Cyclic voltammograms of n-GaN

Now, let’s look at the case of the n-GaN/solution interface. Figure 3.10 (a) shows a cyclic voltammogram of an n-GaN photoanode under illumination. The UV light of the Xe lamp was not reduced by an AM 1.5 filter. An aqueous solution of 0.1 M HBr + 0.2 M H2SO4 was used. The measurement was scanned at 100 mV/s, and started first in the

positive direction and then returned. In order to avoid the reduction of oxygen, the cathodic scan was reversed at -1.25 V.

The concentration of the Br-_/Br

3- redox couple as a function of distance from the GaN

surface is schematically illustrated in Figure 3.10. The potential of the anodic current onset, the anodic peak maximum, the cathodic current onset and the cathodic peak maximum are denoted by Uon,a, Up,a, Uon,c and Up,c, respectively. The formation of one

Br3- requires three Br- (Eq. (3.20)).For better visualization, the concentration of Br3- is

multiplied by three. In the forward scan, for U < Uon,a, the concentration of Br- at the

surface equals to the bulk solution, and there is no Br3- at the surface. In the anodic scan,

Br- _{is oxidized to Br}

3-, contributing to the photocurrent, as described by

3

The oxidation of Br- _{in this case is a hole-capture process. Photo-holes transfer from VB}

of n-GaN to Br- _{states. This process is different from the electron injection from Br}- _states

At the beginning of the photocurrent onset, the anodic current depends on the competition between charge transfer and surface recombination [19], which depends on the electric field of the SCR. The kinetics of Br- _{oxidation is not a limiting step. As the}

scan potential becomes more positive, the electric field of the SCR is enhanced and the photocurrent increases; The [Br-_{] at the surface becomes lower than the bulk solution.}

As the electric field of the SCR is strong enough to separate all the charges, all photo- holes are used to oxidize Br-_{. For U > U}

p,a, the photocurrent is controlled by both the mass

Figure 3.10: (a) a j-U plot of an n-GaN photoanode under illumination. Solution: 0.1 M HBr and 0.2 M Na2SO4; v:

100 mV/s. (b) [Br-_{] and [Br}

3-] as a function of distance

from the electrode surface under different conditions. [Br3-] is multiplied by three for better visualization.

Chapter 3. n-GaN/solution interface

transport of [Br-_{] and the concentration of photo-holes. The photocurrent decreases with}

increasing scan time because the concentration gradient of the [Br-_{] at the surface}

decreases over time. The [Br-_{] at the surface is not zero in our case. The concentration of}

photo-holes is not high enough to oxidize all the Br- _{at the surface.}

In the reverse scan, Br3- is reduced back to Br- at the surface by electrons of the CB.

Electrons of the VB cannot reduce Br3- due to the potential difference. At U = Up,c, Br3-

is fully consumed at the surface. For U < Up,c, the cathodic current depends on the

diffusion of Br3-. The current density as a function of scan time can be described by the

Cottrell equation. As the CB bends upward, surface concentration of majority carriers increases. The availability of carriers is not a limiting step but the availability of oxidants, which is similar to the case of the metal/electrolyte interface.

For all the measurements in Chapter 4-Chapter6, weak UV illumination and high concentration of reducing agents (> 0.1 M) were used in the measurement. Therefore, the concentration of reducing reagents at the surface of the GaN electrode and in the bulk solution are the same. The j-U characteristic in the anodic scan is determined by the semiconductor physics; the electrochemistry at the interface is under kinetic control.

Figure 3.11 shows cyclic voltammograms (CV) of n-GaN under weak UV illumination. An aqueous solution of 0.1 M HBr + 0.2 M H2SO4 was used; the solution was not purged

with N2 before the measurements. The j-U curves were scanned continuously for 7 times.

Each scan went first positive, reversed at Uj, as indicated in the figure, and then went

negative. The subscript j indicates the scan sequence, ascending from 1 to 7. Two cathodic peaks, located at -0.47 V and -1.07 V, are observed in the reverse scans.

Under illumination of supra-bandgap light, photo-holes are generated and are drifted to the GaN surface by the electric field of the SCR. For U > -0.31 V, photo-holes oxidize reactants in the solution, resulting in the photocurrent. In the presence of Br-_{, which is a}

strong reducing agent, photo-holes are predominately used to oxidize Br- _{to Br}

3-. In the

reverse scan, surface band bending reduces, and the concentration of electrons at the surface increases. For U < -0.32 V, Br3- is reduced back to Br- by surface electrons,

of O2 is observed at -1.07 V. The reduction of protons starts for U > ~-1.15 V.

The inset of Figure 3.11 shows the maximum current density of the Br3- reduction peak

(jBr₃-_/Br-_{) as a function of integrated anodic charge in each scan (Qanodic). The scan rate}

and the scan step are 100 mV/s and 10 mV, respectively. Due to the difficulty of decoupling the reduction peak of Br3- and the reduction peak of O2, 𝑗𝐵𝑟₃-/Br- is used here.

The |𝑗𝐵𝑟3-/Br-| increases as the Qanodic increases. This indicates that the reduction peak

maximum is related to the quantity of Br3- which is oxidized in the anodic scan. For the

scan reversed at U7, the reduction peak of Br3- is barely observed. U7 is more positive

than the onset potential by only 0.02 V, very few Br3- are generated in the forward scan.

When Br3- is generated in the anodic scan, Br3- starts to diffuse from the GaN surface.

Therefore, not all Br3- can be reduced back to Br-. This explains why the jBr3-/Br--Qanodic

slope decreases when the Uj becomes more positive.

Figure 3.11: Cyclic voltammograms of n-GaN under illumination, which were scanned continuously for 7 times and were reversed at Uj at each scan. Subscript j

denotes the scan sequence, ascending from 1 to 7. The inset shows the peak current of the Br3- reduction (U = ~-

0.5 V) as a function of integrated anodic charge for each scan. Solution: 0.1 M HBr and 0.2 M Na2SO4; v: 100

Chapter 3. n-GaN/solution interface

Since dissolved oxygen was not purged out of the solution before the measurements, the reduction peak of O2 is observed in the cathodic scan, as is confirmed in scan 7. The

current maximum of the oxygen reduction peak also increases with Uj. This indicates that

not all photo-holes are used to oxidize Br- _{and some of photo-holes oxidize water.}

The cyclic voltammograms of the n-GaN photoanode in Figure 3.11 show the consistent photocurrent density in the forward scan and in the reverse scan. However, for some n-GaN samples, the photocurrent in the forward and in the reverse scan are different. Figure 3.12 (a) shows a cyclic voltammogram of an n-GaN under illumination in a solution of 0.1 M HBr and 0.2 M Na2SO4. The measurement was recorded at a scan rate

of 100 mV/s. The onset potential is at ~-0.3 V and the plateau photocurrent is reached at ~0.0 V. For -0.3 V < U < 0.0 V, the photocurrent in the forward and reverse scan are different. This effect is not observed in the n-GaN sample in Figure 3.11. We do not know the crystalline quality difference between the n-GaN samples used in Figure 3.11 and Figure 3.12. Both samples have a comparable doping concentration (5×1018 _cm-3_).

Figure 3.12 (b) shows the current density-time (j-t) plots of the n-GaN at different biases. The measurements were first performed in the dark, then under illumination and finally in the dark. The illumination was controlled by a shelter. For current biased at 0.10

Figure 3.12: (a) A cyclic voltammogram of n-GaN under illumination. (b) Current density-time (j-t) plots of the n- GaN at different biases, which were first measured in the dark, then under illumination and finally in the dark. Solution: 0.1 M HBr and 0.2 M Na2SO4; v: 100 mV/s.

V and 0.25 V, no current is observed in the dark. Once the sample is illuminated, the current increases sharply. The photocurrent at 0.25 V is higher than 0.10 V as a result of SCR expansion. After the light is shielded, the current drops sharply to zero. At -0.25 V, when the sample is illuminated, the current increases sharply and a current spike is observed. The photocurrent reaches the steady state after ~0.5 s. Once the light is sheltered, a cathodic current spike is observed. This current spike decays to zero after ~0.5 s. The transient current in the j-t measurement confirms that the photocurrent difference between the forward scan and the reverse scan in Figure 3.12 (a) is a time effect.

The transient effect in the j-U and j-t measurements have been reported for different photoanodes [30]-[33]. When the illumination intensity is suddenly increased, photo- holes can accumulate at the surface due to low kinetics of charge transfer to solution [32]. In the presence of Br- _{in the solution and under weak UV illumination, Faradaic charge}

transfer should not be rate-limiting in our case. The charging of surface states or traps in the bulk can lead to the transient current [33]. Once photo-holes are suddenly present, electrons in traps may recombine with photo-holes efficiently, contributing to the transient current. Similarly, once photoexcitation stops, the process is reversed and negative transient current may be generated.