Chapter 3 CuBi2O4/RGO
Figure 3.10. Incident Photon-to-Current Conversion Efficiency (IPCE) spectra of CuBi2O4 and CuBi2O4/RGO-4.
To gain insights into the electron recombination kinetics of the fabricated PEC devices, transient open-circuit potential (Voc) decay measurements were carried out in 0.5 M Na2SO4. Voc is the potential generated at the photoelectrode with respect to the reference electrode when no current was drawn from the circuit. Figure 3.11 (a) shows that the photo-voltage generated in CuBi2O4/RGO-4 is more than the pristine CuBi2O4, which indicates that there is an increase of charge accumulation at the conduction band of CuBi2O4/RGO-4 than the pristine CuBi2O4. Under the open circuit condition, when the light is turned off, the decay in VOC is solely due to the recombination of photo-generated charge carriers. The decay rate of VOC is directly proportional to the rate of recombination. The slower VOC decay furnished by CuBi2O4/RGO- 4 indicates a better charge separation and reduced recombination upon RGO loading. To comprehend the enhanced PEC performance of CuBi2O4/RGO-4 in comparison to that of CuBi2O4, the electron lifetime of the two photocathodes was determined through open-circuit potential decay (OCPD) curve in Figure 3.11 (b) under the dark conditions using an equation formulated by Zaban.34-36
350 400 450 500 550 600 650 700 0
2 4 6 8 10 12
CuBi2O4/RGO-4 CuBi2O4
IPC E (%)
Chapter 3 CuBi2O4/RGO
54 Part of this chapter has been published in Sustainable Energy Fuels, 2019, 3, 1554
π = β ππ΅π
π / (ππππΆ
ππ‘ ) (3.1) Where Ο is the electron lifetime, kB is the Boltzmann constant, T is the temperature, e is a charge of an electron. The photoelectron lifetime in CuBi2O4/RGO-4 is longer than pristine CuBi2O4. It proves that the addition of RGO over CuBi2O4 enhances charge separation in CuBi2O4/RGO- 4 photocathode.
Figure 3.11 (a) Open circuit potential decay curve of CuBi2O4 and CuBi2O4/RGO-4. (b) Average Electron lifetime determines from open circuit potential decay.
3.3.7 Electrochemical Impedance Spectroscopy (EIS) Analysis
In order to understand the role of RGO modification over CuBi2O4, electrochemical impedance measurements were carried out. Figure 3.12 (a) shows the Nyquist plot of photocathodes measured at -0.4 V vs Ag/AgCl. The Nyquist plots of both the photocathodes are composed of two semicircles, where the semicircle at the low-frequency region is assigned to the charge transfer resistance at the photocathode/electrolyte interface and the semicircle at the high-frequency region is related to charge transfer resistance inside the bulk.37-39 Photocathode with smaller diameter of semicircle signifies an efficient charge separation, lower charge transfer resistance as well as the faster interfacial charge transfer kinetics at photocathode/electrolyte. Pristine CuBi2O4 shows the larger diameter of a semicircle, which
0.82 0.84 0.86 0.88 0.90 0.92 0.94 0.96 0
200 400 600 800 1000 1200 1400 1600
OCP (V) vs RHE
CuBi2O4 CuBi2O4/RGO-4
Electron Life Time (sec)
0 20 40 60 80 100 120 140 160 180 200 0.81
0.84 0.87 0.90 0.93 0.96 0.99 1.02 1.05
ON
OCP (V) vs RHE
CuBi2O4/RGO-4 CuBi2O4
Time (sec)
OFF (a) (b)
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Chapter 3 CuBi2O4/RGO
decreases with the introduction of RGO. Incorporating RGO on CuBi2O4 improves interfacial charge transfer kinetics between photocathode and electrolyte as well as helps in improving the charge separation in CuBi2O4/RGO-4 photocathode. The Nyquist plot of CuBi2O4 and CuBi2O4/RGO-4 photocathode was fitted to an equivalent circuit shown in the inset of Figure 3.12 (a), where RS is the series resistance; Rbulk is the charge transfer resistance inside the bulk of the semiconductor, Rct is charge transfer resistance between photocathode and electrolyte interface. Table 3.1 shows the fitting values of all the parameters of the equivalent circuit. It can be seen that the Rbulk value for CuBi2O4 significantly decreases from 673.6 β¦ to 183.7 β¦, with the introduction of RGO (CuBi2O4/RGO-4), indicating an extraction of an electron from CuBi2O4 to RGO, thereby improving the charge separation efficiency. Decrease in Rct value of CuBi2O4/RGO-4 as compared to CuBi2O4, from 4990 β¦ to 2982 β¦, which indicates that there is an enhancement in charge transfer kinetic from CuBi2O4/RGO-4 photocathode to the electrolyte solution in comparison to pristine CuBi2O4.
To verify the enhanced charge transportation of CuBi2O4, upon introduction of RGO, Mott-Schottky analysis was performed. Figure 3.12 (b) shows the Mott-Schottky plot for pristine CuBi2O4 and CuBi2O4/RGO-4 photocathodes under the dark condition in 0.5 M Na2SO4 at an applied frequency of 1 kHz. Negative slopes for both pristine as well as RGO modified photocathode signifies that both CuBi2O4 and CuBi2O4/RGO-4 photocathode has p- type conductivity. Charge carrier density of both the photocathodes was obtained from the following formula 6, 40
1
πΆ2 = 2
π΄2ππ·πΙΙ0[π β ππΉπ΅βππ
π] (3.2) where C is the capacitance of the semiconductor, A is the area of the photocathode, ND is the charge carrier density of photocathode, e is the charge of the electron, Τ0 is the permittivity of the vacuum, Τ is the dielectric constant of the photocathode (value of Τ for CuBi2O4 is 98)12, V
Chapter 3 CuBi2O4/RGO
56 Part of this chapter has been published in Sustainable Energy Fuels, 2019, 3, 1554
is the applied bias, VFB is the flat-band potential, k is the Boltzmann constant, and T is the temperature. Charge carrier density for CuBi2O4 and CuBi2O4/RGO-4 photocathodes were 3.76 x 1020 cm-3 and 7.92 x 1020 cm-3 respectively obtained from the slopes of the Mott-Schottky plot. There was a more than two-fold increase in the charge carrier density with the introduction of RGO over CuBi2O4 as compared to pristine CuBi2O4 which is in good agreement with the current density of CuBi2O4 and CuBi2O4/RGO-4 as shown in Figure 3.9 (a).
Figure 3.12. (a) The Nyquist plot of CuBi2O4 and CuBi2O4/RGO-4 photocathodes, inset of the figure shows the circuit diagram used to fit the obtained data. (b) MottβSchottky plot of CuBi2O4 and CuBi2O4/RGO-4 photocathodes in 0.5 M Na2SO4 at 1 kHz.
Table 3.1. Fitting parameter obtained after fitting the Nyquist plot
Figure 3.12 shows the Bode phase plot of CuBi2O4 and CuBi2O4/RGO-4 using which lifetime of photo-excited electrons can be calculated by using the formula 41
Οe = 1/(2Οfmax) (3.3) Οe is the lifetime of the photo-excited electron, fmax is the maximum pick frequency. In Figure 3.13, the fmax for CuBi2O4/RGO-4 and CuBi2O4 is 60.26 and 82.67 and the lifetime of photo- excited electrons was calculated to be 2.6 msec and 1.9 msec respectively. RGO acting as
0 1 2 3 4 5 6 7 8 9
0 1 2 3 4 5 6 7 8 9
-Z" (kο)
Z' (kο)
CuBi2O4 CuBi2O4 Fitted CuBi2O4/RGO-4 CuBi2O4/RGO-4 Fitted
RS
Rbulk Rct
Cbulk CEE
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0.0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
1/C2 *109 (F-2 cm4 )
Potential (V vs RHE) CuBi2O4/RGO-4 CuBi2O4
(a) (b)
Photocathode RS (ο) Rbulk (ο) Rct(ο) Carrier Density (cm-3) CuBi2O4/RGO-4 15.28 183.7 2982 7.92 X 1020
CuBi2O4 26.29 673.6 4990 3.76 x 1020
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electron sink suppress electron-hole recombination by rapidly accepting a photo-induced electron from CuBi2O4 surface, improving electron lifetime.
Figure 3.13. Bode phase plot of CuBi2O4 and CuBi2O4/RGO-4 showing an enhanced lifetime of photo-excited electrons in CuBi2O4/RGO-4 then pristine CuBi2O4.
3.3.8 Stability and Faradaic Yield Measurements
Figure 3.14 (a) shows the amount of hydrogen gas evolved and the corresponding Faradaic efficiency of CuBi2O4/RGO-4 photocathode. Hydrogen evolved was monitored through online GC, technique at a fixed potential (-0.6 V vs Ag/AgCl) through a chronoamperometry under 1 Sun illumination of light for 1 hour in 0.5 M Na2SO4 electrolyte.
Faradaic efficiency is the ratio of experimental hydrogen production rate and hydrogen production rate calculated theoretically. Impressive Faradic efficiency of 91.7 % was achieved for CuBi2O4/RGO-4 photocathode which indicates that the photocurrent generated in the modified electrode in Figure 3.9 (a) attributed purely to the water reduction. Apart from the catalytic activity, operational stability is an equally important parameter for practical applications. Therefore, a long-term stability test of each pristine and modified CuBi2O4/RGO- 4 photocathode was carried out under continuous illuminations at 0 VRHE. Figure 3.14 (b)
10-1 100 101 102 103 104 105 0
20 40 60 80