<p>The pH clock is critical for identifying acid-sensitive substances and elucidating the mechanisms of chemical processes using spectral techniques. Effectively controlling the rate of H⁺ release with inorganic acids is challenging due to their fast acidification property. In addition, the strong corrosiveness of inorganic acids and the slowing rate of acidification in organic acids with decreasing pH further limit their applicability in fine spectral analysis. Therefore, developing a simple, safe, acidifying agent capable of controlling H⁺ release with a well-defined identification window is crucial for advancing spectral detection technologies. This study presents niobium oxide dichloride (NbOCl₂) nanosheets as a novel acidifying agent that not only regulates the rate of H⁺ release but also has a smooth extinction spectrum, making it suitable for monitoring acid-responsive behavior. The results demonstrate that NbOCl₂ is an excellent platform for pH clocks. Using spectral dynamics and first derivative images of the time-resolved extinction data, we have quantified the key factors associated with the wavelength of the extinction spectrum. Transient absorption results further indicated that H⁺ released from NbOCl₂ nanosheets reduced absorption, with its carrier dynamics exhibiting pronounced size dependence. These properties suggest NbOCl₂ nanosheets to be an ideal candidate as an acidifying agent.</p>