Plasmonic Photocatalysis of Urea Oxidation and Visible-Light Fuel Cells

Article abstract of DOI:10.1016/j.chempr.2019.06.014

The intense electric (E-) field associated with the localized surface plasmon resonance (LSPR) of noble-metal nanoantennas provides a rational strategy for enhancing photoinduced charge transfer in photocatalysts. Here, we demonstrate E-field-enhanced direct photocatalytic urea oxidation and a visible-light-driven direct urea fuel cell (LDUFC) with tris(bipyridine)ruthenium(II) ([Ru(bpy)₃]²⁺)-enabled plasmonic nanopigments that contain a phospholipid membrane self-assembled around a Ag nanoparticle (NP) whose LSPR overlaps the [Ru(bpy)₃]²⁺ metal-to-ligand charge transfer (MLCT). In the hierarchical plasmonic nanopigment design, the membrane serves as scaffold and spacer to localize [Ru(bpy)₃]²⁺ in an electromagnetic “sweet spot” where substantial plasmonic enhancement of photoexcitation is achieved while strong metal-associated quenching of the reactive excited state is avoided. The demonstration of plasmon-enhanced photocatalytic urea oxidation and the implementation of the LDUFC represent important advancements toward improved light-driven waste-water treatment and efficient solar energy conversion. Plasmon resonances in noble-metal nanoparticles (NPs) provide unique enhancement mechanisms for both charge- and energy-transfer processes and thus offer a tunable strategy for improving the versatility and efficiency of versatile photocatalysts through their large optical cross-sections. The nanopigment architecture developed in this study represents a broadly applicable platform for improving intramolecular charge-transfer-facilitated photocatalysis and provides a blueprint for efficient and specific biomimetic nanoreactors that store absorbed light energy in chemical bonds. We apply [Ru(bpy)₃]²⁺-containing plasmonic nanopigments to implement a spontaneous visible-light and/or solar-driven fuel cell that converts the ubiquitous waste molecule urea into electric energy. This plasmon-enhanced fuel cell marks an important step in the development of next-generation green energy and water-purification technologies that can help to preserve valuable resources and reduce environmental pollution. An increasing world population requires new renewable energy technologies and efficient strategies to minimize environmental pollution. This work introduces plasmonic nanoreactors that can convert the ubiquitous waste molecule urea into electrical energy through absorption of light. Our work utilizes the electromagnetic enhancement of silver nanoparticles to enhance the efficacy of a molecular photocatalyst in a hierarchical nanopigment architecture to enable light-driven urea oxidation and direct urea fuel cells. We demonstrate substantial plasmonic enhancement on both the photoexcitation and photoreactivity.

Full text of DOI:10.1016/j.chempr.2019.06.014

Article
Plasmonic Photocatalysis of Urea Oxidation
and Visible-Light Fuel Cells
Xingda An, David Stelter, Tom
Keyes, Bjorn M. Reinhard
¨
bmr@bu.edu
HIGHLIGHTS
Efficient photocatalysis of urea
oxidation and light-driven fuel cell
Plasmonic E-field enhancement of
intramolecular charge transfer
Self-assembled hierarchical
plasmonic nanopigment
architecture
Plasmonic enhancement of
[Ru(bpy)3]2+ quantum yield
An increasing world population requires new renewable energy technologies and
efficient strategies to minimize environmental pollution. This work introduces
plasmonic nanoreactors that can convert the ubiquitous waste molecule urea into
electrical energy through absorption of light. Our work utilizes the
electromagnetic enhancement of silver nanoparticles to enhance the efficacy of a
molecular photocatalyst in a hierarchical nanopigment architecture to enable
light-driven urea oxidation and direct urea fuel cells. We demonstrate substantial
plasmonic enhancement on both the photoexcitation and photoreactivity.
An et al., Chem 5, 1–15
August 8, 2019 ª 2019 Elsevier Inc.
https://doi.org/10.1016/j.chempr.2019.06.014

Please cite this article in press as: An et al., Plasmonic Photocatalysis of Urea Oxidation and Visible-Light Fuel Cells, Chem (2019), https://doi.org/
10.1016/j.chempr.2019.06.014
Article
Plasmonic Photocatalysis of Urea
Oxidation and Visible-Light Fuel Cells
Xingda An,1,2 David Stelter,1 Tom Keyes,1 and Bjorn M. Reinhard1,2,3,
*
¨
SUMMARY
The Bigger Picture
Plasmon resonances in noble-
metal nanoparticles (NPs) provide
unique enhancement mechanisms
for both charge- and energy-
transfer processes and thus offer a
tunable strategy for improving the
versatility and efficiency of
The intense electric (E-) field associated with the localized surface plasmon reso-
nance (LSPR) of noble-metal nanoantennas provides a rational strategy for
enhancing photoinduced charge transfer in photocatalysts. Here, we demon-
strate E-field-enhanced direct photocatalytic urea oxidation and a visible-
light-driven direct urea fuel cell (LDUFC) with tris(bipyridine)ruthenium(II)
([Ru(bpy)3]2+)-enabled plasmonic nanopigments that contain a phospholipid
membrane self-assembled around a Ag nanoparticle (NP) whose LSPR overlaps
the [Ru(bpy)3]2+ metal-to-ligand charge transfer (MLCT). In the hierarchical plas-
monic nanopigment design, the membrane serves as scaffold and spacer to
localize [Ru(bpy)3]2+ in an electromagnetic ‘‘sweet spot’’ where substantial plas-
monic enhancement of photoexcitation is achieved while strong metal-associ-
ated quenching of the reactive excited state is avoided. The demonstration of
plasmon-enhanced photocatalytic urea oxidation and the implementation of
the LDUFC represent important advancements toward improved light-driven
waste-water treatment and efficient solar energy conversion.
versatile photocatalysts through
their large optical cross-sections.
The nanopigment architecture
developed in this study represents
a broadly applicable platform for
improving intramolecular charge-
transfer-facilitated photocatalysis
and provides a blueprint for
efficient and specific biomimetic
nanoreactors that store absorbed
light energy in chemical bonds.
We apply [Ru(bpy)3]2+-containing
plasmonic nanopigments to
INTRODUCTION
Solar energy conversion through photocatalysis of chemical bond formation and
photo fuel cells are examples of photo-harvesting processes that benefit from opti-
mized photocatalysts. Intriguingly, the excitation of localized surface plasmon res-
onances (LSPRs) in noble-metal nanoparticles (NPs) provides unique opportunities
for enhancing photochemistry through different forms of plasmonic catalysis.
While plasmon decay-induced hot charge-carrier formation has been demon-
strated to contribute to noble-metal direct heterocatalysis1–3 and catalysis facili-
tated by metal-semiconductor junctions,4–8 the resonant electric (E-) fields gener-
ated from LSPRs offer a powerful alternative approach for amplifying molecular
transitions-mediated photocatalysis.9–15 A major challenge for E-field-driven plas-
monic catalysis concepts is, however, the need to balance E-field-enhanced ab-
sorption rates with the quenching of the photoexcited state through the metal.
In practice, for NPs with diameters of around 45 nm, as used in this work, separa-
tions between the NP surface and photocatalysts < 5 nm are required to substan-
tially boost absorption through the strong local E-field,4,16–19 but for even smaller
separations (<2 nm), strong quenching of excited states through the metal surface
becomes dominant.20–22 The viability of plasmonic E-field enhancement of photo-
catalysis has been demonstrated primarily with semiconductor photocatalysts.11–15
For less robust but potentially more versatile and selective photoreactive molecu-
lar catalysts, the effect of the E-field enhancement is less well understood,9,10 and
the realization of enhanced hybrid systems is challenged by the need for reliable
positioning of the photocatalysts at desirable separations from the metal NP
surface.
implement a spontaneous visible-
light and/or solar-driven fuel cell
that converts the ubiquitous waste
molecule urea into electric
energy. This plasmon-enhanced
fuel cell marks an important step
in the development of next-
generation green energy and
water-purification technologies
that can help to preserve valuable
resources and reduce
environmental pollution.
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In this manuscript, we explore the quantitative optimization of plasmonic catalysis
mediated by E-field-enhanced intramolecular metal-to-ligand charge transfer
(MLCT) for direct photocatalysis of urea oxidation reaction (UOR). We evaluate the
design of a hierarchical nanopigment architecture that incorporates (1) a molecular
photocatalyst, (2) a spherical Ag NP as an effective nanoantenna, and (3) a lipid
membrane layer as both the scaffold and molecular spacer. The self-assembled lipid
layer provides a facile and effective strategy to facilitate the localization of the pho-
tocatalyst at a separation between 2–4 nm from the Ag surface, which represents a
‘‘sweet spot’’ for plasmonic enhancement because it is close enough to the NP sur-
face to provide high E-field intensity for the resonant enhancement of photoexcita-
tion but sufficiently far away to limit the quenching of photoexcited states. For the
photocatalyst, we chose trisbipyridine-ruthenium(II), [Ru(bpy)3]2+, which derives its
photoreactivity from a light-mediated MLCT absorption23,24 that overlaps the
LSPR of the Ag NPs at around 430 nm. [Ru(bpy)3]2+ and analogs play an important
role in photoelectrochemical (PEC) conversions either as photosensitizers25–27 or
as photocatalysts in solar-to-fuel conversion.28–30 The light-harvesting efficiency of
[Ru(bpy)3]2+ is, however, fundamentally limited by the moderate extinction coeffi-
cient of its MLCT band (4,600–11,000 M–1cm–1),31,32 which is much lower than those
of high-performance dyes (>200,000 M–1cm–1).33,34 We test the hypothesis that the
E-field-mediated enhancement of the MLCT photoexcitation of [Ru(bpy)3]2+ in plas-
monic nanopigments can close this gap and achieve a measurable increase in
photoreactivity. We implement a robust visible-light fuel cell that achieves energy
conversion through photo-oxidation of the ubiquitous waste molecule urea. To
the best of our knowledge, our work presents the first example of a direct photoca-
talytic UOR as well as a first light-driven direct urea fuel cell (LDUFC).
RESULTS AND DISCUSSION
Assembly and Structural Characterization of Plasmonic Nanopigments
Artificial lipid membranes and micelles that mimic the hierarchical structure of bio-
logical photosynthetic systems have long been important components of man-
made reactive and energy conversion systems.35–38 In our design of nanopigments,
we utilized a self-assembled lipid membrane architecture around a Ag NP core to
immobilize [Ru(bpy)3]2+ in the near field of the Ag NP. Lipid wrapping was achieved
through modified one-pot synthesis39–42 (see Supplemental Experimental Proced-
ures). We adopted a lipid membrane composition based on a simplified biological
membrane model containing 45 mol % zwitterionic lipid DPPC (1,2-dipalmitoyl-sn-
glycero-3-phosphocholine), 5 mol % negatively charged lipid DOPS (1,2-dioleoyl-
sn-glycero-3-phospho-L-serine), 40 mol % lubricant molecule cholesterol, and
10 mol % [Ru(bpy3)]2+. These lipids were tethered to the NP core through an inter-
mediate octadecanethiol (ODT) layer chemisorbed to the Ag NP cores (Figure 1A).
In order to understand the equilibrium of [Ru(bpy3)]2+ in the lipid membrane,
we calculated the potential of mean force (PMF) of a [Ru(bpy3)]2+ complex from
a steered molecular dynamics simulation by using Jarzynski’s equality (Equa-
tion S1)43–45 (Figure 1B; see also Supplemental Experimental Procedures). We first
reconstructed an area of the nanopigments with Ag atoms, ODT, lipid layer, and
[Ru(bpy3)]2+ at experimental proportions. [Ru(bpy3)]2+ was allowed to equilibrate
in the local lipid membrane environment. We then pulled the molecule along the
1Department of Chemistry, Boston University,
Boston, MA 02215, USA
cross-section of the lipid membrane to calculate the nonequilibrium work (W) as a
function of distance to the metal NP surface. W is related to the equilibrium free-en-
ergy difference (DF, PMF) using an exponential average (Equation S1). The resulting
PMF plot exhibits a minimum at around 2 nm, indicating a preferential localization
of [Ru(bpy3)]2+ close to the membrane surface at the interface formed by the hydro-
philic lipid headgroups and the hydrophobic lipid tails.
2Photonics Center, Boston University, Boston,
MA 02215, USA
3Lead Contact
*Correspondence: bmr@bu.edu
https://doi.org/10.1016/j.chempr.2019.06.014
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Figure 1. Design of the Nanopigment
(A) Scheme for the nanopigment hierarchical structure. Zoomed-in view: simulated surface
morphology diagram of the nanopigment. Water molecules are not shown.
(B) Plot of potential of mean force (PMF) of the [Ru(bpy)3]2+ molecule over lipid membrane cross-
sectional distance from NP surface.
(C) Normalized UV-vis absorbance spectra of 44 nm Ag NP colloid and [Ru(bpy)3]2+ water solution.
Inset: molecular structure of [Ru(bpy)3]Cl2.
We utilized Ag NP cores as effective nanoantennas. Because of their large polariz-
ability46 and optical cross-sections47,48 (absorption cross-section > 3 3 10–9 m2;
see Supplemental Experimental Procedures for calculations), Ag NPs can efficiently
generate localized incident E-field intensity within a few nanometers of their sur-
face.49,50 We chose NPs with an average hydrodynamic diameter of 44.23 G
0.62 nm (from dynamic light scattering [DLS]) because NPs with much smaller
diameters generate lower E-field intensities and suffer from higher dissipative los-
ses,7,49,51,52 whereas larger NPs typically show retardation and high radiative los-
ses.53,54 As expected, the LSPR absorbance spectrum of the Ag NP cores showed
good spectral overlap with the [Ru(bpy)3]2+ MLCT band at 415–450 nm (Figure 1C).
To validate the successful nanopigment formation, we added small amounts of fluo-
rescent lipid dye (Lissamine rhodamine DSPE) into the lipid membrane and imaged
the nanopigments with correlated dark-field and fluorescence microscopy (Fig-
ure 2A). The optical colocalization of dark-field (top, NP cores) and fluorescence
(middle, membrane) signals confirmed a successful membrane assembly around
the Ag NPs. We used transmission electron microscopy (TEM) and high-resolution
TEM (HRTEM) to characterize the surface and membrane morphology of the plas-
monic nanopigments. A uniform lipid membrane was clearly visible around the NP
core in the TEM (Figure S1A) and HRTEM (Figure 2B) images. A statistical analysis
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Figure 2. Structural Characterizations of the Nanopigments
(A) Darkfield image (top), fluorescence image (middle), and merge (bottom) of an area with nanopigments under inverted microscope. Scale bars: 1 mm.
(B) HRTEM image of a nanopigment particle. Scale bar: 10 nm. See also Figure S1.
(C) STEM image of nanopigments with scan area (orange rectangle) for EDX element mapping scans. Scale bar: 50 nm.
(D and E) EDX elemental scan maps with (D) Ag L edge and (E) Ru L edge. Scale bars: 5 nm.
(F) DLS results of hydrodynamic diameters of nanopigments and nanocomposite controls (columns) and zeta potentials of corresponding liposomes
before incorporation of [Ru(bpy)3]2+ (scatter plot).
(G) Inductively coupled plasma-mass spectrometry (ICP-MS) results of incorporated 101Ru concentrations of nanopigments and nanocomposite controls.
(F and G) All percentages are mol %; the group with 5% DOPS represents standard nanopigment composition (red). Error bars represent standard
deviations of three measurements.
of the thickness of the membrane for 70 particles showed an average membrane
width of 3.8 G 1.4 nm, defining an upper boundary for the distance between
[Ru(bpy3)]2+ and Ag NPs. In further proof of [Ru(bpy3)]2+ binding, we mapped Ag
and Ru across individual nanopigment particles by using energy-dispersive X-ray
spectroscopy (EDX) under scanning transmission electron microscopy (STEM)
mode. The resulting elemental maps confirmed the coexistence of Ag (Figure 2D)
and Ru (Figure 2E) within the boundary of the particle, providing additional experi-
mental proof of a successful integration of [Ru(bpy3)]2+
.
Experimental Characterization of [Ru(bpy)3]2+ Localization in the Lipid
Membrane
Because of its amphiphilic nature, the nanopigment membrane provides both elec-
trostatic and hydrophobic binding regions for [Ru(bpy3)]2+. To probe the contribu-
tions of the two factors, we analyzed [Ru(bpy3)]2+ binding to two control groups:
(1) nanocomposites with different surface charges but hydrophobic tail groups iden-
tical to those of the nanopigments and (2) nanocomposites that lack the hydropho-
bic region but have surface charge similar to that of the nanopigments. We obtained
(1) by varying the amount of negatively charged lipid DOPS or by replacing DOPS
with cationic lipid EPC (1,2-dioleoyl-sn-glycero-3-ethylphosphocholine); we
achieved (2) by passivating Ag NPs cores with thiolated polyethylene glycol (PEG)
with carboxyl end groups (HS-PEG-COOH, Mw = 1,000). The size, surface charge,
and amount of membrane-bound 101Ru are summarized in Figures 2F and 2G.
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Before the incorporation of [Ru(bpy)3]2+, the zeta potential of the liposomes became
more negative as the amount of DOPS increased and more positive as EPC
increased. With increasing negative potential, we observed an overall increase
in 101Ru concentration, indicating a contribution from Coulomb attraction in
[Ru(bpy)3]2+ binding. NPs without DOPS (0 mol %) deviated from this overall trend.
This group exhibited substantial aggregation due to the lack of interparticle electro-
static repulsion, and we attributed the high levels of 101Ru in this case to the inclusion
of catalyst during agglomeration. Intriguingly, the PEG-passivated nanocomposite
control showed a 48% decrease in 101Ru concentration in the absence of the hydro-
phobic tail group of the lipid layer relative to the nanopigments even though it had a
similar zeta potential. These results are in good agreement with the membrane equi-
librium simulations and PMT calculations in Figures 1A and 1B and indicate a struc-
tural model in which [Ru(bpy3)]2+ is partially embedded in the membrane. COOÀ
anions from head groups of membrane DOPS pinned the Ru(II) cationic center to
the interface between hydrophobic and hydrophilic membrane regions, whereas
one or more bipyridine ligands were integrated into the membrane through hydro-
phobic interaction with lipid tail groups.
We varied the [Ru(bpy3)]2+ input percentage in the liposomes used for nanopigment
assembly to characterize the [Ru(bpy3)]2+ loading capacity of the lipid layer (Fig-
ure S2). Nanopigments with 10 mol % [Ru(bpy3)]2+ demonstrated good binding ef-
ficacy and monodispersity and were therefore used for the following analysis unless
otherwise noted.
Plasmonic Enhancement of the MLCT in Nanopigments
In order to quantify the effect of the plasmonic E-field enhancement on MLCT photo-
excitation, we collected the UV-visible (UV-vis) absorbance spectra of water suspen-
sions of nanopigments and controls (Figure 3A). The 101Ru (382 ppb) and 107Ag
(3937 ppb) concentrations were identical to the measured value in nanopigments
in applicable groups. We calculated an extinction coefficient of 4,230 MÀ1 cmÀ1
for aqueous solution of [Ru(bpy3)]2+ (orange, inset) at its MLCT absorbance
maximum, which is in good agreement with previously reported values.31 Impor-
tantly, the nanopigments (red) exhibited a clear enhancement in absorbance in
the MLCT range. We corrected the absorbance of the nanopigments for the contri-
bution from the Ag NP by subtracting the absorbance of ‘‘bare’’ Ag colloid (black) at
the absorbance maximum of 424 nm. The remaining absorbance, which corresponds
to the E-field-enhanced MLCT band, was 35-fold higher than that of [Ru(bpy3)]2+
.
The molar extinction coefficient of the MLCT band in the nanopigments reached
148,000 MÀ1 cmÀ1, comparable to the benchmark of high-performance dyes.33,34
For the control group with Au cores (green), no noticeable enhancement of the
MLCT absorbance was observed; its spectrum contained a peak only at 540 nm,
characteristic of the LSPR for 40 nm Au NPs. This stark contrast to nanopigments
with Ag cores underlines that spectral overlap between the MLCT band and LSPR
absorbance is essential for strong resonant enhancement. A simple mixture control
of Ag NP colloid and [Ru(bpy3)]2+ solution (blue) generated only a weak enhance-
ment of the MLCT band. This finding corroborates that the preferential localization
of the Ru(II) complex within the evanescent field of the NPs is a necessary factor for
the plasmonic enhancement of MLCT.
The nanopigment architecture with the lipid membrane as molecular spacer effec-
tively limits the reactivity loss of [Ru(bpy3)]2+ through metal-associated pathways.
To assess the extent of quenching on reactive photoexcited states, we measured
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Figure 3. Photophysical Characterizations
(A) UV-vis absorbance spectra of nanopigment colloid (red), ‘‘bare’’ Ag NPs (black), membrane-
wrapped Ag NPs without photocatalyst (LipoAg, brown), aqueous solution of [Ru(bpy)3]2+ (orange,
inset), mixture of Ag NPs and aqueous [Ru(bpy)3]2+ solution (blue), and control nanopigments with
Au NP cores (green).
(B) Scheme for the photoexcitation and interstate transitions of [Ru(bpy)3]2+
.
(C and D) Photoluminescence decay patterns and monoexponential fit curves (red) for (C)
nanopigments and (D) [Ru(bpy)3]2+ water solution.
the photoluminescence (PL) lifetime and quantum yield (QY) for an aqueous solution
of [Ru(bpy3)]2+ as well as for the nanopigments. The measured lifetime is determined
by the phosphorescence of the Ru*(II) triplet state that is generated through rapid
intersystem crossing (ISC) from the initial short-lived photoexcited singlet state.24
The triplet state was sufficiently long lived to allow reactive relaxation pathways
and participate in charge or energy transfers (Figure 3B). The measured PL lifetime
of the aqueous suspensions of nanopigments was 371.7 ns (Figure 3C), only slightly
shorter than the 377.4 ns measured for the [Ru(bpy)3]2+ solution (Figure 3D). The
essentially identical lifetimes excluded a significant quenching of the excited state
through the Ag NPs in the plasmonic nanopigments. We also performed lifetime
measurements for [Ru(bpy)3]2+ located closer to the metal NP. In this case,
[Ru(bpy)3]2+ was loaded onto Ag NPs functionalized with HS-PEG3-COOH (Mw =
238.3) (see Supplemental Experimental Procedures). The deprotonated carboxylic
acid group of the short polymer localized [Ru(bpy)3]2+ at a separation of around
1 nm55,56 from the Ag surfaces. The monoexponential fit of the emission tail revealed
a PL lifetime of 2.35 ns for these samples, indicative of strong quenching (Figure S3).
QY measurements of the photoluminescence revealed a QY = 4.55% for the
plasmonic nanopigments compared with QY = 1.95% for free [Ru(bpy)3]2+ and
QY = 0.02% for [Ru(bpy)3]2+ tethered by HS-PEG3-COOH. We conclude that the lipid
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membrane around the Ag NPs in the nanopigments succeeds in positioning the
[Ru(bpy3)]2+ photocatalyst in a ‘‘sweet spot’’ where substantial E-field enhancement
of MLCT photoexcitation is achieved without strong quenching of the reactive
excited state.
Characterizing Photoredox Reactivity and Stability of Nanopigments for Urea
Oxidation Photocatalysis
We characterized the photoredox potentials of the nanopigments and the effects of
plasmonic field enhancement on the photocatalysis with linear sweep voltammetry
(LSV) in a three-electrode setup (Figure 4). We drop-casted nanopigment suspen-
sions and controls onto glassy carbon working electrodes and measured the current
density in the voltage range of 0–1.2 V (versus Ag/AgCl unless otherwise noted) with
illumination from a focused 430 nm light-emitting diode (LED) or in the dark. During
voltammetric measurements, the applied bias induced the oxidation of photogener-
ated Ru*(II) into Ru(III) (Figure 4A), which was then available to facilitate oxidation
processes.57,58
To assess the redox properties of the Ru species, we first used an electrolyte solution
of 0.1 M NaOH + 0.01 M tripropylamine (TPrA), where the baseline was defined by
water oxidation (Figure 4B). TPrA was added as a sacrificial reductant to rapidly
reduce excessive Ru(III) to Ru*(II), which then replenished steady-state Ru(II) through
radiative relaxation.59 The LSV curve for nanopigments with light (red) exhibited an
early reaction onset ꢀ350 mV as well as a drastic increase in photocurrent density
relative to the dark curve (black), which confirms efficient photo-oxidation of water.
The measurable oxidation peak at 650 mV in nanopigments with light could be as-
signed to the oxidation of the photogenerated Ru*(II) to Ru(III) and was therefore
much less noticeable for both nanopigments in the dark and the control group of
lipid-wrapped Ag NPs with no [Ru(bpy)3]2+ (‘‘LipoAg’’; +/À light: orange/blue). We
also included a simple mixture control of Ag NPs colloid and [Ru(bpy)3]2+ solution
(‘‘mixture’’; +/À light: purple/green) with concentrations identical to those of the
nanopigments. The Ru*(II) oxidation peak was present in the ‘‘mixture’’ with illumina-
tion but had a noticeably lower photocurrent density (Jphoto) than the nanopigments.
To quantify the plasmonic enhancement, we compared the Jphoto associated with
the Ru*(II) oxidation peak by subtracting the dark current density from that with light
at 650 mV. We obtained a Jphoto as high as 7.03 mA/cm2 for the nanopigments with
enhancement factors (EFs) of 5.3 and 59 relative to the ‘‘mixture’’ and [Ru(bpy)3]2+
controls, respectively (Figure 4B inset).
Given the favorable photo-oxidation potentials of photogenerated Ru*(II) and Ru(III)
in the nanopigments, we next chose the UOR as a test system for the plasmonic
catalysis. Urea (CO(NH2)2) is a common waste molecule from nitrogen cycles of
mammals. Urea possesses high energy and hydrogen densities60 and great poten-
tials in waste-to-energy conversions. In UOR, anodic urea oxidation (CO(NH2)2
+
6OHÀ / N2 + 5H2O + CO2 + 6eÀ) is coupled to cathodic H2 evolution in neutral
or alkaline conditions (2H2O + 2eÀ / H2 + 2OHÀ), resulting in the overall reaction
CO(NH2)2 + H2O / N2 + 3H2 + CO2.60–62 Nickel-based oxides and hydroxides have
received much attention as electrocatalysts to accelerate the sluggish 6-electron re-
action of UOR.60,61,63–65 Although prior studies have demonstrated photosensitiza-
tion of Ni UOR electrocatalysts by semiconductors,64,66 direct photocatalysis of UOR
has not been reported to the best of our knowledge.
We performed LSV in an alkaline urea electrolyte with a human-urine equivalent
0.33 M urea in 0.1 M NaOH + 0.01 M TPrA63 (Figure 4C). A sharp onset of current
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Figure 4. Electrochemical Characterizations
(A) Scheme for the photocatalytic reaction mechanism of nanopigments in voltammetric
measurements.
(B and C) LSV curves in (B) 0.1 M NaOH + 0.01 M TPrA and (C) alkaline urea electrolyte.
(D and E) Tafel plots in (D) 0.1 M NaOH + 0.01 M TPrA and (E) alkaline urea electrolyte.
(F) Chronoamperometry curves for nanopigments (red) and just [Ru(bpy)3]2+ (magenta) in alkaline
urea electrolyte under 430 nm LED illumination and with 750 mV potential.
Color code: nanopigments +/À light, red/black; LipoAg +/À light, orange/blue; ‘‘mixture’’
control +/À light, purple/green; [Ru(bpy)3]2+ +/À light, magenta/wine (insets).
density for the nanopigments with light was observed at 450 mV (red), leading to an
extended oxidation plateau for Ru(III)-mediated urea oxidation. The shift of the re-
action onset to lower voltages and the substantial increase in current density
compared to nanopigments without illumination (black) confirm superb photocatal-
ysis of UOR. We compared the Jphoto associated with photogenerated Ru*(II)-Ru(III),
which was determined to be 650 mV in Figure 4B discussions. Jphoto of the nanopig-
ments was enhanced by factors of 5.4 and 50 relative to the ‘‘mixture’’ and free
[Ru(bpy3)]2+ control, respectively, which are in good agreement with those obtained
in NaOH + TPrA, indicating consistent and robust catalytic properties of the
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photogenerated Ru(III). Ru(III)-facilitated urea oxidation peaked at 750 mV, where
we recorded a maximum Jphoto of 56.0 mA/cm2. This corresponds to an excellent
applied bias photon-to-current efficiency (Equation S4) of 4.1%, comparable to pre-
viously reported state-of-the-art p-Si hydrogen evolution photocathodes (2.9%)67
and n-Si/NiOx water oxidation photoanodes (1.34%).68
In order to reflect photoelectrochemical kinetics and to relate the reaction rates and
potentials, we constructed Tafel plots for nanopigments and control groups in both
NaOH + TPrA (Figure 4D) and alkaline urea electrolytes (Figure 4E) (see Supple-
mental Experimental Procedures). A lower Tafel slope typically indicates less poten-
tial input required for unit increase of current density and hence better catalytic
properties.69 The illuminated nanopigments showed the lowest Tafel slopes among
all tested conditions. We calculated values of 188 mV/dec in NaOH + TPrA and
120 mV/dec in alkaline urea electrolyte, close to the 120 mV/dec standard for
high-surface-area photocatalysts.68,69
The scaffolding function of the lipid-wrapped architecture also contributee to pro-
moting structural and photoelectrochemical stability of the [Ru(bpy3)]2+ photocata-
lyst. We evaluated the catalytic stability of nanopigments and of [Ru(bpy3)]2+ with
chronoamperometry at 650 mV in alkaline urea electrolyte under constant illumina-
tion (Figure 4F). Over an experimental duration of 6 h, Jphoto remained much more
stable at significantly higher values for nanopigments than for [Ru(bpy)3]2+. For
the latter, Jphoto drastically decreased over the first 10 min of illumination as a result
of photobleaching and depletion of steady-state Ru(II).
Validation of E-Field-Enhanced Plasmonic Catalysis Mechanism
To elucidate the mechanisms of the enhanced UOR photocatalysis of nanopigments,
we first investigated the effect of incident wavelength and power. As shown in Fig-
ure 5A, Jphoto in NaOH + TPrA and alkaline urea electrolytes generally followed the
UV-vis absorbance spectrum of the nanopigments (red) with strong enhancement for
incident wavelengths between 395 and 450 nm, which overlaps the plasmon reso-
nance of the nanopigments. We performed light-power studies by using a 430 nm
LED (Figure 5B) as well as a sunlight simulator (Figure 5C) to better imitate the per-
formance of the nanopigments in a solar cell. A linear dependency of Jphoto on the
incident power was obtained for both light sources. These data confirm that the
observed increase in nanopigment photoreactivity is indeed related to the plas-
mon-enhanced optical absorption provided by the NPs.
Given the relatively large size of the Ag NPs used in this work, we attribute the
observed gain in photocatalysis primarily to the E-field-enhanced MLCT photoexci-
tation and not to hot charge-carrier-associated processes.51,52 To validate this
assumption, we investigated the catalysis of nanocomposites in which the Ag sphere
cores were replaced with triangular Ag nanoplates (Figure 5D, insets). The nano-
plates sustained two LSPR modes at 415 and 525 nm with similar dissipation.46 In
both cases, the decay of the plasmon could generate hot charge carriers, but only
the former mode had good spectral overlap with the [Ru(bpy)3]2+ MLCT. After the
lipid membrane was loaded with [Ru(bpy3)]2+, the 415 nm band of the Lipo-Ag nano-
plate-Ru nanocomposite control was broadened by the MLCT of [Ru(bpy3)]2+
,
whereas the 525 nm peak remained distinct (Figure 5D). We collected the LSV curves
with illumination at either 430 or 565 nm to selectively excite the two LSPR modes. As
shown in Figures 5E and 5F, Jphoto increased only for the mode that peaked at
415 nm close to the MLCT band but not for the plasmon mode at 525 nm, even
though the latter mode had larger extinction. These results confirm that the
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10.1016/j.chempr.2019.06.014
Figure 5. Electrochemical Characterization of Plasmonic Enhancement Mechanism
(A) Plots of photocurrent density (Jphoto) as a function of wavelength of the incident light for
nanopigments in NaOH + TPrA (black triangles) and alkaline urea (blue squares). Jphoto was
calculated from the LSV peaks at 650 mV in NaOH + TPrA and 750 mV in alkaline urea. The
absorbance spectrum of the nanopigments (red) is shown for comparison. Error bars represent
standard deviations of three measurements.
(B) Plot of photocurrent densities as a function of incident power density (430 nm LED) in NaOH +
TPrA (black triangles) and alkaline urea (blue squares) electrolytes. 100% power density refers to the
power used for all LSV measurements in Figures 4, 5A, 5E, and 5F. Error bars represent standard
deviations of three measurements. See also Supplemental Experimental Procedures.
(C) Plot of photocurrent densities over different incident powers of a sunlight simulator in NaOH +
TPrA (black triangles) and alkaline urea (blue squares) electrolytes. Error bars represent standard
deviations of three measurements. See also Figure S4.
(D) UV-vis absorbance spectra of Ag nanoplates (black) and membrane-wrapped Ag nanoplates
with [Ru(bpy)3]2+ (Lipo-Ag nanoplates-Ru) (red). Inset: TEM of Ag nanoplates. Scale bar: 50 nm.
(E) LSV curves for Lipo-Ag nanoplate-Ru in 0.1 M NaOH + 0.01 M TPrA electrolyte without
illumination (black), with 430 nm LED illumination (red), and with 565 nm LED illumination
(green). Inset: TEM image of a Lipo-Ag nanoplate-Ru particle with a visible lipid membrane.
Scale bar: 10 nm.
(F) LSV curves for Lipo-Ag nanoplate-Ru controls in alkaline urea electrolyte in the dark (black), with
430 nm LED illumination (red), and with 565 nm LED illumination (green).
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10.1016/j.chempr.2019.06.014
enhanced photocatalysis of nanopigments is derived from an E-field-driven MLCT
excitation enhancement mechanism.
Interestingly, the increase in Jphoto for nanopigments over free [Ru(bpy3)]2+ solution
(EF = 50) was higher than the enhancement of absorbance (EF = 35). We attribute the
larger gain in photoreactivity to an additional photothermal effect sustained by Ag
NP cores. We calculated a temperature increase of 2.89 K in the immediate vicinity of
the nanopigments for the chosen experimental conditions (see Supplemental Exper-
imental Procedures). Such an increase in local temperature enhances the catalytic
turnover and shifts the reaction toward products.70 We emphasize that the increase
in local temperature of 2.89 K does not affect the structural stability of the nanopig-
ments given that the temperature remains well below the glass transition tempera-
tures of the membrane components (>42ꢁC).39
Implementation and Characterization of Visible LDUFC
Achieving overall urea oxidation in a direct urea fuel cell has the exciting appeal
of simultaneous energy conversion from an abundant source and waste-water
treatment. On the basis of the versatile redox properties of the nanopigments, we
implemented a LDUFC in which nanopigments served as both the cathode and
the photoanode material. An electrolyte of 0.33 M urea + 0.1 M NaOH was used
for characterizing the cell performance. We focused incident light onto the working
electrode to generate photoexcited Ru*(II), which underwent spontaneous oxidation
at the photoanode (Ru*(II) À eÀ / Ru(III)). The cathode reaction Ru(II) + eÀ / Ru(I)
occurred on the non-illuminated ‘‘dark’’ electrode (Figure 6A), giving an overall cell
potential of +0.52 V.23 The strong oxidizer Ru(III) facilitated urea oxidation at the
photoanode, whereas Ru(I) reduced O2 or CO2 to regenerate Ru(II) at the cathode.
We monitored the open circuit potential (OCP) of the LDUFC with illumination both
from a 430 nm LED (Figure 6B) and from a sunlight simulator at 1-sun illumination
(100 mW/cm2) (Figure 6C) for proof-of-concept solar energy harvesting. We re-
corded the photovoltage, reflected by the difference in OCP between light the on
and off stages, with alternating light on and off intervals of 150 and 350 s, respec-
tively. We exchanged the electrolyte after 3 h to guarantee comparable kinetics in
the reactions. For both illumination conditions, the cell OCP stabilized after an initial
decrease in magnitude and then performed steadily with photovoltages between 12
and 15 mV for more than 3 h. We recorded the cell short-circuit current density by
using chronoamperometry with the same cell setup and focused LED illumination.
As shown in Figure 6D, a consistent oscillation in photocurrent density was centered
around 0.25 mA/cm2. We attribute this oscillatory behavior to the photoelectro-
chemical cycling of the photoanode.71 Under continuous illumination, Ru(II) was
rapidly consumed but more slowly replenished by the catalyzed 6-electron UOR, re-
sulting in a consumption-replenishing cycle for the steady-state population.
To further illuminate the nature of the electrode reactions, we purged the cell with
nitrogen and recorded cell OCP under oxygen deficient conditions (Figure 6E). In
the beginning of the measurement, after an activation stage in the first 10 min,
the photovoltage showed an initial increase in magnitude. Subsequent addition of
air triggered a measurable increase in the light response. Overall, lower OCP values
were observed with nitrogen purging. These observations are consistent with our
model of LDUFC reactions in which O2 and CO2 are reduced at the cathode. In
the absence of O2, an increasing amount of CO2 generated through the anode
photoreaction of urea oxidation dominated the cathode reaction in a self-acceler-
ating manner. When O2 became available, the reaction rate at the cathode
Chem 5, 1–15, August 8, 2019
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10.1016/j.chempr.2019.06.014
Figure 6. Characterizations of the LDUFC
(A) LDUFC whole-cell setup and cell reactions scheme.
(B and C) Open circuit potential (OPC) measurements for nanopigments in 0.33 M urea + 0.1 M
NaOH electrolyte with (B) 430 nm LED illumination or (C) sunlight simulator at 1-sun power
illumination. Light on and off cycles are indicated by green and blue arrows, respectively.
(D) Measurement of short-circuit current density under constant illumination with 430 nm LED.
(E) OPC measurements for nanopigments in 0.33 M urea + 0.1 M NaOH electrolyte with 430 nm LED
illumination and N2/air purging.
increased, resulting in an overall increase of OCP as well as a stronger light-induced
modulation of OCP.
In summary, we have demonstrated efficient plasmonic E-field enhancement of
[Ru(bpy)3]2+-mediated photocatalytic urea oxidation in a hierarchical nanopigment
architecture. The nanopigments contain a lipid layer self-assembled around a
ꢀ45 nm Ag nanoantenna with favorable spectral overlap between the Ag NPs
LSPR and [Ru(bpy)3]2+ MLCT. Our experimental and theoretical analyses suggest
that the amphiphilic nature of the self-assembled membrane facilitates both
12
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electrostatic and hydrophobic interactions with [Ru(bpy)3]2+ to achieve localization
of the photocatalyst at an electromagnetic ‘‘sweet spot’’ approximately 2–4 nm
from the metal surface. At the sweet spot, strong E-field intensity provides substan-
tial enhancement of photocatalyst excitation, but quenching of the excited reactive
states in the photocatalyst through the metal is limited. The nanopigments achieved
efficient direct photocatalysis of urea oxidation with photocurrent densities more
than 50 times higher than for [Ru(bpy)3]2+ and an excellent photoelectrode efficiency
of 4.1%. We also implemented a LDUFC with LED and sunlight simulator and
demonstrated steady on/off photovoltaic performance and OCPs for over 4 h. Our
proof-of-principle implementations of UOR and LDUFC promise great achievements
in solar energy conversion and waste-water treatment. Future efforts focusing on ex-
panding the spectrum of reactions catalyzed by photocatalytic nanopigments con-
taining [Ru(bpy)3]2+ or other transition-metal complexes will clarify the full potential
of the approach and may lead to entirely new classes of plasmon-enhanced biomi-
metic nanoreactors.
EXPERIMENTAL PROCEDURES
Detailed experimental procedures are included in Supplemental Experimental
Procedures.
DATA AND CODE AVAILABILITY
All relevant data are available from the authors upon request.
SUPPLEMENTAL INFORMATION
Supplemental Information can be found online at https://doi.org/10.1016/j.chempr.
2019.06.014.
ACKNOWLEDGMENTS
This work was partially supported by the National Science Foundation through
grants CHE-1609778 and CHE-1808241. The authors thank Prof. Chen Yang and
Dr. Melissa Cardona for discussions of electrochemical characterizations and Prof.
Allison Dennis for access to the luminescence lifetime spectrometer.
AUTHOR CONTRIBUTIONS
X.A. designed and conducted the experiments. D.S. designed and conducted the
membrane molecular dynamics simulations. T.K. supervised the simulations.
B.M.R. conceived the idea and supervised the experiments. All authors discussed
the results and contributed to the final manuscript.
DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: February 18, 2019
Revised: May 11, 2019
Accepted: June 19, 2019
Published: July 11, 2019
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