Gold-based Nanomaterials: Spectroscopy, Microscopy and Applications in Catalysis and Sensing

The birth of nanotechnology era has revolutionized materials science, catalysis and field of optoelectronics. Novel and unique phenomena emerge when material dimensions are reduced to ultra-small size regime and enter nanometre (2-100 nm) realm. Such novel materials are expected to replace bulk materials, offering lower cost of manufacturing and enabling progress in many areas such as solar cell, drug delivery, quantum communication and computing, catalysis and sensing applications. With the progress in nanomaterial synthesis and fabrication, the need for the state-of-art characterization techniques became obvious; such techniques help to establish a complete understanding of the nature and interactions of nanosized materials. In this thesis, the first part focuses on the synthesis of gold and ruthenium clusters, namely Au8, Au9, Au101, Ru3, Ru4 and AuRu3, using the well-established synthetic protocols in the literature. Apart from the standard lab-based characterization techniques such as nuclear magnetic resonance (NMR), UV-visible spectroscopy (UV-vis) and Fourier Transform Infra-red (FTIR), a less explored but useful technique far infra-red (far IR) spectroscopy, available at the Australian Synchrotron (AS), was employed to investigate the vibrational modes in these clusters. Peaks in the experimental far IR spectra were assigned unambiguously to specific vibrations by comparing with the ones generated via DFT calculations with the help of collaborators, group of Professor Gregory Metha, University of Adelaide. For the Au9 cluster, three significant gold core vibrations are observed at 157, 177 and 197 cm-1 in the experimental spectrum. In the case of the Ru3 cluster, only a single ruthenium core vibration is identified within the spectrum, at 150 cm-1 with the calculated force constant, k = 0.33 mdyne/Å. The Ru4 cluster exhibits two metal core vibrations at 153 and 170 cm-1 with force constants of 0.35 and 0.53 mdyne/Å, respectively. Substitution with a gold atom yielding a mixed metal AuRu3 cluster shifts the core transitions toward higher wavenumbers at 177 and 299 cm-1 with an increase in force constants to 0.37 and 1.65 mdyne/Å, respectively. This is attributed to the change in chemical composition and geometry of the metal cluster core. A combination of the DFT calculations and high quality synchrotron-based experimental measurements allowed the full assignment of the key transitions in these clusters. Next, these clusters were fabricated into heterogeneous catalysts by depositing on different metal oxide nanopowders. Synchrotron X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) studies were performed at the Australian Synchrotron and the Photon Factory synchrotron in Japan to investigate the electronic structure of Au8, Au9 and Au101 on TiO2 catalysts. The XPS analysis reveals that “as-deposited” Au8 and Au9 retain some un-aggregated clusters while Au101 show bulk-like gold. These findings are in line with TEM observations, where the aggregates (large particles, > 2 nm) of Au8, Au9 and Au101 are hardly seen under HRTEM. UV-visible diffuse reflectance spectroscopy (UV-vis DRS) studies show the absence of localised surface plasmon resonance (LSPR) peaks in these “as-deposited” clusters, suggesting they are below 2 nm in size. Importantly, the XAS spectrum of “as-deposited” Au9 clusters estimates that 60% of pure, un-aggregated Au9 clusters and 40% of bulk gold in the sample. Upon calcination under O2 and combined O2 and H2 (O2-H2), Au8, Au9 and Au101 clusters form larger nanoparticles (> 2 nm) with the appearance of LSPE peak in UV-vis DR spectra. In addition, majority of the phosphine ligands (that stabilise the gold core) dislodge and form phosphine oxide-like species by interacting with oxygen on the TiO2 surface. The third part focused on testing the catalytic performance of the supported Au8, Au9, Au101, Ru3, Ru4 and AuRu3 clusters on different TiO2, SiO2, ZnO and ZrO2 in benzyl alcohol oxidation. Au101-based catalysts display the highest catalytic activity with a turn-over frequency (TOF) up to 0.69 s-1. The high catalytic activity is attributed to the formation of large Au nanoparticles (> 2 nm) that coincides with the partial removal of capping ligands. Au8 and Au9 clusters which contain NO3- counter anions are found to be inactive in benzyl alcohol oxidation. Further work shows that the presence of NO3- species diminishes the catalytic activity. Monometallic ruthenium clusters, Ru3 and Ru4, are found to be inactive yet the bimetallic AuRu3 clusters are active in benzyl alcohol oxidation, suggesting the synergistic effect between ruthenium and gold metal. Investigation of catalytic testing parameters reveals that tuning selectivity of the product is possible through manipulating the reaction temperature. Finally, a joint experiment with Prof. Wojtek Wlodarski’s group at RMIT, Melbourne was undertaken to test the sensing ability of Au9 clusters for hydrogen detection. Au9 clusters were deposited onto radio-frequency (RF) sputtered WO3 films at two different concentrations; 0.01(S1) and 0.1(S2) mg/mL. It was found that the optimal temperatures for sensor S1 and S2 were 300 °C and 350 °C, respectively. The sensor with lower Au9 concentration (S1) displays a faster response and recovery time, and a higher sensitivity toward H2. HRTEM studies reveal that the sensor S1 contain a significant population of sub-5 nm Au nanoparticles which might be responsible for a faster rate of H2 adsorption and dissociation. The key finding in this study suggest that the addition of catalytic layer such as ultra-small Au9 clusters results in improved sensitivity and dynamic performance (response and recovery time) of H2 sensors. In summary, this thesis demonstrated that cluster-based nanomaterials have wide range of applications spanning from catalysis to sensing. Further improvements in material synthesis and use of multiple complimentary characterization techniques allowed better understanding of the nature of the key active species (metal nanoparticles) assisting design of catalysts and sensors with enhanced performance

Factors influencing the catalytic oxidation of benzyl alcohol using supported phosphine-capped gold nanoparticles

Open Access Article. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.Two phosphine-stabilised gold clusters, Au101(PPh3)21Cl5 and Au9(PPh3)8(NO3)3, were deposited and activated on anatase TiO2 and fumed SiO2. These catalysts showed an almost complete oxidation of benzyl alcohol (>90%) within 3 hours at 80 °C and 3 bar O2 in methanol with a high substrate-to-metal molar ratio of 5800 and turn-over frequency of 0.65 s−1. Factors influencing catalytic activity were investigated, including metal–support interaction, effects of heat treatments, chemical composition of gold clusters, the size of gold nanoparticles and catalytic conditions. It was found that the anions present in gold clusters play a role in determining the catalytic activity in this reaction, with NO3− diminishing the catalytic activity. High catalytic activity was attributed to the formation of large gold nanoparticles (>2 nm) that coincides with partial removal of ligands which occurs during heat treatment and catalysis. Selectivity towards the formation of methyl benzoate can be tuned by selection of the reaction temperature. The catalysts were characterised using transmission electron microscopy, UV-vis diffuse reflectance spectroscopy and X-ray photoelectron spectroscopy

Gold Photocatalysis in Sustainable Hydrogen Peroxide Generation

Hydrogen peroxide (H2O2) is a mild and green oxidant widely employed in organic syntheses, medical sector, disinfection, pulp bleaching, environmental remediation and biological processes. However, its production via the expensive, multiple steps and energy intensive anthraquinone process renders it less sustainable. Photocatalysis is a viable, sustainable and promising strategy to produce H2O2 from green sources: water and molecular O2. This article presents the key developments of photocatalytic H2O2 production using gold (Au) nanoparticles supported on semiconductor photocatalysts. Several photocatalytic systems containing Au nanoparticles and the roles of Au nanoparticles in enhancing the photocatalytic H2O2 production including increasing the visible light absorption, facilitating the charge carrier separation and transfer, and as a catalytic Au active site are discussed. Factors defining the photocatalytic activity such as the effects of Au particle size and loading, localised surface plasmon resonance, mixed-gold component, and design of photocatalysts are reviewed. Finally, the challenges and prospect for further developments of Au photocatalysis in sustainable H2O2 synthesis as well as other related applications are highlighted

Benzyl alcohol oxidation using gold catalysts derived from Au8 clusters on TiO2

Atomically-precise gold clusters have gained attraction in catalysis due to high fraction of low-coordinated gold atoms, unique structural geometry and ligand effect. Phosphine-ligated gold clusters offer an advantage in the light of the labile gold-phosphorous bond for easy ligand removal. Here, heterogeneous gold catalysts were prepared by depositing atomically-precise phosphine-ligated gold clusters, Au8(PPh3)8(NO3)2 onto anatase-phase TiO2 nanoparticles. The catalysts were then calcined under two different conditions: O2(Au8/TiO2:O2) and O2 followed by H2(Au8/TiO2:O2–H2) at 200°C, to dislodge phosphine ligands from the Au core. It was found that Au8/TiO2:O2–H2 catalyst showed a decent catalytic activity in benzyl alcohol oxidation while Au8/TiO2 and Au8/TiO2:O2 were completely inactive. Such results imply that small-size gold clusters (2–3nm) alone do not always contribute to high catalytic activity of gold catalysts. It is suggested that the presence of NO3− species defines the catalytic activity of supported gold clusters in benzyl alcohol oxidation in the case of these catalysts and reinforces our initial claim in the previous work

A review of state of the art in phosphine ligated gold clusters and application in catalysis

Atomically precise gold clusters are highly desirable due to their well-defined structure which allows the study of structure–property relationships. In addition, they have potential in technological applications such as nanoscale catalysis. The structural, chemical, electronic, and optical properties of ligated gold clusters are strongly defined by the metal–ligand interaction and type of ligands. This critical feature renders gold–phosphine clusters unique and distinct from other ligand-protected gold clusters. The use of multidentate phosphines enables preparation of varying core sizes and exotic structures beyond regular polyhedrons. Weak gold–phosphorous (Au–P) bonding is advantageous for ligand exchange and removal for specific applications, such as catalysis, without agglomeration. The aim of this review is to provide a unified view of gold–phosphine clusters and to present an in-depth discussion on recent advances and key developments for these clusters. This review features the unique chemistry, structural, electronic, and optical properties of gold–phosphine clusters. Advanced characterization techniques, including synchrotron-based spectroscopy, have unraveled substantial effects of Au–P interaction on the composition-, structure-, and size-dependent properties. State-of-the-art theoretical calculations that reveal insights into experimental findings are also discussed. Finally, a discussion of the application of gold–phosphine clusters in catalysis is presented

Photocatalytic H2O2 Production Over Photocatalysts Prepared By Phosphine-protected Au101 Nanoparticles on WO3

Photocatalytic H2O2 synthesis is an appealing and feasible strategy to replace the energy- intensive, tedious, and waste-generating anthraquinone process. Often, pure metal oxides show low activity in photocatalytic H2O2 production and therefore metal co-catalysts are required to improve the photoactivity. This work investigated photocatalytic H2O2 production using monodisperse gold nanoclusters Au101(PPh3)21Cl5 supported on WO3. From HRTEM imaging, the Au101 size in the uncalcined samples is in the cluster regime (<2 nm) and after calcination at 200 °C the size increases to ca. 4.5 nm. The roles of Au101 have been identified to reduce the charge carrier recombination and provide the active sites for O2 reduction which significantly enhances the photoactivity. Both uncalcined and calcined Au101/WO3 photocatalysts produce over 75 mM g-1 h-1 of H2O2 under UV light irradiation while the pure WO3 is inactive. At early times (up to 30 min), the production rate of H2O2 from calcined Au101/WO3 reaches 173 mM g-1 h-1 and is almost double the rate of the uncalcined catalyst (93 mM g-1 h-1). The higher photoactivity of calcined versus uncalcined Au101/WO3 can be attributed to the aggregated Au101 and removal of phosphine ligands from the Au core as verified by HRTEM and XPS. The reaction rate decreases over time which is attributed to the reverse reaction. Using a simple kinetic model, the rate constant of the H2O2 formation (kf) for uncalcined and calcined Au101/WO3 are 2.07 and 6.31 mM h-1, while the rate constant of the H2O2 decomposition (kd) for uncalcined and calcined Au101/WO3 are 0.49 and 2.93 h-1, respectively. This work highlights a simple preparation of highly active photocatalysts to produce H2O2 derived from Au101 clusters and WO3

Identification of the Vibrational Modes in the Far-Infrared Spectra of Ruthenium Carbonyl Clusters and the Effect of Gold Substitution

High-quality far-IR absorption spectra for a series of ligated atomically precise clusters containing Ru₃, Ru₄, and AuRu₃ metal cores have been observed using synchrotron radiation, the latter two for the first time. The experimental spectra are compared with predicted IR spectra obtained following complete geometric optimization of the full cluster, including all ligands, using DFT. We find strong correlations between the experimental and predicted transitions for the low-frequency, low-intensity metal core vibrations as well as the higher frequency and intensity metal–ligand vibrations. The metal core vibrational bands appear at 150 cm^–1 for Ru₃(CO)₁₂, and 153 and 170 cm^–1 for H₄Ru₄(CO)₁₂, while for the bimetallic Ru₃(μ-AuPPh₃)­(μ-Cl)­(CO)₁₀ cluster these are shifted to 177 and 299 cm^–1 as a result of significant restructuring of the metal core and changes in chemical composition. The computationally predicted IR spectra also reveal the expected atomic motions giving rise to the intense peaks of metal–ligand vibrations at <i>ca</i>. 590 cm^–1 for Ru₃, 580 cm^–1 for Ru₄, and 560 cm^–1 for AuRu₃. The obtained correlations allow an unambiguous identification of the key vibrational modes in the experimental far-IR spectra of these clusters for the first time

Toward Control of Gold Cluster Aggregation on TiO₂ via Surface Treatments

Well-defined Au–TiO₂ materials were synthesized by deposition of triphenylphosphine-protected Au₉ clusters on TiO₂ (Aeroxide P-25), pre-treated in eight different ways and subsequently exposed to two post-treatments. X-ray photoelectron spectroscopy and UV–vis diffuse reflectance spectroscopy studies showed that in most cases the PPh₃ ligand shell was removed upon deposition even before post-treatment, coinciding with some cluster aggregation. However, clusters deposited on TiO₂ treated using H₂SO₄ and H₂O₂ showed remarkable resistance to aggregation, even after high-temperature calcination, while clusters on H₂-treated TiO₂ showed the greatest resistance to aggregation under ozonolysis

Chemically synthesised atomically precise gold clusters deposited and activated on titania. Part II

Synchrotron XPS was used to investigate a series of chemically synthesised, atomically precise gold clusters Au(n)(PPh3)y (n = 8, 9 and 101, y depending on the cluster size) immobilized on anatase (titania) nanoparticles. Effects of post-deposition treatments were investigated by comparison of untreated samples with analogues that have been heat treated at 200 °C in O2, or in O2 followed by H2 atmosphere. XPS data shows that the phosphine ligands are oxidised upon heat treatment in O2. From the position of the Au 4f(7/2) peak it can be concluded that the clusters partially agglomerate immediately upon deposition. Heating in oxygen, and subsequently in hydrogen, leads to further agglomeration of the gold clusters. It is found that the pre-treatment plays a crucial role in the removal of ligands and agglomeration of the clusters.David P. Anderson, Rohul H. Adnan, Jason F. Alvino, Oliver Shipper, Baira Donoeva, Jan-Yves Ruzicka, Hassan Al Qahtani, Hugh H. Harris, Bruce Cowie, Jade B. Aitken, Vladimir B. Golovko, Gregory F. Metha and Gunther G. Andersso