.8 eV should be considered for the obtained NiO nanowires. It is worth pointing out that the presence of sulfur-related residuals may affect also the observed optical properties of NiO nanowires, though the details remain at this stage. Electrochemical Properties. Figure 11 shows typical examples of cyclic voltammograms (CVs) obtained for the asprepared Paraotwayite-type α-Ni(OH)2 nanowires in 1 M NaOH aqueous solution during the first 20 scans between 0.0 and 0.8 V (vs Ag/AgCl). A broad cathodic current peak emerges in the applied potential range of 0.23−0.27 V (vs Ag/ AgCl), and an anodic current peak appears around 0.48−0.51 V, which can be attributed to the electrochemical redox reaction:43

(hv − Eg )η hv

(4)

where hv is the energy of the incident photon and η = 1/2 or 2 when the electron transitions are direct-allowed and indirectallowed, respectively. With an appropriate η, a plot of (αhv)1/η versus hv is linear near the edge, and the intercept of the line on the abscissa gives the optical Eg. Figure 9 shows the UV−

Figure 9. UV−visible-near-infrared absorption spectrum for the assynthesized Paraotwayite-type α-Ni(OH)2 nanowires. Inset is the (αhv)1/2 vs hv curve.

Ni(OH)2 + OH− ↔ NiOOH + H 2O + e−

(5)

As the number of the redox cycle increases, it can also be noticed that the oxidation peak position shifts slightly from 0.49 to 0.51 V along with an increase of anodic current density; in addition, the reduction peak position shifts from 0.27 to 0.23 V, with also an increased cathodic current density. It reveals clearly that the electrochemical redox reaction (eq 5) is primarily dominated by the insertion/diffusion of OH− ions. The standard electrode potential of the electrochemical redox

visible-near-infrared absorption spectrum for the as-synthesized Paraotwayite-type α-Ni(OH)2 nanowires. The materials are characterized by two absorption bands at 402 and 670 nm, and an absorption edge near the band gap transition. The two absorption bands are tentatively attributed to the intra-3d transitions related to Ni2+ ions, according to the previous study on Ni(OH)2 and NiO thin films.41 The inset of Figure 9 is the 17299

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nanowires exhibit an enhanced electrochemical performance, such as fast kinetics, when compared to NiO nanowires. Since the electrochemical ion insertion/extraction reaction, for example, eq 5 or eq 6, is strongly correlated to the length of the diffusion path of the ions, the structural openness (i.e., large interlayer space and interlayer species with high mobility) of the layer-structured Paraotwayite-type α-Ni(OH)2 nanowires will facilitate the diffusion of the hydroxyls during the redox reactions. Moreover, the intensity of the cathodic and anodic currents in Paraotwayite-type α-Ni(OH)2 nanowires are much larger than those of the NiO nanowires, revealing that the energy density in the Paraotwayite-type α-Ni(OH)2 nanowires is higher than that in the NiO nanowires. We have calculated their interfacial capacitances on the basis of the corresponding CV curves by summing the charge current in the positive and negative scan directions and dividing the sum by twice the scan rate.44 The calculated capacitance is 3.96 and 0.38 μF/cm2 for the as-prepared Paraotwayite-type α-Ni(OH)2 nanowires and the corresponding NiO nanowire counterparts. NiO nanoplatelet arrays have previously been reported as supercapacitor electrodes;14 consequently, Paraotwayite-type α-Ni(OH)2 nanowires with a higher energy density than NiO nanowires may also represent an interesting material system for energystorage applications. The as-prepared Paraotwayite-type α-Ni(OH)2 nanowires exhibit also an interesting anodic electrochromism4 and can change color from light green to dark brown during the oxidation cycles, which can be attributed to the Ni2+ → Ni3+ transition (see eq 5).14,41 Figure 13 shows the corresponding transmission spectra obtained before and after the electrochemical test. Upon the cathodic scan, interestingly, the nanowire electrode does not completely bleach to its original state (i.e., light green); even by keeping the cathodic scan at 0.0−0.2 V (vs Ag/AgCl) for about 5 min, part of the nanowires remain still light brown in color. We suspect that some

Figure 11. CV curves of the as-prepared Paraotwayite-type αNi(OH)2 nanowires in 1 M NaOH aqueous solution. Arrows show the scan direction. Scan rate: 50 mV/s.

reaction of Ni(OH)2 nanowires can be calculated to be ∼0.37 V (vs Ag/AgCl). A comparison between the electrochemical properties of the as-prepared Paraotwayite-type α-Ni(OH)2 nanowires and the corresponding NiO nanowire counterparts (see Figure 6, pattern c, and Figure 7b) was also performed in this work, as shown in Figure 12. The NiO nanowires show also one redox couple with an oxidation peak at ∼0.52 V (vs Ag/AgCl) and a reduction peak at ∼0.18 V. The involved electrochemical redox reaction can be given as14 NiO + OH− ↔ NiOOH + e−

(6)

with a standard electrode potential of ∼0.35 V (vs Ag/AgCl), which is very close to that of the Paraotwayite-type α-Ni(OH)2 nanowires. It implies that the corresponding OH− ion insertion reactions are thermodynamically similar in Paraotwayite-type αNi(OH)2 and in NiO. However, Paraotwayite-type α-Ni(OH)2

Figure 12. CV curves of (a) the as-prepared Paraotwayite-type α-Ni(OH)2 nanowires and (b) the NiO nanowires. Note that the Y axis has different scales. Electrolyte: 1 M NaOH aqueous solution. Scan rate: 50 mV/s. The corresponding crystal structures are also shown for comparison (interlayer species are removed for simplification). 17300

dx.doi.org/10.1021/jp405149d | J. Phys. Chem. C 2013, 117, 17294−17302

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the support from the Research Council of Norway and several partners through the Research Centre on Zero Emission Buildings (ZEB).

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Figure 13. Transmission spectra of the as-prepared Paraotwayite-type α-Ni(OH)2 nanowire electrode at (a) original and (b) colored states, respectively. Insets show the corresponding samples.

Paraotwayite-type α-Ni(OH)2 may irreversibly convert to NiO during the electrochemical cycles, probably due to their structural instability (see Figure 5). It is also worth pointing out that, during the electrochemical cycling, the corresponding NiO nanowire counterparts (e.g., Figure 12b) did not exhibit the color change from colorless to dark brown, as reported previously.41,43,45 It seems that the electrochemical properties of NiO nanowires have somehow been modified due to the existence of impurities (e.g., sulfur, Figure 7d). Further investigations are still underway for the detailed electrochromism of Paraotwayite-type α-Ni(OH)2 and NiO nanowires.

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REFERENCES

CONCLUSIONS

Single-crystalline Paraotwayite-type α-Ni(OH)2 nanowires with diameters of 20−30 nm and lengths up to several micrometers have been prepared by using a simple hydrothermal reaction of NiSO4 and NaOH at 160 °C. The as-prepared nanowires crystallize in a layered monoclinic structure (unit cell parameters: a = 0.78867(1) nm, b = 0.29661(3) nm, c = 1.35164(2) nm, and β = 91.1°) with sulfate anions and water molecules being sandwiched in the two-dimensional Ni(OH)2 principle layers. Paraotwayite-type α-Ni(OH)2 nanowires have a mean chemical composition of Ni(OH)1.64(SO4)0.18·0.3H2O, which can transform into NiO nanowires through dehydration and decomposition. The obtained NiO nanowires are S-doped. An indirect-allowed electron transition with a band gap energy of about 3.8 eV has been obtained for the as-prepared Paraotwayite-type α-Ni(OH)2 nanowires, compared to a directallowed electron transition (Eg ∼ 3.8 eV) for the corresponding NiO nanowire counterparts. Thanks to their intrinsic layered structure, the as-prepared Paraotwayite-type α-Ni(OH)2 nanowires exhibit an improved electrochemical performance, such as fast kinetics and high energy density, compared to the NiO nanowire counterparts. Moreover, Paraotwayite-type α-Ni(OH)2 nanowires show an anodic electrochromism and can change color from light green to dark brown, which is related to the redox couple of Ni2+ → Ni3+. The interesting structural, optical, and electrochemical properties of the as-prepared Paraotwayite-type α-Ni(OH)2 nanowires may suggest potential applications of these 1D nanomaterials in photoelectrochemical and energy-storage devices. 17301

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