All materials and chemicals used in this study were of analytical grade and sourced from Sigma-Aldrich, Supelco, J.T. Baker, Daejung Chemicals, Junsei Chemical, Next Chimica, Danil Syschem, Sundo Group, and Daesung Industrial Gases. These included titanium dioxide (Aeroxide P25 TiO₂, ≥99.5%), chloroplatinic acid hydrate (H₂PtCl₆·xH₂O, ≥99.9%), sodium fluoride (NaF, ≥99.0%), perchloric acid (HClO₄, 60%), phosphoric acid (H₃PO₄, 85.0%), acetonitrile (C₂H₃N, ≥99.9%), sodium hydroxide (NaOH, ≥97.0%), dicrotophos (C₈H₁₆NO₅P, ≥99.5%), dichlorvos (C₄H₇Cl₂O₄P, ≥98.0%), trichlorfon (C₄H₈Cl₃O₄P, ≥98.0%), acephate (C₄H₁₀NO₃PS, ≥98.0%), ethoprophos (C₈H₁₉O₂PS₂, ≥95.0%), methacrifos (C₇H₁₃O₅PS, ≥95.0%), fenamiphos (C₁₃H₂₂NO₃PS, ≥98.0%), methanol (CH₃OH, ≥99.5%), argon gas (Ar, ≥99.999%), sodium phosphate (Na₃PO₄, ≥96.0%), silver nitrate (AgNO₃, ≥99.95%), copper(II) nitrate trihydrate (Cu(NO₃)₂·3H₂O, ≥99.0%), nickel(II) chloride (NiCl₂, ≥98.0%), palladium(II) chloride (PdCl₂, ≥99.0%), and chloroauric acid hydrate (HAuCl₄·xH₂O, ≥99.995%). All experiments were conducted using deionized water (resistivity = 18.2 MΩ·cm; total organic carbon ≤5 ppb) obtained from a Milli-Q EQ 7008 ultrapure water system.

The surface platinization of TiO₂ (TiO₂@Pt, 0.5 wt%) was performed using the photodeposition method with chloroplatinic acid hydrate as the Pt precursor [36]. First, TiO₂ (0.5 g) was dispersed in 477.36 mL of water in a beaker via sonication for 10 min. Chloroplatinic acid hydrate (2.64 mL, 4.88 mM) and methanol (20 mL, 24.54 M) were then added to the suspension. The mixture was stirred continuously and exposed to UV irradiation for 3 h using a 300 W xenon (Xe) arc lamp (Oriel) equipped with a cutoff filter (λ > 295 nm). The resulting photocatalysts were separated using a 0.45 μm polyvinylidene fluoride (PVDF) filter, washed three times with water, and dried at 80°C for 5 h. The concentration of Pt ions in the filtrate, measured using an inductively coupled plasma optical emission spectrometer (ICP-OES, Thermo Scientific iCAP 6300), was negligible, indicating that all the added Pt ions had been deposited onto the TiO₂ surface. The mass ratio of Pt to TiO₂ was estimated to be 0.5 wt%. Silver nitrate, copper(II) nitrate trihydrate, nickel(II) chloride, palladium(II) chloride, and chloroauric acid hydrate were used as metal precursors for the synthesis of other metal-deposited TiO₂ photocatalysts. Surface fluorination of TiO₂ (or TiO₂@Pt) was achieved through a simple ligand exchange process by adding sodium fluoride and lowering the pH of the TiO₂ (or TiO₂@Pt) suspension (>Ti–OH + F⁻ → >Ti–F + OH⁻, pK_F = 6.2) [37].

X-ray diffraction (XRD) patterns of bare TiO₂ and surface-modified TiO₂ were recorded using a Malvern Panalytical X’Pert Pro multipurpose diffractometer with Cu Kα radiation (λ = 1.54 Å) at an accelerating voltage of 40 kV and a current of 30 mA. Surface compositions and atomic valence states were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha) using Al Kα radiation (1486.6 eV), with C 1s (284.8 eV) as the calibration standard.

The photocatalytic activity was evaluated based on both the degradation of organophosphorus pesticides and H₂ production. A photocatalyst suspension (typically 15 mg of bare TiO₂ or TiO₂@Pt in 25.5 mL of water) was prepared by dispersing the photocatalyst powder via sonication. For surface anion modification, sodium fluoride and sodium phosphate (typically 1 mM) were added to the suspension to induce fluorination and phosphation, respectively. An aliquot of the organophosphorus pesticide stock solution (typically 2 mM, 4.5 mL) was then added to achieve the desired initial concentration (typically 300 μM). The pH was adjusted using either perchloric acid (HClO₄, 1.0 M) or sodium hydroxide (NaOH, 0.5 M) solutions (typically pH 3.0). The resulting suspension, containing the organophosphorus pesticide, was transferred to a bottle-shaped glass reactor (total volume = 55.5 mL, headspace = 25.0 mL) and sealed with a rubber septum. Argon (Ar) gas was purged through the system for 30 min to remove dissolved oxygen and establish an anoxic environment. The reaction was initiated by irradiating the catalyst suspension with UV light from a 300 W Xe arc lamp (Oriel), equipped with a 320 nm cutoff filter and a water-circulating infrared filter.

Photocatalytic experiments for organophosphorus pesticide degradation and H₂ production were conducted separately under identical experimental conditions. Photogenerated H₂ was sampled from the reactor headspace using a 100 μL glass syringe (Hamilton 81030). Liquid samples were collected using a 1 mL plastic syringe, filtered through a 0.45 μm polytetrafluoroethylene (PTFE) filter to remove catalyst particles, and immediately analyzed. All experiments were performed in duplicate or more, and the results are presented as average values with standard deviations.

The production of H₂ was analyzed using gas chromatography (GC, Agilent 7890B) equipped with a 5 Å molecular sieve column and a thermal conductivity detector, with Ar as the carrier gas. The concentrations of organophosphorus pesticides were determined using high-performance liquid chromatography (HPLC, Agilent 1120) equipped with a Zorbax 300SB C-18 column (4.6 mm × 150 mm) and a UV–visible detector. The detection wavelength was set to 207 nm, and the eluent consisted of a 50:50 (v/v) mixture of 0.1% phosphoric acid solution and acetonitrile.

The XRD patterns of bare TiO₂, F⁻/TiO₂, TiO₂@Pt, and F⁻/TiO₂@Pt samples showed no significant differences (Fig. 1a), indicating that surface modification with Pt and/or F⁻ does not alter the crystalline phase of TiO₂. This is consistent with the previous result observed for the TiO₂@Pt-coated film after soaking in the NaF solution [38]. In both bare and surface-modified TiO₂ samples, the diffraction peaks at 25.3°, 36.9°, 37.8°, 38.5°, 48.1°, 54.0°, 55.1°, and 62.7° correspond to the (101), (103), (004), (112), (220), (104), (211), and (204) planes of the anatase phase (JCPDS file No. 21–1272). Additionally, small peaks at 27.4°, 36.1°, and 41.1° are assigned to the (110), (101), and (111) planes of the rutile phase (JCPDS file No. 77–0441). The absence of Pt-related peaks in the XRD patterns of TiO₂@Pt and F⁻/TiO₂@Pt is likely due to the small size and low loading of Pt nanoparticles [39].

The chemical composition and valence states of bare TiO₂, F⁻/TiO₂, TiO₂@Pt, and F⁻/TiO₂@Pt samples were characterized using XPS. In the high-resolution Ti 2p spectrum of bare TiO₂, two prominent peaks at 459.3 eV and 465.0 eV are attributed to Ti⁴⁺ 2p^3/2 and Ti⁴⁺ 2p_1/2, respectively (Fig. 1b) [40]. The O 1s spectrum of bare TiO₂ can be deconvoluted into two peaks at 530.5 eV and 531.8 eV, corresponding to lattice oxygen (O_L) and surface-adsorbed oxygen species (O_S), respectively (Fig. 1c) [41]. Surface platinization and/or fluorination did not affect the valence states of TiO₂, as evidenced by the similarity in Ti 2p and O 1s spectra among F⁻/TiO₂, TiO₂@Pt, F⁻/TiO₂@Pt, and bare TiO₂ (Fig. 1b and c).

In the binding energy range of 70–80 eV, the spectra of bare TiO₂ and F⁻/TiO₂ differed from those of TiO₂@Pt and F⁻/TiO₂@Pt (Fig. 1d). A peak at 76.2 eV, observed in all samples, corresponds to the Ti 3s energy loss peak [42]. However, two additional peaks at 71.0 eV and 74.3 eV were detected in the TiO₂@Pt and F⁻/TiO₂@Pt samples. These peaks correspond to Pt⁰ 4f_7/2 and Pt⁰ 4f_5/2, respectively [43], confirming the presence of metallic Pt on the TiO₂ surface. In the binding energy range of 682–688 eV, a peak at 684.8 eV (F 1s) was clearly observed in both F⁻/TiO₂ and F⁻/TiO₂@Pt (Fig. 1e), indicating the presence of surface-bound F⁻ [44]. This result confirms that surface fluorination of both bare TiO₂ and TiO₂@Pt was successfully achieved via ligand exchange in the presence of F⁻ (1 mM) at low pH (3.0).

The photocatalytic activity of F⁻/TiO₂@Pt for the degradation of organophosphorus pesticides, along with concurrent H₂ production, was investigated and compared to that of bare TiO₂, F⁻/TiO₂, and TiO₂@Pt under UV irradiation. Dicrotophos was used as a model organophosphorus pesticide. In the bare TiO₂ suspension, 42.4 μM of dicrotophos (300 μM) was degraded after 4 h of UV irradiation (Fig. 2a). The presence of F⁻ and Pt on the TiO₂ surface enhanced the degradation, with 92.4 μM and 251.5 μM of dicrotophos degraded in F⁻/TiO₂ and TiO₂@Pt suspensions, respectively, after 4 h. In contrast, complete degradation was achieved after 3 h of UV irradiation when F⁻/TiO₂@Pt was used as the photocatalyst. In terms of the pseudofirst-order degradation rate constant (k), surface fluorination and platinization increased the k value by 2.4-fold (0.036 → 0.085 h⁻¹) and 13.5-fold (0.036 → 0.485 h⁻¹), respectively. Notably, the dual surface modification with Pt and F⁻ resulted in an impressive 46.4-fold enhancement in the degradation rate of dicrotophos (0.036 → 1.669 h⁻¹).

Photocatalytic H₂ production in the presence of dicrotophos occurred only when Pt was present on the TiO₂ surface (i.e., TiO₂@Pt and F⁻/TiO₂@Pt), while bare TiO₂ and F⁻/TiO₂ remained inactive for H₂ production (Fig. 2b). Consistent with the degradation trend observed in TiO₂@Pt and F⁻/TiO₂@Pt, F⁻/TiO₂@Pt exhibited higher H₂ production. Specifically, H₂ production in F⁻/TiO₂@Pt (17.87 μmol after 4 h) was 2.7 times higher than in TiO₂@Pt (6.60 μmol after 4 h). In the absence of dicrotophos, the production of H₂ was significantly reduced in both TiO₂@Pt and F⁻/TiO₂@Pt (Fig. 2c). The presence of dicrotophos (300 μM) enhanced H₂ production by 4.1-fold for TiO₂@Pt and by 5.0-fold for F⁻/TiO₂@Pt. These results suggest that dicrotophos plays a crucial role as a hole scavenger, thereby enhancing H₂ production.

The positive effect of Pt was more pronounced than that of F⁻ in both dicrotophos degradation and H₂ production. Since the experiments for simultaneous degradation and H₂ production were conducted under anoxic conditions, conduction band electrons (e_CB⁻) were prone to recombine with valence band holes (h_VB⁺) in the absence of Pt (i.e., in bare TiO₂ and F⁻/TiO₂), resulting in limited degradation and H₂ evolution. However, the presence of Pt on the TiO₂ surface (i.e., TiO₂@Pt and F⁻/TiO₂@Pt) suppresses electron-hole recombination and facilitates electron transfer by forming a Schottky barrier [45]. In this situation, the highly electronegative F⁻ on the TiO₂@Pt surface further reduces electron-hole recombination by strongly retaining e_CB⁻ [46]. Platinization and fluorination are expected to act synergistically to inhibit electron-hole recombination, thereby facilitating electron transfer to H⁺ for H₂ production and promoting hole transfer to dicrotophos or H₂O for hydroxyl radical (^•OH) generation, which contributes to dicrotophos degradation. As a result, simultaneous dicrotophos degradation and H₂ production occurred most efficiently with F⁻/TiO₂@Pt.

We also investigated H₂ production in the presence of other organophosphorus pesticides, including dichlorvos, trichlorfon, acephate, ethoprophos, methacrifos, and fenamiphos, along with their degradation (Table 1). Both degradation efficiency and H₂ production varied significantly depending on the type of organophosphorus pesticide; however, general patterns were observed. In the presence of rapidly degraded organophosphorus pesticides such as trichlorfon, dicrotophos, and dichlorvos (degradation efficiency = 83.8–100%), H₂ production was high in both TiO₂@Pt and F⁻/TiO₂@Pt (6.60–19.11 μmol). Conversely, with more slowly degraded organophosphorus pesticides such as acephate, ethoprophos, and fenamiphos (degradation efficiency = 7.6–53.5%), H₂ production was low (2.04–5.82 μmol). Notably, F⁻/TiO₂@Pt consistently exhibited higher photocatalytic activity than TiO₂@Pt for both the degradation of organophosphorus pesticides and H₂ production across all tested compounds. This performance highlights F⁻/TiO₂@Pt as a more practical photocatalyst for dual-function photocatalysis targeting organophosphorus pesticides.

The kinetics of dicrotophos degradation and H₂ production in TiO₂@Pt and F⁻/TiO₂@Pt were investigated as functions of F⁻ concentration, catalyst dosage, Pt loading, and pH. Increasing the F⁻ concentration enhanced both the photocatalytic degradation of dicrotophos and H₂ production. In F⁻/TiO₂@Pt, dicrotophos was completely degraded within 2 h at F⁻ concentrations above 2 mM, within 3 h at 1 mM, and within 4 h at concentrations above 100 μM. In contrast, 48.5 μM of dicrotophos remained in TiO₂@Pt without F⁻ after 4 h (Fig. 3a). Simultaneously, 6.60, 8.64, 12.58, 16.02, 17.87, 19.95, and 22.08 μmol of H₂ were produced at F⁻ concentrations of 0, 0.1, 0.2, 0.5, 1.0, 2.0, and 5.0 mM, respectively, after 4 h of UV exposure (Fig. 3b). The degradation rate constant (k) and H₂ production gradually increased, reaching saturation at an F⁻ concentration of 2 mM (Fig. 3c). This behavior is likely due to the saturation of F⁻ adsorption on TiO₂@Pt at this concentration. F⁻/TiO₂@Pt consistently produced more H₂ than TiO₂@Pt across a broad range of F⁻ concentrations (0.1–5.0 mM), with relative H₂ production values (F⁻/TiO₂@Pt over TiO₂@Pt) greater than 1 (Fig. 3d). These results demonstrate that surface fluorination is an efficient and facile strategy for simultaneous dicrotophos degradation and H₂ production, without the need for precise control of F⁻ concentration.

We also evaluated the degradation efficiency of dicrotophos and the production of H₂ after 4 h of UV irradiation using TiO₂@Pt and F⁻/TiO₂@Pt at various catalyst dosages. Complete degradation of dicrotophos was achieved with F⁻/TiO₂@Pt at dosages of 0.5 g/L or higher, whereas the maximum degradation remained below 87% with TiO₂@Pt (Fig. 4a). Photocatalytic H₂ production was also consistently higher in the presence of F⁻ across all TiO₂@Pt dosages up to 2.0 g/L (Fig. 4b). However, the extent of the positive effect of surface fluorination increased up to a catalyst dosage of 0.5 g/L, then significantly declined at catalyst dosages above 1.0 g/L. The relative H₂ production values (F⁻/TiO₂@Pt over TiO₂@Pt) increased from 2.0 to 2.7 as the catalyst dosage rose from 0.125 g/L to 0.5 g/L, and then decreased to 1.5 and 1.3 at catalyst dosages of 1.0 g/L and 2.0 g/L, respectively (Fig. 4c). This marked decline in the extent of the positive effect of surface fluorination at catalyst dosages above 1.0 g/L is possibly due to an insufficient F⁻ concentration relative to the amount of catalyst. Since the F⁻ concentration was kept constant at 1 mM regardless of catalyst dosage, less F⁻ was adsorbed per unit of TiO₂@Pt at higher catalyst dosages. This reduced surface coverage of F⁻ likely contributed to the diminished positive effect of surface fluorination on H₂ production, as observed in Fig. 3d.

Fig. 5 illustrates the effect of Pt loading on dicrotophos degradation and H₂ production using TiO₂@Pt and F⁻/TiO₂@Pt after 4 h of UV irradiation. In both cases, the degradation of dicrotophos and the production of H₂ initially increased and then decreased with increasing Pt loading, with the highest activity observed at 1.0 wt% Pt (Fig. 5a and b). Pt loadings above 1.0 wt% likely act as a shield against UV light [47], inhibiting light absorption by TiO₂ and the generation of electron-hole pairs, thereby reducing the photocatalytic activity for both dicrotophos degradation and H₂ production. Notably, surface fluorination of TiO₂@Pt enhanced H₂ production across all Pt loadings, with the relative H₂ production values (F⁻/TiO₂@Pt over TiO₂@Pt) exceeding 2 in every case. The most pronounced enhancement was observed at a Pt loading of 2 wt% (Fig. 5c). The reduced relative H₂ production values at Pt loadings of 3.5 wt% and 5.5 wt%, compared to 2.0 wt%, are likely due to the fact that excessive Pt loading can reduce the number of available sites for F⁻ adsorption on TiO₂@Pt.

In TiO₂-based photocatalysis for pollutant degradation and H₂ production, pH plays a crucial role by significantly influencing the chemical and physical properties of both TiO₂ and the target pollutant, as well as their interactions [48]. Consequently, the rates of degradation and H₂ production can be highly sensitive to pH. For this reason, we examined the effect of pH on the photocatalytic performance of F⁻/TiO₂@Pt and TiO₂@Pt. For both F⁻/TiO₂@Pt and TiO₂@Pt, the degradation efficiency of dicrotophos decreased as the pH increased from 3.0 to 6.0. When the pH increased further from 6.0 to 11.0, the degradation efficiency increased for TiO₂@Pt but remained nearly constant for F⁻/TiO₂@Pt (Fig. 6a). Additionally, H₂ production in the presence of dicrotophos increased with rising pH for TiO₂@Pt, whereas it decreased for F⁻/TiO₂@Pt (Fig. 6b). Overall, the synergistic effect of surface F⁻ and Pt on both dicrotophos degradation and H₂ production diminished with increasing pH and nearly disappeared at pH 11.0. The relative H₂ production (F⁻/TiO₂@Pt over TiO₂@Pt) was 2.7, 1.8, 1.6, 1.3, and 1.1 at pH 3.0, 4.5, 6.0, 9.5, and 11.0, respectively (Fig. 6c). At higher pH, the TiO₂@Pt surface becomes more negatively charged [49]. Consequently, the ligand exchange between surface hydroxyl groups on TiO₂@Pt and F⁻ (i.e., the surface fluorination of TiO₂@Pt) is hindered by electrostatic repulsion [50]. The theoretical surface coverage of F⁻ (>Ti–F), defined as the ratio of the number of surface sites occupied by F⁻ to the total number of surface sites, decreased with increasing pH and approached zero under basic conditions (Fig. 6c). This phenomenon explains the decreasing synergistic effect of surface F⁻ and Pt on dicrotophos degradation and H₂ production as the pH increases.

We also synthesized various metal-deposited TiO₂ photocatalysts using Ag, Cu, Ni, Pd, and Au, and investigated the effect of surface fluorination on dicrotophos degradation and H₂ production. Regardless of the type of metal deposited, the presence of F⁻ significantly enhanced dicrotophos degradation (Fig. 7a). After 4 h of UV irradiation, surface fluorination increased the degradation efficiency from 21.2% to 73.4% for TiO₂@Ag, from 12.0% to 35.2% for TiO₂@Cu, from 12.2% to 43.6% for TiO₂@Ni, from 17.6% to 70.8% for TiO₂@Pd, from 48.3% to 87.2% for TiO₂@Au, and from 83.8% to 100% for TiO₂@Pt. In the absence of F⁻, all metal-deposited TiO₂ photocatalysts produced less than 6.60 μmol of H₂ (Fig. 7b), whereas F⁻/TiO₂@Au, F⁻/TiO₂@Pd, and F⁻/TiO₂@Pt each produced more than 8.73 μmol (Fig. 7c). The relative H₂ production values (with F⁻ over without F⁻) were greater than 1 in all cases (Fig. 7d), indicating that the beneficial effect of surface fluorination is not limited to TiO₂@Pt, but is also evident across various metal-deposited TiO₂ systems for the simultaneous degradation of organophosphorus pesticides and H₂ production.

Phosphate (PO₄³⁻) is another anion that can be used to modify the TiO₂ surface to enhance its photocatalytic performance [51]. PO₄³⁻ can be adsorbed onto the TiO₂ surface via bidentate complexation between the titanium atom in TiO₂ and the oxygen atom in PO₄³⁻ [52]. The effect of surface phosphation on TiO₂@Pt was investigated and compared with that of surface fluorination in terms of both dicrotophos degradation and H₂ production (Fig. 7e and f). Although the presence of PO₄³⁻ enhanced both processes, its positive effect was less significant than that of F⁻. After 4 h, the degradation efficiency reached 98.2% with PO₄³⁻ and 100% with F⁻. The difference in enhancement was more pronounced in H₂ production: PO₄³⁻/TiO₂@Pt and F⁻/TiO₂@Pt produced 9.93 and 17.87 μmol of H₂, respectively, after 4 h of UV exposure. The superior performance of F⁻ compared to PO₄³⁻ can be attributed to its higher electronegativity relative to the oxygen atom in PO₄³⁻, which facilitates more efficient electron transfer to H⁺ by more effectively suppressing electron-hole recombination.