Simultaneous ozone and granular activated carbon for advanced treatment of micropollutants in municipal wastewater effluent

The main objective of this study was to compare the efficacy of ozone (O₃) and O₃ with granular activated carbon (GAC) (O₃/GAC) at pilot-scale for the enhanced removal of micropollutants (MPs) from wastewater effluent. The results revealed enhanced removal of tris (2-carboxylethyl) phosphine (TCEP), sucralose, and meprobamate during the O₃/GAC treatment experiments compared to the sum of their removal during isolated ozonation and GAC adsorption experiments. The long-term O₃/GAC experiment showed the promotive effect of GAC substantially decreased after 20 h of O₃ exposure. This decreased performance correlates with changes to GAC surface properties caused by O₃. After 6 h of operation, O₃ initially led to an increase in Brunauer-Emmett-Teller (BET) surface area on the GAC improving the elimination level of investigated MPs (except N-nitrosomorpholine (NMOR)). However, after 20 h of exposure, O₃ ultimately caused structural damages to the GAC surface, decreased the BET surface area in the final stages of the experiment, and a 4-fold increase in O_1s:C_1s ratio on the GAC surface was observed due to an increase in surface acidic functional groups caused by O₃ treatment.

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.

Covalent Surface Modification of Gallium Arsenide Photocathodes for Water Splitting in Highly Acidic Electrolyte

Efficient water splitting using light as the only energy input requires stable semiconductor electrodes with favorable energetics for the water-oxidation and proton-reduction reactions. Strategies to tune electrode potentials using molecular dipoles adsorbed to the semiconductor surface have been pursued for decades but are often based on weak interactions and quickly react to desorb the molecule under conditions relevant to sustained photoelectrolysis. Here, we show that covalent attachment of fluorinated, aromatic molecules to p-GaAs(1 0 0) surfaces can be employed to tune the photocurrent onset potentials of p-GaAs(1 0 0) photocathodes and reduce the external energy required for water splitting. Results indicate that initial photocurrent onset potentials can be shifted by nearly 150 mV in pH -0.5 electrolyte under 1 Sun (1000 W m^-2 ) illumination resulting from the covalently bound surface dipole. Though X-ray photoelectron spectroscopy analysis reveals that the covalent molecular dipole attachment is not robust under extended 50 h photoelectrolysis, the modified surface delays arsenic oxide formation that results in a p-GaAs(1 0 0) photoelectrode operating at a sustained photocurrent density of -20.5 mA cm^-2 within -0.5 V of the reversible hydrogen electrode.

Water reduction by a p-GaInP2 photoelectrode stabilized by an amorphous TiO2 coating and a molecular cobalt catalyst

Producing hydrogen through solar water splitting requires the coverage of large land areas. Abundant metal-based molecular catalysts offer scalability, but only if they match noble metal activities. We report on a highly active p-GaInP2 photocathode protected through a 35-nm TiO2 layer functionalized by a cobaloxime molecular catalyst (GaInP2-TiO2-cobaloxime). This photoelectrode mediates H2 production with a current density of ∼9 mA cm(-2) at a potential of 0 V versus RHE under 1-sun illumination at pH 13. The calculated turnover number for the catalyst during a 20-h period is 139,000, with an average turnover frequency of 1.9 s(-1). Bare GaInP2 shows a rapid current decay, whereas the GaInP2-TiO2-cobaloxime electrode shows ≤5% loss over 20 min, comparable to a GaInP2-TiO2-Pt catalyst particle-modified interface. The activity and corrosion resistance of the GaInP2-TiO2-cobaloxime photocathode in basic solution is made possible by an atomic layer-deposited TiO2 and an attached cobaloxime catalyst.

Phosphonic Acid Modification of GaInP2 Photocathodes Toward Unbiased Photoelectrochemical Water Splitting

The p-type semiconductor GaInP2 has a nearly ideal bandgap (∼1.83 eV) for hydrogen fuel generation by photoelectrochemical water splitting but is unable to drive this reaction because of misalignment of the semiconductor band edges with the water redox half reactions. Here, we show that attachment of an appropriate conjugated phosphonic acid to the GaInP2 electrode surface improves the band edge alignment, closer to the desired overlap with the water redox potentials. We demonstrate that this surface modification approach is able to adjust the energetic position of the band edges by as much as 0.8 eV, showing that it may be possible to engineer the energetics at the semiconductor/electrolyte interface to allow for unbiased water splitting with a single photoelectrode having a bandgap of less than 2 eV.

Co-solvent enhanced zinc oxysulfide buffer layers in Kesterite copper zinc tin selenide solar cells

Performance deficiencies from the too large conduction band offset between Cu₂ZnSnSe₄/ZnOS heterojunctions are abated by the inclusion of a co-solvent during aqueous growth of the buffer layer.

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 (Voc) 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 J/V 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.