## 5 Conclusions  
The Zr, ZrVHf and TiZrV NEG films were prepared by DC magnetron sputtering. The activation process of the NEG films was investigated by in situ SRPES after heating at 80 \({}^{\circ}\)C for 6 h, and 120, 180, 200, 250 and 300 \({}^{\circ}\)C for 0.5 h, respectively, under UHV conditions.  
From the emergence of the metal states and the change in O 1s and C 1s bonds, the activation temperature for the TiZrV film was found to be 120 \({}^{\circ}\)C--the lowest activation temperature ever reported on TiZrV. The activation temperature for Zr and ZrVHf films were 300 \({}^{\circ}\)C and 200 \({}^{\circ}\)C. The Zr, ZrVHf and TiZrV NEG films were able to be fully activated after the heating treatment at 300 \({}^{\circ}\)C, 200 \({}^{\circ}\)C and 120 \({}^{\circ}\)C, respectively, for a longer time.  
There were three noteworthy phenomena: (1) as the heating temperature increased, the transformation of the metal-C bond followed the order of V-C, Ti-C, Zr-C, Hf-C; (2) in this work, it was found that for different NEG films containing the same elements, the order for reduction difficulty of the same element oxides, Zr oxide and V oxide in different films, followed Zr film \(>\) ZrVHf film \(>\) TiZrV film; and (3) it was also found that the order of difficulty in the reduction of Ti, Zr, V and Hf oxides in the same alloy NEG film followed HfO\({}_{2}\)\(>\) ZrO\({}_{2}\)\(>\) TiO\({}_{2}\)\(>\) V\({}_{2}\)O\({}_{5}\).  
We propose that the above phenomena can be explained by thermodynamics of a solid solution. The overall diffusion coefficient of the residual gas atoms in single crystals follows the order of V \(>\) Ti \(>\) Zr \(>\) Hf. Since the alloy films prepared by sputtering is nanocrystalline or even amorphous, grain boundary diffusion plays a decisive role in the reduction. Due to the crystal structure difference, V cannot form an infinite solution with Zr, V and Hf. During the film growth process and heating treatment, V and Ti tend to precipitate more at the grain boundaries. The V and Ti precipitates act as a protection layer for Zr and Hf in the bulk. Consequently, the reduction of ZrO\({}_{2}\) is easier at relatively lower temperature in TiZrV films. At a similar grain size, TiZrv has a lower activation temperature than ZrVHf. TiZrV is suitable for applications requiring a low operating temperature.  
Conceptualization, Y.Y. and P.H.; methodology, T.H. and Y.M.; software, Y.Y. and S.C.; validation, J.W.; formal analysis, Y.Y.; investigation, Y.Y., Z.C. and X.P.; resources, F.S.; data curation, Y.Y., B.L. and X.W.; writing--original draft preparation, Y.Y.; writing--review and editing, Y.Y. and J.W.; visualization, Y.Y.; supervision, H.D.; project administration, P.H.; funding acquisition, P.H. and H.D. All authors have read and agreed to the published version of the manuscript.  
This work is supported by High Energy Photon Source (HEPS), a major national science and technology infrastructure.  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.