Surface oxidation of Cu(100) has been investigated by variable temperature scanning tunneling microscopy and quantitative x-ray photoelectron spectroscopy as a function of ${\mathrm{O}}_2$ pressure ($8.0\times {10}^{-7}$ and $3.7\times {10}^{-2}\mathrm{mbar}$ ) at $373\mathrm{K}$ . Three distinct phases in the initial oxidation of Cu(100) have been observed: (1) the formation of the mixed oxygen chemisorption layer consisting of $\mathrm{Cu}\left(100\right)\text{-}c(2\times 2)\text{-}\mathrm{O}$ and $\mathrm{Cu}\left(100\right)\text{-}(2\sqrt{2}\times \sqrt{2})R45°\text{-}\mathrm{O}$ domains, (2) the growth of well-ordered $(2\sqrt{2}\times \sqrt{2})R45°\text{-}\mathrm{O}$ islands, and (3) the onset of subsurface oxide formation leading to the growth of disordered ${\mathrm{Cu}}_2\mathrm{O}$ . We demonstrate that the $(2\sqrt{2}\times \sqrt{2})R45°\text{-}\mathrm{O}$ reconstruction is relatively inert in the low pressure regime. The nucleation and growth of well-ordered two-dimensional Cu–O islands between two $(2\sqrt{2}\times \sqrt{2})R45°\text{-}\mathrm{O}$ domains is revealed by time-resolved scanning tunneling microscopy experiments up to 0.5 ML of oxygen. The formation of these islands and their nanostructure appear to be critical to the onset of further migration of oxygen atoms deeper into copper and subsequent ${\mathrm{Cu}}_2\mathrm{O}$ formation in the high pressure regime. The reactivity of each phase is correlated with the surface morphology and the role of the various island structures in the oxide growth is discussed.

Amorphous titania (am.-TiO₂) has gained wide interest in the field of photocatalysis, thanks to exceptional disorder-mediated optical and electrical properties compared to crystalline TiO₂. Here, we study the effects of intrinsic Ti³⁺ and nitrogen defects in am.-TiO₂ thin films via the atomic layer deposition (ALD) chemistry of tetrakis(dimethylamido)titanium(IV) (TDMAT) and H₂O precursors at growth temperatures of 100–200 °C. X-ray photoelectron spectroscopy (XPS) and computational analysis allow us to identify structural disorder-induced penta- and heptacoordinated Ti⁴⁺ ions (Ti_5/7c⁴⁺), which are related to the formation of Ti³⁺ defects in am.-TiO₂. The Ti³⁺-rich ALD-grown am.-TiO₂ has stoichiometric composition, which is explained by the formation of interstitial peroxo species with oxygen vacancies. The occupation of Ti³⁺ 3d in-gap states increases with the ALD growth temperature, inducing both visible-light absorption and electrical conductivity via the polaron hopping mechanism. At 200 °C, the in-gap states become fully occupied extending the lifetime of photoexcited charge carriers from the picosecond to the nanosecond time domain. Nitrogen traces from the TDMAT precursor had no effect on optical properties and only little on charge transfer properties. These results provide insights into the charge transfer properties of ALD-grown am.-TiO₂ that are essential to the performance of protective photoelectrode coatings in photoelectrochemical solar fuel reactors.

Visually black, electrically leaky, amorphous titania (am-TiO₂) thin films were grown by atomic layer deposition (ALD) for photocatalytic applications. Broad spectral absorbance in the visible range and exceptional conductivity are attributed to trapped Ti³⁺ in the film. Oxidation of Ti³⁺ upon heat treatment leads to a drop in conductivity, a color change from black to white and crystallization of am-TiO₂. ALD grown black TiO₂, without any heat treatment, is subject to dissolution in alkaline photoelectrochemical conditions. The best photocatalytic activity for solar water splitting is obtained for completely crystalline white TiO₂.

Surface oxidation of Cu(100) in O2 has been investigated in situ by x-ray photoelectron spectroscopy, x-ray induced Auger electron spectroscopy (XAES), and scanning tunneling microscopy (STM) as a function of surface temperature (TS=303–423K) and O2 pressure (pO2=3.7×10−2–213mbars). Morphology of the oxide on the surface and in the near surface layers is characterized by utilizing STM and the inelastic electron background of the XAES O KLL signal. Analysis of the peak shape of the XAES Cu LMM facilitates the quantification of Cu, Cu2O, and CuO surface concentrations. The authors conclude that the surface oxidation of Cu(100) proceeds in three distinct steps: (1) Dissociative adsorption of O2 and the onset of Cu–(2√2×√2)R45°–O (θO=0.5ML) surface reconstruction, (2) initial formation of Cu2O and the appearance of 1.8Å high elongated islands that also adopt the Cu–(2√2×√2)R45°–O structure, and (3) formation of highly corrugated Cu–O islands which together with the surface reconstruction strongly enhance the reactivity of the surface towards further oxide formation. Both Cu2O and CuO formations are enhanced by increased surface temperature, but no pressure dependence can be seen.

As the Earth's atmosphere contains an abundant amount of water as vapors, a device which can capture a fraction of this water could be a cost-effective and practical way of solving the water crisis. There are many biological surfaces found in nature which display unique wettability due to the presence of hierarchical micro–nanostructures and play a major role in water deposition. Inspired by these biological microstructures, we present a large scale, facile and cost-effective method to fabricate water-harvesting functional surfaces consisting of high-density copper oxide nanoneedles. A controlled chemical oxidation approach on copper surfaces was employed to fabricate nanoneedles with controlled morphology, assisted by bisulfate ion adsorption on the surface. The fabricated surfaces with nanoneedles displayed high wettability and excellent fog harvesting capability. Furthermore, when the fabricated nanoneedles were subjected to hydrophobic coating, these were able to rapidly generate and shed coalesced droplets leading to further increase in fog harvesting efficiency. Overall, ∼99% and ∼150% increase in fog harvesting efficiency was achieved with non-coated and hydrophobic layer coated copper oxide nanoneedle surfaces respectively when compared to the control surfaces. As the transport of the harvested water is very important in any fog collection system, hydrophilic channels inspired by leaf veins were made on the surfaces via a milling technique which allowed an effective and sustainable way to transport the captured water and further enhanced the water collection efficiency by ∼9%. The system presented in this study can provide valuable insights towards the design and fabrication of fog harvesting systems, adaptable to arid or semi-arid environmental conditions.