Layer-by-Layer Assembly and Photocatalytic Activity of Titania Nanosheets on Coal Fly Ash Microspheres

In order to address the problem with titania distribution and recovery, series of Ti 0.91 O 2 /CFA photocatalysts (Ti 0.91 O 2 /CFA-n, n = 2, 4, 6, and 8) were fabricated by assembling Ti 0.91 O 2 nanosheets on coal fly ash (CFA) microspheres via the layer-bylayer assembly (LBLA) process and characterized by scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), N 2 sorption, and ultraviolet-visible absorption (UV-vis) techniques. The SEM images and UV-vis spectra illustrated that Ti 0.91 O 2 nanosheets were immobilized successfully on the CFA by the LBLA approach and changed the characteristics of CFA noticeably. The photocatalytic activity of Ti 0.91 O 2 /CFA was evaluated by the photodegradation of methylene blue (MB) under UV irradiation. The results demonstrated that Ti 0.91 O 2 /CFA-6 showed the best photocatalytic activity among the series of Ti 0.91 O 2 /CFA irradiated for 60min, with a decoloration rate above 43%. After photocatalysis, the Ti 0.91 O 2 /CFA could be easily separated and recycled from aqueous solution and Ti 0.91 O 2 nanosheets were still anchored on the CFA.


Introduction
Fabrication of multilayers of colloidal particles through electrostatic attraction has been studied since 1966 [1].Generally, the composites consist of organic or inorganic particles as cores and inorganic nanocoatings as shells.For the great potentials of core-shell composites in photonics and catalysis areas, many researches have been focused on the preparation of core-shell structure via the layer-by-layer assembly (LBLA) approach in recent years [2][3][4].The LBLA method has been applied to assemble charged thin films of various materials on oppositely charged templates.The inorganic coatings are deposited on the templating cores via electrostatic attraction, so the shell coatings can be adsorbed on the cores uniformly and firmly.Titanium oxide is popular with researchers for its excellent performance of high photocatalytic activity, chemical inertness, stability against photocorrosion, and costeffectiveness [5].Titania nanosheet (Ti 1−x O 2 4− ), synthesized by chemical delamination and exfoliation of precursor, is of typical 2D structure and the lateral size of it ranges from hundreds of nanometers to a few micrometers and the thickness of the individual layer is approximately 1 nm [6].The titania nanosheets after exfoliation are negatively charged, so titania nanosheets can be assembled on substrates via the LBLA approach coupled with cationic polyelectrolyte [7].Multilayer titania nanosheets possess unique properties besides photocatalytic activity [8], such as optical absorption properties [7,9], high anisotropy [10], photoelectrochemical properties [11], and high thermal resistance [12].
Titania nanosheet hollow shells and hollow spheres consisting of titania and graphene nanosheets have been fabricated by the layer-by-layer assembly method in previous studies [13,14].However, the titania nanosheet hollow spheres are hard to separate spontaneously and recycle from aqueous suspension after photocatalysis, which limits their practical application in wastewater treatment.In order to improve the convenience of separation and recovery of nano-TiO 2 , photocatalysts by immobilizing nano-TiO 2 on some substrates were recently prepared, such as on glass, polymer, and active carbon [15][16][17].However, most substrate materials are expensive, and cheap and stable substrates are desired.In our early work, photocatalysts of TiO 2 immobilized on coal fly ash (CFA) were prepared [18][19][20].CFA is one of the solid wastes generated from thermal power plants.CFA chosen as 2 International Journal of Photoenergy a supporter has advantages as follows: (1) CFA particles are microspheres and easy to precipitate in water, so photocatalysts supported by CFA are easy to recycle from aqueous solution after reaction; (2) CFA, consisting primarily of Al 2 O 3 and SiO 2 , as a supporter can inhibit recombination of electron and hole effectively [21,22]; (3) the cost of preparation and a source of environment pollution can be reduced.However, it is difficult to control uniform distribution of TiO 2 on CFA [17,23], which greatly restrains the activity of the photocatalyst and the availability of substrate.Because the lateral size of titania nanosheets is in the micron range and CFA particle size is less than 100 m, the problem can be solved by loading titania nanosheets on CFA through electrostatic attraction via layer-by-layer assembly method at ambient temperature [24].The LBLA method involves electrostatic sequential deposition of negatively charged titania nanosheets onto substrate along with an oppositely charged polymer.Based on this principle, an assembly of layered titanate on CFA is expected to achieve very well.
In this paper, the titania nanosheet (Ti  O (0.4 g) was shaken vigorously (300 rpm) in 100 mL aqueous solution of TBAOH (0.004 M) for 2 weeks, so the layered hosts underwent osmotic swelling and exfoliating into nanosheets.The colloid produced stood for storage.CFA, calcined at 800 ∘ C, was immersed in a bath of 1/1 methanol/HCl and concentrated H 2 SO 4 with stirring for 20 min each and then washed with copious water to remove acid residue.The pretreated CFA was dried for use later.Deionized water was used throughout the experiments.

Fabrication
Procedure.Core/shell composites (CFA was employed as a substrate core and Ti 0.91 O 2 nanosheets were inorganic shell) were fabricated by electrostatic sequential deposition approach [13,14].For Ti 0.91 O 2 nanosheets were negatively charged, in order to load Ti 0.91 O 2 nanosheets onto the CFA by electrostatic deposition, the surface of CFA was modified by cationic polyelectrolytes (PEI and PDDA).The detailed fabrication procedure of layer-by-layer assembly (LBLA) method is as follows: 5 g CFA was dispersed in 200 mL PEI solution (2.5 g⋅L −1 , pH = 9) and further stirred for 60 min to ensure PEI was adsorbed on the CFA surface for introducing positive charges.After the mixture was centrifuged at 3000 rpm for 20 min, the supernatant was removed and then the CFA was washed dried in a drying oven under 60 ∘ C. Subsequently, the PEI-coated CFA (CFA/PEI) was dispersed in Ti 0.91 O 2 colloidal suspension (200 mL) and further stirring was carried out for 60 min.Small flocculated aggregates formed in the solution during this process for electrostatic attraction.The resulting CFA/PEI/Ti 0.91 O 2 was washed by water for 2 cycles to remove excess Ti 0.91 O 2 nanosheets.After drying, it was dispersed in 200 mL PDDA solution (20 g⋅L −1 , pH = 9), as the same as the PEI modifying process, and CFA/PEI/Ti 0.91 O 2 /PDDA was fabricated.The procedure for PDDA/Ti 0.91 O 2 alternative deposition was repeated  frequencies to synthesize a multilayer assembly-PEI/Ti 0.91 O 2 /(PDDA/Ti 0.91 O 2 ) m ( = 1, 3, 5, and 7).In this study, the photocatalysts as-prepared were abbreviated as Ti 0.91 O 2 /CFA- ( = 2, 4, 6, and 8), where n represented the loading frequency of Ti 0.91 O 2 nanosheets by LBLA.

Characterizations.
The morphologies of the samples were observed by a field emission scanning electron microscope (SEM) and the Ti elemental mapping was detected with an energy dispersive X-ray spectrometer (EDX) (S-4800, Japan).X-ray diffraction (XRD) data of the samples were collected using diffractometer (X'Pert PRO, Holland) with Cu K irradiation.The morphology of the exfoliated nanosheets was observed by transmission electron microscope (TEM, H-7650, Japan).UV-vis absorption spectra of all samples were recorded by a Shimadzu spectrophotometer (UV-2450, Japan).The chemical composition of CFA was characterized by quantitative X-ray fluorescence spectrum (XRF) analysis (Axios mAX, Holland).The N 2 adsorption-desorption isotherms, Brunauer-Emmett-Teller (BET) surface area, and Barrett-Joyner-Halenda (BJH) pore size distribution were obtained by surface area and porosity analyzer (ASAP 2020 M+C, America).

Evaluation of Photocatalytic Activity.
The photocatalytic reaction was carried out in a photochemical reaction system as described before [18].Briefly, the initial concentration and volume of MB were 20 mg/L and 50 mL, respectively.The dosage of Ti 0.91 O 2 /CFA- was 0.2 g.A 500 W UV lamp with major emission at 365 nm was used as a light source, and the irradiation time was 60 min.After reaction and settling for a while, the upper solution was centrifuged at 3000 rpm for 20 min to eliminate fine particles.Then the absorbance of supernatant was analyzed at the wavelength of 664 nm by a UV-vis spectrophotometer.

Results and Discussion
3.1.SEM Analysis of Samples.SEM was used to define the morphology of the Ti 0.91 O 2 /CFA microstructure.The images present CFA before (Figures 1(a) and 1(b)) and after (Figures 1(c)-1(h)) Ti 0.91 O 2 nanosheets were loaded by the LBLA approach.From Figures 1(a) and 1(b), it can be found that the surface of the CFA is smooth and barren.There are no obvious attachments on the CFA surface after rinsing with H 2 SO 4 and CH 3 OH, which indicates that the impurities on the surface had been removed.After Ti 0.91 O 2 nanosheets were deposited on the CFA by LBLA, some aggregates consisting of Ti 0.91 O 2 multilayer nanosheets were adhered to the CFA (Figures 1(c)-1(h)).With the increase of loading frequency, the area covered by corrugation was enlarged, and multilayers can be seen.It can be observed in Figure 1(c) that only very few aggregates are anchored on the surface of the CFA after twice repeated layer-by-layer loading.When the loading took place 4 times, the area of agglomerates was obviously enhanced (Figure 1(e)).Under high magnification (Figure 1(f)), there are more nanosheet agglomerates than with two loadings (Figure 1(d)).When the layer-by-layer loading frequency was further raised to 8, the agglomerates almost covered the CFA surface (Figures 1(g

) and 1(h)).
There is an obvious difference between the Ti 0.91 O 2 nanosheet agglomerates here and the TiO 2 nanoparticle agglomerates loaded on CFA.The Ti 0.91 O 2 agglomerates are of approximate rectangle sheet shape (Figure 2), and their counterparts (anatase TiO 2 ) are nanosphere agglomerates of 3D structure [19].This is ascribed to the different loading methods.The conventional loading methods for fabricating TiO 2 include the sol-gel-adsorption method and the hydrothermal method [26,27], and the TiO 2 agglomerates prepared are usually granular after calcination.In this study, the Ti 0.91 O 2 /CFA was fabricated without calcination under high temperature, so Ti 0.91 O 2 was able to keep its original crystalline phase and did not transform into anatase or rutile.Therefore, the Ti 0.91 O 2 nanosheets were still lamellar after loading on CFA.From the images of the morphology of the Ti 0.91 O 2 /CFA, it is certain that the Ti 0.91 O 2 nanosheets are successfully immobilized on the surface of CFA with layer-by-layer approach, although the amount of Ti 0.91 O 2 is smaller compared to our earlier work [19].

UV-Vis Absorption Spectra of Samples.
The UV-vis absorption spectra demonstrate the optical absorption characteristics of the CFA and Ti 0.91 O 2 /CFA samples.In Figure 3, the absorbances of the CFA do not change much along the range of wavelengths (200 nm-800 nm), except for a weak and broad peak at around 370 nm.Compared to the spectrum of the CFA, the spectra of the Ti 0.91 O 2 /CFA illustrate a characteristic absorption change along the whole range of wavelengths, with two strong absorption peaks at around 266 nm and 375 nm.The peak at around 375 nm was dependent on the immobilized multilayer nanosheets which were not completely exfoliated into monolayer nanosheets.So the absorption peak has a micro red-shift.The peak of the absorption curve around 266 nm is in accord with the multilayer titania nanosheets which had been studied [7], but the absorption peak is broader than that in the study of Sasaki et al. [28].This may be attributed to the fact that the selected substrates are different.The previous substrates were quartz glass platelets and silicon wafer chips (1 × 5 cm 2 ), which had a flat surface and different size from CFA.However, the substrate in this study was CFA of microsphere shape and the particle size of CFA was below 100 m.Therefore, the assembly style of titania nanosheets on the substrate was not exactly the same.This led to the discrepancy.It can be also observed that the absorption peak of Ti 0.91 O 2 /CFA-6 around 266 nm is stronger than the others.It may be explained that the peak around 266 nm was previously found to be unilamellar Ti 0.91 O 2 nanosheets [7], and there were more unilamellar Ti 0.91 O 2 nanosheets on the surface of the CFA than other types.On Ti 0.91 O 2 /CFA-2, 4, 8, the incompletely exfoliated nanosheets made up a larger proportion than Ti 0.91 O 2 /CFA-6, which weakened the intensity of the peak around 266 nm.4(a)-4(c).In Figure 4(a), CFA primarily contains mullite (Al 4.5 Si 1.5 O 9.75 ) and quartz (SiO 2 ), which is similar to the previous study [29].After Ti 0.91 O 2 were loaded, there was no obvious difference between CFA and Ti 0.91 O 2 /CFA.It could be two reasons:

Crystal
(1) Ti 0.91 O 2 nanosheets were adhered to the surface of CFA through electrostatic force, not entering the crystal lattices of mullite and quartz; (2) the amount of deposited Ti 0.91 O 2 nanosheets was small, so the influence on overall diffraction peak was little relatively.Therefore, no prominent changes in crystalline patterns of the CFA were observed clearly.However, there are still tiny differences between CFA and Ti 0.91 O 2 /CFA in the range of 2 among 26 ∘ -27 ∘ (Figure 4(b)) and 39 ∘ -40 ∘ (Figure 4(c)), which are partially enlarged details of Figure 4(a).At 2 = 26.6 ∘ , before Ti 0.91 O 2 were loaded, the peak of the CFA is very weak; after LBLA was carried out for several frequencies, there is a small peak, which is higher than the CFA sample.At 2 = 39.5 ∘ , the intensity of the CFA peak decreases remarkably after Ti 0.91 O 2 were immobilized.Figure 4(d) depicts the XRD pattern of protonic titanate before exfoliation.The peak at 9.5 ∘ is a characteristic of protonic titanate, and it disappears on Ti 0.91 O 2 /CFA, which can prove that slight changes of Ti 0.91 O 2 /CFA XRD pattern at 2 = 26.6 ∘ and 39.5 ∘ resulted from the loaded titania nanosheets via LBLA, not protonic titanate.This confirms that LBLA method is feasible to immobilize Ti 0.91 O 2 nanosheets.

𝑁 2 Adsorption-Desorption and Pore Distribution of Samples.
The BET surface area of the Ti 0.91 O 2 /CFA as-prepared is summarized in Table 2. Before Ti 0.91 O 2 were loaded, the BET surface area of CFA was 2.62 m 2 /g.After Ti 0.91 O 2 nanosheets were loaded, the BET surface area of Ti 0.91 O 2 /CFA-2 declined instead, so did the surface area of Ti 0.91 O 2 /CFA-4.When the loading frequency was 6, the specific surface area of Ti 0.91 O 2 /CFA-6 reduced to the minimum which is about  1.26 m 2 /g; the specific surface area increases slightly while the loading frequency rose to 8, however, still below 2 m 2 /g.The specific surface area decreased gradually with the increase in loading Ti 0.91 O 2 nanosheets, similar to the results reported previously [30,31].The reason may be that Ti 0.91 O 2 nanosheets immobilized by LBLA on the surface of the CFA were a few and lamellar, so it was difficult to form porous structure, which could not change the surface area much.
The specific surface area of Ti 0.91 O 2 /CFA-6 was the smallest among the samples.Combined with absorption analysis, this can be attributed to the monolayer nanosheets.Unilamellar Ti 0.91 O 2 nanosheets were loaded more regularly than multilayer (incompletely exfoliated nanosheets) on the surface of CFA, so the surface structure of Ti 0.91 O 2 /CFA-6 is less complicated than others with a smaller surface area.
Figure 5 shows the N 2 adsorption-desorption isotherms of CFA and Ti 0.91 O 2 /CFA samples.It can be seen that all the samples can be assigned as an isotherm of type 2 in the IUPAC classification [32,33].The fact that the hysteresis loop is difficult to observe and volume adsorbed is very small reveals that the samples are typically nonporous characteristic by LBLA [34].Moreover, it is shown that the adsorption capacity of the Ti 0.91 O 2 /CFA is lower than the CFA.With the loaded Ti 0.91 O 2 nanosheets increasing, adsorption capacity and the hysteresis loop of Ti 0.91 O 2 /CFA became low and small gradually, respectively, which implies that immobilization of Ti 0.91 O 2 nanosheets resulted in forming new structure (not 3D network), but did not increase the pore volume.
The pore size distribution of the CFA and Ti 0.91 O 2 /CFA in Figure 6 illustrates that pores of all samples are micropores and mesopores in a wide distribution range (<2 nm to 50 nm).Pores below 3 nm of all samples take a large amount with small pore volume.This can be explained by the fact that the specific surface area was very small and the samples were nearly nonporous.After Ti 0.91 O 2 nanosheets were anchored by LBLA on CFA, the mesopore distribution  Before the photocatalytic reaction began, the adsorption experiment in the dark lasted for 60 min to reach adsorption equilibrium [35].The absorbance of MB is proportional to its concentration, so the decoloration rate can be calculated by the equation as follows: (%) = 100( 0 −   )/ 0 % = 100( 0 −   )/ 0 %, where  0 and   are the absorbances of MB solution at initial and  time, respectively.
Figure 7(a) shows MB was eliminated only ∼10% by adsorption of samples and did not degrade in the blank experiment when the UV light was switched off.When the (1) (1)  A poor photocatalytic characteristic of CFA originated from its chemical component (Table 1), which was the same as our early work [18], so Ti 0.91 O 2 contributes to photocatalytic activity principally.This demonstrates again that it is successful to load Ti 0.91 O 2 nanosheets on CFA via LBLA.
Generally, the photocatalytic activity is enhanced with increasing the Ti 0.91 O 2 layer-by-layer loading frequency.With two frequencies loading, the decoloration rate of the Ti 0.91 O 2 /CFA-2 achieved 24.7% correspondingly; when Ti 0.91 O 2 loading accomplished 4 frequencies, the photocatalytic activity of the Ti 0.91 O 2 /CFA improved remarkably, with a decoloration rate of 41.8%, due to more Ti 0.91 O 2 nanosheets immobilized.While the loading frequency was raised to 6 further, the activity improved a little, up to a maximum decoloration rate about 43.2%.Nevertheless, the activity of Ti 0.91 O 2 /CFA-8 was lower than Ti 0.91 O 2 /CFA-4, 6.It is understandable that the amount of Ti 0.91 O 2 nanosheets anchored on CFA influences the photocatalytic activity.The more Ti 0.91 O 2 nanosheets were loaded, the higher activity the sample had.However, the excessive Ti 0.91 O 2 nanosheets would block the UV illumination into the interior of photocatalyst, with a depressed availability of UV irradiation, which was consistent with other studies [36][37][38].Meanwhile, the transfer of charge carrier may be also limited.It should be noted that Ti 0.91 O 2 /CFA-6 exhibited the highest photocatalytic activity whose specific surface area and pore volume are the smallest.It may be also explained that there are more Ti   profile with a stronger peak at 266 nm, which had been discussed in UV-vis and BET analysis.Although the decoloration rate (43.2%) was not too high when Ti 0.91 O 2 /CFA- were used as photocatalysts, Ti 0.91 O 2 /CFA- are very easy to separate and recycle from aqueous suspension due to the weight of CFA, which facilitates the recycle and reuse, as mentioned in our previous publication [18][19][20].
Figure 8    supporters uniformly [40,41].However, from Ti 0.91 O 2 /CFA as-prepared in our work, it can be seen that Ti element is welldistributed, which can improve the availability of substrate.It is a unique feature of Ti 0.91 O 2 /CFA.In a word, the unilamellar nanosheets can be easily and compactly immobilized on the CFA by LBLA, which is in favor of maintaining the stability of Ti 0.91 O 2 /CFA.

Conclusion
We fabricated a novel photocatalyst successfully via the layer-by-layer assembly method.Ti 0.91 O 2 nanosheets were immobilized on CFA by using sequential modification of cationic polyelectrolyte and Ti

3. 5 .
Photocatalytic Activity.The photocatalytic activity of Ti 0.91 O 2 /CFA was investigated by the degradation of MB solution as a test reaction.Blank experiment (MB solution without CFA and Ti 0.91 O 2 /CFA) was served as a comparison.
1−x O 2 4− ) referred to is Ti 0.91 O 2 0.36− and is abbreviated as Ti 0.91 O 2 .This study describes the fabrication of Ti 0.91 O 2 /CFA by the LBLA approach for controlling the uniform distribution of Ti 0.91 O 2 .The layered Ti 0.91 O 2 nanosheets were obtained by swelling and exfoliation of layered protonic titanate (-FeOOH type) in (C 4 H 9 ) 4 NOH (TBAOH) solution and were negatively charged.The surface of CFA was modified by cationic polyelectrolyte beforehand.Ti 0.91 O 2 nanosheets were adsorbed on the surface of CFA due to the electrostatic attraction and immobilized continually by repeating the LBLA procedure.Ti 0.91 O 2 nanosheets anchored served as a shell and CFA could be regarded as core.The photocatalytic activity of Ti 0.91 O 2 /CFA was also evaluated by the decoloration of methylene blue (MB).
Ti 1.825 O 4 ⋅H 2 O) was prepared from cesium titanate (Cs 0.7 Ti 1.825 O 4 ) by acid exchange for 3 days.And then, H 0.7 Ti 1.825 O 4 ⋅H 2 Structure of Samples.XRD was used to investigate phase structure changes of the CFA before and after Ti 0.91 O 2 were immobilized by LBLA.CFA and Ti 0.91 O 2 /CFA crystalline patterns are shown in Figures
0.91 O 2 monolayer nanosheets (with high photocatalytic activity by themselves and not blocking UV illumination) on the surface of Ti 0.91 O 2 /CFA-6 than others for its UV absorbance spectral illustrates the SEM and EDX micrographs of the Ti 0.91 O 2 /CFA-6 before (Figures 8(a) and 8(b)) and after photocatalysis (Figures 8(c) and 8(d)).It is obvious that there are almost no changes in the appearance of Ti 0.91 O 2 /CFA-6, and Ti 0.91 O 2 nanosheets can be still found on the surface Ti 0.91 O 2 /CFA as presented in Figure 8(e) which shows the distribution of Ti-containing species on the surface of CFA microsphere.This indicates that there is another difference between Ti 0.91 O 2 /CFA and conventional TiO 2 cluster immobilized on substrates.In conventional studies, it was difficult to control TiO 2 cluster loaded on International Journal of Photoenergy

Table 2 :
BET surface area and pore volume of CFA and Ti 0.91 O 2 /CFA.Samples CFA Ti 0.91 O 2 /CFA-2 Ti 0.91 O 2 /CFA-4 Ti 0.91 O 2 /CFA-6 Ti 0.91 O 2 /CFA-8 0.91 O 2 nanosheets.Since Ti 0.91 O 2 nanosheets have special properties, Ti 0.91 O 2 /CFA exhibits different characteristics, which can be concluded as follows.(1) Difference in morphology depends on the loading frequency.The amount of Ti 0.91 O 2 nanosheets on the CFA surface increases with the layer-by-layer assembly frequency increasing.Ti 0.91 O 2 nanosheets distribute uniformly on the surface of CFA whose availability can be improved.(2) CFA exhibits weak optical absorption in whole range of wavelength compared with Ti 0.91 O 2 /CFA, which proves that the immobilization of Ti 0.91 O 2 nanosheets by LBLA can enhance the UV-absorption of the CFA.Ti 0.91 O 2 /CFA has a UV-absorption peak around 266 nm.(3) Ti 0.91 O 2 /CFA shows photocatalytic activity.Significant enhancement in decoloration of MB can be achieved by Ti 0.91 O 2 /CFA compared to barren CFA.Ti 0.91 O 2 /CFA is also easy to recycle and of mechanical stability, causing little secondary pollution.