Wafer-Level Artificial Photosynthesis for CO2 Reduction into CH4 and CO Using GaN Nanowires

Article abstract of DOI:10.1021/acscatal.5b00776

We report on the first demonstration of high-conversion-rate photochemical reduction of carbon dioxide (CO₂) on gallium nitride (GaN) nanowire arrays into methane (CH₄) and carbon monoxide (CO). It was observed that the reduction of CO₂ to CO dominates on as-grown GaN nanowires under ultraviolet light irradiation. However, the production of CH₄ is significantly increased by using the Rh/Cr₂O₃ core/shell cocatalyst, with an average rate of ～3.5 μmol g_cat^-1 h^-1 in 24 h. In this process, the rate of CO₂ to CO conversion is suppressed by nearly an order of magnitude. The rate of photoreduction of CO₂ to CH₄ can be further enhanced and can reach ～14.8 μmol g_cat^-1 h^-1 by promoting Pt nanoparticles on the lateral m-plane surfaces of GaN nanowires, which is nearly an order of magnitude higher than that measured on as-grown GaN nanowire arrays. This work establishes the potential use of metal-nitride nanowire arrays as a highly efficient photocatalyst for the direct photoreduction of CO₂ into chemical fuels. It also reveals the potential of engineered core/shell cocatalysts in improving the selectivity toward more valuable fuels.

Full text of DOI:10.1021/acscatal.5b00776

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WaferꢀLevel Artificial Photosynthesis for CO2
Reduction into CH and CO Using GaN Nanowires
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Bandar AlOtaibi , Shizhao Fan , Defa Wang , Jinhua Ye , and Zetian Mi
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Department of Electrical and Computer Engineering, McGill University, 3480 University Street,
Montreal, Quebec H3A 0E9, Canada
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TU-NIMS Joint Research Center, School of Materials Science and Engineering, Tianjin University, 92
Weijin Road, Nankai District, Tianjin 300072, China
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Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), 92 Weijin Road,
Nankai District, Tianjin 300072, China
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Environment and Energy Materials Division, National Institute for Materials Science, 1-1 Namiki,
Tsukuba, Ibaraki, 305-0044 Japan
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E-mail: zetian.mi@mcgill.ca; Phone: 1 514 398 7114
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ABSTRACT
We report on the first demonstration of high conversion rate photochemical reduction of carbon dioxide
CO ) on gallium nitride (GaN) nanowire arrays into methane (CH ) and carbon monoxide (CO). It was
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observed that the reduction of CO to CO dominates on asꢀgrown GaN nanowires under ultraviolet light
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irradiation. However, the production of CH is significantly increased by using Rh/Cr O core/shell coꢀ
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ꢀ1 ꢀ1
catalyst, with an average rate of ~3.5 ꢁmol gcat h in 24 hours. In this process, the rate of CO to CO
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conversion is suppressed by nearly an order of magnitude. The rate of photoreduction of CO to CH
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ꢀ1 ꢀ1
can be further enhanced and can reach ~14.8 ꢁmol gcat h by promoting Pt nanoparticles on the lateral
mꢀplane surfaces of GaN nanowires, which is nearly an order of magnitude higher than that measured
on asꢀgrown GaN nanowire arrays. This work establishes the potential use of metalꢀnitride nanowire
arrays as a highly efficient photocatalyst for the direct photoreduction of CO into chemical fuels. It also
2
reveals the potential of engineered core/shell coꢀcatalysts in improving the selectivity towards more
valuable fuels.
Keywords: carbon dioxide photoreduction; artificial photosynthesis; solar fuels; methane; gallium
nitride; nanowires
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It has been envisioned that the chemical transformation of carbon dioxide (CO ) may provide a
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promising solution to simultaneously address some of the grand challenges we face, including energy
storage, energy resource shortage, as well as the negative environmental impact of the increasing
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greenhouse gas emissions . In this regard, the photocatalytic reduction of CO using solar energy,
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ꢀ12
among other approaches, has drawn considerable attention . Unlike the photoelectrochemical CO2
reduction approach, photocatalytic route generally involves only the use of CO , sunlight and water
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without any external electrical bias. This process mimics the biological photosynthesis in plants. Light is
absorbed by the semiconductor photocatalyst, and the photogenerated energetic electrons can reduce
CO to compounds of high standard enthalpy of combustion. Various semiconductor photocatalysts,
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including metal oxide, oxynitride, sulfide and phosphide have been investigated over the last few
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, 9
decades . However, molecular CO has a very low electron affinity and is chemically inert and very
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stable. The electrochemical potential for CO singleꢀelectron reduction is ~ ꢀ1.9 V vs. NHE . The
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thermodynamic barrier for CO photoreduction is relatively high for the photoꢀexcited electrons due to
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the large affinity of most semiconductor photocatalysts, thereby limiting the efficiency of CO2
conversion. Moreover, depending on the mechanism of transferring the energetic electrons, the
reduction of CO may follow different pathways, leading to multiple products when a single material
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catalyst is used. Consequently, the overall efficiency of the CO photocatalytic reduction may ultimately
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be limited by the complexity involved in the practical separation of products. The achievement of
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efficient and selective production of highly valuable fuel compounds is critical for viable CO2
photoreduction systems.
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The conduction band minimum (CBM) of GaN is more negative than those of most metal oxides,
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and hence GaN is kinetically more probable for the reduction of highly stable CO molecules
.
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Moreover, it can meet the thermodynamic requirements to reduce CO to CO and hydrocarbons in a
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large range of pH solutions
. IIIꢀnitride materials have also shown excellent stability in aqueous
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solution
. Recent studies have further demonstrated that nonꢀpolar metalꢀnitride surfaces are
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extremely reactive for producing H directly from water under UV and visible light illumination
,
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which is essentially required for the hydrogenation of CO . The use of nanowires can further enhance
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the efficiency due to the drastically reduced defect densities, increased reaction sites, and superior
charge carrier transfer properties at the semiconductor/electrolyte interface.
In this work, we have demonstrated, for the first time, photochemical reduction of CO to CH and
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CO on GaN nanowire arrays. It was observed that the reduction of CO to CO dominates on asꢀgrown
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GaN nanowires under ultraviolet light irradiation. With the use of Rh/Cr O core/shell coꢀcatalyst,
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however, the production of CH is significantly increased, with an average rate of ~3.5 ꢁmol g h in
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4 hours, and the rate of CO to CO conversion is suppressed by nearly an order of magnitude. The rate
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of photoreduction to CH can be further enhanced and can reach ~14.8 ꢁmol g h by promoting Pt
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nanoparticles on the lateral mꢀplane surfaces of GaN nanowires, which is nearly one order of magnitude
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higher than that measured on asꢀgrown GaN nanowire arrays. Compared to conventional powders, such
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waferꢀlevel nanostructured semiconductors can effectively promote the solidꢀgas CO reduction
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Results and Discussion
In this experiment, catalystꢀfree GaN nanowire arrays were grown directly on nꢀtype Si(111)
substrate by radio frequency plasmaꢀassisted molecular beam epitaxy under nitrogen rich conditions.
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The detailed growth conditions were described elsewhere . The scanning electron microscopy image
of GaN nanowires grown on Si is shown in Figure 1a. The wire diameters are in the range of 80 to 120
nm with lengths in the range 800 to 900 nm. The nanowires are vertically aligned on Si substrate with
their sidewalls being nonpolar mꢀplane. Previous studies have shown that the Fermi level is unpinned on
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clean GaN surfaces . The presence of negligible surface states in the midꢀbandgap is attributed to the
strong ionic characteristic of GaN where the electrostatic energy contributing to the binding energy
between atoms at the surface is nearly equivalent to that in the bulk, thereby leading to the bunching of
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surface states close to the bandedges rather than the midꢀbandgap
. The absence of polarization on
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the mꢀplane can further reduce the interface states introduced by spontaneous polarization . In
addition, compared to conventional planar structures grown on lattice mismatched substrates, IIIꢀnitride
nanowires grown on foreign substrates are largely free of dislocations due to the effective lateral stress
relaxation and the absence of an epitaxial relationship between the nanowires and the underlying
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substrate . This leads to significantly reduced nonradiative recombination. These attributes have been
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previously studied
. The incorporation of metal particles on the GaN surface can significantly reduce
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the upward band bending of unintentionally
nꢀtype doped GaN
due to the neighboring
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inhomogeneous Schottky’s barriers . That is, electrons experience less opposing electrical field,
thereby leading to more efficient electron transfer. Previous studies have shown that such nanowire
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arrays are highly reactive for water splitting and H generation
. The electrochemical potentials for
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the reduction of CO to CH and CO are schematically shown in Figure 1b. The energy band gap of IIIꢀ
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nitrides (AlGaInN) can be varied from ~ 6.2 eV to 0.65 eV. It is of interest in noticing that photoꢀ
excited electrons in the conduction band of GaN can be transferred to adsorbed CO molecules to drive
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various reductive half reactions, which potentially leads to very poor product selectivity.
It has been recognized that coꢀcatalysts such as transition metals play an important role in
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enhancing the efficiency and product selectivity for CO photoreduction
. Transition metal
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nanoparticles can form Schottky contact with GaN. The surface charge properties, including band
bending can therefore be engineered by incorporating metal nanoparticles with different work functions
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on the nanowire surfaces
. Among the various nanoparticle coꢀcatalysts, Rh and Pt have work
functions ~ 5 eV and 5.5 eV, respectively, which are positioned in the fundamental energy bandgap of
GaN. The larger work function of Pt may be responsible for more facile photoꢀexcited electron transfer
from the GaN nanowire, and thus higher reaction rate could be expected from Pt if the diffusion of
electrons to nanoparticles is the rateꢀdetermining step. In addition to the enhanced charge separation and
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extraction, the coꢀcatalyst metal particles can further reduce the active energy barrier of CO reduction
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Water is commonly used as an atomic hydrogen donor for CO reduction. In this regards, GaN was
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ꢂꢀ
previously reported as excellent photocatalyst for water oxidation
. The anion radical CO has
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been first demonstrated to form on rhodium (Rh) catalysts , which is essential for CO to CO reduction.
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High activity was also shown by promoting Rh catalysts. In this regard, we have investigated the use of
Rh nanoparticles as a promoter for hydrogenation of CO to CH instead of the thermodynamically
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unfavorable oneꢀelectron reduction of CO . However, we found that promoting only Rh particles on the
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surface of GaN nanowires did not improve CO photoreduction into neither CO nor CH (Supporting
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S2). Since the H O reforming rate is faster than that of CO
, the hydrogenation of CO into
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hydrocarbons may significantly be suppressed. Therefore, an amorphousꢀlike Cr O shell capsuling Rh
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particle is utilized as O molecular diffusion barrier preventing the onsite backward formation of H O.
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In this regard, Rh metalꢀcore nanoparticle enhances the forward reaction of CO and H O reduction,
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whereas the Cr O amorphousꢀshell (i) suppresses the high rate backward reaction to form H O from H
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and O2 and (ii) acts as an efficient CO adsorption sites . This noble metal on metalꢀoxide supporter
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was realized earlier to be beneficial in CO reduction
. A similar view is adopted for Pt nanoparticles
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on GaN nanowires. The configuration of the chemisorbed CO on Pt is favorable for hydrogenation .
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High resolution transmission electron microscopy (TEM) was used to characterize the Rh/Cr O3
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core/shell nanostructures photodeposited on the GaN nanowires. Figure 2a illustrates a high resolution
TEM image of Rh/Cr O nanoparticles photodeposited on GaN nanowires. It is clearly shown that
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Rh/Cr O nanoparticles were deposited on the lateral surfaces of GaN nanowires. The highꢀangle
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annular darkꢀfield image shown in Figure 2b confirms the uniform deposition of Rh/Cr O core/shell
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nanostructures on GaN nanowires. It is observed that the GaN nanowire is single crystalline and free of
structural defects. The single crystallinity and absence of structural defects have also been observed in
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our previous studies
. For the Rh/Cr O core/shell structure, the metallic Rh core is well crystallized,
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with an average diameter of ~3 nm, while the Cr O shell is amorphous. The low magnification TEM
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image of Pt nanoparticles on GaN nanowire surface is shown in Figure 2c. The diameters of Pt
nanoparticles are 1ꢀ2 nm. The TEM image shows well distributed Pt nanoparticles along a GaN
nanowire. The photoꢀreduction processes of CO on Rh/Cr O and Pt decorated GaN nanowires are
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schematically shown in Figure 2d.
We have first performed CO photoreduction experiments on asꢀgrown GaN nanowire array as well
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as GaN nanowires with the presence of Rh/Cr O nanoparticles. For both samples, the major products
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observed were H , CO and CH . Figure 3a shows the measured CH evolution over 24 hours for both
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samples. It is seen that the rate of CH formation increased from 1.3 ꢁmol g
h on asꢀgrown GaN
cat
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nanowires to 3.5 ꢁmol gcat h on GaN decorated by Rh/Cr O nanoparticles. This enhancement could
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be attributed to the effective adsorption and hydrogenation of CO molecules on Rh/Cr O . Shown in
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Figure 3b is the measured CO evolution over 24 hours for both samples. It is seen that a significant
ꢀ1
amount of CO, ~ 1130 ꢁmol gcat was measured from the asꢀgrown GaN nanowires in 24 hrs, while a
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drastically reduced amount of CO, ~ 120 ꢁmol gcat was measured on Rh/Cr O ꢀdecorated GaN
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nanowires. It is clearly seen that promoting Rh/Cr O nanoparticles on GaN increases the product
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selectivity of CH over CO. That is, the conversion rate of CO into CO, which is a twoꢀelectron
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reduction process, is dramatically suppressed by one order of magnitude, while the production of CH4,
which is an eightꢀelectron reduction process, has been increased by three fold. Also seen in Figure 3b,
the CO evolution on Rh/Cr O ꢀdecorated nanowires has been inhibited after the first two hours,
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implying that the photoꢀexcited electrons are effectively collected by Rh/Cr O core/shell nanoparticles,
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with no apparent reductive reaction occurring on the active sites of the exposed nanowire lateral
surfaces without Rh coverage. Shown in Figure 3c is the H evolution rate during these photochemical
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CO reduction experiments (Supporting S1). The high H formation is due the favourable simple twoꢀ
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electron reduction of H formed from the oxidation process of water. The measured photocatalytic
activities over 24 hours suggest that it is water, rather than GaN, was being oxidized (Supporting
Information S3). This is further confirmed by detailed Xꢀray photoelectron spectroscopy studies that
showed the presence of GaN surfaces after the reaction (Supporting Information S3).
While it has been incomprehensibly observed the reduction of CO towards either CH or CO on
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rhodium
transformation to surface carbonate species
to CH on metalꢀcontaining oxide in the presence of protons
, previous studies have shown that the adsorption of CO molecule on Cr O can lead to the
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. The carbonate species can undergo reduction reactions
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. In this case, the adsorbed CO can be an
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intermediate determining step toward CH production . Another pathway, which is likely to occur
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when H is promoted, is that formate ions are formed by CO adsorption on the metalꢀoxide shell
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surfaces
. Rhodium was found to promote the formation of formate ions due to the migration of
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atomic hydrogen to react with CO molecule on the surface of metal oxide . The pathway that CH can
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be formed from formate species without CO being an intermediate and the reaction elementary steps can
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be briefly described as follows ,
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CO + H O → HCOOH
(1)
(2)
(3)
(4)
2
2
HCOOH → HCHO + ½ O2
HCHO + H O → CH OH + ½ O
2
2
3
CH OH → CH + ½ O
2
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3, 74
Here, CO can also be formed, but to a less extent, from the dissociation of formate species
.
While both pathways may occur simultaneously, it is expected that one process may dominate,
depending on the experimental conditions. For example, the formate pathway may become significant
after a sufficient amount of H is produced, since formate can form easily in H environment. This could
2
2
be the reason that CO is produced during the first 2 hours and then is largely inhibited for Rh/Cr O
2
3
decorated GaN nanowires, as shown in Figure 3b. In either pathway the effective adsorption of CO on
62, 65, 75
Cr O may facilitate its further reduction to methane
. It is worth mentioning that O was also
2
2
3
detected during the course of these experiments where the source of oxygen could be the combination of
water oxidation and/or other reactions.The photoreduction rate of CO into CO and CH on GaN
2
4
nanowires and Rh/Cr O ꢀdeocrated nanowires, respectively, were higher than previously reported
2
3
7
6ꢀ81
nanostructured materials
. Moreover, the main products of CO and CH on GaN and Rh/Cr O ꢀ
4 2 3
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deocrated wires increase linearly with illumination time. The nearly constant reduction rate of CO to
2
CH and CO over 24 hour suggests the stability of GaN nanowires during CO photoreduction.
4
2
Further studies were also performed by promoting Pt nanoparticles on GaN nanowire surfaces.
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ꢀ1 ꢀ1
Figure 4 shows the measured CH evolution (~ 14.8 ꢁmol g
h ) over 24 hours on Ptꢀdecorated GaN
cat
4
nanowires, which is nearly one order of magnitude higher than that produced on bare GaN. The CO
evolution rate, on the other hand, has not been significantly improved by promoting Pt nanoparticles,
shown in the inset of Figure 4. Similarly, evolutions of both CO and CH exhibit linear increase,
4
indicating stable photocatalytic CO reduction. Comparing with some previously reported Pt promoted
2
8
2ꢀ89
nanomaterials, Ptꢀdecorated GaN nanowires shows higher activity
. It is worth mentioning that the
effective separation of hot carriers, together with the reduction of CO activation barrier by Pt metal coꢀ
2
5
3
catalyst, may be the major reasons for this enhancement . Besides activating the CO molecule,
2
efficient H generation and adsorption on Pt nanoparticles could promote and accelerate CH
2
4
90ꢀ92
formation
. The available states on Pt has less energy than the GaN conduction band minimum, and
therefore photoexcited hot electrons can easily transfer to Pt metal. These electrons can be effectively
involved in the multiꢀelectron CO reduction process. It was previously reported that CO can
2
2
5
0
disassociate on Pt to form CO and O at room temperature . The reduction of CO to CO by Ptꢀ
2
2
7
, 55
catalyzed TiO was suggested to be a twoꢀelectron process , wherein the elementary reaction steps are
2
described as follows:
+
ꢀ
CO + H + e → HCO
(5)
2
2 (ads)
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ꢀ
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HCO2 (ads) + H + e → CO + H O
(6)
(7)
(8)
2
+
+
2
2
H O + 2h → 2OH
+ 2H
(ads)
2
OH (ads) → ½ O + H O
2
2
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In this process, holes can be provided simultaneously by GaN nanowires. Hence, the reaction is likely to
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occur close to the interface between GaN and Pt nanoparticles. CO can be adsorbed on Pt and react
7
further with atomic hydrogen to form methane ,
ꢀ
+
CO + 6e + 6H → CH + H O
(9)
4
2
In this regard, the multiꢀelectron reduction could be the rateꢀdetermine step for CH formation. On the
4
other hand, the evolutions of CO on Ptꢀdecorated nanowire and on asꢀgrown nanowires are on the same
order. This may indicate that electronꢀtransfer rate is comparable to (or larger than) the kinetics limit of
CO formation rate. For the case of Ptꢀdecorated GaN, the diffusion of electrons to nanoparticles could
be the rateꢀdetermining step due to the Coulomb repulsion induced by accumulated electrons on Pt
94
nanoparticles . Further investigation is required to determine the reaction pathway and the rateꢀ
determine step.
Conclusion
In summary, we have demonstrated the photocatalytic reduction of CO on GaN nanowires, with
2
CH and CO being the major products. The reaction exhibits relatively high conversion rate and stability
4
over 24 hours. Promoting Rh/Cr O core/shell nanoparticles showed an enhancement of the reaction rate
2
3
and selectivity towards CH over CO. It was also found that Pt nanoparticles could significantly enhance
4
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the activity of CO reduction to CH . Future work includes the development of InGaN nanowire
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photocatalysts for the reduction of CO into useful chemical fuels under visible light irradiation. The use
2
of engineered nanoꢀstructured coꢀcatalysts, together with variations of the surface charge properties of
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nanowires, to manipulate and control the CO photoreduction pathways will also be investigated.
2
METHODS
S1. Molecular Beam Epitaxial (MBE) Growth
The vertically aligned GaN nanowires were grown on a low resistivity (< 0.005 ꢃcm) nꢀtype
Si(111) substrate by radio frequency plasmaꢀassisted MBE under nitrogenꢀrich conditions without any
foreign metal catalyst. Prior to loading into the MBE chamber, the Si(111) substrate was cleaned with
acetone and methanol to remove any organic contaminants. Subsequently, Si substrate was immersed in
1
0% hydrofluoric acid (HF) to remove native oxide. Before the growth initiation, in situ oxide
desorption was performed at ~770 °C until the formation of a clean Si (111) 7×7 reconstructed surface
was confirmed by reflection highꢀenergy electron diffraction (RHEED). Thermal effusion cell was used
as a source for Ga. Approximately one monolayer of a Ga seeding layer was deposited to promote the
nucleation of the nanowires before growth initiation. Nitrogen ions were delivered from a radio
frequency plasma source. The growth parameters include a substrate temperature of ~750 °C, a nitrogen
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flow rate of 1.0 standard cubic centimeters per minute (sccm), a forward plasma power of ~350 W, and
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ꢀ8
a Ga beam equivalent pressure (BEP) of ~6×10 Torr.
S2. Photodeposition of Co-catalyst
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The deposition of Rh/Cr O was carried out using a twoꢀstep photoꢀdeposition process. The
2
3
advantage of this technique is that the metal particles are preferentially reduced on the reactive sites on
the surface of the photocatalyst (i.e. GaN nanowires lateral nonꢀpolar surfaces) by photogenerated
electrons. First, GaN nanowire photocatalyst sample was placed in a Pyrex chamber. To deposit Rh
particles, a 2 ꢁL of 0.2 mM sodium hexachlororhodate(III) (Na RhC , Sigma Aldrich), 12 mL of
3
l6
CH OH (i.e. scavenger), and 60 mL of MilliꢀQ (~ 18 Mꢃ) water were poured in the Pyrex chamber with
3
quartz window. The cell was irradiated using 300 W xenon lamp (PerkinElmer, PE300BF) for 15 mins
after evacuation. Similarly, Cr O deposition was followed with 2 ꢁL of 0.2 mM potassium chromate
2
3
(
K CrO , Sigma Aldrich) as a precursor. The sample was then irradiated for 30 mins. Pt nanoparticles
2 4
photodeposition was carried out in the same manner as Rh. 4 ꢁL of 1.5 mM Chloroplatinic acid solution
was added to CH OH and water and illuminated for 30 mins.
3
S3. Scanning Transmission Electron Microscopy (STEM)
For STEM HAADF imaging, a Hitachi HD2700 Csꢀcorrected dedicated STEM was used with a
cold field emission emitter operated at 200 kV and with an electron beam diameter of ~ 0.1 nm.
S4. Photochemical CO Reduction
2
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The CO reduction and product evaluation experiments were performed in an airꢀtight gas
2
2
circulation system (~ 450 mL). The waferꢀbased GaN nanowires (area ~ 3.5 cm ) were carefully washed
before putting at the bottom of the Pyrex reaction cell. The circulation system was first well evacuated
0
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and then filled with pure CO until reaching a pressure of 80 kPa. 2 mL of distilled water was then
2
added into the reaction cell. The added water was vaporized under the illumination of a 300 W Xe lamp
(
ILC Tech, CERMAX LXꢀ300), which was also used as the light source for the reaction. The products
were measured by a gas chromatograph (GCꢀ14B, Shimadzu) equipped with a flame ionization detector
followed by hydrogenation reactor for CO evaluation. The absence of carbon residues on the samples
was confirmed by control experiments. None of the reported products were detected without adding
1
3
CO reactant. In addition, CO was used as reactant to further confirm that the source of C in CO and
2
2
CH was from CO (Supporting S4).
4
2
ACKNOWLEDGMENT:
This work was supported by the Natural Sciences and Engineering Research Council of Canada and the
Climate Change and Emissions Management (CCEMC) Corporation. B. AlOtaibi acknowledges King
Abdullah foreign scholarship program.
Supporting Information Available: Description of the material. This material is available free of charge
via the Internet at http://pubs.acs.org.
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Figure 1. (a) A 45 °C tilted scanning electron microscope image of GaN nanowires grown on Si(111)
substrate. (b) Band edge positions of GaN in comparison with the work function of Rh and
Pt with respect to the vacuum energy level. Also shown are the CO /CO and CO /CH
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annular dark field (HRꢀHAADF) image of Rh/Cr O core/shell decorated GaN nanowire. (c)
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