# Toward Defect Engineering Strategies to Optimize Energy and Electronic Materials
## 5 Tuning Metal Oxides for Catalysis  
### Hydrogen Doping in WO\({}_{3}\)  
Tungsten oxide, WO\({}_{3}\), can be used as a catalyst and catalyst support. To expand the range of applications to gas sensors and optoelectronics, it is necessary to control its intrinsic and/or extrinsic defect processes [74; 75; 76; 77; 78; 79; 80; 81; 82]. Importantly, although undoped and stoichiometric WO\({}_{3}\) is insulating (band gap of 3-3.3 eV [76]), oxygen hypostoichiometric tungsten oxide has electrical conductivity and optical absorption (blue colour because of the gap narrowing) [78]. Hydrogen is an important dopant in metal oxides as it (a) can modify the band gap; (b) occupy interstitial sites with only limited distortion in the lattice; (c) induce insulator-to-conductor transitions; and (d) interact with intrinsic defects (for example, oxygen vacancies and interstitials) [76; 79; 80; 81; 82]. Understanding H doping and its impact in defect processes remains an active research area in many technologically important metal oxides including WO\({}_{3}\) and TiO\({}_{2}\). Considering WO\({}_{3}\), [PERSON] et al. [76] used DFT calculations to study the structure and formation of defects. Figure 8 represents the configurations of defects in WO\({}_{3}\) reported in the study of [PERSON] et al. [76]. The calculations show that the O atoms that surround a W vacancy (V\({}_{\rm W}\)) have only small distortions from their original positions, conversely to surrounding W atoms towards an O vacancy (V\({}_{\rm O}\)) which exhibit significant distortions [76]. In WO\({}_{3}\), the oxygen interstitials form at the body center (O\({}_{\rm i}\)-1) or at the vicinity of a W atom (O\({}_{\rm i}\)-2)--refer to Figure 8 [76]. The W interstitial (W\({}_{\rm i}\)) atom is stable at the body center having a larger bond length (W-O bond length of 2.08 A) as compared to the perfect structure bond length (1.93 A) [76]. Figure 8 represents the three possibilities for H\({}_{\rm i}\): (a) bond to a single O atom (O-H distance of 0.98 A, H\({}_{\rm i}\)-1); (b) be located at the face center (O-H distances of 1.01 A and 1.67 A, H\({}_{\rm i}\)-2); and (c) form the (HO)\({}_{\rm i}\) interstitial defect (O-H distances of 1.02 A and 1.50 A) [76]. Figure 7: Arsenic concentration profiles with respect to depth for As-doped (blue line 3 \(\times\) 10\({}^{13}\) As/cm\({}^{2}\) with 50 keV implantation energy) and F + As codoped (red line As: 3 \(\times\) 10\({}^{13}\) As/cm\({}^{2}\) with 50 keV implantation energy; F: 1 \(\times\) 10\({}^{15}\) F/cm\({}^{2}\) with 35 keV implantation energy) Ge. Figure 9 reports the formation energies of intrinsic and H-related defects in WO\({}_{3}\) with respect to the Fermi level (O-rich and O-poor limits) for a range of charge states [76]. It can be observed that at the O-poor limit, the formation energy of V\({}_{\text{O}}\) is much lower as compared to V\({}_{\text{W}}\) and even negative for most of the values of the Fermi level (refer to Figure 8) [76]. For the O-rich limit, when the Fermi level is high, V\({}_{\text{W}}\) becomes more energetically favourable compared to V\({}_{\text{O}}\)[76]. For the oxygen interstitial defects, the formation energy of O\({}_{\text{i}}\)-1 is higher than that of O\({}_{\text{i}}\)-2 (refer to Figure 8 for the configurations and Figure 9 for the formation energies) and this is because the O\({}_{\text{i}}\)-1 does not form chemical bonds [76]. Both oxygen interstitials have positive formation energies. Conversely, the W\({}_{\text{i}}\) defect (+6 charge state) has a negative formation energy in the O-poor limit (refer to Figure 9) [76]. Interestingly, the W\({}_{\text{i}}\) formation energy is higher compared to the oxygen interstitials for O-rich conditions and when the Fermi level exceeds the middle of the band gap W\({}_{\text{i}}\) (refer to Figure 9) [76]. Figure 8: Configurations of defects in WO\({}_{3}\) considered using density functional theory by [PERSON] et al. [76]. W, O, and H atoms are represented by black, red, and green spheres, respectively.