Download Wafer-Level Artificial Photosynthesis for CO2 Reduction into CH4

Supporting Information
Wafer-Level Artificial Photosynthesis for CO2 Reduction into
CH4 and CO Using GaN Nanowires
Bandar AlOtaibi1, ShizhaoFan1, Defa Wang2,3, Jinhua Ye2,4, and Zetian Mi1*
1
Department of Electrical and Computer Engineering, McGill University, 3480 University Street, Montreal,
Quebec H3A 0E9, Canada
2
TU-NIMS Joint Research Center, School of Materials Science and Engineering, Tianjin University, 92 Weijin
Road, Nankai District, Tianjin 300072, China
3
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), 92 Weijin Road, Nankai District,
Tianjin 300072, China
4
International Center for Materials Nanoarchitectonics (WPI-MANA) and Environmental Remediation Materials
Unit, National Institute for Materials Science (NIMS), 1-1 Namiki , Tsukuba , Ibaraki 305-0044 , Japan
*
: E-mail: [email protected]; Phone: 1 514 398 7114
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S1. Hydrogen production rate
In these photocatalytic experiments, H+ is formed via the oxidation of water molecules where OH- can be
formed on the surface. Water oxidation has been previously reported on GaN1-5. Still, the water oxidation
mechanism is debatable among researchers6-8. With the input energy from photons in CO2 photoreduction, the
bond C=O can be broken to form C-H bonds, which leads to the formation of different fuels such as methane
(CH4). H2 formation is a very competitive process in CO2 photochemical and photoelectrochemical reduction. This
is due to the use of water as an atomic hydrogen donor for CO2 reduction, whereby water acts as holes’ scavenger.
The competition of H+ reduction to form H2 with CO2 reduction is due to its favourably simple two-electron
transfer process. For this reason, it is common that photocatalytic CO2 reduction using water is accompanied by
high yield hydrogen generation.
Figure S1. Measured H2 evolution rate on as-grown GaN nanowires and Rh, Rh/Cr2O3 and Pt decorated GaN
nanowires.
Figure S1 shows a comparison of maximum H2 evolution rate on as-grown GaN nanowires and GaN
nanowires decorated with Rh, Rh/Cr2O3 core/shell and Pt nanoparticles. The maximum H2 evolution rate was
observed on Pt decorated GaN nanowires where it reaches 3.5 μmol/hr. This relatively high evolution rate is
attributed to the excellent catalytic activity of Pt co-catalyst which is well-known to promote hydrogen evolution
reaction (HER). The H2 evolution rate was observed to be lower (~0.4 μmol/hr) for GaN nanowires without Pt
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nanoparticles. Rh/Cr2O3 core/shell decorated GaN nanowires exhibit relatively high H2 evolution rate (~0.6
μmol/hr), whereas Rh decorated GaN shows much lower H2 evolution rate. This could be attributed to the
backward reaction of water on Rh nanoparticles. Similar observation using Rh and Rh/Cr2O3 in overall water
splitting was previously reported9.
S2. CO2 photoreduction on Rh nanoparticles decorated GaN nanowires.
Promoting only Rh particles on the surface of GaN nanowires did not improve CO2 photoreduction into neither
CO nor CH4. Comparison in yield of CO and CH4 between as-grown GaN nanowires and Rh decorated nanowires
is shown in Figure S2. Trivial yield of CH4 was detected using CO2 photoreduction on Rh nanoparticles decorated
GaN nanowires.
Figure S2. CO and CH4 yield on as-grown GaN nanowires and Rh decorated GaN nanowires.
This CH4 yield was significantly enhanced using Rh/Cr2O3 core/shell co-catalysts on GaN. The formation of
H2O is found to be faster than the hydrogenation of CO2, which makes it another competing process with CO2
reduction. The backward formation of H2O is also undesirable in the field of water splitting in which Rh/Cr2O3
were found to be beneficial for eliminating such backward reactions9. Cr2O3 is a permeable material for H+ 9. That
is, only H+ ions can easily diffuse to Rh, shown schematically in Figure S3, leading to the significant elimination of
H2O formation. The formation of formate (HCO-2) on Rh supported metal oxide was observed by a few studies10,
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. It was believed the formation of formate could occur on the metal-support oxide (e.g. Al2O3 or MgO). And the
atomic hydrogen came from adjacent Rh metal. In this regards, Scheme 1 represents a simple plausible adsorption
of format species on Cr2O3.
Figure S3. Schematic of H+ diffusion to Rh particles on an Rh/Cr2O3 core/shell decorated GaN nanowire.
However, using Rh/Cr2O3, the number of photogenerated electrons participating in the formation of CO and
CH4 is less than those without Rh/Cr2O3 even though the co-catalysts can be used as an electrons sink, which can
facilitate in separating electrons and holes in the photocatalysts to reduce the recombination process. However, the
selectivity of Rh/Cr2O3 toward eight-electron reduction such CH4 rather than two-electron reduction as for CO
increases the probability of recombination and backward reactions.
Scheme S1. Simple representation of the adsorption of formate species on Cr2O3 shell and protons on Rh core.
S3. Stability of GaN nanowires in CO2 photoreduction
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The duration of the performed experiments is 24 hours. The decomposition of GaN will result in abnormal
N2 gas evolution and observable products degradation over time. However, these have not been observed during
the course of the experiment. After each photochemical experiment, scanning electron microscope (SEM) images
were usually taken. Careful evaluation from these images shows no sign of degradation of the GaN nanowires.
Figure S4 shows an SEM image of as-grown GaN nanowires after 24 hour of CO2 photochemical experiment. The
stability of GaN nanowires in photochemical and photoelectrochemical reactions has also been discussed in other
previous studies4, 12, 13.
Figure S4. SEM image of as-grown GaN nanowires after 24 hours of CO2 photoreduction.
In this study, one can exclude the oxidation of GaN nanowires based on the following analysis. The
oxidation and decomposition of GaN proceeds according to the following equation.
2GaN + 6h+ → 2Ga+ + N2
(1)
Or
GaN + 3h+ → Ga+ + 0.5N2
(2)
From Figure 3c in the text, the H2 evolution rate on Pt loaded GaN is 3.5 µmol/hr. The course of the reported
experiments was 24 hours. Therefore, the number of moles of electrons that participated in H2 formation would be
(3.5 µmol/hr × 24 hr × 2 =) 168 µmol, which corresponds to the holes that participated in oxidation reactions. If
these holes participated in the oxidation of GaN, according to equation (2), that would require (168 µmol /3 =) 56
µmol of GaN. However, only ~ 8 µmol of GaN was used during these experiments. The same calculation of the
lowest H2 evolution rate presented in Figure 3c leads to the same conclusion, i.e. the self-oxidation of GaN cannot
explain the measured activities.
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X-ray photoelectron spectroscopy (XPS) studies of GaN nanowires before and after reaction were also
performed. Full XPS spectra of a fresh sample of as-grown GaN nanowires and samples after CO2 reduction
experiments show no significant differences. Shown in Figure S5 are the Ga2p and N1s peaks for the fresh sample
and a sample after CO2 reduction experiments. The position of the Ga2p peak is ~1118.7 eV. N1s can be
decomposed to different components, one of which is GaN, with its peak located at ~ 397.5 eV. The other
components of N1s peak could be attributed to the nitridization of the substrate during the growth process. As the
substrate used is Si, O1s peak is also present. The trivial difference in XPS patterns, together with the SEM images
of nanowires (Figure S4) between a fresh sample and a sample after 24 hours of the CO 2 reduction experiment
provides clear evidence that the CO2 reduction on GaN nanowires is a photocatalytic process.
Figure S5. XPS spectra for Ga2p and N1s for a fresh sample and a sample after reaction grown on the same Si substrate.
S4. Confirming CO2 photoreduction using isotopically labeled 13CO2
Previously, it has been reported that CO and CH4 formation may not be due to CO2 reduction but rather some
carbon residues14. In the experiments reported here, none of the products were detected without filling the chamber
with highly pure CO2 reactant. We have further performed detailed experiments with the use of 13CO2 as reactant
to confirm the source of CO and CH4. Photochemical
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CO2 reduction was performed with the same conditions
described in the main text. The sample was illuminated as described in the main text. Gas samples were analyzed
using Agilent 5975 GC-MS instrument (EI). Figure S6 shows a gas sample recorded by GC-MS of Pt nanoparticles
decorated GaN nanowires after 3 hour of UV illumination. Figure S6 (a) shows the GS-MS gas chromatography
spectrum where
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CH4 and
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CO contents are indicated. Figure S6 (b) shows the mass spectrum illustrating the
presence of 13CH4. Also, H2O and N2 and O2 are detected, as shown in Figure S6 (a). The presence of 13CO is also
confirmed as shown in Figure S6 (c).
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Figure S6. (a) Gas chromatography spectrum indicating 13CH4 and 13CO contents of a sample taken after 3 hr of UV
irradiation on Pt-decorated GaN. (b) Mass spectrum illustrating the presence
presence 13CO.
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CH4. (c) Mass spectrum of illustrating the
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