Photoreduction of Cu2+ ions to Cu metal by titanium (IV) oxide (TiO2) was conducted in the presence of silica-surfactant hybrid under a sulfuric acid condition. After irradiation, the dark-red color, reflections due to Cu metal in the X-ray diffraction pattern, and peaks due to Cu 2p1/2 and 2p3/2 in the X-ray photoelectron spectrum revealed the precipitation of Cu metal in the product. In addition, an increase in the BET surface area of 591 m2/g for the product from those of 36 and 45 m2/g for the silica-surfactant and TiO2 as well as a decrease in the intensities of C-H stretching band in the Fourier-transform infra-red spectra implied the removal of surfactants during the reaction. These characteristics were never obtained using TiO2 solely. Therefore, this study indicated that photoreduction of Cu2+ ions to Cu metal by TiO2 was facilitated under sulfuric acid conditions, where the surfactants extracted from silica–surfactant hybrids by protons in the acidic condition were successfully photo-oxidized by TiO2. Thus, this study presented a new usage of the conversion of silica-surfactant hybrid into mesoporous silicas.
Research Article
Photoreduction of copper ions using silica–surfactant hybrid and titanium (IV) oxide under sulfuric acid conditions
https://doi.org/10.21203/rs.3.rs-1535730/v1
This work is licensed under a CC BY 4.0 License
Version 1
posted
You are reading this latest preprint version
Mesoporous silica
Silica-surfactant hybrid
Heavy metal ion
Photoreduction
Titanium oxide
The discovery of mesoporous silicas prepared by a supramolecular templating approach in the 1990s [1,2] prompted studies of their fundamental properties and practical applications, leading to the generation of mesoporous silicas with varied mesostructures that originated from the mesostructures of silica–surfactant hybrids that form via self-assembly of surfactants such as alkyltrimethylammonium salts with concurrent silica precipitation [1–6] Porous structures thus form upon the removal of surfactants via calcination using an electric furnace or via acid extraction under mild conditions [1–6]. The latter process generates a waste solvent that contains surfactants. Therefore, we have focused on adding this surfactant-containing waste to wastewater to promote the removal of pollutants for environmental purification applications.
Here, we report the photoreduction of Cu2+ ions using silica–surfactant hybrids and titanium (IV) oxide (TiO2) under sulfuric acid conditions. TiO2 is an extensively studied photocatalyst [7–9] that promotes the photo-oxidation of organic compounds and the photoreduction of heavy-metal ions to metals [7–10]. The latter process can be used to produce visible-light-responsive photocatalysts and antibacterial materialsb[7–10], and the generated heavy metals can be photo-oxidized by TiO2 [11]. Notably, heavy-metal ions are present in sulfuric-acid-containing wastewater from mines [12,13]. Sulfate ions can be adsorbed onto the surface of TiO2 to decrease its photocatalytic activity [14]. Therefore, surfactants extracted from silica–surfactant hybrids by sulfuric acid are promising organic compounds that can be degraded by photo-oxidation by TiO2, promoting the photoreduction of heavy-metal ions by TiO2 in the presence of sulfuric acid. Thus, the photocatalytic activity of TiO2 has the potential to both degrade surfactants present as additional waste and remove heavy-metal ions as metal precipitates.
In the present study, we conducted the photoreduction of Cu2+ ions because their concentration is easily measured in the visible-light spectrum when they are complexed with ammonia [15]. To easily detect the particle morphology, in accordance with a previous study [16], we prepared spherical silica–surfactant hybrid particles via the homogeneous precipitation of both silica and hexadecyltrimethylammonium chloride (C16TAC) in a methanolic solution containing ammonia. A field-emission scanning electron microscopy (FE-SEM) image of the product is shown in Figure S1. For comparison, the as-synthesized spherical particles (ASP) were calcined in a similar fashion to those in the previous study [16] that forms calcined spherical particles (CSP), a nanoporous silica. The TiO2 was a standard photocatalyst (P25, Degussa). After both P25 (20 mg) and ASP (80 mg) were dispersed in a 10 mmol/L copper (II) sulfate (CuSO4) solution (5 mL) adjusted to pH 4 by the addition of sulfuric acid, the dispersion was irradiated with a He–Xe lamp for 4 h though a quartz plate. During the irradiation, the dispersion was appropriately stirred for a moment every hour. After the reaction, the resultant solid was centrifuged at 5000 rpm for 10 min and then washed with distilled water, to obtain the product denoted herein as Cu–P25–ASP. The resultant supernatant showed an increase in pH from 4 to 5 and contained bubbles on the surface. The obtained solid was dried under a reduced pressure for 1 day. For comparison, these procedures were also applied to separate starting solutions containing P25 (20 mg), ASP (80 mg), and C16TAC (20 mg) individually.
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 [19] 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 [20].
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) [7] relative to those of Cu2+/Cu+ (0.15 V), Cu2+/Cu (0.34 V), and Cu+/Cu (0.52 V) [21].
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 [22]. 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 [23], 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 [4]. 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.
We demonstrated the photoreduction of Cu2+ ions to Cu metal by TiO2 under sulfuric acid conditions, where the surfactant molecules extracted from silica–surfactant hybrid spheres were successfully photo-oxidized by TiO2. Therefore, the present study might provide a new strategy for environmental purification, where the addition of another waste to wastewater efficiently promotes both degradation and removal of surfactants and heavy metals.
Acknowledgement
This work was conducted using research equipment (TEM, STEM, EDX measurements) in the Materials Characterization Central Laboratory of Waseda University, which was shared in the MEXT Project for promoting public utilization of advanced research infrastructure (program for supporting the construction of core facilities, Grant No. JPMXS0440500021). Our sincere thanks to Prof. Kazuo Miyamura and Dr. Akinori Honda, and researchers at the Tokyo University of Science, who carried out the CHN measurements.
Author Contributions
Shingo Machida: conceptualization, investigation, writing—original draft, supervision. Reo Kato: data curation, invstigation; Kaishi Hasegawa; data curation. Takahiro Gotoh; data curation. Ken-ichi Katsumata, writing—review and editing. Atsuo Yasumori: project administration.
Conflicts of Interest
The authors declare no conflict of interest.
- T. Yanagisawa, T. Shimizu, K. Kuroda, C. Kato, The preparation of alkyltrimethylammonium-kanemite complexes and their conversion of microporous materials. Bull. Chem. Soc. Jpn 63, 988–992 (1990)
- C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism. Nature 359, 710–712 (1992)
- Q. Huo, D.I. Margolese, U. Ciesla, D.G. Demuth, P. Feng, T.E. Gier, P. Sieger, A. Firouzi, B.F. Chmelka, F. Schüth, G.D. Stucky, Organization of organic molecules with inorganic molecular species into nanocomposite biphase arrays. Chem. Mater. 6, 1176–1191 (1994)
- K. Shiba, N. Shimura, M. Ogawa, Mesoporous silica spherical particles. J. Nanosci. Nanotechol 13, 2483–2394 (2013)
- M. Ogawa, Mesoporous silica layer: preparation and opportunity. Chem. Rec 17, 217–232 (2017)
- E. Rastegari, Y.-J. Hsiao, W.-Y. Lai, Y.-H. Lai, T.-C. Yang, S.-J. Chen, P.-I. Huang, S.-H. Chiou, C.-Y. Mou, Y. Chien, An update on mesoporous silica nanoparticles applications in nanomedicine. Pharmaceutics 13, 1067 (2017)
- A. Fujishima, T.N. Rao, D.A. Tryk, Titanium dioxide photocatalysis. J. Photochem. Photobiol C: Photochem. Rev. 1, 1–21 (2000)
- K. Nakata, A. Fujishima, TiO2 Photocatalysis: design and applications. J. Photochem. Photobiol C: Photochem. Rev. 13, 169–189 (2012)
- X. Meng, X. Gao, Photocatalysis for Heavy Metal Treatment: A Review. Processes 9, 1721 (2021)
- C. Liao, Y. Li, S.C. Tjong, Visible-light active titanium dioxide nanomaterials with bactericidal properties. Nanomater. 10, 124 (2020)
- M. Janczarek, E. Kowalska, On the origin of enhanced photocatalytic activity of copper-modified titania in the oxidative reaction systems. Catalysts 7, 317 (2017)
- L. Tong, R. Fan, S. Yang, C. Li, Development and status of the treatment technology for acid mine drainage. Min. Metall Explor 38, 315–327 (2021)
- G. Rodríguez, F.M. Baena-Mreno, S. Vázquez, F. Arroyo-Torrovlvo, L.F. Vilches, Z. Zhang, Remediation of acid mine drainage. Environ. Chem. Lett. 17, 1529 (2019)
- D.S. Bhantkhande, V.G. Pangarkar, A.A.C.M. Beenackers, Photocatalytic degradation for environmental applications-a review. J. Chem. Technol. Biotechnol. 77, 102–116 (2001)
- D. Guspita, A. Ulianas, Optimization of complex NH3 with Cu2+ Ions to determine levels of ammonia by UV-Vis spectrophotometer. J. Phys. Conf. Ser. 1481, 012040 (2020)
- N. Shimura, M. Ogawa, Preparation of surfactant templated nanoporous silica spherical particles by the Stöber method. Effect of solvent composition on the particle size. J. Mater. Sci. 42, 5299–5306 (2007)
- M. Miyagawa, M. Yonemura, H. Tanaka, Lustrous copper nanoparticle films: photodeposition with high quantum yield and electric conductivity. Chem. Phys. Lett. 665, 95–99 (2016)
- P. Pallavicini, G. Dacarro, P. Grisoli, C. Mangano, M. Patrini, F. Rigoni, L. Sangaletti, A. Taglietti, Coordination chemistry for antibacterial materials: a monolayer of a Cu2+ 2.2’-bipyridine complex grafted on a glass surface. Dalton Trans. 42, 4522–4560 (2013)
- R. Kato, N. Shimura, M. Ogawa, Controlled photocatalytic ability of titanium dioxide particle by coating with nanoporous silica. Chem. Lett. 37, 76–77 (2008)
- S. Brunauner, P.H. Emmett, E. Teller, Adsorption gases in multimolecular layers. J. Am. Chem. Soc. 407, 309–319 (1938)
- K. Chen, D. Xue, Reaction route to the crystallization of copper oxides. Appl. Sci. Convergence Technol. 23, 14–26 (2014)
- Y. Umemura, E. Shinohara, A. Koura, T. Nishioka, T. Sasaki, Photocatalytic decomposition of an alkylammonium cation in a Langmuir-Blodgett film of a titania nanosheet. Langmuir 22, 3870–3877 (2006)
- L. Mercier, J. Pinnavia, Access in mesoporous materials: advanced of a uniform pore structure in the design of a heavy metal Ion adsorbent for environmental remediation. Adv. Mater. 9, 500–503 (1997)
- H. Hattori, Y. Ide, T. Sano, Microporous titanate nanofibers for highly efficient UV-protective transparent coating. J. Mater. Chem. A 2, 16381–16388 (2014)
- K. Saito, M. Kozeni, M. Sohmiya, K. Komaguchi, M. Ogawa, Y. Sugahara, Y. Ide, Unprecedentedly enhanced solar photocatalytic activity of a layered titanate simply integrated with TiO2 nanoparticles. Phys. Chem. Chem. Phys. 18, 30920 (2016)
No competing interests reported.