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Article

Low-Temperature Synthesis of Cu-Doped Anatase TiO2 Nanostructures via Liquid Phase Deposition Method for Enhanced Photocatalysis

by
Mitsuhiro Honda
1,*,
Tsuyoshi Ochiai
2,
Popy Listiani
1,
Yuma Yamaguchi
1 and
Yo Ichikawa
1
1
Graduate School of Engineering, Nagoya Institute of Technology, Nagoya 466-8555, Japan
2
Materials Analysis Group, Kawasaki Technical Support Department, Kanagawa Institute of Industrial Science and TEChnology (KISTEC), Kawasaki 213-0012, Japan
*
Author to whom correspondence should be addressed.
Materials 2023, 16(2), 639; https://doi.org/10.3390/ma16020639
Submission received: 5 December 2022 / Revised: 29 December 2022 / Accepted: 5 January 2023 / Published: 9 January 2023
(This article belongs to the Special Issue Design and Applications of Functional Materials)
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Abstract

:
Titanium dioxide (TiO2) photocatalysis can harness the energy from sunlight, providing a solution to many green- and energy-related problems. In this study, we aimed to produce Cu doped TiO2 (Cu-TiO2) structures at a low temperature (~70 °C) under atmospheric pressure based on liquid phase deposition. The products prepared with Cu nitrate exhibited anatase-phase TiO2 with the presence of Cu, and the particles showed a waxberry-like structure. Changing the Cu nitrate concentration allowed control of the atomic concentration; we confirmed ~1.3 atm.% of Cu ions in the product when we applied 10 mM in the precursor solution. By doping Cu, the light absorption edge shifted to 440 nm (~2.9 eV), and we proved the photocatalytic reaction through action spectral measurement. We observed the decomposition of acetaldehyde into CO2 on Cu-TiO2 photocatalysts, which produced optimized improvements in photocatalytic activity at Cu dopant levels between 0.2 and 0.4 atm.%. This study demonstrates that the liquid phase deposition technique can be used for doping metallic ions into TiO2, which shows promise for preparing novel and unique nanomaterials as visible light photocatalysts.

1. Introduction

Photocatalysis is capable of harnessing the energy from sunlight, being a solution to many green- and energy-related problems facing the world [1,2]. Titanium dioxide (TiO2), in the anatase phase, is the most representative photocatalyst due to its strong oxidizing power as well as its physical and chemical stability [3,4,5,6]. The qualities of TiO2 have led to its diverse applications in, for example, self-cleaning surfaces, water splitting, disinfection, and air/water purification [7,8,9,10].
The trigger of photocatalysis is the absorption of photons, which are trapped by the photocatalyst to generate electron–hole pairs, subsequently inducing chemical reactions at the surface [11]. However, TiO2 can only respond to photons with energy larger than 3.2 eV, which is ultraviolet (UV) light, which only accounts for approximately 5% in sunlight [12]. This is considered to be one of the critical limitations for efficient photocatalysis. Thus, to date, extending the wavelength range of the photoactivation of TiO2 photocatalysts toward the visible-light region is a crucial task to enhance the use efficiency of solar energy.
Elemental doping is an effective strategy to increase the visible light sensitivity of TiO2 [13,14]. Doping can narrow the bandgap of TiO2 by downshifting the conduction band bottom or introducing new energy levels within the gap [15,16,17,18,19]. Such states, owing to doping ions, can be used to trap electrons or holes to separate carriers from the bands, thus allowing more carriers to successfully diffuse to the surface. Among the diverse doping elements, copper (Cu) is one of the attractive choices for visible-light harvesting and increased photocatalytic efficiency [20,21,22,23,24,25]. Although the experimental evidence for the enhancement in photocatalytic activity in TiO2 through copper doping is inconclusive, copper doping can modify the large bandgap of TiO2 to optimize its optical properties for visible-light harvesting. Mingmongkol et al. doped TiO2 with Cu ranging from 0.1 to 1.0 wt.%, which resulted in the bandgap decreasing from 3.20 eV for undoped TiO2 to 3.12 eV for 1.0 wt.% Cu-doped TiO2 [26]. Mathew et al. reported that the visible light absorption property of Cu-TiO2 increased and the band gap reduced to 2.8 eV with 0.5 mol% Cu [23]. The results of density functional theory (DFT) studies suggested that the introduction of Cu+ and Cu2+ ions creates oxygen vacancies by replacing Ti4+ ions in the TiO2 lattice. The enhancement in photocatalytic activity owing to doping Cu is driven by reduced charge carrier recombination in Cu-TiO2. Cu dopants (Cu0, Cu+, Cu2+) can create multiple bands to extend electron–hole (e−, h+) pair recombination; therefore, Cu-TiO2 exhibits improved electron–hole separation with photoexcitation, resulting in the increase in photocatalytic efficiency [27,28]. The purposes of doping Cu into TiO2 photocatalyst are: (i) modifying its large bandgap and electronic structure to optimize its optical properties for visible-light harvesting; (ii) improving each step in the charge kinetics to reduce the massive recombination of photogenerated carriers. Photocatalytic activity is optimized at Cu dopant levels between 0.1 and 2 atm.% [27,28,29,30]. Above a certain threshold, increasing the Cu dopant concentration diminishes photocatalytic activity through a combination of enhanced recombination and shading [28,30]. The diversity in the reported behavior of Cu dopants and their effects on photocatalytic activity seem to largely stem from diverse materials synthesis methods that result in Cu dopants in nonequilibrium sites as well as from the diverse approaches through which photocatalytic activity is assessed.
Cu has been added to TiO2 in different mole percentages for various photocatalytic applications such as hydrogen production, CO2 reduction, decomposition of organic contaminants, and antibacterial/bacterial killing [20,21,22,23,24,25]. Golon et al. synthesized Cu-TiO2 via a sol gel method; the doped sample, which was calcined at 600 °C, displayed high photocatalytic activity for phenol degradation [25]. In another study, the gel of Ti(IV) isopropoxide and Cu nitrate was nucleated for 12 h in the dark, the mass was then dried in an oven at 100 °C for 4 h, and the resulting solid mass was finally crushed and calcined at 500 °C for 3 h [31]. Among all the photocatalysts examined in the study, the sample with the lowest Cu concentration produced the most CH4 (1081 μL h−1 g−1) and the highest doping showed the least activity (200 μL h−1 g−1) under similar conditions. In most cases, the synthesis procedures require high-temperature calcination, so are not suitable for forming photocatalyst films on the platforms of technologically important substrates such as plastics and glasses. Another attractive approach to synthesize Cu-doped nanocrystalline TiO2 is the hydrothermal method; this method can be employed under self-produced pressures at low temperatures [32,33]. Typically, the precursor solution is transferred to a Teflon-lined stainless-steel autoclave and heated at ~180 °C for 12 h. The operating temperature is above the melting point of plastics, and the pressure inside a reactor is known to be in the mega pascals range; therefore, this technique is incompatible with forming a film on a flexible substrate such as plastics [34]. The precipitation method was used to synthesize Cu-doped nanocrystalline TiO2; this method can be performed using low-cost materials and easier manufacturing methods at industrial scales. Recently, A.M. Alotaibi et al. used aerosol-assisted chemical vapor deposition (AACVD) to deposit highly photoactive thin films of Cu-doped anatase TiO2 on glass substrates [22]. In their study, a solution of titanium isopropoxide, copper nitrate, and ethyl acetate was atomized using a piezoelectric device, and the generating aerosol was transferred with N2 flow onto the glass substrate heated at 470 °C. AACVD provides a facile route for producing highly photoactive Cu-doped TiO2 thin films. In such films, the interactions between substitutional and interstitial Cu in the anatase lattice can explain the extended exciton lifetimes and the enhanced photocatalytic and antibacterial ability. Thus, although many researchers have focused on the preparation of Cu-TiO2 using sol–gel and hydrothermal techniques, studies on synthesis or coating techniques with operation at low temperatures or under normal pressure are scarce.
In this context, in this study, we aimed at producing Cu-TiO2 structures at low temperature (<~80 °C). For this, we employed the liquid phase deposition (LPD) technique as a method to produce TiO2 nanostructures. LPD is a simple and economical approach to generate TiO2 with an anatase phase at room temperature, which was first reported by Deki in 1996 [35]. LPD synthesis is based on the hydrolysis of ammonium hexafluoro titanate [(NH4)2TiF6], with boric acid used as the fluorine ion scavenger, forming TiO2 via a ligand exchange reaction [35,36]. As this reaction proceeds at room temperature and atmospheric pressure, LPD is applicable for non-heat-resistant materials such as polystyrene, glass, etc. [37,38,39]. In this study, based on the LPD technique, we dissolved Cu salts into a precursor solution for Cu doping, after which we microscopically and spectroscopically characterized the products to prove the possibility of controlling the Cu-doping level.

2. Materials and Methods

2.1. Sample Preparation

Following the typical LPD procedure described in a previous study [39], we produced TiO2 samples through a temperature-assisted liquid phase deposition process, whose procedure is schematically illustrated in Figure 1. For the precursors, we used (NH4)2TiF6 (AHFT) (FUJIFILM Wako, 1st Grade) and H3BO3 (FUJIFILM Wako, 1st Grade). We separately dissolved both materials in distilled water at 0.1 M and 0.3 M for AHFT and boric acid, respectively. This ratio allowed the production of the anatase phase of TiO2. For doping copper ions, we then mixed the solutions with several types of Cu salts under vigorous stirring for 15 min. Here, as Cu salts, we applied Cu(NO3)2·3H2O (FUJIFILM Wako, Wako Special Grade), CuCl2 (Kojundo Chemical Laboratory Co., Ltd., Tokyo, Japan), and Cu(CH3COO)2 (FUJIFILM Wako, Wako Special Grade). Next, we fixed a glass substrate (50 × 50 mm) in a beaker so that the coating surface was kept downward to avoid particle deposition on this surface, which we then placed in an oven at 70 °C for 3 h. In a previous study, a temperature of 70 °C and a reaction time of 3 h was favorable for improved crystallinity and photocatalysis. Prior to use, we thoroughly cleaned the glass substrates by ultra-sonication in acetone (99%) and performed hydrophilic treatment with a table-top UV/Ozone Processor (SSP16-110, SEN LIGHTS). We changed the concentration of Cu salts up to 10 mM for producing different doping levels. Here, we did not apply doping levels higher than 10 mM to avoid excess doping, which weakens photocatalytic activity. After the reaction proceeded for 3 h, we removed the substrate, which we rinsed in distilled water and dried with a blower. We obtained the powders after separation by centrifugation (3500 rpm) for 15 min, which we washed by dispersion in distilled water, centrifuged three times, and then dried at room temperature.

2.2. Characterization

We characterized the substrate samples with a Raman spectroscope (NRS-3300, JASCO), UV–vis spectrometry (UV mini 1240, Shimadzu), a scanning electron microscope (SEM) (JSM-7800F, JEOL), an electron probe micro-analyzer (EPMA) (JXA-8530F, JEOL), and X-ray photoelectron spectroscopy (XPS) (PHI5000 VersaProbe, ULVAC-PHI Inc.). We determined the optical band gap of the nanoparticles using diffuse reflection spectroscopy (DRS) with a fiber optic reflectance sphere (#58-583, Edmund OPTICS). We calculated the band gap energies using the Kubelka−Munk (K-M) method based on the diffuse reflectance spectra, where F(R) = (1 − R)2/2R [40].

2.3. Photocatalysis

We observed the photocatalytic activity of the Cu-TiO2 substrate through decomposing acetaldehyde gas. We performed the photocatalysis experiment based on a JIS R1701-2. We tested powder products for action spectral measurement through the degradation of methylene blue (MB, Nacalai Tesque, Inc., Nakagyo-ku, Kyoto, Japan) as a model organic compound. We prepared the solution for a reaction by adding 25 mg of the sample into 50 mL of 10 ppm MB solution in a glass beaker, which we then mixed via ultrasonication for several minutes. We initiated the photocatalytic reactions with xenon lamp (MAX505, Asahi Spectra) illumination, where we extracted the specific wavelength (320, 360, 400, 440, 480, and 520 nm) by using bandpass filters for action spectral measurement. During the reaction, we continuously stirred the solution with a magnetic stirrer at 300 rpm. After extracting the MB solution via centrifugation, we monitored the time-dependent decay of MB absorbance centered at 665 nm after 30, 60, 90, and 120 min of irradiation using a UV–Visible Spectrophotometer (Shimadzu UV mini-1240).

3. Results and Discussion

3.1. Choice of Cu Salts for Doping

Figure 2 presents the SEM images of undoped and doped samples prepared with different types of Cu salt: Cu(NO3)2·3H2O, CuCl2, CuSO4·5H2O, or Cu(CH3COO)2. In these experiments, we fixed the concentration of these salts at 10 mM. Most of the products had waxberry-like structures; in Figure 2, the size increased when using Cu salts. Among several types of Cu salts, the products with Cu acetate exhibited fewer protrusions, which was due to the inhibition of crystal growth. In addition, this morphology suggests poor photocatalytic activity due to is small surface area. Thus, crystal growth should be improved with the use of Cu nitrate or chloride, whereas Cu acetate is less effective in terms of forming fine structures.
To further understand the molecular vibrational state, we applied Raman spectroscopy. Figure 3 shows the Raman spectra of undoped and doped samples prepared with different types of Cu salt (10 mM). We assigned the Raman peaks at 152, 404, 513, and 634 cm−1 to the Eg, B1g, A1g, and Eg modes of the anatase phase, respectively [39,41]. The strongest Eg vibrational mode (at 152 cm−1) seen in all samples was caused by symmetric O-Ti-O stretching vibrations in TiO2, indicating the formation of an anatase phase in the LPD process even with dissolving Cu salts in a precursor solution. The XRD patterns also confirmed the crystallographic nature of anatase TiO2, whereas the doped samples exhibited poor crystallinity comparing with the undoped ones (Figure S1). Based on the Scherrer equation, we calculated the crystallite size of undoped TiO2 to be ~50 nm; doped TiO2 is expected to be smaller according to the wide FWHM values [39,42]. Analyzing the XRD patterns of doped samples is difficult due to spectral noise; therefore, we performed curve-fittings in Eg Raman mode to evaluate the local structure of TiO2. Table 1 shows that the FWHM values increased with the addition of copper chloride and copper acetate; only the copper nitrate precursor solution had an FWHM value comparable to that of the undoped TiO2. These FWHM values could be explained by the structure of crystals. In general, the spectral line shape of crystal lattice vibrations (or phonons) is inversely proportional to the lifetime of the phonons [43]. In the case of an ideal harmonic crystal, the line shape is infinitesimally narrow. Experimental evidence shows that the Raman line always has a finite width, indicative of the presence of phonon-vibrational decay. The broadening of the Raman line width is attributed to the phonon scattering (decay) owing to the presence of impurities and/or defect sites and crystallite edges in the crystallites. Therefore, in our experiments, we expected the use of copper chloride and acetate to induce lattice disorders or size reduction to generate more impurities and/or defect sites and crystallite edges. However, copper nitrate, as shown in Table 1, produced fewer effects that hindered the atomic arrangement, resulting in maintaining the crystallinity of the products. Therefore, we concluded the use of Cu nitrate is more appropriate for Cu doping into TiO2 with LPD process from the viewpoint of improved crystallinity, which may enable efficient photocatalysis.

3.2. Concentration Dependence on Crystal Structures

We changed the concentration of Cu nitrate from 0.1 to 10 mM, and we observed the morphology of the powder products and coatings by SEM. Figure 4a–c and Figure 5a–d show SEM images of the powder and coating samples prepared with different concentrations of Cu nitrate (0.1, 1.0, and 10 mM). The size histogram measured with SEM is shown in Figure 6. The size of the particles was 1.4–1.5 µm, on average, for 0.1 and 1.0 mM. However, the particle size decreased as the concentration increased up to 10 mM. This was likely due to the excess doping, which inhibited crystallization.
For the coatings, we observed plate-like structures with large cracks between the plates on the glass substrate. The formation of large cracks between plates is an often-observed phenomenon when forming thick films [44]. The surface texture of a plate seemed to be smooth with the absence of protrusions, which considerably differed from that of the powder sample. We think this was caused by crystal growth from densely distributed nuclei. In the LPD process, nucleation starts from the hydroxyl groups, which act as nucleation sites. Without hydrophilization treatment of the substrate, we observed the decrease in density of nucleation sites, generating a waxberry-like structure with a sparse distribution (Figure S2).
We found that the change in Cu concentration affected the surface morphology of the Cu-TiO2 coatings, as shown in Figure 5b–d. The plate grew larger as the concentration increased from 0.1 to 1.0 mM. When we applied 10 mM, a granular structure formed, and plate-like structures did not form. We expected the addition of Cu ions into the precursor solution to enhance the crystallization to generate larger plates, which we observed for the produced powder samples (Figure 2). However, at the higher concentration (10 mM), plate-like structures and cracks were absent. The absence of cracks was due to the inhibition of nucleation and further crystal growth by the copper dopants dissolved in the solution. For the synthesis of TiO2 thin films, we employed an aqueous solution of ammonium hexafluoro titanate ((NH4)2TiF6) and boric acid (H3BO3) as the titanium fluoride complex and F− scavenger, respectively. The titanium fluoride complex is hydrolyzed to titanium hydroxide and free F− ions. The produced F− ions can then be scavenged by H3BO3 [35,36,39]. The hydroxyl groups at the surface of a glass substrate induce hydrolyzation to form TiO2 films. When Cu nitrate is dissolved in a solution, Cu2+ is subject to diffusion over the hydroxylated surface [45]. Therefore, the diffusion of Cu ions over a hydroxylated surface is expected to diminish the hydrolysis (ligand-exchange) reaction of titanium fluoride complexes, resulting in a reduction in the density of TiO2 particles. The particles are isolated; therefore, less agglomeration occurs, so the cracks disappear.
Figure 7a,b represent the Raman spectra of powder and coating samples, respectively. Through the comparison of the spectral patterns in Figure 7a, we found that all samples were anatase-phase TiO2. Despite the weak peaks in the coating samples, as shown in Figure 7b, the presence of the principal Eg vibrational mode (at 155 cm−1) suggested the formation of anatase TiO2 on a substrate. Notably, the spectral width (FWHM) and peak wavenumber should be dependent on the Cu doping concentration.
Figure 8a,b depict the dependence of peak wavenumber and FWHM values on the concentration of Cu nitrate, respectively. In both the coating and powder cases, the strongest Eg peak blue-shifted as the Cu concentration increased from 0.1 to 10 mM. The blue shift in the Eg vibrational mode probably occurred due to the substitution of smaller Ti cations (0.605 Å) by larger Cu cations (0.73 Å). As larger Cu2+ ions replace smaller 4+ ones in their position, anatase TiO2 crystal is expected to be subject to compressive stress [46]. With higher doping concentrations, we observed a monotonical increase in the FWHM values of the Eg vibrational mode. The replacement of Cu by Ti ions was expected to induce disarrangement of Ti-O networks, which would result in the lower crystallinity of the products.
The EDS and EPMA elemental mapping images for undoped and Cu doped samples are presented in Figure 9 and Figure 10, respectively, which demonstrate that all the three elements (Ti, O, and Cu) were uniformly distributed in both the doped powder and coating samples. Thus, using LPD enables the preparation of doped TiO2 nanomaterials with a homogeneous distribution of doping ions.

3.3. Doping Levels and Optical Properties

We measured the atomic concentration of Cu in the powder and coating samples with EDS and EPMA; the results are shown in Figure 11. With increasing Cu nitrate concentration from 0 to 10 mM, the atomic concentration changed from 0 to 0.8 and 1.3 atm.% in the powder and coating samples, respectively.
Figure 12 presents the Kubelka–Munk (K–M) plots we obtained through diffusion reflectance measurements for the samples without (black) and with Cu doping (red, pink, and blue). We determined the bandgap of the material by extrapolating the linear part of [F(R)*hν]1/2 to zero when plotted against E(hν,) as shown in Figure 12. (see Table 2) The bandgap of undoped TiO2 powders is shown in Figure 12 at 3.30 eV, which was slightly larger than that of bulk TiO2 with an anatase phase (3.2 eV) [47]. As the crystallite size of the doped samples was less than 50 nm, the larger bandgap observed here might have been due to the particle size effect [48,49,50]. The subgap region showed an Urbach-like tail at 3.10–3.25 eV. This indicated a distribution of shallow states just below the conduction band minimum (CVM), though the origin of shallow gap states is still unclear [51]. When we applied 0.1 mM Cu nitrate, the bandgap remained at 3.30 eV; the tail observed with undoped TiO2 was invisible in the pink spectrum. The absence of the tail indicated less disorder (e.g., oxygen vacancy) formed in the TiO2 lattice, which agreed well with the results of SEM observation (Figure 5), Raman spectroscopy (Figure 8), and XRD analysis (Figure S1). In this case, with a single compensating oxygen vacancy, the empty 3d state of the Cu2+ ions lies above the CBM of the TiO2 host, so that the dopant does not impact the magnitude of the bandgap. With higher levels of Cu doping (1.0 and 10.0 mM), we observed two features, as shown in Figure 12: (i) red-shifting of the band originating from TiO2; (ii) creating additional states just below the original band. The red-shift of the bandgap to 3.15 eV (1.0 mM) and 2.90 eV (10.0 mM) could be explained by the coexistence of Cu2+ ions and oxygen vacancies. Mathew et al. studied the electronic states of Cu-TiO2 based on the density functional theory simulation, in which the introduction of Cu+ and Cu2+ ions by replacing Ti4+ ions in the TiO2 lattice created oxygen vacancies to extend the valence band maximum (VBM). Contrary to the case with 0.1 mM, Cu-derived states emerged between the valence band edge and extended the VBM to higher energies, leading to a decrease in the band gap. In addition, gap states were created between the VBM and CBM, which was evident as the presence of an absorption tail ranging from 3.05 to 3.25 eV for 1.0 mM and from 2.50 to 3.20 eV for 10.0 mM Cu nitrate. Cu-TiO2 with 10 mM Cu nitrate exhibited optical response in the visible region (wavelengths shorter than 430 nm), which is beneficial for the efficient use of solar light for photocatalysis.

3.4. XPS Analysis

Figure 13 presents the XPS spectra of coating samples synthesized with 10 mM Cu nitrate. Figure 13a,b indicate the narrow scans for Ti 2p and Cu 2p, respectively. We fitted the XPS data to highlight the peak position and FWHM values, which are summarized in Table S1. The fitted curves are presented as dotted curves in Figure 13. We attributed the two strong symmetrical peaks at around 464.1 and 458.4 eV to Ti 2p1/2 and Ti 2p3/2, respectively. The peak separation of 5.7 eV shown in the Ti 2p doublet agrees well with the energy reported for TiO2 nanoparticles with an anatase phase [52]. Those peaks were 1.1 eV lower than those in anatase TiO2, which is generally caused by the presence of a higher anionic vacancy. The peak of the Ti 2p3/2 spectrum located between Ti4+ (459.2 eV) and Ti3+ (457.5 eV) [53]; however, we could not obtain a good fit with two components of Ti4+ and Ti3+, potentially due to the complex defect states of Cu/O vacancies. The Cu2p spectrum on deconvolution exhibited an intense peak at 932.4 eV and a shorter peak at 931.7 eV, which we ascribed to the Cu2+ and Cu+ states, respectively [31]. The absence of the satellite peak (942.6 eV) suggested that the Cu was doped in the TiO2 anatase lattice and did not form a surface CuO layer [54]. The doped Cu existing in oxidation state of Cu+ resulted in the formation of single oxygen vacancies. However, we did not observe the signal corresponding to Ti3+ sites in the Ti 2p region, potentially because that the Cu dopant present in the TiO2 lattice as either Cu2+/Cu+ had lower valency and higher electronegativity compared with those of Ti4+. To conserve the lattice charge as a result of the incorporation of aliovalent Cu ions in the anatase TiO2 matrix, ionic vacancies (O vacancies) in several sites may have formed.

3.5. Photocatalysis

We observed the photocatalytic activity of undoped and Cu doped TiO2 through decomposing acetaldehyde gas into CO2 and H2O. Figure 14 shows the concentration decay of acetaldehyde with light irradiation. In all cases, we confirmed the reduction of acetaldehyde gas and generation of CO2 (Figure S3) gas. Cu-TiO2 with 0.1 and 1.0 mM Cu nitrate exhibited a higher rate of reaction, whereas we found lower activity for the sample with 10 mM Cu nitrate doping. We calculated the average rate constants and standard deviations for undoped and Cu-doped TiO2, and the obtained values are summarized in Table 3. The Cu doping through the LPD process is described in Table 3, which increased photocatalytic efficiency (1.3 times). The undoped TiO2 prepared via LPD exhibited considerable MB degradation activity, as confirmed by a photocatalytic activity 3–4 times higher than that of the commercial photocatalyst P25-Degussa in our previous study [39]. Therefore, the photocatalytic activity of Cu-TiO2 in this study should be 4–5 times higher than that of the commercial photocatalyst P25-Degussa. Enhanced activity is most often driven by reduced charge carrier recombination in Cu-TiO2 materials [27,28]. This arises as the result of photogenerated electrons facilitating the reduction Cu2+ + e− → Cu+, thus extending the valence band hole lifetimes at surfaces, which are able to react with adsorbed species to form active radicals. In our case, the results of XPS analysis confirmed the presence of Cu2+. The results of the Raman study and XRD analysis confirmed the absence of Cu2O structures. These results do not necessarily preclude the existence of amorphous or fine secondary oxide precipitates, which may improve exciton lifetime through electron capture in the secondary phase or enhance activity through an increased surface area [55,56,57]. The contribution of the mechanism discussed above was dominant in the 0.1 mM doped Cu-TiO2; an improvement in the light absorption of TiO2 resulted in the promotion of photocatalysis in the 1.0 and 10.0 mM cases. The light absorption edge extended to 3.15 and 2.90 eV for each sample, as shown in the K-M plots (Figure 12 and Table 2).
To reveal the contribution of the response in the visible light region, we evaluated the action spectra of Cu-TiO2 (10.0 mM). Figure 15 shows the light absorption and action spectrum of Cu-TiO2. Here, we applied 10 mM Cu nitrate, and we performed further annealing treatment to easily observe photocatalytic reaction. As shown in Figure 15, the action spectrum confirmed that the decomposition of MB occurred with 440 ± 10 nm or shorter wavelengths (corresponding to 2.76–2.88 eV energy). Thus, the Cu-TiO2 produced in our experiments responded to visible light to induce photocatalysis, which agrees with the bandgaps shown in Figure 12.

4. Conclusions

In this study, we found that after properly choosing a Cu dopant source for crystal growth, liquid phase deposition could be used produce Cu-doped TiO2 at low operating temperatures (around 70 °C) under atmospheric pressure, with a controllable level of incorporated ions. We dissolved several different types of Cu salts in a precursor solution as a source of Cu ions; we found that Cu nitrate was the best choice for generating Cu-doped TiO2 with improved crystallinity. The results of Raman spectroscopy revealed the formation of anatase TiO2 in all samples with and without Cu ion doping. Based on the SEM measurements, we observed that the structure of the powder samples was micron-sized particles with a waxberry-like structure, which maintained their structure after increasing the Cu nitrate concentration up to 10 mM. The coating samples produced with less than 1.0 mM displayed plate-like structure with cracks, whereas we found individually distributed particles by increasing the doping concentration to 10.0 mM. We think the plate-like structure formed due to the crystal growth from densely distributed nuclei. Additionally, excess doping of Cu ions inhibited the nucleation on the substrate to reduce the density of TiO2 particles, resulting in the disappearance of plates and cracks. The results of elemental mapping revealed the homogeneous distribution of Cu atoms across the doped particles and coatings, and we could control the doping level of Cu cations up to ~1.4 atm.% when we applied 10 mM Cu(NO3)2. Based on the results of the Raman and XPS studies, we concluded that the Cu ions were incorporated in the lattice of TiO2 in the Cu+ or Cu2+ states, which resulted in a slight shrinkage of the cell of the produced material. Finally, we enhanced the photocatalytic performance through decomposing acetaldehyde gas on Cu-TiO2 coating samples, which could be explained by their more efficient light absorption in the visible region or the efficient carrier diffusion in the products. Above a certain threshold (1.0 mM doping), increasing the Cu dopant concentration diminished the photocatalytic activity. We found that improvements in photocatalytic activity were optimized at Cu dopant levels between 0.2 and 0.4 atm.%. This approach is potentially applicable for coating visible light photocatalysts onto a technologically important substrate with a low melting-point, such as glass substrates, plastics, and polymers.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/ma16020639/s1, Figure S1: XRD patterns of undoped (black) and Cu-doped TiO2 (colors). In panel (a), different copper salts, nitrate, chloride, and acetate are denoted by blue, green, and red, respectively. In panel (b), red, pink, and blue colors indicate different concentrations, 0.1, 1.0 and 10.0 mM, respectively.; Figure S2: SEM images of TiO2 coated on a glass substrate (a) with and (b) without hydrophilization treatment by UV illumination; Figure S3: Production of CO2 during the decomposition of acetaldehyde. Table S1: Peak positions and FWHM values obtained by fitting XPS data.

Author Contributions

M.H.: Conceptualization, Methodology, Data Analysis, and Writing. T.O.: Editing and Photocatalysis Evaluation. P.L.: Sample Preparation and Characterization. Y.Y.: Sample Preparation and Characterization. Y.I.: Supervision and Editing. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financially supported by The Hibi Science Foundation and The Iketani Science and Technology Foundation (0341180-A).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The Raman spectroscopy, SEM-EDS, and EPMA measurements were supported by the Equipment Sharing Division, Organization for Co-Creation Research and Social Contributions, Nagoya Institute of Technology.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Fujishima, A.; Zhang, X.; Tryk, D.A. TiO2 Photocatalysis and Related Surface Phenomena. Surf. Sci. Rep. 2008, 63, 515–582. [Google Scholar] [CrossRef]
  2. Dhakshinamoorthy, A.; Navalon, S.; Corma, A.; Garcia, H. Photocatalytic CO2 Reduction by TiO2 and Related Titanium Containing Solids. Energy Environ. Sci. 2012, 5, 9217. [Google Scholar] [CrossRef]
  3. Carcel, R.A.; Andronic, L.; Duta, A. Photocatalytic Activity and Stability of TiO2 and WO3 Thin Films. Mater. Charact. 2012, 70, 68–73. [Google Scholar] [CrossRef]
  4. Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37–38. [Google Scholar] [CrossRef] [PubMed]
  5. Mino, L.; Negri, C.; Santalucia, R.; Cerrato, G.; Spoto, G.; Martra, G. Morphology, Surface Structure and Water Adsorption Properties of TiO2 Nanoparticles: A Comparison of Different Commercial Samples. Molecules 2020, 25, 4605. [Google Scholar] [CrossRef]
  6. Augugliaro, V.; Bellardita, M.; Loddo, V.; Palmisano, G.; Palmisano, L.; Yurdakal, S. Overview on Oxidation Mechanisms of Organic Compounds by TiO2 in Heterogeneous Photocatalysis. J. Photochem. Photobiol. C Photochem. Rev. 2012, 13, 224–245. [Google Scholar] [CrossRef] [Green Version]
  7. Lonnen, J.; Kilvington, S.; Kehoe, S.C.; Al-Touati, F.; McGuigan, K.G. Solar and Photocatalytic Disinfection of Protozoan, Fungal and Bacterial Microbes in Drinking Water. Water Res. 2005, 39, 877–883. [Google Scholar] [CrossRef]
  8. Herrmann, J.-M. Heterogeneous Photocatalysis: Fundamentals and Applications to the Removal of Various Types of Aqueous Pollutants. Catal. Today 1999, 53, 115–129. [Google Scholar] [CrossRef]
  9. Hager, S.; Bauer, R. Heterogeneous Photocatalytic Oxidation of Organics for Air Purification by near UV Irradiated Titanium Dioxide. Chemosphere 1999, 38, 1549–1559. [Google Scholar] [CrossRef]
  10. Ochiai, T.; Aoki, D.; Saito, H.; Akutsu, Y.; Nagata, M. Analysis of Adsorption and Decomposition of Odour and Tar Components in Tobacco Smoke on Non-Woven Fabric-Supported Photocatalysts. Catalysts 2020, 10, 304. [Google Scholar] [CrossRef]
  11. Takanabe, K. Photocatalytic Water Splitting: Quantitative Approaches toward Photocatalyst by Design. ACS Catal. 2017, 7, 8006–8022. [Google Scholar] [CrossRef]
  12. Takanabe, K.; Domen, K. Toward Visible Light Response: Overall Water Splitting Using Heterogeneous Photocatalysts. Green 2011, 1, 313–322. [Google Scholar] [CrossRef]
  13. Khlyustova, A.; Sirotkin, N.; Kusova, T.; Kraev, A.; Titov, V.; Agafonov, A. Doped TiO2: The Effect of Doping Elements on Photocatalytic Activity. Mater. Adv. 2020, 1, 1193–1201. [Google Scholar] [CrossRef]
  14. Nur, A.S.M.; Sultana, M.; Mondal, A.; Islam, S.; Robel, F.N.; Islam, A.; Sumi, M.S.A. A Review on the Development of Elemental and Codoped TiO2 Photocatalysts for Enhanced Dye Degradation under UV–Vis Irradiation. J. Water Process Eng. 2022, 47, 102728. [Google Scholar] [CrossRef]
  15. Lu, C.-M.; Sharma, R.K.; Lin, P.-Y.; Huang, Y.-H.; Chen, J.-S.; Lee, W.-C.; Chen, C.-Y. Characteristics of Doped TiO2 Nanoparticle Photocatalysts Prepared by the Rotten Egg White. Materials 2022, 15, 4231. [Google Scholar] [CrossRef] [PubMed]
  16. Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Visible-Light Photocatalysis in Nitrogen-Doped Titanium Oxides. Science 2001, 293, 269–271. [Google Scholar] [CrossRef] [PubMed]
  17. 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 2020, 10, 251. [Google Scholar] [CrossRef] [Green Version]
  18. Yu, H.; Irie, H.; Hashimoto, K. Conduction Band Energy Level Control of Titanium Dioxide: Toward an Efficient Visible-Light-Sensitive Photocatalyst. J. Am. Chem. Soc. 2010, 132, 6898–6899. [Google Scholar] [CrossRef]
  19. Huang, F.; Yan, A.; Zhao, H. Influences of Doping on Photocatalytic Properties of TiO2 Photocatalyst. In Semiconductor Photocatalysis—Materials, Mechanisms and Applications; IntechOpen: London, UK, 2016. [Google Scholar]
  20. Karunakaran, C.; Abiramasundari, G.; Gomathisankar, P.; Manikandan, G.; Anandi, V. Cu-Doped TiO2 Nanoparticles for Photocatalytic Disinfection of Bacteria under Visible Light. J. Colloid Interface Sci. 2010, 352, 68–74. [Google Scholar] [CrossRef]
  21. Krishnakumar, V.; Boobas, S.; Jayaprakash, J.; Rajaboopathi, M.; Han, B.; Louhi-Kultanen, M. Effect of Cu Doping on TiO2 Nanoparticles and Its Photocatalytic Activity under Visible Light. J. Mater. Sci. Mater. Electron. 2016, 27, 7438–7447. [Google Scholar] [CrossRef]
  22. Alotaibi, A.M.; Williamson, B.A.D.; Sathasivam, S.; Kafizas, A.; Alqahtani, M.; Sotelo-Vazquez, C.; Buckeridge, J.; Wu, J.; Nair, S.P.; Scanlon, D.O.; et al. Enhanced Photocatalytic and Antibacterial Ability of Cu-Doped Anatase TiO2 Thin Films: Theory and Experiment. ACS Appl. Mater. Interfaces 2020, 12, 15348–15361. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  23. Mathew, S.; Ganguly, P.; Rhatigan, S.; Kumaravel, V.; Byrne, C.; Hinder, S.J.; Bartlett, J.; Nolan, M.; Pillai, S.C. Cu-Doped TiO2: Visible Light Assisted Photocatalytic Antimicrobial Activity. Appl. Sci. 2018, 8, 2067. [Google Scholar] [CrossRef] [Green Version]
  24. Slamet, N.H.W.; Purnama, E.; Kosela, S.; Gunlazuardi, J. Photocatalytic Reduction of CO2 on Copper-Doped Titania Catalysts Prepared by Improved-Impregnation Method. Catal. Commun. 2005, 6, 313–319. [Google Scholar] [CrossRef]
  25. Colón, G.; Maicu, M.; Hidalgo, M.C.; Navío, J.A. Cu-Doped TiO2 Systems with Improved Photocatalytic Activity. Appl. Catal. B 2006, 67, 41–51. [Google Scholar] [CrossRef]
  26. Mingmongkol, Y.; Trinh, D.T.T.; Phuinthiang, P.; Channei, D.; Ratananikom, K.; Nakaruk, A.; Khanitchaidecha, W. Enhanced Photocatalytic and Photokilling Activities of Cu-Doped TiO2 Nanoparticles. Nanomaterials 2022, 12, 1198. [Google Scholar] [CrossRef] [PubMed]
  27. Xia, X.H.; Gao, Y.; Wang, Z.; Jia, Z.J. Structure and Photocatalytic Properties of Copper-Doped Rutile TiO2 Prepared by a Low-Temperature Process. J. Phys. Chem. Solids 2008, 69, 2888–2893. [Google Scholar] [CrossRef]
  28. Li, G.; Dimitrijevic, N.M.; Chen, L.; Rajh, T.; Gray, K.A. Role of Surface/Interfacial Cu2+ Sites in the Photocatalytic Activity of Coupled CuO−TiO2 Nanocomposites. J. Phys. Chem. C 2008, 112, 19040–19044. [Google Scholar] [CrossRef]
  29. López, R.; Gómez, R.; Llanos, M.E. Photophysical and Photocatalytic Properties of Nanosized Copper-Doped Titania Sol–Gel Catalysts. Catal. Today 2009, 148, 103–108. [Google Scholar] [CrossRef]
  30. Zhang, W.; Li, Y.; Zhu, S.; Wang, F. Copper Doping in Titanium Oxide Catalyst Film Prepared by Dc Reactive Magnetron Sputtering. Catal. Today 2004, 93–95, 589–594. [Google Scholar] [CrossRef]
  31. Bhattacharyya, K.; Mane, G.P.; Rane, V.; Tripathi, A.K.; Tyagi, A.K. Selective CO2 Photoreduction with Cu-Doped TiO2 Photocatalyst: Delineating the Crucial Role of Cu-Oxidation State and Oxygen Vacancies. J. Phys. Chem. C 2021, 125, 1793–1810. [Google Scholar] [CrossRef]
  32. Razali, M.H.; Ahmad-Fauzi, M.N.; Mohamed, A.R.; Sreekantan, S. Hydrothermal synthesis and characterisation of Cu doped TiO2 nanotubes for photocatalytic degradation of methyl orange. Adv. Mat. Res. 2014, 911, 126–130. [Google Scholar] [CrossRef]
  33. Wu, M.-C.; Wu, P.-Y.; Lin, T.-H.; Lin, T.-F. Photocatalytic Performance of Cu-Doped TiO2 Nanofibers Treated by the Hydrothermal Synthesis and Air-Thermal Treatment. Appl. Surf. Sci. 2018, 430, 390–398. [Google Scholar] [CrossRef]
  34. Feng, S.-H.; Li, G.-H. Hydrothermal and Solvothermal Syntheses. In Modern Inorganic Synthetic Chemistry; Elsevier: Amsterdam, The Netherlands, 2017; pp. 73–104. [Google Scholar]
  35. Deki, S.; Aoi, Y.; Yanagimoto, H.; Ishii, K.; Akamatsu, K.; Mizuhata, M.; Kajinami, A. Preparation and Characterization of Au-Dispersed TiO2 Thin Films by a Liquid-Phase Deposition Method. J. Mater. Chem. 1996, 6, 1879. [Google Scholar] [CrossRef]
  36. Maki, H.; Okumura, Y.; Ikuta, H.; Mizuhata, M. Ionic Equilibria for Synthesis of TiO2 Thin Films by the Liquid-Phase Deposition. J. Phys. Chem. C 2014, 118, 11964–11974. [Google Scholar] [CrossRef]
  37. Herbig, B.; Löbmann, P. TiO2 Photocatalysts Deposited on Fiber Substrates by Liquid Phase Deposition. J. Photochem. Photobiol. A Chem. 2004, 163, 359–365. [Google Scholar] [CrossRef]
  38. Yu, H.; Lee, S.C.; Yu, J.; Ao, C.H. Photocatalytic Activity of Dispersed TiO2 Particles Deposited on Glass Fibers. J. Mol. Catal. A Chem. 2006, 246, 206–211. [Google Scholar] [CrossRef]
  39. Listiani, P.S.; Honda, M.; Oya, H.; Horio, Y.; Ichikawa, Y. Optimization of Hydrolysis Temperature in Liquid Phase Deposition for TiO2 Photocatalysis. Jpn. J. Appl. Phys. 2022, 61, 075508. [Google Scholar] [CrossRef]
  40. Li, H.; Liu, B.; Yin, S.; Sato, T.; Wang, Y. Visible Light-Driven Photocatalytic Activity of Oleic Acid-Coated TiO2 Nanoparticles Synthesized from Absolute Ethanol Solution. Nanoscale Res. Lett. 2015, 10, 415. [Google Scholar] [CrossRef] [Green Version]
  41. Challagulla, S.; Tarafder, K.; Ganesan, R.; Roy, S. Structure Sensitive Photocatalytic Reduction of Nitroarenes over TiO2. Sci. Rep. 2017, 7, 8783. [Google Scholar] [CrossRef]
  42. Patterson, A.L. The Scherrer Formula for X-Ray Particle Size Determination. Phys. Rev. 1939, 56, 978–982. [Google Scholar] [CrossRef]
  43. Durante, O.; Di Giorgio, C.; Granata, V.; Neilson, J.; Fittipaldi, R.; Vecchione, A.; Carapella, G.; Chiadini, F.; DeSalvo, R.; Dinelli, F.; et al. Emergence and Evolution of Crystallization in TiO2 Thin Films: A Structural and Morphological Study. Nanomaterials 2021, 11, 1409. [Google Scholar] [CrossRef] [PubMed]
  44. Liu, M.; Yu, S.; He, L.; Ni, Y. Recent Progress on Crack Pattern Formation in Thin Films. Soft Matter 2022, 18, 5906–5927. [Google Scholar] [CrossRef] [PubMed]
  45. Louwerse, M.J.; Piccinini, M.; De Jong, K.P. Diffusion Mechanisms for Ions over Hydroxylated Surfaces: Cu on γ-Al 2 O 3. J. Phys. Chem. C 2019, 123, 18502–18507. [Google Scholar] [CrossRef] [Green Version]
  46. Liu, D.; Flewitt, P.E.J. Chapter 5. Raman Measurements of Stress in Films and Coatings. Prop. Inorg. Organomet. Compd. 2004, 45, 141–177. [Google Scholar]
  47. Scanlon, D.O.; Dunnill, C.W.; Buckeridge, J.; Shevlin, S.A.; Logsdail, A.J.; Woodley, S.M.; Catlow, C.R.A.; Powell, M.J.; Palgrave, R.G.; Parkin, I.P.; et al. Band Alignment of Rutile and Anatase TiO2. Nat. Mater. 2013, 12, 798–801. [Google Scholar] [CrossRef] [PubMed]
  48. Brus, L. Electronic Wave Functions in Semiconductor Clusters: Experiment and Theory. J. Phys. Chem. 1986, 90, 2555–2560. [Google Scholar] [CrossRef]
  49. Thompson, T.L.; Yates, J.T. TiO2-Based Photocatalysis: Surface Defects, Oxygen and Charge Transfer. Top. Catal. 2005, 35, 197–210. [Google Scholar] [CrossRef]
  50. Honda, M.; Saito, Y.; Kawata, S. Individual TiO2 Nanocrystals Probed by Resonant Rayleigh Scattering Spectroscopy. Appl. Phys. Express 2014, 7, 112402. [Google Scholar] [CrossRef]
  51. Reckers, P.; Dimamay, M.; Klett, J.; Trost, S.; Zilberberg, K.; Riedl, T.; Parkinson, B.A.; Brötz, J.; Jaegermann, W.; Mayer, T. Deep and Shallow TiO2 Gap States on Cleaved Anatase Single Crystal (101) Surfaces, Nanocrystalline Anatase Films, and ALD Titania Ante and Post Annealing. J. Phys. Chem. C 2015, 119, 9890–9898. [Google Scholar] [CrossRef]
  52. Zhu, L.; Lu, Q.; Lv, L.; Wang, Y.; Hu, Y.; Deng, Z.; Lou, Z.; Hou, Y.; Teng, F. Ligand-Free Rutile and Anatase TiO2 Nanocrystals as Electron Extraction Layers for High Performance Inverted Polymer Solar Cells. RSC Adv. 2017, 7, 20084–20092. [Google Scholar] [CrossRef] [Green Version]
  53. Jackman, M.J.; Thomas, A.G.; Muryn, C. Photoelectron Spectroscopy Study of Stoichiometric and Reduced Anatase TiO2 (101) Surfaces: The Effect of Subsurface Defects on Water Adsorption at Near-Ambient Pressures. J. Phys. Chem. C 2015, 119, 13682–13690. [Google Scholar] [CrossRef]
  54. You, M.; Kim, T.G.; Sung, Y.-M. Synthesis of Cu-Doped TiO2 Nanorods with Various Aspect Ratios and Dopant Concentrations. Cryst. Growth Des. 2010, 10, 983–987. [Google Scholar] [CrossRef]
  55. Liu, M.; Qiu, X.; Miyauchi, M.; Hashimoto, K. Cu(II) Oxide Amorphous Nanoclusters Grafted Ti3+ Self-Doped TiO2: An Efficient Visible Light Photocatalyst. Chem. Mater. 2011, 23, 5282–5286. [Google Scholar] [CrossRef]
  56. Hu, Q.; Huang, J.; Li, G.; Chen, J.; Zhang, Z.; Deng, Z.; Jiang, Y.; Guo, W.; Cao, Y. Effective Water Splitting Using CuOx/TiO2 Composite Films: Role of Cu Species and Content in Hydrogen Generation. Appl. Surf. Sci. 2016, 369, 201–206. [Google Scholar] [CrossRef]
  57. Francisco, M.S.P.; Mastelaro, V.R.; Nascente, P.A.P.; Florentino, A.O. Activity and Characterization by XPS, HR-TEM, Raman Spectroscopy, and BET Surface Area of CuO/CeO2-TiO2 Catalysts. J. Phys. Chem. B 2001, 105, 10515–10522. [Google Scholar] [CrossRef]
Materials 16 00639 g001 550
Figure 1. Schematic illustration of experimental procedure for producing Cu-doped TiO2 powders and coatings via liquid phase deposition.
Figure 1. Schematic illustration of experimental procedure for producing Cu-doped TiO2 powders and coatings via liquid phase deposition.
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Figure 2. SEM images of (a) undoped and (b,d) Cu-doped TiO2 particles (10 mM): (b–d) nitrate, chloride, and acetate, respectively. Scale bar indicates 200 nm.
Figure 2. SEM images of (a) undoped and (b,d) Cu-doped TiO2 particles (10 mM): (b–d) nitrate, chloride, and acetate, respectively. Scale bar indicates 200 nm.
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Figure 3. Raman spectra of undoped (black) TiO2 and TiO2 doped with nitrate (blue), chloride (green), and acetate (red). The spectral intensity was normalized.
Figure 3. Raman spectra of undoped (black) TiO2 and TiO2 doped with nitrate (blue), chloride (green), and acetate (red). The spectral intensity was normalized.
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Figure 4. SEM images of Cu-doped TiO2 particles with different dopant concentrations: (a) 0.1, (b) 1, and (c) 10 mM. Scale bar indicates 200 nm.
Figure 4. SEM images of Cu-doped TiO2 particles with different dopant concentrations: (a) 0.1, (b) 1, and (c) 10 mM. Scale bar indicates 200 nm.
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Figure 5. SEM images of Cu-doped TiO2 coatings with different dopant concentrations: (a) 0, (b) 0.1, (c) 1, and (d) 10 mM. Scale bar indicates 2 μm.
Figure 5. SEM images of Cu-doped TiO2 coatings with different dopant concentrations: (a) 0, (b) 0.1, (c) 1, and (d) 10 mM. Scale bar indicates 2 μm.
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Materials 16 00639 g006 550
Figure 6. Size distribution of particles prepared with different concentrations of Cu nitrate: (a) 0.1, (b) 1, and (c) 10 mM.
Figure 6. Size distribution of particles prepared with different concentrations of Cu nitrate: (a) 0.1, (b) 1, and (c) 10 mM.
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Materials 16 00639 g007 550
Figure 7. Raman spectra of Cu-TiO2 (a) powder and (b) coating samples. Black line indicates undoped TiO2. Red, pink, and blue lines correspond to Cu concentrations of 0.1, 1.0, and 10 mM, respectively.
Figure 7. Raman spectra of Cu-TiO2 (a) powder and (b) coating samples. Black line indicates undoped TiO2. Red, pink, and blue lines correspond to Cu concentrations of 0.1, 1.0, and 10 mM, respectively.
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Figure 8. Concentration dependence on (a) peak wavenumber of Raman Eg mode and (b) spectral width. Red and blue indicate coating and powder samples, respectively.
Figure 8. Concentration dependence on (a) peak wavenumber of Raman Eg mode and (b) spectral width. Red and blue indicate coating and powder samples, respectively.
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Figure 9. SEM and EDS mapping of (a) undoped and (b) Cu doped TiO2. Cu nitrate concentration was 10 mM.
Figure 9. SEM and EDS mapping of (a) undoped and (b) Cu doped TiO2. Cu nitrate concentration was 10 mM.
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Figure 10. Elemental mapping of (a) undoped and (b) Cu doped TiO2 coated on glass substrate. Cu nitrate concentration was 10 mM.
Figure 10. Elemental mapping of (a) undoped and (b) Cu doped TiO2 coated on glass substrate. Cu nitrate concentration was 10 mM.
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Figure 11. Atomic concentration of Cu incorporated into powder and coating samples.
Figure 11. Atomic concentration of Cu incorporated into powder and coating samples.
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Figure 12. Tauc plots of samples prepared with 0 (black), 0.1 (red), 1.0 (pink), and 1.0 (blue) mM Cu nitrate.
Figure 12. Tauc plots of samples prepared with 0 (black), 0.1 (red), 1.0 (pink), and 1.0 (blue) mM Cu nitrate.
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Figure 13. XPS spectra of Cu-doped TiO2 prepared with 10 mM Cu nitrate. (a,b) Narrow scans for Ti 2p and Cu 2p, respectively. (b) Green, red, and gray lines indicate Cu2O, CuO, and Cu(NO3)2, respectively.
Figure 13. XPS spectra of Cu-doped TiO2 prepared with 10 mM Cu nitrate. (a,b) Narrow scans for Ti 2p and Cu 2p, respectively. (b) Green, red, and gray lines indicate Cu2O, CuO, and Cu(NO3)2, respectively.
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Figure 14. Photocatalytic decomposition of acetaldehyde by undoped (black) and Cu doped TiO2 (colors) coated on a glass substrate. Red, pink, and blue curves denote 0.1, 1.0, and 10.0 mM Cu nitrate used for synthesis, respectively.
Figure 14. Photocatalytic decomposition of acetaldehyde by undoped (black) and Cu doped TiO2 (colors) coated on a glass substrate. Red, pink, and blue curves denote 0.1, 1.0, and 10.0 mM Cu nitrate used for synthesis, respectively.
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Figure 15. Action and absorption spectrum of Cu-TiO2 prepared with 10.0 mM of Cu nitrate. Thermal treatment under nitrogen for 1 h was performed after synthesis. Inset shows photograph of doped sample.
Figure 15. Action and absorption spectrum of Cu-TiO2 prepared with 10.0 mM of Cu nitrate. Thermal treatment under nitrogen for 1 h was performed after synthesis. Inset shows photograph of doped sample.
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Table
Table 1. Dependence of FWHM values on Cu salt species.
Table 1. Dependence of FWHM values on Cu salt species.
SoluteFWHM
(cm−1)
-30.2
Cu(NO3)230.0
CuCl231.8
Cu(CH3COO)234.5
Table
Table 2. Concentration dependence on atomic concentration of Cu and bandgaps.
Table 2. Concentration dependence on atomic concentration of Cu and bandgaps.
Conc. (mM)Atm.%Bandgap (eV)
0.00.03.30
0.10.23.30
1.00.43.15
101.32.90
Table
Table 3. Average rate constant for undoped TiO2 and Cu-TiO2.
Table 3. Average rate constant for undoped TiO2 and Cu-TiO2.
Cu Nitrate
(mM)		Rate Constant k
(ppm/min.)		Std. Error
(ppm/min.)
0.00.001310.00025
0.10.001640.00011
1.00.001560.00012
100.000680.00006
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Honda, M.; Ochiai, T.; Listiani, P.; Yamaguchi, Y.; Ichikawa, Y. Low-Temperature Synthesis of Cu-Doped Anatase TiO2 Nanostructures via Liquid Phase Deposition Method for Enhanced Photocatalysis. Materials 2023, 16, 639. https://doi.org/10.3390/ma16020639

AMA Style

Honda M, Ochiai T, Listiani P, Yamaguchi Y, Ichikawa Y. Low-Temperature Synthesis of Cu-Doped Anatase TiO2 Nanostructures via Liquid Phase Deposition Method for Enhanced Photocatalysis. Materials. 2023; 16(2):639. https://doi.org/10.3390/ma16020639

Chicago/Turabian Style

Honda, Mitsuhiro, Tsuyoshi Ochiai, Popy Listiani, Yuma Yamaguchi, and Yo Ichikawa. 2023. "Low-Temperature Synthesis of Cu-Doped Anatase TiO2 Nanostructures via Liquid Phase Deposition Method for Enhanced Photocatalysis" Materials 16, no. 2: 639. https://doi.org/10.3390/ma16020639

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