Silicon nanowires as photoelectrodes for carbon dioxide fixation

Article abstract of DOI:10.1002/anie.201202569

Lights on: When illuminated, p-type Si nanowires donate photogenerated electrons to aromatic ketones, producing reactive radicals that can harvest CO₂ to yield α-hydroxy acids (see scheme). The reaction scheme closely resembles that of natural photosynthesis and gives up to 98 % yield and selectivity. Products obtained by this reaction include important precursors for nonsteroidal anti-inflammatory drugs, such as ibuprofen and naproxen. Copyright

Full text of DOI:10.1002/anie.201202569

Angewandte
Chemie
DOI: 10.1002/anie.201202569
CO2 Photochemistry
Silicon Nanowires as Photoelectrodes for Carbon Dioxide Fixation**
Rui Liu, Guangbi Yuan, Candice L. Joe, Thomas E. Lightburn, Kian L. Tan,* and
Dunwei Wang*
Natural photosynthesis harvests the energy in solar light to
power chemical reactions and uses CO2 as the carbon source.
Because light as an energy source is free and abundant,
chemical reactions similar to photosynthesis have major
fundamental and practical implications.[1,2] Indeed, significant
efforts have been attracted to this research goal. The majority
of attention for photochemical reactions that transform CO2
has focused primarily on conversion into fuels.[3,4] How to
Scheme 1. The key carboxylation steps in natural photosynthesis and
those reported herein.
learn from photosynthesis and devise reaction routes for the
synthesis of useful organic compounds receives surprisingly
little consideration.[5] Drawing inspiration from the mecha-
nisms found in dark reactions of photosynthesis and using p-
type Si nanowires as a photocathode, herein we show that
highly specific reactions can be readily carried out to produce
a-hydroxy acids by the photoreduction of aromatic ketones,
and subsequent CO2 fixation. Powered by solar light, this
reaction closely resembles natural photosynthesis, and is
different from its electrochemical analogues. The carboxyla-
tion products of two of the substrates examined serve as
precursors to nonsteroidal anti-inflammatory drugs
(NSAID), ibuprofen and naproxen.[6]
In nature, photosynthesis is carried out in two distinct
stages: light and dark reactions. During the light-promoted
stage, the energy in photons is harvested and stored in
chemicals, such as NADPH (nicotinamide adenine dinucle-
otide phosphate) and ATP (adenosine-5’-tiphosphate), which
are subsequently used to sequester carbon dioxide for the
synthesis of complex sugar monomers. At the heart of the
Calvin cycle (dark reactions) is the conversion of ribulose-1,5-
bis-phosphate (RuBP) into an intermediate b-keto acid
(Scheme 1), which ultimately fragments to 3-phosphoglycer-
ate (3PG), the core building block for sugars.[7] By not directly
reducing CO2, this process avoids producing C in a variety of
oxidation states and gains a critical advantage of high
selectivity.[8] This chemistry inspired us to propose a strategy
to perform carboxylation reactions using light as a direct
energy source and CO2 as a carbon source. As shown in
Schemes 1 and 2, our reaction route is in close resemblance to
natural photosynthesis but different from existing approaches
that seek to directly photoreduce CO2. It solves a critical
challenge of poor selectivity inherent to the direct photo-
reduction of CO2 which arises from the nature of the
multielectron transfer processes. Our strategy has the poten-
tial to meet the selectivity requirement necessary for synthetic
targets more complex than fuels, opening doors to a wide
range of light-powered chemical reactions[9–12] that have not
been studied to date.
We used Si nanowires (SiNWs) as the light-harvesting
electrode because they are efficient in converting solar energy
into electrical energy, easy to make, and remarkably stable
under reductive conditions.[13–17] To examine their suitability
for organic synthesis, we first conducted a reaction that has
been previously performed electrochemically, the formation
of benzilic acid through CO2 fixation by benzophenone.[18,19]
The key difference of the result reported herein is that light
serves as an important source of energy input. Our goal for
this initial set of experiments was to determine whether the
energy levels of Si are correctly aligned for the reduction of
benzophenone to the radical anion, the key step in the
carboxylation reaction. We would then apply this knowledge
to the CO2 photofixation with ketone-based substrates.
Information important to our considerations includes the
electrochemical potential of the solution (determined by the
Tafel technique in dark as ꢀ0.12 V; all potentials are relative
to Ag/AgI/Iꢀ reference, which was 0.60 V more positive than
the saturated calomel electrode; SCE) and the Fermi level of
Si (measured by the Mott–Schottky plot as 0.74 V, see
Supporting Information). From this information and the
known doping levels of Si (1015 cmꢀ3 B-doped; 1: 10–
20 Wcm), we constructed the energetics of the benzophenone
system as shown in Figure 1. Under equilibrium conditions,
a large degree of band bending (0.86 V in magnitude) on the
surface creates a substantial depletion layer where photo-
generated charges can be separated with high efficiencies
[*] R. Liu,[+] G. Yuan,[+] C. L. Joe, Dr. T. E. Lightburn, Prof. Dr. K. L. Tan,
Prof. Dr. D. Wang
Merkert Chemistry Center, Department of Chemistry, Boston
College
2609 Beacon St., Chestnut Hill, MA 02467 (USA)
E-mail: kian.tan@bc.edu
dunwei.wang@bc.edu
[+] These authors contributed equally to this work.
[**] We thank S. Shepard and M. Miao for their technical assistance.
Financial support is provided by NSF (DMR 105576 to D.W.) and in
part by NIGMS (R01GM087581 to K.L.T.). Mass spectrometry
instrumentation at Boston College is supported by funding from the
NSF (DBI-0619576).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201202569.
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high-efficiency PEC operations. Important to this
discussion, the saturation current density scales
with illumination intensity in a linear fashion,
supporting that the charge-separation mechanism
agrees with that proposed in Figure 1, and that
charge collection is effective. The sharp turn-on of
the
photocurrent
density
(a
slope
of
70.1 mAcmꢀ2 Vꢀ1 was measured in the linear
region between ꢀ1.00 and ꢀ1.20 V) was compara-
ble to that measured on Si in the more extensively
studied [CoCp2]+/0 (Cp = C5H5) system,[22] in which
charge-transfer resistance from Si to the electro-
lyte is low, as well as that of Pt (a slope of
72.0 mAcmꢀ2 Vꢀ1 between ꢀ1.15 and ꢀ1.50 V).
The comparison indicates illuminated Si is a suit-
able candidate for aromatic ketone reduction.
Indeed, under typical operation conditions
(ꢀ1.20 V, 100 mWcmꢀ2 AM 1.5 illumination),
benzophenone is carboxylated at a faradic effi-
Scheme 2. Proposed mechanism of the light-driven carboxylation reactions;
h+ =hole.
Figure 1. Energetics of using p-type SiNWs for benzophenone reduc-
tion. Under equilibrium conditions in the dark, a substantial band
bending (0.86 V in magnitude) forms, providing a basis for efficient
charge separation. When illuminated, the separated charges create
a build-in field to help power the benzilic acid formation by carbox-
ylation. CB=conduction band, VB=valence band, EF =Fermi level,
E
F,p =quasi Fermi level of holes, EF,n =quasi Fermi level of electrons,
ꢀqE(A/Aꢀ)=reduction potential of benzophenone.
Figure 2. Photoelectrochemical characteristics of benzilic acid forma-
tion by p-type SiNWs. a) Compared with Pt and planar Si substrates,
p-type SiNWs have less-negative turn-on voltages. Data obtained
under 100 mWcm2 AM 1.5 G illumination. b) Photocurrent density
versus voltage plots under different illumination conditions. Inset: A
cross sectional SEM view of the electrode; scale bar: 1 mm. c) Light-
powered carboxylation of benzophenone.
when illuminated. This understanding was indeed consistent
with the photoelectrochemical (PEC) measurements
(Figure 2). Both p-type and n-type SiNWs with different
doping levels were investigated, and moderately doped p-type
SiNWs were found to be the most suitable photocathodes (see
Supporting Information).
Several additional characteristics of the PEC data are
noteworthy. In the absence of light, no photocurrent was
detected for applied potentials up to ꢀ2.4 V, nor did we
obtain any carboxylation products (Figure 2a). In contrast,
when illuminated, a high saturation current density is
measured at relatively low negative applied potentials
(31.1 mAcmꢀ2 at ꢀ1.20 V, Figure 2). These results suggest
that the reaction as shown in Scheme 1 and Scheme 2 is
indeed powered by light. Control experiments where SiNWs
were replaced by Pt only gave 2.00 mAcmꢀ2 under identical
applied potential and illumination conditions. The current
level also approaches what is theoretically possible by Si
(43.0 mAcmꢀ2) under the same lighting conditions,[20, 21]
further highlighting the feasibility of using the system for
ciency of 94% and in over 98% yield of isolated a-hydroxy
acid product (2a; Figure 2c). We note that to avoid direct
reduction of CO2, which would alter the proposed reaction
mechanism and produce undesired by-products, it is impor-
tant to limit the operating potentials at or more positive than
ꢀ1.2 V. Additional control experiments also suggest that the
reaction proceeds by a 2-step single-electron transfer process
(Supporting Information).[23,24] Dimerization of the starting
material as well as reduction to the secondary alcohol is often
observed in electrochemical coupling of ketones with carbon
dioxide but is absent in our experiments as confirmed by
analysis of the crude photoelectrochemical reaction mixture
1
by H NMR spectroscopy. This result is consistent with the
high yield of isolated product.[25] Lastly, we note that the
anode, Al is oxidized to produce Al3+ (Scheme 2) and
dissolves in the reaction medium.[24]
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Further analysis of the benzophenone carboxylation
reaction showed that SiNWs exhibit a less-negative turn-on
voltage (ꢀ0.52 V at > 1 mAcmꢀ2) than planar Si (ꢀ0.63 V)
but lower saturation current density (32.1 mAcmꢀ2 vs.
34.4 mAcmꢀ2, Figure 2a). It has been reported that the high
surface area of nanostructures such as SiNWs, may result in
increased charge recombination at the semiconductor/solu-
tion interface, leading to reduced saturation current densities
without considering light-trapping mechanisms.[16,26,27] How-
ever, the recombination mechanism would also predict
reduced open-circuit potentials, implying a more-negative
turn-on voltage should be measured on SiNWs than on planar
Si. To account for the apparent discrepancies, we suggest that
the observed trend is indicative of improved charge-transfer
kinetics on SiNWs. That is, the multifaceted nature of SiNWs
favors charge transfer from Si to benzophenone, resulting in
lower overpotentials. Our hypothesis is supported by control
experiments in which the turn-on voltages were compared
with the length of SiNWs (L varying between 0 and 10 mm).
Within a limited range ð0 < L ꢁ 6mmÞ, the turn-on voltage
changes with the surface roughening factor monotonically;
the recombination-induced open-circuit potential reduction
dominates for longer SiNWs (L > 6 mm), and more-negative
turn-on voltages were measured (Supporting Information). A
similar effect has also been observed in SiNWs-based water-
splitting reactions,[28,29] although more details about the
reasons remain, to our knowledge, unclear.
For practical applications, the stability of the photoelectr-
odes against photocorrosion and other mechanisms that may
degrade their performance, such as oxidation, is an important
concern. For the reported process, the SiNWs are operating
under reductive conditions, so we considered oxidation of
SiNWs less likely and instead focused our attention on
assessing the stability. Recycling of the photoelectrode made
of SiNWs up to four times showed no measurable differences
in the PEC performance (Figure 3). Importantly, the rate,
yield, and selectivity were reproduced over each successive
experiment, consistent with the SiNWs remaining intact over
the course of the reaction (over 34 h). If we assume every Si
surface atom as an active site, a peak turn-over frequency
(TOF) of 25.8 sꢀ1 is estimated (see Supporting Information
for more details).
To demonstrate the synthetic utility of the reaction we
applied the method to 2-acetyl-6-methoxynaphthalene and 4-
isobutylacetophenone, which are precursors for the anti-
flammatory drugs naproxen and ibuprofen.[19,24] We observed
consistently high yields and selectivities for both substrates
(Figure 4); furthermore, the performance is comparable to
that reported for electrochemical carboxylation techniques
Figure 4. Summary of selectivity for and yield of isolated NSAID
precursors.
where electricity was the only source of energy input and
graphite or mercury were the electrodes (see Supporting
Information for a detailed comparison). We emphasize that
the photoelectrochemical syntheses reported herein were
carried out at potentials up to 670 mV less-negative than
those reported using electrochemical approaches, the differ-
ence being provided by solar light. Our strategy showed that
solar-light photon energies can indeed be harnessed to
promote the synthesis at lower applied potentials.[30]
In conclusion, we demonstrated a chemical reaction that is
powered by light, the most abundant energy source on the
surface of earth, and uses CO2, an inexpensive and readily
available source of carbon. Significantly, these reactions
produce organic targets that can be readily used to synthesize
NSAIDs, such as ibuprofen and naproxen. Although the
energy-harvesting aspect of natural photosynthesis has been
widely exploited in reactions, such as H2O splitting or CO2
reduction, for fuel production, learning from nature and using
the harvested photoenergy for complex-molecule synthesis is
an underdeveloped area. One of the most important merits
offered by the reaction strategy is the ease with which
electron exchange (donation for photocathode or withdrawal
for photoanode) takes place between the photoelectrode and
the organic substrates. It has the potential to greatly broaden
the scope of artificial photosynthesis. While in the present
proof of concept demonstration an additional electrochemical
potential is still necessary, the energy input from the
harvested light plays a critical role. As such, our approach is
a step forward in the use of light to power complex organic
molecule syntheses.
Figure 3. Stability of SiNW photoelectrodes. No significant difference
is observed for four consecutive runs under identical operating
conditions. y axis: electron mole per starting material mole. Reaction
yields as determined by 1H NMR spectroscopy: 1st run, over 98%;
2nd run 97%; 3rd run, 98%; 4th run, over 98%.
Experimental Section
Photoelectrode fabrication: The preparation of SiNWs was reported
previously.[31] Once prepared, the substrates containing SiNWs were
immersed in HF (aqueous, 5%) for 2 min and then dried in a stream
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of N2. Al (300 nm) was then sputtered onto the backside of the
substrates by radio frequency magnetron sputtering (AJA Interna-
tional, Orion 8, MA, USA). They were then annealed in Ar (flow
rate: 5000 standard cubic centimeter per minute, SCCM) at 4508C for
5 min. Afterward, tinned Cu wires were fixed to the Al film by Ag
epoxy (SPI supplies, PA, USA). Lastly, non-conductive hysol epoxy
(Loctite, OH, USA) was used to seal the entire substrates except the
regions where SiNWs resided.
Photoelectrochemical Synthesis: In a dry box tetrabutylammo-
nium bromide (0.644 g, 2.00 mmol), benzophenone (0.22m benzo-
phenone in acetonitrile; 2.00 mL, 80.0 mg, 0.439 mmol), and aceto-
nitrile (18 mL) were added to a flame-dried, three-neck, 25 mL
round-bottom flask equipped with magnetic stir bar. One of the
following was placed in each neck of the round-bottom flask: SiNWs
working electrode, aluminum counter electrode, Ag/AgI/Iꢀ reference
electrode. The reaction vessel was brought out of the dry box and CO2
was bubbled through the solution with an oil bubbler outlet. A
constant potential of ꢀ1.2 V was applied to the reaction mixture and
reaction current was monitored during the synthesis time. Light was
shined onto the reaction which was vigorously stirred overnight.
Without illumination, the reaction current was negligible.
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Received: April 2, 2012
Published online: May 22, 2012
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[30] Our result is in line with a recent publication in Angew. Chem.
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was published during the final stage of preparation of this
manuscript.
Keywords: aromatic ketones · carboxylation · CO2 fixation ·
photosynthesis · Si nanowires
.
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