Fig. 1: Diverse coordination environments in the (1 × 4) reconstructed surface of anatase-TiO₂(001).

a Structural model of the (1 × 4) reconstructed surface. Upper panel: side view, lower panel: top view. The ball-and-stick model sketches three kinds of ridges. Left: the ADM ridge (O_TOP-ADM), right: the AOM ridge (O_TOP-AOM), middle: the mixed ADM and AOM ridge with a ratio of 1:1. The O_TOP, O_SIDE, O_TERRACE, and O_BULK atoms are correspondingly colored in upper and lower panels. An O_TOP-AOM becomes an O_TOP-ADM when it is missing an O atom marked by the red arrows. The surface O_V defects at terrace (O_V-TERRACE and O_V-3C) and ridge (O_V-RIDGE) sites are marked by the black arrows. b The empty-state STM images of high bias (3.0 V, 350 pA) and high-resolution (1.2 V, 350 pA) with the same area, measured at 80 K. High-resolution STM image exhibits clear terrace and ridge structures. The white arrow indicates an intrinsic dark point defect³². c Line profiles extracted from the corresponding colored lines in b.The red curve from high bias STM image shows the nonuniform electronic distributions, and the cyan one from the high-resolved image presents two different heights. d A set of in-situ empty-state STM (1.5 V, 10 pA) and the NC-AFM images at a frequency shift of −29 Hz within the same area, measured at 5 K, using a W tip. e Line profiles extracted from the corresponding colored lines in d. The red curve from STM image shows the nonuniform electronic distributions, and the blue one from AFM presents two distinct contrasts, labeled as ADM (blue arrow) and AOM (yellow arrow), according to the relative height in their models. f Left panel: a representative XPS spectrum in the energy range 0–30 eV measured at excitation with light energy of hυ = 200 eV, sample kept at 20 K, showing the main features of GSs, VBs, and semi-core levels. Right panel: schematic diagram of the main contributions from Ti 3d, O 2p, and O 2s.