DOI: 10.1002/chem.201601642
Communication
&
Carbon Dioxide Reduction
A Monolithically Integrated Gallium Nitride Nanowire/Silicon Solar
Cell Photocathode for Selective Carbon Dioxide Reduction to
Methane
Yichen Wang, Shizhao Fan, Bandar AlOtaibi, Yongjie Wang, Lu Li, and Zetian Mi*[a]
namically favorable conduction band edge position to reduce
Abstract: A gallium nitride nanowire/silicon solar cell pho-
tocathode for the photoreduction of carbon dioxide (CO2)
is demonstrated. Such a monolithically integrated nano-
wire/solar cell photocathode offers several unique advan-
tages, including the absorption of a large part of the solar
spectrum and highly efficient carrier extraction. With the
incorporation of copper as the co-catalyst, the devices ex-
hibit a Faradaic efficiency of about 19% for the 8eÀ pho-
toreduction to CH4 at À1.4 V vs Ag/AgCl, a value that is
more than thirty times higher than that for the 2eÀ re-
duced CO (ca. 0.6%).
CO2 to various hydrocarbons, and superior charge carrier trans-
port properties.[5] Moreover, owing to the ionic bonding char-
acter and the absence of surface states in the middle of the
bandgap, Ga(In)N is much more stable compared to other III-V
compound semiconductors.[5g,6] The surface recombination ve-
locity (ca. 104 cmsÀ1) is also smaller than other III-V materials.
In this context, Yotsuhashi et al. have studied the reduction
of CO2 using planar GaN photocathodes grown on sapphire
substrate.[4b–e] More recently, significantly enhanced photocata-
lytic activities have been demonstrated with the use of Ga(In)N
nanowire photocatalysts and photoelectrodes.[6,7] Such nano-
structured photoelectrodes can exhibit enhanced light absorp-
tion and carrier extraction efficiency, which is due to the large
surface-to-volume ratios. Moreover, the spacing amongst nano-
wires can facilitate the diffusion of electrolyte and improve the
photoelectrocatalytic performance.[5g,8] Nearly defect-free
Ga(In)N nanowire photoelectrodes can also be readily grown
on foreign substrates, including Si and SiOx,[9] thereby making
it possible for the direct integration with Si solar cell technolo-
gy. Such monolithically integrated nanowire/Si solar cell photo-
cathodes can offer significantly enhanced efficiency, greatly re-
duced cost, and scalable manufacturing compared to previous-
ly reported devices. To date, however, the use of a metal ni-
tride nanowire photocathode for CO2 reduction has not been
reported.
With the increased fossil fuel consumption, the level of atmos-
pheric CO2 has shown a steady increase during the past de-
cades. In this regard, the conversion of CO2 into hydrocarbon
fuel via solar-powered photocatalysis or photoelectrocatalysis
(PEC) approach has been intensively studied, as it can simulta-
neously decrease the amount of CO2 in atmosphere and
reduce the use of conventional fossil fuel.[1] Compared to pho-
tocatalysis, a PEC system offers several important advantages,
including efficient charge carrier separation and collection of
the oxidation and reduction products at the anodic and catho-
dic electrode, respectively. Various photocathodes, including Si,
GaP, CuInS2 CuO/Fe2O3, InP, and ZnTe have been developed for
the reduction of CO2.[2] Their performance, however, has re-
mained very limited, owing the poor light absorption and/or
rapid corrosion and degradation. Moreover, much of the work
has been focused on the conversion of CO2 into the 2eÀ re-
duced species such as carbon monoxide (CO), and the 8eÀ
photoreduction of CO2 to methane (CH4) has remained ex-
tremely challenging, which is due to the kinetic barriers associ-
ated with the multiple proton coupled electron transfer (PCET)
process.[1h,3] Recently, the use of metal nitrides, including
Ga(In)N, for CO2 reduction has attracted considerable atten-
tion.[4] Compared to conventional metal oxides, Ga(In)N pos-
sesses several unique advantages, including tunable energy
bandgap across nearly the entire solar spectrum, thermody-
Herein, we report on the first demonstration of a metal ni-
tride nanowire/Si solar cell photocathode for the reduction of
CO2 under simulated sunlight illumination. Unique to such
a monolithically integrated photocathode is that the Si solar
cell substrate can harvest a large part of the solar spectrum
and the GaN nanowires can significantly enhance the extrac-
tion of photo-generated electrons. With the incorporation of
Cu co-catalyst on GaN nanowire arrays, we have measured the
direct conversion of CO2 into CH4 and CO. At À1.4 V vs Ag/
AgCl, the Faradaic efficiency can reach about 19% for the 8eÀ
photoreduction to CH4, which is more than 30 times higher
than that for the 2eÀ reduced CO (ca. 0.6%). Detailed studies
further suggest the synergistic effect of GaN nanowires and Cu
co-catalyst in significantly enhancing the selectivity for the
photoreduction of CO2 to CH4. The GaN nanowire/Si solar cell
photocathode also showed excellent stability towards CO2 re-
duction.
[a] Dr. Y. Wang, S. Fan, B. AlOtaibi, Y. Wang, Dr. L. Li, Prof. Z. Mi
Department of Electrical and Computer Engineering, McGill University
3480 University Street, Montreal, QC H3A 0E9 (Canada)
The photocathode (Figure 1a) consists of a planar n+-p Si
solar cell wafer and n-GaN nanowire arrays. The Si solar cell
functions as both the substrate and the photon absorber. It
Supporting information for this article can be found under:
Chem. Eur. J. 2016, 22, 8809 – 8813
8809
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