A surfactant free method for rutile TiO2 microspheres-graphene oxide composite and its photocatalytic performance

Accepted Manuscript A surfactant free method for rutile TiO2 microspheres-graphene oxide composite and its photocatalytic performance Guiming Peng, Ruian Du, Quanming Peng, Suqin Wu, Changlin Yu PII:

S0254-0584(18)30361-4

DOI:

10.1016/j.matchemphys.2018.04.092

Reference:

MAC 20587

To appear in:

Materials Chemistry and Physics

Received Date: 7 August 2017 Revised Date:

29 March 2018

Accepted Date: 25 April 2018

Please cite this article as: G. Peng, R. Du, Q. Peng, S. Wu, C. Yu, A surfactant free method for rutile TiO2 microspheres-graphene oxide composite and its photocatalytic performance, Materials Chemistry and Physics (2018), doi: 10.1016/j.matchemphys.2018.04.092. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical abstract:

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A Surfactant Free Method for Rutile TiO2 Microspheres-Graphene Oxide Composite and Its Photocatalytic Performance Guiming Peng,1,2,3* Ruian Du,1 Quanming Peng,3 Suqin Wu,1 Changlin Yu1* School of Metallurgy and Chemical Engineering, Jiangxi University of Science and

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1

Technology, Ganzhou 341000, P. R. China 2

School of Materials and Energy, Guangdong University of Technology, 100 Waihuan

CAS Key Laboratory of Renewable Energy, Guangzhou Institute of Energy

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3

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Xi Road, Guangzhou Higher Education Mega Center, Guangzhou 510006, P.R. China

Conversion, Chinese Academy of Sciences, Guangzhou 510640, P. R. China

* Corresponding author. address:

[email protected]

(G.P.),

phone:

+86-18379878908;

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Email

Abstract

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[email protected] (C. Y.), phone: +86-13763908974

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Titanium dioxide (TiO2) nanostructures and their graphene related composites have attracted tremendous interests due to their wide applications in organic contaminant photodegradation and renewable energy conversion and storage. In this report, rutile TiO2 microspheres with diameter of ～2.2 µm and TiO2 microspheres /graphene oxide composite were synthesized through a simple surfactant free solvent-thermal method. SEM images at different synthesis stages disclosed that the TiO2 microspheres initialized from radically assembled TiO2 nanorods clusters, and grew 1

ACCEPTED MANUSCRIPT up through the nanorod root fusion due to the lateral growth of the nanorods. Photodegradation experiments demonstrated the TiO2/GO composite exhibited more pronounced dye degradation performance than TiO2 microspheres owing to the excess

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charge transport route afforded by GO. In addition, photochemical stability test indicates the photo-oxidation of GO occurs in TiO2/GO composite upon UV light irradiation.

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Keywords: rutile titanium dioxide; microsphere; graphene oxide; photo-catalysis;

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photochemical stability 1. Introduction

Titanium dioxide (TiO2), a material with low cost, nontoxicity, and remarkable physical and chemical stability, has been intensively studied in renewable energy

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conversion and storage,[1-6] organic pollutant degradation,[7, 8] and sensing.[9] The synthesis of TiO2 nanostructures with controllable morphology is significant to tailor

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its practical applications. So far, various kinds of morphologies, such as nanowires or nanorods,[10] nanoplates,[11] nanospindles,[12, 13] et al. have been achieved for

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TiO2. Generally, as the addition of surfactants helps to generate emulsion system,[14, 15] or they are able to be favorably adsorbed onto certain crystal facet,[16, 17] surfactants are widely used to tune the morphology of TiO2 nanostructures.[18, 19] However, the use of surfactants itself together with the extra energy consumption in removal of these surfactants after materials synthesis is not a favorite choice from the sustainable

environmental

protection

viewpoint.

Thus,

synthesis

of

TiO2

nanostructures that requires no usage of surfactants is more advocated. 2

ACCEPTED MANUSCRIPT In addition, the humble conductivity within TiO2 skeletons is still one of the issues that hamper its practical applications. To overcome this issue, compositing TiO2 with conductive materials is believed to be an effective way. Graphene, a

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one-atom-thick carbon nanomaterial with good chemical stability, exhibits extraordinary conductivity. Hybridizing TiO2 with graphene is able to overcome the inferior conductivity of TiO2, which facilitates electron transport from TiO2 and

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reduces the charge recombination.[20] So far, tremendous efforts have been devoted

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to TiO2/carbonaceous nanomaterial composites for photo-catalysts,[21-24] energy conversion and storage.[25-27] Physical blend and in situ growth are two ways to synthesize such composites. Compared to the physically blend method, in situ growth of TiO2 starting from graphene sheets via chemical bonds provides better materials

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stability and more efficient charge transport within the composite because of the intimate contact.[13] The oxygen-containing functionalities on graphene oxide sheets enable the possibility to decorate graphene sheets with metal oxides.[28]

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Despite tremendous TiO2 nanostructures have been reported, many of them are

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using "black-box" synthesis methods without disclosing the morphology evolution process. The direct observation of nanostructure growth process would give more insights for the morphology tuning and better understanding of the material growth behavior. In addition, for the TiO2/nanocarbon photocatalysts, the photochemical stability is an important concern that deserves more attention because of the photocatalytic oxidation ability of TiO2. In this report, ～2.2 µm sized rutile TiO2 microspheres and TiO2 microspheres/graphene oxide composite (TiO2/GO) were 3

ACCEPTED MANUSCRIPT synthesized through a simple surfactant free solvent-thermal method. The growth process of the TiO2 microspheres was investigated by scanning electron microscopy (SEM) imaging at different synthesis durations. Subsequently, photo-degradation

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experiments of methylene blue (MB) and methyl orange (MO) were performed and discussed. Lastly, the long-term photochemical stability of TiO2/GO composite upon UV light illumination was also investigated.

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2. Experimental section

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2.1 Synthesis of TiO2 spheres and TiO2/GO composite

For TiO2 microspheres synthesis, typically, 0.1 mL titanium isopropoxide (TTIP) (99%, Aladdin) was firstly added into 20 mL HCl solution (1.5 mol/L) which was prepared by diluting HCl aqueous solution (3 mol/L) with methanol in 1:1 volume

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ratio. After 5 min vigorous stirring, the mixture was transferred to a 30 mL Teflon-lined stainless steel autoclave. The reaction was run by putting the sealed

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autoclave at 150 °C for 4 hours. After cooling to room temperature, the material was collected by centrifugation followed by washing with deionized water to remove the

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acid residue.

TiO2/GO synthesis was obtained following the same procedures with some

modifications. 2 mL GO aqueous solution (0.02 mg/mL) was firstly dispersed into 18 mL HCl solution (1.5 mol/L) which was made by diluting HCl aqueous solution (3 mol/L) with the methanol. 0.1 mL TTIP was injected into the mixture under stirring. Then the mixture was transferred to a 30 mL Teflon-lined stainless steel autoclave and was heated at 150 °C for 4 hours. After cooling to room temperature, the material was 4

ACCEPTED MANUSCRIPT collected by centrifugation followed by removal of the acid residue by washing with deionized water. 2.2 Materials characterization

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The synthesized materials were characterized by field emission scanning electron microscopy (SEM, FEI MLA650F), transmission electron microscopy (TEM, FEI

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Tecnai 20), x-ray diffractometry (XRD, Bruker D8-Advance), and Raman spectroscopy (Renishaw inVia). Specific surface areas of the catalysts were

adsorption/desorption at 77 K.

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determined by Brunauer Emmett Teller (BET, Micromeritics ASAP2010) nitrogen

2.3 Dye photo-degradation experiments

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Firstly, a suspension of 15 mg of TiO2 microspheres in 25 mL methyl orange (MO) (or methylene blue (MB)) aqueous solution (2.7×10-5 mol/L) in a quartz vessel was stirred under dark condition for 30 minutes. After that, the mixture was

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illuminated by 254 nm UV light with the lamp power efficiency of 12 Walts. The

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concentration of MO solution was monitored by UV-vis (Mapada UV6300) during photodegradation. The photocatalytic dye degradation ability of TiO2/GO was evaluated following the same method. The total organic carbon (TOC) measurements during the photocatalytic study were performed on a multiN/C2100 TOC analyzer. All the dye degradation experiments were conducted in ambient temperature. 2.4 TiO2/GO photochemical stability measurement A stirring TiO2/GO aqueous suspension (0.02 mg/L) of 20 mL was irradiated by 5

ACCEPTED MANUSCRIPT 254 nm UV light with the lamp power efficiency of 12 Walts. The experiments were performed in air. UV-vis spectra of the suspension were collected every 20 minutes during the whole UV light irradiation experiments. Control experiment was run by

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addition of 5 mL ethanol into the above TiO2/GO suspension while other conditions kept unchanged.

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3. Results and discussion

After material synthesis following the method detailed in Experimental section,

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monodispersed TiO2 spheres with the diameter centered at ~2.2 µm were obtained (Figure 1(a-b) and Figure S1). It is noted that the TiO2 spheres keep well separated without aggregation. With close observation, it is found that the surface of the TiO2 spheres is not smooth, where some bumps can be observed (inset in Figure 1a).

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To gain insight of the evolution mechanism of TiO2 microspheres, the role of HCl concentration was investigated by varying HCl concentrations in the synthesis

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solution. When using more diluted HCl solution (0.5 mol/L), no microspheres but shapeless TiO2 nanostructures with serious aggregation were obtained (Figure 1c).

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However, for the case when HCl concentration was 3 mol/L, flower-like TiO2 nanoclusters composed of TiO2 nanorods were synthesized (Figure 1d). The morphology differences when using different concentrated HCl solutions is ascribed to the different suppression degrees of TTIP hydrolysis rate by HCl. As is known, the presence of acids in the synthesis solution inhibits the hydrolysis of TTIP, and thus it is beneficial for the crystallization of the resultant TiO2. In the case of using 0.5 mol/L HCl solution, TTIP still reacts too fast, leading to the formation of shapeless 6

ACCEPTED MANUSCRIPT aggregated TiO2 nanostructures. Whereas, the hydrolysis of TTIP in HCl solution with its concentration of 3 mol/L takes place much more slowly, resulting in the formation of more open morphology, “flower-like” TiO2 nanorod clusters, which actually can be

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seen as the pre-stage of TiO2 microspheres (detailed below). Therefore, moderate TTIP hydrolysis rate controlled by HCl concentration is critical for the successful growth of TiO2 microspheres. It is necessary to mention that TiO2 microspheres also

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cannot be obtained when replacing HCl solution with the same concentrated H2SO4 or

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acetic acid aqueous solution (Figure S3). It can be concluded that the presence of HCl solution takes effect in two ways during the formation of TiO2 microspheres: (1) control the hydrolysis rate of TTIP; (2) the chloride ions induce the formation TiO2

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nanorod clusters,[5, 29] and finally direct the formation microspheres.

Figure 1. (a) SEM images of TiO2 nanostructures synthesized in 1.5 mol/L HCl solution. (b) size distribution of the TiO2 microspheres in panel (a). (c) SEM images of TiO2 nanostructures synthesized in 0.5 mol/L HCl solution. (d) SEM images of 7

ACCEPTED MANUSCRIPT TiO2 nanostructures synthesized in 3 mol/L HCl solution. Influence of alcohols were investigated by replacing methanol with ethanol and isopropanol during synthesis, respectively. Results showed that, instead of

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microspheres, broad size-distributed mulberry-like TiO2 nanostructures were obtained in both cases (Figure S2). This is because the alcohols of different polarities alter the

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reaction rate and the nanomaterial growth fashion.

Figure 2. (a) SEM images of TiO2 nanostructures obtained with different synthesis durations. (b) The proposed growth mechanism of the TiO2 spheres. (c) The TEM

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image of a TiO2 sphere with the synthesis duration of 4 hours. The scale bars are 500

The growth process of TiO2 microspheres were observed by SEM imaging the

TiO2 nanostructures at different synthesis stages (Figure 2a). Interestingly, at the stage of 0.5 hour, clusters radically assembled by conical nanorods were discerned. As the synthesis duration was elongated, the conical nanorods and the clusters grew larger and the interstices between the nanocones were gradually filled and fused starting from the root area, and finally evolved into a microsphere. In addition, the tips of the 8

ACCEPTED MANUSCRIPT nanocones still can be differentiated from the microspheres after four-hours synthesis, which were manifested as the small white spots (Figure 2a and the inset in Figure 1a). More clearly, the assembly process of the microspheres is depicted in Figure 2b. The

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proposed evolution process is further evidenced by the TEM image of a TiO2 microsphere, where radical nanorod profiles could be discerned and the sphere is more densely fused in the center (Figure 2c).

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The facile method can be also transferred to the in situ growth of TiO2 spheres on

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GO nanosheets. Figure 3 presents the SEM images of TiO2/GO composite. As can be seen, TiO2 spheres hybridize well with GO, and no free GO can be observed. On the other hand, part of the TiO2 spheres is exposed, rather than they are wrapped by GO sheets, indicating the growth of TiO2 microspheres starts from the GO sheets via

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chemical bonds, not physically attachment.

Figure 3. (a) SEM image of TiO2/GO composite. (b) SEM image of a TiO2 sphere on GO nanosheets.

XRD patterns of the TiO2 spheres and TiO2/GO were shown in Figure 4a. The

patterns are indexed well with the rutile phase TiO2 (JCPDS 01-076-0320). The diffraction peaks at 27.4°, 36.0°, 41.0°, 43.8°, 54.0°, 56.3°, 62.3°, 68.6°, and 69.3° are assigned to the (110), (101), (111), (210), (211), (220), (002), (301), and (112) of 9

ACCEPTED MANUSCRIPT rutile TiO2, respectively.[5] Raman spectra further confirm the TiO2 spheres are rutile phase. The Raman peaks at 231, 438, and 607 cm-1 are pertaining to the multi-photon, Eg, and A1g modes (Figure 4b), respectively.[10, 30] In addition, relative to that of

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TiO2 spheres, the Ag peak of TiO2 microspheres in TiO2/GO is blue-shifted to 431 cm-1, and the multi-photon peak is indiscernible, implying the strong interaction between the TiO2 spheres and GO. Moreover, the peaks at 1325 cm-1 and 1598 cm-1

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are associated with the disordered sp2 carbon (D) and well-ordered graphite (G)

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separately.[13, 31] The D/G intensity ratio reflects the extent of structural disorder of the graphitic material.[32] The D/G ratio decreased to 1.23 after incorporation with TiO2 compared to that of the pristine GO (D/G = 1.31). This means the sp2 carbon in GO is recovered to some extent during the in situ hydrothermal synthesis of TiO2

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microspheres. The partial reduction of GO is because of the presence of the reductive hydrocarbon moieties in TTIP molecules and the consumption oxygen-containing functionalities by formation of linkages with TiO2 nanostructures. Generally, the

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partial restoration of the sp2 graphitic structure of GO benefits its application in

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photocatalytic degradation of organic pollutants by accelerating charge separation.

Figure 4. (a) XRD patterns, (b) Raman spectra of TiO2 and TiO2/GO composite. 10

ACCEPTED MANUSCRIPT The specific surface area of both TiO2 and TiO2/GO were evaluated by the N2 adsorption-desorption isotherms measurements at 77 K. Overall, both materials exhibit the typical type-IV isotherms with a distinct hysteresis loop (Figure 5),

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indicating their mesoporous nanostructural characteristics. The corresponding Brunauer-Emmett-Teller (BET) specific surface areas are 60.8 m2/g for TiO2 microspheres, and 74.2 m2/g for TiO2/GO, larger than that of P25 (~50 m2/g).[33] The

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increase in surface area after hybridization with GO is attributed by GO. The high

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surface area suggests that the surface of TiO2 microspheres is accessible for organic

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dyes when utilized for photo-catalytic dye degradation.

Figure 5. N2 adsorption-desorption isotherm curves of (a) TiO2 nanopheres and (b)

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TiO2/GO composite.

Photo-catalytic performance of the TiO2/GO was tested by the photo-degradation

of MO and MB in aqueous solution by shining with 254 nm UV light. The experimental details are given in Experimental section. As can be seen, the MB is degraded faster than MO on both catalysts surface. However, the TiO2/GO exhibits superior degradation rates for both MO and MB with respect to that of TiO2 microspheres (Figure 6a and Figure 6b). The TOC measurements further confirm the 11

ACCEPTED MANUSCRIPT photo-catalytic mineralization of the organic dyes on the surface of both catalysts as time progresses. as well as the better photocatalytic activity of TiO2/GO than bare TiO2 (Figure 6c and Figure 6d). It is noted that the UV-vis and TOC results coincide

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largely with each other about the degradation rates. The superior photo-catalytic performance for the composite is mainly ascribed to the following reason. The incorporated partially reduced GO can accelerate the photo-generated electron

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separation and thus reduce carrier recombination. This allows a longer life time for

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to react with the organic dyes.

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the oxidative species (i.e. photo-generated holes, hydroxyl radicals, oxygen radicals)

Figure 6. The photodegradation performance of the catalysts toward (a) MO and (b) MB under 254 nm UV irradiation. (c) The photodegradation performance of MO evaluated by TOC measurement. (d) The photodegradation performance of MB 12

ACCEPTED MANUSCRIPT evaluated by TOC measurement. Previous reports together with our recent work show that the graphene material in anatase TiO2/graphene composites can be decomposed with long-term UV light

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exposure.[13, 34] However, the photochemical stability of rutile TiO2/graphene composite is not disclosed. Therefore, photochemical stability of the rutile TiO2/GO composite is studied here by monitoring the absorbance spectra of the catalyst during

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continuous UV light irradiation in air. By checking the UV-vis spectra during the

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experiments, it is observed that the absorbance of the catalyst beyond 400 nm decreased gradually as the UV light illumination progresses (Figure 7a). The inset in Figure 7a shows the decrease of the absorbance at 550 nm as a function of irradiation time. As the absorbance in visible light region is mainly attributed by GO from

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TiO2/GO. Thus the decrease in absorbance in the range above 400 nm indicates GO is partially oxidized when shining the TiO2/GO composite aqueous mixture in air with 254 nm UV light. This is because there are heteroatoms and functional groups bonded

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to the GO sheets, like hydrogen atoms and oxygen-containing groups, which makes a

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GO sheet could be envisioned as polyaromatic hydrocarbons. Upon UV light illumination, the photo-generated electrons can react with oxygen molecules to form O2•, or be taken away by the oxygen molecules from air.[35] This allows a longer life time for the photo-generated holes. Highly oxidative species like photo-generated holes, hydroxyl radicals and O2•, can be formed, and subsequently they attack and oxidize the attached GO sheets. To elucidate the oxidation mechanism, 5 mL ethanol which acts as hole capture agent was added into the TiO2/GO suspension. Obviously, 13

ACCEPTED MANUSCRIPT the absorbance change turns over that the absorbance above 400 nm increases upon UV light irradiation (Figure 7b), meaning the GO was reduced with the presence of hole scavenger upon UV light illumination in this situation. This agrees well with the

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case when compositing graphene with anatase TiO2 nanoparticles.[36] The GO reduction when photo-generated hole are erased strongly supports the fact that

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photo-generated holes can decompose carbon nanomaterials.

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Figure 7. (a) UV-vis spectra of TiO2/GO composite aqueous suspension upon 254 nm UV light illumination. (b) UV-vis spectra of TiO2/GO composite aqueous suspension

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with the presence of ethanol upon 254 nm UV light illumination. 4. Conclusions

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Conclusively, rutile TiO2 microspheres with diameter around 2.2 µm and TiO2/GO composite were synthesized through a surfactant free solvent-thermal method. SEM images at different synthesis stages revealed that the TiO2 microspheres come forth starting from the radically assembled TiO2 nanorods clusters. As the nanorods grow larger, the clusters become gradually fused from the root, and finally evolve into the microspheres. Photo-catalytic degradation experiments showed that the TiO2/GO exhibited superior photo-catalytic degradation ability than the TiO2 14

ACCEPTED MANUSCRIPT individual microspheres towards both MO and MB, owing to the fast electron transfer and reduced charge recombination contributed by partially reduced GO. Catalyst photochemical stability study showed that the GO in TiO2/GO can be oxidized by the

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photo-generated holes, implying extra caution should be taken when utilize graphene based composites under light illumination conditions. Acknowledgement

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This work was supported by CAS Key Laboratory of Renewable Energy

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(Y707ka1001), Natural Science Foundation of Jiangxi province (20171BAB213010), Department of Education of Jiangxi province (GJJ160670), and Jiangxi University of Science and Technology (jxxjbs 15023). References

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ACCEPTED MANUSCRIPT platform for photocatalysis? Mineralization of reduced graphene oxide with UV-irradiated TiO2 nanoparticles, Chem. Mater. 26 (2014) 4662-4668. [35] Y. Zhang, Z.-R. Tang, X. Fu, Y-J. Xu, TiO2-graphene nanocomposites for

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gas-phase photocatalytic degradation of volatile aromatic pollutant: Is TiO2-graphene truly different from other TiO2-carbon composite materials, Acs Nano 4 (2010) 7303-7314.

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[36] G. Williams, B. Seger, P.V. Kamat, TiO2-graphene nanocomposites. UV-assisted

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photocatalytic reduction of graphene oxide, Acs Nano 2 (2008) 1487-1491.

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ACCEPTED MANUSCRIPT 1. Rutile TiO2 nanospheres were obtained via a surfactant free method. 2. The growth mechanism of the TiO2 nanospheres was revealed. 3. TiO2 nanospheres/graphene oxide (TiO2/GO) composite was synthesized.

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5. Photochemical stability of the TiO2/GO was discussed.

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4. Photocatalytic performance of TiO2 nanospheres and TiO2/GO was discussed.