It is easy to publish positive results and celebrate successes. "Negative results," which in the case of chemistry are mostly syntheses or processes that do not deliver the desired product or do not show any reaction at all, disappear into the drawer. This is partly justified because reliable evidence that something does not work is only rarely possible.
What was investigated in our case? To etch silicon wafer surfaces for applications in the semiconductor and photovoltaic industries, hydrofluoric acid mixtures with added oxidizing agents are used. The goal of the study was to replace the oxidizing agent nitric acid with perchloric acid to avoid the formation of toxic nitrogen oxides and nitrate-containing wastewater. The fact that perchloric acid and its salts can act as strong oxidizers is evident from the fact that perchlorates are used as rocket propellants and severe explosions have occurred during investigations of perchlorates in the presence of oxidizable substances.
And yet: In all 130 (!) attempts to etch silicon using mixtures of hydrofluoric acid and perchloric acid, no change in the surfaces was observed. Respect at this point to Ann-Lucia Neumann for her perseverance! This behavior was observable even when passivation layers were removed beforehand and silicon-hydrogen surfaces were present, which, for example, is already oxidized by atmospheric oxygen. This was clearly demonstrated by spectroscopic investigations (NMR, DRIFTS).
The explanation for the described results is actually quite straightforward: The extreme oxidizing power of perchloric acid is a thermodynamic property that can be suppressed by kinetic barriers. Such a barrier is caused by a very good distribution of charge on the perchlorate ion and a particularly good shielding of the chlorine atom, which is the actual oxidizing atom.
The reviewers and editors were nonetheless convinced by the very careful work, so our "negative results" can now be read in the article Investigations on the dissolution behavior of silicon in aqueous HF-HClO4-mixtures in RSC Advances (Open Access). [1]
[1] A.-L. Neumann, A. Stapf, N. Schubert, N. Zomack, E. Kroke, Investigations on the dissolution behavior of silicon in aqueous HF-HClO4-mixtures, RSC Adv., 2025, 15, 15796–15800 (DOI: 10.1039/d5ra00859j).