Chapter 3 Structure and Annealing Studies of Electrodeposited Cu 2 O Films…
3.4. Conclusions
The annealing effect in our system did not increase the grain size as we hoped.
There was a possible structural transition. The Cu2O films after annealing revealed reduced grain size and notable morphology change. The resistivity for the pH 9 films was decreased, but the resistivity for the pH 11 films was increased. However, both films improved their conductivity at 350 ℃. The resistivity of pH 9 film annealed at 350 ℃ for 30 min had the best conductivity with a resistivity of 3.96× 10-4 Ωcm. Its conductivity was even better than the pH 11 films.
Chapter 4
Photoelectrochemical Properties of Cuprous Oxide
4.1. Introduction
Results from material characterizations on the Cu2O films have been discussed in chapter 3. Their photoelectrochemical properties are presented here. We studied the hydrogen evolution capability of Cu2O films using a semiconductor-electrolyte cell, and evaluated its stability under extensive light illumination.
4.2. Experimental
4.2.1. Setup
Photoelectrochemical analysis was conducted by a three-electrode arrangement.
The Cu2O films discussed in Chapter 3 were served as the working electrode. We used a platinum foil and Ag/AgCl (KCl saturated) as the counter and reference electrode, respectively. The electrolyte was 0.5 M Na2SO4 solution, which is commonly used in a semiconductor-electrolyte cell or water splitting reaction. The electrolyte was pre-purged with N2 gas for 30 min before the measurement in order to eliminate any impurity or oxidant in the electrolyte. A 100 W halogen lamp (OSRAM, HLX64625) was used as the light source and it was positioned at 15 cm away from the working electrode. The photocurrent was detected by a SOLAR TRON-SIC1287. Fig 4.1
provide the photograph of the photoelectrochemical setup.
Figure 4.1 The scheme and the photoelectrochemical setup.
4.2.2. Photoelectrochemical measurement
The photoelectrochemical properties of Cu2O were recorded by illuminating the Cu2O films under a constant bias of -0.3 V (versus Ag/AgCl) for 1 h. The supporting bias was employed because the established inefficiency of Cu2O to oxidize water according to previous studies. It is noted that from previous studies, the Cu2O was stable under this bias. The range of potentiodynamic analysis started from 0 to -0.8 V (vs. Ag/AgCl) at a 5 mVsec-1 scanning rate and the light was chopped every 5 sec.
Evaluation on the stability was carried out in potentiostatic measurement under -0.3 V for 5 h.
4.3. Results and discussion
Ag/AgCl) bias for 5 h. The results are presented in Fig 4.2. The fluctuating currents in both Fig 4.2a and 4.2b were the dark currents for the as-deposited pH 9 and 11 film, respectively. They were measured under -0.3 V without illumination. We observed that only under a constant illumination we revealed a stable output. Both pH 9 and pH 11 films exhibited a stable output with negligible degradation for at least for 5 h. The average current density for the films deposited from pH 9 and pH 11 were -29.4 and -66.5 μAcm-2, respectively.
We examined the Cu2O electrode after illumination by XRD and SEM. From XRD patterns presented in Fig 4.3a and Fig 4.4a, we did not observe any impurity from in both films. This indicated the Cu2O films were stable in our electrolyte either with or without illumination. The stability of Cu2O lasted at least for 5 h. The SEM images of pH 9 films (Fig 4.3b) exhibited no change either with or without illumination after for 5 h. The surface of pH 11 films (Fig 4.4b) appeared slightly corroded after the reaction, which may due to the vigorous output of photocurrent.
Unfortunately, we did not find any differences from the XRD pattern.
a)
Figure 4.2 I-t curves of a) pH 9 and b) pH 11 films under potentiostatic measurements at -0.3 V for 5 h.
a) potentiostatic measurements at -0.3 V for 5 h with and without illumination.
a)
20 30 40 50 60 70
0 2000 4000
60000 20 30 40 50 60 70
800
1600 20 30 40 50 60 70
0 40000 80000
pH11
2-Theta
Dark
In te n si ty ( A .U .)
Light
b)
Fig 4.4 a) XRD patterns and b) SEM images of pH 11 films before and after potentiostatic measurements at -0.3 V for 5 h with and without illumination.
B. Photo-Responses Characterizations
The Cu2O is a non-stoichiometic semiconductor. Its photoelectrochemical property depends on the concentration of copper vacancies and oxygen vacancies. The copper vacancies induced a p-type character and oxygen vacancies induced a n-type character. We electrodeposited the Cu2O films in different pH environments which rendered different copper vacancies in the films. Furthermore, the process during electrodeposition had to be strictly controlled because the vacancies in the Cu2O were changing easily. Not only the pH value was expected to affect the composition of Cu2O, but also the deposition temperature, concentration of electrolyte, and any factors which were likely to affect the deposition rate and grain growth.
Comparison in the photoelectrochemical performances for the as-deposited pH 9 and pH 11 films under -0.3 V is shown in Fig 4.5. Both dark currents were fluctuating violently implying that the Cu2O was inefficient to reduce H2O without illumination at -0.3 V. This unsettled current output suggested that there were some reactions on the Cu2O electrode surface. We observed that the photocurrents from films deposited in pH 11 bath had a better performance than that of pH 9 films. The results indicated the ability of photoelectrolysis water between these two films had a large distance.
This implied that the Cu2O electrodeposited in higher pH value could get a better performance.
The photocurrent of pH 11 film was much larger than the pH 9 film. This result may be due to the grain size differences in these two films. From Chapter 3, the grain size for the as-deposited pH 9 film was under 1 µm. On the other hand, the grain size for the pH 11 film was distributed from 1 to 8 µm. Most electron-hole recombination occurred at the grain boundary areas. It would reduce the energy conversion efficiency and also affect the conductivity of the semiconductor. Therefore, the grain
size could be the main effect affecting the efficiency. The resistivity measurements also agreed with this result. The resistivity for the as-deposited pH 9 and pH 11 films were 0.206 and -3.318 log (Ω cm), respectively.
Furthermore, the difference of Cu vacancy distribution should also be considered.
However, the effect of vacancy distribution was hardly identified. The Cu2O deposited in higher pH environment was expected to contain a higher concentration of O and had more Cu vacancies in its lattice, which exhibited a better p-type character. It might be another reason that the Cu2O deposited in pH 11 bath had a better performance.
Additionally, in order to confirm how different the pH value of deposition bath would affect its photoelectrochemical properties, we prepared the films electrodeposited in the bath at pH 7. The photoelectrochemical properties of the Cu2O electrodes were studied by a linear sweep voltammetry with chopped illumination (Fig 4.6). The results demonstrated that the films deposited in pH 11 bath had a strong response to light, implying that it was a well-fabricated p-type semiconductor. The films deposited in pH 9 and 7 exhibited steady responses with illumination until the bias was larger than -0.3 V, and the responses were both weak. However, the pH 9 film performed slightly better than pH 7. We concluded that the pH environment for electrodeposited Cu2O was related to the structure and photoelectrochemical ability of Cu2O. At a higher pH value, as a better p-type Cu2O was obtained.
Unfortunately, we could not compare these two films directly in the following section. These two films had different preferred orientations and thicknesses. Their resistivities and grain sizes were also different. Therefore, the photocurrent for these two films after annealing treatments were not be compared together.
0 1000 2000 3000 4000
Figure 4.5 Potentiostatic curves of as-deposited pH 9 and 11 films under -0.3 V with or without illumination.
Figure 4.6 LSV curves with chopped illumination of pH 7, 9, and 11 films. The scan rate was 5 mVsec-1.
4.3.2. Annealed Cuprous Oxide Films from pH 9 Bath
A. Effect of Annealing Temperatures
The photoelectrochemical measurements for the annealed pH 9 films are presented in Fig 4.7. The measurements were under -0.3 V in constant illumination for 1 h. The average photocurrents for the annealed Cu2O are summarized from the potentiostatic measurements, and presented in Fig 4.7b. The average currents were 10.19, -23.78, -22.72, -33.58, and -72.27 μAcm-2 for pH 9 films annealed at 150, 200, 250, 300, and 350 ℃ for 30 min, respectively. The average current for the as-deposited pH 9 film was -28.59 μAcm-2. The photocurrent of pH 9 films after annealing increased as the annealing temperature was increased. Although, at first the film annealed at 150 ℃ reveled no p-type character. The photocurrent presented in Fig 4.7a displayed a very unstable current which fluctuated strongly. However, the performances for the annealed pH 9 films were enhanced by annealing temperature.
The resistivity of the annealed pH 9 films was decreased as the annealing temperature was increased, which agreed with the photocurrents. We assumed that the film annealed at 150 ℃ produced a damaged interface between the Cu2O and stainless steel. As the temperature was increased, the contact at the interface may be reconstructed by inter diffusion. The film annealed at the highest temperature, 350℃, revealed the best performance.
B. Effect of Annealing Time
In Fig 4.8, we summarized the pH 9 films annealed at different temperatures for 10, 30, and 60 min, respectively. Their results exhibited similar curves like before.
They both became worse at the temperature 150 ℃ and then the photocurrents were increased as the temperature was increased. The annealing effect at 150 ℃ did not show any improvement but more damages. We assumed that the contact of Cu2O annealed at 150 ℃ became poor. However, the crystallinity was only slightly improved. The interface may reconstruct by diffusion at higher temperatures. The average current densities for all samples are summarized in Table 4.1. The annealed pH 9 film became better as the temperature was increased, implying that the annealing process was effectively improving the crystallinity and conductivity. The results agreed with what we confirmed in the chapter 3.
a)
b)
Figure 4.7 a) I-t curves and b) average current density of pH 9 films annealed at different temperatures for 30 min under potentiostatic measurements at -0.3 V and illumination for 1 h.
Table 4.1 Average current density (μAcm-2) of pH 9 annealed at different temperatures and annealing times under potentiostatic measurements at -0.3 V and illumination for 1 h.
Time 150 ℃ 200 ℃ 250 ℃ 300 ℃ 350 ℃
10 min 7.53 -10.98 -13.02 -21.40 -51.69
30 min 10.19 -23.78 -22.72 -33.58 -72.27
60 min 3.48 -11.19 -25.94 -28.40 -141.13
Figure 4.8 Average current density of pH 9 annealed at different temperatures and annealing times under potentiostatic measurements at -0.3 V and illumination for 1 h.
4.3.3. Annealed Cuprous Oxide Film from pH 11 Bath
A. Effect of Annealing Temperatures
The photoelectrochemical measurements for the pH 11 films annealed for 30 min are presented in Fig 4.9. The average photocurrents were -81.7, -32.5, -53.7, -41.2, and -66.7 μAcm-2 for pH 11 films annealed from 150 to 350 ℃, respectively (Fig 4.9b). The photocurrent for the as-deposited pH 11 film was -60.9 μAcm-2. These values did not increase as the temperature was increased and it was not a steady trend.
The trend displayed a rocky curve, which revealed the best performance at 150 ℃.
However, from the results of Chapter 3, we had realized that the pH 11 samples had a special trend after annealing. The resistivity for the pH 11 Cu2O films was increased as the temperature was increased to 300 ℃, then abruptly decreased at temperature up to 350 ℃. And its grain size was decreased as the temperature was increased.
However, the photocurrent exhibited neither trend of resistivity nor grain size. At this moment, we could not conclude the trend for the annealing effect yet. After all, the photoelectrochemical performance reflected the quality of the Cu2O films.
B. Effect of Annealing Time
In Fig 4.10, we present the pH 11 films annealed at different temperatures and annealing times, and the average current densities are summarized in Table 4.2. These results presented a similar curve like time effect. According to the results of Chapter 3, the pH 11 films after annealing went through a structural transition. The resistivity was getting worse as the annealing time was increased except at 350 ℃ (Fig 3.21).
The grain size was also decreased after annealing. Again, we could not conclude how the re-crystallization would affect on their photoelectrochemical properties yet.
However, the degree of the resistivity reduction was not as much as the resistivity improvement of pH 9 films. In fact, the worst conductivity of pH 11 film annealed at 300 ℃ was still better than the pH 9 film annealed at the temperature less than 300 ℃.
The results indicated we could not conclude the annealing effect on pH 11 film yet.
The variation of photocurrent was negligible. However, the trend for the photocurrent revealed a slight decrease at 300 ℃ and then increased again at 350 ℃, which agreed with the resistivity of annealed pH 11 films.
a)
b)
Figure 4.9 a) I-t curves and b) The average current density of pH 9 annealed at
different temperatures for 30 min under potentiostatic measurements at -0.3 V and illumination for 1 h.
Table 4.2 Average current density (μAcm-2) of pH 11 annealed at different temperatures and annealing times under potentiostatic measurements at -0.3 V and illumination for 1 h.
Time 150 ℃ 200 ℃ 250 ℃ 300 ℃ 350 ℃
10 min -47.20 -29.70 -26.79 -13.01 -51.83
30 min -81.75 -32.54 -53.73 -41.24 -66.71
60 min -61.12 -32.79 -53.44 -55.22 -119.03
Figure 4.10 Average current density of pH 11 annealed at different temperatures and annealing times under potentiostatic measurements at -0.3 V and illumination for 1 h.
4.4. Materials Characterizations after
We examined the Cu2O films after photoelectrochemical reactions by XRD (Fig 4.11). We did not found any impurity after light illumination for 1 h. As we concluded previously, the Cu2O was stable in our system.
However, we did find evidence of transformation after reaction from the pH 11 films annealing at 350 ℃ for 60 min, as shown in SEM images (Fig 4.12). We observed that there were small cubes appeared on the surface. Because we did not observe any impurity from the XRD patterns, we assumed these cubes were possibly still Cu2O. There appearance was similar to those from chemical route. These cubes implied that the Cu2O could re-crystallize at the surface. This behavior has not been mentioned in previous studies.
The phenomenon was only observed on pH 11 films undergoing annealing at 300
℃ for 30 and 60 min, and films annealed at 350 ℃ for 30 and 60 min after photoelectrochemical measurements. This indicated that it only occurred on the pH 11 films after annealing at high temperature, but did not take place on the pH 9 films.
The surface of pH 9 annealed at 350 ℃ for 60 min after reaction is displayed in Fig 4.13.
Photoelectrochemical measurements with and without illumination on pH 11 films which annealed at 350 ℃ for 60 min were conducted, with SEM images shown in Fig 4.14. We prepared the films before and after illumination for 10 min. The results demonstrated that after illumination for 10 min we could find small cubes formed on the surface, indicating that the reaction at the surface became re-crystallized. On the other hand, the film without illumination (Fig 4.15) did not reveal any transformations for 30 min. We prolonged the reaction to 150 min, and the result is presented in Fig 4.15c. The films appeared destructed after reaction by
forming new compounds. We examined the films after 150 min without illumination by XRD (Fig 4.16) and the results confirmed the presence of Cu.
Our results implied that only the films under illumination would form cubes on the surface. Otherwise the Cu was reduced by the overpotential. However, both phenomenons were observed on the films after annealing at high temperature. We did not observe any cube or Cu formation on the as-deposited Cu2O films (Fig 4.3 and 4.4). It implied that the surface of Cu2O film after annealing might be more active.
The pH value of 0.5 M Na2SO4 was 6.5. According to the pourbaix diagram (Fig 4.17), under -0.3 V (vs. Ag/AgCl) the Cu2O would be reduced to Cu. It could be the reason that we observed Cu formation at films without illumination. On the other hand, the cubes formation on the surface could be caused by the re-crystallization of Cu2O.
However, the exact nature of this cube formation is unknown.
Figure 4.11 XRD patterns of pH 9 and pH 11 films which were both annealed at 350
℃ for 1 h and after photoelectrochemical measurements of illumination for 1 h with -0.3 V.
Figure 4.12 SEM images of pH 11 film which was annealed at 350 ℃ for 1 h and
after a photoelectrochemical measurement of illumination for 1 h under -0.3 V. The second image is a magnified view of the surface.
Figure 4.13 SEM images of pH 9 film which was annealed at 350 ℃ for 1 h and after
a photoelectrochemical measurement of illumination for 1 h under -0.3 V. The second image is a magnified view of the surface.
Figure 4.14 SEM images on the surface of pH 11 film which was annealed at 350 ℃
for 1 h; a) after annealing, b) after photoelectrochemical measurement for 10 min illumination under -0.3 V, and c) a magnified view of (b).
Figure 4.15 SEM images on the surface of pH 11 film which was annealed at 350 ℃
for 1 h; a) after annealing, b) after photoelectrochemical measurement for 30 min without illumination under -0.3 V, and c) after photoelectrochemical measurement for 150 min without illumination under -0.3 V.
Figure 4.16 XRD patterns of pH 11 film which was annealed at 350 ℃ for 1 h before
and after photoelectrochemical measurement for 150 min without illumination under -0.3 V.
Figure 4.17 Pourbaix diagram of Cu2O.
4.5. Conclusions
The as-deposited pH 9 film revealed poor conductivity before the annealing process. The conductivity after annealing was as good as the pH 11 films. The improvement was also confirmed by the photocurrent. The results suggested that the resistivity of Cu2O film was the primary factor affecting its photoelectrochemical properties. After all, the annealing process was an effective treatment to improve the crystallinity of Cu2O.
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