J. Phys. Chem. B 2001, 105, 3039-3044
3039
Spectroelectrochemistry of Nanostructured NiO Gerrit Boschloo* and Anders Hagfeldt Department of Physical Chemistry, Uppsala UniVersity, Box 532, S-751 21 Uppsala, Sweden ReceiVed: September 27, 2000; In Final Form: December 8, 2000
Transparent nanostructured NiO electrodes have been prepared by heating Ni(OH)2 sol-gel films at a temperature of 300-320 °C. Nanostructured NiO (bunsenite) behaves as a p-type semiconductor and has an indirect band gap of 3.55 eV. It shows a strong anodic electrochromic effect, as it changes color from transparent to brown-black upon application of positive potentials. This effect is caused by oxidation of Ni atoms located at the NiO/electrolyte interface. Electrochemical oxidation reactions are highly reversible in both aqueous and nonaqueous electrolytes. In aqueous electrolyte, the half-potentials show a Nernstian pH dependence, whereas in nonaqueous electrolytes, the type of cation present determines the shape and position of the cyclic voltammogram.
Introduction
Experimental Section
In recent years there has been a great interest in the electrochemistry of nanostructured metal oxide films prepared from colloidal solutions. Nanostructured films combine a small primary particle size (1-100 nm) with a high porosity (typically ∼50%), resulting in a large surface area, typically 2-3 orders of magnitude greater than the projected surface area. This gives nanostructured electrodes great advantages over conventional electrodes in many applications such as photoelectrochemical solar cells1-3 and electrochromic windows.4,5 The metal oxides that have been employed are typically wide band gap n-type semiconductors such as TiO2 1,4-6, SnO2 2, and ZnO.7 There is considerable scientific and technological interest in developing nanostructured metal oxides with p-type semiconductivity. There are, however, relatively few metal oxides that tend to be p-type. Nickel(II) oxide, NiO, is one of those and was chosen for this study, as it is a stable wide band gap material. It has been demonstrated that thin-film NiO can be used as a transparent p-type (semi)conducting layer.8 Nickel oxides exhibit anodic electrochromism, and are being studied for application in smart windows.9 It should be noted that a large part of this research is focused on Ni(OH)2, a material with good ion intercalation properties and widely used in rechargeable batteries. Crystalline NiO, on the other hand, has a dense structure that does not allow for intercalation of chargecompensating anions. Nevertheless, films containing nanocrystalline NiO, prepared by various methods such as evaporation,10 sputtering,11-13 electrodeposition,14 and sol-gel techniques,15 show good electrochromic properties. Recently, nanostructured NiO electrodes prepared by sol-gel and electrodeposition techniques have been applied as electrochemical supercapacitors16,17 and dye-sensitized photocathodes.18 In this study, nanostructured NiO films of good optical quality have been prepared using sol-gel techniques. These films show excellent electrochromic properties as reversible oxidation of the surface leads to a strong and rapid dark brown coloration. The (spectro)electrochemical properties are studied in a range of aqueous and nonaqueous electrolytes.
Nanostructured NiO films were prepared as follows: 4.75 g (0.020 mol) NiCl2‚6H2O (Merck) was dissolved in 100 mL deionized water (Barnstead purification system), and 45 mL of a 1.0 M NaOH solution was added under strong stirring. A green Ni(OH)2 precipitate formed immediately. After settling overnight, the clear supernatant was removed and the precipitate (∼70 mL) was dialyzed against deionized water. Next, the suspension was concentrated using a rotary evaporator until it was visibly viscous. A small amount of acetic acid (10 drops of 1.75 M HAc) and up to 30% ethanol were added to improve spreading and drying characteristics. The sol was spread using a glass rod on conducting SnO2:F glass (TEC 15, Libby Owens Ford) or microscope glass (Menzelglas), which was masked by adhesive tape (Scotch Magic, 40 µm thickness). The film was dried in air and fired at 300-320 °C for 15 min in a hot air stream. The heating and cooling times were typically about one minute. The film thickness was measured using a Sloan Dektak 3 profilometer. For the TEM study, part of a nanostructured NiO film was scraped off from the substrate, suspended in ethanol, and sonicated for 15 min. One drop was placed on a TEM grid. A Hewlett-Packard diode array spectrometer (model 8453) was used for UV-visible NIR characterization. Electrochemical experiments were performed using a EG&G 273 potentiostat and a one-compartment, 3-electrode cell (a quartz optical cell in spectroelectrochemical measurements) with a nanostructured NiO working electrode and a Pt wire counter electrode. In aqueous electrolytes, a Ag/AgCl/3 M KCl reference electrode (CH Instruments) was used and potentials are reported against Ag/AgCl. In nonaqueous electrolytes, a Ag/Ag+ reference electrode (CH Instruments) was used and potentials are reported versus the ferrocene/ferrocenium redox couple. Electrolytes were made using ultrapure water, 3-methoxypropionitrile (Fluka), potassium chloride (Merck), lithium triflate, lithium perchlorate, and tetrabutylammonium triflate (all from Aldrich), which were used as supplied.
* Corresponding author. Email:
[email protected] Results and Discussion Structural and Optical Characterization. Nanostructured NiO films were obtained by firing Ni(OH)2 gel films at a
10.1021/jp003499s CCC: $20.00 © 2001 American Chemical Society Published on Web 03/27/2001
3040 J. Phys. Chem. B, Vol. 105, No. 15, 2001
Boschloo and Hagfeldt
Figure 1. X-ray diffractogram of nanostructured NiO (film thickness 0.8 µm) on glass. The reference spectrum of NiO (bunsenite) is shown below.
Figure 3. UV-visible spectra of nanostructured nickel oxide films. (1) Nanostructured NiO (0.80 µm thickness) on glass, measured in air and corrected for glass absorption. (2) Nanostructured NiO (0.75 µm) on conducting glass, measured in electrolyte (0.2 M KCl + 0.02 M phosphate buffer, pH 6.8) at open circuit potential. Corrected for cuvette, electrolyte, and conducting glass absorption. (3) Nanostructured NiO (as (2)), after polarization at -0.20 V. (4) Nanostructured Ni(OH)2 on glass. Same sample as (1), but before firing. Inset: data of (1) used for the determination of the indirect band gap of NiO.
Figure 2. Transmission electron micrograph of nanostructured NiO, removed mechanically from a glass substrate. The bar corresponds to 50 nm.
temperature of 300-320 °C. The conversion of Ni(OH)2 to NiO occurs at about 250 °C.14,17,19,20 X-ray diffraction reveals that cubic NiO (bunsenite) is formed, see Figure 1. From the peak broadening, an average NiO crystallite size of 4.5 nm is calculated using the Scherrer equation. In addition to the bunsenite peaks, a small but sharp peak is observed at 32°, which appears to be a NaCl residue. Figure 2 shows a transmission micrograph of nanostructured NiO. The individual NiO nanocrystals are strongly agglomerated, with agglomerate sizes larger than 30 nm. The thickness of the NiO films in this study was in the range of 0.3 to 1.5 µm. Thicker films tended to crack during the heat treatment or had a poor optical quality. UV-visible spectra of nanostructured nickel oxide on glass and conducting glass are shown in Figure 3. Before firing, only Ni(OH)2 is present, which is fully transparent to the eye (trace 4 in Figure 3). On closer inspection, a weak absorption throughout the spectrum with maxima at 380 and 670 nm is observed, which can be attributed to intra-3d transitions in NiII ions. After firing at 300-320 °C, nanostructured NiO is formed (traces 1 and 2 in Figure 3). A strong increase in absorption in the UV region is observed at wavelengths smaller than 360 nm, while a broad absorption appears in the visible and near-infrared region. The latter gives the otherwise clear (i.e., not light scattering) film a brown color. This color disappears completely when a small negative potential is applied to nanostructured NiO electrode in an electrochemical cell, see Figure 3, trace 3. During the heat treatment in air, part of the NiII atoms at the surface of the nanostructured electrode are oxidized to NiIII,
which gives rise to a brown or black coloration.20 This process is fully reversed by the electrochemical treatment. A small residual absorbance throughout the visible region with a maximum at 720 nm persists in the bleached film, which can be assigned to intra-3d transitions of NiII in the cubic crystal field of NiO.21,22 The strong absorption in the UV region is attributed to band gap absorption in NiO.21 The optical transition type and the band gap Eg can be determined using Equation 1
hν - Eg ∝ (Rhν)n
(1)
where hν is the photon energy, R is the absorption coefficient, and n is either 2 for a direct transition or 1/2 for an indirect transition.3 The inset in Figure 3 shows that a linear relation is found for n) 1/2 between 3.6 and 4.0 eV (linear fit with R > 0.998). From the inflection point, the band gap is determined to be 3.55 eV. No linear relation was observed for the same data when n ) 2 was used (not shown). Cyclic Voltammetry. Figure 4a shows typical examples of cyclic voltammograms (CVs) obtained for nanostructured NiO in aqueous electrolyte. Two broad but distinct peaks are visible in both the anodic and the cathodic scan, which are due to redox reactions I and II. Similar peaks have been observed in CVs of single crystalline Li-doped NiO and were attributed to the oxidation of NiII atoms at the surface to NiIII and NiIV.23 The peak currents in the CVs of nanostructured NiO scale linearly with the scan rate (Figure 4b), which is in agreement with the occurrence of surface redox reactions. The ratio of the peak currents of reaction I and II is in the order of 0.5 to 0.6, whereas it was slightly more than 1 for the single crystal.23 If all surface Ni atoms would first oxidize to NiIII and then to NiIV, peak currents (or rather the area under the peaks) should be equal. Clearly, the situation is more complex, as will also be demonstrated in the experiments in aprotic electrolyte (see below). The anodic and cathodic charges that pass during the cyclic voltammograms (Figure 4a) are about 12 mC cm-2 for a 750
Spectroelectrochemistry of Nanostructured NiO
J. Phys. Chem. B, Vol. 105, No. 15, 2001 3041
Figure 5. Half potentials ((Epa + Epc)/2 in a 10 mV s-1 cyclic scan) for reactions I and II as function of pH. Linear regression fits are shown.
SCHEME 1: Surface Oxidation of NiO in Aqueous Electrolyte
Figure 4. (a) Cyclic voltammograms of a nanostructured NiO electrode (0.75 µm) in aqueous electrolyte (0.2 M KCl + 0.01 M KH2PO4 + 0.01 M K2HPO4, pH 6.8). Scan rates (mV s-1) are indicated. (b) Peak currents versus scan rate.
nm thick nanostructured NiO electrode. This charge is almost independent of scan rate (
