After irradiation, the P25–ASP mixed dispersion was dark-red (Figure 1), whereas the other dispersions and solutions were not. The X-ray diffraction (XRD) pattern of theCu–P25–ASP shows reflections attributable to P25 and ASP as well as Cu metal; no reflections attributable to copper oxides are observed (Figure 2). Transmission electron microscopy (TEM) and scanning TEM (STEM) images of the Cu–P25–ASP show spherical particles (ASP) and ~30 nm particles of P25. The latter particles exhibit relatively dark and bright regions (Figure 3aandb). The light regions contain high concentrations of Cu, as evident in the corresponding energy-dispersive X-ray (EDX) mapping image (Figure 3d). The X-ray photoelectron spectrum of the Cu–P25–ASP (Figure S2) shows peaks at binding energies of 953.5, 943.0, 933.0, and 932.5 eV, which are attributed to Cu 2p1/2, Cu2+ satellite, Cu2+ 2p3/2, and Cu 2p3/2, respectively [17,18]. The ratios of the decrease in Cu2+ concentration relative to those in the starting solutions are reported in Table 1. The dispersion that contained both P25 and ASP resulted in the largest decrease, whereas the Cu2+ concentration hardly decreased for the P25 dispersion or the C16TAC solution. By contrast, the ASP dispersion resulted in a 40% decrease in Cu2+ concentration.
Table 1. Ratios reflecting the decrease in Cu2+ concentration after irradiation of the additives combined with the starting solution.
The intensities of the C–H stretching bands19 at 2922 and 2872 cm− 1 and the C–N stretching band  at 1460 cm− 1 in the Fourier transform infrared (IR) spectrum of the mixed solid of P25 (20 mg) and ASP (80 mg) are substantially lower than those in the spectrum of the Cu–P25–ASP (data not shown). The C and N contents of 2.9 and 0.13 mass%, respectively, in the Cu–P25–ASP are lower than those in the mixed solid before irradiation (18 and 0.95 mass%, respectively). Figure 4 shows N2 adsorption/desorption isotherms for the ASP, P25, Cu–P25–ASP, and CSP, as recorded at − 196°C. According to these isotherms, the specific surface areas of the ASP, P25, Cu–P25–ASP, and CSP were calculated to be 36, 45, 591, and 806 m2/g using the Brunauer–Emmett–Teller (BET) method .
Collectively, the product appearance (Fig. 1), XRD patterns (Fig. 2), XPS spectrum (Figure S2), and electron microscopy and elemental analyses (Fig. 3) reveal that Cu2+ was photo-reduced, resulting in the precipitation of Cu metal in the P25–ASP mixed product after irradiation. Copper oxides are absent, consistent with the lower redox potential of the conduction band of TiO2 (− 0.52 V)  relative to those of Cu2+/Cu+ (0.15 V), Cu2+/Cu (0.34 V), and Cu+/Cu (0.52 V) .
Alkylammonium ions are well known to be degraded by •OH and •O2 radicals, which can be generated by reaction of H2O and O2 molecules with holes photogenerated in the valence band of TiO2 . In the present study, the IR spectra, N2 adsorption/desorption isotherms (Fig. 4), and elemental analyses indicate a decrease in the concentration of hexadecyltrimethylammonium ions (C16TMA+) in the ASP after the reaction. Because a decrease in the concentration of Cu2+ was observed when ASP alone were used for the present reaction (Table 1) and because the cation-exchange reactions of silanol groups on silicas are well known , exchange reactions of C16TMA+ with protons and/or Cu2+ ions in the CuSO4 acidic solution are highly likely. The presence of Cu2+ detected by XPS (Fig. 4) thus results from the Cu2+ adsorbed on the ASP which are not accessible on P25 surfaces (Fig. 3) or on the white regions observed after irradiation (Fig. 1); the latter situation can be improved by modifying the experimental conditions. The dispersion never became dark-red as shown in Fig. 1 when the present reaction was conducted with P25 alone; it remained white. In addition, the Cu2+ decrease ratio for the Cu–P25–ASP is higher than those for the P25 and ASPs. Notably, the solution after irradiation was still under sulfuric acid conditions (refer to the experimental conditions). Therefore, photo-oxidation of C16TMA+ extracted from ASP is evident in the present reaction, which successfully promoted the photoreduction of Cu2+ under sulfuric acid conditions.
After the dispersion containing P25 and ASPs was irradiated for 4 h, slight bubbling was observed on the surface of the dispersed particles, suggesting the presence of undegraded C16TMA+. In addition, C16TMA+ and/or C16TAC was incompletely extracted from the ASP according to the residual alkyl chains in Cu–P25–ASP, as revealed by the IR spectra and CHN analyses; this interpretation is further supported by the lower porosity of Cu–P25–ASP compared with that of CSP, as determined from the N2 adsorption/desorption isotherms (Fig. 4). Notably, silica–surfactant hybrid spheres are prepared using alkyltrimethylammonium salts with shorter side-chain lengths . In addition, the synthesis of silica–surfactant hybrids has been used to coat silica–surfactant layers onto various inorganic surfaces to form inorganic solid/silica–surfactant core–shell particles [5,19]. Increasing the amounts of TiO2 and TiO2-based compounds [24–25] added to the present reaction is also feasible. However, the present product containing Cu metal might find applications where visible-light-responsive photocatalysts and antibacterial materials [7–14] have been used. We plan to conduct such studies in the future.