Carbon nanosheet-titania nanocrystal composites from reassembling of exfoliated graphene oxide layers with colloidal titania nanoparticles

Journal of Solid State Chemistry 197 (2013) 329–336

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Carbon nanosheet-titania nanocrystal composites from reassembling of exfoliated graphene oxide layers with colloidal titania nanoparticles Yong-Jun Liu a, Mami Aizawa a, Wen-Qing Peng a, Zheng-Ming Wang a,b,n, Takahiro Hirotsu c a Energy Storage Materials Group, Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba, Ibaraki 305-5869, Japan b Adsorption and Decomposition Technology Research Group, Environmental Management Technology Research Institute, National Institute of Advanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba, Ibaraki 305-5869, Japan c Health Research Institute, National Institute of Advanced Industrial Science and Technology, 2217-14 Hayashi-cho, Takamatsu, Kagawa 761-0395, Japan

a r t i c l e i n f o

abstract

Article history: Received 29 May 2012 Received in revised form 19 July 2012 Accepted 5 August 2012 Available online 16 August 2012

Nanoporous composites of carbon nanosheets (CNS) and titania nanoparticles (NPs) were synthesized by reassembling of delaminated graphite oxide (GO) layers with titania clear sol (TCS), and their structural and porous properties were examined by various physico-chemical methods such as XRD, TG/DTA, FT-IR, Raman, FE-SEM/TEM, and low temperature N2 adsorption. It was found that the facile approach, which utilizes the electrostatic attraction between the negatively charged GO layers and the positively charged TCS particles, leads to a well composed CNS and ultrafine TiO2 NPs material whose titania amount reaches up to 71 wt%. The titania phase in these composite materials is mainly anatase, which is resistible against high temperature calcination, but also contains a little amount of rutile and brookite depending on synthesis condition. The porosity of the composite is improved and partially affected by the size distributions of TiO2 NPs. The unique structure, better porosity, and compatible surface affinity of these composites bring about an adsorption concentration-promoted photocatalytic effects toward organic dyes by successfully combining both properties of CNS and titania NPs. & 2012 Elsevier Inc. All rights reserved.

Keywords: Graphene oxide Carbon nanosheet Titania Exfoliation Reassembling Composite

1. Introduction Carbon nanosheets (CNSs) such as graphene with a single atomic layer or a stack structure of several graphene layers have attracted a great deal of concerns due to their extraordinary electrical and thermal conductivities, mechanical stiffness, and chemical specialty and stability [1–3]. Composing CNS with functional metal oxide nanoparticles (NPs) or metallic NPs can generate exceptional combined properties which cannot be realized separately by NPs or CNSs themselves. It is well known that, resembling clay minerals, graphite oxide (GO) is rich of intercalation chemistry, which facilitates exfoliation and ion-exchange properties toward varieties of organic and inorganic materials [4–13]. GO can thus be a favorite precursor to obtain composites with other functional materials. Recently, graphene or CNS composites are becoming a hot research topic among scientific and technology communities due to their hopeful applications in the fields of catalysis, adsorption/gas storage, capacitor/battery, fuel cell, sensing, and so forth [14,15]. Increasing numbers of publications have been devoted to the hybridization of graphene with other inorganic materials such as silica [16], titania [17–19], iron

n

Corresponding author. Fax: þ81 29 861 8866. E-mail address: [email protected] (Z.-M. Wang).

0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2012.08.020

oxides [20], tin oxide [21], and metallic nanoparticles such as platinum [22], palladium [23], gold [24], silver [25], etc., where GO or GO-derived graphene materials were used as the precursor. So far we have reported nanoporous carbon-silica composites from GO precursor via surfactant-mediated synthesis route which possess remarkable porosity and special surface affinity [14,26–29]. Through intercalation and the corresponding posttreatment procedures, we were possible to utilize the layers of GO as the templates for loading of highly dispersed 1 D tubular titania or palladium nanoparticles with a significant amount [30–32]. Titania is a promising functional material applicable not only for environmentally benign photocatalysis but also for photovoltaic devices, solar cells, and so forth [33–36]. Hybridization of titania with a carbon substrate can facilitate a coupled effect from semiconductive titania and carbon layers with special chemical affinity or electrical conductivity. For example, hydrophobic carbon-supported photoactive TiO2 materials are reported to have a promoted adsorption concentration-assisted photocatalytic effect [37]. TiO2-hybridized graphene materials can be used as an efficient transparent electrode in a dye-sensitized solar cell [38]. In previous reports where TiO2 NPs were hybridized with nanoporous carbons such as activated carbons [37,39,40], TiO2 NPs are supported only on the external surface of carbon because they are loaded mainly through interaction with surface functional groups on the periphery of pore entrances. By the contrast, TiO2 NPs can

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be encapsulated in between carbon nanosheets when using GO as the supporting precursor if the basal plane oxygen containing groups (OCGs) such as –OH groups on GO can be efficiently used as the loading sites. These composite structures are expected to be more effective for achieving a better hybridization performance in an adsorption concentration-promoted photocatalytic process and are favorable especially in a process which requires charge transfer through the roles of CNSs. The delamination–reassembling technique is a unique method to introduce guest molecules into interlayer spaces of host layered materials [10,41,42]. In this method, contacting of the delaminated layers of a layered material with a guest species having an opposite charge reassembles the layers to a new guest species-intercalated layered structure. We reported that the delamination–reassembling method is a facile way to synthesize TiO2 nanocrystal-CNS composites [43]. Herein, we report in detail the synthesis conditions, formation mechanism, structural and porous properties, and liquid phase adsorption and photocatalytic behavior of these composites.

2. Experimental 2.1. Materials GO was synthesized from natural graphite by Hummers– Offeman’s method [44]. Its chemical formula was determined as C8H1.44O1.4 by elemental analysis (after subtracting water contents estimated from the weight loss below 393 K) and cation exchange capacity (CEC) to be 5.4 mmol/g by alkali titration [27]. In this work, GO was used as synthesized without further surface modification or surface functionalization and we put our main weight on the examination of synthesis conditions of TiO2 NPs for the purpose of controlling crystalline size, type and compositions of titania. For the synthesis of titania clear sol (TCS), special grade titanium isopropoxide (Ti(i-OC3H7)4, TTIP) was purchased from Wako Pure Chemical Industries, Ltd. and used as Ti precursor. First, 8.52 g of TTIP was slowly added into 0.36 M HCl under vigorous stirring. The molar ratio of HCl to TTIP was changed from 0.36 to 4. The obtained white slurry was then peptized by stirring at 323 K for 3 h, till which the transparent TCS was formed. Secondly, 0.2 g of GO was dispersed in 20 ml of 0.05 M NaOH solution and subjected to ultrasonic treatment for 30 min. The GO-dispersed solution was then added dropwise into the TCS and the mixture was continuously stirred at 323 K for 3 h. The addition amounts of titanium and GO in the solution were controlled by maintaining the molar ratio of titanium over the amount of exchangeable H þ in GO (the multiplication of CEC and the amount of GO) to be 22.3. After reaction, the mixed solution was washed with de-ionized water for several times during which excess titania sol in the supernatant solution was removed alternatively by centrifugation and decantation. Finally, the composite powders were collected after drying at 313 K overnight in a vacuum oven. These samples were denoted as NPGC-xTi where x stands for the molar ratio of HCl to TTIP. The NPGC-xTi samples were further calcined at 823 K for 2 h in vacuum to obtain the heat-treated samples which are denoted NPGC-xTi-823. For comparison and formation mechanism studies, several reference materials are prepared. In contrast to the above procedure where GO was previously dispersed (delaminated) in diluted alkali solution, the composite sample from non-delaminated GO (i.e. that without pre-dispersion in alkali solution) was prepared by directly adding GO powder into TCS while other procedures and conditions are the same as the above for NPGC-xTi. These samples were denoted as GCT-x (before calcination) and GCT-x-823 (after calcination at 823 K). The background pure

titania powders were synthesized by evaporating H2O from TCS prepared at x ¼1.44, which were further calcined at various temperatures for 2 h either in air or in vacuum. The obtained titania powders were denoted as TiO2-aT and TiO2-vT where a, v, and T stand for air atmosphere, vacuum atmosphere, calcination temperature, respectively. In addition, one part of the NPGC-xTi and GCT-x samples were further subjected to hydrothermal treatment by the following procedures. After re-dispersion of the sample powder in 10 ml of de-ionized water, the composite powder-immersed solution was sealed in a Teflon-lined stainless steel autoclave having an internal volume of 50 ml and the autoclave was kept in an oven at 423 K for 3 h. The hydrothermally treated samples were collected after centrifugation, washed with de-ionized water, and further calcined at 823 K. These samples were denoted as NPGC-xTi-823-HTt and GCT-x-823-HTt where HT and t stand for the hydrothermal treatment condition and treatment time, respectively. 2.2. Characterization X-ray diffraction patterns (XRD) of samples were measured by a Rigaku Co. made RU-300 system using CuKa radiation (l ¼0.15406 nm) in a 2y range of 51–901. The operating tube voltage and current were 40 kV and 80 mA, respectively. Data were collected at a scanning speed of 21 min  1 and a sampling angle interval of 0.021. The thermal gravimetry (TG) and differential thermal analysis (DTA) were carried out by a Rigaku Co. made Thermo 2000 TG/ DTA system under a constant air flow with a rate of 100 ml min  1. Temperature was raised from room temperature to 573 K at a ramp rate of 1 K min  1 and from 573 up to 1073 K at 5 K min  1. The diffuse reflectance infrared Fourier transform (DRIFT) spectra of samples were measured by a Nicolet NEXUS 470-type FT-IR spectrometer with a MCT detector. Each spectrum was recorded from 256 scans at a resolution of 2 cm  1 after CO2 species physisorbed completely disappeared from the spectra through continuous purge of nitrogen gas of 99.99% purity. RAMAN spectra were collected with a Kaiser Optical Systems Inc. made HoloProbe 532 system containing continues wave frequency-doubled Nd-YAG laser light with a wavelength of 532 nm and a CCD detector. Laser light with a power of 1 mW was focused onto the sample for 60 s and the spectra were collected by two scans at a resolution of 5 cm  1. The morphologies of samples were observed by a Topconmade DS-720 type field-emission electron microscope (FE-SEM) with a ZrO-W thermal field-emission electron gun and an in lens observation system. The accelerating electron voltage was 5 kV. Transmission electron microscopy (TEM) was performed by a Topcon EM-002B apparatus under an accelerating electron beam voltage of 120 kV. Sample powders after grounding were dispersed in water and one drop of the solution was dripped onto a polymer-coated copper grid, which was used as the observation object of TEM after drying at room temperature. The nitrogen adsorption isotherms were measured at 77 K by a Belsorp Co. made Belsorp 18A-type volumetric apparatus. Samples were degassed at 823 K below 1 mPa for 2 h before adsorption. 2.3. Liquid-phase adsorption and photocatalytic reaction Methyl orange (MO) was used as a model organic molecule to examine adsorption properties and photocatalytic activities of the composite samples in the liquid phase. For the adsorption experiment, 10 mg of sample was added into a 50 ml conical flask containing 20 ml of MO solution. The initial concentration (C0) of MO is 50 mg L  1. The conical flask was then hermetically sealed and immersed in a water bath whose temperature was

Y.-J. Liu et al. / Journal of Solid State Chemistry 197 (2013) 329–336

maintained at 303 K. This system was covered for assuring a dark condition. After continuous agitation for a time interval, the supernatant solution was collected by centrifugation and its MO concentration (C) was measured by a JASCO Co. made V650 type UV–vis spectrometer using absorbance at 453 nm. The adsorption amount of MO, WMO, ad, in mg g  1 was calculated by the following equation: W MO,ad ¼ ðC 0 C 24 Þ  V=M

ð1Þ

where C24 is the MO concentration in the supernatant solution after adsorption in the dark for 24 h, V and M are the solution volume (20 ml) and sample weight (10 mg), respectively. For photocatalytic experiment, Erlenmeyer tube with a quartz cover was used as a reactor to assure penetration of the UV light. The sample weight, solution concentration and volume were the same as those in adsorption experiment. After continuous agitation under dark condition for 24 h to allow adsorption equilibrium, photocatalytic reaction was set up by switching on six sterilizing lamps (254 nm) situated on the top of the water bath, each with a power of 15 W. After reaction for a time interval, the light irradiation was stopped and the powder sample was separated from solution by centrifugation. The MO concentration in the supernatant solution at each time interval was measured using the same method as the above.

3. Results and discussion

331

structure-derived peak is not detected for the collected hybrid particles (NPGC-1.44Ti) within the detection limitation of the present XRD measurement conditions. Instead, broad diffraction peaks characteristic of anatase are observed (Fig. 1(b)), which reflects the ultrafine crystalline property of the clear sol. Calcination of NPGC-1.44Ti at an increasingly higher temperature leads to more evident and sharper peaks due to anatase, indicating the gradual growth of anatase crystal with increasing calcination temperature (Fig. 1(c–g)). Besides those from anatase phase, a small peak at 2y ¼  30.71 characteristic of brookite phase and the obvious peaks due to rutile phase were also detected in the XRD pattern of NPGC-1.44Ti-T with TZ823 K. Fig. 2(A) and (B) show the XRD patterns of the calcined composite samples synthesized at various conditions. The different synthesis conditions do not realize the layer regularity but do affect the crystalline structure of titania contained in the composites. A higher ratio of hydrochloride over titanium (x) in preparation of clear sol (Fig. 2(A)) is favorable for the growth of rutile phase in the composites in agreement with previous researches which proposed that electrophilicity of H þ and nucleophilicity of Cl  promote the formation of rutile structure, where parallel chains of octahedrons are linked through sharing vertices with another chain of octahedrons, rather than the anatase structure, in which edge-sharing of octahedrons plays an important role in crystal formation [47,48]. The diffraction peaks due to rutile for GCT-1.44-823 (Fig. 2(B)) are much more evident as compared to those for NPGC-1.44Ti-823, indicating that composite structure

3.1. XRD pattern

: anatase : rutile : brookite

Because of the negatively charged properties of GO layers, it was found that the TiO2 NPs with positive surface charges in TCS (in acidic solution) [45,46] are easily stuck on the basal plane of the exfoliated GO to form flocculation of hybridized particles which are finally separated and collected by centrifugation. Fig. 1 shows XRD patterns of the NPGC-1.44Ti and NPGC-1.44Ti-T samples in comparison with that of the pristine GO. The pristine GO presents a sharp (001) peak with an interlayer distance of 0.79 nm (2y ¼11.141) and detectable (10) peaks with repeating distances around 0.21 nm (2y ¼42.521) in its XRD pattern, characteristic of a good layered structure. However, the layer

x=4

1.44 1.08 0.72

(101) (110)

Intensity / a.u.

: anatase : rutile : brookite

(g)

0.36

(e)

Intensity / a.u.

(f) (e)

(d)

(d) (c)

(c) (b)

(b)

(001)

(a) 10

(10) 10

30

(a) 50 2θ

70

90

Fig. 1. XRD patterns of (a) GO, (b) NPGC-1.44Ti, and NPGC-1.44Ti-T samples where T¼ (c) 523, (d) 623, (e) 723, (f) 823, and (g) 923 K.

30

50 2θ

70

90

Fig. 2. Comparisons of XRD patterns among (A) NPGC-xTi-823 samples prepared at different x and (B) samples prepared at various conditions. (a): NPGC-1.44Ti823, (b): GCT-1.44-823, (c): NPGC-1.44Ti-823HT3, (d): NPGC-1.44Ti-823HT6, and (e): GCT-1.44-823HT3.

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Y.-J. Liu et al. / Journal of Solid State Chemistry 197 (2013) 329–336

layers play an important role in partitioning titania NPs from contacting each other, thus slowing down the growing speed of titania size and the formation of rutile. Table 2 further shows the values of LA(101) and AF for the composite samples synthesized at different conditions. The LA(101) value are almost the same for NPGC-xTi-823 prepared at different x. While NPGC-xTi-823 prepared at a small x value contain pure antase phase, rutile phase appears for NPGC-xTi-823 at x Z1.08 with the fraction gradually increasing with x. Hydrothermal treatment does not seriously influence the sizes of anatase NPs, but slightly promote the formation of rutile phase. GCT-1.44-823 has a greater fraction of rutile as compared to NPGC-1.44Ti-823 possibly due to the fact that this sample synthesized from non-delaminated GO does not achieve a good composition structure between TiO2 NPs and CNS.

has an effect on the crystalline growth mechanism of titania contained inside it. Furthermore, similar to high temperature treatment, hydrothermal treatment has a negative effect on maintaining pure anatase phase no matter whether GO has been exfoliated or not. If TiO2 NPs and CNS are well composed, CNS can function as nano-partitions to separate TiO2 NPs and prevent them from aggregating to become larger sized particles upon high temperature calcination. In order to quantitatively examine this effect, Table 1 compares the values of the crystal size of anatase, LA(101), and the anatase fraction, AF, for NPGC-1.44Ti-T with those for TiO2-aT and TiO2-vT obtained under different atmospheres. Here, LA(101) was calculated by the following Scherer’s equation: LAð101Þ ¼ 0:9l=BcosyB ðnmÞ

ð2Þ

where l is the wavelength of CuKa irradiation (0.15406 nm), B is the full width at half maximum, and yB is the diffraction angle of the anatase (101), and AF was obtained by the following empirical equation [49]: AF ¼ 100  IAð101Þ =ðIAð101Þ þ 1:26IRð110Þ Þ ðwt%Þ

3.2. TG/DTA, DRIFT and RAMAN spectra Fig. 3 shows the TG and DTA curves of the composite samples synthesized at different conditions. The TG/DTA curve of NPGC1.44Ti is characteristic of a large weight loss below 393 K due to the hydroscopic property of GO and two exothermic DTA peaks due to GO layer decomposition (a slight peak around 473 K) and carbon burning around 600 K, respectively. By the contrast, all the calcined samples show only the DTA peak due to carbon burning, demonstrating that GO layers are transformed to carbon nanosheets due to the releasing of OCG on GO during the calcination step in synthesis. Furthermore, the DTA peak due to carbon burning gradually shifts to a high temperature side by hydrothermal treatment and by increasing the hydrothermal treatment time, indicating that hydrothermal treatment leads to more sufficient liberation of OCG remained on CNS, thus enhancing the stability of carbon layers against oxidizing burning. On the other hand, GCT-1.44-823 exhibits a larger weight loss and a great DTA exothermic peak due to carbon burning, indicative of a great carbon content and comparatively small titania content. Table 2 shows the TiO2 content of the samples prepared at various conditions, which are obtained from the weight of the residual white powders after the TG programs. Because of the oxygen and hydrogen components in GO, TiO2 content of NPGC-1.44Ti is comparatively smaller than those of the calcined samples. The differences in TiO2 contents among the composite samples prepared at different x values or that by further hydrothermal treatment are negligible, indicating that titania content is likely determined by the OCG amounts on GO with which TCS can interact and the x value and hydrothermal treatment condition mainly affect the crystal phase constitution of titania NPs. In comparison with NPGC-1.44Ti-823, the TiO2 content in

ð3Þ

where IA(101) and IR(110) are peak intensities of anatase (101) and rutile (110) reflections, respectively. As shown in Table 1, the fast crystal growth in air leads to formation of a larger fraction of rutile with a very large LA(101) value from a low calcination temperature (as low as 523–623 K) for the TiO2-aT samples. Vacuum condition seriously retards the growths of rutile phase and the growth of particle size of anatase in TiO2-vT probably because of the serious lack of oxygen in the condition. As compared to the TiO2-aT and TiO2-vT samples, the crystalline sizes of NPGC-1.44Ti-T are ultrafine until a very high T (923 K) and there exists only the single anatase phase up to 723 K. This indicates that a good composition structure of CNS and TiO2 NPs was formed in the NPGC-1.44Ti-T samples, in which carbon

Table 1 Comparisons of titania crystal phases in composites and pure titania obtained at different calcination atmosphere and temperatures. T (K)

523 623 723 823 923

LA(101) (nm) (AF (wt%)) TiO2-aT

TiO2-vT

NPGC-1.44Ti-T

4.6 11 16 25 37

4.1 4.9 11 22 40

3.4 3.6 5.0 5.7 8.2

(50) (58) (65) (67) (19)

(100) (81) (81) (68) (40)

(100) (100) (100) (78) (27)

Table 2 Values of LA(101), AF, TiO2 content, pore parameters, MO adsorption and photodegradation rate constant. SBET (m2 g  1)

V0 total (ml g  1)

Vmeso (ml g  1)

Vmicro (ml g  1)

Dmeso (nm)

WMO, ad (mg g  1)

rn

56 71 71

398 293 347

0.33 0.29 0.32

0.25 0.29 0.29

0.10 0.00 0.03

3.7 3.5 3.7

36 64 83

79 78 48 70

67 70 68 73

335 323 310 281

0.31 0.26 – 0.28

0.33 0.22 0.44 0.26

– 0.04 – 0.02

3.7 3.7 3.7 3.8

70 53 59 53

– 0.8 0.13 (r1) 0.9 0.7 0.8 0.7

3.7

63

37

212

0.26

0.31

–

4.0

33

4.9

100

100

86

0.12

0.15

–

3.5

28

Sample

LA(101) (nm)

AF (wt%)

NPGC-1.44Ti NPGC-0.36Ti-823 NPGC-0.72Ti-823

3.9 6.5 5.9

100 100 100

NPGC-1.08Ti-823 NPGC-1.44Ti-823 NPGC-4Ti-823 NPGC-1.44Ti823HT3 GCT-1.44-823

5.9 5.7 6.7 5.5

TiO2-v623 n

TiO2 content (wt%)

r1 in parenthesis stands for the 1st-order rate constant in  10

3

lg

1

h

1

, those without r1 mark are the zero-order constant r0 in  10  3 h  1.

0.05 (r1) 0.7

Y.-J. Liu et al. / Journal of Solid State Chemistry 197 (2013) 329–336

Weight loss / wt%

20

(e) (d)

(c) (b)

(a) (f)

4

Heat flux / mV/g

(a) (b) (c) (d) (e) (f) 237

473

673 T/K

873

1370

Fig. 3. (A) TG and (B) DTA curves of (a) NPGC-1.44Ti, (b) NPGC-1.44Ti-823, NPGC1.44Ti-823HTt samples for t¼ (c) 1, (d) 3, (e) 6 h, and (f) GCT-1.44-823.

(e) (d)

K-M function

1296 866 3393

1746

333

3612 cm  1 (acidic –OH) and O–H bending band or C–OH stretching band around 1150–1250 cm  1, –COOH groups in the vicinity of layer edges, characteristic of –CQO group at 1746 cm  1 and also O–H stretching band around 3612 cm  1 (overlapping with that from acidic –OH), and the epoxy groups on the basal plane around 1065 cm  1 [26,50]. Contacts of TCS with non-delaminated GO only leads to a slight reduction in peak intensities due to – COOH or acidic –C–OH species at 1746 and 3612 cm  1 (relative to the neighbor peaks at 1620 and 3456 cm  1 due to physisorbed water) (Fig. 4(b)), indicating that titania particles are partially attached on GO through interaction with one part of the edge corner –COOH or acidic –C–OH group in GCT-1.44. By the contrast, the peak at 3612 cm  1 completely disappears and the peaks at 1746 cm  1 and around 1150–1250 cm  1 are seriously weakened (as judged from the changes in relative peak intensity with respect to that at 1620 cm  1 due to physisorbed water) for NPGC-1.44Ti (Fig. 4(c)), manifesting the sufficient interaction of the positively charged TCS particles with both the basal plane –C– OH species and the edge corner –COOH groups in this sample. The more intensified Ti–O–Ti peak below 900 cm  1 for NPGC-1.44Ti as compared to GCT-1.44 is attributed to a greater content of titanium in this sample. Therefore, coinciding with the above TG/DTA results, FT-IR results support the consideration that sufficient interaction of TCS with OCGs on the delaminated GO results in a greater content of TiO2 NPs which are consequently well composed with CNS in the NPGC-xTi-823 samples. Fig. 5 shows Raman spectra of the pristine GO and the composite samples synthesized at different conditions. Despite a greater TiO2 amount as compared to GCT-1.44-823, no signals due to titania are detected in the spectrum of NPGC-1.44Ti (Fig. 5(b)), suggesting that titania NPs in this sample are well dispersed inside GO layers which lowers the detection sensitivity. On the other hand, titania signals are detectable for the sample after calcination (Fig. 5(c)) and become increasingly stronger by further hydrothermal treatment (Fig. 5(e)), which can be attributed to crystalline growth (Table 2) and detachment of titania NPs from carbon layers during decomposition of GO layers. An enhanced peak at 153 cm  1 characteristic of brookite [51] can be observed on the spectra of the two samples, indicative of existence of brookite structure inside them in good agreement with the XRD results. The merged peaks from both anatase at 399, 519 and 639 cm  1 and rutile at 447 and 612 cm  1, respectively [52] in the spectra of GCT-1.44-823, NPGC-1.44Ti-823, and NPGC-1.44Ti823HT3 can be another evidence for the co-existence of anatase and rutile phases in these samples. Similar to the pristine GO, all

(c)

Titania

1350 1593 D′/G D

153

(b) (a)

(e) 1620

300 2000 Wavenumber / -1cm

1065 1000

Fig. 4. DRIFT spectra of (a) GO, (b) GCT-1.44, (c) NPGC-1.44Ti, (d) NPGC-1.44Ti823, (e) NPGC-1.44Ti-823HT3.

GCT-1.44-823 synthesized from non-delaminated GO is much smaller, suggesting that only one part of OCG on GO take part in interaction with the TCS particles and serve as the loading sites for TiO2 NPs. Fig. 4 shows DRIFT spectra of the pristine GO and the composite samples prepared at different conditions. The spectrum of GO exhibits absorption bands from –C–OH groups on the basal plane, characteristic of O–H stretching band around

Intensity

3456 3612

(d) 447 639 519 399

0

(c)

612

500

(b) (a) 1500 1000 Raman shift / cm-1

2000

2500

Fig. 5. Raman spectra of (a) GO, (b) NPGC-1.44Ti, (c) NPGC-1.44Ti-823, (d) GCT1.44-823, (e) NPGC-1.44Ti-823HT3.

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of the composite samples exhibit D band around 1350 cm  1, characteristic of disordered structure, and G band around 1593 cm  1 [53] with a broad width in the carbon range of the Raman spectra. In comparison with those of other samples, the peak intensity of D band relative to G band is obviously enhanced for NPGC-1.44Ti-823-HT3, indicative of an increase in the amount of edges or deficient sites [54] on carbon nanosheets. Accordingly, this result agrees very well with those of the above TG/DTA and FT-IR that hydrothermal treatment likely leads to more sufficient detachment of oxygen from CNS. 3.3. Microscopic structure and porosity Fig. 6(a) shows a FE-SEM image of the NPGC-1.08Ti-823 sample from which one can observe edges of stacked plate-like structures with the surface deposited by a lot of white dots possibly being of titania nanoparticles. The same image can be more clearly observed in a TEM image (Fig. 6(b)) in which black dotted particles are uniformly distributed along carbon platelets. A high-resolution TEM image (Fig. 6(c)) further confirms a stacking of carbon platelets with nanosized titania particles dispersed inside them. The size of titania NPs are around 5 nm or smaller, roughly in agreement with the Scherer’s results. Some particles are found to have lattice fringes with an interplane distance of 0.29 nm, characteristic of the (121) reflection of brookite crystal [43]. Hence, the microscopic data also evidences a well composed structure of CNS and mixed phases (anatase, rutile, brookite) ultrafine titania NPs. Fig. 7 shows N2 adsorption isotherms at 77 K on the composite and pure titania samples. NPGC-1.44Ti exhibits the greatest N2 adsorption among these samples, whose N2 adsorption isotherm is of a hybrid shape of types I and II, [55,56] indicative of a good composition structure of GO layers and TCS spacers, which contains both micropores and mesopores. NPGC-1.44Ti-823 with the unique composition structure of CNS and titania NPs still maintains a great amount of N2 adsorption as compared to GCT-1.44-823, further indicating that non-delamination condition is not beneficial for realizing a good composition structure. The shape of N2 adsorption isotherm on NPGC-1.44Ti-823 changes to the typical type IV, suggesting that decomposition of GO layers during calcination of NPGC-1.44Ti mainly leads to the loss of micropores. Additionally, structural rearrangement by hydrothermal treatment slightly reduces N2 adsorption. The composite samples have a much greater N 2 adsorption as compared to the pure titania samples, further indicating the formation of a good composition structure.

Table 2 shows the values of specific surface area, SBET, from the Brunauer–Emmett–Teller (BET) method and mesopore volume, Vmeso, and width, Dmeso, from the Barret–Joyner–Halenda (BJH) method [55], the total pore volume, Vtotal, from the amount of N2 adsorption at P/P0 ¼0.95, and the micropore volume, Vmicro, from the subtraction of Vmeso from Vtotal. All the samples have an average mesopore width around 3.5–4 nm. While the Vmeso value changes a little, calcination and hydrothermal treatment of NPGC1.44Ti leads to the loss of microporosity and a smaller SBET value. The SBET value of NPGC-1.44Ti-823 is one and half fold greater than that of GCT-1.44-823 obtained from non-delaminated GO and about four fold greater than that of the pure TiO2-v623 due to a good composition structure. Among the NPGC-xTi-823 samples, the SBET and V0, total values become the greatest for NPGC-0.72Ti-823 where no rutile phase is found, but become smaller for samples prepared at xZ1.08 where rutile phase appears. This result suggests that the crystal phase constitution may be a factor affecting porosities of the composites because structural rearrangement in the process of forming mixed phases of TiO2 likely leads to a more heterogeneous distribution of particle sizes and thus a partial loss of the function of TiO2 NPs as a spacer. 3.4. Adsorption and photocatalytic properties Fig. 8 shows time courses of adsorption and photodecomposition of MO on the NPGC-xTi-823 samples in comparison with those on GCT-1.44-823 and the pure TiO2-v623. During

(A) 200 (a) V(N2) / ml-STPg-1

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150

100

(b)

(d)

(c) (e)

50

(f) 0

0.2

0.6 0.4 P/P0

0.8

Fig. 7. N2 adsorption isotherms at 77 K on (a) NPGC-1.44Ti, (b) NPGC-1.44Ti-823, (c) NPGC-1.44Ti-823HT3, (d) GCT-1.44-823, (e)TiO2-v623, and (f) TiO2-v823.

5 nm 500 nm

1.0

10 nm

Fig. 6. (a) FE-SEM and (b, c) TEM images of NPGC-1.08Ti-823.

Y.-J. Liu et al. / Journal of Solid State Chemistry 197 (2013) 329–336

Dark

UV

1.0

C/C0

0.8 0.6

0.4 0.2 0 0

24 48 Time course / h

72

Fig. 8. Time courses of MO adsorption and decomposition on NPGC-xTi-823 at x¼ 0.36 (J), (b) 0.72 (&), (c) 1.08 (m), (d) 1.44 (K), and (e) 4 (~), (f) GCT-1.44823 (X), and (g) TiO2-v623 (W).

adsorption at the dark condition, MO concentration on TiO2-v623 and GCT-1.44-823 sharply decreases at the initial 3 h and become constant after then, indicating a fast adsorption process. The MO adsorption on GCT-1.44-823 is slightly enhanced as compared to that on TiO2-v623, which can be attributed to the carbon component and the better porosity. Regarding the light irradiation results, while that on TiO2-v623 shows a zero order linear relationship, the time course of concentration reduction of MO on GCT-1.44-823 is non-linear, indicative of a higher order (concentration-dependent) degradation. This result implies that an improved adsorptivity of MO on the carbon component of GCT1.44-823 establishes an adsorption concentration-promoted photocatalytic effect. As compared to the GCT-1.44-823 and TiO2-v623 samples, reductions in MO concentration during adsorption on NPGC-xTi823 are much more evident, indicating that the hydrophobic CNS and the good composition structure, which brings about a more favorable surface affinity and a better porosity, are more favorable for adsorbing organic molecules. In addition, unlike the fast adsorption on GCT-1.44-823 and TiO2-v623, MO concentration on the NPGC-xTi-823 samples experiences a sharp drop at the initial stage and continues to decrease gradually even after 24 h. This phenomenon suggests that, following an initial fast uptake of MO, there likely exist a rate-determining intra-particle diffusion process inside the composite structure. The light irradiation time course of MO concentration on NPGC-0.72Ti-823, which has the greatest SBET value and contains pure anatase phase, is characteristic of a concentration-dependent higher order relationship, indicating that the process is via an adsorption concentrationpromoted mechanism. On the other hand, time courses of photodegradation on other NPGC-xTi-823 samples show a zero-order relationship, resembling that on the pure TiO2-v623 sample. Table 2 shows the adsorption amount WMO, ad and the photodegradation rate constants of the samples. The value of WMO, ad increases up to the greatest 83 mg g  1 for NPGC-0.72Ti823, being more than two fold greater than those on GCT-1.44823 and TiO2-v623, and reversely decreases with further increasing x at x40.72. The changes in WMO, ad is basically the same tendency as their orders of SBET or Vmeso values, indicating that porosity is an important factor for MO adsorption. The 1st order photodegradation rate constant (r0) of NPGC-0.72Ti-823 is enhanced by 2.6 times as compared to that of GCT-1.44-823, owing to the two and half times enhanced MO adsorption due to the good composition structure (the greatest SBET, high Ti content, and pure anatase phase). In the case of GCT-1.44-823, one part of

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MO adsorbed may not be supplied for photodecomposition on the consequence of separation of loading sites of TiO2 NPs (at the edges of CNSs) and adsorption sites of MO (in pores or basal planes of CNSs). Thus, its composite structure cannot facilitate an as good adsorption concentration-promoted photocatalytic effect as that of NPGC-0.72Ti-823. On the other hand, characteristic of the zero-order reaction, rate constants (r0) of the NPGC-xTi-823 samples other than that for x¼ 0.72 are independent of their WMO, ad values, and comparable with each other and with that of the pure TiO2 sample (TiO2-v623) too. This result suggests no establishment of adsorption concentration-promoted photocatalysis for these samples although they show reasonable adsorption and photocatalytic properties. The main reason responsible for the difference between these samples and NPGC-0.72Ti-823 is probably related to the delicate differences in their composite structures featured by porosity and crystal phases (anatase and its fraction), and the complexity in local surface structures which influence surface diffusion of MO molecules from adsorption sites to photoactive ones.

4. Conclusions Contacting of negatively charged delaminated GO layers and positively charged TCS particles gives a good nanoporous composite of CNS and titania NPs. XRD results show that the composites contain ultrafine TiO2 NPs whose crystal phase are either pure anatase or mixed phases of anatase, rutile, and brookite, depending on the synthesis conditions. Greater HCl to TTIP ratio, higher calcination temperature, and hydrothermal treatment are negative factors for maintaining pure anatase phase. The titania NPs resist against high temperature calcination as compared to the pure titania NPs, due to the partition effect of CNS in the composites. TG/DTA and FT-IR results demonstrate that the composite materials from delaminated GO contains a larger amount of titania due to the sufficient interaction of TCS particles and OCGs (both the edge corner –COOH and the basal plane C– OH) on GO in the initial reassembling process. These methods together with RAMAN spectra confirm that hydrothermal treatment leads to sufficient detachment of OCG from CNS and the increase in edge amounts on CNS, which enhances the stability of CNS against burning in air. FE-SEM and TEM results demonstrate the good composition structure in which nanosized TiO2 particles (around 5 nm) are encapsulated in between or loaded on the top surface of CNS. Low temperature N2 adsorption shows the good porosity for these composites which is partially dependent on the crystal phases inside them. The composite with the best porosity and pure anatase phase exhibits adsorption concentrationpromoted photocatalysis toward organic dyes due to the successful combination of the surface compatible property of CNS and photoactivity of titania NPs.

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