Zinc oxide (ZnO) and niobium oxide (NbO<SUB>x</SUB>) with a nano-island structure were deposited by a sputtering method on Al-coated glass substrates. Cells with a (ZnO or NbO <SUB>x</SUB>)/Al/glass|KNO <SUB>3</SUB>aq.|Al/ glass structure were assembled, and electrochemical and photoelectrochemical properties were evaluated. The ZnO and NbOx electrodes had higher electrode potentials than the counter Al/glass electrode, and electron flows from the counter electrode to the ZnO and NbO <SUB>x</SUB> electrodes through the external circuit were commonly confirmed. In the ZnO-based cell, only faint photocurrent generation was seen, where Zn and Al elution from the ZnO electrode was found. In the NbOxbased cell, however, stable generation of electricity was successfully achieved, and electrode corrosion was not recognized even in microscopic observations. A photoelectrochemical conversion model was proposed based on potential-pH diagrams. In the case of nano-island structures formed at shorter NbO <SUB>x</SUB> deposition time, it was concluded that the photoelectrochemical reactions, which were proceeded in the immediate vicinity of the boundary among nano-islands, substrate, and electrolyte solution, were predominant for the photoelectrochemical conversion, and in the case of film structures with longer deposition time, the predominant reactions took place at the film surface.
In recent years, various energy conversion devices, such as photovoltaic, thermoelectric, and piezoelectric devices, have been extensively studied because fossil fuel may run out within the next several decades. Among the devices, it is guessed that demand for the photovoltaic solar cells rises rapidly, because solar energy is semipermanent and is used anywhere under sunlight. However, there are various problems in solar cells, such as material cost, process cost, and power generation efficiency, and a number of attempts have been made to solve these problems [
In the authors’ research group [
Anyway, PECs have not been put to practical use yet, because the conversion efficiency had been not enough. In order to improve the conversion efficiency, various nano-textures have been studied so far: for example in ZnO and NbOx, nanotubes [
In the present study, not only nano-island but also continuous film structures of NbOx deposits were fabricated by changing deposition time to clarify the nano-texture dependence on the electrochemical and photoelectrochemical properties, in which surface states of ZnO and NbOx/Al/glass electrodes and Al/glass substrate were observed by atomic force microscopy (AFM), and valence states of niobium ions were also investigated by optical absorption measurement. The electrochemical and photoelectrochemical reactions in the ZnO- and NbOx- based PECs were discussed based on the experimental results.
Al thin films were deposited on a glass (SCHOTT Nippon K.K., Glass code: D263T) substrate by a radio frequency (RF) magnetron sputtering (SHINKO SEIKI CO., LTD, Type: SRV4320), where the deposition time was extended to 40 min, obtaining 100 nm of Al films. The Al-coated glass substrate was also used as a counter electrode of PEC. ZnO or NbOx was deposited on the Al/glass substrate by RF magnetron reactive sputtering (ULVAC JAPAN, Ltd., Model: YH-500A or DIAVAC LIMITED, Type: DS-412Z, respectively).
Al | ZnO | NbOx | |
---|---|---|---|
Substrate | Glass | Al/glass | Al/glass, quartz glass |
Target | ϕ101.6 mm, Al (99.9%) | ϕ50 mm, Zn (99.99%) | ϕ50 mm, Nb (99.99%) |
Ar gas | 99.99%, 1.0 ccm | 99.99%, 1.8 ccm | 99.99%, 6.0 ccm |
O2 gas | - | 99.99%, 2.4 ccm | 99.99%, 6.0 ccm |
Orbital speed of substrate holders | 1800 rpm | - | - |
Distance from substrate to target | 100 mm | 60 mm | 60 mm |
RF power, and frequency | 50 W, 13.56 MHz | 200 W, 13.56 MHz | 200 W, 13.56 MHz |
Back pressure | <8.0 × 10−5 Pa | <6.7 × 10−4 Pa | <6.7 × 10−4 Pa |
Deposition pressure | 9.3 × 10−2 Pa | 0.39 Pa | 0.39 Pa |
Deposition rate | 0.042 nm/s | 1 nm/s | 0.17 nm/s |
Deposition time | 2400 s | 3, 17 s | 10 - 300 s |
KNO3 was dissolved in a distilled water (Wako Pure Chemical Industries, Ltd., CAS No.: 7732-18-5, 200 ml), obtaining 0.1 mol/L electrolyte solution. During the measurements, the electrolyte solution was kept at a constant temperature of 25˚C, and the measurements were done in a dark place.
Firstly, the electrode potential was measured. The ZnO or NbOx/Al/glass electrode and a carbon counter electrode (NaRiKa Corporation, CAT. NO. B10-2050-09) were immersed in the electrolyte solution. The electrodes were connected to a potentiostat (Bio Logic, SP-50), and an Ag/AgCl electrode in saturated KCl solution (HOKUTO DENKO Co., HX-R6) was also connected as a reference electrode. The potential of the electrodes with respect to the reference electrode was determined by measuring open circuit voltage for 100 s. Subsequently, electrochemical stability, that is, corrosion resistance of the electrodes was evaluated, where a constant load discharge (CLD) mode with an electrical resistance of 100 kΩ installed in the potentiostat was used. At this time, Al/glass soaked in a separate beaker was used as a counter electrode, and the beakers were connected with a KNO3 salt bridge. Elution of Nb, Zn and Al in the electrodes was investigated after immersing the electrodes for 1 hour. Inductively coupled plasma measurement (SEIKO and VARIAN Inst., Vista-PRO CCD Simultaneous ICP-OES) was used for the detection of the elements eluted in the electrolyte solution.
In the evaluation of photoelectrochemical properties, photocurrent generation was firstly measured by using the PEC shown in
Finally, electricity generation property was investigated by measuring current and voltage generated by the light irradiation. In the measurement, the cell setup given in
In the previous report [
In the previous study [
Electrode potentials are shown in
Electrode | Deposition time (s) | Electrode potential (V vs. Ag/AgCl) |
---|---|---|
Al/glass | - | -0.70 |
ZnO/Al/glass | 3 | -0.55 |
17 | -0.46 | |
NbOx/Al/glass | 10 | -0.19 |
15 | -0.25 | |
17 | -0.19 | |
20 | -0.07 | |
30 | -0.04 | |
40 | 0.00 | |
60 | 0.07 | |
100 | 0.11 | |
150 | 0.19 | |
200 | 0.20 | |
300 | 0.23 |
electrode potentials of ZnO- and NbOx-deposited electrodes are higher than those of the Al/glass electrodes. In NbOx/Al/glass electrode, the electrode potential increases with increasing the deposition time, and similar trend is also seen in ZnO/Al/glass electrode. During the potential measurements, the electrode potential of NbOx-de- posited electrodes was stable and almost constant with small fluctuation. In the case of ZnO-deposited electrodes, however, the electrode potential decreased continuously during the measurements, and the electrode with shorter deposition time had higher rate of decrease in electrode potential. The decrease in the electrode potential during the measurements suggests the two possible changes in the ZnO-deposited electrodes, that is, the reduction of Zn from 2+ to 0 in valence number and the dissolution of ZnO deposits into the electrolyte solution.
After electrochemical corrosion tests, the elution of Al and Zn from ZnO/Al/glass electrodes is confirmed, and such the elution is not observed in NbOx/Al/glass electrodes. The elution is discussed later.
In the previous study of ZnO nano-islands [
Test | ZnO deposition time (s) | Al (ppb) | Zn (ppb) |
---|---|---|---|
Corrosion | 3 | 7 | 72 |
17 | 8 | 80 | |
Photocurrent generation | 3 | 12 | 116 |
17 | 28 | 143 |
and photocurrent measurements. After the photocurrent measurements, Zn and Al were confirmed in the electrolyte solution, and the concentrations were larger as compared with the case without light irradiation. Therefore, it is supposed that the electrochemical reactions resulting in the elution of Zn and Al are accelerated by their radiation. In case of the NbOx-based PECs, neither Al nor Nb elution was detected in the photovoltaic electrode. However, Al elution from the counter electrodes was confirmed in both PECs.
In both PECs, electrons flowed from the counter electrode into the photovoltaic electrode through the external circuit during light irradiation. The direction of electron flow was the same even in the electrochemically-unst- able ZnO-based PECs. According to the direction of electron flow, following reduction and oxidation reactions are expected at the photovoltaic and counter electrodes, respectively.
Photovoltaic electrode:
Counter electrode:
However, the current flow observed in the present study is opposite to the direction commonly observed in wet-type solar cells with semiconductor electrodes, such as TiO2 [
opposite change in J-V curve is commonly observed, that is, steep and gradual decreases in J are seen at lower and higher V regions, respectively.
Then, fill factor, FF is calculated from the equation, FF = Pmax/(Jsc × Voc), where Pmax is the maximum power density, Jsc is the short circuit current density, and Voc is the open circuit voltage.
In
After the photoelectrochemical measurement (
NbOx deposits, a leading edge is commonly confirmed at ca. 2.9 eV, which is close to the absorption edge of Nb2O5 reagent powder. For the NbOx deposits, a steep increase in the slope of absorption curves is also recognized at 3.5 - 3.7 eV, which is close to the band gap of 3.4 eV generally known for Nb2O5. In our measurement, however, the optical band gap of Nb2O5 reagent powder is ca. 3.1 eV, which is smaller than the energy gap of 3.4 eV given in literatures, and it is hence suggested that the NbOx deposits have different electronic state or atomistic structure from Nb2O5 crystal. Actually, no crystalline diffraction peaks are observed in XRD patterns of the NbOx deposits, indicating that the NbOx deposits are in amorphous state. It is supposed that the amorphous state of the NbOx deposits is due to oxygen deficiency, and it is also expected that part of Nb ions are reduced to 4+ or 2+ from 5+. The reagent powders of crystalline NbO2 and NbO consisting of lower valency of niobium ions show strong absorption in almost the entire region. The NbOx deposits indicate quite different absorption spectra from NbO2 and NbO. Therefore, it is difficult to estimate the valence state of niobium ions in the NbOx deposits, but it is supposed that most of Nb ions are present as 5+.
As mentioned, the elution of Al and Zn from ZnO/Al/glass electrodes is observed regardless of light irradiation,
and such the elution is not recognized in NbOx/Al/glass electrodes, which are explainable by the electrochemical corrosion based on potential-pH diagrams [
deposition times of 3 and 17 s, respectively. Nano-dips with the depth of ca. 1 nm at brinks of nano-islands (vertical lines in
As shown in
As above mentioned, during light irradiation, electrons flow from the counter electrode into the photovoltaic electrode through the external circuit, and at the same time, very small and few bubbles are formed. Moreover, Al elution from the counter electrodes is observed in the ZnO- and NbOx-based PECs. Hence, electrochemical reactions given in Equations (1) and (2) are suggested, that is, reduction of nitrate ions and H2 generation by reduction reactions at the photovoltaic electrodes and Al elution and O2 generation by oxidation reactions at the counter electrode.
In the case of NbOx deposits, the electrons consumed in the reduction reactions are probably provided from the NbOx deposits, in which the electrons are excited into the conduction band by absorbing the irradiated light. It is also supposed that the reduction reactions occur at the interface between NbOx deposits and electrolyte
solution. However, the maximum power density is not proportional to the NbOx deposition time, and it is hence suggested that the reduction reactions occur only at the restricted surface of the NbOx deposits. It is conse- quently considered that most of the electrons excited by optical absorption are consumed to recombine with the holes created at the same time, and the residual holes are presumably filled with the electrons supplied from the external circuit through the Al film on the substrate electrode.
NbOx is probably an n-type semiconductor as well as Nb2O5. When a semiconductor electrode is soaked in an electrolyte solution with different electrostatic potential, a potential bending occurs in the electrode surface, which induces migrations of electrons and holes. In case of the NbOx-based PEC, it is expected that electrons are concentrated at the surface of NbOx deposits, and if present, holes migrate toward the underlying Al film. In the case of nano-islands, the thickness is too small to bend the surface potential, resulting in almost no potential gradient, which is similar to a flat-band state. In flat-band state, mobility and migration distance of electrons and holes are quite small, and it is hence expected that only the holes produced very near the Al film recombine with electrons which are supplied through the Al film. It is consequently supposed that the photoelectrochemical reactions proceed in the immediate vicinity of the boundary among NbOx nano-island, Al film, and electrolyte solution.
Then, the boundary is estimated from the two-dimensional AFM phase images given in
As shown in
surface, because the maxima in Jsc and Pmax of nano-island state are larger than those of film state (
In the case of ZnO, various photo corrosion reactions by holes have been proposed.
Han et al. [
Rao et al. [
Fruhwirth et al. [
A corrosion reaction, Al + 3h+ → Al3+ is also assumed [
When combined with the results from the AFM observation (
According to a potential-pH diagram of Nb system [
ZnO and NbOx were deposited on Al-coated glass substrates by an RF magnetron reactive sputtering, and photoelectrochemical cells, PECs were constructed, in which an Al/glass was used as a counter electrode and KNO3 solution was chosen as an electrolyte. Electrochemical and photoelectrochemical properties of the PECs were evaluated.
Under the light irradiation from a solar simulator, faint and unstable photocurrent generation was seen in ZnO-based PECs. As for the NbOx electrode, however, stable generation was successfully achieved. Very small and few bubbles were generated at both electrodes, and Al elution was found at the counter electrodes. The maximum power output was not proportional to the deposition time of NbOx, and larger output was obtained when the NbOx deposits were not in film, but in nano-island structures.
The photoelectrochemical properties were discussed based on the electrochemical properties. The ZnO and NbOx electrodes had higher electrode potentials than the counter Al/glass electrode, and electron flows from the counter electrode to the ZnO or NbOx electrodes through the external circuit were commonly confirmed. It was hence supposed that the bubbles generated during the light irradiation were H2 at ZnO and NbOx electrodes and O2 at the counter Al/glass electrode. In the ZnO-based PEC, the elution of Zn and underlying Al from the ZnO electrode was observed in a dark place, and the elution rate increased during the light irradiation. After the light irradiation, precipitates on the ZnO electrode surface were found in the AFM observations. According to the potential-pH diagrams, it was suggested that the eluted Zn2+ and Al3+ ions were re-precipitated as hydroxides of Zn(OH)2 and Al(OH)3 on the electrode surface, which was due to the change in pH of the electrolyte solution, being resulted from the OH− generating and H+ consuming reactions. During the repetition of photo irradiation, the Zn(OH)2 precipitates changed into thin ZnO layer, resulting in photocurrent generation. At the NbOx electrode, elution was observed neither in a dark place nor during the light irradiation, and quite little change was observed on the NbOx electrode surface. From the optical absorption spectra, it was suggested that Nb ions in NbOx deposits were almost present as Nb5+, but they had different electronic state or atomistic structure from Nb2O5 crystal. Furthermore, it was also suggested that the electronic state and atomistic structure of NbOx deposits were unchanged and independent of the deposition time. The maximum power output was correlated not with the deposition time but with the length of the boundary between the NbOx nano-islands and the underlying Al film. Second maximum in power output was also obtained at the longer NbOx deposition time, where thin film of NbOx deposits was formed. Larger power generation was obtained with the NbOx deposits with nano- island structure.
Based on the experimental findings, the following photoelectrochemical reaction mechanism was suggested: due to the light irradiation, electrons in the NbOx deposits were excited into the conduction band, and were used in the reduction of