The investigation of structure and photocatalytic degradation of

Subscriber access provided by University of Sydney Library

Article

The investigation of structure and photocatalytic degradation of organic pollutant for protonated anatase/titanate nanosheets during thermal treatment Dingze Lu, Minchen Yang, Kiran Kumar Kondamareddy, Pei Wu, and Neena D ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04081 • Publication Date (Web): 27 Feb 2018 Downloaded from http://pubs.acs.org on February 28, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Sustainable Chemistry & Engineering is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

The investigation of structure and photocatalytic degradation of

organic

pollutant

for

protonated

anatase/titanate

nanosheets during thermal treatment

Dingze Lua, b*, Minchen Yangb, Kondamareddy Kiran Kumarb,c, Pei Wub, Neena Db

a

School of Science, Xi’an Polytechnic University, No.19 of Jinhua South Road, Beilin District,

Xi’an 710048, PR China b

Department of Physics and Key Laboratory of Artificial Micro- and Nano-structures of Ministry

of Education, Wuhan University, No.299 of Bayi Road, Wuchang District, Wuhan 430072, PR China. c

Department of Physics, Veltech Dr. RR. & Dr. SR. R&D Institute of science and technology,

Avadi-600062, Chennai,Tamilnadu, India

* Corresponding author. Tel.: +86-29-8233-0277; Fax: +86-29-8233-0277; E-mail address: [email protected] (D.Z. Lu);

1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT

The mesoporous protonated TiO2-derived nanosheets were fabricated through an alkaline hydrothermal method. The samples with higher specific-surface area of 378 m2/g are proved to consist of anatase and titanate phases. The X-ray diffraction studies and Raman spectroscopy techniques are employed for investigating structural transformation of the nanosheets caused by thermal treatment. During the thermal treatment from room temperature to 550 °C, the titanate phase gradually transformed to anatase phase, resulting in the destruction of the nanosheet structure. Particularly, the anatase phase is not observed to grow at the temperatures below 300 °C. The pore volume and specific-surface area decrease under thermal treatment, which is ascribed to dehydration. The efficiency of gaseous and liquid phase based photocatalytic activity of the materials is assessed by employing the photodegradation of gaseous benzene and rhodamine B (RhB), respectively. The photocatalytic activities of the samples are strongly influenced by conversion of the phase structure, pore volume and specific surface area during the thermal treatment. This shows a significant impact on the efficiency of separation and transfer of photo-generated charge carriers.

KEYWORDS: Nanosheets; Thermal treatment; Mesoporous; Structural transformation; Photocatalytic activity.

2


Page 2 of 46

Page 3 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION

Since Kasuga et al.1-2 have synthesized a typical TiO2-rooted nanomaterial of novel morphology through a simple wet chemical method. Extensive work has been contributed for such new nano-structure because of its widely unexploited applications in various areas.3-10 Crystalized structure of nanotubes is a controversial topic of recent time, and different strategies were put forward to synthesize the crystalline nanotubes.11-14 The debate is being persisted. Nevertheless, the multi stage method involve the transformation from 3dimension—2dimension —1dimension) for the formation of the tube-like structure has been popularly acknowledged12,

15-17

during which the original TiO2 is initially removed into products of lamellar and then allowed to roll down for achieving nanotubes. Apparently, the lamellar structure (2dimensional) is essential for generation of nanotubes. Though the present work is not aimed to discuss the crystallization of nanotubes, we hold the idea that the crystalized structure of the ultimate product might rely on some factors including temperature of reaction, concentration and nature of alkaline and the precursors of titanium.13,

18-20

Particularly we focused on the fantastic

intermediate lamellar structure. We have reported by our earlier work,21 that the formation of TiO2 based nanosheets is governed by the duration of reaction which restricts the peeled lamina from being scrolled. The TiO2-based nanosheets exhibit several properties better than the final products of nanotubular, including high adsorption and photocatalytic activity for molecules of dye. Higher specific surface 3



area of the nanosheets facilitates large area of contact with pollutant molecules and exposure to the radiation. Furthermore, the lamellar structure provides thin sheets which can lead to a higher quantum yield, as the photo-generated carrier stably accomplish the face of nanosheets prior to recombination.21-24 In other words, the structure of 2D-nanosheets plays a significant role in augmenting the photocatalytic activity. Thermal treatment is considered to be a convenient and effective post-treatment to promote photocatalytic activity of TiO2-based materials. The effect of thermal treatment on the morphology, structure and efficiency of photocatalytic activity for various TiO2-based photocatalysts have been widely studied, especially for nanoparticles25-27 and nanotubes.20, 28-31 However, the investigations on the thermal treatment for the 2D TiO2-based nanosheets are sparse.32-34 Since TiO2-derived nanosheets exhibit better photocatalytic performance than particles or nanotubes for some specific pollutants, further study of thermal treatment for those 2D-nanosheets are necessary to broaden their potential applications. In the present work, TiO2-based nanosheets were prepared by alkali hydrothermal method and we elucidate the effect of thermal treatment over the structural conversion and the properties which are the consequences of simple thermal treatment. We investigated the thermally influenced liquid-phase and gaseous-phase based photocatalytic activity of the materials by the photocatalytic degradation of RhB and gaseous benzene, respectively.

4


Page 4 of 46

Page 5 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


EXPERIMENTAL

Synthesis

1.0 g of commercially available anatase TiO2 was dispersed into 60 mL of NaOH aqueous solution (10 mol/L) by stirring at ambient temperature for half hour. After that, the suspensions were transferred to Teflon based autoclave, to treat hydrothermally at 130 °C for 3 h, and then allowed to cool down to room temperature using water bath. The precipitate is separated by centrifugation and then cleaned with water which is further distilled repeatedly until the pH 7.0. The precipitate was immersed in the solution of HCl (0.1 mol/L) and stirred for 2 h, which is followed by centrifugation-washing step second time. The samples were dried at 80 °C overnight using oven and then milled into a fine powder. For comparison, the TiO2-derived nanotubes were also prepared using the same procedure, while the hydrothermal reaction time was 24 h during the preparation.

Characterization of the samples

The thermal stability of the samples was investigated using thermogravimetric analysis (TGA) of the samples performed on a Netzsch STA449C (simultaneous TG/DTA/DSC) apparatus. The structural details were analyzed through Raman spectral studies carried out on Raman spectra (Japan, LabRAM HR,Horiba) provided 5



by wavelength of excitation as 488 nm. The powder diffraction of X-ray (XRD) pattern was recorded by an advance X-ray diffractometer of Bruker D8 using a 40 kV voltage and 40 mA current. The morphological studies were performed using a scanning electron microscope (SEM)-(FEI Sirion HRSEM, Netherlands) and a high-resolution transmission electron microscope (HRTEM)-(JEOL JEM-2010-HT, Japan). The adsorption apparatus of nitrogen (JW-BK, China) is used to measure the porosity and the surface area by recording the nitrogen adsorption-desorption isotherms over the samples.

Evaluation of the photocatalytic activity

The liquid-phase related photocatalytic activity is studied using photocatalytic removal of RhB from aqueous based solution. The suspensions for photocatalytic activity were made by dispersing the powders of catalyst (0.2 g) in RhB (10 mg/L, 100 mL) liquid solution. Initially the solutions were magnetically stirred in dark for about 1 h till attainment of a balance of adsorption-desorption and then were radiated by a mercury lamp of 160 W and they were kept in constant ambient conditions. The absorption of solution of RhB was measured using a Shimadzu UV-2550 spectrometer at the specified time intervals of 20 min on irradiation. The photocatalytic efficiency was evaluated by performing photocatalytic mineralization of gaseous benzene under ambient conditions. In order to prepare the photocatalysts, initially the samples (of 0.5 g) were disperse ultrasonically in distilled 6


Page 6 of 46

Page 7 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


water taken in glass-made dish (diameter: 15.5 cm) and evaporated at 80 °C (about 5 h) resulting in to a thin dried coating of the photocatalyst on the glass dish. The sample-coated glass dish was placed into a reactor made up of stainless steel (7.2 L). Benzene (2 µL) introduced into the reactors through a micro-syringe was turned into gaseous benzene in the presence of an in built electric fan due to its higher volatility. The vapor of benzene was allowed to reach adsorption equilibrium with catalysts in the darkness before irradiation by the same mercury lamp. The reactor was carried out with a gas chromatograph (Huaai GC9560, China) in order to detect concentration of the generated CO2. A small part of benzene was inevitably absorbed by the internal walls of the reactor under ambient conditions. Subsequently, the rate of mineralization was estimated in accordance with the produced CO2, by taking the ratio of amount of CO2 produced and the predicted amount of CO2 generated from introduced benzene.

Study of photo-generated hydroxyl radicals (·OH)

The generation of ·OH radicals over the surface of photocatalysts upon the illumination was confirmed by the photo-luminescence (PL) studies for which coumarin was used as a probe molecule. The coumarin instantly reacts with ·OH and generates a highly fluorescent coumarin-OH (7-hydroxycoumarin) adduct.35-37 The intensity of PL for 7-hydroxy-coumarin is found to be directly proportional to the quantity of ·OH radicals generated over the surface of the photocatalysts. The experiment was performed in the same way as that employed for evaluating the 7



liquid-stage photocatalytic activity but with the solution (1 mM, 100 mL) of coumarin replaces the aqueous solution of RhB. The trend of the intensity of PL at a wavelength of 450 nm was recorded for 7-hydroxycoumarin using the excitation wavelength of 340 nm on fluorescence spectrophotometer (Hitachi F-4600).

RESULTS AND DISCUSSIONS

Figure 1. The TGA/DTA thermograms for the protonated TiO2-derived nanosheets.

The TGA/DTA thermogram for the protonated TiO2-derived nanosheets (Fig. 1) represents weight loss with increase in temperature of the sample. The thermogram was recorded over the range of temperatures from 25 to 700 °C. The protonated TiO2-derived nanosheets undergo weight loss by two different processes of different weight loss rates. An initial weight loss of about 13% has occurred between 25°C and 150°C (process I) which corresponds to the evaporation of free water molecules absorbed by the nanosheets, during their synthesis and from the ambient atmosphere 8


Page 8 of 46

Page 9 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


after the synthesis owing to their mesoporous structure and high surface area.16, 38 A second weight loss about 6% appears between 150 °C and 450 °C (process II) which might be a consequence of dehydration of the nanosheets during the evolution of anatase structure.16

Figure 2. Comparative Raman spectra of the protonated anatase/titanate nanosheets and the protonated titanate nanotubes.

Raman spectroscopy is an idealistic approach to characterize the TiO2-derived materials as it can classify various crystalline phases of these materials. The Raman spectra recorded for the TiO2-debased nanosheets and nanotubes are shown in figure 2. Raman spectra of the nanotubes exhibits several Raman bands at around 190, 280, 386, 455, 673, and 832 cm−1, all of which can be assigned to the titanate phase.16, 39, 40 No anatase active mode is observed for the spectra of nanotubes, suggesting a complete transformation of the raw TiO2 into nanotubular titanate. The similar Raman bands can also be seen for the Raman spectra of nanosheets, apart from additional 9



bands around 155, 401, and 633 cm−1 assigned to the anatase phase.41, 42 The results evidently suggest that the as-synthesized sample actually consists of anatase and titanate phases. It is considered that the anatase phase was derived from the incomplete transformation of the raw TiO2 into lamellar titanate. The generation of TiO2-based nanotubes under hydrothermal conditions involves a transformation of three dimensional structures into one dimensional via two dimensional structures as an intermediate12,

15

, indicating that the nanotubes are smart nanosheets. By the

reduction of duration of the reaction, we have controlled the transformation from 2D to 1D. However, the transformation from 3D to 2D was also unavoidably restrained due to the limited reaction time. Therefore, a small part of raw anatase remained, achieving the anatase/titanate nanosheets.

10


Page 10 of 46

Page 11 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Raman spectra of the as-prepared nanosheets for various temperatures from room temperature up to 550 °C.

In order to monitor thermal behavior of the protonated anatase/titanate nanosheets, the Raman measurements were performed on heating the samples under the ambient conditions from room temperature to ~550 °C. The in situ Raman spectra are shown in figure 3. The spectra obtained at room temperature exhibit several characteristic Raman bands as well as a broad band between 600~750 cm−1. The bands at around 155 and 401 cm−1 are assigned to the vibrational modes of anatase phase, while those at 280, 455, and 832 cm−1 correspond to the titanate phase.39, 40 For the temperatures below 300 °C, the characteristic peak of anatase phase at 155 cm−1 shows no 11



significant enhancement which indicates that the anatase phase does not grow at low temperatures. However, the broad band between 600~750 cm−1 dissociate into two bands of which one takes the position at 633 and the other at 704 cm−1 with the raise of temperature. The earlier is assigned to the anatase phase, while the latter to the surface vibration mode of the protonated titanate. This titanate band (704 cm−1) became stronger significantly which can be ascribed to desorption of moisture40 and then turns weaker when the temperature rises up to 450 °C, while the band at 633 cm−1 becomes much stronger remarkably. However, at 550 °C, the bands correspond to the titanate phase totally disappear, suggesting the complete transformation from titanate phase to anatase phase.

Figure 4. In situ XRD patterns of the as-prepared nanosheets at various temperatures (from room temperature up to 550 °C).

The XRD was used to investigate the structural transformation of hydrothermally synthesized protonated anatase/titanate nanosheets. Figure 4 exemplifies the in situ 12


Page 12 of 46

Page 13 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


XRD patterns of the as-prepared nanosheets for various temperatures. It is reported that the protonated titanates of different crystal structures, exhibit Bragg peaks similar to that of anatase phase.16, 25, 31 Because of the low crystallinity of the protonated titanates, the corresponding peaks are overlapped by that of anatase phase. For example, the two peaks located between 24~28° are superimposed by the anatase (101) diffraction peak at 25.3°. However, the diffraction peak at around 10° obtained at room temperature is easily distinguishable for the protonated anatase/titanate nanosheets.16, 25, 31 This, confirms again that the nanosheets actually consist of anatase phase and titanate phase. As the temperature rises, the characteristic peak at 10° gradually turns weak, suggesting the decrease of the titanate phase. During the thermal treatment, the titanate phase undergoes dehydration and forms anatase structure. It can be found that there is no substantial change in the peaks of anatase phase at the temperatures below 300 °C when compared the relative intensities of the peaks correspond to the anatase and the titanate phase. However, the intensity of these peaks increase dramatically for the temperatures above 300 °C and up to 550 °C, only the peaks of anatase phase can be observed while the peaks correspond to the titanate phase disappear completely. It suggests the complete transformation from titanate phase to anatase phase, which is consistent with the Raman spectral studies. Further the XPS patterns shown in Figure S2a confirm that there is a strong interfacial chemical interaction between titanate and titania that forms heterojunction.

13



Figure 5. SEM images of the protonated anatase/titanate nanosheets (a) Low magnification (b) high magnification; and transmission electron microscope images of the samples (c) before heat treatment and (d) the samples obtained at 550 °C during the thermal treatment.

Furthermore, the structural transformation is monitored using scanning electron microscopy and transmission electron microscopy. Uniform peanut-like clusters (Figure 5a) of nanosheets were obtained by thermal treatment carried out at 130 °C for 3 h. The SEM images at high magnification (Figure 5b) and TEM image (Figure 5c) displays peanut-like clusters which are made up of a large number of nanosheets like mackerel scales with a diameter around 500 nm. The nanosheets are considered to be peeled off and grown from TiO2 precursor.12, 15 The conjoined nanosheets exhibit slight curving or scrolling due to the short reaction time (3 h). Figure 5d depicts the TEM image of the sample prepared using thermal treatment at 550 °C. The sample composed of short nanorods with around 10 nm diameters and a few tens of nanometers length. In other words, the anatase/titanate nanosheets were decomposed 14


Page 14 of 46

Page 15 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


after thermal treatments which can be attributed to the dehydration of the nanosheets during the thermal process. The anatase/titanate nanosheets are not very stable, which are considered to be an intermediate structure during the transformation from TiO2 precursor to titanate nanotubes. Desorption of moisture due to the thermal treatments can change the interface energy of those nanosheets with large surface area which results in to reconstruction of crystallinity.

Figure 6. The N2 adsorption−desorption isotherms of the samples obtained at different temperatures.

In order to survey the structural transformation of the prepared protonated anatase/titanate nanosheets, a part of the samples were separated during the thermal process for N2 adsorption-desorption studies. Figure 6 represents the nitrogen adsorption-desorption isotherms of the samples acquired at various temperatures (room temperature, 300, 350, 450, and 550 °C). It is found that the sample displays obvious hysteresis behavior. The isotherm of every sample is of type IV (IUPAC 15



classification), with a hysteresis loop of type H3, implying the presence of mesoporous structures.29, 39 As the temperature is rising, the hysteresis loop gradually shifts towards the region of higher pressure and area of the loops becomes smaller with the temperature.

Figure 7. The curves of pore-size distribution measured from the desorption branch of isotherms for the samples obtained at different temperatures (room temperature, 300, 350, 450, and 550 °C).

Figure 7 presents the curves of pore-sized distribution figured out from the desorption branch of N2 adsorption and desorption isotherms for the sample. The 16


Page 16 of 46

Page 17 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


position of the peaks for pore-size distribution gradually shifted to the larger pore diameters with the temperature. The tendency of variation is in accordance with the change in the overall pore volume and surface area of the sample. The Brunauer—Emmett—Teller (BET) surface area (SBET) as well as overall pore volume are figured out from the data of N2 adsorption-desorption isotherm for all the samples and summarized in Table 1. It can be found that the structural parameters of the sample are temperature dependent. The SBET values and the total pore volume sharply decrease with temperature during the dehydration of titanate phase and the transformation of anatase phase which possibly corresponds to the contraction of the air-gaps and merging of nanosheets. It can be interpreted in terms of the acceleration of mass transfer processes at higher temperature. Because the anatase/titanate nanosheet structure is actually unstable, its tiny crystal grains may easily rearrange in a tight and ordered pack through sliding over grain boundaries. With the rearrangement, the air-gaps within or outside of the tiny crystal grains are gradually deform, minimize, and finally disappear. The pore structure strongly influences the absorption capacity of the photocatalysts to the pollutants. The energy band gap of TiO2 is determined as 3.16 eV (Fig. S1). A red-shift in the UV-vis diffuse reflectance spectra (DRS) is observed which can be attributed to the formation of trititanate on TiO2 surface. The energy band gap for the as-prepared nanosheets decreased to 3.02 eV, that is significantly lower than that of TiO2 and results into the extension of light absorbance from 392 nm to 411 nm, which promotes the of utilization of light. The samples obtained at 450 oC showed intermediate ability 17



of light absorbance that of nanosheets and the raw TiO2.

Figure 8. The degradation of RhB over the raw TiO2, P25 and the samples obtained at different temperatures.

The RhB was utilized as a pollutant to investigate the liquid-stage photocatalytic activity of the sample prepared at various temperatures. The efficiency of photocatalytic activity of the raw TiO2 and P25 was also evaluated in order to compare with that of protonated titanate nanotubes. Figure 8 represents the photocatalytic removal of RhB over different samples. It is inferred that the molecules of RhB were partially degraded by the raw TiO2, P25 and the protonated titanate nanotubes with the rates of degradation recorded after 120 min irradiation were about 61.4%, 71.4% and 78.7%, respectively. Dramatically the protonated anatase/titanate nanosheets completely degraded the RhB after 120 min irradiation, showing higher photocatalytic activity than that of three materials. In general the photocatalytic activity is primarily attributed to crystalline phase and adsorption capability to 18


Page 18 of 46

Page 19 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


pollutant by the photocatalyst. The poorer photocatalytic activity of the titanate nanotubes is ascribed to the lack of high crystallization of anatase phase, the most effective phase of TiO2. However, the raw anatase TiO2 and P25 also showed much lower photocatalytic activity than the protonated anatase/titanate nanosheets. It can be attributed to its lower pore volume and surface area. Adsorption capability of the photocatalyst for pollutant is proportional to the specific-surface area, while the large pore volume facilitates a rapid diffusion of the products of degradation during the reaction. Furthermore, the nanosheets exhibited temperature dependent photo catalytic activity which slowly declined with the temperature and the rate of degradation of RhB drops to 50% when the temperature has risen to 550 °C. The experimental data is observed to be obeying the expression ln(c(t)/c0)=−kt (first order kinetics), Where c0 is the concentration of RhB measured initially, c(t), the concentration of RhB measured at a certain time t after irradiation, and k the rate constant of photo-catalytic interaction (measured from the slope of the plots).29,

43

The corresponding rate

constants of the reactions for different photocatalysts are listed in Table 1. The higher rate constant indicates greater photocatalytic activity. The variation in k value suggests that the efficiency of photocatalytic degradation for the protonated anatase/titanate nanosheets decrease with the temperature. The thermal treatment not only results in to a reduction in the specific surface area and the pore volume but also promotes the phase transformation from titanate to anatase. These adversely affect the photocatalytic degradation of RhB. The results are in good agreement with that of the 19



previous reports.44-47 In addition, the conduction and valence band positions for trititanate are lower than that of TiO2 as shown in Figure S4, and hence photo-excited electrons can be captured by trititanate from TiO2, while the generated holes transfer to the valence band of TiO2. Thus, the efficiency of charge separation for the composite TiO2/trititanate nanostructure remarkably increased relative to that of TiO2. It can be inferred that the formation of heterostructure for the samples led to improve photocatalytic activity of TiO2. Furthermore abundant hydroxyl groups over surface of trititanate can enhance the adsorption capacity of the catalyst, which also promotes the degradation of the pollutant.

Figure 9. The mineralization of benzene evaluated from generation of CO2 in the vicinity of the raw TiO2, P25 and the samples obtained at different temperatures.

The gaseous-phase photocatalytic activity of the raw TiO2, P25 and the samples obtained at different temperatures was investigated by using the mineralization of 20


Page 20 of 46

Page 21 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


gas-like benzene under the irradiation provided by a mercury lamp of 160 watt. Figure 9 shows the mineralization of benzene evaluated from generation of CO2 in the presence of different samples for 4 h irradiation. The rate of mineralization for raw TiO2, P25 and the as-prepared protonated anatase/titanate nanosheets are 8.1%, 53.2%, and 10.8% respectively, while that of photocatalysts obtained at 300, 350, 450 and 550 °C increases to about 58.3%, 65.3%, 90.4%, and 70.1% respectively under the identical reaction conditions. The photocatalysts exhibited strikingly higher efficiency of photo degradation than that of unheated one, upon thermal treatment. The rate of mineralization for benzene increases with temperature up to 450 °C, implying that the thermal

treatment

tremendously

strengthened

the

gas-stage

photocatalytic

performance of the protonated anatase/titanate nanosheets. Furthermore, the rate of mineralization slows down for the temperatures above 450 °C.

Figure 10. Formation of 7-hydroxycoumarin in the suspensions of the raw TiO2, P25 and photocatalysts obtained at different temperatures.

21



The ·OH radicals originated from the oxidation of hydroxyl (OH−) ions (or water molecules) assisted by the photo-induced holes (h+) or from the multi-step reduction of •O2− radicals assisted by the photo-excited electrons (e-) have been regarded as the main dynamic species that contribute photo-catalytic interaction.48-51 In order to classify the effect of heat treatment, the efficiency of photo-catalytic generation of ·OH- radicals was investigated by detecting the time-dependent production of 7-hydroxycoumarin. Figure 10 presents intensity of fluorescence at around 450 nm as a function of illumination time for the samples obtained at different temperatures. It can be found that the intensity of fluorescence for the raw TiO2 and P25 were relatively low. In general, intensity of fluorescence is in proportion to the number of of photo-induced hydroxyl radicals. The intensity for all samples increases with span of irradiation by visible light. Nevertheless the rate of generation of ·OH radicals slowly declines with calcination temperature. It is because of that the titanate/titania hetero-junction was used to separate photo-generated election and hole and then enhance the photocatalytic generation efficiency of ·OH radical. The titanate/titania hetero-junction was destructed by the increasing temperatures, reducing the photocatalytic generation of ·OH radical. The trend is in consistence with variation in photocatalytic degradation of RhB, indicating that the photo-generated ·OH radical plays predominant role in the photocatalytic activity of RhB. Besides, the amount of surface OH- ions over the raw TiO2, nanosheets and the samples obtained at different temperatures was studied by XPS in Figure S2b. After the calcination, the intensity of the peak corresponds to Ti−OH group for the prepared sample decreased gradually 22


Page 22 of 46

Page 23 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


when compared with that of the nanosheets. This is in accordance with that of the photocatalytic activity of RhB. Besides, the surface photocurrent spectra and electrochemical impedance spectra were measured for the raw P25, nanosheets, and photocatalysts obtained at different temperatures (Figure S5). The results are in good agreement with that of fluorescence studies. A variation is detected for the benzene and RhB degradation on annealing. It is found that photocatalytic degradation for RhB depends upon the formation rate of hydroxyl radical, which also exerts a critical part in benzene mineralization.35, 52, 53 Nevertheless, for degradation of benzene there are some more pathways which refer to the straight oxidation of holes.52 The photo-generated hole along with the hydroxyl radical participate in the oxidization of benzene. It is well known that high crystallinity is advantageous for the photo-generated holes in transferring them to the surface of photocatalysts. In other words, thermal treatment promotes the contribution of holes to the mineralization of benzene. As a result, the photocatalytic activity for mineralization of gas-like benzene is contributed by both the hydroxyl radical as well as the photo-generated holes, and their synergistic effect resulted in the highest photocatalytic activity at 450 °C. In addition, excellent recyclability for the samples obtained at 450 oC (h-450) are exhibited in Figure S3. The photocatalytic efficiency after the usage for six times is about 90.86%. Thus the composite catalyst exhibits a highly reproducible photocatalytic activity. It demonstrates that the present photocatalysts are easy recoverable

and

reusable

which

makes

the

photocatalytic

23


activity

more


environmentally benign and practical.

CONCLUSION

Protonated anatase/titanate nanosheets with a higher specific-surface area of 378 m2/g were synthesized through an alkaline hydrothermal method with subsequent washing by acid. During the thermal treatment, dehydration of the nanosheets resulted in a complete structural phase transformation from titanate to anatase and destroying of the laminated structure. The anatase phase has not been found to grow at temperatures below 300 °C. Increase in the temperature lead to an improvement in the crystallinity and the declination of the pore volume as well as the surface area. These changes have shown a marked impact on the photo-generated charge carrier separation and transfer. Thus they influenced liquid-phase and gaseous-phase photocatalytic activity of the materials in different ways. The liquid-phase photocatalytic activity for RhB of the materials exhibited a downfall with the increasing temperature, while the gaseous-phase photocatalytic activity for benzene initially increased and then decreased when the temperature exceeds 450 °C.

SUPPORTING INFORMATION

Supplementary information includes Figure S1-S5 showing data on the UV-vis DRS, XPS spectra of Ti 2p and O 1s, cyclic runs for the photocatalytic degradation, 24


Page 24 of 46

Page 25 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


schematic diagram of the transfer, surface photocurrent curves and electrochemical impedance spectroscopy.

ACKNOWLEDGEMENT

This work was financially supported by the National Basic Research Program of China (973 Program, Nos. 2009CB939704 and 2009CB939705). We also appreciate the enthusiastic help and instructive suggestion given by Prof. Dahai Wang and Yong Liu.

REFERENCES

(1) Kasuga T.; Hiramatsu M.; Hoson A.; Sekino T.; Niihara K. Formation of titanium oxide nanotube. Langmuir 1998, 14, 3160-3163. (2) Kasuga T.; Hiramatsu M.; Hoson A.; Sekino T.; Niihara K. Titania nanotubes prepared by chemical processing. Adv. Mater. 1999, 11, 1307-1311. (3) Doong R.A.; Liao C.Y. Enhanced visible-light-responsive photodegradation of bisphenol A by Cu, N-codoped titanate nanotubes prepared by microwave-assisted hydrothermal method. J hazard mater 2017, 322, 254-262. (4) Kim S.; Kim M.; Hwang S.H.; Lim S.K. Enhancement of photocatalytic activity of titania-titanate nanotubes by surface modification. Appl. Catal. B: Environ. 2012, 123, 391-397. 25



(5) Ramírez G.; Recio F.J.; Herrasti, P.; Ponce-de-León C.; Sirés I. Effect of RVC porosity on the performance of PbO2 composite coatings with titanate nanotubes for the electrochemical oxidation of azo dyes. Electrochim Acta 2016, 204, 9-17. (6) Kang K.Y.; Lee Y.G.; Kim S.; Seo S.R.; Kim J.C.; Kim K.M. Electrochemical properties of carbon-coated TiO2 nanotubes as a lithium battery anode material. Mater.

Chem. Phys. 2012, 137, 169-176. (7) Zhang Y.; Tang Z.R.; Fu X.; Xu Y.J. Nanocomposite of Ag–AgBr–TiO2 as a photoactive and durable catalyst for degradation of volatile organic compounds in the gas phase. Appl Catal B 2011, 106(3), 445-452. (8) Zhang Y.; Chen Z.; Liu S.; Xu Y.J. Size effect induced activity enhancement and anti-photocorrosion of reduced graphene oxide/ZnO composites for degradation of organic dyes and reduction of Cr(VI) in water. Appl Catal B 2013, 140, 598-607. (9) Zhang N.; Yang M.Q.; Liu S.; Sun Y.; Xu Y.J. Waltzing with the versatile platform of graphene to synthesize composite photocatalysts. Chem. Rev 2015, 115(18), 10307-10377. (10) Yuan L.; Yang M.Q.; Xu Y.J. Tuning the surface charge of graphene for self-assembly synthesis of a SnNb2O6 nanosheet–graphene (2D–2D) nanocomposite with enhanced visible light photoactivity. Nanoscale 2014, 6(12), 6335-6345. (11) Wu D.; Liu J.; Zhao X.N.; Li A.D.; Chen Y.F.; Ming N.B. Sequence of events for the formation of titanate nanotubes; nanofibers; nanowires; and nanobelts. Chem.

Mater. 2006, 18, 547-553. (12) Yao B.D.; Chan Y.F.; Zhang X.Y.; Zhang W.F.; Yang Z.Y.; Wang N. Formation 26


Page 26 of 46

Page 27 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


mechanism of TiO2 nanotubes. Appl. Phys. Lett. 2003, 82, 281-283. (13) Tsai C.C.; Teng H.S. Structural features of nanotubes synthesized from NaOH treatment on TiO2 with different post-treatments. Chem. Mater. 2006, 18, 367-373. (14) Zhang T.; Rahman Z.U.; Wei N.; Liu Y.; Liang J.; Wang D. In situ growth of single-crystal TiO2 nanorod arrays on Ti substrate: Controllable synthesis and photoelectro-chemical water splitting. Nano Res 2017, 10(3), 1021-1032. (15) Lu D.; Chai W.; Yang M.; Fang P.; Wu W.; Zhao B.; Xiong R.; Wang, H. Visible light induced photocatalytic removal of Cr(VI) over TiO2-based nanosheets loaded with surface-enriched CoOx nanoparticles and its synergism with phenol oxidation. Appl. Catal. B: Environ. 2016, 190, 44-65. (16) Gao T.; Fjellvag H.; Norby P. Crystal Structures of Titanate Nanotubes: A Raman Scattering Study. Inorg. Chem. 2009, 48, 1423-1432. (17) Kukovecz A.; Hodos N.; Horvath E.; Radnoczi G.; Konya Z.; Kiricsi I. Oriented crystal growth model explains the formation of titania nanotubes. J. Phys. Chem. B 2005, 109, 17781-17783. (18) Yoshida R.; Suzuki Y.; Yoshikawa S. Effects of synthetic conditions and heat-treatment on the structure of partially ion-exchanged titanate nanotubes. Mater.

Chem. Phys. 2005, 91, 409-416. (19) Panahian Y.; Arsalani N.; Synthesis of Hedgehoglike F-TiO2(B)/CNT Nanocomposites for Sonophotocatalytic and Photocatalytic Degradation of Malachite Green (MG) under Visible Light: Kinetic Study. J Phys Chem A 2017, 121, 5614-5624. 27



Page 28 of 46

(20) Naoi K.; Kurita T.; Abe M.; Furuhashi T.; Abe Y.; Okazaki K.; Miyamoto J.; Iwama E.; Aoyagi S.; Naoi W.; Simon P. Ultrafast Nanocrystalline-TiO2(B)/Carbon Nanotube

Hyperdispersion

Prepared

via

Combined

Ultracentrifugation

and

Hydrothermal Treatments for Hybrid Supercapacitors. Adv Mater 2016, 28, 6751-6757. (21) Chen F.T.; Fang P.F.; Gao Y.P.; Liu Z.; Liu Y.; Dai Y.Q. Effective removal of high-chroma crystal violet over TiO2-based nanosheet by adsorption-photocatalytic degradation. Chem. Eng. J. 2012, 204, 107-113. (22) Jang J.S.; Choi S.H.; Park H.; Choi W.; Lee J.S. A composite photocatalyst of CdS nanoparticles deposited on TiO2 nanosheets. J Nanosci Nanotechno 2006, 6(11), 3642-3646. (23) Chen F.; Fang P.; Liu Z.; Gao Y.; Liu Y.; Dai Y.; Luo H.; Feng J. Dimensionality-dependent photocatalytic activity of TiO2-based nanostructures: nanosheets with a superior catalytic property. J Mater Sci 2013, 48(15), 5171-5179. (24) Liu L.; Ji Z.; Zou W.; Gu X.; Deng Y.; Gao F.; Tang C.; Dong, L. In situ loading transition metal oxide clusters on TiO2 nanosheets as co-catalysts for exceptional high photoactivity. ACS Catal 2013, 3(9), 2052-2061. (25) Pap Z.; Karacsonyi E.; Cegled Z.; Dombi A.; Danciu V.; Popescu I.C.; Baia L.; Oszko A.; Mogyorosi K. Dynamic changes on the surface during the calcination of rapid heat treated TiO2 photocatalysts. Appl. Catal. B: Environ. 2012, 111, 595-604. (26) Khalid N.R.; Ahmed E.; Ikram M.; Ahmad M.; Phoenix D.A.; Elhissi A.; Ahmed W.; Jackson M.J. Effects of Calcination on Structural, Photocatalytic Properties of 28


Page 29 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TiO2 Nanopowders Via TiCl4 Hydrolysis. J. Mater. Eng. Perform. 2012, 22, 371-375. (27) Jimmy C.Y.; Lin J.; Lo D.; Lam S.K. Influence of thermal treatment on the adsorption of oxygen and photocatalytic activity of TiO2. Langmuir 2000, 16, 7304-7308. (28) Zhang M.; Jin Z.S.; Zhang J.W.; Guo X.Y.; Yang H.J.; Li W.; Wang X.D.; Zhang Z.J. Effect of annealing temperature on morphology, structure and photocatalytic behavior of nanotubed H2Ti2O4,OH,2. J. Mol. Catal. A: Chem. 2004, 217, 203-210. (29) Yu J.; Yu H.; Cheng B.; Trapalis C. Effects of calcination temperature on the microstructures and photocatalytic activity of titanate nanotubes. J. Mol. Catal. A:

Chem. 2006, 249, 135-142. (30) Lee C.K.; Wang C.C.; Lyu M.D.; Juang L.C.; Liu S.S.; Hung S.H. Effects of sodium content and calcination temperature on the morphology, structure and photocatalytic activity of nanotubular titanates. J. Colloid Interf. Sci. 2007, 316, 562-569. (31) Cortes-Jacome M.A.; Ferrat-Torres G.; Ortiz L.; Angeles-Chavez C.; Lopez-Salinas E.; Escobar J.; Mosqueira M.L.; Toledo-Antonio J.A. In situ thermo-Raman study of titanium oxide nanotubes. Catal. Today. 2007, 126, 248-255. (32) Chan C.K.; Porter J.F.; Li Y.G.; Guo W.; Chan, C.M. Effects of calcination on the microstructures and photocatalytic properties of nanosized titanium dioxide powders prepared by vapor hydrolysis. J Am Ceram Soc 1999, 82(3), 566-572. (33) Kim G.S.; Ansari S.G.; Seo H.K.; Kim Y.S.; Shin H.S. Effect of annealing 29



temperature on structural and bonded states of titanate nanotube films. J Appl Phys 2007, 101, 24314. (34) Zhang M.; Jin Z.S.; Zhang J.W.; Guo X.; Yang J.; Li W.; Wang X.; Zhang Z. Effect of annealing temperature on morphology, structure and photocatalytic behavior of nanotubed H2Ti2O4(OH)2. J. Mol. Catal. A 2004, 217(1-2), 203-210. (35) Ishibashi K.; Fujishima A.; Watanabe T.; Hashimoto K. Detection of active oxidative species in TiO2 photocatalysis using the fluorescence technique.

Electrochem. Commun. 2000, 2, 207-210. (36) Szabo-Bardos E.; Petervari E.; El-Zein V.; Horvath A. Photocatalytic decomposition of aspartic acid over bare and silver deposited TiO2. J. Photochem.

Photobiol. A: Chem. 2006, 184, 221-227. (37) Kim J.; Choi W. TiO2 modified with both phosphate and platinum and its photocatalytic activities. Appl. Catal. B: Environ. 2011, 106, 39-45. (38) Jiang F.; Zheng S.R.; An L.C.; Chen H. Effect of calcination temperature on the adsorption and photocatalytic activity of hydrothermally synthesized TiO2 nanotubes.

Appl. Surf. Sci. 2012, 258, 7188-7194. (39) Kőrösi L.; Papp S.; Csapó E.; Meynen V.; Cool P.; Dékány I. A short solid-state synthesis leading to titanate compounds with porous structure and nanosheet morphology. Micropor. Mesopor. Mater. 2012, 147, 53-58. (40) Kim S.J.; Yun Y.U.; Oh H.J.; Hong S.H.; Roberts C.A.; Routray K.; Wachs I.E. Characterization of Hydrothermally Prepared Titanate Nanotube Powders by Ambient and In Situ Raman Spectroscopy. J. Phys. Chem. Lett. 2010, 1, 130-135. 30


Page 30 of 46

Page 31 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(41) Swamy V.; Kuznetsov A.; Dubrovinsky L.S.; Caruso R.A.; Shchukin D.G.; Muddle B.C. Finite-size and pressure effects on the Raman spectrum nanocrystalline anatse TiO2. Phys. Rev. B 2005, 71, 184302. (42) Mazza T.; Barborini E.; Piseri P.; Milani P.; Cattaneo D.; Bassi A.L.; Bottani C.E.; Ducati C. Raman spectroscopy characterization of TiO2 rutile nanocrystals.

Phys. Rev. B 2007, 75, 45416. (43) Krejčíková S.; Matějová L.; Kočí K.; Obalová L.; Matěj Z.; Čapek L.; Šolcová O. Preparation and characterization of Ag-doped crystalline titania for photocatalysis applications. Appl. Catal. B: Environ. 2012, 111-112, 119-125. (44) Amano F.; Prieto-Mahaney O.O.; Terada Y.; Yasumoto T.; Shibayama T.; Ohtani, B. Decahedral single-crystalline particles of anatase titanium (IV) oxide with high photocatalytic activity. Chem Mater 2009, 21(13), 2601-2603. (45) Kominami H.; Matsuura T.; Iwai K.; Ohtani B.; Nishimoto S.I.; Kera, Y. Ultra-highly active titanium (IV) oxide photocatalyst prepared by hydrothermal crystallization from titanium (IV) alkoxide in organic solvents. Chem lett 1995, 24(8), 693-694. (46) Ide Y.; Inami N.; Hattori H.; Saito K.; Sohmiya M.; Tsunoji N.; Komaguchi K.; Sano T.; Bando Y.; Golberg D.; Sugahara Y. Remarkable charge separation and photocatalytic efficiency enhancement through interconnection of TiO2 nanoparticles by hydrothermal treatment. Angew. Chem. Int. Edit. 2016, 55(11), 3600-3605. (47) Saito K.; Kozeni M.; Sohmiya M.; Komaguchi K.; Ogawa M.; Sugahara Y.; Ide, Y. Unprecedentedly enhanced solar photocatalytic activity of a layered titanate simply 31



integrated with TiO2 nanoparticles. Phys Chem Chem Phys 2016, 18(45), 30920-30925. (48) Jin S.F.; Li Y.Z.; Xie H.; Chen X.; Tian T.T.; Zhao X.J. Highly selective photocatalytic and sensing properties of 2D-ordered dome films of nano titania and nano Ag2+ doped titania. J. Mater. Chem. 2012, 22, 1469-1476. (49) Zhu S.; Liang S.; Gu Q.; Xie L.; Wang J.; Ding Z.; Liu P. Effect of Au supported TiO2 with dominant exposed {001} facets on the visible-light photocatalytic activity.

Appl. Catal. B: Environ. 2012, 119-120, 146-155. (50) Liu Y.; Wei J.H.; Xiong R.; Pan C.X.; Shi J. Enhanced visible light photocatalytic properties of Fe-doped TiO2 nanorod clusters and monodispersed nanoparticles. Appl. Surf. Sci. 2011, 257, 8121-8126. (51) Gurkan Y.Y.; Turkten N.; Hatipoglu A.; Cinar Z. Photocatalytic degradation of cefazolin over N-doped TiO2 under UV and sunlight irradiation: Prediction of the reaction paths via conceptual DFT. Chem. Eng. J. 2012, 184, 113-124. (52) D'Hennezel O.; Pichat P.; Ollis D.F. Benzene and toluene gas-phase photocatalytic degradation over H2O and HCL pretreated TiO2: by-products and mechanisms. J. Photochem. Photobiol. A: Chem. 1998, 118, 197-204. (53) Einaga H.; Futamura S.; Ibusuki T. Heterogeneous photocatalytic oxidation of benzene; toluene; cyclohexene and cyclohexane in humidified air: comparison of decomposition behavior on photoirradiated TiO2 catalyst. Appl. Catal. B: Environ. 2002, 38, 215-225.

32


Page 32 of 46

Page 33 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure Captions

Figure 1. The TGA/DTA thermograms for the protonated TiO2-derived nanosheets.

Figure 2. Comparison of the Raman spectra of the protonated anatase/titanate nanosheets and the protonated titanate nanotubes.

Figure 3. In situ Raman spectra of the as-prepared nanosheets at various temperatures from room temperature up to 550 °C.

Figure 4. In situ XRD patterns of the as-prepared nanosheets at various temperatures from room temperature up to 550 °C.

Figure 5. Low magnification (a) and high magnification (b) SEM images of the protonated anatase/titanate nanosheets; and TEM images of the samples before thermal treatment (c) and obtained at 550 °C during the thermal treatment (d).

Figure 6. The N2 adsorption−desorption isotherms of the samples obtained at different temperatures.

Figure 7. The curves of pore-size distribution measured from the desorption branch of isotherms for the samples obtained at different temperatures (room temperature, 300, 33



350, 450, and 550 °C).

Figure 8. The degradation of RhB over the raw TiO2, P25 and the samples obtained at different temperatures.

Figure 9. The mineralization of benzene evaluated from generation of CO2 in the vicinity of the raw TiO2, P25 and the samples obtained at different temperatures.

Figure 10. Formation of 7-hydroxycoumarin (monitored by fluorescence emission) in the suspensions of the raw TiO2, P25 and photocatalysts obtained at different temperatures.

34


Page 34 of 46

Page 35 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Tables Table 1 Effects of the thermal treatment on the structure properties and photocatalytic activity of the protonated anatase/titanate nanosheets. SBET c (m2/g) Vpore d (cm3/g) ke (h−1)

As-prepareda

300 °Cb

350 °Cb

450 °Cb

550 °Cb

378 0.738 2.48

258 0.636 1.69

198 0.553 0.90

136 0.428 0.64

58 0.326 0.35

a

As-prepared protonated anatase/titanate nanosheets before thermal treatment. The samples taken out at different temperatures during the thermal treatment. c The specific surface area. d The total pore volume. e The reaction rate constant of RhB degradation. b

35



Figure 1. The TGA/DTA thermograms for the protonated TiO2-derived nanosheets. 57x40mm (600 x 600 DPI)


Page 36 of 46

Page 37 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Comparison of the Raman spectra of the protonated anatase/titanate nanosheets and the protonated titanate nanotubes. 62x47mm (600 x 600 DPI)



Figure 3. In situ Raman spectra of the as-prepared nanosheets at various temperatures from room temperature up to 550 °C. 109x144mm (600 x 600 DPI)


Page 38 of 46

Page 39 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. In situ XRD patterns of the as-prepared nanosheets at various temperatures from room temperature up to 550 °C. 64x49mm (600 x 600 DPI)



Figure 5. Low magnification (a) and high magnification (b) SEM images of the protonated anatase/titanate nanosheets; and TEM images of the samples before thermal treatment (c) and obtained at 550 °C during the thermal treatment (d). 82x60mm (300 x 300 DPI)


Page 40 of 46

Page 41 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. The nitrogen adsorption−desorption isotherms of the samples obtained at different temperatures. 59x43mm (600 x 600 DPI)



Figure 7. The corresponding pore-size distribution curves calculated from the desorption branch of the isotherms by the Barrett—Joyner—Halenda (BJH) method for the samples. 114x158mm (600 x 600 DPI)


Page 42 of 46

Page 43 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8. The degradation of RhB over the raw TiO2, P25 and the samples obtained at different temperatures. 62x47mm (600 x 600 DPI)



Figure 9. The mineralization of benzene evaluated from generation of CO2 in the presence of the raw TiO2, P25 and the samples obtained at different temperatures. 61x45mm (600 x 600 DPI)


Page 44 of 46

Page 45 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 10. Formation of 7-hydroxycoumarin (monitored by fluorescence emission) in the suspensions of the raw TiO2, P25 and photocatalysts obtained at different temperatures. 61x46mm (600 x 600 DPI)



Synopsis: In situ investigation of structure and photocatalytic degradation of gaseous benzene and rhodamine B solution for protonated anatase/titanate nanosheets during thermal treatment was reported. 49x35mm (600 x 600 DPI)


Page 46 of 46

The investigation of structure and photocatalytic degradation of

Recommend Documents