Design of robust 2,2'-bipyridine ligand linkers for the stable immobilization of molecular catalysts on silicon(111) surfaces.

The attachment of the 2,2'-bipyridine (bpy) moieties to the surface of planar silicon(111) (photo)electrodes was investigated using ab initio simulations performed on a new cluster model for methyl-terminated silicon. Density functional theory (B3LYP) with implicit solvation techniques indicated that adventitious chlorine atoms, when present in the organic linker backbone, led to instability at very negative potentials of the surface-modified electrode. In prior experimental work, chlorine atoms were present as a trace surface impurity due to required surface processing chemistry, and thus could plausibly result in the observed surface instability of the linker. Free energy calculations for the Cl-atom release process with model silyl-linker constructs revealed a modest barrier (14.9 kcal mol-1) that decreased as the electrode potential became more negative. A small library of new bpy-derived structures has additionally been explored computationally to identify strategies that could minimize chlorine-induced linker instability. Structures with fluorine substituents are predicted to be more stable than their chlorine analogues, whereas fully non-halogenated structures are predicted to exhibit the highest stability. The behavior of a hydrogen-evolving molecular catalyst Cp*Rh(bpy) (Cp* = pentamethylcyclopentadienyl) immobilized on a silicon(111) cluster was explored theoretically to evaluate differences between the homogeneous and surface-attached behavior of this species in a tautomerization reaction observed under reductive conditions for catalytic H2 evolution. The calculated free energy difference between the tautomers is small, hence the results suggest that use of reductively stable linkers can enable robust attachment of catalysts while maintaining chemical behavior on the electrode similar to that exhibited in homogeneous solution.

Atomic force microscopy: Emerging illuminated and operando techniques for solar fuel research.

Integrated photoelectrochemical devices rely on the synergy between components to efficiently generate sustainable fuels from sunlight. The micro- and/or nanoscale characteristics of the components and their interfaces often control critical processes of the device, such as charge-carrier generation, electron and ion transport, surface potentials, and electrocatalysis. Understanding the spatial properties and structure-property relationships of these components can provide insight into designing scalable and efficient solar fuel components and systems. These processes can be probed ex situ or in situ with nanometer-scale spatial resolution using emerging scanning-probe techniques based on atomic force microscopy (AFM). In this Perspective, we summarize recent developments of AFM-based techniques relevant to solar fuel research. We review recent progress in AFM for (1) steady-state and dynamic light-induced surface photovoltage measurements; (2) nanoelectrical conductive measurements to resolve charge-carrier heterogeneity and junction energetics; (3) operando investigations of morphological changes, as well as surface electrochemical potentials, currents, and photovoltages in liquids. Opportunities for research include: (1) control of ambient conditions for performing AFM measurements; (2) in situ visualization of corrosion and morphological evolution of electrodes; (3) operando AFM techniques to allow nanoscale mapping of local catalytic activities and photo-induced currents and potentials.

Stable solar-driven oxidation of water by semiconducting photoanodes protected by transparent catalytic nickel oxide films.

Reactively sputtered nickel oxide (NiOx) films provide transparent, antireflective, electrically conductive, chemically stable coatings that also are highly active electrocatalysts for the oxidation of water to O2(g). These NiOx coatings provide protective layers on a variety of technologically important semiconducting photoanodes, including textured crystalline Si passivated by amorphous silicon, crystalline n-type cadmium telluride, and hydrogenated amorphous silicon. Under anodic operation in 1.0 M aqueous potassium hydroxide (pH 14) in the presence of simulated sunlight, the NiOx films stabilized all of these self-passivating, high-efficiency semiconducting photoelectrodes for >100 h of sustained, quantitative solar-driven oxidation of water to O2(g).

Photoelectrochemical Behavior of a Molecular Ru-Based Water-Oxidation Catalyst Bound to TiO2-Protected Si Photoanodes.

A hybrid photoanode based on a molecular water oxidation precatalyst was prepared from TiO2-protected n- or p+-Si coated with multiwalled carbon nanotubes (CNT) and the ruthenium-based water oxidation precatalyst [RuIV(tda)(py-pyr)2(O)], 1(O) (tda2- is [2,2':6',2″-terpyridine]-6,6″-dicarboxylato and py-pir is 4-(pyren-1-yl)-N-(pyridin-4-ylmethyl)butanamide). The Ru complex was immobilized by π-π stacking onto CNTs that had been deposited by drop casting onto Si electrodes coated with 60 nm of amorphous TiO2 and 20 nm of a layer of sputtered C. At pH = 7 with 3 Sun illumination, the n-Si/TiO2/C/CNT/[1+1(O)] electrodes exhibited current densities of 1 mA cm-2 at 1.07 V vs NHE. The current density was maintained for >200 min at a constant potential while intermittently collecting voltammograms that indicated that over half of the Ru was still in molecular form after O2 evolution.

Organoimido-Polyoxometalate Nonlinear Optical Chromophores: A Structural, Spectroscopic, and Computational Study.

Ten organoimido polyoxometalate (POM)-based chromophores have been synthesized and studied by hyper-Rayleigh scattering (HRS), Stark and Resonance Raman spectroscopies, and density functional theory (DFT) calculations. HRS ß0 values for chromophores with resonance electron donors are significant (up to 139 × 10-30 esu, ∼5 times greater than that of the DAS+ cation), but systems with no donor, or the -NO2 acceptor show no activity, in some cases, despite large DFT-predicted ß-values. In active systems with short (phenyl) π-bridges, ß0 values comfortably exceed that of the purely organic structural analogue N,N-dimethyl-4-nitroaniline (DMPNA), and intrinsic ß-values, ß0/N3/2 (where N is the number of bridge π-electrons) thus appear to break empirical performance limits (ß0/N3/2 vs λmax) for planar organic systems. However, ß0 values obtained for extended systems with a diphenylacetylene bridge are comparable to or lower than that of their nitro analogue, N,N-dimethyl-4-[(4-nitrophenyl)ethynyl]-aniline (DMNPEA). Resonance Raman spectroscopy confirms the involvement of the POM in the electronic transitions, whether donor groups are present or not, but Stark spectroscopy indicates that, in their absence, the transitions have little dipolar character (hence, NLO inactive), consistent with DFT-calculated frontier orbitals, which extend over both POM and organic group. Stark and DFT also suggest that ß is enhanced in the short compounds because the extension of charge transfer (CT) onto the POM increases changes in the excited-state dipole moment. With extended π-systems, this effect does not increase CT distances, relative to a -NO2 acceptor, so ß0 values do not exceed that of DMNPEA. Overall, our results show that (i) the organoimido-POM unit is an efficient acceptor for second-order NLO, but an ineffective donor; (ii) the nature of electronic transitions in arylimido-POMs is strongly influenced by the substituents of the aryl group; and (iii) organoimido-POMs outperform organic acceptors with short π-bridges, but lose their advantage with extended π-conjugation.

Atomic force microscopy with nanoelectrode tips for high resolution electrochemical, nanoadhesion and nanoelectrical imaging.

Multimodal nano-imaging in electrochemical environments is important across many areas of science and technology. Here, scanning electrochemical microscopy (SECM) using an atomic force microscope (AFM) platform with a nanoelectrode probe is reported. In combination with PeakForce tapping AFM mode, the simultaneous characterization of surface topography, quantitative nanomechanics, nanoelectronic properties, and electrochemical activity is demonstrated. The nanoelectrode probe is coated with dielectric materials and has an exposed conical Pt tip apex of ∼200 nm in height and of ∼25 nm in end-tip radius. These characteristic dimensions permit sub-100 nm spatial resolution for electrochemical imaging. With this nanoelectrode probe we have extended AFM-based nanoelectrical measurements to liquid environments. Experimental data and numerical simulations are used to understand the response of the nanoelectrode probe. With PeakForce SECM, we successfully characterized a surface defect on a highly-oriented pyrolytic graphite electrode showing correlated topographical, electrochemical and nanomechanical information at the highest AFM-SECM resolution. The SECM nanoelectrode also enabled the measurement of heterogeneous electrical conductivity of electrode surfaces in liquid. These studies extend the basic understanding of heterogeneity on graphite/graphene surfaces for electrochemical applications.

Tunable Chiral Second-Order Nonlinear Optical Chromophores Based on Helquat Dications.

Fourteen new dipolar cations have been synthesized, containing methoxy or tertiary amino electron donor groups attached to helquat (Hq) acceptors. These Hq derivatives have been characterized as their TfO- salts by using various techniques including NMR and electronic absorption spectroscopies. UV-vis spectra show intense, relatively low energy absorptions with λmax ≈ 400-600 nm, attributable to intramolecular charge-transfer (ICT) excitations. Single-crystal X-ray structures have been solved for two of the chromophores, one as its PF6- salt, revealing centrosymmetric packing arrangements (space groups Pbca and P1̅). Molecular quadratic nonlinear optical (NLO) responses have been determined directly by using hyper-Rayleigh scattering (HRS) with a 800 nm laser, and indirectly via Stark (electroabsorption) spectroscopy for the low energy absorption bands. The obtained static first hyperpolarizabilities ß0 range from moderate to large: (9-140) × 10-30 esu from HRS in MeCN and (44-580) × 10-30 esu from the Stark data in PrCN. The magnitude of ß0 increases upon either extending the π-conjugation length or replacing a methoxy with a tertiary amino electron donor substituent. Density functional theory (DFT) and time-dependent DFT calculations on selected tertiary amino chromophores confirm that the low energy absorptions have ICT character. Relatively good agreement between the simulated and experimental UV-vis absorption spectra is achieved by using the CAM-B3LYP functional with the 6-311G(d) basis set. The ßtot values predicted by using DFT at the same level of theory are large ((472-1443) × 10-30 esu in MeCN). Both the theoretical and experimental results show that para-conjugation between Hq and electron donor fragments is optimal, and enlarging the Hq unit is inconsequential with respect to the molecular quadratic NLO response.

Helquat Dyes: Helicene-like Push-Pull Systems with Large Second-Order Nonlinear Optical Responses.

Helquat dyes combine a cationic hemicyanine with a helicene-like motif to form a new blueprint for chiral systems with large and tunable nonlinear optical (NLO) properties. We report a series of such species with characterization, including determination of static first hyperpolarizabilities ß0 via hyper-Rayleigh scattering and Stark spectroscopy. The measured ß0 values are similar to or substantially larger than that of the commercial chromophore E-4'-(dimethylamino)-N-methyl-4-stilbazolium. Density functional theory (DFT) and time-dependent DFT calculations on two of the new cations are used to probe their molecular electronic structures and optical properties. Related molecules are expected to show bulk second-order NLO effects in even nonpolar media, overcoming a key challenge in developing useful materials.

Synthesis and Characterization of Atomically Flat Methyl-Terminated Ge(111) Surfaces.

Atomically flat, terraced H-Ge(111) was prepared by annealing in H2(g) at 850 °C. The formation of monohydride Ge-H bonds oriented normal to the surface was indicated by angle-dependent Fourier-transform infrared (FTIR) spectroscopy. Subsequent reaction in CCl3Br(l) formed Br-terminated Ge(111), as shown by the disappearance of the Ge-H absorption in the FTIR spectra concomitant with the appearance of Br photoelectron peaks in X-ray photoelectron (XP) spectra. The Br-Ge(111) surface was methylated by reaction with (CH3)2Mg. These surfaces exhibited a peak at 568 cm(-1) in the high-resolution electron energy loss spectrum, consistent with the formation of a Ge-C bond. The absorption peaks in the FTIR spectra assigned to methyl "umbrella" and rocking modes were dependent on the angle of the incident light, indicating that the methyl groups were bonded directly atop surface Ge atoms. Atomic-force micrographs of CH3-Ge(111) surfaces indicated that the surface remained atomically flat after methylation. Electrochemical scanning-tunneling microscopy showed well-ordered methyl groups that covered nearly all of the surface. Low-energy electron diffraction images showed sharp, bright diffraction spots with a 3-fold symmetry, indicating a high degree of order with no evidence of surface reconstruction. A C 1s peak at 284.1 eV was observed in the XP spectra, consistent with the formation of a C-Ge bond. Annealing in ultrahigh vacuum revealed a thermal stability limit of ∼400 °C of the surficial CH3-Ge(111) groups. CH3-Ge(111) surfaces showed significantly greater resistance to oxidation in air than H-Ge(111) surfaces.

Catalytic hydrogen evolution from a covalently linked dicobaloxime.

A dicobaloxime in which monomeric Co(III) units are linked by an octamethylene bis(glyoxime) catalyzes the reduction of protons from p-toluenesulfonic acid as evidenced by electrocatalytic waves at -0.4 V vs. the saturated calomel electrode (SCE) in acetonitrile solutions. Rates of hydrogen evolution were determined from catalytic current peak heights (k(app) = 1100 ± 70 M(-1) s(-1)). Electrochemical experiments reveal no significant enhancement in the rate of H(2) evolution from that of a monomeric analogue: The experimental rate law is first order in catalyst and acid consistent with previous findings for similar mononuclear cobaloximes. Our work suggests that H(2) evolution likely occurs by protonation of reductively generated Co(II)H rather than homolysis of two Co(III)H units.

Photoelectrochemical behavior of hierarchically structured Si/WO3 core-shell tandem photoanodes.

WO3 thin films have been deposited in a hierarchically structured core-shell morphology, with the cores consisting of an array of Si microwires and the shells consisting of a controlled morphology WO3 layer. Porosity was introduced into the WO3 outer shell by using a self-assembled microsphere colloidal crystal as a mask during the deposition of the WO3 shell. Compared to conformal, unstructured WO3 shells on Si microwires, the hierarchically structured core-shell photoanodes exhibited enhanced near-visible spectral response behavior, due to increased light absorption and reduced distances over which photogenerated carriers were collected. The use of structured substrates also improved the growth rate of microsphere-based colloidal crystals and suggests strategies for the use of colloidal materials in large-scale applications.

Catalysis of proton reduction by a [BO4]-bridged dicobalt glyoxime.

We report the preparation of a dicobalt compound with two singly proton-bridged cobaloxime units linked by a central [BO4] bridge. Reaction of a doubly proton-bridged cobaloxime complex with trimethyl borate afforded the compound in good yield. Single-crystal X-ray diffraction studies confirmed the bridging nature of the [BO4] moiety. Using electrochemical methods, the dicobalt complex was found to be an electrocatalyst for proton reduction in acetonitrile solution. Notably, the overpotential for proton reduction (954 mV) was found to be higher than in the cases of two analogous single-site cobalt glyoximes under virtually identical conditions.

Synthesis, structures, and optical properties of ruthenium(II) complexes of the tris(1-pyrazolyl)methane ligand.

Four new complex salts [Ru(II)Cl(Tpm)(L(A))2][PF6]n [Tpm = tris(1-pyrazolyl)methane; n = 1, L(A) = pyridine (py) 1 or ethyl isonicotinate (EIN) 2; n = 3, L(A) = N-methyl-4,4'-bipyridinium (MeQ(+)) 3 or N-phenyl-4,4'-bipyridinium (PhQ(+)) 4] have been prepared and characterized. Electronic absorption spectra show intense d → π* metal-to-ligand charge-transfer (MLCT) absorption bands, while cyclic voltammetry reveals a reversible Ru(III/II) wave, accompanied by quasireversible or irreversible L(A)-based reductions for all except 1. Single crystal X-ray structures have been obtained for 1•Me2CO, 2, and 3•Me2CO. For 2-4, molecular first hyperpolarizabilities ß have been measured in acetonitrile solutions via the hyper-Rayleigh scattering (HRS) technique at 800 nm. Stark (electroabsorption) spectroscopic studies on the MLCT bands in frozen butyronitrile allow the indirect estimation of static first hyperpolarizabilities ß0. The various physical data obtained for 3 and 4 are compared with those reported previously for related cis-{Ru(II)(NH3)4}(2+) species [ Coe, B. J. et al. J. Am. Chem. Soc. 2005 , 127 , 4845 ]. TD-DFT calculations on the complexes in 1-4 confirm that their lowest energy absorption bands are primarily Ru(II) → L(A) MLCT in character, while Ru(II) → Tpm MLCT transitions are predicted at higher energies. DFT agrees with the Stark, but not the HRS measurements, in showing that ß0 increases with the electron-accepting strength of L(A). The 2D nature of the chromophores is evidenced by dominant ßxxy tensor components.

Noncovalent immobilization of electrocatalysts on carbon electrodes for fuel production.

We show that molecular catalysts for fuel-forming reactions can be immobilized on graphitic carbon electrode surfaces via noncovalent interactions. A pyrene-appended bipyridine ligand (P) serves as the linker between each complex and the surface. Immobilization of a rhodium proton-reduction catalyst, [Cp*Rh(P)Cl]Cl (1), and a rhenium CO2-reduction catalyst, Re(P)(CO)3Cl (2), afford electrocatalytically active assemblies. X-ray photoelectron spectroscopy and electrochemistry confirm catalyst immobilization. Reduction of 1 in the presence of p-toluenesulfonic acid results in catalytic H2 production, while reduction of 2 in the presence of CO2 results in catalytic CO production.

Heck coupling of olefins to mixed methyl/thienyl monolayers on Si(111) surfaces.

The Heck reaction has been used to couple olefins to a Si(111) surface that was functionalized with a mixed monolayer comprised of methyl and thienyl groups. The coupling method maintained a conjugated linkage between the surface and the olefinic surface functionality, to allow for facile charge transfer from the silicon surface. While a Si(111) surface terminated only with thienyl groups displayed a surface recombination velocity, S, of 670 ± 190 cm s(-1), the mixed CH3/SC4H3-Si(111) surfaces with a coverage of θSC4H3 = 0.15 ± 0.02 displayed a substantially lower value of S = 27 ± 9 cm s(-1). Accordingly, CH3/SC4H3-Si(111) surfaces were brominated with N-bromosuccinimide, to produce mixed CH3/SC4H2Br-Si(111) surfaces with coverages of θBr-Si < 0.05. The resulting aryl halide surfaces were activated using [Pd(PPh3)4] as a catalyst. After activation, Pd(II) was selectively coordinated by oxidative addition to the surface-bound aryl halide. The olefinic substrates 4-fluorostyrene, vinylferrocene, and protoporphyrin IX dimethyl ester were then coupled (in dimethylformamide at 100 °C) to the Pd-containing functionalized Si surfaces. The porphyrin-modified surface was then metalated with Co, Cu, or Zn. The vinylferrocene-modified Si(111) surface showed a linear dependence of the peak current on scan rate in cyclic voltammetry, indicating that facile electron transfer had been maintained and providing evidence of a robust linkage between the Si surface and the tethered ferrocene. The final Heck-coupled surface exhibited S = 70 cm s(-1), indicating that high-quality surfaces could be produced by this multistep synthetic approach for tethering small molecules to silicon photoelectrodes.

Heptametallic, octupolar nonlinear optical chromophores with six ferrocenyl substituents.

New complexes with six ferrocenyl (Fc) groups connected to Zn(II) or Cd(II) tris(2,2'-bipyridyl) cores are described. A thorough characterisation of their BPh4(-) salts includes two single-crystal X-ray structures, highly unusual for such species with multiple, extended substituents. Intense, visible d(Fe(II))→π* metal-to-ligand charge-transfer (MLCT) bands accompany the π→π* intraligand charge-transfer absorptions in the near UV region. Each complex shows a single, fully reversible Fe(III/II) wave when probed electrochemically. Molecular quadratic nonlinear optical (NLO) responses are determined by using hyper-Rayleigh scattering and Stark spectroscopy. The latter gives static first hyperpolarisabilities ß0 reaching as high as approximately 10(-27)  esu and generally increasing with π-conjugation extension. Z-scan cubic NLO measurements reveal high two-photon absorption cross-sections σ2 of up to 5400 GM in one case. DFT calculations reproduce the π-conjugation dependence of ß0, and TD-DFT predicts three transitions close in energy contributing to the MLCT bands. The lowest energy transition has octupolar character, whereas the other two are degenerate and dipolar in nature.

Thermally stable N2-intercalated WO3 photoanodes for water oxidation.

We describe stable intercalation compounds of the composition xN(2)·WO(3) (x = 0.034-0.039), formed by trapping N(2) in WO(3). The incorporation of N(2) significantly reduced the absorption threshold of WO(3); notably, 0.039N(2)·WO(3) anodes exhibited photocurrent under illumination at wavelengths ≤640 nm with a faradaic efficiency for O(2) evolution in 1.0 M HClO(4)(aq) of nearly unity. Spectroscopic and computational results indicated that deformation of the WO(3) host lattice, as well as weak electronic interactions between trapped N(2) and the WO(3) matrix, contributed to the observed red shift in optical absorption. Noble-gas-intercalated WO(3) materials similar to xN(2)·WO(3) are predicted to function as photoanodes that are responsive to visible light.

Experimental and Theoretical Comparison of Potential-dependent Methylation on Chemically Exfoliated WS2 and MoS2.

Reductant-activated functionalization is shown to enhance the methylation of chemically exfoliated MoS2 (ceMoS2) and ceWS2 by introducing excess negative charge to facilitate a nucleophilic attack reaction. Relative to methylation in the absence of a reductant, the reaction produces a twofold increase in coverage of ceWS2, from 25 to 52% coverage per WS2. However, at every potential, the methyl coverage on ceWS2 was ∼20% lower than that on ceMoS2. We applied grand canonical density functional theory to show that at constant potential, more negative charge is present on 1T'-MoS2 than on 1T'-WS2, making methylation both thermodynamically and kinetically more favorable for 1T'-MoS2 than 1T'-WS2. This effect was moderated when the reactions were compared at constant charge, emphasizing the importance of comparing the reactivity of materials at nominally identical electrode potentials.

Photoelectrochemical hydrogen evolution using Si microwire arrays.

Arrays of B-doped p-Si microwires, diffusion-doped with P to form a radial n(+) emitter and subsequently coated with a 1.5-nm-thick discontinuous film of evaporated Pt, were used as photocathodes for H(2) evolution from water. These electrodes yielded thermodynamically based energy-conversion efficiencies >5% under 1 sun solar simulation, despite absorbing less than 50% of the above-band-gap incident photons. Analogous p-Si wire-array electrodes yielded efficiencies <0.2%, largely limited by the low photovoltage generated at the p-Si/H(2)O junction.

Combining very large quadratic and cubic nonlinear optical responses in extended, tris-chelate metallochromophores with six pi-conjugated pyridinium substituents.

We describe a series of nine new complex salts in which electron-rich Ru(II) or Fe(II) centers are connected via pi-conjugated bridges to six electron-accepting N-methyl-/N-arylpyridinium groups. This work builds upon our previous preliminary studies (Coe , B. J. J. Am. Chem. Soc. 2005, 127, 13399-13410; J. Phys. Chem. A 2007, 111, 472-478), with the aims of achieving greatly enhanced NLO properties and also combining large quadratic and cubic effects in potentially redox-switchable molecules. Characterization has involved various techniques, including electronic absorption spectroscopy and cyclic voltammetry. The complexes display intense, visible d --> pi* metal-to-ligand charge-transfer (MLCT) bands, and their pi --> pi* intraligand charge-transfer (ILCT) absorptions in the near-UV region show molar extinction coefficients as high as ca. 3.5 x 10(5) M(-1) cm(-1). Molecular quadratic nonlinear optical (NLO) responses beta have been determined by using hyper-Rayleigh scattering at 800 and 1064 nm and also via Stark (electroabsorption) spectroscopic studies. The directly and indirectly derived beta values are very large, with the Stark-based static first hyperpolarizabilities beta(0) reaching as high as ca. 10(-27) esu, and generally increase on extending the pi-conjugation and enhancing the electron-accepting strength of the ligands. Cubic NLO properties have also been measured by using the Z-scan technique, revealing relatively high two-photon absorption cross sections of up to 2500 GM at 750 nm.