Water Adsorption on ZnO(0001): Transition from Triangular Surface Structures to a Disordered Hydroxyl Terminated phase

Abstract

We present room temperature scanning tunneling microscopy and photoemission spectroscopy studies of water adsorption on the Zn-terminated ZnO(0001) surface. Data indicates that the initial adsorption is dissociative leaving hydroxyl groups on the surface. At low water coverage, the adsorption occurs next to the oxygen-terminated step edges, where water is believed to bind to zinc cations leaving off hydrogen atoms to under-coordinated oxygen anions. When increasing the water dose, triangular terraces grow in size and pits diminish until the surface is covered with wide irregular terraces and a large number of small pits. Higher water exposure (20 Langmuir) results in a much more irregular surface. Hydrogen, which is produced in the dissociation reaction is believed to have an important role in the changed surface structure at high exposures. The fact that adsorbed water completely changes the structure of ZnO (0001) is an important finding toward the understanding of this surface at atmospheric conditions.

Polarity effects in the x-ray photoemission of ZnO and other wurtzite semiconductors

“…10,11 Three stabilization mechanisms are commonly proposed: ͑a͒ a fractional negative charge transfer from the O-polar to Zn-polar face, ͑b͒ a surface reconstruction which removes 25% of the surface atoms, and ͑c͒ the adsorption of hydroxide ͓hydrogen͔ on the Zn-polar ͓O-polar͔ face. [10][11][12][13] In the case of the Znpolar face, some form of condition-dependent ͑i.e., pH, temperature, H 2 / H 2 O partial pressure͒ competition between the formation of triangular islands and pits with O-terminated step edges and the adsorption of hydroxyl ͑OH͒ groups may occur. 12,13 The O-polar face, except under rare circumstances, has been shown to exhibit an unreconstructed ͑1 ϫ 1͒ H-termination.…”

“…[10][11][12][13] In the case of the Znpolar face, some form of condition-dependent ͑i.e., pH, temperature, H 2 / H 2 O partial pressure͒ competition between the formation of triangular islands and pits with O-terminated step edges and the adsorption of hydroxyl ͑OH͒ groups may occur. 12,13 The O-polar face, except under rare circumstances, has been shown to exhibit an unreconstructed ͑1 ϫ 1͒ H-termination. 14,15 Polarity-related differences in the XPS spectra of ZnO were investigated using c-axis, m-plane ͑1100͒, and a-plane ͑1120͒ bulk ZnO single crystal wafers ͑10ϫ 10ϫ 0.5 mm 3 ͒ grown using both the hydrothermal ͑HT͒ and pressurizedmelt ͑PM͒ techniques.…”

“…Since the step edges of these features are O-terminated, they lower the effective Zn/O ratio on the Znpolar face. 10,12 The termination of the ZnO surface can be determined from the O 1s core-level spectra. Figure 2 shows O 1s spectra taken at different photon energies ͑1486 to 600 eV͒ from organic-solvent cleaned HT ZnO samples at the Australian Synchrotron.…”

“…For h = 1486 eV, the dominant emission is at 530.8 eV due to oxygen in the bulk, with a high binding energy shoulder at 532.3 eV associated with the presence of OH groups. 12,14 The intensity of the OH contribution increases significantly with decreasing photon energy indicating that all three faces contain a significant amount of OH bonding localized at the surface. Since the hydroxyl termination at the Zn-polar face is ZnO͑0001͒Zn-OH compared to ZnO͑0001͒O -H for the O-polar face, [12][13][14] hydroxylation of the ZnO surface effectively places additional O atoms at the Zn-polar face, also lowering the Zn/O ratio.…”

The Review of Semiconductor Gas Sensor for Nox Detcting

“…A convergence of the Madelung energy can be achieved by these existing efficient stabilization mechanism of these surfaces. By the way the gas sensing properties of ZnO may be affected by these stabilization mechanisms (Bagus, Illas, Pacchioni, & Parmigiani, 1999;Chevtchenko et al, 2006;Dulub, Diebold, & Kresse, 2003;Kresse, Dulub, & Diebold, 2003;Lahiri, Senanayake, & Batzill, 2008;Önsten et al, 2010). The electrons are current carrier for ZnO because it is an n-type semiconductor.…”