## 1 Introduction
Due to this, the recombination of carriers on the surface of the photocatalyst is negligible, and thus, efficient charge carrier separation is provided. The carrier lifetimes of photogenerated charge carriers are determined by time-resolved spectroscopy as time-resolved absorption/diffuse reflectance spectroscopy (TAS/TDRS) [6], time-resolved photoluminescence spectroscopy (TPLS) [7] and time-resolved IR spectroscopy (TIRS) [8]. The measurements are based on analysis of the optical absorption, reflection or emission peaks of the material. Unfortunately, their position and intensity critically depend on the nature of the semiconductors (e.g., size, crystal phase and morphology), excitation wavelength, irradiation time and the measurement circumstances, which may disturb the measurements and give unreliable results. Thus, the use of more sensitive and reliable techniques is required, such as those based on photothermal effect [9; 10; 11; 12; 13]. Among them, photothermal beam deflection spectrometry (BDS) is of high interest as it is suitable for the characterization of opaque and weakly absorbing samples. Furthermore, an additional advantage of the BDS technique is that it enables simultaneous determination of spectroscopic (e.g., extinction coefficients and band gap energies) and thermal parameters (e.g., thermal diffusivity, thermal conductivity and thermal effusivity) as well as other important parameters which are closely related to these properties (e.g., porosity) [14; 15]. In the BDS technique, the examined sample is irradiated by a modulated excitation laser beam. The absorbed photon energy is converted into heat, inducing temperature oscillations (TOs) in the sample and further in an adjacent gas layer close to its surface. Subsequently, the generation of TOs leads to the creation of a density gradient in the adjacent gas layer and a change in its index of refraction. These changes are detected by a probe laser beam deflection measured by a position sensitive detector, which generates phase and amplitude signals related to the photothermal parameters of the sample. The BDS method has already been applied for determination of the optical, structural and transport properties of different materials that, in turn, define their photocatalytic activity [12; 15]. The aim of this work was to develop Cu- and/or Zr-modified TiO\({}_{2}\) nanoparticles and characterize them by the use of the BDS technique, UV-Vis spectrophotometry, X-ray diffraction (XRD)and energy-dispersive X-ray (EDX) to determine their thermo-optical and transport parameters. The modification of TiO\({}_{2}\) materials exhibits induced spectral shifts into the visible spectral region, thus enhancing the absorption capacity of a photon under solar light. Additionally, a number of variables that affect the photocatalytic activity were examined. By knowing these parameters, it is possible to conclude about their potential application for photodegradation of various chemical and microbiological pollutants. Therefore, creating a proof of concept was the first step in testing the photodegradation activity of the characterized samples. In this regard, we used reactive blue 19 (RB19) dye as a test sample in the initial proof of concept and the thermal lens spectrometry (TLS) technique [16] in order to monitor the rate of photocatalytic reaction.