## 3 Results
For the ZrVH film (Figure 2b), the intensity of the O-C and -OH group almost fully reduced after heating at 180 \({}^{\circ}\)C. After heating at 80 \({}^{\circ}\)C, the lattice-O peak at 530.5 eV showed a mixture of ZrO\({}_{2}\), V\({}_{2}\)O\({}_{3}\), V\({}_{2}\)O\({}_{5}\) and HfO\({}_{2}\). After heating at 120 \({}^{\circ}\)C, V\({}_{2}\)O\({}_{5}\) reduced to V\({}_{2}\)O\({}_{3}\) at 530.7 eV [28], which can be cross validated by the V 2p spectrum of the ZrVHf film mentioned later. After heating at 200 \({}^{\circ}\)C, the lattice-O peak started to shift to a lower binding energy, suggesting the reduction in V-O. After heating at 250 \({}^{\circ}\)C, the lattice-O peak had two components--ZrO\({}_{2}\) at 531.0 eV and HfO\({}_{2}\) at 530.4 eV [28]. The intensity of the ZrO\({}_{2}\) and HfO\({}_{2}\) components is partly reduced after heating at 300 \({}^{\circ}\)C. The shifting of the lattice-O peaks suggests that the ease of breaking the metal-O bond follows the order of V-O, Zr-O, Hf-O. After heating at temperatures above 250 \({}^{\circ}\)C, most O was bonded to Zr and Hf. For TiZrV film (Figure 2c), after heating at 80 \({}^{\circ}\)C, metal-O bonds showed components of V\({}_{2}\)O\({}_{5}\)/TiO\({}_{2}\) at 530.0 eV, V\({}_{2}\)O\({}_{3}\) at 530.7 eV and ZrO\({}_{2}\) at 531.0 eV [28]. The intensity of all components of the O 1s peak was significantly reduced at only 120 \({}^{\circ}\)C, suggesting that 120 \({}^{\circ}\)C may be the activation temperature for the TiZrV film. After heating at 300 \({}^{\circ}\)C, the intensity of the lattice-O was significantly reduced. Figure 3 shows the SRPES spectra of the Zr 3d core-level peak of different NEG films recorded after heating at various temperatures. In all these cases, the deconvolution of Zr 3d spectra resulted in two peaks which corresponded to the Zr 3d5/2 and Zr 3d3/2, respectively. In Figure 3a, for the 80 \({}^{\circ}\)C degassed Zr film, the binding energy of the Zr 3d5/2 is 182.2 eV, a value that corresponds to fully oxidized Zr (IV) [29]. After heating at 250 \({}^{\circ}\)C, the peak intensity of Zr (IV) significantly reduced, with a possible amount of Zr (sub) [30]. At high temperatures of 300 \({}^{\circ}\)C, the Zr (IV) peak shifted to a lower binding energy and consisted of several elementary peaks, with the majority corresponding to metallic Zr (0) at 178.8 eV [29], suggesting that Zr (IV) reduced to a large extent. Compared to the Zr film, the reduction in Zr (IV) in the ZrVHf film occurred at a lower temperature of 200 \({}^{\circ}\)C, as can be seen in Figure 3b. Furthermore, in the TiZrV film as shown in Figure 3c, the Zr (IV) was partly reduced to Zr (sub) and Zr (0) after heating at a temperature as low as 120 \({}^{\circ}\)C. The tail at the binding energy of 187.4 eV is attributed to Auger peaks. It should be noted that since the binding energy of Zr-C is very close to that of ZrO (sub), the corresponding peaks could be a mixture of both. Figure 4 shows SRPES spectra of the V 2p, Ti 2p and Hf 4f core-level peaks of ZrVHf and TiZrV films recorded after heating at various temperatures. The peak position resolved in Figure 4 is consistent with references [31; 32; 33]. In Figure 4a-c, the reduction in the fully oxidized states of V 2p and Ti 2p in ZrVHf and TiZrV films occurred after heating at a temperature of only 120 \({}^{\circ}\)C.  
The integrated peak areas can be used to determine the content ratios of different states to understand the activation process. In Figure 4a, at 120 \({}^{\circ}\)C, there is no V (0), but V (sub). In Figure 4b, at 120 \({}^{\circ}\)C, V (0) and V (sub) appear with the percentages of 68.7 at% and 31.3 at%, respectively. Therefore, the activation of V in TiZrV is easier than that in ZrVHf.