Promoting Ni(II) Catalysis with Plasmonic Antennas

Full text of DOI:10.1016/j.chempr.2019.07.022

Article
Promoting Ni(II) Catalysis with Plasmonic
Antennas
Pengfei Han, Tana Tana, Qi
Xiao, Sarina Sarina, Eric R.
Waclawik, Daniel E. Go´ mez,
Huaiyong Zhu
s.sarina@qut.edu.au (S.S.)
hy.zhu@qut.edu.au (H.Z.)
HIGHLIGHTS
Plasmonic antenna enhances the
catalytic activity of Ni2+
complexes by light
Reactants are concentrated at the
catalyst surface by plasmonic
electromagnetic field
Hot electrons are transferred via a
molecular bridge of the benzyl
ring of the reactants
Enhanced chemisorption by
irradiation significantly increases
the reaction rate
Plasmonic metal NPs such as Au and Ag are proven to channel visible-light photon
energy to nearby metal complexes effectively and allow the conventional metal
cationic catalysts to work at moderate temperatures under light irradiation. A key
finding is that reactant molecules can be significantly concentrated at the active
sites simply by light and thus increase the chance of activation. The merging of
plasmonic antenna and metal cationic catalysis will be an inspiring strategy for
designing efficient photocatalytic systems.
Han et al., Chem 5, 1–21
November 14, 2019 ª 2019 Elsevier Inc.
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Article
Promoting Ni(II) Catalysis
with Plasmonic Antennas
2
3,4
Pengfei Han,1,2 Tana Tana,2 Qi Xiao,3 Sarina Sarina,2,5, Eric R. Waclawik, Daniel E. Go´ mez,
*
and Huaiyong Zhu2,
*
SUMMARY
The Bigger Picture
Plasmonic metal nanoparticles
(NPs) absorb visible light
Plasmonic catalysis has drawn significant interest recently, as the catalysis can
be driven by visible light. Here, we show a new tactic to apply low-flux visible-
light irradiation on plasmonic metal nanoparticles (NPs) to initiate catalysis
with surface-bound transition-metal complexes under mild conditions. Ni2+
complexes (as catalytic reaction sites) and Au or Ag NPs were immobilized on
g-Al2O3 nanofibers to produce plasmonic-antenna-promoted catalysts. The
light irradiation on Au or Ag NPs enhanced photocatalytic activity of the Ni2+
complexes for reductive cleavage of C–O bond by 18-fold or 17-fold, respec-
tively. The intense electromagnetic near-fields of the plasmonic metal NPs
significantly increased the chemisorption of the reactant to the Ni2+ active sites.
The light-excited hot electrons transfer via a molecular bridge of the aromatic
ring of the reactants. The light-enhanced chemisorption plays a key role in
this photocatalyst’s structure that comprises a plasmonic antenna and catalyti-
cally active metal complex sites.
intensely; this produces hot
electrons on their surface and also
generates localized
electromagnetic fields near the
surfaces. This property is
successfully applied to enhance
the catalytic performance of
typical metal-cationic-catalyst-
Ni2+ complexes by low-flux
visible-light irradiation. Here, Ni2+
complexes are immobilized in
close proximity to Au (or Ag) NPs,
which serve as the light-harvesting
antennas and trigger the activity
of Ni2+ catalysts effectively for the
C-O bond cleavage reaction by
17- to 18-fold enhancement. The
intense electromagnetic near-
fields of irradiated plasmonic NPs
enable the reactant molecule to
be significantly concentrated at
active sites and play a key role in
the photo-accelerated reaction
rates. This research could inspire a
new paradigm to modify the
conventional metal cationic
catalysis by visible light.
INTRODUCTION
Direct photocatalysis using plasmonic metal (gold, silver, copper, or aluminum)
nanoparticles (NPs) under visible-light irradiation, also called plasmonic catalysis,
has drawn significant interest in the last decade.1–13 When plasmonic metal NPs
are illuminated, they can efficiently absorb visible light due to the excitation of
localized surface plasmon resonance (LSPR), where the conduction electrons of
the NPs collectively oscillate with the electromagnetic (EM) field of incident light.14
The plasmon-resonant excitation generates energetic hot electrons that can trigger
chemical reactions of a reactant adsorbed on the plasmonic NPs, under mild reac-
tion conditions.1–7,12,13 However, only a limited number of the transformations can
be directly catalyzed by NPs of plasmonic metals, especially when compared to
the range of transformations accessible with transition-metal complexes in homoge-
neous catalysis systems. For specific reactions, photocatalysts of alloy NPs of a plas-
monic and a transition metal of inherent catalytic activity for specific reactions have
been developed to expand the application of the plasmonic photocatalysis to a
wider range of selective organic synthesis reactions.15
Transition-metal complexes are widely used for the homogeneous catalytic synthe-
sis of many important organic compounds.16–19 Combining the optical function of
plasmonic metal NPs with the inherent bond-forming or bond-breaking ability of
transition-metal complexes may enable the transition-metal complexes to catalyze
reactions with the light energy harvested by plasmonic metal NPs under mild condi-
tions. With traditional catalysis reactions, intense thermal heating is often required
to bridge reaction activation barriers and achieve sufficient catalytic efficiency.
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Figure 1. The Schematic Structure of the Photocatalyst
Ni2+ complexes and plasmonic metal NPs are immobilized on g-Al2O3 nanofibers. The randomly oriented nanofibers form connected cages of irregular shapes.
Right: the photocatalytic reaction investigated in the present study.
When plasmonic metal NPs are introduced as antennas to direct energy to the active
reaction site, there may be no need for such intensive heating. This difference in the
way catalysis is expected to prove beneficial for selective synthesis. In a combined
system, the plasmonic NP surfaces do not act as catalytically active sites themselves;
instead, the NPs facilitate chemical transformations via the transfer of energy and
light-generated hot electrons to reactive transition-metal complex sites, as schemat-
ically illustrated in Figure 1. The adsorption and activation of reactants at the com-
plex sites of the new catalysts are different from those on metal NP surface.
It is well known that the LSPR light absorption of plasmonic NPs can generate EM
near-fields, with intensities much higher than that of the incident radiation.4,6–11
The plasmon field enhancement can significantly change the light-matter interaction
in excitonic systems.14,20 For instance, the field intensity in a narrow junction be-
tween Ag NPs has been predicted to be 106 times that of the incident light alone.4
These narrow junctions between closely spaced NPs are the so-called hot spots.21,22
Whether the high intensity of EM fields at the hot spots can be utilized for catalysis by
surface-bound active sites has not been investigated. We consider that the strong
EM fields at hot spots may radically change the interaction between reactant and
transition-metal complexes when the complexes are immobilized within the hot
spots. A strong interaction may direct the energy of the incident light to the reaction
site. Also, since the number density of plasmonic NPs is larger at hot spots than other
regions in a sample, the number of hot electrons at hot spots is also predicted to be
higher. These properties may facilitate a chemical transformation and thus improve
1College of Chemistry and Chemical
the catalytic efficiency of the metal complexes.
Engineering, Hunan University, Changsha
410082, P.R. China
2School of Chemistry, Physics and Mechanical
Engineering, Queensland University of
Technology, Brisbane, QLD 4001, Australia
Electron transfer may contribute to the catalysis process by changing the oxidation
state of the transition metal complexes, a key mechanism that can promote catalysis
of many reactions.23 In a combined system, the transfer of energy and hot electrons
is determined by energy alignment: the energy of photoexcited electrons have to be
sufficiently high to be injected into the metal complex sites (via a medium) and
change the metal oxidation state. The light harvested by plasmonic NP antennas
promotes conduction electrons to energy levels above the Fermi level,4,12,14 and en-
ergy distribution of hot electrons depends on the energy of the incident photons and
the light absorption mechanism of the plasmonic metal NPs.
3CSIRO Manufacturing, Bayview Ave, Clayton,
VIC 3168, Australia
4RMIT University, Melbourne, VIC 3000, Australia
5Lead Contact
*Correspondence: s.sarina@qut.edu.au (S.S.),
hy.zhu@qut.edu.au (H.Z.)
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In addition to the intraband electron excitation in the plasmonic NPs due to the LSPR
effect, interband excitation of d / sp transitions by visible light can generate elec-
tron-hole (e–-h+) pairs in Au NPs, which can participate in a chemical reaction of
adsorbate on the NP surface.24 The interband excitation in NPs of non-plasmonic
metals can also facilitate chemical reactions.25 It is of great interest to know whether
the interband excitation can promote catalytic performance of the metal complexes
at a distance. Small Au NPs possess only weak LSPR absorption and the light absorp-
tion of the NPs exposed to short wavelengths (<450 nm) induces predominantly in-
terband excitation.12 This property of small Au NPs can therefore be used to inves-
tigate whether interband excitation can promote the catalysis even when the NP
surfaces are not catalytically active sites themselves. Hence, the mechanisms of
the systems with the plasmon-antenna-promoted catalysts will be different from
those of the plasmonic metal NP catalysts and homogeneous metal complex cata-
lysts. These features are of great interest from a fundamental research perspective.
To verify the efficacy of the proposed strategy of exploiting the antenna-effect of
plasmonic metal NPs and to develop protocols for transition-metal-catalyzed
reactions under mild conditions, we designed a structure as illustrated in Figure 1.
In this structure, Ni2+ complexes and Ag or Au NPs are immobilized on g-Al2O3
nanofibers. Immobilizing Ni2+ complexes to the g-Al2O3 supports maintains the
complexes at fixed locations relative to the metal NPs, allowing us to investigate
the influence of the high intensity of EM near-fields and transfer of hot electrons
from the metal NPs to Ni2+ ions. Such a structure can achieve stable photocatalytic
performance and make the catalysts recyclable.
Ni2+ complexes have been extensively used as homogeneous catalysts.26–29 Nickel
is a common, inexpensive transition metal and has been used to catalyze reductive
cleavage of C–O bonds.30–35 This reaction was chosen as a model reaction because
it is the essential reaction step for the production of high-value aromatic chemicals
from biopolymer lignin.36 Aryl ether C–O bonds are relatively unreactive; for
example, the dissociation energy of the aryl ether C–O bonds in the a-O-4 linkage
is 218 kJ mol .
ꢀ1 31 So catalysts usually function at high temperatures and hydrogen
pressures (>120oC and added hydrogen pressure),37 inevitably yielding saturated
hydrocarbons. To avoid undesired hydrogenolysis of aromatic rings and to achieve
selective cleavage of the aryl ether C–O bonds, the reaction should ideally be con-
ducted under mild reaction conditions (low temperature and pressure). Hence, the
catalysts must be highly active and selective to the C–O bond. Cleavage of C–O
bonds in aryl ethers with a photocatalyst [Ir(ppy)2(dtbbpy)]PF6 at room temperature
was reported recently.38 Although these types of organometallic homogeneous
photocatalysts are efficient, they are generally expensive and their use requires
costly recycling processes.
In the present study, photocatalysts with immobilized Ni2+ complexes and plas-
monic metal NPs were applied to the hydrogenolysis of benzyl phenyl ether that
has an a-O-4 linkage.39 We demonstrate that by varying the metal NP loading on
the alumina support, the number of the plasmonic hot spots could be substantially
altered. The hot electrons generated by both interband excitation of small Au NPs
and intraband excitation (via LSPR effect) of Ag NPs can promote the catalysis. A sig-
nificant observation is evidence for a light-induced, enhanced chemisorption of the
reactant molecules at the catalytic active Ni2+ sites. This is beneficial to the catalytic
performance of the Ni2+ sites. We also find that transfer of the hot electrons from the
plasmonic metal NPs to the Ni2+ complexes via a ‘‘bridge’’ of the aromatic ring of the
reactant is essential to the performance of the photocatalysts.
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RESULTS AND DISCUSSION
Photocatalyst Structure
The photocatalysts were prepared following the procedures shown in Figure S1. The
prefix number of the sample name indicates the plasmonic metal content in weight per-
centage (wt %), and ASN (alumina-slilane-NH2) is the support of Al2O3 nanofibers
grafted with a silane containing an amino group. The metal content in the catalysts
was measured by inductively coupled plasma optical emission spectrometry (ICP-
OES), confirming the contents of Ni and plasmonic metal (Table S1). For example,
the 2.5Au-ASN-Ni2+ catalyst contains 2.5 wt % of Au NPs, and Ni2+ ions are immobilized
on the ASN support.
The g-Al2O3 nanofibers are ꢁ5 nm thick and 100 nm long, which were sintered to form a
highly porous framework of randomly oriented fibers.40 The cage-like nanofiber config-
uration (see Figure 1) could confine the NPs formed within the structure and allow reac-
tant molecules to readily diffuse to the Ni2+ complex reaction sites in the vicinity of the
metal NPs through inter-fiber voids. Plasmonic NPs (Ag or Au) and Ni2+ were immobi-
lized on the alumina fibers in the procedure schematically illustrated in Figure 1. The
transmission electron microscopy (TEM) images indicate that the metal NPs were
dispersed throughout the g-Al2O3 fiber support (Figures 2A and 2D). The supported
Au NPs were relatively small, most of them smaller than 5 nm (Figure 2B and inset of Fig-
ure 2A), the mean particle size is 2 nm. The particle size distributions of the Ag NPs (Fig-
ure 2E and inset of Figure 2D) were broader, most of the Ag NPs were smaller than 15 nm
and the mean particle size is about 9 nm. Line scan analysis (energy dispersion X-ray
spectroscopy [EDX]) of plasmonic metal and Ni compositional fluctuations in Figures
2C and 2F indicate that the Ni2+ ions were immobilized on the bare support between
the metal NPs, instead of accumulating on the surface of the plasmonic metal NPs.
The other structural and chemical properties of the catalyst were characterized by
nitrogen adsorption, Fourier transform infrared spectroscopy (FTIR), X-ray diffrac-
tion (XRD), and X-ray photoelectron spectroscopy (XPS) and are shown in Table S2
and Figures S2–S6. The results verify that the photocatalysts possess the assembled
structure as we designed.
Diffuse reflectance ultraviolet-visible (DR UV-vis) spectra of the samples are shown in
Figures 2G and 2H. g-Al2O3 fibers and grafted g-Al2O3 fibers (ASN) have no obvious
absorption in the visible-light region (wavelength > 400 nm), while two broad bands
peaked at wavelengths of 378 and 652 nm are clearly observed for Al2O3-silane-
NH2-Ni2+ (ASN-Ni2+). These two bands can be assigned to the 3A2g(F) / 3T1g(P) and
3
3A2g(F) / T1g(F) transitions for distorted octahedral Ni2+ complexes.41,42 These ab-
sorption bands in 2.5Au-ASN-Ni2+ and 4.3Ag-ASN-Ni2+ samples overlap with the
strong absorption of the supported metal NPs in the samples. Both 4.3Ag-ASN-Ni2+
and 4.3Ag-ASN samples have absorption bands centered at 405 nm, which are the
LSPR absorption of Ag NPs.8 The 2.5Au-ASN-Ni2+ catalyst shows a broad light absorp-
tion and the absorption intensity decreases when the wavelengths are longer than
550 nm. No sharp characteristic LSPR peak of Au NPs at ꢁ530 nm43 is observed. This
could be attributed to that the small Au NPs (with a mean size of 2 nm). The interaction
of Au NPs with the amino group of the grafted silane also changes the electron density
of the small Au NPs and their light absorption due to LSPR effect44,45 more significantly
than that of the relatively larger Ag NPs (with mean size of 9 nm).
Photocatalytic Performance for Cleavage of Aryl Ether C–O Bonds
Figure 3 shows the catalytic performance for the C–O bond cleavage of benzyl
phenyl ether (one of the a-O-4 model compounds) by the photocatalysts prepared
4
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Figure 2. Characterization of the Photocatalysts
(A and D) TEM images of 2.5Au-ASN-Ni2+ (A) and 4.3Ag-ASN-Ni2+ photocatalysts (D); the insets are the particle-size distribution of the Au NPs and Ag
NPs, obtained from measurement over 300 isolate particles in the TEM images.
(B and E) High-resolution TEM images of Au and Ag NPs on 2.5Au-ASN-Ni2+ (B) and 4.3Ag-ASN-Ni2+ (E).
(C and F) EDX line profile analysis of 2.5Au-ASN-Ni2+ (C) and 4.3Ag-ASN-Ni2+ (F).
(G and H) the UV-vis spectra of 2.5Au-ASN-Ni2+ (G) and 4.3Ag-ASN-Ni2+ (H).
with different amounts of plasmonic metal NPs (parameters given in the caption).
Both the 2.5Au-ASN-Ni2+ and 4.3Ag-ASN-Ni2+ catalysts gave high conversion of
the ether (98% and 96%, respectively) under visible-light irradiation of 0.96 W
cmꢀ2, while no reaction was observed in the dark. The selectivity toward production
of phenol was around 50%, indicating that a homolytic C–O bond scission of benzyl
phenyl ether occurred. There are no products of undesired hydrogenolysis of aro-
matic rings; the high selectivity is because the moderate conditions of the photoca-
talytic process induce over-reduction.
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Figure 3. Catalytic Performances of the Assembled Photocatalysts for C–O Bond Cleavage of Benzyl Phenyl Ether under Visible-Light Irradiation and
in the Dark
Reaction conditions: benzyl phenyl ether (0.05 mmol), KOH (0.15 mmol), and 2 mL of isopropanol (IPA) as solvent, mixed with 20 mg of catalysts in argon
atmosphere. Light source was a halogen lamp with intensity of 0.96 W cmꢀ2, and the reaction was conducted at 80ꢂC G 2ꢂC for the systems with Ag NPs
and 90ꢂC G 2ꢂC for the systems with Au NPs (the reactions with ASN-Ni2+ catalyst were conducted at 80ꢂC G 2ꢂC and 120ꢂC G 2ꢂC in the left and right
panels, respectively) for 24 h. At these temperatures, a high reaction rate was achieved, while hydrogenation of the aromatic rings did not take place.
Liquid specimens were sampled from the reaction mixture and measured by gas chromatography (GC). Error bars associated with conversion are the
standard error of three sets of unique measurements.
The catalytic activities of a sample prepared with Ni2+ complexes but without the
metal NPs, a sample with the metal NPs but no Ni2+ ions yielded very low conversion
(6% or nil). This highlights a significant synergistic effect occurred when the Ni2+
complexes were immobilized in the presence of plasmonic metal NPs: when the light
absorption by the metal NPs combines with the inherent catalytic capability of Ni2+
complexes, this results in superior photocatalytic activity of the composite. It is also
evident that conversion of the reactant increased with increased metal NP loading of
the catalyst. The synergistic effect becomes significant when the metal NP content
is high.
Simply the mixture of Ni(NO3)2 and 4.3Ag-ASN exhibited negligible activity.
Hence, the possibility that Ni2+ species in liquid phase are active for the catalytic
reaction is excluded. The mixture of Ni(NO3)2 and 2.5Au-ASN exhibited a low con-
version of about 20%. It appears that the gold NPs can interact with Ni2+ species in
liquid while Ag NPs do not. The detailed reason is unknown. In those control exper-
iments, the overall contents of Ni2+ ions and silver in the mixture of Ni(NO3)2 and
4.3Ag-ASN are the same as those in the 4.3Au-ASN-Ni2+. This is also the case for
the Au sample. The significant differences in catalytic performance between the
mixtures and our designed catalysts suggest that the interaction between metal
NP-ASN and immobilize Ni2+ ions has substantial impact on the overall photocata-
lytic process, the reaction is a complete heterogeneous catalysis process for
Ag-ASN-Ni2+ catalysts and proceeds mainly by heterogeneous catalysis process
for Au-ASN-Ni2+ catalysts. Plasmonic NPs considerably enhance the catalytic
activity of Ni2+ complexes only when they are immobilized in the close proximity
to the NPs.
6
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Figure 4. Impact of the Plasmonic Hot Spots on the Photocatalysis
The time course for the photocatalytic activities of xAg-ASN-Ni2+ (A) and xAu-ASN-Ni2+ (B) in C–O
bond cleavage of benzyl phenyl ether (1.11 W cmꢀ2 of halogen light intensity at 80ꢂC for Ag-Ni2+
system; 0.96 W cmꢀ2 of halogen light intensity at 90ꢂC for Au-Ni2+ system). Error bars associated
with conversion are the standard error of three sets of unique measurements.
Impact of the Plasmonic Hot Spots
It was reported that photothermal effect of plasmonic NPs could elevate the temper-
ature of the NPs, which may play a critical role in the catalysis.46 To understand if
local heating will contribute to the Ni2+ catalysis, we conducted the photo-reaction
using the ASN-Ni2+ catalyst at elevated temperatures (80ꢂC –120ꢂC); only 2% of
conversion increase of C–O bond cleavage was observed (see Table S3), which
means that a 40ꢂC increase in reaction temperature could not enhance the catalytic
activity of the Ni2+complexes in our reaction system. Therefore, photothermal effect
caused by irradiation could not drive the reaction in the present study. The hot elec-
trons from the illuminated metal NPs may play the critical role in activating the aryl
ether C–O bonds as is the case for some other plasmonic catalysts reported in the
literature.1–13 But the Au and Ag NPs are not the active catalytic sites in the present
study, as evidenced by the experimental data (see Figure 3). Hence, these catalysts
may be expected to operate differently from the well-known mechanism where light-
generated hot electrons promote reactions.2–8
We compared the catalytic performance enhancement of Ni2+ when the number
density of metal NPs are different by using catalysts with different metal NP contents
and maintaining the metal NP content in the reaction systems and other experi-
mental conditions identical. For instance, the reaction vessel was loaded with
66 mg of 1.3Ag-ASN-Ni2+ and 20 mg of 4.3Ag-ASN-Ni2+. The quantities of the
two catalysts were calculated from the Ag content (Table S1). Figure 4A shows
that the percent conversion of reactant in the reaction driven by 4.3Ag-ASN-Ni2+
is significantly higher than by 1.3Ag-ASN-Ni2+ although the content of immobilized
Ni2+ ions in the latter is 3.8 times of that in the former (derived from data in Table S1).
The turnover number (TON) with respect to the Ni2+ sites in 4.3Ag-ASN-Ni2+
was calculated from the conversion, being >18 times higher than that in 1.3Ag-
ASN-Ni2+. Given that the Ag NP content in the two systems is the same, the overall
Ag NP content in the catalytic system cannot be the reason for the large difference in
the catalytic performance.
The major difference between the two photocatalytic systems shown in Figure 4A is
that the Ag NPs are more closely packed in 4.3Ag-ASN-Ni2+. The junctions between
closely spaced NPs can generate ‘‘hot spots’’ where the strong EM field coupling of
the closely spaced NPs generates huge EM field enhancement.4,6 In a unit volume
of catalyst, the Ag NPs in the 4.3Ag-ASN-Ni2+ is more than three times the Ag
NPs in the 1.3Ag-ASN-Ni2+ catalyst, and therefore, there are a greater number of
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nano-junctions between Ag NPs. The results in Figure 4A can be interpreted as a
manifestation of the fact that the photocatalytic activity heavily depends on the num-
ber of Ag NPs’ nano-junctions in the photocatalysts. An environment of high number
density of Ag NPs can significantly promote catalytic performance of the immobi-
lized Ni2+ complexes around the particles.
The effect of catalyst mass on photocatalytic activity was examined, and the results
are provided in Table S4. The conversion increased as more catalysts were used. But
the conversion did not increase linearly with catalyst mass. The deviation from the
linearity may be attributed to the screen effect.
Figure S7 shows EDS mapping results of the 4.3Ag-ASN-Ni2+ photocatalyst, which
reveals the existence of closely spaced NPs, consequently supporting the role
played by plasmonic hot spots.21,22 Analysis of the results (Figure S7) indicates the
distribution of plasmonic hot spots regions overlapped with concentrations of sur-
face-bound Ni2+ complexes co-located in the plasmonic hot-spot regions.
A similar phenomenon can be found in the xAu-ASN-Ni2+ photocatalytic systems as
seen in Figure 4B. However, the effect of high number density of Au NPs on the cat-
alytic performance is weaker than that of Ag NPs. The performance improvement
caused by increasing the number density of small Au NPs (from 0.7 wt % Au to 8.4
wt % Au; see TEM images in Figure S8) is less than that caused by increasing the
number density of Ag NPs (from 1.3 wt % Ag to 4.3 wt % Ag; see TEM images in
Figure S9).
We performed simulations of the near-field enhancement of single Au NP (2 nm) and
dimer Au NPs (2 nm) with 1 nm gap between them (Figure S10) by using an electro-
static eigenvalue method; the details of the simulation methodology are from Davis
and Go´ mez.47 Under 400 nm excitation, the intensity of EM fields at the surface of an
isolated Au NP is ꢁ10 times larger than the field intensity of the incoming photon
flux, while the enhancement of EM field occurring in the middle of the dimer is
ꢁ12 times. In contrast, the EM field localization upon LSPR excitation of Ag NPs is
significantly higher (Figure S11). The intensity of EM fields at the surface of an Ag
NP (9 nm) is ꢁ300 times larger than the field intensity of the incoming light, while
the enhancement of EM field occurring at the hot spot of the dimer is ꢁ8,000 times.
The hot spot regions will have high concentrations of energetic electrons (because
the high number density of the plasmonic NPs) and interaction of the EM near-field
with reactant in these regions is likely to be very strong. Thus, these sites are consid-
ered most important for plasmonic photocatalysis.
We note that our wet-chemistry synthesis approach results in distributions of sizes of
the plasmonic metal particles and a range of inter-particle distances in the catalysts
as previously reported.1,2 Uniform metal particle size and inter-particle distance is
not a prerequisite for formation of plasmonic hot spots.46 The 4.3Ag-ASN-Ni2+ cata-
lyst has many more hot spots, compared to 1.3Ag-ASN-Ni2+ as shown in Figure S9.
The intense field enhancement at the hot spots between the metal NPs clearly influ-
ences the catalytic performance of Ni2+ complexes under mild reaction conditions,
and the results of Figure 3 represent a statistical average which is dominated by the
response augmented by plasmonic hot spots.
The existence of hot spots has also been experimentally observed. Highly intense
EM near-fields affect the properties of molecules within them;48 for example, the in-
tensity of signals in surface-enhanced Raman scattering (SERS) of the molecules.48–50
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Figure 5. Light-Enhanced Adsorption
(A) Surface-enhanced Raman spectra of benzyl phenyl ether adsorbed on 2.5Au-ASN-Ni2+, 1.3Ag-
ASN(-Ni2+), and 4.3Ag-ASN-Ni2+ photocatalysts. The characteristic vibration of the C–O bond from
the reactant is marked with an asterisk. The samples were prepared by mixing 10 mg of catalysts
with 0.5 mL of the ether in IPA (0.5 3 10ꢀ2 M) for 6 h at room temperature and dried at 60ꢂC in a
vacuum oven overnight. The instrument is equipped with a laser with 532 nm of wavelength.
(B) The comparison of adsorption properties of catalysts for benzyl phenyl ether. In a typical
procedure, 20 mg of catalyst was added to 2 mL of adsorbate/IPA (0.5 3 10ꢀ2 M) solution, filled with
argon, and sealed. The adsorption was conducted for 10 h at 80ꢂC, the same temperatures
designated for the reaction. Only the ether and IPA can be detected by GC, for KOH is absent in the
experiment. Inset schematically shows optical plasmonic trapping of the reactant.
SERS predominantly originates from EM near-field enhancement, and its intensity is
approximately proportional to the square of the EM field intensity. Thus, the inten-
sity of SERS active peaks of reactant is expected to be commensurate with intensity
of the enhanced EM near-field around Ag NPs experienced by the reactant mole-
cules. The Raman spectra of benzyl phenyl ether adsorbed on the catalysts with
different Ag NP contents were compared (seen in Figure 5A).
The results show that when the aryl ether molecules were adsorbed on the catalysts
with a high content of Ag NPs (4.3Ag-ASN-Ni2+), peaks at 1,153 cmꢀ1 ascribed to the
aryl ether C–O bond vibration become much more intense, even when compared to
their concentrated counterpart (pure benzyl phenyl ether). No SERS signals were de-
tected with the catalyst samples prepared with a lower Ag NP content (1.3Ag-ASN-
Ni2+ and 1.3Ag-ASN) or with small Au NPs. Given that the photocatalysts have a
similar specific surface area (Table S2), the adsorption of the aryl ether molecules
on the alumina support has no obvious effect on the Raman signal intensity. The
SERS results can be attributed to three possibilities: (1) there is a higher concentra-
tion of the aryl ether adsorbed on the samples with a higher Ag NP content; (2) the
EM near-field is much more intense (there are more hot spots) in the photocatalysts
with increased number density of Ag NPs, and this significantly enhances the Raman
signal of adsorbed molecules; or (3) the pronounced Raman signals are due to syn-
ergistic effect of the two reasons above. The near-field enhancement of the sample
with small Au NPs (the 2.5Au-ASN-Ni2+) is relatively weak as indicated by the simu-
lation results and the fact that there is no LSPR absorption peak at about 520 nm
wavelength. This is consistent with the result that no enhanced Raman signal is
observed.
Light-Enhanced Chemisorption
The reactant adsorption is a key step prior to surface reaction in heterogeneous
catalysis, the impact of light irradiation on the adsorption has not been extensively
investigated to date. To explore whether the light irradiation can affect reactant
adsorption, we conducted adsorption experiments of benzyl phenyl ether on
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different catalysts in the dark and under light irradiation with different light inten-
sities. The results are summarized in Figure 5B. The adsorption amount is given by
the percentage of the initial concentration of the reactant before light irradiation,
which was adsorbed by the catalyst under irradiation. The adsorption capacity is
calculated based on the Ni amount collected in Table S1. In the experiment, the
cleavage reaction did not proceed as KOH was not added. No chemicals other
than those added were detected. The adsorption capacity (AC) was calculated as
ꢀ
ꢁ
C0 ꢀ C
ACð%Þ = 0:01 3
3 100;
C0nNi
where 0.01 is the total molar amount of benzyl phenyl ether, C0 and C are the con-
centrations of the ether before and after adsorption experiment, respectively, and
nNi is moles of nickel (calculated by the data shown in Table S1). The adsorption
properties of ASN-Ni2+, 1.3Ag-ASN-Ni2+, and 4.3Ag-ASN-Ni2+ catalysts are
compared in Figure 5B.
The initial concentration used for the adsorption experiment (0.01 M) was much
lower than that used in the reaction. The amount of Ni2+ (the adsorption site) in these
samples was 0.011–0.013 mmol, and as discussed below, only the adsorption on a
fraction of the Ni2+ sites, which are in the intense EM fields, was influenced by light
irradiation. When a high initial concentration of reactant was added, it was difficult to
accurately detect the concentration changes cause by light-induced adsorption.
It is also worth noting that only trace amounts of benzyl phenyl ether were adsorbed
on the illuminated 4.3Ag-ASN catalyst, the sample without Ni2+ but with high Ag NP
content. This implies that the EM near-fields alone are not leading to increased reac-
tant adsorption. One needs to consider the interaction between the ether and Ni2+
complex sites of the catalysts too. We measured FTIR spectra of the catalysts after
the ether adsorption experiments (Figure S12). For the ASN-Ni2+ samples that ad-
sorbed the ether in the dark or under light irradiation of intensity of 1.11 W cmꢀ2
,
the position of the peak located at 1,151 cmꢀ1 (which is assigned to the C–O
bond stretch mode of the ether) remains unchanged (Figures S12A and S12D).
However, for both catalysts containing Ag NPs and the surface-bound Ni2+ com-
plexes (1.3Ag-ASN-Ni2+ and 4.3Ag-ASN-Ni2+), this band is blue-shifted to a higher
wavenumber under light-irradiation conditions. This indicates an enhanced interac-
tion can occur between the ether and Ni2+ sites when hot spots are activated (Fig-
ures S12E and S12F). Also, the intensity of the band at 1,350 cmꢀ1 (the vibration
mode of NO3ꢀ that coordinates with Ni2+ sites) decreased markedly after the sample
was illuminated (Figures S12B and S12C). This can be explained if NO3ꢀ is displaced
by the ether during adsorption under visible-light irradiation. The light-induced
adsorption of the ether is irreversible since it remained adsorbed after the light
was switched off. The specific adsorption sites (Ni2+ sites), the coordination of the
adsorbate to the sites, the requirement of activation energy (light harvested by
the plasmonic metal NPs) for the adsorption, and irreversible adsorption process
suggested chemisorption of the ether at Ni2+ sites.
The adsorption of the ether increases significantly with the light intensity increases,
on the photocatalyst 4.3Ag-ASN-Ni2+ (Figure 5B). The adsorption of the ether on
1.3Ag-ASN-Ni2+ catalyst (with much less hot spots) is considerably lower, regardless
of the light intensities used. These data demonstrated that the ether adsorption de-
pends on the number density of Ag NPs and intensity of the incident light but not on
the overall number of Ni2+ complexes in a sample. The higher light intensity and the
10
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Figure 6. The Dependence of Photocatalytic Activities of 2.5Au-ASN-Ni2+ and 4.3Ag-ASN-Ni2+
Photocatalysts for Benzyl Phenyl Ether Cleavage on the Intensity of the Light Irradiation
The reactions were conducted at 80ꢂC and 90ꢂC for Ag NP and Au NP systems, respectively. Error
bars associated with conversion are the standard error of three sets of unique measurements.
Ag NPs density determine the stronger EM near-fields generated close to the active
sites Ni2+. 1.3Ag-ASN-Ni2+ sample contains slightly more Ni2+ complexes than the
4.3Ag-ASN-Ni2+ sample (Table S1). It follows that the chemisorption only appears at
the Ni2+ sites subject to the EM fields of high intensity. In 1.3Ag-ASN-Ni2+, a small
number of sites with the sufficient EM field intensity for chemisorption as the sample
has small number of hot spots. Increasing the incident light intensity amplifies the
intensity of the EM near-fields, and as the number of such sites increases, so does
the chemisorption. Such an increase is insignificant for 1.3Ag-ASN-Ni2+ compared
with that for 4.3Ag-ASN-Ni2+. To the best of our knowledge, there have been no re-
ports on light-enhanced chemisorption of molecules with irradiation of continuous-
wave visible-light of moderate intensity (ꢁ1 W cmꢀ2). Very recently, we reported that
a plasmonic alloy system can selectively concentrate the reactant molecules to cata-
lyst surface and thus increase the reaction rate by several orders under light.51 It is
proven that light irradiation generates an optical plasmon force that can add to
van der Waals force and selectively attract reactant molecules to the active sites.
In the plasmonic antenna system in the present study, similar optical plasmon force
contributes to concentrate the ether to catalyst surface and thus enhances the chem-
isorption of ether on Ni2+ sites. This provides essential knowledge on how visible-
light irradiation significantly promotes the catalytic performance of metal cations
by plasmonic metal.
Light-Intensity Dependence of Catalytic Performance
Following the chemisorption, the dependence of photocatalytic performance on the
intensity of the light source is also investigated in Figure 6. The temperature of the
reaction mixture was carefully maintained at 90ꢂC or 80ꢂC G 2ꢂC to guarantee that
thermal effects could be discounted. It is clear that the higher the light intensity, the
greater is the contribution of light irradiation to driving the reaction. When the light
intensity is 0.96 W cmꢀ2, over 90% of the conversion is due to light irradiation for the
C–O bond cleavage of benzyl phenyl ether. Clearly, light irradiation is the principle
driving force of the reaction. Higher light intensity generates more hot electrons in
metal NPs and also generate stronger EM field to increase the reactant chemisorp-
tion; thus, irradiation-induced enhancement of chemisorption and reactant conver-
sion are correlated.
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Figure 7. Effects of Light Wavelength for Benzyl Phenyl Ether Cleavage and the Energy
Alignment
(A and B) The dependence of photocatalytic activities of 2.5Au-ASN-Ni2+ (A) and 4.3Ag-ASN-Ni2+
(B) on the light-irradiation wavelength. Error bars associated with conversion and AQY (y axis on the
right-hand side) are the standard error of three sets of unique measurements. The light intensity for
all wavelengths is the same (0.2 W cmꢀ2). The UV-vis spectrum of the photocatalyst is also provided
for comparison.
(C) Energy diagram of d band and Fermi level of Au and Ni2+/Ni0 reduction potential.
(D) Energy diagram of d band and Fermi level of Ag and Ni2+/Ni0 reduction potential.
Correlating the light intensity dependence of the chemisorption and the reactant
conversion with observation that the catalytic performance is not regulated by the
overall Ni2+ content in the photocatalyst system (Figure 4A), we infer that the C–O
bond cleavage predominantly takes place on the Ni2+ sites within the hot spots,
where the ether molecules chemically adsorbed. The adsorbed ether molecules
will experience an intense oscillating EM field, which could facilitate the activation
of the molecules for the cleavage reaction. The hot spots can capture both light en-
ergy and reactant molecules, have higher concentration of hot electrons and thus,
are an ideal location for the catalysis.
Besides, as can be seen in Figure 6, increasing the light intensity to higher than
0.76 W cmꢀ2 resulted in a dramatic increase in the conversion of the C–O bond
cleavage reaction catalyzed by 2.5Au-ASN-Ni2+ (or 4.3Ag-ASN-Ni2+). There exists
an energy threshold to activation of benzyl phenyl ether molecule. It is between
0.59 and 0.76 W cmꢀ2 for 2.5Au-ASN-Ni2+ catalyst at 90ꢂC and between 0.76 and
0.96 W cmꢀ2 for 4.3Ag-ASN-Ni2+ at 80ꢂC. The intensity threshold is a common
feature in photocatalysis systems mediated by plasmonic NPs and influenced by
various factors.52
Influence of Light-Irradiation Wavelength (Energy Alignment)
The catalytic performance variation with the wavelength reveals the energy align-
ment of the combined system. The action spectrum (Figures 7A and 7B) shows the
dependence of the irradiation wavelength on the conversion efficiency of the
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photocatalytic reactions. The reaction rates were converted to the apparent quan-
tum yields (AQYs), which were calculated as
Ylight ꢀ Ydark
AQY ð%Þ =
3 100;
n
where the Ylight and Ydark are the amount of reactant being converted under light irra-
diation and dark conditions, respectively;15 n is the number of incident photons. The
number of reactant being cleaved in the dark, Ydark, was subtracted from that
observed when the system was irradiated, Ylight, to clearly illustrate the contribution
of light irradiation to the overall conversion. The AQY is a wavelength-dependent
quantity that is given by the ratio of the number of molecules produced to the num-
ber of incident photons. By definition, it is a quantity that is independent of the in-
tensity of the incident radiation, and its trend provides physical insight on the mech-
anism, accounting for the conversion of photonic energy into chemical potential
energy.
For the Ag-Ni2+ photocatalytic system, the highest conversion rate was achieved
when illuminating with a 400 nm peak wavelength (Figure 7B). According to the
UV-vis spectra of the photocatalyst in Figure 2H, the most intense LSPR absorption
of Ag NPs in this photocatalyst occurs at 405 nm, well matched with the LED wave-
length with which the highest cleavage reaction rate conversion was observed. Also,
the AQY spectrum generally matches the absorption of photocatalyst (Figure 7B).
Given the temperature of the reaction mixture and the irradiation energy of each
wavelength were carefully maintained identical, the significant AQY variation with
irradiation wavelength corroborates that the reaction was mainly driven by light.
The results evidence that the enhancement of the catalytic performance is caused
by the LSPR absorption of Ag NPs. The LSPR absorption in the range between 400
and 600 nm can generate hot electrons with sufficient energy to reduce Ni2+ in
the complexes.
For the 2.5Au-ASN-Ni2+ photocatalyst, the AQYs show a mismatch with the catalyst
light absorption: the highest conversion rate was achieved when illuminating with a
400 nm peak wavelength (single-color LED source with band width G 10 nm), while
there is no absorption peak at this wavelength (Figure 7A). Hence, light absorption is
not the sole factor determining the catalytic reaction over the small Au NPs. The high
AQY at 400 nm suggests that there is an energy alignment governing the electron
transfer: only hot electrons of sufficient energy can trigger the reaction that takes
place at Ni2+ complex sites.
According to the diagram shown in Figure 7C, photons with 397 nm wavelength
(ꢁ3.12 eV) are able to excite d electrons of Au NPs to the energy level for Ni2+ reduc-
tion. Photons with longer wavelengths have insufficient energy to reduce Ni2+ ions.
On the other hand, a much lower reaction rate is observed under irradiation with
wavelength < 380 nm (Figure 7A). It has been reported that visible-light illumination
can make the reduction potential of Ni2+ complex shift negatively.53 We also found
that the UV irradiation shifted Ni2+/Ni0 reduction potential by 0.2 eV increasing the
difficulty in reduction of Ni2+ complexes. Another possible reason is that light with
wavelength around 380 nm causes the 3A2g(F) / 3T1g(P) transition for the distorted
octahedral Ni2+ complex (which exhibits light absorption peaked at 378 nm). A frac-
tion of light can be absorbed for this transition; accordingly, fewer hot electrons are
generated. Consequently, the reduction of Ni2+ complex, which is critical for the
C–O bond cleavage as discussed later, proceeds at a slow rate. Therefore, there is
a narrow energy window at about 3.12 eV, in which photons generate hot electrons
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by interband electron excitation in the Au NPs to reduce Ni2+ ions driving the reac-
tion. The energy alignment in Figures 7C and 7D may not be strict as the alignment is
affected by other factors such as temperature, pH, and solvent. However, it provides
a good approximation of the alignment. As shown in Figures 7C and 7D, isopropanol
acts as a sacrificial electron donor and is oxidized into acetone on the metal
particles,2,15 accomplishing the electron-hole separation in high efficiency.
In contrast, the energy level of d band in silver is much lower than that in gold54 so
that the hot electrons generated by interband transition in Ag NPs do not have suf-
ficient energy to reduce Ni2+ ions directly (Figure 7D).
Interband Electron Excitation of Small Au NPs
The catalysts with Au NPs or Ag NPs have similar architecture but distinct differences
in LSPR absorption characteristics. The weak LSPR absorption of the small Au NPs
(seen in Figure 2G) can be expected to generate a much weaker EM near-field.
The EM field enhancement at the hot spots of small Au NP dimer is also predicted
to be weak (Figure S10). Hence, the LSPR absorption is possibly too weak to be
the main driving force for promoting the reaction. Light-excited interband transition
(from d band to sp band) in the small Au NPs should be main mechanism for gener-
ating hot electrons that induce the catalytic reaction.
Interestingly, light-induced adsorption phenomenon occurred in 2.5Au-ASN-Ni2+
photocatalytic system as well (Figure S13). It is known that metal NPs produce large
field gradients in a wide wavelength range (not only LSPR wavelength)55 as the oscil-
lating EM field of incident light always changes the charge distribution of metal NPs.
Also, electrical charges tend to accumulate in sharp edges and high curvature sur-
face of metal particles (lightening rod effect). The higher charge density at surface
of the smaller particles results in sharper gradient around the particles. The two ef-
fects are applicable to small Au NPs, leading to the increasing chemisorption under
light irradiation.
Molecular Bridge for the Hot Electron Transfer and Ni2+ Reduction
The above observation implies that the reaction is operated via the transfer of en-
ergy and light-excited hot electrons from metal NPs to Ni2+ active sites, modulating
the oxidation state of Ni2+ in the process. In many reactions catalyzed by the transi-
tion-metal complexes, the change in the oxidation state of the transition metal en-
ables the bond-breaking or bond-forming processes (oxidative addition and reduc-
tive elimination).23
The electron paramagnetic resonance (EPR) spectra of the photocatalysts shown in
Figure 8 support our inference. The EPR spectra of the photocatalysts (4.3Ag-ASN-
Ni2+ and 2.5Au-ASN-Ni2+) with the reactant benzyl phenyl ether and solvent isopro-
panol (IPA) after purple light irradiation (400 nm of wavelength) exhibits a six-line
signal (traces b and d in Figure 8), which is similar to that observed from the ASN-
Ni2+ sample after reduction in H2 atmosphere at 200ꢂC (trace e in Figure 8). The
g-factor (ꢁ2.01) is close to the reported Ni NPs with a characteristic of paramagnetic
behavior.56 So a fraction of the nickel in this H2 reduced sample exists in the Ni0
state. The paramagnetic signals are not observed in the spectra of other samples,
even the system of the photocatalysts in IPA solvent after light irradiation (without
the ether, see Figure S14). The results indicate that in the presence of the ether, a
Ni2+ / Ni(0) transformation takes place in the photocatalyst under irradiation of
400 nm. The hot electrons generated on the illuminated metal NPs migrate to the
Ni2+ complex and reduce Ni2+ ion to Ni0. However, the results shown in trace d in
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Figure 8. EPR Spectra of the Photocatalytic System before and after Irradiation under Light with
400 nm Wavelength
The concentration of a-O-4 reactant is 0.1 M; the EPR measurement was conducted at room
temperature. Inset: schematically shown the reactant molecular serves as a ‘‘molecular bridge’’ for
the transfer of the photo-induced hot electrons from the Ag(Au) NPs to Ni2+ sites.
Figure S14 that direct electron transfer from Ag NPs to the Ni2+ complex is not evi-
denced by the EPR spectrum of the photocatalyst dispersed in IPA solvent in the
absence of a-O-4 reactant under irradiation of 400 nm wavelength. It seems that
the ether molecules are involved in the reduction of Ni2+ to Ni0. Surprisingly, we
observed the Ni2+ / Ni0 reduction after illumination, when the photocatalyst was
dispersed in benzotrifluoride (BTF) or toluene (traces f-i in Figure 8) in the absence
of the ether, but this phenomenon was not observed when BTF was replaced with
N,N-dimethyl-formamide (DMF) (traces j and k in Figure 8). It has been reported
that for the C-C stretching modes of unsaturated hydrocarbons, temporary electron
transfer from plasmonic metal NPs into the normally unoccupied anti-bonding p* or-
bitals can happen in a SERS system.57 Therefore, the aromatic ring in the molecules
of the aryl ether, BTF, and toluene could serve as a ‘‘molecular bridge’’ for the trans-
fer of the photo-induced hot electrons from the plasmonic metal NPs to Ni2+ sites
(inset in Figure 8). The hot electrons have sufficient energy to migrate to the unoc-
cupied anti-bonding p* orbitals of the aromatic ring.
For the Au-Ni2+ system, the ether molecules are also necessary for the charge trans-
fer process to produce Ni0 state species (comparing the results of trace d in Figure 8
and trace a in Figure S14). So generation of hot electrons on Au NPs surface is a pre-
requisite but not sufficient condition. The transfer of the hot electrons plays a critical
role in the Ni2+ reduction.
No products were detected when toluene or BTF was used as the solvent (Table S5),
suggesting that the IPA is necessary in the reaction as the hydrogen source for the
reductive cleavage. The reducing agent IPA adsorbed on the surface of Ag NPs or
Au NPs releases H atoms at the surface and is itself oxidized as a sacrificial electron
donor in the presence of KOH and acetone was detected.2 Using IPA as reduction
agent avoids high pressure H2 that has a safety risk. The aryl ether molecules adsorbed
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at Ni2+ complex sites, and molecules with aromatic rings could assist transfer of the
light-generated hot electrons from the illuminated NPs to Ni2+ sites. The reduced
Ni0 state is chemically unstable in the reaction environment because the reductive po-
tential of Ni0/NiII (ꢀ1.2 V versus SCE)28 is more negative than that of benzyl phenyl
ether (ꢀ0.62 V versus SCE, seen in Figure S15). Therefore, the reduced Ni0 sites could
catalyze the cleavage of the C–O bonds in the aryl ether compound. Given the depen-
dence of the chemisorption on the EM near-field intensity and the required molecular
bridge for the hot electron transfer, the reaction should take place predominately at
the Ni2+ complex sites which are simultaneously close to the metal NPs and subject
to intense EM fields, i.e., within plasmonic hot spots.
The efficiency of electron transfer by benzyl phenyl ether bridge should be also
correlated to the ratio of NP to Ni2+. The mean density of the Ag NPs and Au NPs
are estimated and summarized in the Table S6 and correlated to mean densities
of APTMS and Ni complexes. The distributions of supported NPs and Ni2+ com-
plexes based on the mean densities are presented as below. The densities are aver-
aged data, summarized from TEM images in Figures 2, S8, and S9. The Ag NPs den-
sity in 4.3Ag-ASN-Ni2+ sample is 8 times of Ag NPs density in 1.3Ag-ASN-Ni2+
sample. The density of the Ag and Au NPs determines the ratio of NP:Ni2+. The
higher the NP:Ni2+ ratio, the higher the efficiency of electron transfer by benzyl
phenyl ether bridge. Thus, the electron transfer efficiency in 4.3Ag-ASN-Ni2+ is
much higher than in 1.3Ag-ASN-Ni2+ because there are 36 Ni2+ corresponds to
per Ag NP in 4.3Ag-ASN-Ni2+ and 508 Ni2+ corresponds to per Ag NP in 1.3Ag-
ASN-Ni2+. Similarly, the Au NP:Ni2+ ratio in 8.4Au-ASN-Ni2+ is much higher (1:10)
than the other two catalysts, 1:17 and 1:47. The electron transfer by benzyl phenyl
ether bridge in 8.4Au-ASN-Ni2+ and 2.5Au-ASN-Ni2+ catalysts is more efficient
than that in 0.7Au-ASN-Ni2+ catalyst.
The Mechanism of the Cleavage Reaction
On the basis of these results, we propose a tentative reaction pathway for the cleav-
age, the main features are depicted in Figure 9. The aryl ether molecules first diffuse
within the framework of the photocatalyst and physically adsorb onto the catalyst
surface (the adsorption observed in the dark; Figure 5B). Visible-light irradiation in-
duces chemisorption of the ether at Ni2+ complex sites (i-ii), the ether molecules co-
ordinate to Ni2+ ions replacing the NO3ꢀ ions. Then photo-generated hot electrons
transfer to Ni2+ sites (via the unoccupied anti-bonding p* orbitals of the aromatic
ring of the ether) and reduce the ions to Ni0 species (iii). The reduced Ni0 species
can catalyze the C–O bond cleavage, as described by literature reports,37,58 yielding
the final products (v). Here, IPA is oxidized on the plasmonic metal NPs (iv), similar to
the mechanism reported.2 The IPA oxidation releases hydrogen and electrons,
which are needed in hydrogenolysis. The synergistic effect between the optical an-
tenna function of plasmonic metal NPs and the catalytic ability of Ni2+ sites domi-
nates the transformation of the aryl ether. The synergistic effect is accredited to
light-induced chemisorption of reactant molecules and the generation and function
of the hot electrons as shown in Figure 9.
The stability and recyclability of the catalysts investigated. The silane was grafted
onto the Al2O3 support in toluene at 110ꢂC, so that the Al–O–Si bond is stable at
the reaction temperatures (80ꢂC and 90ꢂC). We compared the contents of Si in
4.3Ag-ASN-Ni2+ and reused 4.3Ag-ASN-Ni2+ photocatalysts measured by the
EDX (Figure S16), they are similar (2.2–2.4 wt %). The recycle stability of the photo-
catalyst is illustrated in Figure S16. The 4.3Ag-ASN-Ni2+ catalyst was reused for 3 cy-
cles of C–O bond cleavage of benzyl phenyl ether under light irradiation. The
16
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Figure 9. Proposed Mechanism for the Photocatalytic C–O Bond Cleavage of Benzyl Phenyl Ether with the Photocatalyst
product conversion moderately decreased during recycling, and the leaching of
Ni2+ ions during the reaction caused the decrease in Ni content of the used catalyst.
The soluble Ni2+ ions were not active for the hydrolysis at 80ꢂC. The decrease could
also be due to the partial loss of Ag NPs on the external surface of aggregates of
randomly oriented fibers and leaching of nickel, thus lowering the chemisorption
of reactant amount and formation possibility of Ni0 species, as seen in the EDS map-
ping result displayed in the figure.
CONCLUSIONS
In summary, with the designed photocatalyst structure we have demonstrated that
Au NPs or Ag NPs can act as optical antennas to absorb visible-light and promote
the catalytic performance of Ni2+ complex immobilized in the hot spots. The new
plasmonic-antenna-promoted catalysts with 2.5 wt % of small Au NPs (ꢁ 2 nm) or
4.3 wt % of Ag NPs (ꢁ 9 nm) exhibited superior catalytic activity for the cleavage
of relative stable C–O bonds in benzyl phenyl ether under visible-light irradiation
and mild reaction conditions without reduction of the aromatic rings. The results
signify a new mode to activate chemical reactions by combining the advantages
of plasmonic metal NPs and the chemical bond formation ability of transition-metal
complexes, which is complementary to known plasmonic catalysis and transition-
metal complex catalysis.
We have provided strong evidence suggesting that the catalytic performance is
enhanced by the synergy of the following two effects: hot-electron transfer to the
catalytically active Ni2+ complex sites and light-enhanced chemisorption of reactant
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species. Both of these effects are greatly augmented by the strong EM near-field
localization attainable with plasmonic hot spots. The energy alignment is commonly
a key issue for the hot-electron transfer in the new photocatalysts. We believe these
results will stimulate further research into the creation of novel plasmonic-antenna-
promoted catalysts that exploit the high chemical reactivity of transition-metal
complexes.
EXPERIMENTAL PROCEDURES
Materials
The chemicals were purchased from commercial suppliers and used as provided:
benzyl phenyl ether (Sigma-Aldrich, >98%), isopropanol (Sigma-Aldrich, >99.5%,
anhydrous), toluene (Fisher, >99.99%, GC assay), N,N-dimethyl formamide
(Sigma-Aldrich, >99.8%, anhydrous), a,a,a-trifluorotoluene (Sigma-Aldrich, >99%,
anhydrous), acetic acid (Ajax Finechem, >99.7%), nitric acid (Ajax Finechem, 68%–
70%), C12-14H25-29O(CH2CH2O)5H surfactant (Sigma-Aldrich), (3-aminopropyl)trime-
thoxysilane (Sigma-Aldrich, >97%), potassium hydroxide (Sigma-Aldrich, >99.99%),
phenol (Ajax Finechem, AR), silver nitrate (Merck, AR), gold chloride trihydrate
(Sigma-Aldrich, >99.9%), nickel(II) nitrate hexahydrate (Scharlau, >98%), sodium
borohydride (Sigma-Aldrich, >98%), sodium aluminate (Sigma-Aldrich, anhydrous),
H2 (Supagas, >99.999%), and Ar (Supagas, >99.99%)
Catalyst Preparation
The photocatalysts were prepared following the illustration shown in Figure S1.
g-Al2O3 nanofibers were prepared following a procedure reported previously59
and used as support of the catalysts. Boehmite (with a chemical formula of AlOOH)
nanofibers were prepared from NaAlO2 and then converted to g-Al2O3 fibers by
calcination at 450oC for 5 h. The details of the fiber preparation are provided in
the Supplemental Information. Amino groups were then grafted on 3.0 g of
g-Al2O3 fibers by refluxing in 50 mL of toluene solution of 9.7 mmol of (3-amino-
propyl)trimethoxysilane (APTMS) for 40 h. The solid sample was collected by
washing and filtrating with H2O and ethanol, and finally dried at 60oC (Al2O3-
silane-NH2 is abbreviated to ASN).
Plasmonic Ag or Au NPs were prepared on ASN support by an impregnation-reduc-
tion procedure. For example, in the synthesis of 1.3 wt % of Ag NPs on ASN support,
1.0 g of the ASN support was dispersed into 133 mL of deionized water under
vigorous stirring with a magnetic stirrer for 20 min. 27.8 mL of 0.01 M AgNO3
aqueous solution was then added to the suspension and stirred for further 20 min.
Next, 74 mL of NaBH4 (0.038 M) aqueous solution was added dropwise to the sus-
pension over 30 min under continuous stirring. The suspension was aged overnight,
and then the solid was separated by washing with water and dried at 60ꢂC under vac-
uum. The obtained sample was labeled as 1.3Ag-ASN.
The Ni2+ ions were introduced via complexation with the free amine groups of the
silane grafted on the g-Al2O3 fibers. The procedure for introducing Ni2+ ions to
Ag-ASN samples is as follows: 0.5 g of obtained sample with supported Ag NPs
(xAg-ASN, where x denotes the weight percentage of silver in the catalysts) was
mixed with 30 mL of Ni(NO3)2 aqueous solution (0.017 M) by a shaker for 24 h at
room temperature. Then the solid was washed with water 3 times before drying at
60oC under vacuum. The catalysts obtained were labeled as xAg-ASN-Ni2+. This
procedure immobilizes Ni2+ complexes on the sample surfaces, including the narrow
gaps between metal NPs, where hot spots are likely to be generated. The g-Al2O3
nanofibers are ꢁ5 nm thick and 100 nm long, which were sintered to form a highly
18
Chem 5, 1–21, November 14, 2019

Please cite this article in press as: Han et al., Promoting Ni(II) Catalysis with Plasmonic Antennas, Chem (2019), https://doi.org/10.1016/
j.chempr.2019.07.022
porous framework of randomly oriented fibers.40 The fibers possessed a large spe-
cific surface area of 260 m2$gꢀ1, as seen in Table S2 and Figure S2, where most of the
surface area was available for grafting of the silane possessing an NH2 group to form
Ni2+ complexes and immobilizing plasmonic NPs. Moreover, the cage-like structure
(see Figure 1) could confine the NPs formed within the structure and allow reactant
molecules to readily diffuse to the Ni2+ complex reaction sites in the vicinity of the
metal NPs through inter-fiber voids.
Photocatalytic Reactions
The photocatalytic reaction was conducted in a light reaction chamber. A 10 mL Pyrex
glass tube was used as the reaction container. After adding the reactants and catalyst,
the tube was filled with argon and sealed with a rubber septum cap. Then the tube was
placed above a magnetic stirrer with stirring, and illuminated under a halogen lamp (Phi-
lips Industries: 500W, wavelength in the range of 400–750 nm). An air conditioner was
set to the light reaction chamber to control the reaction temperature. Reactions were
also conducted in the dark at the same temperature for comparison. The reaction tem-
perature in the dark was maintained the same as the reaction under irradiation. All the
reactions in the dark were conducted using an oil bath placed on a magnetic stirrer. The
tube was wrapped with aluminum foil to avoid exposure of the reaction to light. After the
reaction, the mixture was collected and filtered through a Millipore filter (pore size
0.45 mm) to remove the solid photocatalyst. The products were analyzed by an Agilent
6890 gas chromatography (GC) with HP-5 column. An Agilent HP5973 mass spectrom-
eter was used to identify the product. All the products concentrations were calibrated
with an external standard method.
Catalyst Characterization
XRD patterns of the samples were recorded on a Philips PANalytical X’Pert PRO
˚
diffractometer using CuKa radiation (l=1.5418 A). The working power was 40 kV
and 40 mA. The diffraction data were collected from 10ꢂ to 80ꢂ with a resolution
of 0.01ꢂ (2q). FT-IR measurements were conducted on PerkinElmer Spectrum.2
The samples were prepared in KBr pellets and stabilized under controlled relative
humidity before acquiring the spectrum. Attenuated total reflectance Fourier trans-
form infrared (ATR-FTIR) spectra were collected by a Nicolet-5700 spectrometer in
the wavenumber range between 4,000 and 620 cmꢀ1 at resolution 4 cmꢀ1. The
particle size and morphology of the catalyst samples was characterized with a
JEOL2100 transmission electron microscope, equipped with
a Gatan Orius
SC1000 CCD camera. Nitrogen physisorption isotherms were measured at
ꢀ196ꢂC on the Tristar II 3020. Prior to each measurement, the sample was
degassed at 120ꢂC for 24 h under vacuum. The specific surface areas of the sam-
ples were calculated by using the Brauner-Emmet-Teller (BET) method and the ni-
trogen adsorption data in a relative pressure (P/Po) range between 0.05 and 0.2.
Varian Cary 5000 spectrometer was used to collect the data for the diffuse reflec-
tance UV-visible (DR-UV-vis) spectra of the samples. EPR spectra were recorded
with a Burker EPR ELEXSYS 500 spectrometer operating at a frequency of 9.5
GHz in the X-band mode. Metal content in catalysts were obtained by inductively
coupled plasma optical emission spectrometry (ICP-OES) carried out on a R4 Per-
kinElmer ICP-OES 8300DV instrument. Prior to analysis, the powder samples were
dissolved in HNO3 (70%) and diluted by deionized water. Electrochemical mea-
surements were conducted with Bio-Logic SAS potentiostat model VSP. All Raman
spectra were taken with a 532 nm excitation laser, 10 s exposure, one-time accumu-
lation, and a Raman imaging setup based on a Renishaw Invia Raman microscope
was used.
Chem 5, 1–21, November 14, 2019
19

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j.chempr.2019.07.022
SUPPLEMENTAL INFORMATION
Supplemental Information can be found online at https://doi.org/10.1016/j.chempr.
2019.07.022.
ACKNOWLEDGMENTS
We acknowledge financial support from the Australian Research Council
(DP150102110). The electron microscopy work was performed through a user proj-
ect supported by the Central Analytical Research Facility (CARF), Queensland Uni-
versity of Technology.
AUTHOR CONTRIBUTIONS
P.F.H. performed all the experiments. H.Y.Z. and S.S. supervised the project. E.W.
and Q.X. provided valuable discussion and revised the manuscript. D.G. provided
simulation of EM field. The manuscript was written through contributions of all
authors.
DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: March 24, 2019
Revised: June 12, 2019
Accepted: July 25, 2019
Published: August 22, 2019
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