You may have to measure it yourself. Maybe you can follow procedures in these references:
Isoelectric point-controlled preferential photodeposition of platinum on Cu2O-TiO2 composite surfaces. Wang, M. Liu, Y.; Li, D.; Tan, J.; Huang, W. Chinese Chemical Letters 2019, 30(5), 985–988 DOI: 10.1016/j.cclet.2019.01.017
Determination of isoelectric points of metals and metallic alloys by adhesion of latex particles. Lefèvrea, G.; Čerovića, L.; Milonjić, S.; Fédoroff, M.; Finnec, J.; Jaubertied, A. Journal of Colloid and Interface Science 2009, 337(2), 449–455 DOI: 10.1016/j.jcis.2009.05.005