Chapter 5. Electrocatalytic Activity of NiO on Silicon Nanowires with a Carbon Shell and its
5.4 Result and Discussion
Figure 5.1 illustrates the synthesis of the NiO@SiNW/C on the Si substrate. The process is as follows: a) a piece of the Si substrate was immersed in an etching solution consisting of diluted hydrofluoric acid (HF) and silver nitrate (Ag(NO)3) at room temperature, b) the silver dendrites on the surface of the Si substrate were dissolved in diluted nitric acid (HNO3), resulting in the vertically aligned SiNW on the Si substrate, c) the carbon shell was thermally deposited onto the surface of the SiNW under a mixture of hydrogen (H2) and methane (CH4) gas at a temperature of 1075 °C, resulting in the production of SiNW/C, and d) a drop of nickel solution was dropped onto the SiNW/C, and this was annealed at 450 °C under Ar, finally resulting in the NiO@SiNW/C.
Figure 5.1 Schematic of the fabrication of NiO@SiNW/C. a) Ag deposition on the Si substrates, b) metal assisted chemical etching, c) carbon coating with chemical vapor deposition (CVD) in a mixture
of hydrogen and methane gas, d) NiO decoration onto the carbon shell with a nickel precursor solution.
Figures 5.2 (a)–(c) demonstrate the scanning electron microscopy (SEM) images of the NiO@SiNW/C on the Si substrate. The length and diameter of the SiNWs were ca. 5.5 μm and between 60-180 nm, respectively. As shown in Figures 5.2 (a)–(b), the SiNWs are vertically aligned, densely packed, and bundled as a result of the mutual attraction between them.135-136 The high- magnification SEM image confirms the presence of the NiO nanoparticles decorated on the carbon shell (Figure 5.2 (c) (obtained from Figure 5.2 (b)). Furthermore, we can confirm that the NiO@SiNW/C consists of Si, C, Ni, and O, as shown by the energy dispersive spectrometry (EDS) data in Figure 5.2 (d).
Figure 5.2 Scanning electron microscopy images of a) tilted NiO@SiNW/C, b) vertically aligned NiO@SiNW/C, c) enlarged scale of b), and d) EDS spectra obtained from c).
Figure 5.3 Transmission electron micrographs of a SiNW/C@NiO, and EDS elemental mapping micrographs of Si, Ni, O, and C.
Figure 5.4 a) Low-magnification transmission electron microscopy image of a single NiO@SiNW/C, b) high resolution image of SiNW/C with no NiO decoration (Inset corresponds to FFT-ED pattern of
SiNW), c) high resolution image of NiO@SiNW/C (Inset corresponds to FFT-ED pattern of NiO), and d) XRD pattern of NiO.
Figures 5.4 (a)–(c) show the low-magnification transmission electron microscopy (TEM) images of a single NiO@SiNW/C, and the high-resolution TEM images for the detailed nanostructures of the SiNW/C with and without the NiO decoration, respectively. As shown in the inset of Figure 5.4 (b), the fast Fourier transformed electron diffraction (FFT-ED) pattern of the [011] zone axis confirms that the SiNW is single-crystalline with the [100] direction along the wire axis. As shown in Figure 5.4 (b), the carbon shell on the surface of the SiNW grew along the wire axis and was randomly oriented. It had an interdistance of 0.34 nm, which is consistent with that of the (002) planes of the graphitic layers. From the high-resolution TEM image in Figure 5.4 (c), we confirmed the successful immobilization of the NiO decoration on the surface of the carbon shell; the lattice spacing of the NiO
is separated by 0.2 nm. The X-ray diffraction (XRD) patterns of the sample prepared by dropping nickel solution on bare glass exhibited the characteristic peaks of the (111), (200), and (220) crystal planes at the 2θ degrees of 37.31°, 43.31°, and 62.91°; this is consistent with the standard pattern (JCPDS card No. 47-1049). Thus, both the FFT-ED pattern in the inset of Figure 5.4 (c) and the XRD pattern indicate that the planes of the NiO decorated on the carbon shell can be indexed as (200) planes.
Figure 5.5 XPS spectra for SiNW/C and NiO@SiNW/C. a) Survey scan, b) high resolution C 1s spectrum, c) high resolution Ni 2p3/2 spectrum, d) high resolution O 1s spectrum for NiO@SiNW/C.
We further investigated the functional groups and the chemical oxidation states present in the NiO@SiNW/C using X-ray photoelectron spectroscopy (XPS) analysis; the results are demonstrated in Figure 5.5. The XPS survey scan in Figure 5.5 (a) shows a comparison of the SiNW/C with and without NiO decoration. Peaks corresponding to Si 2p, C 1s, and O 1s for both the SiNW/C and NiO@SiNW/C were detected, and a peak corresponding to Ni 2p was also detected in the
The high-resolution XPS spectra corresponding to C 1s, Ni 2p3/2, and O 1s of the NiO@SiNW/C can be observed in Figures 5.5 (b)–(d). In Figure 5.5 (b), the main C 1s peak at 284.6 eV can be ascribed to the graphite-like sp2 carbon (C-C), and the two small peaks at 285.5 and 288.5 eV reflect the different bonding structures, which could be ascribed to C-O and COOH, respectively. In Figure 3c, the XPS spectra for Ni 2p3/2 were de-convoluted into three peaks. The binding energies at 853.8 and 855.8 eV can be ascribed to Ni2+ and Ni3+, and the shake-up satellite for Ni 2p3/2 can be observed at 861.0 eV. Since the peak area reflects the atomic concentration, we performed a numerical calculation from the curve fitting, and the Ni2+/Ni3+ ratio was determined as 1.66. We can obtain information about the chemical oxidation states from the XPS spectra for O 1s. In Figure 3d, the peaks at 529.7 eV and 531.5 eV can be assigned to divalent137-138 and trivalent NiO,137-140 specifically, nickel (II) oxide (NiO) and nickel (III) oxide (Ni2O3), respectively. Furthermore, the peak at 532.9 eV can be attributed to C-OH and/or C-O-C groups.141 The above explanation for the XPS analysis indicates that two nickel-oxide phases (NiO and Ni2O3) co-exist in the NiO@SiNW/C.
Figure 5.6 The J-V characteristics of dye-sensitized solar cells using a Pt coated reference electrode, SiNW/C, and NiO@SiNW/C as counter electrodes.
The NiO@SiNW/C electrode was directly applied as a counter electrode in the DSSCs. Figure 5.6 shows the J-V characteristics of the DSSCs with a Pt-coated reference electrode, SiNW/C, and NiO@SiNW/C as the counter electrodes. The reference cell from the Pt-coated counter electrode demonstrated an open circuit voltage (Voc) of 0.78 V, a short circuit current density (Jsc) of 15.3
mA/cm2, and a fill factor (FF) of 70% with a power conversion efficiency (η) of 8.47%. The SiNW/C counter electrode cell exhibited a Voc of 0.79 V, Jsc of 15.4 mA/cm2, and a FF of 68% with a power conversion efficiency of 8.40%. The maximum performance was achieved with the NiO@SiNW/C material. This material achieved a performance of 9.10% with a Jsc of 15.9 mA/cm2, a Voc of 0.78 V, and a FF of 73%. These values compare well to the performances of other cells and other cell performances, and are, in fact, superior. The difference in the performance of the NiO@SiNW/C material is mainly owed to the current density.
Table 5.1 Photovoltaic parameters of DSSCs based on FTO/Pt, SiNW/C, and NiO@SiNW/C counter electrodes measured under AM1.5G illumination, and EIS parameters fitted from the equivalent
circuit.
a) Obtained from symmetric cells Counter
Electrode
Voc [V]
Jsc [mA/cm2]
FF
[%]
ŋ
[%]
Rsa) [Ω·cm2]
Rcta) [Ω·cm2]
Zna) [Ω·cm2]
FTO/Pt 0.78 15.3 70 8.47 0.60 0.46 0.27
SiNW/C 0.79 15.4 68 8.40 0.17 0.23 0.29
NiO@SiNW/C 0.78 15.9 73 9.10 0.19 0.07 0.25
Figure 5.7 An analysis of cyclic voltammograms for the Pt coated counter electrode, SiNW/C counter electrode, and NiO@SiNW/C counter electrode.
To understand the influence of the current density on the electrochemical activity, an analysis of the cyclic voltammograms (CV) was conducted for the three different counter electrodes with symmetric cells. The three-electrode system used a Pt gauze and an Ag/AgCl electrode as the counter and reference electrodes, respectively.142 The electrochemical behavior of the electrodes with regard to the I−/I3
− redox reaction was investigated by CV (three-electrode system) measurements in order to clarify the catalytic mechanisms of the Pt-coated, SiNW/C, and NiO@SiNW/C counter electrodes in the Figure 5.7. The oxidation and reduction peaks of the NiO@SiNW/C counter electrode shift to higher and lower potentials compared with those of the Pt counter electrode. Furthermore, the cathodic and anodic peak current densities of the NiO@SiNW/C counter electrode are greater than the current densities of the Pt counter electrode. This implies that the NiO@SiNW/C counter electrode exhibits superior electrochemical activity.24, 142 Since the CV measurements were important in order to analyze the ion diffusivity and the catalytic mechanism in the electrochemical system, an additional CV measurement (two electrode system) was conducted with a scan rate of 100 mVs-1. As shown in Figure 5.8, the current density of the NiO@SiNW/C counter electrode was greater than those of the other counter electrodes. This indicates that the NiO@SiNW/C composite based electrode exhibits superior catalytic properties for the triiodide reduction compared with the other two electrodes.143
Figure 5.8 Cyclic voltammograms (CV) with the symmetric cells of SiNW/C and NiO@SiNW/C composite films compared with FTO/Pt coated on FTO substrates.
In order to investigate the interface characterization of the DSSCs with the three counter electrodes, we employed electrochemical impedance spectroscopy (EIS) with a circuit that has commonly been used in other studies.93, 144 Figure 5.9 shows the Nyquist plot of the symmetrical cells (often known as dummy cells) with the Pt, SiNW/C, and NiO@SiNW/C counter electrodes. The first semicircle at high frequency was assigned to the charge transfer at the interface between the electrode of interest and the electrolyte, while the second semicircle at low frequency was primarily associated with the Nernst diffusion of I3
− within the electrolyte.94-95, 119, 145
The introduction of NiO on the surface of the silicon nanowire arrays with a carbon shell induces an obvious improvement in the electrocatalytic activity, which can be directly determined by the first arc.131, 142-143, 146
We determined that the Rct of the silicon nanowire material was as low as approximately 0.07– 0.1 Ω • cm2, whereas the Rct of the Pt coated counter electrode was approximately 0.22 Ω • cm2. In the Nyquist plot, the SiNW/C and NiO@SiNW/C electrodes exhibited lower series resistance values than the Pt counter electrode. This decrease in the series resistance may induce the increase in the current density. The EIS for the full DSSCs was also conducted with the use of Pt, SiNW/C, and NiO@SiNW/C as the counter electrodes;
this is exhibited in Figure 5.10. This EIS analysis of the full cell exhibited a similar trend to the dummy cells, and the Rct values for the Pt coated, SiNW/C, and NiO@SiNW/C counter electrodes
were approximately 2.3 Ω • cm2, 0.31 Ω • cm2, and 0.3 Ω • cm2, respectively. The detailed resistance values are summarized in Table S1.
Figure 5.9 The interfacial electrochemical properties with Pt coated counter electrode, SiNW/C, and NiO@SiNW/C counter electrodes.
Figure 5.10 The EIS for the full DSSCs with the use of Pt, SiNW/C, and NiO@SiNW/C as the counter electrodes.
Before the wires are connected, the positive Ni2+ on the NiO-decorated SiNW/C are deprived of electrons by a relatively negative carbon. Ni2+ and Ni3+ exhibit a coexistence state. However, when the wire is connected, the electrons along the wire are introduced into the SiNW/C. At this time, the Ni3+
prefer to be in a standard state as Ni2+, and the received Ni3+ electrons transform into Ni2+. These electrons are transported to the electrolyte in order to pass into the oxidation dyes, depending on the electrolyte.