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Article

Facile Synthesis of Potassium-Doped Titanium Oxide Nanostructure (KTiOxs)/AlO(OH) Composites for Enhanced Photocatalytic Performance

by
So Yoon Lee
1,
Tatsuya Matsubara
1,
Daiki Numata
2 and
Ai Serizawa
1,*
1
Department of Materials Science and Engineering, College of Engineering, Shibaura Institute of Technology, Toyosu, Koto-ku, Tokyo 135-8548, Japan
2
Graduate School of Engineering and Science, Shibaura Institute of Technology, Toyosu, Koto-ku, Tokyo 135-8548, Japan
*
Author to whom correspondence should be addressed.
Catalysts 2021, 11(5), 548; https://doi.org/10.3390/catal11050548
Submission received: 16 August 2020 / Revised: 22 April 2021 / Accepted: 23 April 2021 / Published: 25 April 2021
(This article belongs to the Section Catalytic Materials)
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Abstract

:
Generally, nanoparticles (NPs) are used as photocatalysts, which sometimes results in difficulties in the separation and recycling of photocatalysts from suspensions after their application in water and wastewater treatment, which hinders industrial applications of NPs that are too fine to be removed by gravitational settling. This can be solved by using support NPs to overcome these problems. -OH enrich AlO(OH), which is produced by a steam coating process, has been could be used as a possible support, because the -OH groups on the surface can interact with foreign molecules; thus, various composite functional materials can be prepared. Potassium doped titanium oxide NPs, which are produced by a wet corrosion process, namely KTiOxs, have been selected as photocatalysts, because KTiOxs have sufficient K+ ions, thereby expecting the chemical bonding with -OH group from AlO(OH). This study fabricated a novel photocataysis system made by combining KTiOxs as catalysts and AlO(OH) as the catalysts’ support, namely KTiOxs/AlO(OH) composites. The KTiOxs nanowires, obtained from 10 mol/L of a KOH solution treated with Ti and AlO(OH) at 280 °C for 24 h through a steam coating process, yielded the highest surface area and the highest photocatalytic performance.

1. Introduction

The optical and electrical properties and catalytic performance of materials strongly depend on their morphology and structure. The high quality of nanostructures with controlled crystalline, grain, and surface architectures is of great fundamental and technological interest because of their promising new properties, which can be valuable for advanced materials [1,2,3,4,5,6,7,8]. In particular, the hierarchical self-assembly of nanoscale materials (e.g., particles, cluster, wires, belts, sheets, and tubes) into novel architectures of a higher dimensionality is highly desirable because these structures could have new properties that can be meaningful for the fabrication of advanced nanodevices and in other nanotechnology applications [9,10]. Several researchers have studied the optical and electrical properties and catalytic performances of materials using different semiconductors, including single catalysts like TiO2 and ZrO2; double catalysts, like Cu–Fe/TiO2–SiO2, and Cu–ZnO/Pt–K2Ti6O13; and metal and compound oxides, such as CuO, LaCoO3, Ga2O3, and ATaO3, where A represents Na, Li, and K [11,12,13,14,15,16,17].
Titanium oxide (TiOx) materials have attracted considerable interest in many fields of research, including the fields of photovoltaic and photocatalysis [18,19,20,21]. In particular, the photocatalysis of a TiOx semiconductor has been actively studied and applied in environmental decontamination, such as air purification, water disinfection, hazardous waste remediation, and water purification, because of its biological and chemical inertness, strong oxidizing power, cost effectiveness, and long-term stability against photocorrosion and chemical corrosion [22,23,24,25,26]. The TiO2 group is composed of rutile, anatase, and brookite, and their band gap is around 3.2 eV [19,23,27,28,29,30]. In particular, nanostructured alkali metal incorporated titanates containing A–Ti–O (A indicates alkali element and/or H) bonds have been produced as nanotubes, nanorods, nanofibers, and nanosheets.
They have attracted considerable interest because of their unique layered structure. Among them, potassium (K)-incorporated TiO2 is of particular interest because of its specific photochemical properties and artificial cage type structure. Although these K-incorporated nanostructured titanium oxides films (nTOFs) have been one of the leaders in this new class of materials, their synthesis still has limitations to tackle [2]. So far, there is plenty of research into enhancing the photocatalytic activity; one effective way of doing this is to minimize particle size [2,31,32]. Generally, particles are used as photocatalysts, and nanoscale particles present some difficulties in the separation and recycling of photocatalysts from suspensions after their application in water and wastewater treatment, which hinders industrial applications of TiO2 nanoparticles that are too fine to be removed by gravitational settling because of the strong Brownian motion [33,34]. In order to overcome these problems, structures that can contain nanoscale particles have been considered.
Herein, aluminum (Al)-based materials are considered attractive because of their ultra-lightness, cost effectiveness, and so on. Generally, Al-based materials have superior mechanical and physical properties, making them ideal for use as structural materials. These properties include a high strength-to-weight ratio, good formability, high thermal conductivity, and excellent recyclability. In this way, Al-based materials have been used for numerous applications in a wide range of industries, including automobiles, building, and heat-transfer components [35,36].
Among them, boehmite (γ-AlO(OH)) is one of the most important materials and has attracted a lot of attention [37]. Generally, AlO(OH) has many advantages, such as a high resistance, cost effectiveness, and diversity of fabrication for various morphologies. In particular, the most significant property is the following: as AlO(OH) has a large number of -OH groups on the surface, AlO(OH) is inclined to interact with foreign molecules, and thus various functional composite materials can be prepared [38,39].
Based on these material properties, AlO(OH) has the potential to be a support for nanoparticles. In order to use it with high-quality Ti-based catalysts, potassium-doped titanium oxide nanoparticles, namely KTiOxs, are suitable as counter materials, because KTiOxs have sufficient K+ ions, thereby enabling chemical bonding with the -OH group from AlO(OH). In our previous research, KTiOxs nanowires could be obtained on a metal surface through a wet corrosion process (WCP) [2,40]. WCP relies on potassium hydroxide (KOH) treatment of Ti metals, and has demonstrated a high efficiency and high reproducibility in producing a nanostructured surface with a high surface area and with varying potassium doping. The produced KTiOxs nanowires showed high photocatalytic performances [2,40]. WCP is a simple one-step green synthesis method done at room-temperature without any supplementary treatments. The formation of KTiOxs nanowires is directly affected by the solution concentration, which also changes the K content in the nanostructures formed on the surface. Simultaneously, the surface area and photocatalytic activity scales can be changed with the KOH concentration. Thus, WCP is a promising method for the efficient and well-tunable production of KTiOxs nanowires as catalysts.
In order to synthesize AlO(OH) as a support for catalysts, recently, a stem coating process has been created to produce AlO(OH) [41,42]. Generally, the steam coating process allows for producing a dense film on the surface of a metal using the reaction of Al with water, which is synthesized by a pressure such as an autoclave. Note that the steam coating process uses only water; therefore, this is a simple and eco-friendly method to synthesize the oxide layer. The oxide layer can be grown directly on a substrate, so that the oxide layer exhibits excellent adhesion to the substrate. Furthermore, the process can be applied to components with complicated shapes and large substrates. In addition, the steam coating process does not require any pre-treatment, and so the fabrication step can be reduced. AlO(OH) is one of the main hydroxide layers synthesized through this process. It is another big challenge to apply this method for photocatalyst support.
This study aimed to fabricate a novel photocataysist system made using a combination of KTiOxs as the catalyst and AlO(OH) as the catalyst support, namely KTiOxs/AlO(OH) composites, in order to easily separate them from their suspension after their application in water and wastewater treatment. In addition, this study systematically investigated the best optimized conditions for photocatalysis within this system using various conditions of KTiOxs/AlO(OH) composites.

2. Results and Discussions

Figure 1 shows the FE-SEM images of the surfaces of the Al substrates after a steam coating process at 180 °C and 280 °C for 24 h. A thin film was clearly observed on the surface after applying the steam coating process at 180 °C for 24 h (Figure 1a). However, a crystalized thick film was observed on the surface after being subjected to a steam coating process at 280 °C for 24 h (Figure 1b). Both of the films fully covered the surface, and the distribution of the crystal size was small, regardless of the process conditions. The average length and width of the crystals of the thin film was < 220 ± 80 nm and < 20 ± 30 nm, respectively, while the average length and width of the lath-like crystal of thick film was < 650 ± 75 nm and < 65 ± 30 nm, respectively. Note that these values were calculated using n = 20 within the FE-SEM images. It is worthwhile that both the length and width can be controlled spontaneously according to the steam coating process temperature.
Figure 2 shows the XRD profiles of the obtained AlO(OH) films that were prepared through the steam coating process at 180 °C and 280 °C for 24 h. Several peaks corresponding to aluminum hydroxide, including AlO(OH), were observed in all of the specimens. Compared with the AlO(OH) film that was prepared at 180 °C for 24 h, the crystallinity of the AlO(OH) film that was prepared at 280 °C for 24 h was higher.
It can be explained that this phenomenon is the reaction that balanced the water and Al during the chemical reaction of the steam coating process. The chemical reaction between Al and water likely occurs as follows.
2Al + 4H2O → 2AlO(OH) + 3H2
It has been reported that AlO(OH) is at its most stable form from around 280 °C [43]. The reaction environment of the steam coating process was balanced by subcritical water in the liquid phase and saturated steam in the gas phase. Note that the state water is presented in a general pressure–temperature diagram. Subcritical water exists at temperatures between the atmospheric boiling point and the critical temperature of water. Therefore, the most impactful chemical reaction happens through the formation of AlO(OH). Here, this is worthwhile, because the large number of -OH groups on the surface were induced by this reaction, which are expected to interact with foreign molecules in order to fabricate a variety of composite functional materials. Therefore, the steam coating process condition can be completed through this reaction for this work.
Figure 3 shows the surface area of the obtained the AlO(OH) crystals that were prepared at 180 °C and 280 °C for 24 h by AFM. Figure 3a shows the 3D images of the obtained AlO(OH) surfaces. After the steam coating process, synthesized crystals are clearly observed compared with the untreated sample. Based on this observation, a high surface area could be achieved by applying a steam coating process at 280 °C for 24 h, because the microstructures of the lath crystals have a bumpy geometry, which could increase the surface area (Figure 3b). So, we can conclude that a high temperature treatment condition for the steam coating process can increase the surface area.
From here, we would like to demonstrate the results of KTiOxs nanowires as photocatalysts. Figure 4 shows the FE-SEM and TEM images of the KTiOxs nanowires obtained by WCP. Compared with the untreated sample, unique nanowires were observed on the surface of the Ti particles after WCP, as in previous works [2,40]. The initial Ti materials were 200 µm in size. After KOH treatment, the nanowires were synthesized on the Ti surface, and each of the nanowires were around 200 nm in size. Based on these results, it can be confirmed that Ti particles can successfully modify nanostructures through WCP. Figure 4d shows a TEM image of the KTiOx nanowires with a diameter in the range of 10–30 nm. The presence of K in the individual KTiOx nanowires was confirmed by STEM-based EDX mapping. A quantitative analysis yielded a K-concentration of about 45±11 wt.%.
The Raman spectra of the obtained KTiOxs nanowires are presented in Figure 5 New peaks were observed for the potassium titanate phase (280 cm−1), namely KTiOxs, caused mainly by the structural changes in TiO2 [44,45]. The synthesis of the KTiOxs involved the rearrangement of the Ti–O bonds. Potassium atoms were inserted in the space between the Ti–O bonds. The Ti–O–K bonds were formed with some of the broken Ti–O bonds; the other Ti–O bonds were reserved. The peaks at 445, 660, and 903 cm-1 were assigned to different TiO2 phases (anatase, rutile, and brookite) [45,46]. Therefore, this indicates that KTiOxs exhibited the characteristics of a (Ti–O–K) structure unit containing TiO2 phases [45]. A Raman spectroscope with back-scattering configuration was used for examining the structural properties of the films. Raman shifts are affected by the vibration of the electronic polarization of the constituents in the films, which depend on bonding characteristics such as the atomic distance and bonding angle [47]. Therefore, the structure of the product changed when the chemical bond was rearranged. In order to verify the surface chemical composition of the obtained nanostructured oxide films, an X-ray photoelectron spectroscopy (XPS) analysis was performed. The incorporation of K into titanium oxide films could be evidenced by the existence of the K2p peak in the XPS spectrum. The XPS result (Figure 6) shows that the peak of the K element was observed at about 293.5 eV, which was the region of the compound for K+ [2].
Therefore, these results can be expected to make a chemical bonding with -OH group from AlO(OH). The Raman results confirmed that the amount and crystallinity of the obtained KTiOxs nanowires increased at a high concentration for the KOH solution. This result may explain the generation of more chemical bonding between OH− and K+ for suitable composites for photocatalysis.
Figure 7 and Figure 8 show the FE-SEM images and EDX analysis of the obtained KTiOxs/AlO(OH) composites. In the case of the KTiOxs obtained from the 1 mol/L-KOH treatment, it is hard to see the combination of KTiOxs and AlO(OH); however, the composites formed with KTiOxs obtained from the 10 mol/L-KOH treatment are clearly observed. It can be assumed that the K+ ion site hs less 1 mol/L-KOH treated Ti compared with the 10 mol/L-KOH treated Ti, based on the Raman results. Unique nanowires of KTiOxs were observed so, it can be said that this composite was of a combination of KTiOxs and AlO(OH). In detail, an EDX analysis was carried out to confirm this composite. The Ti component is clearly shown in the EDX mapping analysis, indicating that KTiOxs/AlO(OH) composites were successfully obtained during the reaction (Figure 8).
In order to figure out the chemical bonding of the KTiOxs/AlO(OH) composites obtained here, FTIR was carried out (Figure 9). The strong band at 3000–3400 cm−1 and the weak band at 1600–1700 cm−1 can be assigned to the stretching and bending modes of the adsorbed water [48]. Other absorption peaks were observed at around 480–500 cm−1 after the steam coating process, which could be attributed to Al-O symmetric stretching and ring vibration mode in Al-O tetraheda formed through the oxygen atom [49]. In particular, the KTiOxs/AlO(OH) composite reveled the stretching vibration mode of Ti-O (580 cm−1) and Ti-O-Ti (720 cm−1) with Al-O bonding [50]. On the basis of these results, K+ from KTiOxs and the –OH group from AlO(OH) could undergo chemical bonding to fabricate the composite. Therefore, it can be assumed that the initial concept to make the composite using AlO(OH) as a support for KTiOxs is successfully achieved.
In order to examine the photocatalytic activity of the obtained KTiOxs nanowires, the photodegradation of methylene blue (MB) was evaluated under UV light irradiation. The band gap of the obtained KTiOxs is around 2.8~2.9 eV. The UV–VIS absorption spectra presented in Figure 10A demonstrate the decomposition of MB dye, leading to decolorization. For comparison, the absorption spectra of the untreated sample are also presented. The major absorption peaks appeared at about 612 and 664 nm, which is characteristic for MB dye [51]. The major absorption peaks appeared and were used as a direct indication of photocatalytically activated dye degradation. Indeed, all of the obtained KTiOxs specimens showed a higher photocatalytic activity than the untreated samples. This observation was in agreement with the remarkable photocatalytic properties reported for nanostructured titanium oxide materials with an enhanced surface area and increased electron transfer ability [2,52]. Interestingly, the 10 mol/L-KOH-treated Ti specimen showed a higher photocatalytic activity than the 1 mol/L-KOH-treated Ti. The intensity of the MB peaks of the 10 mol/L-KOH treated Ti decreased to a blank level after 30 min of UV exposure. The degradation rate of the 10 mol/L-KOH treated Ti was almost 99% in this study. This result is superior to the reported results (high score of the reported results was 90%) [29,53,54]. Generally, the photocatalytic activity leading to the degradation of dyes depends on the catalyst concentration. The high surface area exerts a synergistic effect on the catalyst concentration. As the higher catalyst concentration creates an increase in adsorbed dye molecules, and a higher photocatalytic performance can be expected [55].
Normally, the photocatalytic process is based on the photogeneration of electron-hole pairs, which will initiate redox reactions with the species adsorbed on the surface of the catalysts. In the photocatalytic process, OH radicals originating from the oxidation of OH- or H2O through the photogenerated electron–hole pairs in the presence of oxygen have been considered as the major reactants responsible for the photocatalytic oxidation of organic materials and the degradation of pollutants [22,23,56]. Consequently, improved electronic properties can be linked to an enhanced photocatalytic activity. In our work, the electronic properties are governed by the amount of Ti–O–K components. From these results, we can conclude that an efficient charge/energy transfer occurs in the KOH-treated samples under photo-irradiation and leads to an improved photocatalytic activity, as evidenced by the drastic diminishment of the UV absorption peaks. These results clearly demonstrate that KTiOx can be a cost-effective, highly efficient, and environmental-friendly photocatalyst.
In order to optimize the best composite conditions for the photocatalytic effect, we prepared four different KTiOxs/AlO(OH) composites, as shown in Figure 11. For comparison, the absorption spectra of bare AlO(OH) specimens prepared by a steam coating process at 180 °C and 280 °C for 24 h were also presented as the control samples. From the results, at first, we can confirm that the KTiOxs/AlO(OH) composites showed a higher photocatalytic activity compared with the bare AlO(OH) specimens because of the existence of KTiOxs. Among the KTiOxs/AlO(OH) composites, the 10 mol/L-KOH treated Ti and the steam coating AlO(OH) composite treated at 280 °C for 24 h showed a remarkable enhanced photocatalytic activity because both the KTiOxs and AlO(OH) had a high surface area. Once again, we assume that a high surface area executes a synergistic effect on the catalyst concentration, because a large amount of dye molecules can be loaded on the surface, resulting in a higher photocatalytic performance [57,58].
In order to optimize the best conditions for the composites, a degradation level calculated by the intensity of the MB was used in this system. The composites used in this study are listed in Table 1.
First, in order to figure out the effect of the surface area of KTiOxs, a higher surface area of AlO(OH) substrates was used. This indicated that the degradation function of MB with 10 mol/L-KOH treated Ti was four times higher than for the 1 mol/L-KOH treated Ti (Figure 11b). From these results, it can be seen that the 10 mol/L-KOH treated Ti catalyst is suitable for a higher photocatalysis performance because of its high surface area. Second, in order to figure out the effect of the surface area of AlO(OH), we used the 10 mol/L-KOH treated Ti as a high performance photocatalyst. From this comparison, a large surface area of AlO(OH) could contain the large amounts of KTiOxs, thereby increasing the photocatalytic activity (Figure 11c). On the basis of these results, the K+ from KTiOxs and OH- from AlO(OH) could create a chemical reaction to make the composite successful, and the best composite condition for the photocatalysis was to use a high surface area for KTiOxs and AlO(OH). Briefly, it can be explained that the best condition for the KTiOxs/AlO(OH) composite is using 10 mol/L-KOH treated Ti as a photocatalyst and steam coating processed AlOOH treated at 280 °C for 24 h as a catalyst support.

3. Materials and Methods

3.1. Preparation of KTiOxs/AlO(OH) Composites for Photocatalysis System

An Al-Mg-Si alloy with a size of 10 mm X 10 mm was used as the substrate for synthesizing AlO(OH) as a support. The chemical composition of the Al alloy is shown in Table 2. The chemical composition is the same as that of AA6061.
The steam coating process was carried out with press micro reactor (MMS-100 OMlabotech) with a volume of 10 m3 using a Teflon container with a 100-mL capacity. AA6061 substrates were placed on a substrate stage, which was made of Teflon tape, 10 mL of ultrapure water was added to the bottom of a Teflon container to produce steam. The autoclave process was carried out by heating up to 180 °C and 240 °C for 24 h, and then was cooled down naturally at room temperature.
In order to synthesize the K-incorporated Ti (KTiOxs) as photocatalysts, Ti microspheres (purity >99.8% and diameter of 125–250 µm) were purchased from Sigma-Aldrich in Japan. The Ti particles were soaked in 5 mL KOH solution with a concentration of 1 and 10 mol/L at room temperature for 24 h. Note that this process is called as WCP. After WCP, all particles were washed with deionized water and dried. These particles contained a Ti core and a shell of KTiOxs nanowires forming a network.
In order to synthesize the KTiOxs/AlO(OH) composites for the photocatalysis system, the obtained the AlO(OH) substrates were attached by a carbon type on the bottom of the container (UG Yamayu, Tokyo, Japan) and mixed with 0.75g of KTiOxs with 4 mL of H2O at 200 rpm for 24 h with shaking. After 24 h, the samples were washed with H2O to remove the unattached KTiOxs. In order to figure out the effect of the surface area of KTiOxs and AlO(OH), we prepared four different composites as follows. The sample names of the composites were recorded in the order of the condition of KTiOxs/AlO(OH), as follows: 1 mol/L-KOH treated Ti/180 °C for 24 h treated Al, 1 mol/L-KOH treated Ti/280 °C for 24 h treated Ti, 10 mol/L-KOH treated Ti/180 °C for 24 h treated Al, and 10 mol/L-KOH treated Ti/180 °C for 24 h treated Ti.

3.2. Evaluation of Photocatalytic Activity of KTiOxs/AlO(OH) Composites

First, in order to evaluate the photocatalytic activity of the KTiOxs that were treated with 1 and 10 mol/L-KOH, with treated Ti as the photocatalysts, 2g of KTiOxs were mixed with 2 mL of methylene blue organic dye (MB; 125 mg/L) solutions. For the photocatalysis, UV irradiation (UV light; GrassyLeDio RX122, Volxjapanv, Tokyoy, Japan) with a 10 cm working distance was carried out for 10, 20, and 30 min. Note that the UV light can react under a 500 nm wavelength. During UV exposure, the mixtures (KTiOxs and MB solutions) were stirred continuously. After we confirmed the activity of the KTiOxs, the photocatalytic activity of the KTiOxs/AlO(OH) composites were carried out. Here, 300 mL of MB dye was drop wised on the surface of KTiOxs/AlO(OH) composites for UV irradiation for 10, 20, and 30 min.

3.3. Characterizations

Changes in the surface structure, shape, and size of the obtained AlO(OH) and KTiOxs nanowires were observed using a field emission scanning electron microscope (FE-SEM; JSM-7610, JEOL Ltd., Tokyo, Japan), which was operated at 15 kV. The crystal phase of the obtained AlO(OH) film was identified using X-ray diffraction (XRD; Smart Lab, Rigaku, Tokyo, Japan), with Cu K radiation (40 kV, 30 mA) within a range of 5°–90° and at a scanning rate of 2θ = 4°/min. The surface area was measured using atomic force microscopy (AFM; AFM5000, HITACHI, Tokyo, Japan) with DFM mode at 20 µm × 20 µm. A Raman spectroscope was used for examining the structural properties of the obtained KTiOxs nanowires. A laser Raman spectrometer (NRS-5100, JASCO, Tokyo, Japan) was used within a range of 100–1500 cm−1 at a 532 nm laser wavelength. For the identification and visualization of the obtained KTiOxs/AlO(OH) composites, an energy-dispersive X-ray (SEM-EDX; JSM-7610, JEOL Ltd., Tokkyo, Japan) analysis was carried out at 20 kV. To investigate the structural properties of the obtained KTiOxs/AlO(OH) composites, Fourier-transform infrared spectroscopy (FT-IR; IRAffinity-1S, Shimadzu Co., Tokyo, Japan) was performed, which was recorded within the range of 400–4000 cm−1, with 20 scans at a resolution of 4 cm−1. After UV irradiation, the photocatalytic activity was evaluated by collecting the UV–VIS absorption spectra (UV–VIS Spectrophotometer; V-630Bio Spectrophotometer, JASCO, Tokyo, Japan) of the solution. The degradation efficiency was recorded as the decreased absorption peak.

4. Conclusions

The preparation of KTiOxs/AlO(OH) composites for photocatalysis was demonstrated. In order to overcome the separation after photocatalysis and water purification using particles catalysts, a support to contain the particle catalysts is needed. Herein, KTiOxs nanoparticles were selected as the photocatalysts, because nanostructured KTiOxs was a promising candidate for photocatalysis in previous work. Based on this concept, AlO(OH) was considered as a support, because the enriched −OH group has a higher potential to create a chemical reaction with the K+ from KTiOxs. In particular, our strategic plan for this project was to enhance the photocatalytic activity as follws: (1) to make a large surface area of support to load more catalysts and (2) to make a large surface area of catalysts to enlarge the concentration of dye molecules. In this contribution, AlO(OH) as a support was prepared by a steam coating process at 180 °C and 280 °C for 24 h to fabricate the different surface areas, and the KTiOxs nanowires were prepared using WCP with 1 and 10 mol/L-KOH solution treatments to elongate the different surface areas. The photocatalytic activity of KTiOxs/AlO(OH) composites was successfully studied as a function of the surface area. The KTiOxs nanowires obtained from the 10 mol/L-KOH solution treated Ti and the AlO(OH) obtained from treatment at 280 °C for 24 h in a steam coating process yielded the highest surface area and also the highest photocatalytic performance. It is notable that two different facile fabrication methods can be used to fabricate high potential composites such as KTiOxs/AlO(OH) for high performance photocatalysis. Therefore, we believe that the KTiOxs/AlO(OH) composites produced by the WCP and steam coating process have great potential for photocatalysis, but also for photovoltaics and bio- and eco-friendly devices, where a high catalytic activity and easy cleaning are required.

Author Contributions

S.Y.L. writing the paper and supervised the experiments. T.M. and D.N. conducted the experiments and A.S. supervised the all process of this research. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by JST under the Program on Open Innovation Platform with Enterprises, Research Institute and Academia (OPERA; no. 18072116), and JSPS KAKENHI, Grant-in-Aid for Scientific Research (B) (no. 19H02482).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. FE-SEM images of specimens subjected to steam coating process under different conditions: (a) 180 °C for 24 h and (b) 280 °C for 24 h.
Figure 1. FE-SEM images of specimens subjected to steam coating process under different conditions: (a) 180 °C for 24 h and (b) 280 °C for 24 h.
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Figure 2. XRD pattern of specimens subjected to a steam coating process under different condition:(a) 180 °C for 24 h and (b) 280 °C for 24 h.
Figure 2. XRD pattern of specimens subjected to a steam coating process under different condition:(a) 180 °C for 24 h and (b) 280 °C for 24 h.
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Catalysts 11 00548 g003 550
Figure 3. AFM observations of the surface area of specimens after the steam coating process under different condition: (a) the 3D images of each of the specimens for the control and for those subjected to steam coating under different conditions of 180 °C for 24h and 280 °C for 24h. (b) The results of surface areas for each of the specimens.
Figure 3. AFM observations of the surface area of specimens after the steam coating process under different condition: (a) the 3D images of each of the specimens for the control and for those subjected to steam coating under different conditions of 180 °C for 24h and 280 °C for 24h. (b) The results of surface areas for each of the specimens.
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Catalysts 11 00548 g004a 550Catalysts 11 00548 g004b 550
Figure 4. FE-SEM images of the specimens subjected to a wet corrosion process under different conditions: (a) control, (b) 1 mol/L-KOH solution treated Ti for 24h, (c) 10 mol/L-KOH solution treated Ti for 24h, and (d) TEM image of KTiOxs nanowires present in the outer shell area of the Ti particles. The dark-field STEM image (DF) and the EDX maps (O-, K-, and Ti-maps) taken from the nanowires indicate the presence of KTiOxs.
Figure 4. FE-SEM images of the specimens subjected to a wet corrosion process under different conditions: (a) control, (b) 1 mol/L-KOH solution treated Ti for 24h, (c) 10 mol/L-KOH solution treated Ti for 24h, and (d) TEM image of KTiOxs nanowires present in the outer shell area of the Ti particles. The dark-field STEM image (DF) and the EDX maps (O-, K-, and Ti-maps) taken from the nanowires indicate the presence of KTiOxs.
Catalysts 11 00548 g004aCatalysts 11 00548 g004b
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Figure 5. Raman profile of after the wet corrosion process (WCP) process.
Figure 5. Raman profile of after the wet corrosion process (WCP) process.
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Figure 6. K2p XPS spectra of the surface of the 10 mol/L-KOH treated Ti.
Figure 6. K2p XPS spectra of the surface of the 10 mol/L-KOH treated Ti.
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Figure 7. FE-SEM images of the specimens after synthesizing the KTiOxs/AlO(OH) composites with different conditions: Note that the sample names of composites are given as the conditions of WCP to produce the KTiOxs and the conditions of the steam coating process to produce AlO(OH): (a) 1 mol/L-KOH solution/180 °C for 24 h, (b) 1 mol/L-KOH solution/280 °C for 24 h, (c) 10 mol/L-KOH solution /180 °C for 24 h, and (d) 10 mol/L-KOH solution/280 °C for 24 h.
Figure 7. FE-SEM images of the specimens after synthesizing the KTiOxs/AlO(OH) composites with different conditions: Note that the sample names of composites are given as the conditions of WCP to produce the KTiOxs and the conditions of the steam coating process to produce AlO(OH): (a) 1 mol/L-KOH solution/180 °C for 24 h, (b) 1 mol/L-KOH solution/280 °C for 24 h, (c) 10 mol/L-KOH solution /180 °C for 24 h, and (d) 10 mol/L-KOH solution/280 °C for 24 h.
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Figure 8. EDX analysis of each sample with element mapping. Note that the sample names of the composites are given as the conditions of WCP to produce the KTiOxs and the conditions of the steam coating process to produce AlO(OH): (a) 1 mol/L-KOH solution/180 °C for 24 h, (b) 1 mol/L-KOH solution/280 °C for 24 h, (c) 10 mol/L-KOH solution/180 °C for 24 h, and (d) 10 mol/L-KOH solution/280 °C for 24 h.
Figure 8. EDX analysis of each sample with element mapping. Note that the sample names of the composites are given as the conditions of WCP to produce the KTiOxs and the conditions of the steam coating process to produce AlO(OH): (a) 1 mol/L-KOH solution/180 °C for 24 h, (b) 1 mol/L-KOH solution/280 °C for 24 h, (c) 10 mol/L-KOH solution/180 °C for 24 h, and (d) 10 mol/L-KOH solution/280 °C for 24 h.
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Figure 9. FTIR spectrum of the prepared substrates before/after synthesizing the KTiOxs/AlO(OH) composites: Note that the sample names of the composites are given as the conditions of WCP to produce the KTiOxs and the conditions of steam coating process to produce AlO(OH): (a) bare AlO(OH) prepared at 280 °C for 24 h and (b) 10 mol/L-KOH solution treated Ti at 280 °C for 24 h.
Figure 9. FTIR spectrum of the prepared substrates before/after synthesizing the KTiOxs/AlO(OH) composites: Note that the sample names of the composites are given as the conditions of WCP to produce the KTiOxs and the conditions of steam coating process to produce AlO(OH): (a) bare AlO(OH) prepared at 280 °C for 24 h and (b) 10 mol/L-KOH solution treated Ti at 280 °C for 24 h.
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Figure 10. (A) Photographs and (B) UV–VIS absorption spectra from the original MB dye, and from the dye exposed to UV light irradiation for 30 min: (a) pure H2O, (b) original MB, (c) 1 mol/L-KOH treated Ti, and (d) 10 mol/L-KOH treated Ti.
Figure 10. (A) Photographs and (B) UV–VIS absorption spectra from the original MB dye, and from the dye exposed to UV light irradiation for 30 min: (a) pure H2O, (b) original MB, (c) 1 mol/L-KOH treated Ti, and (d) 10 mol/L-KOH treated Ti.
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Figure 11. Analysis of the photocatalytic activity: (a) UV–VIS absorption spectra took from the original MB dye, and from the dye exposed to various conditions of KTiOx/AlO(OH) composites after UV light irradiation for 2 h. (b) Effect of photocatalysis depends on the surface area of the KTiOx photocatalysts. (c) Effect of photocatalysis depends on the surface area of AlO(OH).
Figure 11. Analysis of the photocatalytic activity: (a) UV–VIS absorption spectra took from the original MB dye, and from the dye exposed to various conditions of KTiOx/AlO(OH) composites after UV light irradiation for 2 h. (b) Effect of photocatalysis depends on the surface area of the KTiOx photocatalysts. (c) Effect of photocatalysis depends on the surface area of AlO(OH).
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Table
Table 1. The conditions for each of the samples in this study.
Table 1. The conditions for each of the samples in this study.
The Condition of Produce of Al(OH)The Condition of Produce of KTiOxs
180 °C-24 h180 °C for 24 h treated AlX
280 °C-24 h280 °C for 24 h treated AlX
180 °C-24 h (1 M)180 °C for 24 h treated Al1 M-KOH treated Ti
180 °C-24 h (10 M)180 °C for 24 h treated Al10 M-KOH treated Ti
280 °C-24 h (1 M)280 °C for 24 h treated Al1 M-KOH treated Ti
280 °C-24 h (10 M)280 °C for 24 h treated Al1 M-KOH treated Ti
Table
Table 2. Chemical composition of the starting materials used in the present study.
Table 2. Chemical composition of the starting materials used in the present study.
Composition (wt.%)
AlMgSiFeMnCrTiCuZn
Bal.0.8~1.20.4~0.81<0.7<0.150.04~0.35<0.150.15~0.4<0.25
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Lee, S.Y.; Matsubara, T.; Numata, D.; Serizawa, A. Facile Synthesis of Potassium-Doped Titanium Oxide Nanostructure (KTiOxs)/AlO(OH) Composites for Enhanced Photocatalytic Performance. Catalysts 2021, 11, 548. https://doi.org/10.3390/catal11050548

AMA Style

Lee SY, Matsubara T, Numata D, Serizawa A. Facile Synthesis of Potassium-Doped Titanium Oxide Nanostructure (KTiOxs)/AlO(OH) Composites for Enhanced Photocatalytic Performance. Catalysts. 2021; 11(5):548. https://doi.org/10.3390/catal11050548

Chicago/Turabian Style

Lee, So Yoon, Tatsuya Matsubara, Daiki Numata, and Ai Serizawa. 2021. "Facile Synthesis of Potassium-Doped Titanium Oxide Nanostructure (KTiOxs)/AlO(OH) Composites for Enhanced Photocatalytic Performance" Catalysts 11, no. 5: 548. https://doi.org/10.3390/catal11050548

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