Critical Interface States Controlling Rectification of Ultrathin NiO–ZnO p–n Heterojunctions

Herein, we consider the heterojunction formation of two prototypical metal oxides: p-type NiO and n-type ZnO. Elementally abundant, low-cost metal oxide/oxide’ heterojunctions are of interest for UV optical sensing, gas sensing, photocatalysis, charge confinement layers, piezoelectric nanogenerators, and flash memory devices. These heterojunctions can also be used as current rectifiers and potentially as recombination layers in tandem photovoltaic stacks by making the two oxide layers ultrathin. In the ultrathin geometry, understanding and control of interface electronic structure and chemical reactions at the oxide/oxide’ interface are critical to functionality, as oxygen atoms are shared at the interface of the dissimilar materials. In the studies presented here the extent of chemical reactions and interface band bending is monitored using X-ray and ultraviolet photoelectron spectroscopies. Interface reactivity is controlled by varying the near surface composition of nickel oxide, nickel hydroxide, and nickel oxyhydroxide using standard surface-treatment procedures. A direct correlation between relative percentage of interface hydroxyl chemistry (and hence surface Lewis basicity) and the local band edge alignment for ultrathin p–n junctions (6 nm NiO/30 nm ZnO) is observed. We propose an acid–base formulism to explain these results: the stronger the acid–base reaction, the greater the fraction of interfacial electronic states which lower the band offset between the ZnO conduction band and the NiO valence band. Increased interfacial gap states result in larger reverse bias current of the p–n junction and lower rectification ratios. The acid–base formulism could serve as a future design principle for oxide/oxide’ and other heterojunctions based on dissimilar materials

Pentafluorophenoxy Boron Subphthalocyanine (F₅BsubPc) as a Multifunctional Material for Organic Photovoltaics

We have demonstrated that pentafluoro phenoxy boron subphthalocyanine (F₅BsubPc) can function as either an electron donor or an electron acceptor layer in a planar heterojunction organic photovolatic (PHJ OPV) cell. F₅BsubPc was incorporated into devices with the configurations ITO/MoO₃/F₅BsubPc/C₆₀/BCP/Al (F₅BsubPc used as an electron-donor/hole-transport layer) and ITO/MoO₃/Cl-BsubPc/F₅BsubPc/BCP/Al (F₅BsubPc used as an electron-acceptor/electron-transport layer). Each unoptimized device displayed open-circuit photopotentials (<i>V</i>_oc) close to or in excess of 1 V and respectrable power conversion efficiencies. Ultraviolet photoelectron spectroscopy (UPS) was used to characterize the band-edge offset energies at the donor/acceptor junctions. HOMO and LUMO energy level offsets for the F₅BsubPc/C₆₀ heterojunction were determined to be ca. 0.6 eV and ca. 0.7 eV, respectively. Such offsets are clearly large enough to produce rectifying <i>J</i>/<i>V</i> responses, efficient exciton dissociation, and photocurrent production at the interface. For the Cl-BsubPc/F₅BsubPc heterojunction, the estimated offset energies were found to be ca. 0.1 eV. However, reasonable photovoltaic activity was observed, with photocurrent production coming from both BsubPc species layers. Incident and absorbed photon power conversion efficiencies (IPCE and APCE) showed that photocurrent production qualitatively tracked the absorbance spectra of the donor/acceptor heterojunctions, with some additional photocurrent activity on the low energy side of the absorbance band. We suggest that photocurrent production at higher wavelengths may be a result of charge-transfer species at the donor/acceptor interface. Cascade photovoltaics were also fabricated to expand on the understanding of the role of F₅BsubPc in such device architectures

Defect Tolerance in Methylammonium Lead Triiodide Perovskite

Photovoltaic applications of perovskite semiconductor material systems have generated considerable interest in part because of predictions that primary defect energy levels reside outside the bandgap. We present experimental evidence that this enabling material property is present in the halide-lead perovskite, CH₃NH₃PbI₃ (MAPbI₃), consistent with theoretical predictions. By performing X-ray photoemission spectroscopy, we induce and track dynamic chemical and electronic transformations in the perovskite. These data show compositional changes that begin immediately with exposure to X-ray irradiation, whereas the predominant electronic structure of the thin film on compact TiO₂ appears tolerant to the formation of compensating defect pairs of V_I and V_MA and for a large range of I/Pb ratios. Changing film composition is correlated with a shift of the valence-band maximum only as the halide–lead ratio drops below 2.5. This delay is attributed to the invariance of MAPbI₃ electronic structure to distributed defects that can significantly transform the electronic density of states only when in high concentrations

Experimental and Computational Investigation of Acetic Acid Deoxygenation over Oxophilic Molybdenum Carbide: Surface Chemistry and Active Site Identity

Ex situ catalytic fast pyrolysis (CFP) is a promising route for producing fungible biofuels; however, this process requires bifunctional catalysts that favor C–O bond cleavage, activate hydrogen at near atmospheric pressure and high temperature (350–500 °C), and are stable under high-steam, low hydrogen-to-carbon environments. Recently, early transition-metal carbides have been reported to selectively cleave C–O bonds of alcohols, aldehydes, and oxygenated aromatics, yet there is limited understanding of the metal carbide surface chemistry under reaction conditions and the identity of the active sites for deoxygenation. In this paper, we evaluated molybdenum carbide (Mo₂C) for the deoxygenation of acetic acid, an abundant component of biomass pyrolysis vapors, under ex situ CFP conditions, and we probed the Mo₂C surface chemistry, identity of the active sites, and deoxygenation pathways using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. The Mo₂C catalyst favored the production of acetaldehyde and ethylene from acetic acid over the temperature range of 250–400 °C, with decarbonylation pathways favored at temperatures greater than 400 °C. Little to no ethanol was observed due to the high activity of the carbide surface for alcohol dehydration. The Mo₂C surface, which was at least partially oxidized following pretreatment and exposure to reaction conditions (possibly existing as an oxycarbide), possessed both metallic-like H-adsorption sites (i.e., exposed Mo and C) and Brønsted acidic surface hydroxyl sites, in a ratio of 1:8 metallic:acidic sites following pretreatment. The strength of the acidic sites was similar to that for H-Beta, H-Y, and H-X zeolites. Oxygen vacancy sites (exposed Mo sites) were also present under reaction conditions, inferred from DRIFTS results and calculated surface phase diagrams. It is proposed that C–O bond cleavage steps proceeded over the acidic sites or over the oxygen vacancy sites and that the deoxygenation rate may be limited by the availability of adsorbed hydrogen, due to the high surface coverage of oxygen under reaction conditions. Importantly, the reaction conditions (temperature and partial pressures of H₂ and H₂O) had a strong effect on oxygen surface coverage, and accordingly, the relative concentrations of the different types of active sites, and could ultimately result in completely different reaction pathways under different reaction conditions

Evidence for near-Surface NiOOH Species in Solution-Processed NiO_<i>x</i> Selective Interlayer Materials: Impact on Energetics and the Performance of Polymer Bulk Heterojunction Photovoltaics

The characterization and implementation of solution-processed, wide bandgap nickel oxide (NiO_<i>x</i>) hole-selective interlayer materials used in bulk-heterojunction (BHJ) organic photovoltaics (OPVs) are discussed. The surface electrical properties and charge selectivity of these thin films are strongly dependent upon the surface chemistry, band edge energies, and midgap state concentrations, as dictated by the ambient conditions and film pretreatments. Surface states were correlated with standards for nickel oxide, hydroxide, and oxyhydroxide components, as determined using monochromatic X-ray photoelectron spectroscopy. Ultraviolet and inverse photoemission spectroscopy measurements show changes in the surface chemistries directly impact the valence band energies. O₂-plasma treatment of the as-deposited NiO_<i>x</i> films was found to introduce the dipolar surface species nickel oxyhydroxide (NiOOH), rather than the p-dopant Ni₂O₃, resulting in an increase of the electrical band gap energy for the near-surface region from 3.1 to 3.6 eV via a vacuum level shift. Electron blocking properties of the as-deposited and O₂-plasma treated NiO_<i>x</i> films are compared using both electron-only and BHJ devices. O₂-plasma-treated NiO_<i>x</i> interlayers produce electron-only devices with lower leakage current and increased turn on voltages. The differences in behavior of the different pretreated interlayers appears to arise from differences in local density of states that comprise the valence band of the NiO_<i>x</i> interlayers and changes to the band gap energy, which influence their hole-selectivity. The presence of NiOOH states in these NiO_<i>x</i> films and the resultant chemical reactions at the oxide/organic interfaces in OPVs is predicted to play a significant role in controlling OPV device efficiency and lifetime

Ru-Sn/AC for the Aqueous-Phase Reduction of Succinic Acid to 1,4‑Butanediol under Continuous Process Conditions

Succinic acid is a biomass-derived platform chemical that can be catalytically converted in the aqueous phase to 1,4-butanediol (BDO), a prevalent building block used in the polymer and chemical industries. Despite significant interest, limited work has been reported regarding sustained catalyst performance and stability under continuous aqueous-phase process conditions. As such, this work examines Ru-Sn on activated carbon (AC) for the aqueous-phase conversion of succinic acid to BDO under batch and flow reactor conditions. Initially, powder Ru-Sn catalysts were screened to determine the most effective bimetallic ratio and provide a comparison to other monometallic (Pd, Pt, Ru) and bimetallic (Pt-Sn, Pd-Re) catalysts. Batch reactor tests determined that a ∼1:1 metal weight ratio of Ru to Sn was effective for producing BDO in high yields, with complete conversion resulting in 82% molar yield. Characterization of the fresh Ru-Sn catalyst suggests that the sequential loading method results in Ru sites that are colocated and surface-enriched with Sn. Postbatch reaction characterization confirmed stable Ru-Sn material properties; however, upon a transition to continuous conditions, significant Ru-Sn/AC deactivation occurred due to stainless steel leaching of Ni that resulted in Ru-Sn metal crystallite restructuring to form discrete Ni-Sn sites. Computational modeling confirmed favorable energetics for Ru-Sn segregation and Ni-Sn formation at submonolayer Sn incorporation. To address stainless steel leaching, reactor walls were treated with an inert silica coating by chemical vapor deposition. With leaching reduced, stable Ru-Sn/AC performance was observed that resulted in a molar yield of 71% BDO and 15% tetrahydrofuran for 96 h of time on stream. Postreaction catalyst characterization confirmed low levels of Ni and Cr deposition, although early-stage islanding of Ni-Sn will likely be problematic for industrially relevant time scales (i.e., thousands of hours). Overall, these results (i) demonstrate the performance of Ru-Sn/AC for aqueous phase succinic acid reduction, (ii) provide insight into the Ru-Sn bimetallic structure and deactivation in the presence of leached Ni, and (iii) underscore the importance of compatible reactor metallurgy and durable catalysts