Synthesis and Characterization of Titania− Graphene Nanocomposites

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Synthesis and Characterization of Titania-Graphene Nanocomposites Timothy N. Lambert,*,† Carlos A. Chavez,† Bernadette Hernandez-Sanchez,‡ Ping Lu,§ Nelson S. Bell,| Andrea Ambrosini,† Thomas Friedman,⊥ Timothy J. Boyle,‡ David R. Wheeler,# and Dale L. Huber∇ Sandia National Laboratories, Albuquerque, New Mexico, 87185, Departments of: Materials, DeVices and Energy Technologies, Ceramic Processing and Inorganic Materials, Materials Characterization, Nanostructured and Electronic Materials, Nanomaterials Sciences, Biosensors and Nanomaterials, CINT Science ReceiVed: June 10, 2009; ReVised Manuscript ReceiVed: October 2, 2009

In this work, the synthesis and physiochemical characterization of titanium oxide nanoparticle-graphene oxide (TiO2-GO) and titanium oxide nanoparticle-reduced graphene oxide (TiO2-RGO) composites was undertaken. TiO2-GO materials were prepared via the hydrolysis of TiF4 at 60 °C for 24 h in the presence of an aqueous dispersion of graphene oxide (GO). The reaction proceeded to yield an insoluble material that is composed of TiO2 and GO. Composites were characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), Raman spectroscopy, N2 adsorption-desorption, and thermal gravimetric analysis/differential thermal analysis (TGA/DTA). This approach yielded highly faceted anatase nanocrystals with petal-like morphologies on and embedded between the graphene sheets. At higher GO concentrations with no stirring of the reaction media, a long-range ordered assembly for TiO2-GO sheets was observed due to self-assembly. GO-TiO2 composites formed colloidal dispersions at low concentrations (∼0.75 mg/mL) in water and ethanol but were not amenable to forming graphene papers via filtration through Anodisc membranes (0.2 µM pore diameter) due to their high titania concentration. Zeta potential measurements and particle size distributions from dynamic light scattering (DLS) experiments on these materials explain the stability of the TiO2-GO colloidal solutions. Chemical and thermal methods were also used to reduce TiO2-GO to give TiO2-RGO materials. Introduction Graphene, a 2-d monolayer of fused sp2 carbon bonds in a honeycomb-like network, has attracted a great deal of scientific interest due to its outstanding mechanical, electrical, thermal, and optical properties and theoretically high surface area of ∼2600 m2/g.1-4 Graphene-based materials (graphene, graphene oxide, exfoliated graphite, chemically modified graphene, etc.) have therefore found use in a variety of applications such as capacitors,5,6 liquid crystalline displays,7 nanoelectromechanical resonators,8 various types of films/papers/membranes,9-11 polymer composites,12,13 and as scaffolds for environmental sorbents14-16 and catalysts.17 The chemical methods for graphene synthesis/preparation have recently been reviewed,18 with a variety of preparative methods including: chemical vapor deposition, micromechanical exfoliation, epitaxial growth by heating silicon carbide, and by solution methods yielding colloidal suspensions. Solution methods are particularly appealing as they offer potentially low cost, scalable approaches that are amenable to further derivatization and solution processing. As one example, colloidal solutions of graphene-oxide (GO) can be readily obtained from graphite, yield dispersed individual flakes of graphene oxide at low concentrations, and can be rendered water or organic soluble.19 Additionally, GO was found * To whom correspondence should be addressed. Tel.: (505)-284-6967, Fax: (505)-844-7786, E-mail: [email protected]. † Department of Materials, Devices and Energy Technologies. ‡ Department of Ceramic Processing and Inorganic Materials. § Department of Materials Characterization. | Department of Nanostructured and Electronic Materials. ⊥ Department of Nanomaterials Sciences. # Department of Biosensors and Nanomaterials. ∇ Department of CINT Science.

to be amenable to additional functionalization, as well as being easily reduced to a conductive graphene like material [i.e., reduced graphene oxide (RGO) or chemically modified graphene (CMG)].18 Hence, solution methods are a convenient starting point for the development of various graphene-inorganic composites. The dispersion of inorganic nanomaterials onto graphene nanosheets, forming new nanocomposite hybrid materials, could lead to new materials with a myriad of potential applications, such as those listed above. With this in mind, graphene-based nanocomposites with metal nanoparticles (platinum,20-22 palladium,21,23 gold,24 copper25) and metal oxides (titania,26-29 clay,30,31 silica,28,32 polyoxometallates,14 birnessite manganese oxide,33 cobalt oxide,17 zinc oxide34) have recently been reported. Such inorganic nanomaterial-graphene composites can be prepared conceptually from two different solution based approaches: an in situ preparation of the inorganic component in the presence of a graphene dispersion/solution, or from solution mixing of the two previously prepared components. The in situ synthesis approach toward ceramic oxide-graphene materials has been limited to cobalt oxide-,17 manganese oxide-33 and titanium oxide-graphene oxide27,29 composites. Of particular interest, a suspension of GO was intercalated with TiO2 nanoparticles by treatment with titanium isopropoxide (Ti(OPri)4) and then hydrothermally treated with 10 M NaOH to form a GO-TiO2 nanotube composite with a combined surface area of 235 m2/g. The graphene sheet was said to influence the nanotube formation. Calcination at 1023 K led to the formation of titanium nanorod-graphene composites (215 m2/g), which exhibited photocatalytic degradation properties toward methyl orange.27 Most recently, TiO2-graphene composites have also been

10.1021/jp905456f CCC: $40.75  2009 American Chemical Society Published on Web 10/22/2009

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prepared in situ using an anionic surfactant stabilized-graphene approach and examined for their Li-ion insertion properties.29 The solution mixing approach has also been utilized to prepare colloidal TiO2-GO by mixing preformed TiO2 nanoparticles (2-7 nm in size, prepared from the hydrolysis of Ti(OPri)4 in ethanol) and GO in ethanol. UV-irradiated solutions were found to lead to photoreduction of the TiO2-GO to a colloidal TiO2-RGO.26 For this report, the in situ synthesis and physiochemical characterization of nanocomposites comprised of flowerlike anatase TiO2-graphene oxide (TiO2-GO) was undertaken. These new TiO2-GO composites were synthesized via the hydrolysis of TiF4 in the presence of aqueous dispersions of GO. Once isolated, TiO2-GO was found to form stable colloidal solutions across a broad pH range (pH ∼3-10), which is in agreement with data from zeta potential measurements. Subsequent chemical and thermal reduction of this composite material gave anatase-TiO2-RGO and anatase/rutile-TiO2RGO, respectively. Materials applications of TiO2-GO and TiO2-RGO are manifold and could conceivably range from photocatalysts27,35 to new electrical energy storage materials.29,36 Experimental Methods Syntheses of Graphene-TiO2 Nanocomposites. Synthesis of Graphite Oxide. Graphite oxide was prepared following the Hummers method37 using purified natural graphite (SP-1, 30 µm nominal particle size, Bay Carbon, Bay City, MI). Sample Preparation of TiO2-GO (Small Scale Method). GO (15 mg) was added to deionized water (20 mL, in a beaker) and placed in a bath sonicator (125 W) for 30 min to yield a light-brown solution. Titanium fluoride (TiF4) was then added with vigorous stirring to achieve the desired TiF4 concentration (0.005, 0.01, 0.02, 0.04, 0.08, or 0.16 M). The resulting solution was covered and placed in a bath sonicator (125 W) for 1 h and then heated at 60 °C for 24 h in an oven (beaker was covered with a watch glass).38 After this time, the reaction was allowed to cool and the solid products were centrifuged and washed three times with water, once with ethanol and then collected and dried under vacuum. Sample Preparation of TiO2-GO (Large Scale Method). GO (150 mg) was added to deionized water (200 mL, in a beaker or round-bottom flask) and placed in a bath sonicator (125 W) for 30 min to achieve a light-brown solution. TiF4 was then added with vigorous stirring to achieve a TiF4 the desired concentration (0.005, 0.01, 0.02, 0.04, 0.08, or 0.16 M, respectively). The resulting solution was placed in a bath sonicator for 1 h and then covered loosely with a watch glass and heated at 60 °C for 24 h in an oil bath with stirring.38 After this time, the reaction was allowed to cool and the solid products were centrifuged and washed three times with water, once with ethanol, and then collected and dried under vacuum. Sample Preparation of TiO2-RGO. A TiO2-GO composite (100 mg) was suspended in DI water (100 mL) with sonication for 30 min and then hydrazine hydrate (2 mL) was added. The stirred reaction was warmed to 100 °C and heated for 24 h in a round-bottom flask with water condensor. The reaction was cooled to room temperature and the precipitate was collected by filtration through a coarse glass fritted funnel and washed with water (∼500 mL) and then methanol (∼500 mL). Preparation of TiO2-GO Paper Material.9-11 An aqueous colloidal suspension of a TiO2-GO composite was prepared (20 mL @ 0.75 mg/mL) by sonication (125 W) for 1 h. The resulting solution was vacuum filtered through an Anodisc membrane filter (47 mm diameter, 0.2 µM pore size, Whatman).

Figure 1. Representative X-ray diffraction (XRD) patterns for a) graphene oxide (GO), insert shows d001 at 2Θ ) 10.2; b) TiO2-GO composite from 0.04 M TiF4 and TiF4/GO weight ratio ) 6.61; c) TiO2-GO composite from 0.08 M TiF4 and TiF4/GO weight ratio ) 13.2; broad peaks in b) are attributed to the GO in the composite. Parts b and c index to anatase phase of TiO2 (JCPDS file no. 21-1272: space group: I41/amd).

After filtration, the as-prepared materials were further suction dried (for up to 3 days) and then allowed to air-dry for 1-3 days at which time the paper material cracked as shown in Figure S4 of the Supporting Information. Analogous paper material made with only GO was peeled away from the Anodisc membrane following this drying procedure. Hazards. The reaction to prepare TiO2 from TiF4 (eq 1) generates hydrogen fluoride (HF). For information on hazards and mitigation procedures etc., see [http://www.osha.gov/SLTC/ healthguidelines/hydrogenfluoride/index.html]. Characterization. All samples were washed and dried prior to investigation. Powder X-ray Diffraction (PXRD). Powders were mounted directly onto a zero background holder purchased from The Gem Dugout. Samples were scanned at a rate of 0.02°/2 s in the 2θ range of 10-100° on one of two instruments: 1) a PANalytical powder diffractometer employing Cu KR radiation (1.5406 Å) and a RTMS X’Celerator detector, or 2) a Bruker D8 Advance diffractometer in Bragg-Brentano geometry with Cu KR radiation and a diffracted beam graphite monochromator. Phase identification was determined from the PXRD patterns using Jade 8 software suite. Scanning Electron Microscopy (SEM). The samples were dispersed onto carbon tape and imaged using a Zeiss Supra 55

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Figure 2. Scanning electron micrographs for TiO2-GO composites from varied concentrations (M) of TiF4 a) 0.01, b) 0.02, c) 0.04, d) 0.08, and e) 0.16. f) Electron dispersive spectroscopy micrograph for 0.08 M TiF4 and TiF4/GO weight ratio ) 13.2 composite. All were from reactions with aqueous GO dispersion at 0.75 mg/mL. Scale bars for parts a-e are 200 nm.

VP field emitter gun scanning electron microscope (FEGSEM). A Noran EDS detector and Noran System Six software were used for the acquisition of the EDS spectra. TiO2-GO samples were sputter coated with gold-palladium prior to analysis. EDS Data was plotted using Kaleidagraph software. Transmission Electron Microscopy (TEM). An aliquot of the TiO2-GO nanocomposite dispersed in ethanol (EtOH) was placed directly onto and allowed to dry on a holey carbon typeA, 300 mesh, copper TEM grid purchased from Ted Pella, Inc. The resultant particles were studied using a Philips CM 30 TEM with the Thermo Noran System Six Energy Dispersive X-ray (EDX) System, operating at 300 kV accelerating voltage. Brunauer-Emmett-Teller (BET) Surface Area Analysis. N2 adsorption/desorption on composite samples was measured using a Micrometrics ASAP 2020 or a Micrometrics Tristar 3000 sorptometer. Thermal GraWimetric Analysis and Differential Thermal Analysis (TGA/DTA). A TA Instruments SDT Q600 instrument was used for simultaneous DTA and TGA data acquisition. Data was analyzed using the TA Universal Analysis software. Samples (∼5-10 mg) were loaded into an aluminum oxide crucible and heated at a rate of 1 °C/min and held isothermally at the final temperature (1000 or 1100 °C) for 30 min.

Raman Spectroscopy. Raman spectra were recorded using a 514.5 nm Ar-Ion laser line fiber coupled to a Kaiser Optics Mark II holoprobe head with a 125x microscope objective at 10 mW power. The light was dispersed in an Acton SpectraPro-500i 0.5 m spectrometer using a 600 groove/mm grating onto a 1024 × 1024 CCD chip (Princeton Instruments) with a 25 µm pixel size giving a 2 cm-1 spectral resolution. The holographic notch filters (Kaiser Optics) cut off the spectra ∼450 cm-1 from the laser line. The powder samples were dispersed onto microscope slides and spectra were collected for 15 min to improve the signal/noise ratio. Resulting data was plotted and analyzed using Kaleidagraph software. Dynamic Light Scattering (DLS) and Zeta (ζ) Potential Measurements. DLS and (ζ-potential) measurements were performed using a Zetasizer Nano ZS instrument from Malvern. This instrument utilizes laser Doppler velocimetry with phase analysis light scattering to calculate the particle size distribution and the zeta potential with a red laser (633 nm). (ζ-potential) characterization was performed in background electrolyte of 10-3 M KNO3, and titrated with HNO3 and KOH for pH adjustment. Samples characterized were GO, TiO2-GO (TiF4/ GO weight ratio ) 6.61) and TiO2-GO (TiF4/GO weight ratio ) 13.2) composites. The samples were prepared by adding a

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Figure 3. Scanning electron micrographs for TiO2-GO composite from 0.08 M TiF4 and TiF4/GO weight ratio ) 13.2 and with aqueous GO dispersion at 1.5 mg/mL, a) low magnification showing many self-assembled stacks (scale bar ) 10 µM), b) magnification of approximate area from part a outlined in white rectangle (scale bar ) 2 µM), c) magnification of approximate area from 3b outlined in white rectangle (scale bar ) 1 µM), d) further magnification of c (scale bar ) 200 nM).

Figure 4. Transmission electron micrographs for TiO2-GO composite from 0.08 M TiF4 and TiF4/GO weight ratio ) 13.2 and with aqueous GO dispersion at 0.75 mg/mL. a-c) Bright-field images showing morphologies ranging from TiO2 nanoflowers to TiO2 seeds and graphene oxide; d) dark-field image showing that each petal is single crystalline. Part a, scale bar ) 100 nM; parts b-d, scale bar ) 0.2 µM.

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Figure 5. Transmission electron micrographs of TiO2 nanoflower from 0.08 M TiF4 and TiF4/GO weight ratio ) 13.2 and with aqueous GO dispersion at 0.75 mg/mL. a) Bright-field image showing alignment of crystallites and mesoporous structure (scale bar ) 50 nM), b) dark-field image showing alignment of individual crystallites (scale bar ) 50 nM), c) magnification of area from 5b - lattice fringes are present indicating crystalline nature (scale bar ) 10 nM), d) selected area electron diffraction pattern taken from a large number of particles (like the image in part b of Figure 4). Pattern indicates TiO2 is crystalline and indexes to anatase (JCPDS file no. 21-1272: space group: I41/amd).

small portion of solid material (∼100 mg) to approximately 40 mL of 10-3 M KNO3 solution and dispersed using a cup horn cell with an ultrasonic horn. The power was set to 50%, and the samples were dispersed with 0.5 s pulses for at least 2 min before being placed in the Zetasizer Nano ZS instrument.

evaporate forming a film. The reflectance spectra were analyzed using the Kebulka-Munk function approach as reported in the literature.39

The index of refraction and absorption coefficient were measured for each material at ∼633 nm for analysis in the Zetasizer. Single wavelength ellipsometry was performed on an Accurion EP3 ellipsometer using 630.2 nm light and angles of incidence from 40 to 60°. Values of delta and psi were collected for all four ellipsometric zones and were averaged, and these data were fit using an iterative least-squares minimization of the Fresnel equations to determine optical constants. The index of refraction was determined to be n ) 1.85 for the GO material, and the composite GO-TiO2 material had an index of n ) 2.05. Particle size analysis by DLS determines the size for an equivalent spherical diameter particle, and hence the values shown here are best interpreted as relative trends in aggregation rather than quantitative diameters. Band Gap Measurements. Bandgap measurements were obtained from a Shimadzu UV-3600 UV-vis-NIR spectrometer. An integrating sphere was utilized in order to measure diffuse reflectance of the material. Compounds were dispersed in ethanol, carefully dropped onto a glass slide, and left to

The hybrid TiO2-GO composite materials were prepared from the hydrolysis of TiF438,40 in the presence of aqueous dispersions of GO (typically 0.75 mg/mL), as shown in eq 1. Concentrations of TiF4 were varied from 0.005, 0.01, 0.02, 0.04, 0.08 to 0.16 M (i.e., TiF4/GO weight ratio ) 0.82, 1.65, 3.30, 6.61, 13.2, 26.4). These reactions were first performed on smaller scale by oven heating the TiF4-GO solutions in a beaker with no stirring and later in standard glassware with stirring. At these Ti/GO ratios listed above, the reaction proceeded to form a black/brown to gray precipitate that was collected by centrifugation and washed with excess water, ethanol, and then dried. If complete recovery of the GO from the reaction occurs (the supernatant was colorless in the majority of samples) then the percent conversion of TiF4 to TiO2 was generally 2-10% as determined by weight, with higher concentrations yielding the greater percentage conversion.

Results and Discussion

TiF4 + GO + 2H2O h TiO2-GO(s) + 4HF

(1)

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Figure 6. a) Nitrogen sorption isotherms for GO (dashed line) and a TiO2-GO nanocomposite (solid line) from 0.08 M TiF4 and TiF4/GO weight ratio ) 13.2; b) thermogravimetric analysis for (i) GO, (ii) TiO2-GO from 0.005 M TiF4 and TiF4/GO weight ratio ) 0.82, (iii) TiO2-GO from 0.01 M TiF4 and TiF4/GO weight ratio ) 1.65, (iv) TiO2-GO from 0.04 M TiF4 and TiF4/GO weight ratio ) 6.61, (v) TiO2-GO from 0.08 M TiF4 and TiF4/GO weight ratio ) 13.2.

PXRD and SEM. GO (in the solid state this is referred to as graphite oxide) forms a well-ordered layered structure, as indicated by a well-defined d001 peak in its PXRD spectrum, determined to be at 2Θ ) 10.2 for the GO prepared here, as seen in part a of Figure 1. This sharp peak is lacking in the synthesized TiO2-GO composites, indicating disruption of the GO layered stacking, as seen in parts b and c of Figure 1. The fact that diffraction from the GO is still observable (albeit broad in nature and at a lower intensity) at lower Ti/GO ratios (i.e., part b of Figure 1) would suggest that the GO is reassembled in a more heterogeneous manner in the composite. PXRD data for the TiO2-GO composites indicate that at all concentrations the anatase phase of TiO2 (JCPDS file no. 21-1272: space group: I41/amd) was exclusively formed, as seen in parts b and c of Figure 1. Figure 2 demonstrates the heterostructures formed from this reaction as observed with SEM. At low concentrations of TiF4, TiO2 was formed almost exclusively in a highly faceted flowerlike morphology ∼200-400 nm diameter for the cluster, as seen in part a of Figure 2. As the concentration of TiF4 in the reaction was increased (i.e., from parts a-e of Figure 2), the same morphology was observed; however, an elongated

J. Phys. Chem. C, Vol. 113, No. 46, 2009 19817 particle or seedlike morphology that generally ranges from ∼150-250 nm on the long axis and ∼100-150 nm on the short axis was also formed, as seen in part e of Figure 2. Qualitatively, at higher concentrations there was an increase in the number of seedlike structures observed, which is consistent with a higher rate of nucleation for the higher concentration of TiF4 in the reaction. Further inspection reveals that the TiO2 is embedded in between (or more likely, in between several stacks of) GO layers, as seen in Figure 2 and in Figure S1 of the Supporting Information. Interestingly, the hydrolysis reaction to form TiO2 was found to proceed poorly without the presence of the GO, which would suggest the necessity of a template for seeded TiO2 growth. Whereas the structure for GO is somewhat ambiguous,41-44 GO is highly oxygenated with hydroxyls (and epoxides) on the basal plane and carboxylic acids along the edges (and at defect sites), which can act as ligands for a the hard Ti4+ Lewis acid.27 Whereas the formation of HF gas should drive the reaction to the right, it is likely that the growth of TiO2 onto the GO surface (and subsequent flocculation of the TiO2-GO composite) is a larger driving force, as HF is highly soluble in water. Zeta potential measurements given below corroborate this pH driven aggregation. Furthermore, the partial covering of the reaction vessel during the reaction should minimize HF evaporation. A representative EDS spectrum is shown in part f of Figure 2 and demonstrates that Ti and F are present in the composite. Our conjecture is that this is due to F bound to the nanocrystalline surface as Ti-F, although a low percentage (10) during the reduction. XRD of the TiO2-CRGO composite confirms that the TiO2 remains in the anatase phase, as seen in part d of Figure 8. The first indication of conductivity is again from SEM. Whereas TiO2-GO samples must be coated with Pd0/Au0 before analysis, the TiO2-RGO powder is inherently conductive enough for direct imaging in the SEM. BET analysis on N2 adsorption-desorption data gave a surface area of only 81 m2/g. For comparison, a control experiment with the hydrazine reduction of GO to RGO48 gave surface areas of 459 m2/g. This decrease in surface area is unexpected, given that the TiO2 bonded to basal plane oxidation sites is expected to be reductively cleaved, resulting in a material similar to RGO with some entrapped TiO2; however, the loss of smaller TiO2 nanoparticles (as mentioned above) in the composite, as well as a material that is less dispersible than GO would lead to a lower surface area for a composites surface area that is largely dominated by the TiO2. Vapor phase hydrazine reduction53 on carefully deposited dispersed colloidal solutions (part a of Figure 4 for structure) may provide high surface area thin films in the future. Raman Spectroscopy. The chemical reduction of TiO2-GO to TiO2-RGO was also confirmed with Raman spectroscopy, as seen in Figure 9. GO has been reported to exhibit Raman shifts at ∼1594 and 1363 cm-1,41 corresponding to the G- and D-bands, respectively.48,54,55 Upon reduction with hydrazine (to form RGO) these numbers shift to lower values, 1584 and 1352 cm-1, with the G-band taking on a characteristic asymmetric shape. The accompanying increase in D/G ratio for RGO has been explained by the presence of smaller but more numerous sp2 domains in the carbon.48 Similar trends are observed for

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Figure 10. a) Zeta (ζ) potential measurements in 1.0 mM KNO3. (O) GO, (+) TiO2-GO from 0.04 M TiF4 and TiF4/GO weight ratio ) 6.61, (∆) TiO2-GO from 0.08 M TiF4 and TiF4/GO weight ratio ) 13.2, (0) commercial anatase TiO2 nanopowder. Each point represents the average of 12 measurements - standard deviation was typically 3-10% (not shown for clarity); Dynamic light scattering (DLS) data for b) graphene oxide (GO); c) TiO2-GO from 0.04 M TiF4 and TiF4/GO weight ratio ) 6.61; d) TiO2-GO from 0.08 M TiF4 and TiF4/GO weight ratio ) 13.2.

the transformation from TiO2-GO to TiO2-RGO here. The G-band shifts from 1602 to 1583 cm-1, whereas the D-band shifts from 1355 to 1347 cm-1, as seen in parts a and c of Figure 9. The G-band at 1583 cm-1 appears to have the asymmetric shape associated with RGO. A marked increase in the D/G ratio is also observed, consistent with the reduction of the graphene sheets, forming TiO2-RGO, as seen in part c of Figure 9. Anatase TiO2 has six fundamental transitions for optical modes A1g, B1g, and Eg.56 The Raman shifts of Eg ) 639 cm-1 and A1g/B1g ) 516 cm-1 (for GO-TiO2, as seen in part a of Figure 9) and Eg ) 630 cm-1 and A1g/B1g ) 511 cm-1 (for RGO-TiO2, as seen in part b of Figure 9) are similar to reported values for anatase TiO2: Eg ) 633 cm-1 and A1g/B1g ) 515 cm-1.56 (Instrument limitations prevented examination of Raman peaks below 400 cm-1) GO-TiO2 Paper Assembly. It was determined that the GO-TiO2 composites prepared here can be redispersed in water (or EtOH) at 0.75 mg/mL to form stable colloidal solutions. Hence, attempts were made to prepare TiO2-GO papers by filtration of aqueous colloidal suspensions of TiO2-GO through Anodisc membrane filters as reported for GO.9,10,57,58 Aqueous dispersion of composites prepared with a low (TiF4/GO weight ratio ) 0.82) and high concentration (TiF4/GO weight ratio ) 13.2) sample were attempted. Upon filtration, the filtrate was partially cloudy and white in color indicating that some TiO2 nanoparticles had separated from the GO composite and passed though the filter. A black/brown film was initially formed in both cases similar to that for GO filtration; however, upon further drying TiO2-GO materials peeled away from the filter and cracked, as seen in parts b and c of Figure S4 of the Supporting Information. It is our conjecture that the large branched TiO2 particles destabilize the stacking of GO sheets, especially where layers are overlapped as needed to form a continuous paperlike

material. This is consistent with the observation of buckling in the carbon sheet obtained from the low concentration dispersion (TiF4/GO weight ratio ) 0.82, Figure S5 of the Supporting Information). Alternatively, nonisotropic drying effects due to the presence of TiO2 may be to blame, despite extensive drying. The lower loaded TiO2-GO (TiF4/GO weight ratio ) 0.82) produced the less cracked paperlike material more closely resembling that of the GO. Zeta (ζ) Potential Measurements and Dynamic Light Scattering Measurements. To gain insight into the colloidal solution properties of the TiO2-GO composites, ζ-potential and dynamic light scattering (DLS) measurements were undertaken on GO, TiO2-GO (TiF4/GO weight ratio ) 6.61) and TiO2-GO (TiF4/GO weight ratio ) 13.2) and commercial (Aldrich) anatase TiO2 nanopowder, as seen in Figure 10. The development of the ζ-potential as a function of pH relates to the surface groups in the studied material. For GO, the surface groups include hydroxyl, epoxide, and strongly acidic carboxyl groups at the edges,15 with the pKa values of the surface species for GO displaying a strongly acidic character. GO shows a significant ζ-potential59 even under highly acidic conditions (-35 to -40 mV at pH 2.5), which is sufficient for the exfoliation and dispersion of GO in water, as seen in part a of Figure 10. As expected, the ζ-potential increases in strength as the alkalinity of the solution is increased (∼ -60 mV at pH 9.0). The surface modification of GO nanosheets with TiO2 presents little change in the development of the ζ-potential beyond a reduction in the strength of the measured value. The lower loaded TiO2-GO sample (TiF4/GO weight ratio ) 6.61) exhibits a pH profile similar to that of GO, whereas the higher loaded sample (TiF4/ GO weight ratio ) 13.2) has a profile that is more linear in the development of negative surface charge. The nucleation of titania nanoparticles on the GO surfaces appears to mask the

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surface sites and thereby lower the particle charge and ζ-potential but is not significant enough to affect colloidal stability for the redispersed TiO2-GO samples, as seen in part a of Figure 10. However, during the synthesis of TiO2-GO the strongly opposite ζ-potential values for TiO2 and GO and the drastic lowering of the pH with HF are likely the driving force for aggregation of the TiO2-GO composite. Particle size variation is evident as a function of pH for the titania-modified GO materials. As expected for a sample with high ζ-potential values, the same particle size is observed for GO across the entire pH range, shown by the bimodal distribution with peaks at 50-60 nm and ∼300 nm, as seen in part b of Figure 10. The peak fit at 3 µm is small and may be an instrumental artifact in the fitting of the DLS data. In contrast, both TiO2-GO samples analyzed exhibit a pH dependence attributed to the aggregation of the TiO2 with the GO nanosheets. For TiO2-GO (TiF4/GO weight ratio ) 6.61), there is a large wide peak at ∼800 nm that dominates the light scattering of the system at pH 3.66 and a very small component at 100 nm. Raising the pH even slightly causes a decrease in the particle size, which stabilizes at 200 nm with a broad distribution. The higher loaded sample of TiO2-GO (TiF4/GO weight ratio ) 13.2) shows and even more drastic pH dependence of it is particle size. At lower pH values, there is a broad peak at 900 nm, which begins to shift to a lower value as the pH is raised. This shift is complete even as low as pH 5.9. Particle size analysis by DLS determines the size for an equivalent spherical diameter particle, and hence the values shown here are best interpreted as relative trends in aggregation rather than quantitative diameters. To interpret these results, we use the electrostatic interactions between the dissimilar TiO2 nanoparticles and graphite oxide nanosheets as indicated by the ζ-potential response to pH. The presence of positively charged TiO2 among sheets of negative GO would result in attraction and the formation of larger colloidal aggregates. Because of the inability to produce free titania nanoflowers (as mentioned above), the ζ-potential of commerical TiO2 nanopowder was measured for evaluating electrostatic interactions between the two materials, as seen in Figure 10. The isoelectric point (IEP) for this titania is approximately 6.5, suggesting that, under acidic conditions, the TiO2 and GO would exhibit electrostatic flocculation. The synthesized nanoflower titania IEP may be lower, as the aggregation only appears to impact the materials at pH 4.6 or lower. When pH is increased, both materials are negatively charged and can be separated for the most part. Use of strong ultrasonic forces was not attempted in these size measurements, and the inability to restore the 50-60 nm peak present in the unmodified material could result from hydrodynamic effects as the pH was raised, incomplete charging of the bound titania, or a kinetic phenomenon. In summary, the electrostatic interactions between the dissimilar materials explain the flocculation and particle size coherently. Conclusions We have demonstrated the ability to prepare TiO2-GO composites via the hydrolysis of TiF4 at 60 °C in the presence of an aqueous dispersion (∼0.75 mg/mL) of GO. This approach yielded highly faceted anatase nanocrystals, with petal-like morphologies on and embedded between the graphene sheets. At higher GO concentrations (ex., 1.5 mg/mL) with no stirring of the reaction media, long-range ordered assembly for TiO2-GO sheets was observed due to self-assembly. GO-TiO2 composites formed colloidal dispersions (∼0.75 mg/mL) at low

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