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Article

Role of Post-Hydrothermal Treatment on the Microstructures and Photocatalytic Activity of TiO2-Based Nanotubes

by
Mohammad Qamar
1,*,
Shabi Abbas Zaidi
2,
Mohd Rafatullah
3,*,
Mohammad Qutob
3,
Sun-Jae Kim
4 and
Qasem A. Drmosh
1
1
Interdisciplinary Research Center for Hydrogen and Energy Storage, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
2
Analytical Chemistry Program, Department of Chemistry and Earth Sciences, College of Arts and Sciences, Qatar University, Doha P.O. Box 2713, Qatar
3
School of Industrial Technology, Universiti Sains Malaysia (USM), George Town 11800, Penang, Malaysia
4
Department of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul 05006, Korea
*
Authors to whom correspondence should be addressed.
Catalysts 2022, 12(7), 702; https://doi.org/10.3390/catal12070702
Submission received: 3 June 2022 / Revised: 24 June 2022 / Accepted: 24 June 2022 / Published: 27 June 2022
(This article belongs to the Section Environmental Catalysis)
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Abstract

:
The present study demonstrates the thermal stability and photocatalytic activity of TiO2-based nanotubes with respect to post-hydrothermal treatment. Titanate nanotubes were synthesized by adapting an alkali hydrothermal method from TiO2 sol using NaOH as a catalyst. The effect of post-hydrothermal heating on the properties—such as structure, morphology, textural properties, and activity—of as-synthesized one-dimensional titania nanostructure is investigated in detail. The characterizations are carried out using SEM, EDX, TEM, XRD, and a BET surface area analyzer. When heated in the presence of water in an autoclave, the protonated titanate phase of the nanotubes converts to anatase phase. Meanwhile, the tubular morphology is gradually lost as the post-hydrothermal heating duration increases. The photocatalytic activity was assessed utilizing the photo-oxidation of an amaranth dye. It is discerned that the as-prepared nanotubes are photocatalytically inactive but become active after post-hydrothermal processing. The activity trend follows the formation of the active phase—the titanate phase crystallizes into a photocatalytically-active anatase phase during post-hydrothermal heating. The effect of experimental parameters, such as reaction pH, dye concentration, and amount of catalyst, on the dye removal is studied. The findings also highlight that the role of holes/OH• is more prominent as compared to conduction band electron/O2−• for the removal of the dye. In addition, the photocatalyst exhibited excellent stability and reusability.

1. Introduction

Nanotubes have been a popular one-dimensional allotropic form of carbon in the scientific domain. Currently, a wide variety of nanotubes composed of various different elements, such as carbon, titanium, tungsten, zinc, etc., have attracted the attention of scientists [1,2,3,4]. It has been established that a wide range of layered materials can form nanotubes since the discovery of nanotube morphology [5]. Of such materials, titanate nanotubes or TiO2-based nanotubes have shown tremendous potential in diverse applications [6,7] such as environmental threats including water pollution, impacts on marine ecosystems, and livestock wastes that are on the forefront among various other global issues [8,9].
Substantial work has been performed over the last several decades to develop novel materials with an aim to improve photocatalytic activity or to find additional ways to improve the effectiveness of existing photocatalysts [10,11]. It has now been established that by controlling photocatalyst features, such as morphology, shape, size, surface area and its functionalization, and so forth, the photocatalytic activity can be significantly improved [12,13,14,15,16].
Therefore, because of their increased surface area and one-dimensional shape, TiO2 are expected to be a feasible strategy, which offers an effective delocalization method for the charge carriers. Owing to many peculiar characteristics, such high charge-discharge capacity, excellent photocatalytic ability and high surface area, these one-dimensional nanostructures have been employed in electrochemical energy storage systems [17,18] in addition to in photovoltaics and semiconductor photocatalysts as well as catalysts support and gas sensors [19,20,21]. To produce one-dimensional nanostructures of TiO2 (initially called needle-like structures), Kasuga et al. revealed an alkali hydrothermal process [22]. The method is simple, low cost and capable of producing the product in large quantities.
Since the structure of nanotubes produced by the alkali hydrothermal process is similar to that of titanium oxide, the photocatalysis can be one of the most potential applications of TiO2-based nanotubes. However, not much information about the effect of post-hydrothermal treatment on the phase change and photocatalytic performance is available. For instance, TiO2 nanotubes and Fe-, Ni-, Co- and Pt co-doped TiO2 nanotubes were prepared and the photodecomposition of a mixture of NH3, CH4, and CO2 was investigated [23]. In another study, Kim et al. prepared a heterostructure consisting of TiO2 nanotubes and ultrathin 2D ZnIn2S4 nanosheets, and demonstrated efficient and selective photocatalytic CO2 reduction [24]. The surface of TiO2 nanotubes was also modified with g-C3N4 for enhanced photocatalytic reduction of CO2 to CH4 and CO [25]. Peng et al. fabricated PbS nanocrystal-sensitized ultrafine TiO2 nanotubes for efficient and unusual broadband-light-driven hydrogen production [26]. More examples can be presented here to highlight the fact that as-prepared TiO2 nanotubes have been used as-is or in the form of nanocomposites for photocatalytic applications [27,28,29,30]. However, it should be noted that the as-prepared TiO2 nanotubes are photocatalytically-inactive because of their titanate structures. Earlier, our group discussed and confirmed the phase and structural composition of the needle-like morphology, which was produced through a sodium hydroxide-mediated alkali process [31].
The findings of this study suggest that the photocatalytic activity in TiO2-based nanotubes can be generated with post-hydrothermal heat treatment, and this can be highly advantageous in designing nanocomposites with TiO2-based nanotubes. The present study demonstrates the impact of post-hydrothermal heating time on: (1) the structural and the morphological transformations; and (2) the photocatalytic activity of the TiO2-based nanotubes. The post-hydrothermal temperature was kept constant (120 °C), while the heating time was varied between 1 and 24 h. Structural and morphological evolution was followed by XRD and microscopic techniques. The photocatalytic activity has been investigated by studying the degradation rate of amaranth dye as a probe reaction. Furthermore, the degradation rate of the dye has been obtained by varying reaction pH (between ~3.5 and ~10.5), substrate (dye) concentration (between 0.15 and 0.70 mM), and catalyst loadings (between 0.5 and 5 g L−1). In addition, the role of holes/OH• and conduction band electron/O2−• for the removal of the dye were investigated. In addition, the photocatalyst exhibited excellent stability and reusability. Photocatalytic experiments were carried out to investigate the stability and the recyclability of the photocatalytic sample.

2. Results and Discussion

Figure 1a depicts that, after acid treatment, the as-prepared nanotubes possess tri-titanate phase structure with monoclinic symmetry, as previously reported [32]. The ion-exchange process shows a significant influence on the molecular structure of the samples. The XRD patterns of the acid-treated samples differ slightly than those of the untreated. The intensity of the reflections near 10° and 28°, which correspond to (200) and (211) reflections, are substantially reduced after ion-exchange. This is presumably due to different scattering characteristics of the ions (Na+/H+) [31]. Energy dispersive X-ray (EDX) analyses (Figure 1b) indicated that the sodium ions were significantly removed from the as-prepared nanotubes samples.

2.1. Impact of Post-Hydrothermal Treatment on Phase Structure and Shape and Morphology

It has been demonstrated in the literature that post-hydrothermal heat treatment influences the phase, morphology and surface, consequently affecting the photocatalytic characteristics of materials [31,33]. With this view, the nanotubes were heated in an autoclave at 120 °C for different time intervals. The structural and morphological evolution as a function of heating time was investigated by XRD and FESEM. The XRD patterns of the derivatives obtained after heating the protonated tri-titanate nanotubes at 120 °C from 1 to 24 h are shown in Figure 2. As can be seen, the post-hydrothermal processing induced the structural change in nanotubes. Titanate structures undergo recrystallization and formation of TiO2 (anatase phase) was recorded. This change was indicated by the shift in 2θ = 24° to a higher value. An increase in the heating time improved the crystallinity. These findings suggested that the structure of the protonated nanotubes is tri-titanate formed by the sharing of Ti3O72− units of adjacent layers of H2Ti3O7 [31]. These structural changes are presumably being attributed to the ion-exchange process, in which the H+ replaced the Na+. Under thermal activation, the presence of H+ allows the acid-catalyzed condensation of OH groups and formation of water molecules as shown in equation below:
H2Ti3O7 → 3TiO2 + H2O
The dehydration process among the titanate layers may be followed by an in-situ rearrangement of the structural unites. The transformation from titanate to anatase seems to be a topotactic phase transition, which demands low energy to break the bonding in titanate leading to the formation of the anatase phase [34].
Transmission Electron Micrograph (TEM) of protonated nanotubes was also obtained and is illustrated in Figure 3. Microscopic images indicate that both the ends of the nanotubes are open, non-seamless and multiwalled. The outer diameter was calculated to be in the range of 6 to 10 nm, while the inner diameter was between 4 and 6 nm. The lengths were seen to be several hundreds of nanometers. The TEM irradiation can affect the morphology and structure of TiO2 nanotubes, particularly the amorphous phase of TiO2 nanotubes [35,36]. During TEM analysis, we did not observe any morphological or structural changes, presumably due to the fact that the titanate nanotubes are crystalline in nature as indicated by the XRD patterns (Figure 1). The FESEM images of both as-prepared and acid-treated products did not indicate any modification in shape and morphology because of acid treatment. The shape and morphological changes in nanotubes occurring during post-hydrothermal processing were analyzed by FESEM. The images are presented in Figure 4. The processing was carried out at 120 °C for 1, 3, 6, 12, 18 and 24 h. The images suggest that the nanotubes begin to lose their shape and morphology, and convert into nanoparticles when heated in an autoclave at 120 °C. Similar structural evolution is observed earlier when TiO2 nanotubes undergo thermal treatment [31,37,38,39]. The initial 1 h heat treatment has very little impact on the shape and morphology of nanotubes. However, when the sample was heated for 3 h, a major portion of the nanotubes were converted into nanoparticles and almost all the nanotubes were transformed into nanoparticles after 6 h. On the basis of FESEM and the XRD findings, it may be suggested that the nanotubes samples during heat treatment undergo rearrangement of the structural units, altering the phase as well as the shape and morphology of the samples drastically. In our previous study [31], a detailed investigation based on the change in crystal structure and shape and morphology of TiO2-based nanotubes was carried out with respect to heat treatment in air. When the thermal stability of TiO2-based nanotubes was compared in different media, such as air and water, the nanotubes showed significantly less stability in water as compared to air. This may be due to the enhanced dehydration process caused by acid-catalyzed condensation of OH groups by protons.

2.2. Impact of Post-Hydrothermal Processing on Photocatalytic Activity

In addition to structural and morphological changes, variations in the photocatalytic performance of nanotubes as a function of hydrothermal processing time were studied. This was carried out by monitoring the removal or degradation rate of amaranth. The results rates are presented in Figure 5. The values of crystallite size and surface area as a function of post-hydrothermal heat treatment are listed in Table 1. As is clear from Figure 5, the heat treatment has a substantial effect on the photocatalytic activity of the samples for amaranth degradation. It is worth noting that, in the presence of thermal derivatives, the degradation of the dye was drastically improved. The improvement in photocatalytic activity is attributed to the formation of anatase phase, as confirmed by the XRD patterns. The negligible photocatalytic activity of protonated nanotubes may be due to their titanate and mostly amorphous phase. It can be observed from Figure 5 that the photocatalytic activity of the acid treated nanotubes increases with increasing the duration of heat treatment, presumably owing to the improved crystallization of anatase phase. The samples heated at 120 °C for 12 h exhibited the highest activity for the removal of amaranth, followed by a decrease at longer heat treatment. With the increase in heating time, the surface area decreased while the anatase crystallite size increased. Despite the high surface area, the low activity shown by the sample treated for a shorter time can be rationalized to: (1) smaller fraction of photocatalytically-active anatase phase; and (2) poor crystallinity. Although the crystallinity of the samples improved with increasing heating duration, such as 18 and 24 h, the photocatalytic performance of the samples decreased. The decrease in activity can be attributed to a decrease in surface area of the samples. It seems reasonable to infer that an optimal compromise between surface area and crystallinity should be achieved for the highest activity. This was noticed in the sample heated for 12 h. The highest photocatalytic activity shown by samples heated for 12 h can be attributed to an optimal synergistic effect of surface area and crystallinity.
Since the highest photocatalytic activity was obtained with the sample prepared at 120 °C for 12 h, it was used for the study of photodegradation of amaranth in aqueous suspensions under a variety of experimental parameters.

2.3. Impact of pH on the Degradation Rate

The pH of the solution is a critical parameter as it affects the surface charge properties of the photocatalyst and the size of aggregates it forms. The removal of amaranth was investigated at different pH values between 3 and 10.4. The degradation rate as a function of reaction pH is summarized in Table 2. The experimental results showed that the solution pH had less impact on the degradation of dye. The degradation increased slightly with the increase in pH followed by a decrease at higher pH (e.g., 10.4). These results can be explained in terms of zero point charge (pHzpc) of TiO2 at pH ~6.5, below which the TiO2 nanotubes are positively charged. Beyond pH 6.5, the TiO2 nanotubes possess negative charges and the formation of OH− by catalyst generates unfavorable repulsive forces. As a result, amaranth removal decreased at higher pH. Furthermore, the presence of a higher number of OH− in the photocatalytic solution at higher pH might be excluded (screened) in UV analysis.

2.4. Impact of pH on the Degradation Rate

Dye or substrate concentration is another critical factor that can influence the photocatalyst’s activity. It is vital to examine the relationship of the photocatalytic reaction rate on substrate concentrations. With this view, the effect of amaranth concentration ranging from 0.15 to 0.70 mM on the degradation rate was investigated. The results are summarized in Table 2. It is clear from the obtained data that the degradation rate improves with the increase in substrate concentration from 0.15 to 0.50 mM followed by decrease at higher amaranth concentration. An increase in the dye concentration increases the availability of dye molecules onto the surface of photocatalyst, leading to higher degradation. However, a highly concentrated solution of dye can produce intense color, which can prevent the light penetration into the solution or catalyst’s surface. This can severely limit the surface–photon interaction, absorption, activation and production of reactive species onto the surface of the photocatalyst. As a result, the removal of amaranth decreased its concentration exceeded the optimum limit of 0.50 mM.

2.5. Impact of Photocatalyst Loading on the Degradation Rate

Optimum photocatalyst loading is another important experimental parameter that was investigated by varying photocatalyst amount between 0.5 and 5.0 g L−1. The obtained values are presented in Table 2. As expected, the removal of amaranth was enhanced with the increase in photocatalyst amount, indicating typical heterogeneous catalysis behavior as reported in earlier studies [40,41]. An increase in the photocatalyst amount is expected to provide enhanced photocatalytic active centers for photon absorption and generation of reactive radicals, leading to improved removal of amaranth molecules.
The representative photocatalytic experiment (photocatalyst amount = 1 g L−1, amaranth concentration = 0.15 mM, solution pH = 3.5) was repeated three times to obtain measurement error. The variation in results was found to be approximately ±2%.

2.6. Role of Conduction Band Electrons/O2−• and Holes/OH•

When the surface of photocatalysts is illuminated with appropriate photonic energy (equal to or greater than the band gap), an exciton (e− and h+ pair) could be generated. Excited conduction band electrons could readily reduce O2 to O2−• while the holes in the valence band can oxidize water, generating OH• radicals. These radical species are very reactive, though short-lived, and capable of degrading of organic molecules. To determine the role of conduction band electron/O2−•, the photocatalytic removal of amaranth was carried out in the absence of O2 molecules or presence of N2 gas. Before turning on the light, the suspension was bubbled for ~45 min with N2 gas to ensure the removal of air and dissolved O2. The N2 gas bubbling was maintained throughout the reaction. The involvement of valence band holes/OH• was also investigated by conducting the photocatalytic experiment in the presence of a hole scavenger, ammonium oxalate ((NH4)2C2O4). The results are shown in Figure 6a. As can be seen, the removal of amaranth was noticeably dependent on the ambient environment. The removal of the dye in the absence of oxygen was slow. This suggested that O2 served as effective conduction band electron acceptor thereby increasing the photocatalytic removal of amaranth. Similarly, the photocatalytic removal of amaranth was decreased in the presence of the (NH4)2C2O4) hole scavenger. It can be noted that the effect of the hole scavenger was more profound as compared to O2, indicating that the holes or OH• formed during the photocatalytic reaction played a more dominant role in the removal of amaranth dye.
To investigate the stability and the recyclability of the representative photocatalytic sample, photocatalytic experiments were carried out at pH = 3.5 with catalyst loading = 1 g L−1 and dye concentration = 0.15 mM. After the completion of each cycle, the photocatalyst was collected, washed, dried at an elevated temperature, and utilized for the next cycle under the identical photocatalytic reaction conditions. The results for five cycles (runs) are given in Figure 6b. As can be seen, the photocatalyst is stable and recyclable for the removal of amaranth.
Table 3 compares the photocatalytic results obtained in this study with those reported in the literature.

3. Materials and Methods

3.1. Synthesis

The TiO2 precursor was synthesized as reported in our previous work [26]. Briefly, the TiO2 precursor was heated for 5 h in an autoclave reactor (5000 cm3 capacity) at 150 °C under 100 bar of N2 pressure. After that, a little portion of TiO2 sol was mixed with 10 M NaOH and the resultant mixture was placed in a Ni-lined hydrothermal vessel. The heat treatment was carried out at 150 °C for 48 h. After reaction, white product was obtained at the bottom of the vessel. The liquid solution was discarded and the solid part was first washed with deionized water and then vigorously stirred with 0.1 N HCl at 40 °C to thoroughly eliminate the Na+. Then, the product was extensively washed with deionized water until it was free from chloride ions. Finally, the product was filtered, treated with liquid nitrogen and freeze-dried at −57 °C. A small amount of the dried powder was suspended in water (80 mL), stirred for a long time and then transferred to the autoclave and heated at 120 °C for different time intervals.

3.2. Structural Characterization

Different characterization techniques, such as Transmission Electron Microscopy (TEM, JEM 2010, JEOL Ltd., Tokyo, Japan), Scanning Electron Microscopy (SEM, Hitachi S-4700, Tokyo, Japan), Powder X-ray Diffraction (PXRD, with Cu Kα radiation, Siemens-D50050D, Munich, Germany), and a Brunauer–Emmett–Teller surface area analyzer (BET, KICT-SPA3000, MiraeSI Co., Ltd, Gwangju, Korea), were employed to obtain sample information. The Debye–Scherrer equation was used to confirm the (anatase) crystallite size. The main diffraction pattern of anatase phase centered 2θ = 101° was used.

3.3. Evaluation of Photocatalytic Activity

For the photocatalytic reactions, the catalyst (1 g L−1 unless stated otherwise) was added into a Pyrex photocell containing the desired concentration in 500 mL aqueous solution of the dye. The suspension was agitated magnetically for at least 30 min in the dark. This can facilitate the suspension attaining the equilibrium that is necessary in order to account for the lost compounds (due to adsorption of amaranth) during analyses. The absorption spectra recorded before and after stirring were similar, suggesting that the nanotube sample did not have noticeable adsorption capacity toward amaranth dye. The samples were irradiated using ultraviolet radiation, and dye solutions were withdrawn from the cell using a syringe before and after specific reaction times. Before turning on the light, the dye solution was withdrawn and was used as a standard dye solution to compare the concentration of irradiated dye samples.
The degradation rate for the amaranth dye was calculated following the change in absorbance at λ = ~518 nm. The rate constant of each experiment was determined from the initial slope obtained from a plot of the absorbance (ln) vs. reaction time. Finally, the degradation rates were expressed in mol·min−1.

4. Conclusions

The effect of post-hydrothermal treatment on the properties (e.g., phase, shape and morphology, textural properties, photocatalytic activity) of the nanotubes, obtained through an alkali chemical process, was investigated. It was found that the protonated titanate phase of the nanotubes transformed into anatase phase when hydrothermally heated in an autoclave at 120 °C for a certain time period. The post–hydrothermal treatment, at the same time, destructed the tubular morphology and shape, and significantly affected the crystallite size and textural properties of the nanotubes. With increasing post-hydrothermal heating time, the specific surface area was found to decrease while the crystallinity was improved. The as-prepared and acid-treated nanotubes were photocatalytically inactive, and the generation of photocatalytic activity was because of the formation of active anatase phase as a result of post-hydrothermal heat treatment. The highest photocatalytic activity was recorded because of a synergistic effect operative between surface area and crystallinity. The highest removal of amaranth was obtained at pH ~7.9 with its 0.5 mM concentration. The removal of amaranth increased with the increase in photocatalyst amount from 0.5 to 5.0 g L−1. The role of holes/OH• is more prominent as compared to that of the conduction band electrons/O2−• for the removal of the dye. In addition, the photocatalyst was found to be stable and recyclable.

Author Contributions

Conceptualization, M.Q. (Mohammad Qamar) and S.-J.K.; Formal analysis, M.Q. (Mohammad Qamar) and S.A.Z.; Investigation, Q.A.D. and M.Q. (Mohammad Qamar); Methodology, M.Q. (Mohammad Qamar), S.-J.K., Q.A.D. and M.Q. (Mohammad Qutob); Supervision, M.Q. (Mohammad Qamar) and S.-J.K.; Writing original draft, M.Q. (Mohammad Qutob) and S.A.Z.; Review & editing, M.Q. (Mohammad Qamar), M.R. and S.-J.K.; Funding acquisition, M.Q. (Mohammad Qamar) and M.R. All authors have read and agreed to the published version of the manuscript.

Funding

The authors would like to express their appreciation to Ministry of Higher Education Malaysia for Fundamental Research Grant Scheme with Project Code: FRGS/1/2019/STG07/USM/02/12.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgments

The authors acknowledge the support provided by Universiti Sains Malaysia (USM) and Interdisciplinary Research Center for Hydrogen and Energy Storage, King Fahd University of Petroleum & Minerals (KFUPM), Saudi Arabia.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 12 00702 g001 550
Figure 1. (a,b) Powder XRD patterns of titanate nanotubes showing the effect of acid treatment and EDX analyses before and after acid treatment.
Figure 1. (a,b) Powder XRD patterns of titanate nanotubes showing the effect of acid treatment and EDX analyses before and after acid treatment.
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Catalysts 12 00702 g002 550
Figure 2. Powder XRD patterns of the samples obtained after post-hydrothermal heat treatment of nanotubes at 120 °C for different time periods.
Figure 2. Powder XRD patterns of the samples obtained after post-hydrothermal heat treatment of nanotubes at 120 °C for different time periods.
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Catalysts 12 00702 g003 550
Figure 3. TEM of acid treated nanotubes showing the non-seamless open ends and multiwalled morphology.
Figure 3. TEM of acid treated nanotubes showing the non-seamless open ends and multiwalled morphology.
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Catalysts 12 00702 g004 550
Figure 4. FESEM micrographs of nanotubes hydrothermally heated at 120 °C for different durations; (a) as-prepared (acid-treated) nanotubes, (b) 1 h, (c) 3 h, (d) 6 h, (e) 12 h, and (f) 18 h.
Figure 4. FESEM micrographs of nanotubes hydrothermally heated at 120 °C for different durations; (a) as-prepared (acid-treated) nanotubes, (b) 1 h, (c) 3 h, (d) 6 h, (e) 12 h, and (f) 18 h.
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Catalysts 12 00702 g005 550
Figure 5. (a) Change in absorption spectra of amaranth before and after photocatalytic reaction and (b) change in degradation rates in the presence of nanotubes and samples obtained after post-hydrothermal heat treatment at 120 °C for different time intervals.
Figure 5. (a) Change in absorption spectra of amaranth before and after photocatalytic reaction and (b) change in degradation rates in the presence of nanotubes and samples obtained after post-hydrothermal heat treatment at 120 °C for different time intervals.
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Catalysts 12 00702 g006 550
Figure 6. (a) Effect of O2 and hole scavenger on the photocatalytic removal of amaranth and (b) effect of recycling on the photocatalytic performance.
Figure 6. (a) Effect of O2 and hole scavenger on the photocatalytic removal of amaranth and (b) effect of recycling on the photocatalytic performance.
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Table
Table 1. BET and anatase crystallite size of as-prepared and heat-treated samples.
Table 1. BET and anatase crystallite size of as-prepared and heat-treated samples.
SampleBET Surface Area (m2g−1)Anatase Crystallite Size (nm)
As-prepared254*
Acid-treated353*
1 h262~6
3 h182~14
6 h119~21
12 h107~25
18 h94~28
24 h92~32
*—Anatase phase was absent.
Table
Table 2. Photocatalytic degradation of amaranth in aqueous suspensions under different experimental conditions.
Table 2. Photocatalytic degradation of amaranth in aqueous suspensions under different experimental conditions.
ParameterValueDegradation Rates [M min−1 × 10−3]
Solution pH3.50.0120
5.60.0144
7.90.0183
10.40.0156
Substrate concentration (mM)0.150.0120
0.250.0167
0.350.0268
0.500.0323
0.700.0231
Catalyst loading (g L−1)0.50.0080
1.00.0120
2.00.0157
3.00.0198
4.00.0212
5.00.0215
Table
Table 3. Summary of some studies focusing on semiconductor photocatalysis for the degradation of organic pollutants.
Table 3. Summary of some studies focusing on semiconductor photocatalysis for the degradation of organic pollutants.
CatalystCatalyst LoadingPollutantPollutant ConcentrationRate/Degradation Rate References
TiO2 (P25)1.0 g L−1Tebuthiuron0.5 mM0.012 × 10−3 M min−1[42,43]
TiO2 (UV100)1.0 g L−1Tebuthiuron0.5 mM0.004 × 10−3 M min−1[42,43]
TiO2 (UV100)1.0 g L−1Propachlor0.6 mM0.0016 × 10−3 M min−1[42,43]
TiO2 (P25)1.0 g L−1Chlortoluron0.25 mM0.013 × 10−3 M min−1[42,44]
TiO2 (UV100)1.0 g L−1Chlortoluron0.25 mM0.004 × 10−3 M min−1[42,44]
TiO2 (PC500)1.0 g L−1Chlortoluron0.25 mM0.002 × 10−3 M min−1[42,44]
TiO2 (UV100)1.0 g L−1Bromothymol Blue0.25 mM0.009 × 10−3 M min−1[42,45]
TiO2 (P25)1.0 g L−15-Bromouracil1.0 mM0.0028 × 10−3 M min−1[42,46]
TiO2 nanoparticles0.1 g L−1Methylene blue0.09 mM7.0 × 10−4 min−1[47]
TiO2 (P25)0.4 g L−1Paracetamol0.25 mM1.9 × 10−3 min−1[48]
TiO2 (P25)1.0 g L−1Salicylic acid 0.1 mM1.9 × 10−6 M min−1[49]
TiO2 (anatase)0.5 g L−1Salicylic acid 0.36 mM1.7 × 10−3 min−1[50]
Hollow mesoporous TiO2 microspheres0.1 g L−1Acetaminophen0.33 mM4.3 × 10−2 min−1[51]
TiO2–Pd 1 g L−1Diclofenac0.17 mM5.0 × 10−2 min−1[52]
TiO2–Ag4.0 × 10−2 min−1
Graphene/TiO2-Ag composites4 mgAmaranth0.02 mM5.8 × 10−2 min−1[53]
TiO2 nanoparticles1.0 g L−1Acid Blue-290.06 mM0.6 × 10−4 M min−1[54]
CdS1.0 g L−1Acid Blue-290.06 mM4.5 × 10−4 M min−1[54]
CdS-TiO21.0 g L−1Acid Blue-290.06 mM5.8 × 10−4 M min−1[54]
Ag–TiO2 nanofibers0.4 g L−1Methylene blue dye0.03 mM1.29 × 10−2 min−1[12]
TiO2-based nanotubes1.0 g L−1Amaranth0.15 mM0.012× 10−3 M min−1[This study]
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Qamar, M.; Zaidi, S.A.; Rafatullah, M.; Qutob, M.; Kim, S.-J.; Drmosh, Q.A. Role of Post-Hydrothermal Treatment on the Microstructures and Photocatalytic Activity of TiO2-Based Nanotubes. Catalysts 2022, 12, 702. https://doi.org/10.3390/catal12070702

AMA Style

Qamar M, Zaidi SA, Rafatullah M, Qutob M, Kim S-J, Drmosh QA. Role of Post-Hydrothermal Treatment on the Microstructures and Photocatalytic Activity of TiO2-Based Nanotubes. Catalysts. 2022; 12(7):702. https://doi.org/10.3390/catal12070702

Chicago/Turabian Style

Qamar, Mohammad, Shabi Abbas Zaidi, Mohd Rafatullah, Mohammad Qutob, Sun-Jae Kim, and Qasem A. Drmosh. 2022. "Role of Post-Hydrothermal Treatment on the Microstructures and Photocatalytic Activity of TiO2-Based Nanotubes" Catalysts 12, no. 7: 702. https://doi.org/10.3390/catal12070702

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