Aluminum Nanocrystals as a Plasmonic Photocatalyst for Hydrogen

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Aluminum Nanocrystal as a Plasmonic Photocatalyst for Hydrogen Dissociation Linan Zhou, Chao Zhang, Michael McClain, Alejandro Manjavacas, Caroline M. Krauter, Shu Tian, Felix Berg, Henry O. Everitt, Emily A. Carter, Peter Nordlander, and Naomi J. Halas Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b05149 • Publication Date (Web): 22 Jan 2016 Downloaded from http://pubs.acs.org on January 24, 2016

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Title: Aluminum Nanocrystal as a Plasmonic Photocatalyst for Hydrogen Dissociation

Authors: Linan Zhou †1, 4, Chao Zhang †2, 4, Michael J. McClain 1, 4, Alejandro Manjavacas 5, Caroline M. Krauter 6, Shu Tian

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, Felix Berg

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, Henry O. Everitt 9, Emily A. Carter

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, Peter

Nordlander* 2, 3, 4, and Naomi J. Halas* 1, 2, 3, 4. 1 Department of Chemistry, 2 Department of Electrical and Computer Engineering, 3 Department of Physics and Astronomy, and 4 Laboratory for Nanophotonics, Rice University, 6100 Main Street, Houston, Texas 77005, United States. 5 Department of Physics and Astronomy, University of New Mexico, Albuquerque, New Mexico 87131, United States. 6 Department of Mechanical and Aerospace Engineering, 7 Program in Applied and Computational Mathematics, and 8 Andlinger Center for Energy and the Environment, Princeton University, Princeton, New Jersey 08544, United States. 9 Army Aviation and Missile RD&E Center, Redstone Arsenal, Alabama 35898, United States. 10 Johannes Gutenberg University Mainz, D 55099 Mainz, Germany. Keyword: Plasmonic photocatalysis, Aluminum nanocrystal, Hydrogen dissociation, Hot electron, interband transition Abstract: Hydrogen dissociation is a critical step in many hydrogenation reactions central to industrial chemical production and pollutant removal. This step typically utilizes the favorable band structure of precious metal catalysts like platinum and palladium to achieve high efficiency under mild conditions. Here we demonstrate that aluminum nanocrystals (Al NCs), when illuminated, can be used as a photocatalyst for hydrogen dissociation at room temperature and atmospheric pressure, despite the high activation barrier towards hydrogen adsorption and dissociation. We show that hot electron transfer from Al NCs to the anti-bonding orbitals of hydrogen molecules facilitates their dissociation. Hot electrons generated from surface plasmon decay and from direct photoexcitation of the interband transitions of Al both contribute to this process. Our results pave the way for the use of aluminum, an earth-abundant, non-precious metal, for photocatalysis.

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Catalytic hydrogenation is one of the largest branches of heterogeneous catalysis and is very important for numerous industrial reactions, such as the hydrogenation of unsaturated hydrocarbons1, 2 and hydrodechlorination.3, 4 Supported precious metals have been widely used as catalysts because their favorable electronic structures allow hydrogen molecules to easily adsorb onto the metallic surface and dissociate.5 Recently, plasmonic metal nanoparticles have emerged as a new type of photocatalyst for small molecule activation and related chemical reactions.6-8 Characterized by their strong interaction with electromagnetic fields through the localized surface plasmon resonance (LSPR),9-12 plasmonic nanoparticles can photocatalyze chemical reactions by transfer of hot carriers,13,

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generated from LSPR decay, to the available states of adsorbed

molecules, activating molecular dissociation.15-17 In previous work we demonstrated the use of Au nanoparticles for plasmon-enabled hydrogen dissociation despite its intrinsically inert d-band configuration.7, 8 Aluminum, the most abundant metallic element in the Earth’s crust and ten thousand times cheaper than precious metals, has been shown to possess highly promising plasmonic properties.18-21 Most notably, Al exhibits low optical losses from the visible to the ultraviolet (UV) spectral regions due to its high electron density and extends the optically tunable range of plasmonic nanostructures beyond that of gold and silver. Applications of Al plasmonics include optical antennas,22 plasmon-enhanced photodetectors,23 plasmon-enhanced spectroscopy, 24 and display technology.25-27 However, direct photocatalysis with Al nanostructures is less explored due to its unfavorable electronic band structure and thus poor affinity for molecules.28-30 So far, reported experiments have tried to mitigate this effect by using transition metals to dope Al and reduce the adsorption and reaction barrier,31,

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or using deep UV photons for direct

photodecomposition of organic molecules.33 Here we demonstrate that chemically synthesized aluminum nanocrystals (Al NCs) are capable of photocatalyzing hydrogen dissociation under laser illumination without the use of transition metal dopants, deep UV photons, or extreme reaction conditions such as elevated temperature or pressure. Al NCs supported on γ-Al2O3 were prepared as the photocatalyst and the isotopic exchange reaction H2 + D2 = 2HD was used to monitor hydrogen dissociation. First-principles quantum calculations indicate that electron transfer from Al NCs to the antibonding orbitals of hydrogen molecules can weaken the H-H bond and induce molecule dissociation. The wavelength dependence of this reaction resolves two different origins of hot electrons contributing to this process: surface plasmon decay and interband transitions.


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Al NCs were synthesized according to the published protocol34 with minor modifications (see Supporting Information for synthesis method). The Al NCs used in our experiments were nominally 100 nm in diameter, as determined by high-resolution transmission electron microscope (HRTEM) images (Figure 1a, Figure S1) with a 3 nm native oxide layer (Figure 1b). They were dispersed in γ-Al2O3 powder at 5 wt.% to avoid aggregation. This photocatalyst supports a dipolar plasmon mode at around 460 nm, determined by the UV-Vis-NIR extinction spectra of Al NC solutions and by diffuse reflectance spectra of the Al NC/γ-Al2O3 mixture (see Supporting Information Figure S1 and S2). The photocatalyst was then loaded into a customized stainless steel chamber with a quartz window (Harrick Scientific Product Inc.) for H2 dissociation studies. Research purity hydrogen (H2) and deuterium (D2) were introduced as independent reactants to study the hydrogen dissociation reaction. The detection of HD resulting from catalyst illumination indicates the occurrence of hydrogen dissociation (H2 + D2 = 2HD). White light from a supercontinuum fiber laser (Fianium, 450-900 nm, 4 ps, 40 MHz, see Supporting Information Figure S4 for laser spectrum) were used as excitation source. For wavelength dependence measurements, monochromatic light from a tunable Ti: Sapphire laser (Coherent, Chameleon Ultra II, 150 fs, 80 MHz, 680-1080 nm, bandwidth ~10 nm) equipped with a second harmonic generator (Angewandte Physik und Elektronik GmbH, output wavelength 350-530 nm) was used. The HD concentration in the outlet of the chamber was monitored using a quadrupole mass spectrometer (Hiden Analytical Inc.) in real time.


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Figure 1. Hot electron-induced hydrogen dissociation on 5 wt.% Al nanocrystal photocatalyst Al NC/γ-Al2O3. (a) Transmission electron microscope (TEM) image of Al NCs. (b) Highresolution TEM (HRTEM) of portion of an individual Al NC. The crystalline surface of the Al NC and the native amorphous aluminum oxide layer are resolved. (c) Real-time monitoring of HD generation rate with and without laser excitation (Fianium, 450-900 nm, 4 kW/cm2). Dashed blue lines represent on and off times for catalyst illumination. (d) Comparison of Al NC/γ-Al2O3 (red) with pure γ-Al2O3 (black) under laser illumination (Fianium, 450-900 nm, 4 kW/cm2).

We observed H2 and D2 dissociation by measuring the HD generation rate in the presence or absence of a white light excitation (Figure 1c). The HD generation rate was monitored in real time while keeping the reaction chamber at room temperature and atmospheric pressure. Before 4 ACS Paragon Plus Environment

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laser excitation, a low-level, steady-state HD background level was observed due to HD present in the H2 and D2 gases as an impurity, and also due to a very weak spontaneous HD exchange reaction occurring on the photocatalyst. Upon laser excitation (Fianium, 400 mW with laser spot size ~0.1 mm), the HD rate increased rapidly and reached a plateau within five minutes. After the laser was turned off, the HD rate dropped down to the original background level. This process was repeated multiple times with good reproducibility. To confirm that HD generation was due to the presence of Al NCs in the photocatalyst and not the γ-Al2O3, which can also activate hydrogen dissociation under certain conditions,35 the HD generation rates of Al NC/γ-Al2O3 and pure γ-Al2O3 were compared. As shown in Figure 1d, under laser illumination, Al NC/γ-Al2O3 showed a high activity for light-induced HD generation while γ-Al2O3 showed no such reactivity. This indicated that the Al NCs present in the photocatalyst are the active component for hydrogen dissociation.

Figure 2. (a) Direct comparison of photo-induced and thermally-induced hydrogen dissociation. The shaded grey area shows the effect of laser illumination (Fianium, 3


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kW/cm2) at room temperature while the shaded red area shows the effect of heating the photocatalyst to 50oC with the laser off. (b) Predicted local laser-induced heating effect of a single Al NC (diameter = 100 nm) surrounded by porous γ-Al2O3 shell (thickness = 100 nm) as a function of excitation laser wavelength and power density.

During the photoreaction, the temperature of the photocatalyst increased slightly (< 5 K) due to laser-induced heating of the illuminated photocatalyst. To determine whether thermal effects also contributed to the observed reaction, we compared the HD generation rate of the light-induced reaction with the purely thermal catalytic response. The photocatalyst was first illuminated (Fianium, 300 mW) at room temperature, and then heated to 50 oC without laser illumination. The observed HD generation by heating alone was less than 3% of that observed using laser illumination of the catalyst (Figure 2a). We also calculated the local temperature increase from laser-induced heating as a function of laser power density and wavelength. The temperature increase was calculated as the ratio of the energy absorbed by the Al NC (product of the laser power density and the absorption cross section of the Al NC) to its heat capacity36 (see Supporting Information for detailed calculation). The predicted local temperature increase is below 5 K for all of our experimental conditions (Figure 2b). This comparison confirms that the observed hydrogen dissociation reaction on the photocatalyst is light-triggered rather than thermally driven.


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Figure 3. Wavelength dependence of HD generation on an Al NC/γ-Al2O3 photocatalyst. (a) HD production (red circles) on photocatalyst illuminated by monochromatic light [Chameleon (680-980 nm) and its second harmonic light (350-525 nm), 65 mW] for 30 min as a function of excitation wavelength. The error bars represent the standard deviation of multiple measurements. The calculated absorption cross section of a single Al NC surrounded by a porous 100 nm thick γ-Al2O3 shell is shown as black curve. (b) Dependence of HD generation rate on laser power using 800 nm (magenta squares) and 461 nm (blue circles) light as excitation sources. Magenta and blue lines are linear fits of experimental data.

To better understand the light-induced photocatalytic response, we probed the photocatalyst using excitations at different wavelengths. The amount of HD produced by 30 min of light illumination [Chameleon (680-980 nm) and its second harmonic light (350-525 nm), laser power kept at 65 mW for all wavelengths] as a function of excitation wavelength is shown in Figure 3a as red circles (see Supporting Information for a detailed description of the wavelength dependence measurement method). A small HD production peak around 460 nm correlates with the dipolar 7 ACS Paragon Plus Environment

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LSPR mode of the Al NCs where hot electrons are produced by decay of the surface plasmon. However, maximum hydrogen dissociation is observed at illumination wavelengths around 800 nm, which corresponds to the interband transitions in Al. At these photon energies, hot electrons are generated by laser excitation of direct transitions between filled and unfilled sp-derived bands of Al, which has a band edge ~1 eV above the Fermi level37. As a measure of energy transfer from the incident light field to the Al NC, the absorption cross section of a 100 nm Al NC calculated using Mie theory (see Supporting Information for details) is shown for comparison in Figure 3a. The absorption peaks around 460 nm and 800 nm can be assigned to the localized surface plasmon resonance and the Al interband transition, respectively. Comparison between these two sets of data indicates that excitation of the interband transition of Al results in more HD production even though its absorption cross section is smaller than the LSPR region at shorter wavelengths. Recent work on understanding plasmon-induced hot carrier photophysics indicates that the energy alignment of a metal nanostructure with an adsorbed molecule38 or a semiconductor39 interface ultimately determines which processes, direct photoexcitation or plasmon-induced excitations, will generate useful hot carriers. In Al, the interband transition yields hot electrons with energies approximately 1-1.5 eV above the Fermi level, 13, 37 which are energetic enough to transfer to the hydrogen molecule and facilitate dissociation, as supported by first principles quantum mechanics calculations (discussed below). This is an important and unique feature of Al-based photocatalytic systems: carrier generation for interband transitions is expected to be very efficient in Al due to its high density of states. The dependence of the HD generation rate on laser power was measured at 800 nm (interband transition) and 461 nm (LSPR) using Chameleon laser. We observed a linear dependence on the excitation power at both wavelengths, for power densities up to 4 kW/cm2 (Figure 3b). The external quantum yield of HD generation is estimated to be ~1%. In contrast, for the photocatalytic ethylene epoxidation reaction on Ag nanocrystals, a supralinear power dependence of the reaction rate was observed above a laser power density of 300 mW/cm2 and was attributed to a multi-electron process.40 Here, the observed power dependence remains linear at much higher power densities, suggesting that the hydrogen dissociation reaction studied here is triggered by a single hot electron for both LSPR and interband transition excitations. Another unique feature of the Al NC photocatalyst is the native alumina shell that covers the Al NC. In our experiment, the native alumina shell is amorphous, as observed in Figure 1b and confirmed by X-ray powder diffraction measurements of the pure Al NC powder (see Supporting Information). Unlike close-packed alumina layers fabricated by physical vapor deposition or 8 ACS Paragon Plus Environment

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chemical vapor deposition that are commonly used as hydrogen barriers,41, 42 this native alumina shell is relatively porous, likely allowing hydrogen molecules to reach the Al metal by diffusion to accept hot electrons.43-45 Moreover, while Al metal has a high activation barrier for hydrogen adsorption and dissociation,28-30 hydrogen molecules can readily adsorb on alumina under room temperature.35, 46, 47 Thus an important role of the native alumina shell of the Al NCs in the photocatalyst is to provide adsorption sites for H2 and D2 so that the residence time of H2 and D2 on the Al surface is increased, enhancing the probability of hot electron acceptance and hydrogen dissociation. In addition, adsorption of hydrogen molecules on Al and O sites near the Al-Al2O3 interface could stretch the H-H bond, slightly lowering the antibonding orbital and facilitating hot electron injection into hydrogen molecules.48-51 The scenario where hot electrons transfer to hydrogen molecules by tunneling through the alumina layer is less likely because of the low electron tunneling probability through the nanoscale alumina layer compared with our measured quantum efficiency. We performed periodic density functional theory (DFT) and embedded correlated wavefunction (ECW) calculations to gain further insight into the reaction mechanism (computational details are given in the Supporting Information). Assuming that hydrogen molecules can diffuse through the native alumina layer to the Al surface, a closed-packed Al (111) facet was chosen to model the surface of the Al nanoparticles. Adsorption of the hydrogen molecule parallel to the surface at a bridge site is assumed (see Supporting Information, Figure S6). Of particular interest are chargetransfer (CT) processes from sp-derived Al states into the antibonding orbital of H2 that may lead to a weakening of the bond and thereby facilitate dissociation. Prior calculations7 on an isolated hydrogen molecule have shown that H2- has a metastable state located about 1.7 eV above the ground state of the neutral molecule. The bond dissociation energy for this state is 2 eV lower than for the neutral case (2.6 eV vs. 4.6 eV). The ground state potential energy surface (PES) from embedded complete active space selfconsistent field (CASSCF) calculations is shown in Figure 4a. The predicted energy barrier for dissociation of adsorbed hydrogen is approximately 2.5 eV, similar to the bond dissociation energy noted above for an isolated H2- ion.7 Figure 4b displays the CT occurring on the ground state PES, as obtained by a Löwdin charge analysis of the embedded CASSCF ground state. As the molecule approaches the Al surface and the reaction barrier along the indicated minimum energy pathway, charge is transferred from the metal to the molecule (the initial small positive excess charge on the hydrogen molecule at large distances is due to an orthogonalization of the Al sp-band with the bonding orbital of H230). CT is already substantial in the ground state; at the 9 ACS Paragon Plus Environment

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transition state (marked by a star in Figure 4a/4b), the excess charge on the hydrogen molecule is 0.9 electrons. Such a large charge transfer suggests that the electron affinity level of the hydrogen molecule lies very close to the Fermi level of the Al NC substrate (for a Lorentzian level, a charge transfer of one electron would correspond to the affinity level being resonant with the Fermi level). Because the affinity level is close to the Al Fermi level, it will overlap the photoinduced hot electron distribution. The reaction barrier for hydrogen dissociation will clearly be further lowered after injection of a hot electron from the Al nanoparticle into the antibonding orbital.

Figure 4. (a) Embedded CASSCF PES for the dissociation of hydrogen on the Al (111) surface. The star marks the transition state (at an energy of 2.5 eV above the ground-state minimum, 10 ACS Paragon Plus Environment

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indicated by the triangle in the energy bar) and the circles refer to points at which DOSs, electron density difference maps or excited state energies are provided below. (b) Löwdin charge analysis of the embedded CASSCF ground state. The same points are indicated by circles and a star as in (a). (c) DFT-GGA DOS projected onto the orbitals of the hydrogen atoms for two different distances from the surface. The inset shows the broadening of the antibonding state in more detail. The dashed line denotes the Fermi energy. (d) Excited state PESs along the reaction pathway indicated in (a) obtained with embedded CASSCF and averaging over the lowest 20 states. The ground state is indicated by triangles while the excited states are indicated by circles. The reaction coordinate (r. c.) is obtained from the bond length (𝑙) and the distance to the surface (𝑑) according to 𝑟. 𝑐. = �(𝑙 − 𝑙0 )2 + (𝑑 − 𝑑0 )2 , where 𝑙0 = 0.75 Å and 𝑑0 = 5.0 Å are the parameters at the first point. (e) Interaction-induced DFT-GGA electron density differences as the hydrogen molecule approaches the surface given in units of electrons per Å3 ; 𝑑 refers to the surfacemolecule distance while 𝑙 pertains to the H2 bond length. The electronic density of states (DOS) projected onto the orbitals of the hydrogen atoms obtained from periodic DFT-GGA calculations (compare Figure 4c) further corroborates the prediction of significant CT, although DFT will overestimate its extent. As the hydrogen molecule approaches the surface, its bonding and antibonding orbitals are shifted downwards in energy, and hybridization with the sp-band of Al leads to broadening of these peaks. The tail of the broad antibonding state now extends below the Fermi level and the state becomes partially occupied at small distances from the surface. This is consistent with the significantly increased charge on the hydrogen molecule (Figure 4b). DFT-GGA electron density differences (Figure 4e) reveal that the excess negative charge is indeed concentrated outside of the bonding regions of the hydrogen molecule, in accord with a partial occupancy of the antibonding states, while the bonding electron density is decreased. The points on the PES at which the DOS or electron density differences were evaluated are indicated by circles in Figures 4a/4b). We also performed excited-state embedded CASSCF calculations along an approximate minimum energy pathway (indicated in Figure 4a) to assess the existence of low-lying excited states that might aid the dissociation of H2. The results are shown in Figure 4d. Due to the nearlyfree-electron character of Al, a plethora of excited states exists with very low excitation energies. Along the reaction pathway, frequent state crossings and strong couplings are to be expected. This characteristic property of Al hinders the determination of specific reaction barriers for different excited states, as was possible for the dissociation of H2 on Au nanoparticles.7 Nevertheless, a variety of different reaction pathways can exist along these excited state potential energy surfaces, some of which can be expected to be lower than in the ground state. The lowlying excited states offer possibilities for enhancement of the reactivity, consistent with a


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significant contribution from interband transitions in addition to plasmons, as seen in the experiment. Based on the combined experimental and theoretical results, we suggest a similar though not identical reaction mechanism to that proposed for H2 dissociation on Au in an earlier study.7 Hot electrons formed by either the interband transition or decay of the plasmon excitation can further increase the CT already seen in the ground state and lead to an even stronger population of the hydrogen antibonding orbital. Hot electrons formed by excitation of the interband transition are able to contribute to this process, due to their relatively high energy in Al. Moreover, Al features a large number of low-lying excited states because of its distinct nearly-free-electron character. The formation of such a transient negative ion state is stabilized by its image potential in the metal. Its lifetime can be sufficiently long on small Al nanoparticles to allow for an elongation of the hydrogen bond length along the PES of an excited CT state before the electron is returned to the metal surface. Depending on how far the dissociation has proceeded by this stage, it can be completed either in the excited or the ground state, or the molecule relaxes back to its equilibrium ground state structure. We demonstrated the first use of Al NCs as a photocatalyst for hydrogen dissociation. Firstprinciples quantum mechanics calculations support a mechanism based on hot electron transfer from Al NCs to hydrogen molecules, populating their antibonding orbitals and weakening the HH bond. In addition to the well-recognized role of surface plasmon decay as an origin of hot electrons, the interband transitions of Al also yield hot electrons energetic enough to facilitate this reaction. This unique feature of Al NCs is of great interest in manipulating chemical reactions using photons with different energies. Associated Content Supporting Information Al NCs and Al/γ-Al2O3 photocatalyst sample preparation, characterization of structure and morphology, spectroscopic characterization, photocatalysis measurement, electromagnetic simulation and first principles calculation. This material is available free of charge via the Internet at http://pubs.acs.org.

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Corresponding Authors * [email protected] * [email protected] * [email protected]

† L.Z. and C.Z. contributed equally to this work Notes The authors declare no competing financial interest. Acknowledgements This work was supported by the Air Force Office of Science and Research grant FA9550-15-10022. NJH and PN acknowledge support from the Robert A. Welch Foundation under grants C1220 and C-1222. AM acknowledges financial support from the Department of Physics and Astronomy and the College of Arts and Sciences of the University of New Mexico. CMK acknowledges support by a fellowship within the Postdoc-Program of the German Academic Exchange Service (DAAD). All first-principles quantum mechanics calculations were carried out using Princeton’s TIGRESS High Performance Computing resources or the COPPER high performance computing center at the Air Force Office of Scientific Research High Performance Computing Center. Figures 4e and S6 were generated with the help of the 3D visualization program VESTA.52 The authors acknowledge Dr. Fangfang Wen, Bob Y. Zheng, and Dr. John Mark P. Martirez for proofreading the manuscript and thoughtful discussions.

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