suppressed production of chemically reducing radiolytic species and led to AuNR growth trajectories more closely resembling those seen in Supporting Movies 1-2 (See SI Section 1.2 for further experimental details). Indiscriminate, rapid background particle deposition and/or growth of billowing shells around AuNRs without AuNR reshaping signified the presence of adventitious water, while AuNR growth dynamics explored elsewhere in this discussion showed correspondence to plasmonic hot carrier-mediated processes.