Christopher Miller

All Publications

• Amine Hole Scavengers Facilitate Both Electron and Hole Transfer in a Nanocrystal/Molecular Hybrid Photocatalyst. Journal of the American Chemical Society Gebre, S. T., Kiefer, L. M., Guo, F., Yang, K. R., Miller, C., Liu, Y., Kubiak, C. P., Batista, V. S., Lian, T. 2023; 145 (5): 3238-3247

  Abstract

  A well-known catalyst, fac-Re(4,4'-R2-bpy)(CO)3Cl (bpy = bipyridine; R = COOH) (ReC0A), has been widely studied for CO2 reduction; however, its photocatalytic performance is limited due to its narrow absorption range. Quantum dots (QDs) are efficient light harvesters that offer several advantages, including size tunability and broad absorption in the solar spectrum. Therefore, photoinduced CO2 reduction over a broad range of the solar spectrum could be enabled by ReC0A catalysts heterogenized on QDs. Here, we investigate interfacial electron transfer from Cd3P2 QDs to ReC0A complexes covalently bound on the QD surface, induced by photoexcitation of the QD. We explore the effect of triethylamine, a sacrificial hole scavenger incorporated to replenish the QD with electrons. Through combined transient absorption spectroscopic and computational studies, we demonstrate that electron transfer from Cd3P2 to ReC0A can be enhanced by a factor of ∼4 upon addition of triethylamine. We hypothesize that the rate enhancement is a result of triethylamine possibly altering the energetics of the Cd3P2-ReC0A system by interacting with the quantum dot surface, deprotonation of the quantum dot, and preferential solvation, resulting in a shift of the conduction band edge to more negative potentials. We also observe the rate enhancement in other QD-electron acceptor systems. Our findings provide mechanistic insights into hole scavenger-quantum dot interactions and how they may influence photoinduced interfacial electron transfer processes.

  View details for DOI 10.1021/jacs.2c13464

  View details for PubMedID 36706437

  View details for PubMedCentralID PMC9912264

• PM-IRRAS and DFT investigation of the surface orientation of new Ir piano-stool complexes attached to Au(111). Dalton transactions (Cambridge, England : 2003) Miller, C. J., Brunner, F. M., Kelly, H. R., Cheung, P. L., Torquato, N. A., Gembicky, M., Okuno, S., Chan, T., Batista, V. S., Kubiak, C. P. 2022; 51 (46): 17688-17699

  Abstract

  Surface immobilization of organometallic catalysts is a promising approach to developing new catalytic systems that combine molecular catalysts with heterogenous surfaces to probe surface mechanisms. The orientation of the catalyst relative to the surface is one important parameter that must be considered in such hybrid systems. In this work, we synthesize three new sulfide-modified Ir piano-stool complexes with sulfide-modified bipyridine and phenylpyridine ligands for the attachment to Au(111) surfaces. Self-assembled monolayers made from (Cp*Ir(2,2'-bipyridine-4-sulfide)Cl)2[Cl]2 (C1m) and [Cp*Ir(2-phenylpyridine-4-sulfide)Cl]2 (C2m) were characterized by combining polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) with DFT calculations of the minimum energy orientations of the complexes on the surface. We find that the bipyridine and phenylpyridine ligands are oriented at between 73-77° relative to the surface normal, irrespective of the orientation of the other ligands. Additionally, DFT and PM-IRRAS support that there is no orientation preference for C1m and C2m, with both orientations present on the surface.

  View details for DOI 10.1039/d2dt02730e

  View details for PubMedID 36345597

• Electrochemical Reduction of CO₂ Using Group VII Metal Catalysts TRENDS IN CHEMISTRY Barrett, J. A., Miller, C. J., Kubiak, C. P. 2021; 3 (3): 176-187

  View details for DOI 10.1016/j.trechm.2020.12.009

  View details for Web of Science ID 000634598900004

• Heterogenized Molecular Catalysts: Vibrational Sum-Frequency Spectroscopic, Electrochemical, and Theoretical Investigations. Accounts of chemical research Ge, A., Rudshteyn, B., Videla, P. E., Miller, C. J., Kubiak, C. P., Batista, V. S., Lian, T. 2019; 52 (5): 1289-1300

  Abstract

  Rhenium and manganese bipyridyl tricarbonyl complexes have attracted intense interest for their promising applications in photocatalytic and electrocatalytic CO2 reduction in both homogeneous and heterogenized systems. To date, there have been extensive studies on immobilizing Re catalysts on solid surfaces for higher catalytic efficiency, reduced catalyst loading, and convenient product separation. However, in order for the heterogenized molecular catalysts to achieve the combination of the best aspects of homogeneous and heterogeneous catalysts, it is essential to understand the fundamental physicochemical properties of such heterogeneous systems, such as surface-bound structures of Re/Mn catalysts, substrate-adsorbate interactions, and photoinduced or electric-field-induced effects on Re/Mn catalysts. For example, the surface may act to (un)block substrates, (un)trap charges, (de)stabilize particular intermediates (and thus affect scaling relations), and shift potentials in different directions, just as protein environments do. The close collaboration between the Lian, Batista, and Kubiak groups has resulted in an integrated approach to investigate how the semiconductor or metal surface affects the properties of the attached catalyst. Synthetic strategies to achieve stable and controlled attachment of Re/Mn molecular catalysts have been developed. Steady-state, time-resolved, and electrochemical vibrational sum-frequency generation (SFG) spectroscopic studies have provided insight into the effects of interfacial structures, ultrafast vibrational energy relaxation, and electric field on the Re/Mn catalysts, respectively. Various computational methods utilizing density functional theory (DFT) have been developed and applied to determine the molecular orientation by direct comparison to spectroscopy, unravel vibrational energy relaxation mechanisms, and quantify the interfacial electric field strength of the Re/Mn catalyst systems. This Account starts with a discussion of the recent progress in determining the surface-bound structures of Re catalysts on semiconductor and Au surfaces by a combined vibrational SFG and DFT study. The effects of crystal facet, length of anchoring ligands, and doping of the semiconductor on the bound structures of Re catalysts and of the substrate itself are discussed. This is followed by a summary of the progress in understanding the vibrational relaxation (VR) dynamics of Re catalysts covalently adsorbed on semiconductor and metal surfaces. The VR processes of Re catalysts on TiO2 films and TiO2 single crystals and a Re catalyst tethered on Au, particularly the role of electron-hole pair (EHP)-induced coupling on the VR of the Re catalyst bound on Au, are discussed. The Account also summarizes recent studies in quantifying the electric field strength experienced by the catalytically active site of the Re/Mn catalyst bound on a Au electrode based on a combined electrochemical SFG and DFT study of the Stark tuning of the CO stretching modes of these catalysts. Finally, future research directions on surface-immobilized molecular catalyst systems are discussed.

  View details for DOI 10.1021/acs.accounts.9b00001

  View details for PubMedID 31056907

• Selective Reduction of CO₂ to CO by a Molecular Re(ethynyl-bpy)(CO)₃Cl Catalyst and Attachment to Carbon Electrode Surfaces ORGANOMETALLICS Zhanaidarova, A., Ostericher, A. L., Miller, C. J., Jones, S. C., Kubiak, C. P. 2019; 38 (6): 1204-1207

  View details for DOI 10.1021/acs.organomet.8b00547

  View details for Web of Science ID 000462944200004

• CO2 Reduction Catalysts on Gold Electrode Surfaces Influenced by Large Electric Fields. Journal of the American Chemical Society Clark, M. L., Ge, A., Videla, P. E., Rudshteyn, B., Miller, C. J., Song, J., Batista, V. S., Lian, T., Kubiak, C. P. 2018; 140 (50): 17643-17655

  Abstract

  Attaching molecular catalysts to metal and semiconductor electrodes is a promising approach to developing new catalytic electrodes with combined advantages of molecular and heterogeneous catalysts. However, the effect of the interfacial electric field on the stability, activity, and selectivity of the catalysts is often poorly understood due to the complexity of interfaces. In this work, we examine the strength of the interfacial field at the binding site of CO2 reduction catalysts including Re(S-2,2'-bipyridine)(CO)3Cl and Mn(S-2,2'-bipyridine)(CO)3Br immobilized on Au electrodes. The vibrational spectra are probed by sum frequency generation spectroscopy (SFG), showing pronounced potential-dependent frequency shifts of the carbonyl stretching modes. Calculations of SFG spectra and Stark tuning rates based on density functional theory allow for direct interpretation of the configurations of the catalysts bound to the surfaces and the influence of the interfacial electric field. We find that electrocatalysts supported on Au electrodes have tilt angles of about 65-75° relative to the surface normal with one of the carbonyl ligands in direct contact with the surface. Large interfacial electric fields of 108-109 V/m are determined through the analysis of experimental frequency shifts and theoretical Stark tuning rates of the symmetric CO stretching mode. These large electric fields thus significantly influence the CO2 binding site.

  View details for DOI 10.1021/jacs.8b09852

  View details for PubMedID 30468391

• The Effect of Ar/O2 and H2O Plasma Treatment of SnO2 Nanoparticles and Nanowires on Carbon Monoxide and Benzene Detection. ACS applied materials & interfaces Stuckert, E. P., Miller, C. J., Fisher, E. R. 2017; 9 (18): 15733-15743

  Abstract

  As the final piece of a broader study on structure-property performance of SnO2 sensors, this study examines the performance of sensors created from tin(IV) oxide (SnO2) nanowires and nanoparticles as a function of temperature for untreated (UT) devices as well as those treated using Ar/O2 and H2O plasmas. Nanoparticle and nanowire sensors were exposed to air, carbon monoxide (CO), or benzene (C6H6) to determine sensor response (Rair/Rgas) and sensitivity (Rair/Rgas > 1 or Rgas/Rair > 1). Although both Ar/O2 and H2O plasma modification minimally increase sensor sensitivity toward CO and C6H6 under most conditions, this study explores initial plasma parameters of a wide array of plasma precursors to better understand the materials properties and gas-phase species that lead to specific sensing capabilities. In particular, certain Ar/O2 and H2O plasma treatment conditions resulted in increased sensitivity over UT nanomaterials at 25 and 50 °C, but of greatest importance is the knowledge gained from the combined materials, gas-phase, and sensor performance analysis that provide greater insight for effectively selecting future materials and modification systems to achieve optimal gas sensor performance.

  View details for DOI 10.1021/acsami.7b05680

  View details for PubMedID 28441469

• Gas-phase diagnostics during H₂ and H₂O plasma treatment of SnO₂ nanomaterials: Implications for surface modification JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B Stuckert, E. P., Miller, C. J., Fisher, E. R. 2017; 35 (2)

  View details for DOI 10.1116/1.4976534

  View details for Web of Science ID 000397858500047

• In-Depth View of the Structure and Growth of SnO2 Nanowires and Nanobrushes. ACS applied materials & interfaces Stuckert, E. P., Geiss, R. H., Miller, C. J., Fisher, E. R. 2016; 8 (34): 22345-53

  Abstract

  Strategic application of an array of complementary imaging and diffraction techniques is critical to determine accurate structural information on nanomaterials, especially when also seeking to elucidate structure-property relationships and their effects on gas sensors. In this work, SnO2 nanowires and nanobrushes grown via chemical vapor deposition (CVD) displayed the same tetragonal SnO2 structure as revealed via powder X-ray diffraction bulk crystallinity data. Additional characterization using a range of electron microscopy imaging and diffraction techniques, however, revealed important structure and morphology distinctions between the nanomaterials. Tailoring scanning transmission electron microscopy (STEM) modes combined with transmission electron backscatter diffraction (t-EBSD) techniques afforded a more detailed view of the SnO2 nanostructures. Indeed, upon deeper analysis of individual wires and brushes, we discovered that, despite a similar bulk structure, wires and brushes grew with different crystal faces and lattice spacings. Had we not utilized multiple STEM diffraction modes in conjunction with t-EBSD, differences in orientation related to bristle density would have been overlooked. Thus, it is only through a methodical combination of several structural analysis techniques that precise structural information can be reliably obtained.

  View details for DOI 10.1021/acsami.6b06676

  View details for PubMedID 27538262

• Structure and Lewis-base reactivity of bicyclic low-valent germanium and tin complexes bridged by bis(diisopropylphosphino)amine POLYHEDRON Miller, C. J., Chadha, U., Ulibarri-Sanchez, J. R., Dickie, D. A., Kemp, R. A. 2016; 114: 351-359

  View details for DOI 10.1016/j.poly.2016.01.028

  View details for Web of Science ID 000379564400050