Effect of Ag modification on TiO2 and melem/g-C3N4 composite on photocatalytic performances

1 Vol.:(0123456789) Scientific Reports | (2023) 13:5270 | https://doi.org/10.1038/s41598-023-32094-6 www.nature.com/scientificreports Effect of Ag modification on TiO2 and melem/g‑C3N4 composite on photocatalytic performances M. Michalska 1*, V. Matějka 1, J. Pavlovský 1, P. Praus 1,2, M. Ritz 1, J. Serenčíšová 3, L. Gembalová 4, M. Kormunda 5, K. Foniok 1, M. Reli 2 & G. Simha Martynková 6 Here, the comparison of two different semiconductor materials is demonstrated, TiO2 and melem/g‑ C3N4 composites—modified with balls of approximately 5 nm Ag nanoparticles (NPs) as photocatalysts for the degradation of the model dye acid orange 7 (AO7). The melem molecule synthesized here is one of a series of organic compounds consisting of triazine ring compounds with a structure similar to that of melam and melamine. The photodegradation process of AO7 was carried out to examine all powder materials as a potential photocatalyst. Additionally, two different lamps of wavelengths 368 nm (UV light) and 420 nm (VIS light) were applied to compare the photodegradation tests. A new synthesis route for the acquisition of Ag NPs (Ag content 0.5, 1.0 and 2.5 wt%), based on a wet and low temperature method without the use of reducing reagents was proposed. The best photocatalytic performances under UV and VIS light were obtained for both, TiO2 and melem/g‑C3N4 materials (new synthesis route) modified with a very low Ag content—0.5 wt%. The photodegradation activities using UV lamp (3 h, 368 nm irradiation) for samples with 0.5 wt% of Ag: TiO2 and melem/g‑C3N4, in excess of 95 and 94%, respectively, were achieved. The highest photoactive materials melem/g‑ C3N4 with 0.5 and 1 wt% Ag revealed 98% of activity under the VIS lamp after 3 h long irradiation. Our work demonstrates a novel, environmentally acceptable, and cost‑effective chemical strategy for preparation of photocatalysts suitable for degradation of organic contaminants in wastewater treatment. Titanium dioxide of chemical formula TiO2 is one of the prominent materials that has found applications in almost all research areas1, such as electrode material in Li-ion2–4 and Na-ion batteries2,5,6, supercapacitors7,8, electrochemical solar cells9,10, photocatalysts11–13 and so on. Titanium dioxide is recognized as a chemically stable semiconductor material. It shows a wide band gap energy in the range of 3.0–3.2eV and exhibits photocatalytic activity only when exposed to UV light (up to around 390nm)11–13. Electron–hole pairs photogenerated on the TiO2 surface show strong reducing and oxidizing properties11–13. The values of the edge band potentials are suitable for the creation of reactive oxygen species, for example, the oxidation of water to a hydroxyl radical (∙OH), or the oxygen reduction to a superoxide radical anion (·O2−), which further increase the possibility of using this material in photocatalytic processes11–13. To shift the light absorption of TiO2 to the visible range of spectra, different strategies for TiO2 modification were already proposed11–13. One of the possible approach to overcome the drawbacks mentioned above, is to dope the pristine material with different ions (i.e., N, Fe, Ni, Al, Zn, Cu)14–16 or modify the surface of TiO2 with metal, metal oxide, or carbon species13,17–20. Compared to TiO2, the so-called bulk graphitic carbon nitride (g-C3N4, g-CN) has a proper mid-wider band gap energy (2.7–2.8eV) to absorb visible light efficiently21,22. Graphitic carbon nitride can be synthesized through the thermal polymerization of precursors such as urea23, thiourea24, cyanamide25, dicyandiamide23, and melamine26,27. Based on our previous studies on the synthesis of a graphitic carbon nitride material, melamine is OPEN 1Department of Chemistry and Physico-Chemical Processes, Faculty of Materials Science and Technology, VŠB-Technical University of Ostrava, 17. listopadu 2172/15, 708 00 Ostrava-Poruba, Czech Republic. 2Institute of Environmental Technology, CEET, VŠB-Technical University of Ostrava, 17. listopadu 2172/15, 708 00 Ostrava-Poruba, Czech Republic. 3Energy Research Centre, CEET, VŠB-Technical University of Ostrava, 17. listopadu 2172/15, 708 00 Ostrava-Poruba, Czech Republic. 4Department of Physics, Faculty of Electrical Engineering and Computer Science, VŠB-Technical University of Ostrava, 708 00 Ostrava, Czech Republic. 5Faculty of Science, J. E. Purkyně University, Pasteurova 15, 400 96 Usti nad Labem, Czech Republic. 6Nanotechnology Centre, CEET, VŠB-Technical University of Ostrava, 17. listopadu 2172/15, 708 00 Ostrava-Poruba, Czech Republic. *email: [email protected] 2 Vol:.(1234567890) Scientific Reports | (2023) 13:5270 | https://doi.org/10.1038/s41598-023-32094-6 www.nature.com/scientificreports/ the only precursor that yields more g-CN material than other precursors23,27, and thus g-CN synthesis procedure is very efficient, cost-effective, and also scalable. One of the intermediate products during the g-CN precursors polycondensation is melem. Jurgens etal.28 reported that crystalline powder of single-phase melem (2,5,8-triamino-tri-s-triazine) C6N7(NH2)3 was formed in sealed glass ampules by thermal treatment of several precursors (e.g., melamine C3N3(NH2)3, dicyandiamide H4C2N4, ammonium dicyanamide NH4[N(CN)2], or cyanamide H2CN2, respectively) at temperatures up to 450°C. Melem possesses superior thermodynamic stability at the higher temperature range of around 400–500°C in which is formed29, and Dong etal.30 reported that those materials could work under UV as well as VIS light. The wide variety of applications for the graphitic carbon nitride material is due to its nontoxicity, low cost, and ease of preparation21,22,31,32. However, this material has a high rate of recombination of the generated charge carriers resulting in a low photocatalytic efficiency33. Various strategies have been applied including the fabrication of nanomaterials32,33 or porous structures32,33, doping23,26,27,33, metal or non-metal deposition34–38, coupling g-CN with other semiconductors31,34,36,37,39 to enhance visible light-induced photocatalytic activity of g-C3N4. Noble metal deposition, such as Ag nanoparticles (NPs), has received extensive attention due to its stronger electron storage capacity, lower cost, and nontoxicity34–37. Coupling of photocatalysts with silver nanoparticles brings the benefit of the increased excitons lifetime due to the fact the silver makes a Schottky barrier which enables to capture of the electron photoexcited from the valence band of a given semiconductor40. The increase of life-time of electron–hole pairs leads to the increase of the photodegradation activity of a given silver modified photocatalyst. SPR effect of silver on the photodegradation activity of the silver modified photocatalysts is other phenomenon that positively influences the photodegradation activity of such composites in visible light as reported, for example, Wang etal.41. Ag nanoparticles can be further stimulated to produce electrons and holes and act as an electron source. Additionally, they support active species and oxidize Ag to Ag+ due to the surface plasmon resonance (SPR) effect of silver35,42. Surface modified Ag-TiO2 nanoparticles have more effective electron traps than single Ag nanoparticles43. Other benefit of the silver coupling with the photocatalytic materials comprises the possible antibacterial performance of the resulting composite, and in this way, the functionality of the resulting silver modified photocatalyst is significantly widened44. Herein, in this article, the comparison of the results of surface modification of TiO2 and melem/g-C3N4 materials using Ag nanoparticles (NPs) utilized as a photocatalyst for the degradation of the model acid orange 7 dye under UV or VIS irradiation, is demonstrated. A new synthesis route of obtaining ca. 5nm-sized Ag NPs balls (content of Ag: 0.5, 1.0, and 2.5 wt%), based on a wet, simple chemical, and low-temperature method without using reducing reagents was proposed. All powders were extensively characterized using several complementary techniques. For the first time in this work, the new synthesized materials of melem/g-C3N4 were surface modified with Ag NPs and examined with AO7 as a potential photocatalyst. Methods Synthesis of melem/g‑C3N4 composite. The pristine melem/g-C3N4 material was prepared by thermal treatment processing of the melamine precursor in air flow at 550°C. The heating regime comprised the treatment of the 10g of melamine placed in semi-closed alumina crucible with alumina lid at a rate of 3°C/ minto reach 550°C with immediate displacement of the crucible out of the furnace when the 550°C was reached. The resulting bright yellow powder was labelled g-CN-MM. Synthesis of silver modified TiO2 and melem/g‑C3N4 composites. The flowchart of the synthesis of TiO2 and melem/g-C3N4 decorated with Ag nanoparticles is shown in Fig.1. In the first step, silver nitrate (AgNO3, pure, Lachema) was firstly dissolved in an ethanol solution (EtOH, 96%, Merci). Then, the anatase form of TiO2 (Sigma-Aldrich, 99.8%) or the prepared g-CN-MM materials were added to the prepared AgNO3 solutions in proportions to obtain the suspensions with the Ag content of 0.5, 1.0, and 2.5 wt%, respectively. The mixtures were stirred magnetically for several hours to obtain a homogenously Figure1. The flowchart of the synthesis of TiO2-A-n% Ag and of MM-g-CN-n% Ag composites. 3 Vol.:(0123456789) Scientific Reports | (2023) 13:5270 | https://doi.org/10.1038/s41598-023-32094-6 www.nature.com/scientificreports/ dispersed suspension and then dried in air for a few hours at 70°C and then at 150°C. The prepared samples were labelled as TiO2-A-n% Ag and MM-g-CN-n% Ag (where n indicates the amount of Ag 0.5, 1.0, 2.5 wt% in the final composites). At the last step, the composites were ground in an agate mortar to obtain a fine powder. The sample labelled as MM-g-CN was a material received after the proposed low-temperature chemical synthesis based on the treatment of g-CN-MM only in an ethanol solution (without the presence of silver). Characterization. Elemental analysis measurements for two samples (g-CN-MM and MM-g-CN) were performed on a CHN628 Series Carbon/Hydrogen/Nitrogen Determinator, LECO Corporation, United States. The analysis parameters were the following: furnace temperature 950°C, afterburner temperature 850°C, gas flow oxygen O2 (purity 5.0): 1.burn step was 4L/min–10s, 2.burn step was 0.3L/min–180s, 3.burn step was 1L/min–30s and 4.burn step was 4L/min–30s. The average analysis time was about 7min. The calibration standard was used, melamine (p.a. purity, 28.58wt% C, 4.79wt% H, 66.63wt% N; molecular formula C3H6N6) with 8calibration points. Powder X-ray diffraction (XRD) analysis using CuKα radiation source (λ = 0.1541nm) was employed to identify the crystalline phase of all pristine and silver-modified powders. The diffraction patterns were recorded using diffractometer Ultima IV (RIGAKU, Japan). Working conditions of all studies: CuKα radiation (40kV, 40mA); K-beta filter; CBO selection slit—BB; Scintillation counter; continuous scan; Scan speed—4°/min; Step width—0.05°; Scan range—3–60° 2θ; Incident and receiving slit 1–2/3°; Receiving slit 2–0.6mm were used. Raman spectra were measured using a DXR SmartRaman dispersive Raman spectrometer (ThermoScientific, USA) with a CCD detector. The measurement parameters were as follows: excitation laser 780nm, grating 400 lines/mm, aperture 50μm, exposure time 1s, number of exposures 500. An empty sample compartment was used for background measurement. Treatment of spectra: fluorescence correction (6th order). Infrared spectra were measured by the potassium bromide pellet technique in the middle IR region. Exactly 1.0mg of sample was ground with 200mg dried potassium bromide. This mixture was used to prepare the potassium bromide pellets. The pellets were pressed by 8 tons for 30s under vacuum. The infrared spectra were collected using a Nicolet iS50 FT-IR spectrometer (ThermoScientific, USA) with a DTGS detector. The following parameters were used for the measurement: spectral resolution, 4 cm−1; 64 scans; Happ-Genzel apodization. Treatment of spectra: polynomial (second order) baseline subtraction spectrum of pure potassium bromide. DRS (UV–VIS) spectra of dry powder samples were measured at wavelengths ranging from 220 to 800nm at room temperature on a Shimadzu UV-2600 Series with an IRS-2600Plus integrating sphere (Shimadzu Ltd, Japan). Powder BaSO4 (Nacalai Tesque, Inc., Japan) as a reference sample, and an external 2D detector was used. The values of the indirect band gap energy (Eg) were obtained using the Tauc curve. The photoluminescence (PL) emission spectra of the samples were measured by a FLSP920 Series spectrometer (Edinburgh Instrument Ltd, UK) in the wavelength range from 350 to 600nm at room temperature. Spectrometer was equipped with a 450 W non-ozone xenon lamp (Steady state Xe900 lamp) and a R928P detector (PMT detector). The excitation wavelength was set at 325nm, and excitation and emission slits 0.3nm and dwell time 0.5s were set for all samples of MM-g-CN group. For the samples of TiO2-A group, the dwell time was set at 1s and the excitation and emission slits were set at 3nm. Steady-state and time-resolved photoluminescence (PL) experiments were performed using a FLS980 fluorescence spectrometer (Edinburgh Instruments, UK) equipped with a 450 W xenon arc lamp and an EPL375ps pulsed diode laser (λem = 372nm with a pulse width of 66.5ps, a repetition rate of 10MHz and an average power of 75µW (Edinburgh Instruments, UK) as excitation sources. PL decay curves were fitted with a multiexponential function where I(t) is the intensity of photoluminescence, t is the time, Bi coefficients are the time-invariant constants, and τi are the decay times (decay constants). The mean decay time τm was calculated as follows SEM observations were performed using a FEI Quanta 650 as well as 450 FEG (FEI, USA) scanning electron microscope equipped with an X-ray energy dispersive spectroscopy system (EDS) for chemical analysis and elemental mapping. Two different conditions of measurements as follows: Large field Low vacuum SED (LFD) Detector, Electron Beam resolution operating at HV 10kV with Vacuum system of pressure 50Pa (LoVac) for series of TiO2-A-n% Ag, and Everhardt Thronley SED (EDT) Detector, Electron Beam resolution operating at HV 20kV with Vacuum system of pressure 10−4–10−3Pa (HiVac) for series of MM-g-CN-n% Ag were applied. The images were collected at two different magnifications at 50 × and 40 × by using Cr layer for a series of TiO2-A and MM-g-CN composites with Ag, respectively. Additionally, to compare all samples, the SEM images at 5×magnification were investigatedand presented in Supplementary Material. The TEM examinations were investigated using JEOL 2100 transmission electron microscopy apparatus (Jeol Ltd., Tokyo, Japan) with an LaB6 electron gun, operating at 200kV. The TEM images were collected by a Tengra camera (EMSIS GmbH, Münster, Germany). The powders were dispersed in an ethanol solution and, then sonicated for 5min. One drop of the homogenously prepared dispersion was coated on a copper grid with a holey carbon film and dried at ambient temperature. X-ray photoelectron spectroscopy (XPS) was used for characterization of the surface chemical composition of selected samples and to reveal the forms of given elements. Main parts of the XPS analyser include the high (1) I (t)= 3  i=1 Bie−t/τ i (2) τ m= B1τ 2 1+B2τ 2 2+B3τ 2 3 B 1τ1 +B 2τ2 +B 3τ3 4 Vol:.(1234567890) Scientific Reports | (2023) 13:5270 | https://doi.org/10.1038/s41598-023-32094-6 www.nature.com/scientificreports/ vacuum chamber which was equipped with SPECS X-Ray XR50 (Al cathode 1486.6eV was used), hemispherical analyser SPECS PHOIBOS 100 with 5-channels detector SPECS MCD was used for the signal detection. The Flood gun SPECS FG22/35 was not used. The registered spectra were evaluated in CasaXPS software. Photocurrent measurements were performed using a photoelectric spectrometer (Instytut Fotonowy, Krakow, Poland) and a three-electrode configuration, with Ag/AgCl and platinum wire as the reference and counter electrode, respectively. A thin layer of the material (the working electrode) was deposited at the surface of an ITO-coated transparent PET foil (60 Ω/sq resistance, Sigma-Aldrich). The sample (20mg) was finely ground in the agate mortar with a 150mL of ethanol. A thin layer was made out of the suspension using Elcometer Micrometric film applicator (Elcometer 3570/1, UK). The deposited uniform film was then dried at 80°C in a dryer. The electrolyte (0.1mol/L KNO3, pH = 6.1) was purged with argon for 15min prior to and during the measurement. Photocurrents were recorded by irradiating the working electrode from the backside with a xenon lamp in the range of 250 − 450nm with 10nm step, applying voltages in the range between − 0.2 and 1.0V (vs Ag/AgCl). The size of the working electrode is determined by the diameter of the window (1.0cm), so the area of the irradiated surface is A = 1/4π cm2 = 0.785 cm2. Photocatalytic activity measurements. The photocatalytic activity of all prepared samples was evaluated by degradation of acid orange 7 (AO7) dye. Photodegradation of AO7 (sodium 4-[(2E)-2-(2-oxonaphthalen-1ylidene)hydrazinyl]benzenesulfonate) at concentration 7.14∙10−4mol/L was carried out under UV (368nm) as well as VIS (420nm) light irradiation. In a typical experiment, to obtain a homogenously dispersed suspension, 50mg of catalyst was dispersed in a glass beaker in deionized water and magnetically stirred at 300rpm at room temperature. Then, AO7 was added into the as-prepared suspension. To achieve the adsorption and desorption equilibrium of AO7 dye on the catalyst, all mixtures were magnetically stirred at 300rpm, at room temperature in the dark for 60min. Then, the UV lamp (wavelength 368nm) or the VIS lamp (wavelength 420nm) was turned on for UVor VIS-light irradiation, respectively. All measurement processes were conducted at room temperature with constant stirring (300rpm). During the selected time of the experiment (under dark or under UV/VIS light), aliquots of 2ml of the homogenously dispersed suspension were taken using the syringe. The suspensions obtained in each time interval were filtered using a 0.20µm pore size syringe filter (CHROMAFIL GF/RC-20/25 filters, Macherey–Nagel, Germany) to separate the photocatalytic material. The acid orange 7 degradation was controlled by observing the decrease of absorbance by using a spectrometer Helios Epsilon (Thermo Spectronic, USA) at 485nm in a 1cm quartz glass microcuvette against distilled water. The percentage of photocatalytic degradation of AO7 (Asample) for all studies samples was calculated according to the equation: Asample = (1 − (Ceq. AO7/C0 AO7))·100%, where Ceq. AO7 is the equilibrium state of the concentration of AO7, and C0 AO7 is the initial concentration of AO7 after the dark period. To confirm which of the reactive species are responsible for the AO7 photodegradation, the photodegradation tests were further conducted in the presence of ethylenediaminetetraacetic acid (EDTA), 1,4-benzoquinone (1,4BQ) and t-butanol (t-Bu) as the scavengers of the holes (h+), the superoxide radicals (·O2−) and hydroxyl radicals (·OH). The studies were performed under UV and VIS light irradiation for pristine and 2.5 wt% Ag catalysts. Results and discussion Elemental analysis. The elemental analysis of the C, N, and H contents was performed for as prepared g-CN-MM and after its treatment in ethanol—sample MM-g-CN. The results are presented in Table1. Theoretical C/N atomic ratio in melem is 0.60 considering 10 atoms of nitrogen and 6 atoms of carbon in the melem unit (there are also 6 atoms of hydrogen). The C/N molar ratio in g-C3N4 is 0.75 and that ratio in melam is 0.54 considering 6 carbon atoms and 11 nitrogen atoms per melam unit (there are also 9 hydrogen atoms in melam unit). The obtained C/N molar ratios for g-CN-MM and MM-g-CN are close to those ratios for the ideal melem unit which supports the results of XRPD indicating the melem as the major component of both samples. Also, the molar content of hydrogen in melem unit—27.2mol% is closely similar to the contents in g-CN-MM and MM-g-CN. The oxygen is a typical contaminant of graphitic synthesized carbon nitride45 and melem46. It should be mentioned here that our results for elemental analysis are similar to those published by Liu etal.47. In Liu etal. work the metal-free melem/g-C3N4 hybrid photocatalyst was prepared by hydrothermal method from previously synthesized g-C3N4 material treated at 200°C for 12h in water solution and finally dried at 80°C in air47. It should be pointed out here that, contrary to the Liu etal. work47, our g-CN-MM material was synthesized only in one simple process using thermal treatment of melamine in air atmosphere at 550°C. To show the difference between the melamine (2,4,6-triamino-1,3,5-triazine, C3H6N6) precursor and samples g-CN-MM or MM-g-CN composed of melem (2,5,8-triamino-heptazine, C6H6N10) results from the XRD Table 1. Elemental analysis of C, N, H contents of g-CN-MM and MM-g-CN samples. *The amount of oxygen was calculated to 100%. Material Carbon (wt%/mol%) Nitrogen (wt%/mol%) Hydrogen (wt%/mol%) Oxygen* (wt%/mol%) C:N N:C g-CN-MM 32.5/27.1 61.7/44.0 2.70/27.5 3.10/0.019 0.53/0.61 1.90/1.63 MM-g-CN 32.3/26.7 61.8/43.8 2.78/28.1 3.12/0.019 0.52/0.61 1.91/1.64 5 Vol.:(0123456789) Scientific Reports | (2023) 13:5270 | https://doi.org/10.1038/s41598-023-32094-6 www.nature.com/scientificreports/ (Fig.S1A,B), Raman and FTIR (Figs.S2A,B, S3A,B), EDS with SEM (Figs.S7A,B, S8A–G), PL spectroscopy (Fig.S4) and elemental analysis (Table1) are shown in Supplementary materials. X‑ray diffraction analysis. To observe changes in phase and structure of studied samples, the X-ray diffraction analysis was employed. The samples were studied in as received powdered state and ambient conditions placed in a glass holder using reflection mode. The XRD analysis shows that all silver-modified TiO2 powders (Fig.2A) are single-phase materials and were evaluated as tetragonal anatase phase (ICDD PDF: 21-1272) space group: I41/amd. Therefore, silver decorates the surface of pristine anatase TiO2 and does not lead to any phase changes of TiO2 (Fig.2A). The phase of silver is not detectable in any of the TiO2-A-n% Ag samples as a visible peak due to the low percentage of Ag in the samples and the low crystalline state compared to the major anatase phase. Observing the most intensive peaks of TiO2 (101)—details of Fig.2A—we can see slight changes of peak position and intensity, however, the intensity changes are about 15% and the position of the peak maxima shifts just of hundredths of °2θ. The XRD analysis for a series of MM-g-CN materials modified with silver nanoparticles is shown in Fig.2B. The melem phase and g-C3N4 phase were identified based on the work of Liu etal.48, where diffraction data were simulated using a theoretical molecular model of melem. The process of melem/g-C3N4 phase modification with silver is causing changes that visibly broaden the major peaks and the relative intensity of peaks is increasing. Set of non-basal diffractions in the range 20–30° 2θ is exposing several new peaks compared to the pristine MM-g-CN sample. The changes are more frequent with higher concentration of Ag. The silver phase (111) is possible to detect as very small peaks of cubic pure metal visible for all modified samples. Raman and FTIR spectroscopy. To gain more information about the structural features of all materials, Raman spectroscopy analysis was used. The Raman spectra of selected samples of TiO2 series (TiO2-A; TiO2-A-0.5% Ag; TiO2-A-2.5% Ag) are shown in Fig.3A. In the Raman spectra, the most bands belong to lattice vibration of TiO2 (634 cm−1, 511 cm−1, 392 cm−1, 192 cm−1, and 138 cm−1)9,49. Only the very weak band at 1037 cm−1 is due to a stretching vibration of nitrates as a residue after precipitation of Ag from silver nitrate. A similar effect was observed in other studies9,50. Further, to correlate the obtained results from Raman spectroscopy, IR studies were conducted and are shown in Fig.3B. Likewise, only bands of TiO2 (686 cm−1 and 528 cm−1) and a very weak band of nitrates at 1385 cm−1 are seen in Fig.3B. The band assignment of all the above-mentioned bands is given in Table2. Additionally, it is clearly visible that the tetragonal I41/amd structure is maintained when Ag (in the content: 0.5, and 2.5% Ag) is used to modify the surface of TiO2-A powders without qualitatively affecting the Raman as well as IR spectrum. Similar observations were noticed for the surface-modified MM-g-CN with Ag samples and their findings are depicted in Fig.4. The Raman spectra of selected samples of the MM-g-CN series (MM-g-CN; MM-g-CN-0.5% Ag; MM-g-CN-2.5% Ag) are shown in Fig.4A. The significant bands visible in the spectra could be assigned to melem. There are mainly bands of triazine ring (1151 cm−1, 978 cm−1, and 542 cm−1), amino group (466 cm−1 and 454 cm−1), and the lattice vibration of melem (148 cm−1, 104 cm−1, and 83 cm−1)45–47,49–53. The IR spectra of MM-g-CN sample series are shown in Fig.4B. The spectra contain characteristic bands of amino group (3484 cm−1, 3425 cm−1, 3307 cm−1, 3151 cm−1, and 1608 cm−1) and triazine ring (1469 cm−1, and 805 cm−1)47–54. Especially bands at 1608 cm−1 and 1469 cm−1 are Figure2. XRD patterns of: (A) TiO2 samples pure and modified with various amounts of silver. The most intensive peaks have noted lattice planes with Miller indices and detail of anatase (101) most intensive peak slight changes in position, and (B) melem (MM)/g-C3N4 pure and modified with 3 concentrations of Ag in composition, where the most intensive peaks of the evaluated phases are noted lattice planes with Miller indices. 6 Vol:.(1234567890) Scientific Reports | (2023) 13:5270 | https://doi.org/10.1038/s41598-023-32094-6 www.nature.com/scientificreports/ Figure3. Raman (A) and IR (B) spectra of pure TiO2-A and TiO2-A-0.5% Ag and TiO2A-2.5% Ag powders, where most intensive bands are noted. Table 2. The band assignment of spectra of TiO2-A-n% Ag sample series. Raman (cm−1) Assignment 1037 Symmetric stretching vibration of nitrates 634 Lattice vibration of TiO2; Eg (doubly degenerate vibration with inverse symmetry) 511 Lattice vibration of TiO2; A1g (non-degenerated vibration with inverse and mirror symmetry) 392 Lattice vibration of TiO2; B1g (non-degenerated vibration, asymmetric with inverse and mirroring) 192 Lattice vibration of TiO2; Eg (doubly degenerate vibration with inverse symmetry) 138 Lattice vibration of TiO2; Eg (doubly degenerate vibration with inverse symmetry) IR Assignment 1385 Asymmetric stretching vibration of nitrates 686 Stretching vibration of TiO2 528 Stretching vibration of TiO2 Figure4. Raman (A) and IR (B) spectra of pure MM-g-CN and MM-g-CN-0.5% Ag and MM-g-CN-2.5% Ag powders, where most intensive bands are noted. 7 Vol.:(0123456789) Scientific Reports | (2023) 13:5270 | https://doi.org/10.1038/s41598-023-32094-6 www.nature.com/scientificreports/ entirely characteristic for the melem spectrum47–54. The band assignment of all the above-mentioned bands is given in Table3. Both above-mentioned spectra are entirely typical for melem and melamine. The band assignment of both Raman and infrared spectra of melamine is given in TableS1. Diffuse Reflectance spectroscopy studies. Diffuse Reflectance spectroscopy (DRS) analysis was used to determine the effect of modification of two selected semiconductor materials with silver nanoparticles within the registered spectra in the UV–VIS region 220–800nm. That quick, cheap, and non-destructive technique allows getting the values of band gap energy and to observe the effect of an increasing concentration of Ag on the surface of pristine semiconductor materials. Later, the achieved results are correlated with photoluminescence studies (see chapter3.5). The DRS spectra and Tauc curves from which the Eg values were determined for the series of TiO2-A and MM-g-CN powders are shown in Figs.5(A,B), and 6(A,B). The indirect band gap energy values (Tauc spectra obtained from Kubelka–Munk function) evaluated from DRS spectra of TiO2-A and MM-g-CN, both with and without Ag are listed in Tables4 and 5. Table4 and 5 show that Eg values obtained for samples from the TiO2-A as well as MM-g-CN group are almost the same. Here it is worth noting that the content of Ag was not very high, so the effect of increasing Ag concentration on the surface of both materials, and thus the obtained differences of Eg values are negligible. The Eg values obtained for a series of TiO2-A and MM-g-CN samples are similar to the values presented by authors48,55,56. The optical properties of Ag-based TiO2-A-n% materials are shown in Fig.5A. A very sharp absorption edge for pure TiO2-A was observed at about 400nm (Eg around 3.10eV) as an intrinsic absorption of TiO255,56 and the band gap energy 3.25eV was then estimated from the Tauc plot (Fig.5B). For the silver modified TiO2-A samples, the reflectance increases with growing content of Ag which indicates, the samples absorb more light Table 3. The band assignment of spectra of both MM-g-CN-n% Ag sample series. Raman Assignment 1151 Stretching vibration of triazine ring 978 542 Deformation vibration of triazine ring (out of plane) Vibration of triazine ring 466 454 Combination bands of deformation (bending) vibration of -NH2 and deformation (rocking) vibration of -NH2 148 Lattice vibration of melem 104 Lattice vibration of melem 83 Lattice vibration of melem IR Assignment 3484 Asymmetric stretching vibration of -NH2 3425 Symmetric stretching vibration of -NH2 3307 Stretching vibration of -NH2 3151 Combination band of deformation vibration of -NH2 and stretching vibration of C-N 1608 Deformation vibration of -NH2 1469 Symmetric stretching (breathing) vibration of triazine ring 805 Deformation vibration of triazine ring (out of plane) Figure5. Diffuse reflectance UV–Vis spectra (A) and Tauc spectra (B) at wavelengths from 220 to 800nm of TiO2-A-n% Ag. 8 Vol:.(1234567890) Scientific Reports | (2023) 13:5270 | https://doi.org/10.1038/s41598-023-32094-6 www.nature.com/scientificreports/ in the Vis region. Silver-modified TiO2-A-n% Ag samples show a wider band in the DRS spectrum (Fig.5A) with a wide absorption centred at around 510nm (especially visible for TiO2-A-0.5% Ag), the presence of this phenomenon indicates the localized surface plasmon resonances (LSPR) effect caused by the presence of silver nanoparticles55. This effect is significant especially for the material TiO2-A-0.5% Ag. For the samples of MM-g-CN series, the DRS spectra shown in Fig.6A indicate that all materials have an absorption at about 440nm (2.81eV) (Table5), which corresponds well with the band gap energy values of this type of material presented in the literature36,48. Silver-modified MM-g-CN samples show a significant enhancement of absorption in the visible light regions from 400 to 800nm, which is attributed to the SPR (Surface Plasmon Resonance) effect of Ag nanoparticles56. Photoluminescence studies. Photoluminescence (PL) spectra of TiO2-A and TiO2-A-n% Ag, and MM-g-CN and MM-g-CN-n% Ag samples are shown in Fig.7. TiO2-A sample showed the highest intensity of the maximum of PL peak, but with increasing Ag content the intensity of this peak decreased (Fig.7A). The maximum of PL intensity is blue-shifted up to 1 wt% of Ag in comparison to TiO2-A, while the further increase of Ag to 2.5wt% caused the red shift in comparison to the sample with 1 wt% of Ag, but still blue-shifted in comparison to sample TiO2-A, see Table6. The intensity of the PL signal of pure MM-g-CN is very strong, and when the Ag content in the samples increases, the PL intensity subsequently decreases (Fig.7B), while the PL maxima are weakly red-shifted (Table7) from 435 to 438nm. The results obtained from photoluminescence measurement for MM-g-CN samples are aligned with the literature34,35,48. In the Supplementary Material, the deconvolution of the PL spectrum for sample MM-g-CN is presented (Fig.S4). Figure6. Diffuse reflectance UV–Vis spectra (A) and Tauc spectra (B) at wavelengths from 220 to 800nm of pure MM-g-CN and MM-g-CN-n% Ag. Table 4. Indirect band gap energy values (Kubelka–Munk function, Tauc spectra) from DRS spectra of TiO2A-n% Ag. Materials Eg (eV) TiO2-A 3.25 TiO2-A-0.5% Ag 3.24 TiO2-A-1% Ag 3.25 TiO2-A-2.5% Ag 3.25 Table 5. Indirect band gap energy values (Kubelka–Munk function, Tauc spectra) from DRS spectra of MM-g-CN-n% Ag. Materials Eg (eV) MM-g-CN 2.81 MM-g-CN-0.5% Ag 2.80 MM-g-CN-1% Ag 2.81 MM-g-CN-2.5% Ag 2.81 9 Vol.:(0123456789) Scientific Reports | (2023) 13:5270 | https://doi.org/10.1038/s41598-023-32094-6 www.nature.com/scientificreports/ The deconvolution of the PL spectra of MM-g-CN to the three individual components shown in Fig.S4 was performed using the Gaussian function in the OriginPro 8 software. It can be seen that the MM-g-CN sample has a weak peak centred at 396nm, which corresponds to melam, and a peak centred at 435nm, which corresponds to melem.The band at 452nm corresponds to g-C3N4. The obtained results perfectly matched to the work published by Liu etal.48. The authors showed, that melem and g-C3N4 with maxima centred around 435 and 452nm are the main contributors to the registered PL band, and the presence of melam is reflected by the presence of weak PL peak with the maxima centred at 396 nm48. It should be pointed out here, that the materials studied by Liu etal.48 were synthesized under different conditions in comparison to our samples as already mentioned. In general, the high intensity of the PL spectrum of g-C3N4 reflects fast electron–hole recombination, usually resulting in lower photoactivity44. Defects in the g-C3N4 structure cause rapid recombination of the electrons with photoexcited holes37. The information derived from PL spectra supports also the experiments with photodegradation of AO7 as further shown and discussed in subchapter3.9. For photogenerated electrons in g-C3N4 bulk material, the conduction band (CB) has a much greater power for reduction reactions compared to TiO257. The Ag nanoparticles found on the surface of the g-C3N4 bulk material could abate the charge recombination acting as an electron sink, efficiently trapping photogenerated electrons from the conduction band of g-C3N4 (over the band gap energies of 2.83eV for the MM-g-CN-2.5% Ag material can be explained by the recombination process of electrons and holes)22. PL decay study. PL decay curves were recorded for us to understand photocatalytic processes in the studied materials. The lifetimes of photoinduced electrons and holes were calculated by fitting the PL decay curves with selected exponential relationships and by calculation of mean decay times according to Eqs. (1) and (2), respectively. The normalized PL excitation and emission spectra (Figs.S5, S6) of the materials showed broad excitation bands around 300nm and emission bands around 440nm and, therefore, a laser operated at the wavelength of 372nm could be used for the excitation of the studied samples except TiO2-A-2.5% Ag. The best fits of the normalized PL decay curves displayed in Fig.8 were obtained for the 3rd order kinetic model as Figure7. PL spectra of: (A) TiO2-A-n% Ag and (B) MM-g-CN-n% Ag powders. Table 6. Maximum emission bands of pristine TiO2-A and modified with Ag NPs. Materials Maximum emission bands (nm) TiO2-A 437 TiO2-A-0.5% Ag 432 TiO2-A-1% Ag 428 TiO2-A-2.5% Ag 433 Table 7. Maximum emission bands of pristine MM-g-CN and modified with Ag NPs. Materials Maximum emission bands (nm) MM-g-CN 434 MM-g-CN-0.5% Ag 435 MM-g-CN-1% Ag 436 MM-g-CN-2.5% Ag 438 16 Vol:.(1234567890) Scientific Reports | (2023) 13:5270 | https://doi.org/10.1038/s41598-023-32094-6 www.nature.com/scientificreports/ similar values of kinetic constants in the range 18–19 × 10−3 min−1 showed MM-g-CN samples surface modified with 0.5 and 1 wt% Ag. The contribution of holes, superoxide, and hydroxyl radicals was assessed by photodegradation experiments in the presence of their scavengers. In the case of the non-modified TiO2-A samples, the photodegradation activity was the most suppressed by the presence of the hole’s scavenger (Fig.15A), and therefore it is assumed that the holes are the main species that participate in the photodegradation process of AO7 over non-modified TiO2-A (see Fig.15A). It has to be mentioned that the extent of the AO7 photodegradation over non-modified TiO2 was also suppressed in the case of the scavengers of superoxide and hydroxyl radicals but the photoactivity decrease was not so pronounced as in the case of hole scavenger. The effect of the masking of the reactive species on the photodegradation activity in the case of the TiO2-A-2.5% Ag sample is also evidenced in Fig.15A. Masking of holes and superoxide radicals caused the decrease in the photodegradation activity of TiO2-A-2.5% Ag (Fig.15A). In the case of this sample, the superoxide radicals show the dominant role on the photodegradation activity, on the other hand, the hydroxyl radicals had no effect on the photodegradation activity of silver modified TiO2-A, as evidenced in Fig.15A. The same comparison of the effect of scavengers on the photodegradation of AO7 was performed for samples MM-g-CN and MM-g-CN-2.5% Ag (Fig.15B). In general, the non-modified sample MM-g-CN shows a lower photodegradation activity compared to silver-modified MM-g-CN samples (compare Table 11. Overview of materials used for photodegradation of AO7 or MO or AO. MO, Methyl Orange; AO, Orange P3R Reactive-Orange-12; AO7, Acid Orange 7, *Mixture anatase + rutile. Material (surface modified Ag NPs on TiO2 or g-C3N4)Photodegraded dye Condition of irradiation Photocatalytic efficiency Ref. Ag/TiO2anatase MO, AO 30min and 1h UV 300nm 100% after 30min and 1h 64 *TiO2-0.1% Ag *TiO2-0.1–0.3% Ag AO7 1h UV 368nm 1h UV 360nm 40% 43% 13 0.5% Ag/TiO2-rutile MO 1h VIS, 400 W lamp 10% 65 TiO2-Anatase-0.5% Ag TiO2-Anatase-2.5% Ag AO7 1h UV 368nm 1h VIS 420nm 78% 18% This work Ag/g-C3N4 (1:2) MO 1h VIS 420nm 10% 36 Ag/g-CN (4h of melamine annealing at 550°C) MO 1h VIS 420nm 38% 66 Ag/g-C3N4MO 1h VIS 420nm 90% 67 1.0%Ag/g-C3N4MO 1h VIS 420nm 50% 68 MM-g-CN-0.5% Ag MM-g-CN-0.5% Ag AO7 1h UV 368nm 1h VIS 420nm 71% 62% This work Figure15. Effect of the presence of the scavengers on the photodegradation activity of: (A) TiO2-A and TiO2-A-2.5% Ag samples under UV irradiation and (B) MM-g-CN and MM-g-CN-2.5% Ag samples under VIS irradiation. 17 Vol.:(0123456789) Scientific Reports | (2023) 13:5270 | https://doi.org/10.1038/s41598-023-32094-6 www.nature.com/scientificreports/ Fig.14B,D). For both, non-modified and silver-modified samples the superoxide radicals play the major role in AO7 photodegradation. To investigate the stability and reusability of the best selected photocatalysts containing 0.5 wt% Ag, recycling tests of the photodegradation were performed. The TiO2-A-0.5% Ag sample was analyzed after 3h under the UV (368nm) light, and MM-g-CN-0.5% Ag sample was analyzed after 3h under the VIS (420nm) light. The achieved results are presented in Fig.S10. After four cycles, the photocatalytic activity of both materials remained almost at the same level (Fig.S10), what indicates good stability of their photodegradation performance. The photodegradation mechanism for the given sample was derived from the photodegradation experiments in the presence of scavengers (Fig.15) and is presented graphically in Fig.16. For the pristine TiO2 NPs, all three reactive species (h+, ·O2−, ·OH) generated during the UV irradiation participated in the photodegradation of AO7 in the order: h+ > ·O2− > ·OH (Fig.15A). The scheme of the photodegradation process on a TiO2-A sample is provided in Fig.16A, in which the holes (h+) as the most active species are marked by the purple double circle. ∙OH radicals are marked as the dashed purple circle in Fig.15A. In the case of TiO2-Ag NPs, only ·O2− and h+ participated in the degradation of AO7 as evidenced in Fig.15A. The scheme of UV induced photodegradation process for TiO2-Ag NPs is schematically addressed in Fig.16B. Based on the results of the photodegradation experiments with scavengers (Fig.15B), the photodegradation processes were proposed for pristine and silver modified MM-g-CN samples and are schematically pictured in Fig.16C,D. In the case of both MM-g-CN and MM-g-CN-Ag, the superoxide radicals (marked by the single and double orange circles in Fig.16D) are the only species participating in the photodegradation of AO7. Due to the lower Fermi level of Ag compared to the energy of CB of the melem/g-C3N4 composite, the photoexcited electrons in CB of g-C3N4 are transferred to Ag nanoparticles and interact with O2 to form the ·O2− radicals. Similar behaviour was observed in Wang etal. work, where the AgI/BiSbO4 composite was utilized as a photocatalyst for degradation process of organic pollutants under visible light41. The energy necessary for the formation of ∙OH is more positive than VB of g-C3N4, and thus the holes in VB of g-C3N4 cannot react with OHto form the ∙OH radicals (marked as a dotted orange circle and arrow in Fig.16C,D). This fact is in good agreement with the results presented in Fig.15B, which showed that the shielding of ∙OH radicals by t-Bu did not affect the photodegradation activity of the samples in the MM-g-CN group. The same observation was also found by other authors35,70. Ag NPs act as an electron acceptor and inhibit the charge recombination, thus facilitating charge transfer and leading to improved photodegradation activity of the silver modified samples. This statement is in good agreement with the results presented in Fig.15B (effect of scavengers on photodegradation activity). More ·O2− radicals are generated as the Ag content in the composite increases, enhancing the photocatalytic activities of the materials in UV light (see TableS3 in the Supplementary material). The proposed schemes for MM-g-CN and MM-g-CN-Ag are aligned with previously published works71–73. Conclusions The surface of TiO2 and melem/g-C3N4 materials was successfully modified with silver nanoparticles via wet, simple chemical, and low temperature method. TEM findings showed approximately 5nm sized Ag NPs balls homogenously distributed on both modified semiconductor materials. The highest photocatalytic activity was achieved for both materials modified with a very small silver content—0.5 wt%. The photodegradation activities in excess of 95 and 94% after 3h long irradiation with UV lamp were observed for both TiO2 and e-CB e-CB h+VB h+VB TiO2 ∙O2O 2 photodegradaon of AO7 H2O/OH- ∙OH h+VB A ) B h B e-CB e-CB e-CB h+VB h+VB e -CB e -CB TiO 2 Ag ∙O2O2 photodegradaon of AO7 H2O/OH- ∙OH h+VB UV B ) e-CB h + VB e-CB e-CB h+VB h+VB VIS melem/g-C 3 N 4 O2 ∙O2photodegradaon of AO7 H2O/OH- ∙OH h+VB C ) e-CB e-CB e-CB h+VB h+VB VIS e -CB e -CB melem/ g-C 3 N 4 -Ag Ag O2 ∙O2-photodegradaon of AO7 H2O/OH- ∙OH h+VB D ) e-CB Figure16. UV light induced photodegradation mechanism on (A) TiO2 NPs and (B) TiO2-Ag NPs under; VIS light induced photodegradation mechanism on (C) melem/g-C3N4 composites and (D) melem/g-C3N4-Ag composites. 18 Vol:.(1234567890) Scientific Reports | (2023) 13:5270 | https://doi.org/10.1038/s41598-023-32094-6 www.nature.com/scientificreports/ MM-g-CN samples with 0.5 wt% of Ag. Furthermore, the MM-g-CN sample modified with 0.5 and 1 wt% of Ag revealed the highest value of 98% after 3h long irradiation time with VIS light. It should be also pointed here, that every modification with Ag NPs had a positive effect on both compared semiconductors under UV or VIS light processes. These results allow us to expect that the proposed low temperature chemical method which does not include the reducing agent is suitable for the surface modification of both semiconductors utilized as a photocatalyst for the degradation of model acid orange 7 dye. These investigations are in very good agreement with our previous works on surface modification with Ag NPs of anatase (TiO2), lithium titaniumoxide (Li4Ti5O12) as well as activated carbon materials which found an application in organic photovoltaics9, lithium-ion batteries50, and supercapacitors74, respectively. Data availability The data are available on the link: https:// doi. org/ 10. 6084/ m9. figsh are. 21803 877. Received: 2 January 2023; Accepted: 22 March 2023 References 1. Che, X. & Mao, S. S. Titanium dioxide nanomaterials: Synthesis, properties, modifications, and applications. Chem. Rev. 107, 2891–2959 (2007). 2. Devina, W. et al. Carbon-coated, hierarchically mesoporous TiO2 microparticles as an anode material for lithium and sodium ion batteries. Electrochim. Acta 321, 134639 (2019). 3. Su, X. et al. Advanced titania nanostructures and composites for lithium ion battery. J. Mater. Sci. 47, 2519–2534 (2012). 4. Wen, W. et al. Anatase TiO2 ultrathin nanobelts derived from room-temperature-synthesized titanates for fast and safe lithium storage. Sci. Rep. 5, 11804 (2015). 5. Zhang, X. et al. Porous cake-like TiO2 derived from metal-organic frameworks as superior anode material for sodium ion batteries. Ceram. Int. 45, 2398–2402 (2017). 6. Yang, F. et al. TiO2/carbon hollow spheres as anode materials for advanced sodium ion batteries. Electrochim. Acta 178, 871–876 (2015). 7. Kim, J. H., Choi, H. J., Kim, H.-K., Lee, S.-H. & Lee, Y.-H. A hybrid supercapacitor fabricated with an activated carbon as cathode and an urchin-like TiO2 as anode. Int. J. Hydrog. Energy 41, 13549–13556 (2016). 8. Chen, S. et al. Growth of polyaniline on TiO2 tetragonal prism arrays as electrode materials for supercapacitor. Electrochim. Acta 300, 373–379 (2019). 9. Michalska, M. et al. Analysis of the surface decoration of TiO2 grains using silver nanoparticles obtained by ultrasonochemical synthesis towards organic photovoltaics. New J. Chem. 42, 7340–7354 (2018). 10. Wu, Y. C. et al. Clean and flexible synthesis of TiO2 nanocrystallites for dye-sensitized and perovskite solar cells. Sol. Energy Mater. Sol. Cells 159, 336–344 (2017). 11. Guo, Q., Zhou, C., Ma, Z. & Yang, X. Fundamentals of TiO2 photocatalysis: Concepts, mechanisms, and challenges. Adv. Mater. 31, 1–26 (2019). 12. Schneide, J. et al. Understanding TiO2 photocatalysis: Mechanisms and materials. Chem. Rev. 114, 9919–9986 (2014). 13. Michalska, M. et al. The effect of surface modification with Ag nanoparticles on 21 nm TiO2: Anatase/rutile material for application in photocatalysis. Mater. Today Chem. 26, 101123 (2022). 14. Zhang, Y., Cui, W., Yao, W. & Duan, T. One-step hydrothermal synthesis of iron and nitrogen co-doped TiO2 nanotubes with enhanced visible-light photocatalytic activity. CrystEngComm 17, 8368–8376 (2015). 15. Wang, D. H., Jia, L., Wu, X. L., Lu, L. Q. & Xu, A. W. One-step hydrothermal synthesis of N-doped TiO2/C nanocomposites with high visible light photocatalytic activity. Nanoscale 4, 576–584 (2012). 16. Huang, F., Yan, A. & Zhao, H. Influences of doping on photocatalytic properties of TiO2 photocatalyst. Semicond. Photocatal.: Mater. Mech. Appl. 31–80 (2016). 17. Elsellami, L., Dappozze, F., Houas, A. & Guillard, C. Effect of Ag+ reduction on the photocatalytic activity of Ag-doped TiO2. Superlattices Microstruct. 109, 511–518 (2017). 18. Bai, Y. et al. Stability of Pt nanoparticles and enhanced photocatalytic performance in mesoporous Pt-(anatase/TiO2(B)) nanoarchitecture. J. Mater. Chem. 19, 7055–7061 (2009). 19. Yu, Y., Wen, W., Qian, X.-Y., Liu, J.-B. & Wu, J.-M. UV and visible light photocatalytic activity of Au/TiO2 nanoforests with Anatase/ Rutile phase junctions and controlled Au locations. Sci. Rep. 7, 41253 (2017). 20. Zhang, Z. et al. Facile one-step synthesis of TiO2/Ag/SnO2 ternary heterostructures with enhanced visible light photocatalytic activity. Sci. Rep. 8, 10532 (2018). 21. Reddy, K. R. et al. Polymeric graphitic carbon nitride (g-C3N4)-based semiconducting nanostructured materials: Synthesis methods, properties and photocatalytic applications. J. Environ. Manage. 238, 25–40 (2019). 22. Wen, J., Xie, J., Chen, X. & Li, X. A review on g-C3N4-based photocatalysts. Appl. Surf. Sci. 391, 72–123 (2017). 23. Matejka, V. et al. The role of the g-C3N4 precursor on the P doping using HCCP as a source of phosphorus. J. Mater. Res. Technol. 18, 3319–3335 (2022). 24. Hong, Y. et al. A direct one-step synthesis of ultrathin g-C3N4 nanosheets from thiourea for boosting solar photocatalytic H2 evolution. Int. J. Hydrog. Energy 44, 7194–7204 (2019). 25. Shiraishi, Y. et al. Highly selective production of hydrogen peroxide on graphitic carbon nitride (g-C3N4) photocatalyst activated by visible light. ACS Catal. 4, 774–780 (2014). 26. Fang, J. W., Fan, H. Q., Li, M. M. & Long, C. B. Nitrogen self-doped graphitic carbon nitride as efficient visible light photocatalyst for hydrogen evolution. J. Mater. Chem. A 3, 13819–13826 (2015). 27. Skuta, R. et al. On P-doping of graphitic carbon nitride with hexachlorotriphosphazene as a source of phosphorus. Appl. Surf. Sci. 552, 149490 (2021). 28. Jürgens, B. et al. Melem (2,5,8-triamino-tri-s-triazine), an important intermediate during condensation of melamine rings to graphitic carbon nitride: synthesis, structure determination by X-ray powder diffractometry, solid-state NMR, and theoretical studies. J. Am. Chem. Soc. 125, 10288–10300 (2003). 29. Dong, J., Zhang, Y., Hussain, M. I., Chen, Y. & Wang, L.-N. g-C3N4: Properties, pore modifications, and photocatalytic applications. Nanomaterials 12, 121 (2022). 30. Dong, G., Zhang, Y., Pan, Q. & Qiu, J. A fantastic graphitic carbon nitride (g-C3N4) material: Electronic, structure, photocatalytic and photoelectronic properties. J. Photochem. Photobiol. C 20, 33–50 (2014). 31. Alcudia-Ramos, M. A. et al. Fabrication of g-C3N4/TiO2 heterojunction composite for enhanced photocatalytic hydrogen production. Ceram. Int. 46, 38–45 (2020). 19 Vol.:(0123456789) Scientific Reports | (2023) 13:5270 | https://doi.org/10.1038/s41598-023-32094-6 www.nature.com/scientificreports/ 32. Srinivasu, K., Modak, B. & Ghosh, S. K. Porous graphitic carbon nitride: a possible metal-free photocatalyst for water splitting. J. Phys. Chem. C 118, 26479–26484 (2014). 33. Jiang, L. et al. Doping of graphitic carbon nitride for photocatalysis: A review. Appl. Catal. B 217, 388–406 (2017). 34. Jo, W.-K. & Yoo, H.-J. Combination of ultrasound-treated 2D g-C3N4 with Ag/black TiO2 nanostructure for improved photocatalysis. Ultrason. Sonochem. 42, 517–525 (2018). 35. Zhang, W., Zhou, L. & Deng, H. Ag modified g-C3N4 composites with enhanced visible-light photocatalytic activity for diclofenac degradation. J. Mol. Catal. A Chem. 423, 270–276 (2016). 36. Chen, Y., Huang, W., He, D., Situ, Y. & Huang, H. Construction of heterostructured g-C3N4/Ag/TiO2 microspheres with enhanced photocatalysis performance under visible-light irradiation. ACS Appl. Mater. Interfaces 6, 14405–14414 (2014). 37. Sobahi, T. R. & Amin, M. S. Upgrading the photocatalytic achievement of g-C3N4 nanosheets along decoration with Ag@TiO2 nanospheres for the preparation of vitamin B3. Appl. Nanosci. 9, 1621–1636 (2019). 38. Zhou, L. et al. Recent advances in non-metal modification of graphitic carbon nitride for photocatalysis: A historic review. Catal. Sci. Technol. 6, 7002–7023 (2016). 39. Wei, X. et al. Facile insitu synthesis of plasmonic nanoparticles-decorated g-C3N4/TiO2 heterojunction nanofibers and comparison study of their photosynergistic effects for efficient photocatalytic H2 evolution. Nanoscale 8, 11034–11043 (2016). 40. Thambiliyagodage, C. Activity enhanced TiO2 nanomaterials for photodegradation of dyes: A review. Environ. Nanotechnol. Monit. Manag. 16, 100592 (2021). 41. Wang, Z., Jiang, L., Wang, K., Li, Y. & Zhang, G. Novel AgI/BiSbO4 heterojunction for efficient photocatalytic degradation of organic pollutants under visible light: Interfacial electron transfer pathway, DFT calculation and degradation mechanism study. J. Hazard. Mater. 410, 124948 (2021). 42. Ma, S., Sihui, Z., Yanan, J., Qiang, S. & Qixing, Z. Enhanced disinfection application of Ag-modified g-C3N4 composite under visible light. Appl. Catal. B: Environ. 186, 77–87 (2016). 43. Sirivallop, A., Areerob, T. & Chiarakorn, S. Enhanced visible light photocatalytic activity of N and Ag doped and co-doped TiO2 synthesized by using an in-situ solvothermal method for gas phase ammonia removal. Catalysts 251, 10 (2020). 44. Yan, D. et al. Construction of g-C3N4/TiO2/Ag composites with enhanced visible-light photocatalytic activity and antibacterial properties. Ceram. Int. 46, 696–702 (2020). 45. Praus, P. et al. The presence and effect of oxygen in graphitic carbon nitride synthetized in air and nitrogen atmosphere. Appl. Surf. Sci. 529, 147086 (2020). 46. Abdelhafeez, I. A. et al. Green synthesis of ultrathin edge-activated foam-like carbon nitride nanosheets for enhanced photocatalytic performance under visible light irradiation. Sustain. Energy Fuels 3, 1764–1775 (2019). 47. Liu, S. et al. Metal-free melem/g-C3N4 hybrid photocatalysts for water treatment. J. Colloid Interface Sci. 464, 10–17 (2016). 48. Liu, N. et al. From triazine to heptazine: Origin of graphitic carbon nitride as a photocatalyst. ACS Omega 5, 12557–12567 (2020). 49. Ohsaka, T., Izumi, F. & Fujiki, Y. Raman spectrum of anatase, TiO2. J. Raman Spectrosc. 7, 321–324 (1978). 50. Krajewski, M. et al. Li4Ti5O12 modified with Ag nanoparticles as an advanced anode material in lithium-ion batteries. J. Power Sources 245, 764–771 (2014). 51. Komatsu, T. The first synthesis and characterization of cyameluric high polymers. Macromol. Chem. Phys. 202, 19–25 (2001). 52. Marchewka, M. K. Infrared and Raman spectra of melaminium chloride hemihydrate. Mater. Sci. Eng. B 95, 214–221 (2002). 53. Chu, S., Wang, C., Feng, J., Wang, Y. & Zou, Z. Melem: A metal-free unit for photocatalytic hydrogen evolution. Int. J. Hydrog. Energy 39, 13519–13526 (2014). 54. Zheng, H. B. et al. Melem: An efficient metal-free luminescence material. J. Mater. Chem. C 5, 10746–10753 (2017). 55. Murdoch, M. et al. The effect of gold loading and particle size on photocatalytic hydrogen production from ethanol over Au/TiO2 nanoparticles. Nat. Chem. 3, 489–492 (2011). 56. Chen, Y. et al. Meiting, Enhancement of photocatalytic performance with the use of noble-metal-decorated TiO2 nanocrystals as highly active catalysts for aerobic oxidation under visible-light irradiation. Appl. Catal. B: Environ. 210, 352–367 (2017). 57. Low, J., Cheng, B., Yu, J. & Jaroniec, M. Carbon-based two-dimensional layered materials for photocatalytic CO2 reduction to solar fuels. Energy Storage Mater. 3, 24–35 (2016). 58. Zada, A. et al. Surface plasmonic-assisted photocatalysis and optoelectronic devices with noble metal nanocrystals: design, synthesis, and applications. Adv. Funct. Mater. 30, 1906744 (2020). 59. Aizpurua, J. et al. Theory of hot electrons: General discussion. Faraday Discuss. 214, 245–281 (2019). 60. Bai, X. R. et al. Enhancement of visible photocatalytic activity via Ag@C3N4 core–shell plasmonic composite. Appl. Catal. B: Environ. 147, 82–91 (2014). 61. Biesinger, M. C., Lau, L. W. M., Gerson, A. R. & Smart, R. S. C. Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Sc, Ti, V. Cu and Zn. Appl. Surf. Sci. 257, 887–898 (2010). 62. Ferraria, A. M. & Carapeto, A. P. X-ray photoelectron spectroscopy: Silver salts revisited. Vacuum 86, 1988–1991 (2012). 63. Kim, M., Hwang, S. & Yu, J.-S. Novel ordered nanoporous graphitic C3N4 as a support for Pt–Ru anode catalyst in direct methanolfuel cell. J. Mater. Chem. 17, 1656–1659 (2007). 64. Hussain, M. et al. Ag–TiO2 nanocomposite for environmental and sensing applications. Mat. Chem. Phys. 181, 194–203 (2016). 65. Ansari, S. A. et al. Silver nanoparticles and defect-induced visible light photocatalytic and photoelectrochemical performance of Ag@m-TiO2 nanocomposite. Sol. Energy Mater. Sol. Cells 141, 162–170 (2015). 66. Liu, R. et al. Ag-modified g-C3N4 prepared by a one-step calcination method for enhanced catalytic efficiency and Stability. ACS Omega 5, 19615–19624 (2020). 67. Yan, S. et al. Photodegradation performance of g-C3N4 Fabricated by directly heating melamine. Langmuir 25, 10397–10401 (2009). 68. Ge, L. et al. Enhanced visible light photocatalytic activity of novel polymeric g-C3N4 loaded with Ag nanoparticles. Appl. Catal. A: General 409–410, 215–222 (2011). 69. Juang, R.-S., Lin, S.-H. & Hsueh, P.-Y. Removal of binary azo dyes from water by UV-irradiated degradation in TiO2 suspensions. J. Hazard. Mater. 182, 820–826 (2010). 70. Chakhtouna, H. et al. Recent progress on Ag/TiO2 photocatalysts: Photocatalytic and bactericidal behaviours. Environ. Sci. Pollut. Res. 28, 44638–54466 (2021). 71. Yang, Y. et al. Preparation and enhanced visible-light photocatalytic activity of silver deposited graphitic carbon nitride plasmonic photocatalyst. Appl. Catal. B 142–143, 828–837 (2013). 72. Bu, Y., Chen, Z. & Li, W. Using electrochemical methods to study the promotion mechanism of the photoelectric conversion performance of Ag-modified mesoporous g-C3N4 heterojunction material. Appl. Catal. B 144, 622–630 (2014). 73. Lan, H. et al. Melem-based derivatives as metal-free photocatalysts for simultaneous reduction of Cr(VI) and degradation of 5-Sulfosalicylic acid. J. Colloid Interface Sci. 507, 162–171 (2017). 74. Jain, A., Michalska, M., Zaszczyńska, A. & Denis, P. Surface modification of activated carbon with silver nanoparticles for electrochemical double layer capacitors. J. Energy Storage 54, 105367 (2022). Acknowledgements This work was financially supported by the Ministry of Education, Youth and Sports, Czech Republic (contract no. 8F21007) through the research project cooperation between the AtomDeC Consortium (V4-Japan/JRP/2021/96/ 20 Vol:.(1234567890) Scientific Reports | (2023) 13:5270 | https://doi.org/10.1038/s41598-023-32094-6 www.nature.com/scientificreports/ AtomDeC) by funding received from the Visegrad group(V4)-Japan 2021 2nd Joint Call on “Advanced Materials”. This work was supported by the VŠB-TU Ostrava (project No. SP2023/034). This article was prepared within the project „Increase of Capacity and Quality of INEF Center“, identification code CZ.1.05/2.1.00/19.0407, RDI OP, with the financial support from European Regional Development Fund. The authors thank the Large Research Infrastructure ENREGAT (project No. LM2018098) and for the assistance provided by the Research Infrastructure NanoEnviCz, supported by the Ministry of Education, Youth and Sports of the Czech Republic under Project No. LM2018124. Author contributions M.M.: Conceptualization, methodology, formal analysis, investigation, writing—original draft, writing—review and editing, resources, visualization, funding acquisition. V.M.: Conceptualization, methodology, formal analysis, investigation, writing—original draft, writing—review and editing. J.P.: Methodology, formal analysis, investigation, writing—original draft, writing—review and editing. P.P.: Formal analysis, writing—original draft, writing—review and editing. M.R.: Formal analysis, investigation, writing—original draft. J.S.: Formal analysis, investigation, writing—original draft. L.G.: Formal analysis, investigation. M.K.: Formal analysis, investigation, writing—original draft. K.F.: Formal analysis, investigation. M.R.: Formal analysis, investigation, writing—original draft. G.S.M.: Formal analysis, Investigation, writing—original draft, writing—review and editing, visualization. Competing interests The authors declare no competing interests. Additional information Supplementary Information The online version contains supplementary material available at https:// doi. org/ 10. 1038/ s4159802332094-6. Correspondence and requests for materials should be addressed to M.M. Reprints and permissions information is available at www.nature.com/reprints. Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. 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