Efficient solar photoelectrosynthesis of methanol from carbon dioxide using hybrid CuO-Cu2O semiconductor nanorod arrays

Article abstract of DOI:10.1039/c2cc38068d

Solar photoelectrosynthesis of methanol was driven on hybrid CuO-Cu ₂O semiconductor nanorod arrays for the first time at potentials ～800 mV below the thermodynamic threshold value and at Faradaic efficiencies up to ～95%. The CuO-Cu₂O nanorod arrays were prepared on Cu substrates by a two-step approach consisting of the initial thermal growth of CuO nanorods followed by controlled electrodeposition of p-type Cu₂O crystallites on their walls. No homogeneous co-catalysts (such as pyridine, imidazole or metal cyclam complexes) were used contrasting with earlier studies on this topic using p-type semiconductor photocathodes. The roles of the core-shell nanorod electrode geometry and the copper oxide composition were established by varying the time of electrodeposition of the Cu₂O phase on the CuO nanorod core surface.

Full text of DOI:10.1039/c2cc38068d

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Efficient solar photoelectrosynthesis of methanol from
carbon dioxide using hybrid CuO–Cu2O semiconductor
nanorod arrays†
Ghazaleh Ghadimkhani,a Norma R. de Tacconi,*a Wilaiwan Chanmanee,a
Csaba Janakyab and Krishnan Rajeshwar*a
Cite this: Chem. Commun., 2013,
49, 1297
Received 7th November 2012,
Accepted 20th December 2012
DOI: 10.1039/c2cc38068d
www.rsc.org/chemcomm
Solar photoelectrosynthesis of methanol was driven on hybrid alone there are eighteen methanol production plants with a
CuO–Cu2O semiconductor nanorod arrays for the first time at cumulative annual capacity of over 2.6 billion gallons.5 How-
potentials B800 mV below the thermodynamic threshold value ever, the current feedstock for this production is hydrocarbon-
and at Faradaic efficiencies up to B95%. The CuO–Cu2O nanorod based with a sizeable carbon footprint. Further, the high
arrays were prepared on Cu substrates by a two-step approach operating pressures and temperatures needed for steam
consisting of the initial thermal growth of CuO nanorods followed reforming translate into correspondingly high capital invest-
by controlled electrodeposition of p-type Cu2O crystallites on their ment and a long energy pay-back period. Therefore mild
walls. No homogeneous co-catalysts (such as pyridine, imidazole methods for producing methanol from environmentally proble-
or metal cyclam complexes) were used contrasting with earlier matic feedstocks such as CO2 take on added importance.
studies on this topic using p-type semiconductor photocathodes.
From the active semiconductor perspective, with the excep-
The roles of the core–shell nanorod electrode geometry and the tion of the last compound (pyrite) and elements such as Si and
copper oxide composition were established by varying the time of P in the listing above, all the other component elements in
electrodeposition of the Cu2O phase on the CuO nanorod core surface. these semiconductors are either toxic (As, Cd) or not earth-
abundant (Te, Ga, In). Selective photoconversion of CO2 to
Since the pioneering report in 1978,1 there have been many methanol without the use of a co-catalyst is also not common-
studies on the use of p-type semiconductors (e.g., CdTe, GaP, place in these prior studies, and in such cases, methanol was
GaAs, InP, Si, and FeS2) for driving the photoelectrochemical only photogenerated at very high overpotentials. Two-electron
(PEC) reduction of CO2 in aqueous media; see ref. 2–4 for reduction products such as CO6 and formic acid7 have been
reviews of this body of work. Given that CO2 is a greenhouse reported in some cases in place of the (more difficult) six-
gas, using sunlight to convert CO2 to transportation fuel (such electron conversion to methanol. With the use of semiconductor
as methanol) represents a value-added approach to the simul- colloids such as CdS or ZnS, photocatalytic reduction of CO2
taneous generation of alternative fuels and environmental yields mostly formate and/or formaldehyde as the products.8
remediation of carbon emissions from the continued use of
Copper oxide exists as a compound semiconductor in two
conventional fuels. In this communication, we demonstrate different metal oxidation states, Cu2O and CuO; these two
that CuO–Cu2O nanorod arrays prepared on Cu substrates by a compounds have energy band-gaps of B2.0–2.2 eV and
two-step thermal oxidation–electrodeposition hybrid approach 1.3–1.6 eV respectively.9 Additionally both oxides have high
can drive the efficient solar photoelectrosynthetic conversion absorption coefficients over a substantial portion of the solar
of CO2 to methanol. Importantly, unlike in previous studies spectrum and importantly, from a sustainability and toxicity
on this topic,2,3 no co-catalysts (such as pyridine, imidazole, or perspective, the component elements are plentiful and rela-
Ni cyclam2+) were used for this selective conversion process.
tively less toxic than elements such as Cd or As. The conduction
Selective conversion of CO2 to methanol has several redeeming band edge positions lie at very negative potentials (relative to
features especially in light of the fact that large amounts of this other oxides such as TiO2 and WO3)10,11 translating into high
chemical are manufactured worldwide. For example, in the US reducing power for the photogenerated electrons. Given all
these favorable attributes and the fact that copper and its
a Department of Chemistry and Biochemistry, The University of Texas at Arlington,
oxides have shown high electrocatalytic activity for CO2
reduction,12–14 it is rather surprising that there are no reports
Arlington, Texas 76019-0065, USA. E-mail: rajeshwar@uta.edu, ntacconi@uta.edu
b Department of Physical Chemistry and Materials Science, University of Szeged,
(at least to our knowledge) for the use of copper oxide for the
Szeged, H6720, Hungary
photoreduction of CO2. On the other hand there are several
reports on the utilization of both Cu2O and CuO for the
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c2cc38068d
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photogeneration of H2 from water11,15,16 and very recently,
virus-templated CuO ‘‘3-D nanorod arrays’’ were demonstrated17
to afford high photocurrent densities.
For the preparation of the hybrid CuO–Cu2O photocathodes,
CuO nanorod arrays were first prepared following a thermal
procedure.18 Cathodic electrodeposition of Cu2O was then
performed on CuO nanorod arrays from a copper(II) lactate
bath.11,19 The data below thus correspond to electrodeposited
Cu2O nanocrystallites on CuO nanorods (B100 nm diameter)
thermally grown on copper substrates. The electrodeposition
was performed at À0.2 V vs. SHE in 0.4 M CuSO4 + 3 M lactic
acid, pH 9 at 60 1C for pre-selected time periods. Electrodes are
designated as TH for the thermally grown CuO nanorods, and
TH/EDt for the CuO–Cu2O hybrid samples prepared by electro-
deposition for t min (t spanning the range: 1–30 min) on the
CuO nanorod surface (see Table S1, ESI†). The large area
associated with the CuO nanorods provides a surface aspect
ratio enhancement factor of 7–8 depending on the electro-
synthesis time (Fig. S1, ESI†). This morphological factor is
the key to the performance of the Cu2O–CuO nanorod array
photoelectrodes.
Fig. 2 Comparison of photocurrent–potential profiles of CuO–Cu2O hybrid
nanorod arrays obtained with manually-chopped simulated AM 1.5 illumination
(70 mW cmÀ2) in 0.1 M Na2SO4 aqueous solution for Cu2O electrodeposition
times of 0 (TH, a), 10 (TH/ED10, b) and 30 min (TH/ED30, c). Panel d: comparison
of PEC activity for the electrodeposited (ED30) Cu2O electrode in N2 and CO2
saturated solutions. Panel e: comparison of a CuO–Cu2O hybrid nanorod film vs.
the electrodeposited Cu2O electrode in CO2-saturated 0.1 M Na2SO4 aqueous
solution.
Fig. 1a contains a visual representation of the CuO–Cu2O
nanostructures developed for this study. Fig. 1b–e presents included for comparison. In a N2 atmosphere, the photocurrent
representative SEM pictures for the evolution of the CuO–Cu2O is associated with H2 photogeneration from water (blue traces
nanorods from the starting morphology consisting of thermally in Fig. 2a–c) and we have included these data only for com-
grown CuO nanorods (Fig. 1b) to progressively covered CuO–Cu2O parative purposes. Importantly, in the CO2 saturated electro-
nanostructures as the Cu2O electrodeposition time, t, increased in lyte, the photocurrent transients for CuO–Cu2O nanorod films
the sequence: 1 min (Fig. 1c), 10 min (Fig. 1d), and 30 min are seen to reach B3 times higher values than in the
(Fig. 1e) respectively. By adjusting the t values, the Cu2O shell N2-saturated counterpart solution (red traces in Fig. 2b and c).
thickness on CuO nanorods was varied between ca. 100 and Of all the hybrid CuO–Cu2O samples, the TH/ED10 was found
500 nm, although the resulting CuO–Cu2O nanorods were found to be the best performing for the photoelectroreduction of CO2.
to be thicker at their bases which is suggestive of a bottom-up It is worth noting that for PEC generation of H2 on the other
growth process for Cu2O nanocrystallites.
hand, the thermal film performs the best but at more negative
The photoelectroactivity of selected CuO–Cu2O nanorod applied potentials (Fig. 2a). For CO2 photoelectrochemical
electrodes is compared in Fig. 2 under chopped AM 1.5 reduction, the TH films yielded lower photocurrent than
illumination in a 0.1 M Na2SO4 solution for both N2 and CO2 any of the TH/EDt (t varied between 1 and 30 min) hybrid
saturated conditions. Photocurrent profiles for CuO–Cu2O counterparts.
nanorod electrodes, TH/ED10 and TH/ED30, are provided in
To further clarify the particular contribution of the two
Fig. 2b and c respectively; the corresponding profile for a components in the CuO–Cu2O nanorod arrays for the photo-
thermally prepared photoelectrode (TH, Fig. 2a) is also electroreduction of CO2, Fig. 2d compares photocurrent–potential
profiles of a Cu2O electrodeposited film (prepared on a
polished Cu foil as a substrate) in N2- and CO2-saturated
0.1 M Na2SO4 solutions. The Cu2O film showed a dramatic
increase of photocurrent transients in the presence of CO2 and
also a positive shift of the photocurrent onset potential thres-
hold. At À0.25 V vs. SHE, the photocurrent in CO2 solution was
ca. 3.2 times higher than in N2. At less negative potentials the
photocurrent enhancement in the presence of CO2 was much
higher reaching factors of 6–7 at potentials BÀ0.1 V thus
indicating that CO2 is a much better electron scavenger (capturing
the photoelectrons from Cu2O) than water. The comparison of
Fig. 1 Schematic illustration of the two-step synthesis of CuO–Cu2O hybrid TH/ED30 with ED30 in Fig. 2e sheds important light on the role
nanorod arrays for solar photoelectrosynthesis of CH3OH from CO2: (1) thermal
of the CuO interior core. Observe that throughout the potential
range in Fig. 2e, the CuO–Cu2O hybrid outperformed the
electrodeposited ED30 control sample.
growth of CuO nanorods on a Cu foil and (2) cathodic electrodeposition of Cu2O
for selected time periods from a cupric lactate solution, pH 9 (a). Morphology/
composition of a CuO–Cu2O nanorod is sketched on the right side of panel (a).
The photocurrent onset potential threshold was B0.25 V
more positive for the CuO–Cu2O hybrid than the Cu2O ED
SEM images of thermally grown CuO nanorods before (b) and after Cu2O
electrodeposition on them for 1 (c), 10 (d) and 30 min (e) respectively.
c
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photoelectrons into CO2 is likely contributing to the enhanced
photoelectrochemical performance of CuO–Cu2O nanorod
arrays relative to a single Cu2O film. An added (morphological)
factor is the increased surface area of the CuO–Cu2O with
respect to a compact Cu2O ED film. Importantly note that, to
sustain the double injection pathway, the tip of the CuO core
has to be in contact with the electrolyte.
In summary, a simple two-step hybrid thermal growth/
electrodeposition approach has been employed to prepare
and optimize solar photoactive CuO–Cu2O nanorod arrays for
the photoelectrosynthesis of methanol from CO2. Methanol
photogeneration was confirmed by GC-MS analyses of product
evolved at À0.2 V vs. SHE, i.e. at an underpotential relative to the
standard potential of CO2/CH3OH. This last feature is an
important virtue of the p-type semiconductor based photo-
reduction approach as adopted in this study and by other
authors2–4 previously. In contrast the (‘‘dark’’) electrocatalytic
process counterpart for CO2 reduction suffers from the electrical
energy cost incurred from the need for considerable overpotentials
to overcome the kinetic barrier associated with this process.12,14
This cost is simply circumvented in the solar PEC process from
the energy inherent in sunlight.
Fig. 3 Energy band diagram of hybrid CuO–Cu2O nanorod arrays for solar
photoelectrosynthesis of CH3OH from CO2. Semiconductor band edges and
redox potentials are shown vs. the SHE ref. electrode. CB: conduction band;
VB: valence band.
sample (Fig. 2e). This represents an B800 mV shift relative to
the standard potential of the CO2/CH3OH redox couple. Such
lowering of the onset potential for CO2 photoconversion is
associated with the CuO component and is unprecedented at
least to our knowledge. Photoelectrosynthesis of CH3OH was
demonstrated with a TH/ED10 CuO–Cu2O nanorod photo-
electrode (Fig. S2, ESI†). The photoelectrode was placed in an
aqueous solution saturated with CO2, polarized at À0.2 V vs.
SHE, and irradiated with visible light provided by a AM 1.5 solar
simulator. A gas chromatograph equipped with a mass spectro-
meter (GC-MS) was used to detect methanol. Methanol photo-
generation was monitored at m/z = 31 (CH2OH+) and was found
to reach a concentration of ca. 85 mM after 90 min of irradia-
tion. Faradaic efficiencies were in the 94–96% range (con-
sidering that 6eÀ are required to form one molecule of
CH3OH from CO2). Note that a potential of À0.2 V vs. SHE
represents an ‘‘underpotential’’ greater than at least 150 mV
given that the standard potential for the CO2/CH3OH redox
process lies at À0.38 V vs. SHE (see Fig. 3).
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Chem. Commun., 2013, 49, 1297--1299 1299
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