217Acta Chim. Slov. 2022, 69, 217–226 Žener, et al.: Metal and Non-Metal Modified Titania: ... DOI: 10.17344/acsi.2021.7200 Scientific paper Metal and Non-Metal Modified Titania: the Effect of Phase Composition and Surface Area on Photocatalytic Activity Boštjan Žener,1 Lev Matoh,1 Martin Reli,2 Andrijana Sever Škapin3,4 and Romana Cerc Korošec1,* 1 Faculty of Chemistry and Chemical Technology, University of Ljubljana, Večna pot 113, 1000 Ljubljana, Slovenia; 2 Institute of Environmental Technology, VŠB-Technical University of Ostrava, 17. listopadu 15/2172, Ostrava-Poruba, Czech Republic; 3 Slovenian National Building and Civil Engineering Institute, Dimičeva 12, 1000 Ljubljana, Slovenia 4 Faculty of Polymer Technology - FTPO, Ozare 19, 2380, Slovenj Gradec, Slovenia * Corresponding author: E-mail: romana.cerc-korosec@fkkt.uni-lj.si Received: 10-11-2021 Abstract The application of TiO2 photocatalysis in various environmental fields has been extensively studied in the last decades due to its ability to induce the degradation of adsorbed organic pollutants. In the present work, TiO2 powders doped and co-doped with sulfur and nitrogen and modified with platinum were prepared by particulate sol-gel synthesis. PXRD measurements revealed that the replacement of HCl with H2SO4 during synthesis reduced the size of the crystallites from ~ 30 nm to ~20 nm, increasing the surface area from ~44 m2/g to ~80 m2/g. This is consistent with the photocatalytic activity of the samples and the measured photocurrent behavior of the photocatalysts. The results showed that the prop- erties of the powders (i.e., surface area, crystallite size, photocurrent behavior) depend strongly not only on the type but also on the amount of acid and dopants used in the synthesis. Doping, co-doping and modification of TiO2 samples with nitrogen, sulfur and platinum increased their photocatalytic activity up to 6 times. Keywords: Titanium dioxide; powders; doping; photocatalysis; photocurrent; SEM 1. Introduction Titanium dioxide is considered to be one of the most contemporary important materials. It occurs in nature in three polymorphic modifications: anatase, rutile and brookite, among which rutile is the most abundant and thermodynamically stable. On the contrary, anatase has the highest photocatalytic activity, which can be attributed to the highest number of hydroxyl groups on the surface.1 Furthermore, three metastable phases can be produced synthetically, one of which is β-TiO2, which crystallizes in a monoclinic crystal system.2,3 Due to its favourable prop- erties, including its high chemical stability, non-toxicity, low price and high refractive index (value of µ is 2.70 for rutile and 2.55 for anatase) TiO2 is used for a wide number of applications in a variety of fields, for example as a white pigment in paints, plastic, paper, toothpastes and chewing gums, replacing the toxic lead oxides.4 It is also used in the fields of photovoltaics, electrochemistry and photocataly- sis.5–9 Due to its ability to mineralize adsorbed organic pol- lutants to CO2 and H2O, photocatalysis has been re- searched extensively with regard to its application in the fields of water remediation and air purification.10–12 Oxi- dation of adsorbed organic pollutants can occur directly on the surface of the photocatalyst.13 In the case that ad- sorption is not favourable due to the same electric charge on both pollutant and catalyst itself, reactive hydroxyl rad- ical, formed via oxidation of water with holes, can start degradation reactions of pollutants in a solution. To en- hance photocatalytic efficiency, it is necessary to prevent recombination between holes and electrons on route to the surface or on the surface sites.14 218 Acta Chim. Slov. 2022, 69, 217–226 Žener, et al.: Metal and Non-Metal Modified Titania: ... Various studies have been aimed at increasing the photocatalytic activity of TiO2, which can be achieved by increasing the surface area of TiO2, or through metal and non-metal doping. The surface area of the photocatalyst can be increased by decreasing its particle size.15,16 Sam- ples with a smaller crystallite size have a larger number of surface active sites, which should also increase its photo- catalytic activity. Wang et al., however, concluded that an optimum particle size of 11 nm exists for the degradation of chloroform in water. This was attributed to an increased recombination rate, which offsets the ultra-high surface area.17,18 Maira et al. found an optimum particle size of 7 nm for gas phase photooxidation of trichloroethylene. The diminished activity for samples with crystallites, smaller than 7 nm, was attributed to changes in electronic and structural properties.19 The surface area of the photocata- lyst can also be increased by adding polymers during the synthesis. The calcination that follows removes the poly- mer chains, leaving behind a mesoporous framework of TiO2, with an increased surface area.20–23 Doping TiO2 with metals also increases its photocat- alytic activity, by facilitating free electron capture and thus extending the lifetime of photogenerated electron-hole pairs.24 The capture results in an efficient separation of electron-hole pairs, thus inhibiting recombination and in- creasing the photocatalytic activity of TiO2 by enhancing the mass transfer of holes and possibly electrons to the sur- face.25 It has to be mentioned, however, that metal centres can also act as recombination centres and thus lower the photocatalytic activity. TiO2 is usually doped with noble metals, such as Ag, Pt and Pd.26–28 Other metals include Cu, V, Cr, Ni, as well as In.29–31 TiO2 can be synthesized using various synthetic pro- cedures, including hydrothermal, microwave-assisted and sonochemical methods, and miniemulsion techniques.32–35 Sol-gel synthesis offers many benefits compared to the synthetic methods mentioned above, including low cost, simplicity and low preparation temperatures. Because of this, it is a well-established procedure for the preparation of metal oxide nanoparticles.36 The method is based on in- itial hydrolysis of a precursor (e.g. TiCl4, titanium alkox- ides), which is followed by reactions of condensation (ox- olation and olation). Reactions result in the formation of sols, which are defined as stable suspensions of colloidal particles that can polymerize to form gels under certain conditions. On the other hand, stable sol can be deposited by dip- or spin coating onto a substrate and is then subject- ed to a drying process. During this process, the free –OH groups begin to link together, resulting in xerogels.37 In this work, we focussed on the preparation and characterization of sulfur and nitrogen doped and co- doped and platinum modified TiO2 powders. Synthesis, structural properties and photocatalytic efficiency of the corresponding thin film was already published.38 Since chemically equivalent thin films and powders often behave in different way, we have also systematically studied pow- dered samples, which is the main focus of the presented paper. Different analytical methods were used for their characterization and determination of the photocatalytic activity. Prepared samples were characterized by X-ray dif- fraction (XRD), specific surface area (BET) and photocur- rent measurements. Morphology of the powders was ex- amined using a field emission scanning electron microsco- py. The photocatalytic activity of the powders was deter- mined by monitoring the rate of oxidation of isopropanol to acetone using FTIR spectroscopy. 2. Experimental 2. 1. Synthesis The method of synthesis, as used to synthesize metal and non-metal doped and co-doped TiO2 powders, has been described previously in detail elsewhere.38 The synthe- sis procedure is described in the supplementary material. Sample names, types and amounts of dopants and acids added during the synthesis are presented in Table 1. Table 1. Sample names, types and amounts of dopants and acids added during the synthesis. Nominal Amount Dopant; amount (mL) and Sample dopant of dopant type of acid source relative to TiO2 added (atom %) REF / / 18; HCl Urea_15 N; urea 15 18; HCl Thiourea_15 S; Thiourea 15 18; HCl S2 S; H2SO4 / 3.3; H2SO4 S3 S; H2SO4 / 4.95; H2SO4 S3_N0.5 S; H2SO4 / N; NH4NO3 0.5 4.95; H2SO4 S3_N2 S; H2SO4 / N; NH4NO3 2 4.95; H2SO4 S3_urea15 S; H2SO4 / N; urea 15 4.95; H2SO4 S3_thiourea15 S; H2SO4 / S; Thiourea 15 4.95; H2SO4 S; H2SO4 / S3_N0.5+1%Pt N; NH4NO3 0.5 4.95; H2SO4 Pt; H2PtCl6 1 S; H2SO4 / S3_N0.5+2%Pt N; NH4NO3 0.5 4.95; H2SO4 Pt; H2PtCl6 2 S; H2SO4 / S3_N0.5+3%Pt N; NH4NO3 0.5 4.95; H2SO4 Pt; H2PtCl6 3 S; H2SO4 / S3_urea15+1%Pt N; urea 15 4.95; H2SO4 Pt; H2PtCl6 1 S; H2SO4 / S3_urea15+2%Pt N; urea 15 4.95; H2SO4 Pt; H2PtCl6 2 219Acta Chim. Slov. 2022, 69, 217–226 Žener, et al.: Metal and Non-Metal Modified Titania: ... The measured amounts of different dopants were deter- mined with XPS measurements and are given in.38 2. 2. Characterization XRD patterns were measured using a PANalytical X’Pert PRO MPD instrument in the 2θ range of 20–80° with a step of 0.034° using CuKα1 radiation. The average diameters of crystallites and phase compositions (amounts of polymophic modifications in % ) of the samples and corresponding error values were calculated with Rietveld analysis using TopasR software.39 Structural model (ICSD codes 92363 for anatase and 171670 for β-TiO2) was used for the calculations. The specific surface area of the powders was deter- mined through the measurement of nitrogen adsorp- tion-desorption isotherms by a Tristar 3000, Micromerit- ics (USA) instrument. The measurements were performed at –196 °C (77 K). The samples were outgassed under vac- uum for 16 h at 110 °C (383 K). The mass of the samples in the analyser was ≈ 0.1 g. The specific surface area was cal- culated from the adsorption measurements in the relative pressure (p/p0) range of 0.05–0.25. Photocurrent responses were recorded using a three electrode system, where Ag/AgCl and a Pt wire were used as reference and counter electrodes, respectively. The working electrode was prepared by depositing the meas- ured sample on the conductive side of an ITO foil. 0.1 M KNO3 was used as an electrolyte. The measurements were carried out in the range of wavelengths from 250 nm to 450 nm at an applied external potential of 1 V. This ensures that the highest number of photogenerated electrons travel to the working electrode. In this way the charge carriers (free electrons and holes) were successfully separated, thus preventing recombination. Before each measurement, the cell was purged with argon in order to ensure an oxygen free environment.40,41 In the measured range from 250 nm to 450 nm, the wavelength is being changed by 10 nm step. The photocurrent signal drops down when the shutter closes to switch the wavelength for another 10 nm. The morphology and the size of the particles of pre- pared powders were examined using a field-emission scanning electron microscope FE-SEM (FEI InspectTM F50 and Ultra Plus Zeiss). Accelerating voltage was set to 2 kV. Images were obtained with detection of secondary electrons. 2. 3. Photocatalytic Activity Tests The photocatalytic activity of the powder samples under UV and visible light exposure was determined by measuring the rate constant of oxidation of isopropanol to acetone and further oxidation leading to CO2 and H2O as the final products using FTIR spectroscopy. Commercially available Hombikat UV 100 (DE) – anatase nanopowder, with a primary crystal size <10 nm and specific surface area >250 m2/g was used for comparison. The reactions are presented in Equation (2):42 (2) Generally, the first step (k0) is considered to be a zero order reaction, whereas the second step (k1) is considered to be first order reaction. The method is presented in detail elsewhere.43 In the first step, approximately 50 mg of the powder was suspended in 3 mL of 1-butanol. This suspen- sion was then evenly distributed in a standard Petri dish and dried for 2 hours at 50 °C. Each dried sample was then put in a sealed gas-solid flow reactor system and then in- jected with 8 µL of isopropanol. Once the adsorption equi- librium was reached, as indicated by flat line in the isopro- panol concentration profile, the sample was illuminated with a 300 W Xe lamp (Newport Oriel Instrument) with an infrared filter. The spectrum of this Xe lamp is similar to sun illumination. The working distance between the Petri dish and the lamp was 6 cm. The temperature and relative humidity were set to 23 ± 2 °C and 25 ± 5 %, re- spectively. The isopropanol degradation and acetone for- mation and degradation processes (see equation 2) were followed by monitoring the calculated area of their charac- teristic peaks at 951 cm−1 and 1207 cm−1, respectively, in the IR spectra obtained by a FT-IR spectrometer (Per- kin-Elmer Spectrum BX II). The examples of characteristic FTIR output at three diferent reaction times with related explanations are presented in Figure S1 (Supplementary material). 3. Results and Discussion 3. 1. X-Ray diffraction (XRD) XRD patterns of the prepared powders are presented in Figure 1. It can be seen from the results that anatase is the only polymorphic modification present in samples synthesized with HCl (REF, Urea_15 and Thiourea_15). On the contrary, patterns of samples synthesized with H2SO4 include peaks of β-TiO2, which crystallizes in a monoclinic crystal system and cannot be found in nature. Table 2 shows the share of polymorphic phases and calcu- lated diameters of crystallites (both were calculated using the Rietveld analysis) in characterized samples. The results show that the amount of β-TiO2 in samples synthesized with H2SO4 varies from 15.6 ± 0.3 % to 45.4 ± 0.3 %. The highest amounts are present in samples S3_urea15 and S3_thiourea15 (38.7 ± 0.3 % and 45.4 ± 0.3 %), whereas samples S3 exhibits the lowest (15.6 ± 0.3 %). Moreover, crystallites found in samples synthesized with H2SO4 (15.1 ± 0.3 – 23.5 ± 0.2 nm) were smaller compared to samples synthesized with HCl (26.9 ± 0.7 – 30.7 ± 0.8 nm). The same trend was observed in our previously published work, which focused on thin films.38 As with thin film sys- 220 Acta Chim. Slov. 2022, 69, 217–226 Žener, et al.: Metal and Non-Metal Modified Titania: ... tems, this can be attributed to the formation of TiOSO4 in samples synthesized with H2SO4, which inhibits the crys- tallization of anatase. The smallest crystallites were found in samples S3_N2 and S3_urea15 (15.1 ± 0.3 to 17.5 ± 0.2 nm). Decreasing the crystallite size of TiO2 usually results in an increase in surface area, which can have a positive effect on the photocatalytic activity of the samples. In most samples, the calculated crystallite sizes of β-TiO2 are smaller to those of anatase. When comparing the size of crystallites in powders to those in thin films (as published in our previous work), we observed an interesting phenomenon. In the case of samples synthesized with H2SO4 we found larger crystal- lites in powder form (sizes of ~ 15 – 23 nm in powders compared to 8–12 nm in thin films). This can be explained by the unlimited growth of crystallites in powders, unlike in thin films, where growth is limited by the thickness of the film and the substrate. In direct contrast, when analys- ing samples synthesized with HCl we found larger crystal- lites in thin film samples compared to powders (sizes of 40 – 60 nm and ~ 27 – 31 nm, respectively). We attribute this to the partial crystallization and subsequent growth of TiO2 nanoparticles during the thermal treatment after each deposition (the final layer was prepared from three successive depositions, after each deposition thermal treatment was performed at 300 °C).38 Samples with added platinum also exhibit peaks at 40, 47.5 and 67.5° 2θ, which correspond to metallic plati- num. 3. 2. Specific Surface Area (BET) Results of surface area measurements are presented in Table 2. Measurements for six of these samples (REF, Urea_15, Thiourea_15, S3, S3_N0.5 and S3_N0.5+1% Pt) Table 2. The share of different polymorphic modifications, sizes of crystallites in powder samples calculated from XRD patterns using Rietveld re- finement and the specific surface areas for examined samples. Amount Calculated diameters Amount of Calculated diameters BET specific Sample of anatase of anatase β-TiO2 of β-TiO2 surface area (wt%) crystallites (nm) (wt%) crystallites (nm) (m2/g) REF 100 30.7 ± 0.8 / / 44.1 ± 0.4 Urea_15 100 29.6 ± 0.7 / / 24.2 ± 0.2 Thiourea_15 100 26.9 ± 0.7 / / 49.5 ± 0.3 S2 75.1 ± 0.3 21.1 ± 0.1 24.9 ± 0.3 19.0 ± 0.5 63.2 ± 0.3 S3 84.4 ± 0.3 23.5 ± 0.2 15.6 ± 0.3 19.0 ± 0.7 80.2 ± 0.2 S3_N0.5 81.1 ± 0.4 22.3 ± 0.2 18.9 ± 0.4 18.8 ± 0.6 84.5 ± 0.3 S3_N2 69.9 ± 0.3 16.8 ± 0.1 30.1 ± 0.3 15.1 ± 0.3 103.4 ± 0.4 S3_urea15 61.3 ± 0.3 17.5 ± 0.2 38.7 ± 0.3 16.5 ± 0.3 101.7 ± 0.4 S3_thiourea15 54.6 ± 0.3 18.7 ± 0.2 45.4 ± 0.3 19.3 ± 0.3 92.1 ± 0.3 S3_N0.5+1%Pt 82 ± 1 21.0 ± 0.4 18 ± 1 18 ± 1 83.7 ± 0.2 S3_N0.5+2%Pt 77 ± 2 17.9 ± 0.6 23 ± 2 20 ± 3 80.5 ± 0.4 S3_N0.5+3%Pt 74 ± 2 17.1 ± 0.8 26 ± 2 20 ± 3 84.1 ± 0.4 S3_urea15+1%Pt 69 ± 1 21.0 ± 0.6 31 ± 1 22 ± 2 82.3 ± 0.3 S3_urea15+2%Pt 73 ± 1 20.7 ± 0.6 27 ± 1 23 ± 2 84.2 ± 0.4 Figure 1. X-Ray diffraction patterns of powders. The designations A, B and Pt indicate the peaks for the corresponding TiO2 polymor- phic modifications and platinum. 221Acta Chim. Slov. 2022, 69, 217–226 Žener, et al.: Metal and Non-Metal Modified Titania: ... have already been published in our previous article. This study expands on those results.38 In general, samples synthesized with H2SO4 have higher surface areas (63.2 ± 0.3 to 103.4 ± 0.4 m2/g) com- pared to samples synthesized with HCl (24.2 ± 0.2 to 49.5 ± 0.3 m2/g). The higher surface area can be attributed to smaller crystallites found in samples synthesized with the addition of H2SO4, which is also observed from the XRD measurements. It can be seen from the results presented in Table 2 that the addition of urea (sample Urea_15 has specific sur- face area of 24.2 ± 0.2 m2/g) significantly decreases the surface area compared to the undoped sample REF (44.1 ± 0.4 m2/g). By increasing the volume of H2SO4 added we also increase the surface area of the sample (63.2 ± 0.3 m2/g for S2, 80.2 ± 0.2 m2/g for S3). The addition of NH4NO3 (samples S3_N0.5 and S3_N2), urea (sample S3_urea15) and thiourea (sample S3_thiourea15) to sam- ple S3 also increase the surface area of the samples (surface areas 84.5 ± 0.3 to 103.4 ± 0.4 m2/g). This can be explained by an additional decrease in the size of crystallites in these samples as compared to those in sample S3. The specific surface area does not change significant- ly when adding 1 % and 3 % of Pt to sample S3_N0.5. On the contrary, a decrease in surface area is observed when adding 2 % of Pt to sample S3_N0.5 (from 84.5 ± 0.3 m2/g for sample S3_N0.5 to 80.5 ± 0.4 m2/g for sample S3_ N0.5+2%Pt) and when adding 1 % or 2 % of Pt to sample S3_urea15 (from 101.7 ± 0.4 m2/g for S3_urea15 to 82.3 ± 0.3 m2/g and 84.2 ± 0.4 m2/g for sample S3_urea15+1%Pt and S3_urea15 + 2%Pt, respectively). 3. 3. Scanning Electron Microscopy (SEM) Figure 2 shows FE-SEM micrographs of samples REF, S3_N0.5 and S3_thiourea15. Substituting HCl (sample REF) with H2SO4 (samples S3_N0.5 and S3_thiourea15) during the synthesis has resulted in the formation of more porous powders, which is in agreement with the results of BET spe- cific surface area measurements. Furthermore, we have ob- served crystallites of sizes 31–35 nm in the SEM image of sample REF, which confirms the results of the XRD meas- urements. SEM images of samples synthesized with H2SO4 show crystallites ranging in size from 20–22 nm (sample S3_N0.5) and 18–23 nm (sample S3_thiourea15), which is also in agreement with the results of XRD measurements. We can also observe pores with sizes ranging from 70-100 nm (sample S3_N0.5) and 70-80 nm (sample S3_thiourea15). 3. 4. Photocurrent Measurements Figure 3 shows the results of photocurrent measure- ments for different samples. The reason why there is a very low photocurrent response in UV region is due to the fact 150 W Xe lamp was used as the light source. Xe lamps have very low intensity in UV region below 300 nm. By adding urea (sample Urea_15) to the undoped sample REF, the photocurrent response has decreased. We can attribute this to the lower surface area of sample Urea_15, which was observed from BET measurements. Contrarily, the addition of thiourea (sample Thiourea_15) has resulted in an increase in photocurrent response, which is attributed to the higher surface area of the sample Thiourea_15. Figure 2. FE-SEM micrographs of powders: REF, S3_N0.5 and S3_ thiourea15 at 200,000x magnification. Accelerating voltage was set to 2 kV. Images were obtained with detection of secondary elec- trons. 222 Acta Chim. Slov. 2022, 69, 217–226 Žener, et al.: Metal and Non-Metal Modified Titania: ... By substituting HCl with H2SO4 during the synthe- sis (samples S2 and S3) the current has increased. As with sample Thiourea_15, this can be explained by the higher surface area of samples S2 and S3 compared to the sam- ples synthesized with HCl. Because sample S3 has a high- er surface area than sample S2, a higher photocurrent is induced when irradiating the former. The results, as pre- sented in Figure 3b, show that the addition of 0.5 % of NH4NO3 (sample S3_N0.5), urea (sample S3_urea15) or thiourea (sample S3_thiourea15) to sample S3 has result- ed in a decrease in photocurrent response, despite the increase in surface area. The response has increased when adding 2 % of NH4NO3 (sample S3_N2), which also has the highest surface area of all the samples. From this we can deduce that the amount of induced photocurrent de- pends not only on the surface area, but also on the amount of added dopants. Band gap energies for selected samples have already been measured and published by Žener et al. 38 Results for samples with added Pt are presented in Figures 3c and 3d. When comparing these samples with those without the added Pt (samples S3_N0.5 and S3_ urea15), we can see that the addition of 1 % and 2 % of Pt had a positive effect on the photocatalytic response, de- spite the lower surface area of these samples. When irradi- ating the sample with 3 % of added Pt (sample S3_N0.5 + 3%Pt) the amount of induced photocurrent has decreased dramatically, which could be due to the increased Pt block- ing light to the photocatalyst. Moreover, platinum particles can also act as recombination centres. 3. 5. Photocatalytic Activity Concentration profiles for isopropanol and acetone for selected samples are presented in Figure 4. The photo- catalytic activity of powders under UV and visible light irradiation was tested by monitoring the oxidation of iso- propanol to acetone. It can be seen that the isopropanol curve is unstable before the UV and visible light is switched on, which can be attributed to the adsorption of isopropanol onto the surface of the sample and reactor system. After the UV and visible light is switched on the acetone concentration increases, while the isopropanol concentration decreases. Reactions are presented in Equa- tion (2) in the experimental section. In the initial steps of the photocatalysis, we can approximate the reaction to be of zero order, because the photocatalytic oxidation of iso- propanol to acetone is faster than the subsequent oxida- tion of acetone. Zero order kinetics can be described with Equation (3): (3) In this equation c and c0 represent concentration of acetone and the initial concentration of acetone, respec- tively in ppm, while t is time in hours and k0 is the zero-or- der rate constant (units ppm/h). Therefore, the initial slope of the acetone concentration curve is equal to k0 (Equation (3)), which was determined from line equations in Figure 4. This presents a good basis to compare photocatalytic ac- tivity of different TiO2 powders.43 Zero order rate con- stants (k0) for different samples are presented in Table 3. Figure 3. Photocurrent responses recorded under applied external potential of 1 V. 223Acta Chim. Slov. 2022, 69, 217–226 Žener, et al.: Metal and Non-Metal Modified Titania: ... The addition of urea (sample Urea_15) to the un- doped sample REF has resulted in an increase in the pho- tocatalytic activity (k0 = 122 ± 4 ppm/h and 68 ± 1 ppm/h for sample Urea_15 and REF, respectively). In our previ- ously published work, we reported a very high band-gap value in sample REF (3.44 eV). The addition of urea nar- rows the band-gap due to nitrogen doping, and conse- quently increases photocatalytic activity even under UV light.38,44 On the contrary, addition of thiourea (sample Thiourea_15) decreases the photocatalytic activity of the sample (k0 = 50 ± 2 ppm/h) when compared to REF, de- spite the former having a higher surface area (49.5 ± 0.3 m2/g for sample Thiourea_15 and 44.1 ± 0.4 m2/g for REF) and higher photocurrent response. This could be explained by higher recombination rates of the charge carriers in sample Thiourea_15. Additional measurements would, however, be needed to confirm this theory. By using H2SO4 instead of HCl in the synthesis (sam- ples S2 and S3) we obtained samples with smaller-sized crystallites, which results in higher porosity (24.2 ± 0.2 to 49.5 ± 0.3 m2/g for samples synthesized with HCl; 63.2 ± 0.3 Figure 4. Concentration profiles of isopropanol and acetone for: (a) REF; (b) Urea_15; (c) S3; (d) S3_N2; (e) S3_N0.5 + 1%Pt and (f) S3_urea15 + 1%Pt; y in the line equation represents concentration of acetone (in ppm) and x represents time of experiment in reactor (in hours) Table 3. Photocatalytic activities of different samples, determined by observing the oxidation of isopropanol to acetone (rate constant k0). Activity under UV and Sample visible light exposure - k0 (ppm/h) HOMBIKAT UV 100 337 ± 2 REF 68 ± 1 Urea_15 122 ± 4 Thiourea_15 50 ± 2 S2 183 ± 2 S3 310 ± 3 S3_N0.5 260 ± 2 S3_N2 328 ± 3 S3_urea15 304 ± 2 S3_thiourea15 225 ± 2 S3_N0.5+1%Pt 343 ± 2 S3_N0.5+2%Pt 251 ± 3 S3_N0.5+3%Pt 260 ± 3 S3_urea15+1%Pt 420 ± 3 S3_urea15+2%Pt 420 ± 2 224 Acta Chim. Slov. 2022, 69, 217–226 Žener, et al.: Metal and Non-Metal Modified Titania: ... and 80.2 ± 0.2 m2/g for samples S2 and S3, respectively) and a far higher photocurrent response. For these reasons, sam- ples S2 and S3 show much higher activity than samples syn- thesized with HCl (k0 = 183 ± 2 ppm/h for sample S2 and 310 ± 3 ppm/h for sample S3). S3 has a higher surface area and higher photocurrent response compared to S2, and consequently much higher photocatalytic activity. The addition of NH4NO3 (samples S3_N0.5 and S3_ N2), urea (sample S3_urea15) and thiourea (sample S3_ thiourea_15) to sample S3 increases the specific surface areas (84.5 ± 0.3 to 103.4 ± 0.4 m2/g). Despite this, only one sample has a higher photocatalytic activity than the sample without additions (sample S3, k0 = 310 ± 3 ppm/h, sample S3_N2, k0 = 328 /h). This can be explained by the results of the photocurrent measurements, which have shown decreased responses (compared to sample S3) in all but sample S3_N2, which also has the highest surface area (103.4 ± 0.4 m2/g). Higher activity can also be attributed to the addition of nitrogen, which increases the amount of oxygen vacancies and reduces the band gap energy, result- ing in higher photocatalytic activity.38 The decreased activ- ity in samples S3_urea15 and S3_thiourea15 can be attrib- uted to higher percentages of β-TiO2, found in these two samples, since the photocatalytic activity of β-TiO2 is gen- erally much lower than that of anatase.45–47 The addition of Pt to sample S3_N0.5 does not in- crease its surface area, in fact in the case of the sample with 2 % Pt added (sample S3_N0.5 + 2% Pt) it actually de- creases (84.5 ± 0.3 m2/g for sample S3_N0.5 and 80.5 ± 0.4 m2/g for sample S3_N0.5 + 2% Pt). Despite this, the sam- ple S3_N0.5 + 1%Pt shows much higher photocatalytic activity (k0 = 343 ± 2 ppm/h), compared to sample S3_N0.5 (k0 = 260 ± 2 ppm/h). We attribute this to Pt acting as an efficient trap for free electrons, thus inhibiting recombina- tion (confirmed with photocurrent measurements), whilst also improving the free electron transfer to adsorbed pol- lutants. The activity of sample S3_N0.5+3%Pt is equal to the activity of sample S3_N0.5, but the activity of sample S3_N0.5 + 2%Pt has reduced slightly. Doping the sample S3_urea15 with Pt (samples S3_ urea15 + 1%Pt and S3_urea15+2%Pt) significantly lowers its surface area (82.3 ± 0.3 m2/g for sample S3_urea15 + 1%Pt and 84.2 ± 0.4 m2/g for sample S3_urea15 + 2%Pt). Furthermore, results of the photocurrent measurements have shown that the addition of Pt significantly increases the photocurrent response, resulting in far higher photo- catalytic activity in samples S3_urea15 + 1%Pt (k0 = 420 ± 3 ppm/h) and S3_urea15 + 2%Pt (k0 = 420 ± 2 ppm/h) compared to sample S3_urea15 (k0 = 304 ± 2 ppm/h). It was found out that metal and non-metal doping, as well as addition of HPC significantly increase the photocatalytic activity of powders under UV and visible light irradiation. In the case of samples S3_urea15 + 1%Pt and S3_urea15 + 2%Pt the photocatalytic activity is even higher than that of selected anatase sample available on the market: HOMBIKAT UV 100 (k0 = 337 ± 2 ppm/h) (see Table 3).48 4. Conclusions In the present work TiO2 powders, doped with sulfur and nitrogen and modified with platinum were prepared by means of particulate sol-gel synthesis in order to in- crease the photocatalytic activity of undoped sample, while the organic polymer hydroxypropyl cellulose (HPC) was added to increase the surface area of the photocatalyst. By substituting HCl with H2SO4 during the synthesis, the resulting samples contained smaller crystallites (26.9 ± 0.7 – 30.7 ± 0.8 nm for samples synthesized with HCl; 15.1 ± 0.3 – 23.5 ± 0.2 nm nm for samples synthesized with H2SO4). We attributed the smaller crystallite size to the formation of TiOSO4, which inhibits the crystallization of TiO2. Additionally, we observed the presence of β-TiO2 in samples synthesized with H2SO4. The highest percentage of β-TiO2 was found in sample S3_thiourea15 (45.4 ± 0.3 %). The afore-mentioned decrease in the size of crystallites led to a higher specific surface area for samples synthe- sized with H2SO4 (63.2 ± 0.3 to 103.4 ± 0.4 m2/g) com- pared to those synthesized with HCl (24.2 ± 0.2 to 49.5 ± 0.3 m2/g). The addition of NH4NO3, urea and thiourea to sample S3 increased its porosity. Sample S3_N2 had the highest surface area (103.4 ± 0.4 m2/g) and the smallest crystallites of anatase (16.8 ± 0.1 nm). The addition of Pt did not increase the sample’s porosity, in some cases it even decreased it. FE-SEM images confirmed the increased po- rosity of samples synthesized with H2SO4. Additionally, smaller crystallites were found in samples synthesized with H2SO4, confirming the results of X-Ray diffraction. The increased porosity of the samples synthesized with H2SO4 also resulted in greater photocurrent respons- es in these samples compared to those synthesized with HCl. Despite increasing the specific surface area, the addi- tion of NH4NO3, urea and thiourea to S3 yielded a lower photocurrent response, with the exception of sample S3_ N2, which also had the highest surface area. In all but one case the addition of platinum resulted in greater photocur- rent responses. Samples synthesized with H2SO4 exhibit higher pho- tocatalytic activity compared to samples synthesized with HCl, which can be explained by larger surface areas and higher photocurrent responses. Out of all the samples with added NH4NO3, urea or thiourea, only sample S3_N2 has a higher photocatalytic activity compared to sample S3. This is also the only sample in this group which exhibit a greater photocurrent response than sample S3. The addi- tion of platinum to sample S3_urea significantly increased its photocatalytic activity, which is in agreement with the results of photocurrent measurements. Out of all the sam- ples, samples S3_urea15 + 1%Pt and S3_urea15 + 2%Pt showed the highest photocatalytic activity (k0 = 420 ± 3 and 420 ± 2 ppm/h), which was even higher than the activ- ity of the well-known pure anatase photocatalyst HOMBIKAT UV 100 (k0 = 337 ± 2 ppm/h). We were able to significantly increase the photocatalytic activity of pow- 225Acta Chim. Slov. 2022, 69, 217–226 Žener, et al.: Metal and Non-Metal Modified Titania: ... ders under UV and visible light irradiation by increasing the surface area and photocurrent response with non-met- al and metal doping. Acknowledgements The authors acknowledge the financial support from the Slovenian Research Agency (research core funding Nos. P1-0134 and P2-0273, while part of the work was conducted under project No. NC-0002). B. Ž. is grateful to Slovenian Research Agency for the position of young re- searcher enabling him the doctoral study. M. R. also ac- knowledges the Operational Programme Research, Devel- opment and Education, project No. CZ.02.1.01./0.0/0.0/17 _049/0008419 „COOPERATION“. The authors thank to Mojca Opresnik from the National Institute of Chemistry for BET measurements and to Edi Kranjc (also from the National Institute of Chemistry) for performing XRD measurements. The authors also acknowledge dr. Amalija Golobič for her help with Rietveld analysis. 5. References 1. R. Fagan, D. E. McCormack, D. D. Dionysiou, S. C. Pillai, Mater. Sci. Semicond. Process. 2016, 42, 2–14. DOI:10.1016/j.mssp.2015.07.052 2. R. Marchant, L. Brohan, M. Tournox, Mater. Res. Bull. 1980, 15, 1129–1133. DOI:10.1016/0025-5408(80)90076-8 3. S. León-Ríos, R. Espinoza González, S. Fuentes, E. Chávez Ángel, A. Echeverría, A. E. 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PXRD meritve kažejo, da zamenjava HCl s H2SO4 med sinteznim postopkom zmanjša velikost kristalitov iz ~30 nm na ~20 nm, pri čemer se pov- eča tudi specifična površina iz ~44 m2/g na ~80 m2/g. Opažanja korelirajo z izmerjeno fotokatalitsko aktivnostjo vzorcev in izmerjenim fototokom. Rezultati kažejo, da so lastnosti prahov (specifična površina, velikost kristalitov, obnašanje fototoka) odvisne ne le od vrste uporabljene kisline, temveč tudi od njene količine in uporabljenega dopanta. Dopiranje z žveplom, kodopiranje z žveplom in dušikom in modifikacija prahov TiO2 s platino povečajo fotokatalitsko aktivnost tudi do šestkrat. Except when otherwise noted, articles in this journal are published under the terms and conditions of the  Creative Commons Attribution 4.0 International License