The interaction of water vapor with a single crystal ZnO(101̅0) surface was investigated using synchrotron-based ambient pressure X-ray photoelectron spectroscopy (APXPS). Two isobaric experiments were performed at 0.3 and 0.07 Torr water vapor pressure at sample temperatures ranging from 750 to 295 K up to a maximum of 2% relative humidity (RH). Below 10 % RH the ZnO(101̅0) interface is covered with ∼0.25 monolayers of OH groups attributed to dissociation at nonstoichiometric defect sites. At ∼10 % RH there is a sharp onset in increased surface hydroxylation attributed to reaction at stoichiometric terrace sites. The surface saturates with an OH monolayer ∼0.26 nm thick and occurs in the absence of any observable molecularly bound water, suggesting the formation of a 1 × 1 dissociated monolayer structure. This is in stark contrast to ultrahigh vacuum experiments and molecular simulations that show the optimum structure is a 2 × 1 partially dissociated HO/OH monolayer. The sharp onset to terrace site hydroxylation at ∼10 % RH for ZnO(101̅0) contrasts with APXPS observations for MgO(100) which show a sharp onset at 10 % RH. A surface thermodynamic analysis reveals that this shift to lower RH for ZnO(101̅0) compared to MgO(100) is due to a more favorable Gibbs free energy for terrace site hydroxylation.

Ambient pressure X-ray photoelectron spectroscopy (APXPS) was used to quantitatively assess the chemical changes of the top few nanometers of the ionic liquid (IL)–gas interface of 1-butyl-3-methylimidazolium acetate, [BMIM]­[OAc], in the presence of water vapor at room temperature. Above 10–3 Torr the uptake of water into the interfacial region was observed and increases up to a maximum water mole fraction (x w) of 0.85 at 5 Torr. Comparing APXPS to gravimetric analysis measurements, the kinetics of interfacial uptake are rapid compared to bulk water absorption. There is growing evidence from experiments and molecular dynamic simulations that water/IL mixtures undergo a phase transition from being homogeneously mixed to a system composed of nanometer sized, segregated polar and nonpolar regions near x w = 0.7 in the bulk. For x w > 0.6, APXPS C 1s spectra show a sudden change in shape. It is suggested that this observed spectral change in C 1s is due to a similar nanostructuring occurring near the IL–gas interface. Increasing interfacial water gives rise to relative binding energy shifts in O 1s, C 1s, and N 1s regions which increase with x w, thus suggesting that water significantly influences the electronic environment of both the anion and cation.

Long-term thermal stability is one limiting factor that impedes the commercialization of the perovskite solar cell. Inspired by our prior results from machine learning, we discover that coating a thin layer of 4,4′-dibromotriphenylamine (DBTPA) on top of a CH 3 NH 3 PbI 3 layer can improve the stability of resultant solar cells. The passivated devices kept 96% of the original power conversion efficiency for 1000 h at 85 °C in a N 2 atmosphere without encapsulation. Near-ambient pressure X-ray photoelectron spectroscopy (XPS) was employed to investigate the evolution of the composition and evaluate thermal and moisture stability by in situ studies. A comparison between pristine MAPbI 3 films and DBTPA-treated films shows that the DBTPA treatment suppresses the escape of iodide and methylamine up to 150 °C under 5 mbar humidity. Furthermore, we have used attenuated total reflection Fourier transform infrared and XPS to probe the interactions between DBTPA and MAPbI 3 surfaces. The results prove that DBTPA coordinates with the perovskite by Lewis acid−base and cation−π interaction. Compared with the 19.9% efficiency of the pristine sample, the champion efficiency of the passivated sample reaches 20.6%. Our results reveal DBTPA as a new post-treating molecule that leads not only to the improvement of the photovoltaic efficiency but also thermal and moisture stability.

Ambient pressure X-ray photoelectron spectroscopy (APXPS) is a powerful spectroscopy tool that is inherently surface sensitive, elemental, and chemical specific, with the ability to probe sample surfaces under Torr level pressures. Herein, we describe the design of a new lab-based APXPS system with the ability to swap small volume analysis chambers. Ag 3d(5/2) analyses of a silver foil were carried out at room temperature to determine the optimal sample-to-aperture distance, x-ray photoelectron spectroscopy analysis spot size, relative peak intensities, and peak full width at half maximum of three different electrostatic lens modes: acceleration, transmission, and angular. Ag 3d(5/2) peak areas, differential pumping pressures, and pump performance were assessed under varying N2(g) analysis chamber pressures up to 20 Torr. The commissioning of this instrument allows for the investigation of molecular level interfacial processes under ambient vapor conditions in energy and environmental research.

The liquid−vacuum interface was investigated for a ionic liquid (IL) mixture containing 1-ethyl-3-methylimidazolium acetate, [C 2 MIM][OAc], and 1ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [C 2 MIM][TFSI]. Herein, we detail a quantitative connection between molecular simulations and angle-resolved X-ray photoemission spectroscopy for an IL−vacuum interface. Results show that for a mixture with a low concentration of [TFSI] − , the anion [OAc] − is slightly depleted from the interface, whereas the [TFSI] − anion is significantly enhanced relative to the bulk. Both experiments and simulations reveal that the mole fraction of [TFSI] − increases significantly from the bulk value in the top 17 Å. Furthermore, simulations show that [TFSI] − has a preferred orientation at the liquid−vacuum interface.