Highly Stable Photoelectrochemical Water Splitting and Hydrogen ...
Supporting Information
Highly Stable Photoelectrochemical Water Splitting and Hydrogen Generation Using a Double-Band InGaN/GaN Core/Shell Nanowire Photoanode B. AlOtaibi, H. P. T. Nguyen, S. Zhao, M. G. Kibria, S. Fan, and Z. Mi* Department of Electrical and Computer Engineering, McGill University 3480 University Street, Montreal, Quebec H3A 0E9, Canada *
: E-mail: [email protected]; Phone: 1 514 398 7114
S1. Measurement of the incident-photon-to-current-conversion efficiency The incident-photon-to-current-conversion efficiency (IPCE) describes the ratio of photo-generated electrons collected by the electrodes over the number of incident monochromatic photons. In the ideal case, the IPCE can approach unity, i.e. each incident photon is absorbed by the semiconductor electrode and produces one electron-hole pair that contributes to the current of the PEC system. However, in practice, the IPCE is limited by the inefficient light absorption, carrier recombination, and resistive loss in the system. The obtained IPCE values for this study were derived by measuring the current under 300 W Xenon lamp irradiation with different band pass optical filters, shown in Figure S1. During the measurements, the applied potential was scanned from -1 to 1 V with a rate of 20 mV/s. The photocurrent was obtained by subtracting the dark current density from the measured current density for a particular wavelength. The following band pass filters, with center wavelengths at 350, 380, 408, 445,
1
488, 532, 570, and 600 nm were employed, and the measured light intensity was 27, 38, 38, 40, 66, 37, 54, and 150 mW/cm2, respectively. The IPCE is then computed using: %
/
/
100
(1)
Figure S1. Variations of the current density with applied voltage (vs. Ag/AgCl) under illumination conditions with the use of various band pass filters in 1 mol/L HBr.
S2. H2 production under simulated sunlight illumination H2 evolution was measured using two-electrode configuration in 1 mol/L HBr electrolyte as a function of time at 0.2 V (vs. the counter electrode) under simulated solar spectrum using an AM1.5G filter. The produced H2 gas was detected using a gas chromatograph (Shimadzu GC-8A) equipped with a thermal conductivity detector (TCD). Argon gas was used as the carrier gas for GC analysis. An air tight syringe was used for sampling from the vacuum sealed chamber. Figure S2 shows nearly steady H2 evolution measured over 5 hours, with an average generation rate of ~ 35 µmol/hr. The calculated mole 2
of hydrogen, shown in Figure S2 (red solid line), was obtained from Faraday's laws of electrolysis using the measured current based on the following formula: #
(2)
where I is the photocurrent, t is time, F is the Faraday constant (the quantity of charge in coulomb carried by one mole of electrons, i.e. 96484.34 C/mole), and Q is the quantity of charge in coulomb and equals to
. When the current is not constant, the quantity of charge passed through the circuit
equals to the integration of the measured current over time ( ). It is seen that the measured and calculated H2 evolution agrees very well.
Figure S2. H2 evolution in 1 mol/L HBr at 0.2 V vs. the counter electrode under simulated sunlight using AM1.5G filter. The evolution of H2 exhibits a nearly linear increase over time. The H2 evolution is also calculated from the measured current (solid red curve).
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S3.
On the effect of Si substrate In order to evaluate any contribution of the underlying Si substrate towards the measured
photocurrent, a control experiment was carried out on bare Si substrate. The n-type Si(111) substrate was illuminated by 300 W Xenon lamp with AM1.5G filter in 1 mol/L HBr. The sample preparation and experimental conditions were similar to those for InGaN/GaN nanowire photoanode. Shown in Figure S3, the measured current density is less than 2 µA/ cm2 at 0.2 V vs. the counter electrode, which is nearly two to three orders of magnitude smaller than that measured from InGaN/GaN nanowire structures under identical conditions, suggesting negligible contribution of the Si substrate for the measured photo-current.
Figure S3. Measured current density as a function of time at 0.2 V vs. the counter electrode under 300W Xenon lamp illumination with AM1.5G filter in 1mol/L HBr, showing the negligible contribution from Si substrate.
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S4. Stability analysis of InGaN/GaN nanowire photoanode To evaluate the stability of InGaN/GaN nanowires, chronoamperometric experiment over 24 hours was performed. The sample was biased at 0.2 V (vs. the counter electrode) in 1 mol/L HBr in the twoelectrode configuration. The sample was irradiated by 300 W Xenon lamp with a 375 nm long-pass filter. Then InGaN/GaN nanowires were studied using scanning electron microscopy (SEM), shown in Figure S4, which confirms unchanged morphology of the InGaN/GaN core/shell nanowires. The nanowires were further examined using STEM and energy dispersive X-Ray spectrometry (EDXS). Figure S5 shows the HAADF image of a single nanowire and the elemental distribution in the nanowire structure analyzed by EDXS. Variations of the In Lα, Ga Kα, and N Kα signals along the nanowire axial and lateral directions are shown in Figures S5(b)-(e). The axial scan (blue line in Figure S5(a)) along the nanowire clearly shows the three InGaN segments, illustrated Figure S5(b). The In and Ga compositions of the three segments were further analyzed by the lateral EDXS scans (green lines in Figure S5(a)), shown in Figures S5(c)-(e). This clearly demonstrates the stability of InGaN/GaN nanowires and its excellent resistance to photo-oxidation and corrosion.
Figure S4. SEM image of InGaN/GaN core/shell nanowires after 24 hours chronoamperometric experiment.
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Figure S5. (a) High-angle annular dark field image of InGaN/GaN nanowire after 24 hours photoelectrochemical reaction in 1 mol/L HBr at a bias of 0.2 V vs. the counter electrode. (b)-(e) Energy dispersive X-Ray (EDX) line scans along the axial (b) and radial ((c)-(e)) directions of the nanowire.
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Highly Stable Photoelectrochemical Water Splitting and Hydrogen Generation Using a Double-Band InGaN/GaN Core/Shell Nanowire Photoanode B. AlOtaibi, H. P. T. Nguyen, S. Zhao, M. G. Kibria, S. Fan, and Z. Mi* Department of Electrical and Computer Engineering, McGill University 3480 University Street, Montreal, Quebec H3A 0E9, Canada *
: E-mail: [email protected]; Phone: 1 514 398 7114
S1. Measurement of the incident-photon-to-current-conversion efficiency The incident-photon-to-current-conversion efficiency (IPCE) describes the ratio of photo-generated electrons collected by the electrodes over the number of incident monochromatic photons. In the ideal case, the IPCE can approach unity, i.e. each incident photon is absorbed by the semiconductor electrode and produces one electron-hole pair that contributes to the current of the PEC system. However, in practice, the IPCE is limited by the inefficient light absorption, carrier recombination, and resistive loss in the system. The obtained IPCE values for this study were derived by measuring the current under 300 W Xenon lamp irradiation with different band pass optical filters, shown in Figure S1. During the measurements, the applied potential was scanned from -1 to 1 V with a rate of 20 mV/s. The photocurrent was obtained by subtracting the dark current density from the measured current density for a particular wavelength. The following band pass filters, with center wavelengths at 350, 380, 408, 445,
1
488, 532, 570, and 600 nm were employed, and the measured light intensity was 27, 38, 38, 40, 66, 37, 54, and 150 mW/cm2, respectively. The IPCE is then computed using: %
/
/
100
(1)
Figure S1. Variations of the current density with applied voltage (vs. Ag/AgCl) under illumination conditions with the use of various band pass filters in 1 mol/L HBr.
S2. H2 production under simulated sunlight illumination H2 evolution was measured using two-electrode configuration in 1 mol/L HBr electrolyte as a function of time at 0.2 V (vs. the counter electrode) under simulated solar spectrum using an AM1.5G filter. The produced H2 gas was detected using a gas chromatograph (Shimadzu GC-8A) equipped with a thermal conductivity detector (TCD). Argon gas was used as the carrier gas for GC analysis. An air tight syringe was used for sampling from the vacuum sealed chamber. Figure S2 shows nearly steady H2 evolution measured over 5 hours, with an average generation rate of ~ 35 µmol/hr. The calculated mole 2
of hydrogen, shown in Figure S2 (red solid line), was obtained from Faraday's laws of electrolysis using the measured current based on the following formula: #
(2)
where I is the photocurrent, t is time, F is the Faraday constant (the quantity of charge in coulomb carried by one mole of electrons, i.e. 96484.34 C/mole), and Q is the quantity of charge in coulomb and equals to
. When the current is not constant, the quantity of charge passed through the circuit
equals to the integration of the measured current over time ( ). It is seen that the measured and calculated H2 evolution agrees very well.
Figure S2. H2 evolution in 1 mol/L HBr at 0.2 V vs. the counter electrode under simulated sunlight using AM1.5G filter. The evolution of H2 exhibits a nearly linear increase over time. The H2 evolution is also calculated from the measured current (solid red curve).
3
S3.
On the effect of Si substrate In order to evaluate any contribution of the underlying Si substrate towards the measured
photocurrent, a control experiment was carried out on bare Si substrate. The n-type Si(111) substrate was illuminated by 300 W Xenon lamp with AM1.5G filter in 1 mol/L HBr. The sample preparation and experimental conditions were similar to those for InGaN/GaN nanowire photoanode. Shown in Figure S3, the measured current density is less than 2 µA/ cm2 at 0.2 V vs. the counter electrode, which is nearly two to three orders of magnitude smaller than that measured from InGaN/GaN nanowire structures under identical conditions, suggesting negligible contribution of the Si substrate for the measured photo-current.
Figure S3. Measured current density as a function of time at 0.2 V vs. the counter electrode under 300W Xenon lamp illumination with AM1.5G filter in 1mol/L HBr, showing the negligible contribution from Si substrate.
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S4. Stability analysis of InGaN/GaN nanowire photoanode To evaluate the stability of InGaN/GaN nanowires, chronoamperometric experiment over 24 hours was performed. The sample was biased at 0.2 V (vs. the counter electrode) in 1 mol/L HBr in the twoelectrode configuration. The sample was irradiated by 300 W Xenon lamp with a 375 nm long-pass filter. Then InGaN/GaN nanowires were studied using scanning electron microscopy (SEM), shown in Figure S4, which confirms unchanged morphology of the InGaN/GaN core/shell nanowires. The nanowires were further examined using STEM and energy dispersive X-Ray spectrometry (EDXS). Figure S5 shows the HAADF image of a single nanowire and the elemental distribution in the nanowire structure analyzed by EDXS. Variations of the In Lα, Ga Kα, and N Kα signals along the nanowire axial and lateral directions are shown in Figures S5(b)-(e). The axial scan (blue line in Figure S5(a)) along the nanowire clearly shows the three InGaN segments, illustrated Figure S5(b). The In and Ga compositions of the three segments were further analyzed by the lateral EDXS scans (green lines in Figure S5(a)), shown in Figures S5(c)-(e). This clearly demonstrates the stability of InGaN/GaN nanowires and its excellent resistance to photo-oxidation and corrosion.
Figure S4. SEM image of InGaN/GaN core/shell nanowires after 24 hours chronoamperometric experiment.
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Figure S5. (a) High-angle annular dark field image of InGaN/GaN nanowire after 24 hours photoelectrochemical reaction in 1 mol/L HBr at a bias of 0.2 V vs. the counter electrode. (b)-(e) Energy dispersive X-Ray (EDX) line scans along the axial (b) and radial ((c)-(e)) directions of the nanowire.
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