Self-localization of polyacrylic acid molecules on polar ZnO(0001)–Zn surfaces

Abstract

The adsorption of single polyacrylic acid (PAAc) molecules was investigated on stepped hydroxide-stabilized polar ZnO(0001)-Zn surfaces using atomic force microscope (AFM) topography and force distance spectroscopy. Stepped surfaces of ZnO(0001)-Zn were prepared by a wet chemical etching procedure and PAAc molecules were adsorbed from aqueous NaClO(4) solutions. AFM single molecule topography studies could be utilized to show that polyacrylic acid molecules specifically adsorb on the non-polar (10-10) step edge faces at low ionic strengths. The radius of gyration of the dissolved PAAc in aqueous solution was measured by means of static light scattering experiments yielding a radius of gyration of R(g)=136 nm at pH 7.4 in 50 mM NaClO(4)/NaOH solution, which is in good agreement with the size of the adsorbed PAAc molecules as measured using AFM. The obtained results could be rationalized in terms of binding-site configurations at step edges and the effect of the chemical environment on both local electric double layer charge and molecular conformation of the PAAc molecules. The point of zero charge of the ZnO(10-10) surface was measured with chemical force microscopy to be pH(PZC)=10.2 ± 0.2. The specific adsorption of polyacrylic acid at non-polar ZnO step-edges can be explained by coordinative bonds formed between the carboxylic acid group and the Zn-surface atoms. On the hydroxide stabilized polar surface only weak hydrogen bonds can be formed in addition to van-der-Waals forces. Thus a "diffusion and trapping" mechanism keeps the adsorbed PAAc molecules mobile on the ZnO(0001)-Zn surface terraces due to small interaction forces until they are trapped at the (10-10) step faces by stronger coordinative bonds from the carboxylic groups to zinc atoms located in the first atomic layer of the crystal structure.

First-Principles Approach to Calculating Energy Level Alignment at Aqueous Semiconductor Interfaces

“…Therefore, an additional measurement must determine the pH at which the interface is neutral, assuring the equivalent of flat-band conditions on the water side of the junction. This has not been measured for GaN, but it is in the range of pH = 8 to 10 for ZnO [56,57]. Taken together with the measured potential for the band edges [54,55], the data are tightly clustered and relatively independent of facet, covering a range of about 0.3 eV, centered on E V = −7.3 eV relative to vacuum.…”

In Situ Study of the Polar ZnO(0001)−Zn Surface in Alkaline Electrolytes

“…In contrast, the (disordered) hydroxide stabilization of the surface in alkaline media is equally effective at dislocations and elsewhere, leading to smooth topographies without deep etching pits as long as the processing times are kept short (minutes). 42 This clearly demonstrates why an alkaline treatment should be favored over an acidic treatment for cleaning the polar ZnO(0001) surface.…”

Surface modification and immobilization in poly(acrylic acid) of Ag/ZnO for photocatalytic degradation of endocrine‐disrupting compounds

“…[25][26][27] Poly(acrylic acid) (PAA) molecules preferentially adsorb on the nonpolar facet of ZnO forming coordinative bonds between the carboxylic acid groups and Zn-atoms. 27 The photocatalytic activity of pure ZnO has been recently associated with polar facets. 28,29 This is relevant because theoretically the photoactive sites on ZnO-polyacrylates composites could be available for contaminants degradation.…”