We report a facile approach to perform post-growth doping of hematite (α-Fe2O3) nanostructures by depositing titanium (Ti) precursor solution and subsequent annealing in air. Using hematite nanowire photoanodes on fluorine doped tin oxide (FTO) glass substrates as a model system, the doping conditions were carefully optimized and highly photoactive hematite photoanodes were prepared at a more practically acceptable temperature of 650–700 °C than the ≥800 °C commonly used in previous works. A combination of microstructural characterization, elemental analysis, photoelectrochemical (PEC) measurements, and electrochemical impedance spectroscopy (EIS) analysis were employed to confirm the distribution of Ti atoms in hematite nanostructures and the role of Ti dopants in enhancing the photocurrent of hematite photoanodes. It was found that the Ti-treatment increases the donor concentration of hematite by about 10 fold and facilitates majority carrier transport and collection, which may account for the performance enhancement. Moreover, EIS measurements under illumination and Mott–Schottky analysis clearly showed that Ti dopants interact with the surface trap states of hematite, suggesting that surface passivation may also contribute to the improved PEC performance. This facile post-growth doping method can be applied to other hematite nanostructures such as electrochemically deposited hematite films and expanded to other dopants such as zirconium (Zr).

We report here covalent attachment of a catalytically active cobalt complex onto boron-doped, p-type conductive diamond. Peripheral acetylene groups were appended on a cobalt porphyrin complex, and azide-alkyne cycloaddition was used for covalent linking to a diamond surface decorated with alkyl azides. The functionalized surface was characterized by X-ray photoelectron spectroscopy and Fourier transform IR spectroscopy, and the catalytic activity was characterized using cyclic voltammetry and FTIR. The catalyst-modified diamond surfaces were used as "smart" electrodes exhibiting good stability and electrocatalytic activity for electrochemical reduction of CO(2) to CO in acetonitrile solution.

The UV-induced photochemical grafting of terminal alkenes has emerged as a versatile way to form molecular layers on semiconductor surfaces. Recent studies have shown that grafting reactions can be initiated by photoelectron emission into the reactant liquid as well as by excitation across the semiconductor band gap, but the relative importance of these two processes is expected to depend on the nature of the semiconductors, the reactant alkene and the excitation wavelength. Here we report a study of the wavelength-dependent photochemical grafting of alkenes onto single-crystal TiO(2) samples. Trifluoroacetamide-protected 10-aminododec-1-ene (TFAAD), 10-N-BOC-aminodec-1-ene (t-BOC), and 1-dodecene were used as model alkenes. On rutile (110), photons with energy above the band gap but below the expected work function are not effective at inducing grafting, while photons with energy sufficient to induce electronic transitions from the TiO(2) Fermi level to electronic acceptor states of the reactant molecules induce grafting. A comparison of rutile (110), rutile (001), anatase (001), and anatase (101) samples shows slightly enhanced grafting for rutile but no difference between crystal faces for a given crystal phase. Hydroxylation of the surface increases the reaction rate by lowering the work function and thereby facilitating photoelectron ejection into the adjacent alkene. These results demonstrate that photoelectron emission is the dominant mechanism responsible for grafting when using short-wavelength (~254 nm) light and suggest that photoemission events beginning on mid-gap states may play a crucial role.

Photoinduced charge transfer modifies the device properties of illuminated pentacene field effect transistors (FETs) incorporating ZnO quantum dots at the gate insulator/pentacene interface. The transferred charge is trapped on electronic states associated with the ZnO quantum dots, with a steady state population approximately proportional to the rate of organic-inorganic charge transfer. Trapped charge shifts the threshold voltage of the FETs, providing the means to evaluate the rate of organic/inorganic charge transfer and the effects of interface modification. Monolayers of the wide-gap alkane stearic acid and the conjugated oligomer terthiophene attached to the ZnO suppress or permit charge transfer, respectively.

In this proceeding, transient 2D IR spectroscopy is used to study Re dye sensitized TiO2 nanocrystalline thin films. Multiple conformations of the dye on the interfaces are found by equilibrium 2D IR spectrum and transient 2D IR spectrum indicates these different binding conformations have different electron transfer kinetics.

Photochemical grafting of alkenes has emerged as a versatile way to form functionalized surfaces and enables multistep surface chemistry on a range of substrates. However, the self-terminating nature of the reaction remains poorly understood. Here, we use X-ray photoelectron spectroscopy (XPS) to explore multilayer formation during photochemical grafting of functional alkenes on single-crystal rutile (110) TiO2 surfaces. We demonstrate that simple alkenes such as 1-hexene stop at 1 monolayer, but alkenes with an additional alcohol group easily form multilayers. Atomic force microscopy (AFM) and XPS studies show that the multilayers are highly conformal and that the alcohol groups remain chemically accessible even within rnultilayer films. Small amounts of water enhance multilayer formation by facilitating formation of center dot OH or other radical species that initiate oligomerization processes, while oxygen has little effect. We show that the multilayer formation can be reduced or eliminated by using small amounts of a radical scavenger or by modifying the molecular structure of the alkene. Overall these studies show that multilayer formation is controlled by the ability to form alkyl radicals under ultraviolet (UV) illumination.

We have investigated the photochemical grafting of organic alkenes to atomically flat ZnO(10 ̅10) single crystals and ZnO nanorods as a way to produce functional molecule-semiconductor interfaces. Atomic force microscopy shows that photochemical grafting produces highly conformal, smooth molecular layers with no detectable changes in the underlying structure of the ZnO terraces or steps. X-ray photoelectron spectroscopy measurements show that grafting of a methyl ester-terminated alkene terminates near one monolayer, while alkenes bearing a trifluoroacetamide-protected amine form very smooth multilayers. Even with multilayers, it is possible to deprotect the amines and to link a second molecule to the surface with excellent efficiency and without significant loss of molecules from the surface. This demonstrates that the use of photochemical grafting, even in the case of multilayer formation, enables multistep chemical processes to be conducted on the ZnO surface. Photoresponse measurements demonstrate that functionalization of the surface does not affect the ability to induce field effects in the underlying ZnO, thereby suggesting that this approach to functionalization may be useful for applications in sensing and in hybrid organic-inorganic transistors and related devices.

We explore the use of single-bounce infrared reflection–absorption spectroscopy (IRRAS) to characterize molecular and biomolecular layers on doped and undoped diamond thin films on silicon substrates. Experimental measurements of thin layers of poly(methyl methacrylate) (PMMA) as a function of polarization and angle of incidence were used to characterize the intensity, frequency, and symmetry of the vibrational features. Fresnel multilayer reflectivity calculations were used to identify optimized conditions and to understand the observed trends. The measurements were then extended to characterize the nonspecific binding of fibrinogen as a model system. Finally, we present data demonstrating the ability to characterize Escherichia coli antibodies covalently linked to diamond surfaces, including changes in Amide I band due to conformational changes associated with protein denaturation. Our results show that dispersion and reflection effects under different experimental conditions lead to changes in the frequency of the PMMA CO mode and the fibrinogen Amide I band that are comparable to the changes due to different protein conformations. This has significant implications for the use of the Amide I feature to analyze the conformation of proteins on diamond thin film and highlights the utility of Fresnel modeling in the interpretation of FTIR spectra at surfaces.

Strategies to modify metal oxide surfaces are important because of the increasing applications of metal oxides in catalysis, sensing, electronics, and renewable energy. Here, we report the formation of molecular monolayers on anatase nanocrystalline TiO(2) surfaces at near-ambient temperatures by a simple one-step immersion. This is achieved by an analogue of the Williamson ether synthesis, in which the hydroxyl groups of the TiO(2) surface react with iodo-alkane molecules to release HI and form a Ti-O-C surface linkage. The grafted molecules were characterized by Fourier-transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) to confirm the formation of covalently bonded monolayers. Kinetic studies yielded an activation barrier of ∼59 kJ/mol for the grafting reaction. Measurements of hydrolytic stability of the grafted molecules in water show that approximately half the molecules are removed within minutes to hours at temperatures of 25-100 °C with an activation energy of ∼82 kJ/mol, while the remaining molecules are stable for much longer periods of time. These different stabilities are discussed in terms of the different types of Ti-O-C bonds that can form on TiO(2) surfaces.

We have measured the electron injection kinetics of four rhenium bipyridine complexes (Re1C, ReEC, Re1TC, and Re2TC) on TiO2 nanocrystalline films using transient infrared spectroscopy. The self-assembled monolayer formation of these complexes was characterized by UV-visible spectroscopy, infrared reflection absorption spectroscopy, and X-ray photoelectron spectroscopy. These complexes bind to the TiO2 surface through the formation of carboxylate groups, and these self-assembled layers are approximately a monolayer. The kinetics studies address the effect of insulating and conjugated spacers and the length of conjugation on the electron-transfer process. The insulating bridge leads to a slower injection rate and poorer injection yield compared with the conjugated spacers. The electron injection of Re2TC was found to be a fast, high-yielding, and multiple electron injector process. The ground and electronically excited states of the dye complexes were characterized using ground-state and time-dependent density functional theory. We present the role of electronic conjugation in modulating electron injection using a combination of computational and experimental work and find that these metal-based complexes adsorbed on a semiconductor surface can be used to read out the electron injection kinetics through tailored molecular bridges.

Previous work has shown that organic alkenes will graft to covalent semiconductors such as diamond and silicon. Here, we demonstrate that organic alkenes can be grafted to the surfaces of metal oxide semiconductors, including TiO2 in nanocrystalline thin films and as single-crystal anatase(001) epitaxial films grown on SrTiO3(001) substrates. The resulting layers can be used as a starting point for linking biomolecules such as DNA to metal oxide surfaces. Initial results are presented showing that this chemistry can also be applied to graft molecular layers to zirconium oxide thin films. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Transient 2-D IR spectroscopy suppresses the signal of free electrons injected at organic-inorganic interfaces, which dominate standard transient spectra. Consequently, vibrational features are better resolved, permitting study of charge injection from different conformations of dyes.

Many emerging fields such as biotechnology and renewable energy require functionalized surfaces that are "smart" and highly stable. Surface modification schemes developed previously have often been limited to simple molecules or have been based on weakly bound layers that have limited stability. In this Account, we report on recent developments enabling the preparation of molecular and biomolecular interfaces that exhibit high selectivity and unprecedented stability on a range of covalent materials including diamond, vertically aligned carbon nanofibers, silicon, and metal oxides. One particularly successful pathway to ultrastable interfaces involves the photochemical grafting of organic alkenes to the surfaces. Bifunctional alkenes with a suitable functional group at the distal end can directly impart functionality and can serve as attachment points for linking complex structures such as DNA and proteins. The successful application of photochemical grafting to a surprisingly wide range of materials has motivated researchers to better understand the underlying photochemical reaction mechanisms. The resulting studies using experimental and computational methods have provided fundamental insights into the electronic structure of the molecules and the surface control photochemical reactivity. Such investigations have revealed the important role of a previously unrecognized process, photoelectron emission, in initiating photochemical grafting of alkenes to surfaces. Molecular and biomolecular interfaces formed on diamond and other covalent materials are leading to novel types of molecular electronic interfaces. For example, electrical, optical, or electromechanical structures that convert biological information directly into analytical signals allow for direct label-free detection of DNA and proteins. Because of the preferential adherence of molecules to graphitic edge-plane sites, the grafting of redox-active species to vertically aligned carbon nanofibers leads to good electrochemical activity. Therefore researchers could graft electrocatalytic materials to carbon nanofibers to develop new types of selective electrocatalytic interfaces. Extending this chemistry to include metal oxides such as TiO(2) may lead to highly specific and efficient chemical reactions and new materials with useful applications in photovoltaic and photocatalytic energy conversion.

Surface chemical changes or passivation can be used to enhance the electrical performance of device Structures. The solution-based (Nh(4))(2)S-t, treatment of ZnO single crystals of (0001), (10 (1) over bar0), and (000 (1) over bar) orientations as well as ZnO nanorods was studies by using X-ray and Ultraviolet photoelectron spectroscopies (XPS and ups). The (10 (1) over bar0) and (000 (1) over bar)-onented single crystals and ZnO nanorods each showed evidence that the ZnO was consumed, forming a ZnS layer at the surface In contrast, the ZnO (0001) crystal (lid not substantially react with the sulfide Solution. However, the surface hydroxyl species were displaced by a Sulfide passivation layer of 0.21 ML. This process resulted in all apparent increase in upward band bending by similar to 0.1 -0.3 eV. P3HT-ZnO photovoltaic devices fabricated oil O-polar single crystals showed electrical properties which can be correlated with ZnS formation due to the Sulfide treatment.

Well-defined molecular layers can be formed on the surface of nanocrystalline anatase TiO2 by photochemically grafting organic molecules bearing a terminal vinyl group. The molecular layers produced are shown to have minimal oxidation and are able to be patterned and uniformly grafted through optically thick nanocrystalline films. Stability tests show that the layers have excellent stability in deionized water at 80 degrees C, aqueous solutions at pH=1.0 and pH=10.3 at 65 degrees C, and acetonitrile for time scales approaching 1200 h. Degradation of the films in deionized water occurs using a AM1.5 full-spectrum solar simulator as an illumination source but is partially suppressed by filtering with a 400 nm UV blocking filter which blocks the above-bandgap light. A mechanism is proposed for the grafting reaction in which the surface hydroxyl groups trap photoexcited holes, facilitating reaction with the vinyl group.

TiO2 thin films are highly stable and can be deposited onto a wide variety of substrate materials under moderate conditions. We demonstrate that organic alkenes will graft to the surface of TiO2 when illuminated with UV light at 254 nm and that the resulting layers provide a starting point for the preparation of DNA-modified TiO2 thin films exhibiting excellent stability and biomolecular selectivity. By using alkenes with a protected amino group at the distal end, the grafted layers can be deprotected to yield molecular layers with exposed primary amino groups that can then be used to covalently link DNA oligonucleotides to the TiO2 surface. We demonstrate that the resulting DNA-modified surfaces exhibit excellent selectivity toward complementary versus noncomplementary target sequences in solution and that the surfaces can withstand 25 cycles of hybridization and denaturation in 8.3 M urea with little or no degradation. Furthermore, the use of simple masking methods provides a way to directly control the spatial location of the grafted layers, thereby providing a way to photopattern the spatial distribution of biologically active molecules to the TiO2 surfaces. Using Ti films ranging from 10 to 100 nm in thickness allows the preparation of TiO2 films that range from highly reflective to almost completely transparent; in both cases, the photochemical grafting of alkenes can be used as a starting point for stable surfaces with good biomolecular recognition properties.

We use nonlinear 2D IR spectroscopy to study TiO(2) nanocrystalline thin films sensitized with a Re dye. We find that the free electron signal, which often obscures the vibrational features in the transient absorption spectrum, is not observed in the 2D IR spectra. Its absence allows the vibrational features of the dye to be much better resolved than with the typical IR absorption probe. We observe multiple absorption bands but no cross peaks in the 2D IR spectra, which indicates that the dyes have at least three conformations. Furthermore, by using a pulse sequence in which we initiate electron transfer in the middle of the infrared pulse train, we are able to assign the excited state features by correlating them to the ground state vibrational modes and determine that the three conformations have different time scales and cross sections for electron injection. 2D IR spectroscopy is proving to be very useful in disentangling overlapping structural distributions in biological and chemical physics processes. These experiments demonstrate that nonlinear infrared probes are also a powerful new tool for studying charge transfer at interfaces.