Evaluation of sputtered nickel oxide, cobalt oxide and nickel–cobalt oxide on n-type silicon photoanodes for solar-driven O₂(g) evolution from water

Thin films of nickel oxide (NiO_x), cobalt oxide (CoO_x) and nickel–cobalt oxide (NiCoO_x) were sputtered onto n-Si(111) surfaces to produce a series of integrated, protected Si photoanodes that did not require deposition of a separate heterogeneous electrocatalyst for water oxidation. The p-type transparent conductive oxides (p-TCOs) acted as multi-functional transparent, antireflective, electrically conductive, chemically stable coatings that also were active electrocatalysts for the oxidation of water to O₂(g). Relative to the formal potential for water oxidation to O₂, E^(o′)(O₂/H₂O), under simulated Air Mass (AM)1.5 illumination the p-TCO-coated n-Si(111) photoanodes produced mutually similar open-circuit potentials of −270 ± 20 mV, but different photocurrent densities at E^(o′)(O₂/H₂O), of 28 ± 0.3 mA cm⁻² for NiO_x, 18 ± 0.3 mA cm⁻² for CoO_x and 24 ± 0.5 mA cm⁻² for NiCoO_x. The p-TCOs all provided protection from oxide growth for extended time periods, and produced stable photocurrent densities from n-Si in 1.0 M KOH(aq) (ACS grade) under potential control at E^(o′)(O₂/H₂O) for >400 h of continuous operation under 100 mW cm−2 of simulated AM1.5 illumination

Vibrational Sum-Frequency Spectroscopic Investigation of the Structure and Azimuthal Anisotropy of Propynyl-Terminated Si(111) Surfaces

Vibrational sum-frequency generation (VSFG) spectroscopy was used to investigate the orientation and azimuthal anisotropy of the C–H stretching modes for propynyl-terminated Si(111) surfaces, Si—C≡C—CH_3. VSFG spectra revealed symmetric and asymmetric C–H stretching modes in addition to a Fermi resonance mode resulting from the interaction of the asymmetric C–H bending overtone with the symmetric C–H stretching vibration. The polarization dependence of the C–H stretching modes was consistent with the propynyl groups oriented such that the Si—C≡C– bond is normal to the Si(111) surface. The azimuthal angle dependence of the resonant C–H stretching amplitude revealed no rotational anisotropy for the symmetric C–H stretching mode and a 3-fold rotational anisotropy for the asymmetric C–H stretching mode in registry with the 3-fold symmetric Si(111) substrate. The results are consistent with the expectation that the C–H stretching modes of a –CH_3 group are decoupled from the Si substrate due to a −C≡C– spacer. In contrast, the methyl-terminated Si(111) surface, Si–CH_3, was previously reported to have pronounced vibronic coupling of the methyl stretch modes to the electronic bath of bulk Si. Vacuum-annealing of propynyl-terminated Si(111) resulted in increased 3-fold azimuthal anisotropy for the symmetric stretch, suggesting that removal of propynyl groups from the surface upon annealing allowed the remaining propynyl groups to tilt away from the surface normal into one of three preferred directions toward the vacated neighbor sites

Control of the Band-Edge Positions of Crystalline Si(111) by Surface Functionalization with 3,4,5-Trifluorophenylacetylenyl Moieties

Functionalization of semiconductor surfaces with organic moieties can change the charge distribution, surface dipole, and electric field at the interface. The modified electric field will shift the semiconductor band-edge positions relative to those of a contacting phase. Achieving chemical control over the energetics at semiconductor surfaces promises to provide a means of tuning the band-edge energetics to form optimized junctions with a desired material. Si(111) surfaces functionalized with 3,4,5-trifluorophenylacetylenyl (TFPA) groups were characterized by transmission infrared spectroscopy (TIRS), X-ray photoelectron spectroscopy (XPS), and surface recombination velocity (S) measurements. Mixed methyl/TFPA-terminated (MMTFPA) n- and p-type Si(111) surfaces were synthesized and characterized by electrochemical methods. Current density versus voltage and Mott-Schottky measurements of Si(111)–MMTFPA electrodes in contact with Hg indicated that the barrier height, Φb, was a function of the fractional monolayer coverage of TFPA (θTFPA) in the alkyl monolayer. Relative to Si(111)–CH3 surfaces, Si(111)–MMTFPA samples with high θTFPA produced shifts in Φb of ≥0.6 V for n-Si/Hg contacts and ≥0.5 V for p-Si/Hg contacts. Consistently, the open-circuit potential (Eoc) of Si(111)–MMTFPA samples in contact with CH3CN solutions that contained the 1-electron redox couples decamethylferrocenium/decamethylferrocene (Cp*2Fe+/0) or methyl viologen (MV2+/+●) shifted relative to Si(111)–CH3 samples by +0.27 V for n-Si and by up to +0.10 V for p-Si. Residual surface recombination limited the Eoc of p-Si samples at high θTFPA despite the favorable shift in the band-edge positions induced by the surface modification process

Synthesis, Characterization, and Reactivity of Ethynyl- and Propynyl-Terminated Si(111) Surfaces

Ethynyl- and propynyl-terminated Si(111) surfaces synthesized using a two-step halogenation/alkylation method have been characterized by transmission infrared spectroscopy (TIRS), high-resolution electron energy-loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), atomic-force microscopy (AFM), electrochemical scanning–tunneling microscopy (EC-STM) and measurements of surface recombination velocities (S). For the ethynyl-terminated Si(111) surface, TIRS revealed signals corresponding to ethynyl ≡C–H and C≡C stretching oriented perpendicular to the surface, HREELS revealed a Si–C stretching signal, and XPS data showed the presence of C bound to Si with a fractional monolayer (ML) coverage (Φ) of Φ_(Si–CCH) = 0.63 ± 0.08 ML. The ethynyl-terminated surfaces were also partially terminated by Si–OH groups (Φ_(Si–OH) = 0.35 ± 0.03 ML) with limited formation of Si^(3+) and Si^(4+) oxides. For the propynyl-terminated Si(111) surface, TIRS revealed the presence of a (C–H)CH_3 symmetric bending, or “umbrella,” peak oriented perpendicular to the surface, while HREELS revealed signals corresponding to Si–C and C≡C stretching, and XPS showed C bound to Si with Φ_(Si–CCCH_3) = 1.05 ± 0.06 ML. The LEED patterns were consistent with a (1 × 1) surface unit cell for both surfaces, but room-temperature EC-STM indicated that the surfaces did not exhibit long-range ordering. HCC–Si(111) and CH_3CC–Si(111) surfaces yielded S values of (3.5 ± 0.1) × 10^3 and (5 ± 1) × 10^2 cm s^(–1), respectively, after 581 h exposure to air. These observations are consistent with the covalent binding of ethynyl and propynyl groups, respectively, to the Si(111) surface

Vibrational Sum Frequency Generation (VSFG) Spectroscopy Measurement of the Rotational Barrier of Methyl Groups on Methyl-Terminated Silicon(111) Surfaces

The methyl-terminated Si(111) surface possesses a 3-fold in-plane symmetry, with the methyl groups oriented perpendicular to the substrate. The propeller-like rotation of the methyl groups is hindered at room temperature and proceeds via 120° jumps between three isoenergetic minima in registry with the crystalline Si substrate. We have used line-shape analysis of polarization-selected vibrational sum frequency generation spectroscopy to determine the rotational relaxation rate of the surface methyl groups and have measured the temperature dependence of the relaxation rate between 20 and 120 °C. By fitting the measured rate to an Arrhenius dependence, we extracted an activation energy (the rotational barrier) of 830 ± 360 cm^(–1) and an attempt frequency of (2.9 ± 4.2) × 10^(13) s^(–1) for the methyl rotation process. Comparison with the harmonic frequency of a methyl group in a 3-fold cosine potential suggests that the hindered rotation occurs via uncorrelated jumps of single methyl groups rather than concerted gear-like rotation

Experimental and theoretical study of rotationally inelastic diffraction of H_2(D_2) from methyl-terminated Si(111)

Fundamental details concerning the interaction between H_2 and CH_3–Si(111) have been elucidated by the combination of diffractive scattering experiments and electronic structure and scattering calculations. Rotationally inelastic diffraction (RID) of H_2 and D_2 from this model hydrocarbon-decorated semiconductor interface has been confirmed for the first time via both time-of-flight and diffraction measurements, with modest j = 0 → 2 RID intensities for H_2 compared to the strong RID features observed for D_2 over a large range of kinematic scattering conditions along two high-symmetry azimuthal directions. The Debye-Waller model was applied to the thermal attenuation of diffraction peaks, allowing for precise determination of the RID probabilities by accounting for incoherent motion of the CH_3–Si(111) surface atoms. The probabilities of rotationally inelastic diffraction of H_2 and D_2 have been quantitatively evaluated as a function of beam energy and scattering angle, and have been compared with complementary electronic structure and scattering calculations to provide insight into the interaction potential between H_2 (D_2) and hence the surface charge density distribution. Specifically, a six-dimensional potential energy surface (PES), describing the electronic structure of the H_2(D_2)/CH_3−Si(111) system, has been computed based on interpolation of density functional theory energies. Quantum and classical dynamics simulations have allowed for an assessment of the accuracy of the PES, and subsequently for identification of the features of the PES that serve as classical turning points. A close scrutiny of the PES reveals the highly anisotropic character of the interaction potential at these turning points. This combination of experiment and theory provides new and important details about the interaction of H_2 with a hybrid organic-semiconductor interface, which can be used to further investigate energy flow in technologically relevant systems

Lightly Fluorinated Graphene as a Protective Layer for n-Type Si(111) Photoanodes in Aqueous Electrolytes

The behavior of n-Si(111) photoanodes covered by monolayer sheets of fluorinated graphene (F–Gr) was investigated under a range of chemical and electrochemical conditions. The electrochemical behavior of n-Si/F–Gr and np^+-Si/F–Gr photoanodes was compared to hydride-terminated n-Si (n-Si−H) and np+-Si−H electrodes in contact with aqueous Fe(CN)_6^(3-/4-) and Br_2/HBr electrolytes as well as in contact with a series of outer-sphere, one-electron redox couples in nonaqueous electrolytes. Illuminated n-Si/F–Gr and np^+-Si/F–Gr electrodes in contact with an aqueous K_3(Fe(CN)_6/K4(Fe(CN)_6 solutions exhibited stable short-circuit photocurrent densities of ∼10 mA cm^(–2) for 100,000 s (>24 h), in comparison to bare Si electrodes, which yielded nearly a complete photocurrent decay over ∼100 s. X-ray photoelectron spectra collected before and after exposure to aqueous anodic conditions showed that oxide formation at the Si surface was significantly inhibited for Si electrodes coated with F–Gr relative to bare Si electrodes exposed to the same conditions. The variation of the open-circuit potential for n-Si/F–Gr in contact with a series of nonaqueous electrolytes of varying reduction potential indicated that the n-Si/F–Gr did not form a buried junction with respect to the solution contact. Further, illuminated n-Si/F−Gr electrodes in contact with Br_2/HBr(aq) were significantly more electrochemically stable than n-Si−H electrodes, and n-Si/F−Gr electrodes coupled to a Pt catalyst exhibited ideal regenerative cell efficiencies of up to 5% for the oxidation of Br^– to Br_2

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

Reactively sputtered nickel oxide (NiO_x) films provide transparent, antireflective, electrically conductive, chemically stable coatings that also are highly active electrocatalysts for the oxidation of water to O_2(g). These NiO_x 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 NiO_x films stabilized all of these self-passivating, high-efficiency semiconducting photoelectrodes for >100 h of sustained, quantitative solar-driven oxidation of water to O_2(g)

Experimental and theoretical study of rotationally inelastic diffraction of H_2(D_2) from methyl-terminated Si(111)

Fundamental details concerning the interaction between H_2 and CH_3–Si(111) have been elucidated by the combination of diffractive scattering experiments and electronic structure and scattering calculations. Rotationally inelastic diffraction (RID) of H_2 and D_2 from this model hydrocarbon-decorated semiconductor interface has been confirmed for the first time via both time-of-flight and diffraction measurements, with modest j = 0 → 2 RID intensities for H_2 compared to the strong RID features observed for D_2 over a large range of kinematic scattering conditions along two high-symmetry azimuthal directions. The Debye-Waller model was applied to the thermal attenuation of diffraction peaks, allowing for precise determination of the RID probabilities by accounting for incoherent motion of the CH_3–Si(111) surface atoms. The probabilities of rotationally inelastic diffraction of H_2 and D_2 have been quantitatively evaluated as a function of beam energy and scattering angle, and have been compared with complementary electronic structure and scattering calculations to provide insight into the interaction potential between H_2 (D_2) and hence the surface charge density distribution. Specifically, a six-dimensional potential energy surface (PES), describing the electronic structure of the H_2(D_2)/CH_3−Si(111) system, has been computed based on interpolation of density functional theory energies. Quantum and classical dynamics simulations have allowed for an assessment of the accuracy of the PES, and subsequently for identification of the features of the PES that serve as classical turning points. A close scrutiny of the PES reveals the highly anisotropic character of the interaction potential at these turning points. This combination of experiment and theory provides new and important details about the interaction of H_2 with a hybrid organic-semiconductor interface, which can be used to further investigate energy flow in technologically relevant systems