Functional Coating by Atomic Layer Deposition

Electrochromic (EC) materials offer wide-ranging commercial applications, including smart windows, electrochromic mirrors, and programmable static displays. While many conventional electrochromic applications require the encapsulation of the EC materials and other components inside a sealed cell, there are cases where the color appearance of the object’s surface needs to be durable in atmospheric or aquatic environments. Among the current fabrication methods for EC materials, atomic layer deposition (ALD) remains less explored. In this work, we fill these gaps by employing ALD to grow a (Ti,Cr)Ox film as a new subset of EC coatings that operates in direct contact with aqueous solutions. The protective and bipolar coloration properties of the (Ti,Cr)Ox are achieved by the ALD alloying of TiO2 and CrOx nanocrystals. Intrinsic EC performance showed excellent cycle stability of more than 2000 cycles in aqueous electrolytes of high salinity and high corrosion resistance in pH = 0 acid. Furthermore, multicolor modulation has been achieved via a Fabry–Perot optical cavity design. Microstructural and spectroscopic characterizations combined with finite-difference time-domain (FDTD) modeling allow us to correlate the optical appearance with coating electronic and microstructures. This work showed that this ALD-grown ternary EC coating can also act as a surface protection layer and has unique advantages over other EC materials to achieve balanced performance among color tunability, coloration efficiency, and cycle stability

Particulate photocatalysis is a promising approach to solar fuels production at scale. Herein, we present a general design by using conformal coatings and attaching nanoscale cocatalysts to achieve local charge separation and, at the same time, to stabilize photocatalysts that are easily photocorroded otherwise. With spatial charge separation, the nanometer-spaced reductive and oxidative surface sites can coevolve to produce H2 and O2, or to produce H2 and oxidize redox mediators, or to produce O2 and reduce redox mediators. This work investigates the charge separation strategy for the semiconductor/coating/cocatalyst structure both by tuning barrier height energetics and by building numerical models.

Synthesizing nanopores which mimic the functionality of ion-selective biological channels has been a challenging yet promising approach to advance technologies for precise ion–ion separations. Inspired by the facilitated fluoride (F–) permeation in the biological fluoride channel, we designed a highly fluoride-selective TiO2 film using the atomic layer deposition (ALD) technique. The subnanometer voids within the fabricated TiO2 film (4 Å < d < 12 Å, with two distinct peaks at 5.5 and 6.5 Å), created by the hindered diffusion of ALD precursors (d = 7 Å), resulted in more than eight times faster permeation of sodium fluoride compared to other sodium halides. We show that the specific Ti–F interactions compensate for the energy penalty of F– dehydration during the partitioning of F– ions into the pore and allow for an intrapore accumulation of F– ions. Concomitantly, the accumulation of F– ions on the pore walls also enhances the transport of sodium (Na+) cations due to electrostatic interactions. Molecular dynamics simulations probing the ion concentration and mobility within the TiO2 pore further support our proposed mechanisms for the selective F– transport and enhanced Na+ permeation in the TiO2 film. Overall, our work provides insights toward the design of ion-selective nanopores using the ALD technique.

Stable photoelectrochemical solar fuel production requires protective coatings to achieve effective charge separation, transport, and injection at the semiconductor–liquid interfaces, implying that the coating should energetically align its intermediate band (IB) with both the photoabsorber’s band edge and co-catalyst’s potentials. Yet approaches to adjust coating IB positions to accommodate various semiconductor light absorbers for constructing efficient and stable photoelectrodes have not been developed. Herein, three types of transition metal (M = Mn2+, Mn3+, and Cr3+ ions) alloyed TiO2 coatings are discovered using atomic layer deposition (ALD). The IB energetics of these coatings are characterized by X-ray photoelectron spectroscopy and are found to be tunable inside the TiO2 bandgap, through varying ALD growth conditions. By applying these coatings to n-type GaP and integrating with IrOx co-catalysts, the water-oxidation J–E performance is comparable to an uncoated corroding GaP photoanode. It reaches the bulk recombination limit of the GaP and achieves ≈28% absorbed photon to current efficiency under 475-nm light excitation (6.48 mW cm−2) and 100-h stable water oxidation. The outstanding performance and stability are attributed to the efficient charge separation and hole transport, as allowed by the energy alignment of the coating IB and the GaP valence band edge.

Please read more in our work (Zhao et al., 2021), (Zhou et al., 2021), (Shen et al., 2022), and (Yang et al., 2023).