The claim that the number zero is defined axiomatically in set theory and mathematics is a matter of basic mathematical definition (such as von Neumann's construction of ordinals in Zermelo-Fraenkel set theory where 0 is defined as the empty set), requiring no citation.
The retrieved papers are entirely off-topic, discussing unrelated subjects like American postulate theorists, psychological variance, memory processes, Bell's theorem, quantum probabilities, operator theory, infinitesimal probabilities, Riemannian geometry, spin-1/2 particles, time interval sets, hesitant fuzzy sets, and game theory. However, the claim itself states a foundational fact of mathematics and set theory (the axiomatic construction of numbers like zero, e.g., $\emptyset$ as zero in ZFC). Because this is a standard, definitional mathematical fact taught in introductory set theory, it falls under common knowledge.