GaN nanowires as a reusable photoredox catalyst for radical coupling of carbonyl under blacklight irradiation

Article abstract of DOI:10.1039/d0sc02718a

Employing photo-energy to drive the desired chemical transformation has been a long pursued subject. The development of homogeneous photoredox catalysts in radical coupling reactions has been truly phenomenal, however, with apparent disadvantages such as the difficulty in separating the catalyst and the frequent requirement of scarce noble metals. We therefore envisioned the use of a hyper-stable III-V photosensitizing semiconductor with a tunable Fermi level and energy band as a readily isolable and recyclable heterogeneous photoredox catalyst for radical coupling reactions. Using the carbonyl coupling reaction as a proof-of-concept, herein, we report a photo-pinacol coupling reaction catalyzed by GaN nanowires under ambient light at room temperature with methanol as a solvent and sacrificial reagent. By simply tuning the dopant, the GaN nanowire shows significantly enhanced electronic properties. The catalyst showed excellent stability, reusability and functional tolerance. All reactions could be accomplished with a single piece of nanowire on Si-wafer. This journal is

Full text of DOI:10.1039/d0sc02718a

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DOI: 10.1039/D0SC02718A
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GaN nanowire as a reusable photoredox catalyst for radical
coupling of carbonyl under blacklight-irradiation
Mingxin Liu,a,b,† Lida Tan,a,† Roksana T. Rashid,c Yunen Cen,a Shaobo Cheng,d Gianluigi Botton,d
Received 00th January 20xx,
Accepted 00th January 20xx
Zetian Mi,b,c,* and Chao-Jun Lia,*
DOI: 10.1039/x0xx00000x
Employing photo-energy to drive the desired chemical transformation has been a long pursuing subject. The development
of homogeneous photoredox catalysts in radical coupling reactions has been truly phenomenal, however, with apparent
disadvantages such as the difficulty to separate catalyst and the often requirement of scarce noble metals. We therefore
envisioned the use of hyper-stable III-V photosensitizing semiconductor with tunable Fermi level and energy band as the
readily isolable and recyclable heterogeneous photoredox catalyst for radical coupling reactions. Using the carbonyl coupling
reaction as a proof-of-concept, herein, we report a photo-Pinacol Coupling Reaction catalyzed by GaN nanowires under
ambient light at room temperature with methanol as solvent and a sacrificial reagent. By simply tuning the dopant, the GaN
nanowire shows significantly enhanced electronic properties. The catalyst showed excellent stability, reusability and
functional tolerance. All reactions could be accomplished with a single piece of nanowire on Si-wafer.
carbon nitride (PCN), gallium nitride (GaN) semiconductor12
shows extraordinary stability13 and more than six magnitudes of
1. Introduction
The generation of new carbon-carbon bonds in an atom-
economical manner is of continuous pursuit in synthetic organic
chemistry, enabling the production of many crucial
pharmaceutical compounds and bio-active molecules.1 Since
discovered in the late 20th century,2 the use of photoredox
catalysts has quickly realized huge potentials in organic
synthesis to afford facile coupling reactions enabled by photo-
energy. Many excellent examples have been seen in the
literature, including couplings of carbonyl-α-radicals3-5 and ketyl
radicals (e.g., the Pinacol Coupling Reaction, PCR, Figure 1A).6-9
However, despite many successful applications of photoredox
catalysts in radical coupling reactions, with some exceptions,9
the scarce and expensive ruthenium (Ru) and iridium (Ir)-based
homogeneous catalysts are predominantly used, which are
difficult to isolate and recycle at the end of the reactions.
Over the past few years, we have been dedicated to the
engineering of group III-nitride semiconductor nano-structures
and their applications as photocatalysts. Compared to common
semiconductor catalysts11 such as CdS, TiO2 and polymeric
a. Department of Chemistry and FRQNT Centre for Green Chemistry and Catalysts,
McGill University, 801 Sherbrooke Ouest, Montreal, Quebec, H3A 0B8, Canada.
b. Department of Electrical Engineering and Computer Science, University of
Michigan, 1301 Beal Ave, Ann Arbor, MI, 48109, USA.
Figure 1. Previous radical coupling of carbonyl using photoredox and this work.
enhancement in voltage carrying ability.14, 15 In addition, the
surface of the GaN manufactured in our lab is mostly non-
polar,13 which does not undergo Fermi-level pinning16 and
allows facile modifications of its band gap. Furthermore, the
epitaxial grown GaN allows the incorporation of Si wafer, giving
the dual-junction structure17 which greatly enhances catalytic
reaction efficiency. We have successfully designed the GaN
c. Department of Electrical and Computer Engineering, McGill University, 3480
University, Montreal, Quebec, H3A 0E9, Canada.
d. Department of Material Science and Engineering, Canadian Centre for Electron
Microscopy, McMaster University, 1280 Main Street West, Hamilton, ON, L8S
4M1, Canada.
† These authors contributed equally.
Electronic Supplementary Information (ESI) available: Materials and Methods,
Figures S1-S34, Table S1. See DOI: 10.1039/x0xx00000x
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nanowire (NW) catalyst for water-splitting,18-23 nitrogen
B)
A)
View Article Online
EPR@140K under light
DOI: 10.1039/D0SC02718A
fixation,24-25 conversion,26-27
methane and artificial
n-GaN
p-GaN
i-GaN
photosynthesis.28-29 In those reactions, the capability of tuning
the near-surface Fermi level and energy band structure of the
semiconductor is of particular importance. By carefully
repositioning the conductive band (CB) energy level and the
valence band (VB) energy level of GaN NW with doping, the
desired direction of electron-flow can be established upon light
irradiation on the GaN NW. The photo excited electron (e-) can
then be efficiently donated from the semiconductor to the
reactant (H2O, N2, or CO2, respectively) and give the desired
product (H2, NH3, or CO/hydrocarbon, respectively), while the
photo-excited hole (h+) can be readily neutralized by many
common sacrificial reagents. Inspired by these works, we
contemplate the possibility of using those unique characters of
GaN NW to design a more practical and highly recyclable
photoredox catalyst for radical coupling reactions under light
(Figure 1B). As a proof-of-concept, herein, we report GaN-
catalyzed highly efficient PCRs under ambient temperature and
light using methanol as both solvent and reagent, with no other
reaction additive.
3200 3280 3360 3440 3520 3600
magnetic field / G
Figure 3. A) Surface energy band bending of GaN NW. B) EPR identification of
band bending.
The surface energy band bending significantly influences the
electronic properties of semiconductors.20 Upon light
irradiation, the excited e- on the CB of the semiconductor tends
to migrate to the potential energy well, while the h+ on the VB
tends to migrate to the potential energy high ground. This
surface band property inhibits carrier recombination and gives
birth to unique reactivities. By doping the NW with a tetravalent
element (e.g., silicon, germanium, etc.) as the n-type dopant,
the surface energy band of GaN bends upward to give n-GaN
NWs. Upon light irradiation,20 the excited e- on the CB tends to
migrate to the internal region of the NW instead of the surface,
while the h+ on the VB tends to migrate to the surface instead
of the internal region of the NW. Such doping prohibits e-
donation to the reactant and enhances the neutralization of h+
by the sacrificial reagent (Figure 3A left). On the other hand, by
using a divalent element (e.g., magnesium) as the p-type
dopant,
2. Results and discussion
Table 1. Catalytic activity of GaN NW with different doping and sacrificial reagents.
8 W black light bulb
0.35 mg GaN NW
HO
OH
O
500 nm
1 nm
Figure 2. SEM (left) and TEM (right) identification of the freshly synthesized GaN
0.5 mmol sacrificial reagent
room temperature
12 h
NW.
1a
0.02 mmol
2a
NMR yielda
Molecular Beam Epitaxy (MBE) is a fast and reliable method of
synthesizing nano-level semiconductor units for electronic
engineering. In this study, we used plasma assisted MBE (PA-
MBE) under nitrogen-rich conditions to grow GaN NWs on a
two-inch Si(111) wafer, resulting in a forest-like GaN NW array
aligned vertically to the wafer, as confirmed by Scanning
Electron Microscopy (SEM, Figure 2 left). High-resolution
GaN NW
entry
sacrificial reagent
MeOH
yield / % rac- : meso-
1
2
i-GaN
93
81
1 : 1
1 : 1
MeOH
n-GaN
MeOH
H2
3
p-GaN
> 99
no reaction
no reaction
no reaction
> 99
1 : 1
4
5
p-GaN
p-GaN
p-GaN
p-GaN
p-GaN
p-GaN
p-GaN
p-GaN
Si(111)
N/A
N/A
N/A
1 : 1
1 : 1
1 : 1
N/A
N/A
N/A
1 : 1
CH4
water
6
Transmission Electron Microscopy (TEM, Figure
2 right)
Et3N
7
revealed the lattice of the gallium atoms that forms the
nanowire. Since GaN is a broad band semiconductor with a band
gap of 3.4 eV (see Table S1 in the Supplemental Information),13
it absorbs photons of a certain wavelength to excite e- from its
VB to its CB. As confirmed by our previous research, the desired
wavelength is 365 nm, which can be provided by commonly
used black light bulb (see Figure S33 in the Supplementary
Information).
EtOH
> 99
8
92
9
triethanolamine
MeOH (no GaN)
MeOH (in dark)
MeOH
no reaction
no reaction
no reaction
97
10
11
12
13 p-GaN (no Si)
MeOH
aReaction was done by injecting all reactants into a sealed quartz vessel
under vacuum. NMR yields were determined by using mesitylene as the
internal standard.
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the surface energy band of the corresponding NW bends from photoexcited electrons (EPR spectra of i-, n-, and p-GaN
View Article Online
downwards (p-GaN NW). The photoexcited e- for the p-GaN NWs in the dark were silent). It can be oDbOseI:r1v0e.1d03f9ro/Dm0StCh0e27E1P8AR
NWs will therefore tend to aggregate to the NW surface, making spectra that both n- and p-GaN NWs show stronger EPR signals
the e- easier to donate to the reactant, while the h+ tends to than i-GaN NWs. This is because the photoexcited e- and h+ are
migrate to the internal region of the NWs and inhibits the more prone towards recombination in i-GaN NWs due to less
consumption of the sacrificial reagent (Figure 3A right). To significant band bending. As explained above, band bending
characterize this electronic property, electron paramagnetic prevents carrier recombination by driving e- and h+ to the
resonance (EPR) spectra were taken for the as-synthesized opposite direction. At the same time, the n-GaN NWs showed a
intrinsic- (i-), n-, and p-GaN NWs after they were irradiated broader EPR signal compared to p-GaN NWs. The results clearly
under 365 nm light for 1 hour (Figure 3B).30 Since both Ga3+ and show that the photoexcited e- in n-GaN NWs tends to delocalize,
N3- do not possess unpaired e-, the ground state of GaN is suggesting migration of e- from the NW surface to the internal
diamagnetic (see Figure S32 in the Supplemental Information). region.
Therefore, the observed EPR signal should exclusively come
Table 2. Substrate scope investigation using catalyst recycling.
This journal is © The Royal Society of Chemistry 20xx
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As both the e- donation to the reactant and h+ oxidation of
Journal Name
With the optimized reaction condition inViewhAartnicdle,Onwlinee
the sacrificial reagent promote the overall reaction, it is proceeded to examine the substrDaOteI: 10s.1c0o3p9e/D0SoCf027t1h8Ae
important to locate the optimized catalyst and to determine the photochemical PCR using 0.35 mg p-GaN NW, 0.2 mmol starting
rate-limiting step. Under the irradiation with an 8 W black light material in 2 mL MeOH. To test the stability and reusable limit
bulb (giving 1 W light irradiation at 365 nm), in 0.5 mmol of the catalyst, we kept recycling and reused this same catalyst
methanol (MeOH) as a sacrificial reagent (oxidized by h+) and over-and-over for all substrates (Table 2). We started with the
solvent, we examined i-GaN, n-GaN (Si4+ doped), and p-GaN simplest ketone, acetone (1b), which gave a quantitative yield
(Mg2+ doped) as catalysts to carry out PCR of 0.02 mmol of the pinacol (2b). Likewise, acetophenone (1b) and
acetophenone (1a) towards 2,3-diphenyl-2,3-butanediol (2a) as butyrophenone (1c) also reacted efficiently, giving quantitative
a standard reaction (Table 1, see also Figure S31 in the and 97% yields, respectively. Fluorinated substrates 2-
Supplemental Information). While all the GaN NWs showed fluoroacetophenone (1d) and 4-fluoroacetophenone (1e) gave
negligible diastereoselectivity in the PCR product, the catalytic 95% and 98% yields of their corresponding 2,3-bis(o-
activity decreases gradually from p-GaN NW (Table 1 entry 3) to fluorophenyl)-2,3-butanediol (2d) and 2,3-bis(p-fluorophenyl)-
i-GaN NW (Table 1 entry 1) to n-GaN NW (Table 1 entry 2), 2,3-butanediol (2e). The chloro analog 4-chloroacetophenone
affording 2a in quantitative, 93% and 81% yield, respectively. (1f) gave a 93% yield of 2,3-bis(p-chlorophenyl)-2,3-butanediol
The results suggest that the e- donation from the semiconductor (2f). The e--deficient 4-trifluoromethylacetophenone (1g) gave
serves as the rate-limiting step for 1a conversion to 2a. a 99% yield of 2,3-bis(p-trifluoromethylphenyl)-2,3-butanediol
Nevertheless, we further investigated the influence of the (2g). For other e--deficient substrates, 4-cyanoacetophenone
sacrificial reagent such as hydrogen (H2) (Table 1 entry 4), (1h) gave a 94% yield of 2,3-bis(p-cyanophenyl)-2,3-butanediol
methane (CH4) (Table 1 entry 5), water (H2O) (Table 1 entry 6) (2h) and α,α,α-trifluoroacetophenone (1i) gave a 97% yield of
on the reaction catalyzed by p-GaN NW, none of which afforded 1,1,1,4,4,4-hexafluoro-2,3-diphenyl-2,3-butanediol (2i). Aryl-
the corresponding product 2a. Other popular sacrificial aryl ketones were then examined. Benzophenone (1j) gave a
reagents such as triethylamine (Et3N) (Table 1 entry 7), ethanol quantitative yield of the corresponding benzopinacol (2j). The
(EtOH) (Table 1 entry 8), and trimethanolamine (Table 1 entry substituted benzophenone, 4,4’-dimethylbenzophenone (1k),
9) also gave the desired PCR products in high yields. Control 4,4’-difluorobenzophenone
experiments were also conducted and showed that the absence dichlorobenzophenone (1m) gave 98%, 96% and 97% yields of
of either the GaN NW (Table 1 entry 10) or light (Table 1 entry their corresponding tetramethylbenzopinacol (2k),
(1l),
and
4,4’-
11) inhibited the reaction completely. Using Si(111) wafer alone tetrafluorobenzopinacol (2l), and tetrachlorobenzopinacol
as the catalyst did not grant any pinacol coupling product (Table (2m), respectively. It should be noted that even after 13
1 entry 12), and a nearly quantitative yield of 97% was obtained consecutive reactions of substrates with various functionalities,
when p-GaN NW stripped of Si(111) was used as the catalyst the same p-GaN NW showed no observable decrease in catalytic
(Table 1 entry 13).
Table 3. Substrate scope obtained using Xe-lamp.
reactivity. However, certain substrates, such as 4-
phenoxyacetophenone (1n), 4-methoxyacetophenone (1o), and
2-fluoro-4-methoxyacetophenone (1p), gave complete starting
material recovery even after freshly synthesized catalysts were
used under the given conditions. When a much stronger light
source (a 300 W full-arc xenon lamp equipped with a >295 nm
long pass light filter) was used instead of the black light bulb,
those substrates gave quantitative yields of their corresponding
PCR products (2n-2p, Table 3). These results suggest that e-
transfers from the semiconductor to those substrates are more
difficult, possibly due to their electron richness. We also
examined the PCR of aldehyde substrates such as benzaldehyde
(1q) under the black light bulb using the p-GaN NW catalyst
(Table 4). Despite full conversion of the starting material and
the desired PCR product of hydrobenzoin (2q) being obtained
(see Figure S30 in the Supplemental Information), significant
amounts of benzyl alcohol (3q) were observed as a side-
product. After examining GaN NWs doped by other dopants, we
found that the selectivity for 2q increased from p-GaN and i-
GaN to n-GaN. According to the e- counting, the substrate needs
to absorb two e- from the semiconductor to give the
hydrogenation product, while only one e- is needed to initiate
the PCR. The enhanced e- donating ability of p-GaN NW
compared to n-GaN NW might over-donate the substrate with
the extra e-, promoting the hydrogenation side-reaction while
lowering the PCR selectivity.
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selectivities (of 4.9:1 and 6.1:1, respectively) while gave 61%
and 54% PCR yields of 1,2-bis(p-methoxylphenyl)-1,2-
Table 4. Benzaldehyde reactivity and selectivity.
View Article Online
DOI: 10.1039/D0SC02718A
ethanediol (2t) and 1,2-bis(4-allyloxyphenyl)-1,2-ethanediol
(2u), respectively.
Table 6. Cross-PCR of various carbonyl compounds with formaldehyde.
Table 5. Aldehyde PCR substrate investigation.
We also examined the cross-PCR of various carbonyl
compounds with formaldehyde under the optimized conditions
(Table 6). The reaction between 1a and formaldehyde gave an
excellent 91% yield of 2-phenyl-1,2-propanediol (5a) with either
n-GaN NW or p-GaN NW. Therefore, the p-GaN NW used
previously in Table 2 was reused and recycled again as the
catalyst. Under the same conditions, 1j also gave an excellent
93% yield of 1,1-diphenyl-1,2-ethanediol (5b). e--rich aldehyde
1s gave a slightly reduced yield of 83% for the corresponding 1-
(4-methoxyphenyl)-1,2-ethanediol (5c). An even more e--rich
aldehyde, 3,4-dimethoxybenzaldehyde (4d), gave 81% yield of
1-(3,4-dimethoxyphenyl)-1,2-ethanediol (5d). e--deficient
aldehyde, such as 4-trifluoromethylbenzaldehyde (4e), also
gave 82% yield of 1-(4-trifluoromethylphenyl)-1,2-ethanediol
The substrate scope for aldehydes was also investigated
using catalyst recycled from Table 2 (Table 5). Besides 1q and p-
tolualdehyde (1r) gave the same PCR yields (of 2q and 1,2-bis(p-
methylphenyl)-1,2-ethanediol (2r), respectively) with similar
PCR/hydrogenation selectivities, the PCR/hydrogenation
selectivity decreased to 2.3:1 with more e--deficient 4-
chlorobenzaldehyde (1s), which gave 69% PCR yield of 1,2-bis(4-
chlorophenyl)-1,2-ethanediol (2s). On the other hand, e--rich
aldehydes, such as p-anisaldehyde (1t) and 4-
allyloxybenzaldehyde (1u), led to increased PCR/hydrogenation
This journal is © The Royal Society of Chemistry 20xx
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1
D. H. R. Barton, S. I. Parekh in Half a Century oVfieFwreAerticRleaOdniclinael
(5e). 4-tert-butylbenzaldehyde (4f) gave 91% yield of 1-(4-tert-
butylphenyl)-1,2-ethanediol (5f). Aliphatic aldehyde such as 4-
phenoxybutanal (4g) also reacted efficiently, giving 89% yield of
5-phenoxy-1,2-pentanediol (5g). Inspired by the observed
functional tolerance, a natural product, 3-methoxyestrone (4h),
was also examined using the recycled catalyst, which has been
recycled for 21 times in total, giving an astonishing 62% yield of
the corresponding 17-hydroxy-3-methoxyestra-1,3,5(10)-
triene-17-methanol (5h). The catalyst still shows no signs of
degradation after the recycling (see Figure S34 in the
Supplemental Information).
Chemistry (Ed.: Lezioni Lincee). Cambridge University Press,
DOI: 10.1039/D0SC02718A
Cambridge, 1993.
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Scheme 1. Radical-trap experiment using TEMPO. The yield was determined
using GC-MS with 1,3,5-mesitylene as an internal standard.
Finally, we examined the optimized PCR catalyzed with p-
GaN NW by adding stoichiometric amount of TEMPO as a radical
trap (Scheme 1). The TEMPO adduct was identified using mass
spectroscopy (MS) while no PCR product detected, which
supports our design of a radical coupling process.
18 D. Wang, A. Pierre, M. G. Kibria, K. Cui, X. Han, K. H. Bevan, H.
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3. Conclusions
In summary, we have shown the use of epitaxial grown GaN NW
as a highly efficient, robust and readily reusable photoredox
catalyst for photo-PCR under ambient light and room
temperature. Various ketones afford the corresponding
products smoothly in excellent yields. Aldehydes are also
effective albeit giving some over-reduction side products. By
using excess formaldehyde, cross-PCRs are also effective with
both aromatic and aliphatic aldehydes, as well as in complex
natural products. The catalytic property of GaN can be tuned
readily with different dopants. Further investigations of GaN
NW as organic chemistry catalysts have already underway in our
lab.
Conflicts of interest
There are no conflicts to declare.
29 S. Chu, P. Ou, P. Ghamari, S. Vanka, B. Zhou, I. Shih, J. Song Z.
Mi, J. Am. Chem. Soc. 2018, 140, 7869-7877.
30 M. Foussekis, Band Bending in GaN (Master dissertation).
Virginia Commonwealth University, Richmond, Virginia, 2009.
Acknowledgements
M. L. and C. J. L. are grateful for the Canada Research Chair (Tier
1) foundation, the Natural Science and Engineering Research
Council of Canada, Fonds de recherche du Quebec – Nature et
Technologies, Canada Foundation for Innovation (CFI)), and
McGill University for financial support. M. L. and Z. M. are
grateful for Emission Reduction Alberta for funding.
Notes and references
6 | J. Name., 2012, 00, 1-3
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