Quantifying the crystallographic phases present at a surface is an important challenge in fields such as functional materials and surface science. X-ray photoelectron spectroscopy (XPS) is routinely employed in surface characterisation to identify and quantify chemical species through core line analysis. Valence band (VB) spectra contain characteristic but complex features that provide information on the electronic density of states (DoS) and thus can be understood theoretically using density functional theory (DFT). Here we present a method of fitting experimental photoemission spectra with DFT models for quantitative analysis of heterogeneous systems, specifically, mapping the anatase to rutile ratio across the surface of mixed-phase TiO2 thin films. The results were correlated with mapped photocatalytic activity measured using a resazurin based smart ink. This method allows large-scale functional and surface composition mapping in heterogeneous systems, and demonstrates the unique insights gained from DFT-simulated spectra on the electronic structure origins of complex VB spectral features.