Synergetic photocatalysts derived from porous organo Ti–O clusters pillared graphene oxide frameworks (GOFs)

Abstract

Unique nanocomposites comprising thin TiO₂ nanoplates and lamellar carbon sheets were derived from organo Ti–O clusters pillared graphene oxide (GO), a porous layered framework synthesized by intercalating GO layers with organo-Ti reagents via a simultaneous intercalation and hydrolysis process. The nanocomposites display both high adsorption capacity and enhanced photocatalytic activity towards methyl orange (MO), promising as synergetic photocatalysts for the removal of organic contaminates in a water environment.

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Nanocomposites that incorporate two or several parts of components into a functional integrity are attractive for the synergetic effects generated at the interface, which provide a wealth of opportunities for exploring enhanced or even new properties as well as multi-functionalities.^1–6 Optimizing each component in terms of its size, morphology and crystallinity, particularly, coupling these components by engineering the interfacial regions, are important to explore advanced materials with desired electronic, optical, mechanical, and catalytic properties.^1,3 Recent interest has been drawn to the assembly of inorganic nanocrystals with two-dimensional carbon sheets, typically, the graphene, a newly emerging two-dimensional allotrope of carbon, consisting of a single atomic layer. The unique electrical conductivity and gigantic two-dimensional surface of carbon sheets can promote the charge transferring between the inorganic components, producing synergetic effects that are beneficial for enhancing the performance of photocatalysts or photodetective devices.^5,6

Graphene oxide (GO), a none-stoichiometric oxidized derivative of graphene containing abundant functional groups, such as hydroxyl, carboxyl and epoxide groups, can display flexible intercalation behaviours.⁷ Recently, a new kind of graphene oxide framework (GOFs) has been synthesized. Similar to the formation of covalent organic frameworks (COFs), the good reactivity of borinic acid to the hydroxyl group promotes its intercalation into the GO layers, forming new porous materials potential for gas storage application.⁸ We are here motivated to screen from the organo-metal reagent family, which displays good reactivity to water and the hydroxyl group, to design a new type of GOF compounds.⁹ In this study, we report a highly expanded GOF compound formed via intercalation of organo-Ti reagent into GO accompanied by an intra-gallery hydrolysis process. Such new GOFs can be transformed into nanocomposites of TiO₂ and carbon sheets under controlled conditions, which are promising as synergetic photocatalysts towards the treatment of trace organic contaminates in a water environment.

The strategy of this study is illustrated in Scheme 1. Initially, highly hydrated GO was prepared from graphite powder by the modified Hummers method.^10,11 GO powder was ultrasonically dispersed into anhydrous ethanol. The dispersion was then added dropwise into another transparent ethanol solution containing organo-Ti reagent (here, titanium butoxide, Ti(OBuⁿ)₄, was used to exemplify the strategy). The mixture was stirred, and heated at 65 °C for 5 hours. This aging procedure promotes the insertion of neutral Ti(OBuⁿ)₄ into the GO gallery, hereafter reacting with the inter-gallery water molecules and functional groups to form a GO intercalate. The product, identified as organo Ti–O GOF hereinafter, was recovered by centrifugation, washed by anhydrous ethanol for several times, and finally dried at 60 °C in an oven. To prepare the synergetic photocatalyst, as-prepared GO intercalate was re-dispersed in ethanol. After addition of a small amount of water or HF solution, the mixture was sealed into a Teflon-lined autoclave, and then subjected to solvothermal treatment at 180 °C for 24 hours. Such a treatment will promote the removal of a major portion of functional groups and the restoration of some aromatic linkage to form a reduced GO (rGO) layer. At the same time, the pillared GOF structure is destructed, but nonetheless, the intercalated organo Ti–O species are transformed to TiO₂ nanostructures and heterogeneously crystallized on the surface of rGO layers to form a nanocomposite.

Scheme 1 Illustration of the synthesis strategy.

Fig. 1a shows the X-ray diffraction (XRD) pattern of the GO sample. The basal spacing of 0.93 nm indicates that the sample is highly hydrated. The water content determined from thermogravimetric (TG) analysis is 14.8% (see ESI†). The chemical formula based on elemental and TG analyses is C₄O_2.45H_1.37 (H₂O)_0.93. The high oxygen content may be due to a deep oxidation, which generates a rich amount of functional groups. After reaction with Ti(OBuⁿ)₄, the basal peak of GO phase is thoroughly disappeared. The product displays an intensive diffraction peak corresponding to a spacing of 6.3 nm and a weak diffraction very close to the intra-layer 10 diffraction of GO, suggesting the retention of aromatic-like carbonic layers and the expansion of the gallery via a topotactic process. Since the basal spacing is considerably higher than that of ethanol-solvated GO derivatives (in a range of 1.3–1.5 nm),¹² the highly expanded 6.3 nm-phase should not arouse from the swelling of GO in ethanol but is attributable to the intercalation of organo-Ti species within the GO gallery. TEM image (Fig. 1b) shows that the sample consists of folded thin flakers. Corresponding selected area electron diffraction (SAED) pattern shows diffraction rings that can be assigned to the in-plane diffractions of GO, indicating that the host layers are randomly stacked in high disorder. Energy-dispersive X-ray (EDX) analysis reveals the presence of elements of Ti and O along with C. TG and XRD measurements show that the content of Ti-related species, if estimated as the weight of TiO₂ (the residue after TG measurement in air, see ESI†), is about 23.5%.

Fig. 1 XRD patterns (a), TEM image with SAED pattern and EDX spectrum (b), solid state ¹³C-NMR (c), N₂ adsorption–desorption isotherm (d), and a proposed structure model of the organo Ti–O clusters pillared GOF (e). All data are those of the organo Ti–O GOF except for the bottom lines in (a) and (c) representing those of GO.

Fig. 1c shows the solid state ¹³C-nuclear magnetic resonance (NMR) spectra of the GO sample and the intercalated product. For GO, the chemical shift of 72 ppm and a shoulder at 65 ppm represent the hydroxyl and epoxide groups, respectively. The signal at 136 ppm is correlated to the aromatic sp² carbons located in the GO layers. Another peak at 166 ppm can be attributed to the carbonyl group.¹³ After reaction with Ti(OBuⁿ)₄, two new signals located at 17 and 35 ppm appear in the spectra, which can be assigned to the –CH₃ and –CH₂– groups, respectively, strongly supporting the presence of organo species. Importantly, compared with GO, the signal corresponding to the hydroxyl group (75 ppm) is apparently left-shifted. This change indicates that the intercalates strongly interact with the hydroxyl groups on the basal planes of the GO layer. Considering the basal spacing of 6.3 nm, considerably larger than the size of Ti(OBuⁿ)₄, and the good reactivity of organo-Ti reagent towards water and hydroxyl groups, the intercalates are attributed to the organo Ti–O clusters formed from partial hydrolysis of organo-Ti reagents.¹⁴ Presumably, the clusters are bonded with GO layers via a Ti–O linkage, forming a highly expanded pillared framework analogous to the GOF architectures, as proposed in Fig. 1e. N₂ adsorption–desorption isotherm of this sample shows a typical shape of type I partially hybridized with type IV with medium and low pressure hysteresis branches (Fig. 1d), indicative of a complex intra-gallery pore structure containing both mesopores and small size micropores. The specific surface area calculated from the Brunauer–Emmett–Teller (BET) equation is 225 m² g⁻¹, supporting the porous feature of this new GOF compound.

Like compounds of metal–organic frameworks (MOFs), the organo Ti–O GOF can be facially transformed into TiO₂/carbon nanocomposites.¹⁵ A solvothermal reaction in neutral condition is employed to remove the functional groups on GO layers and at the same time promote the transformation of intercalated organo Ti–O clusters into well crystallized TiO₂ nanocrystallites.¹⁴ As shown in Fig. 2a, the reaction product displays diffraction patterns from both anatase-type TiO₂ (tetragonal phase, a = 0.3795(2) nm, c = 9.5156(3) nm) and in-plane structures of graphene or reduced GO layers. The content of the TiO₂ component, determined from TG data, is 54.6% (Fig. 2b). ¹³C-NMR spectra show a prominent signal corresponding to sp² carbons and a weak signal of the epoxide group (see ESI†), indicating that a major part of functional groups are removed, and some of aromatic carbon linkages are restored. The N₂ adsorption result gives a specific surface area of 294 m² g⁻¹, which is slightly greater than that of the precursor. TEM image (Fig. 2c) shows that the sample comprises micro-meter size carbon sheets, on which platelet-like and some of the quasi-spherical TiO₂ nanocrystals are loaded. The lateral size of the nanoplatelet is below 30 nm. The well-developed edges indicate that the nanoplatelets are highly crystallized. A high resolution TEM (HRTEM) image (Fig. 2d) recorded along the 〈111〉 axis shows two sets of lattice fringe. The periodic distance of 0.35(1) nm and 0.34(9) nm correlate to the (101) and (011) lattices, respectively. The cross angle between these two set fringes are measured as 82°, consistent with the theoretical value. The thickness of the nanoplatelets estimated from some vertically stacked platelets is only 3–5 nm. On the other hand, when HF is added in the solvothermal solution, the size of the TiO₂ nanoplatelets in the obtained product can reach 80 nm (Fig. 2a and e). The platelets are rather thin and tend to stack in a face-to-face manner on carbon sheets. HRTEM image recorded from the 〈001〉 axis (Fig. 2f) exhibits a lattice fringe around 0.19(3) nm, which corresponds to the inter-distance between the (100) planes. TG data shows that the weight ratio of TiO₂ is decreased to about 28.5% (Fig. 2b) and is further lowered to 8.8% if 25% excessive amount HF is employed. In addition to the role of selective adsorption on certain surface facets of tetragonal TiO₂ to promote anisotropic growth,¹⁶ HF also contributes to the dissolution of a portion of organo Ti–O clusters, consequently reducing the TiO₂ content in the composite (Fig. 2b).

Fig. 2 XRD patterns (a), TG curves (b), TEM and HRTEM images of the TiO₂/rGO composites prepared at neutral (c and d) and acidic conditions (e and f). Asterisks mark the peaks from carbon.

Fig. 3 Adsorption and photocatalytic decomposition of MO by P-25 and TiO₂/rGO nanocomposites prepared at neutral and acidic conditions. The inset is the adsorption results in the dark.

The TiO₂/rGO composites are evaluated from their adsorption and photocatalytic degradation properties towards methyl orange (MO) (Fig. 3). Compared with the commercial TiO₂ (Degussa (Evonic), P-25), the composites can rapidly adsorb MO, and the specific adsorption capacity is considerably higher than that of the Degussa, P-25. The composite prepared in acidic condition shows a higher adsorption capacity than that in a neutral condition, which is mainly owing to its greater specific surface area (375 m² g⁻¹). The samples display high activity in decomposing MO under ultraviolet irradiation. The apparent rate constants are calculated based on the 1st order kinetics (see ESI†). Though the rate constants for the two composites are slightly lower than (neutral condition, 0.142 min⁻¹) or comparable to (acidic condition, 0.288 min⁻¹) that of P-25 (0.269 min⁻¹), if taking the content of TiO₂ into account, the composites can display a higher activity in the photodecomposition of MO as compared to the pure P-25 and other reported composites prepared by simply mixing GO dispersion with Ti precursors.¹⁷ This result suggests the presence of synergetic effect between the TiO₂ and rGO sheets, similar to that observed in the TiO₂–graphene composite.⁵ A tentative analysis on diffuse reflectance spectra gave a band gap value of 2.8 eV and 2.6 eV for the composites prepared at neutral and acid conditions, respectively, which are smaller than that of anatase (3.2 eV). Hence, besides the role of adsorption concentration of carbon toward hydrophobic substrate molecules, the promoted charge transferring due to the carbon platelets was considered to be a key factor to the enhancement of photoactivity of the composite.¹⁸ Since TiO₂ precursors (organo Ti–O species) are beforehand intercalated in GO layers, the current method is beneficial for achieving well dispersed TiO₂ in the graphene matrix whose platelet morphologies can lead to a more sufficient contact with carbon surface and thus more efficiently improve the electron–hole charge separation as compared to other reported composites. Furthermore, the thin TiO₂ nanoplatelets themselves with preferentially exposed crystalline planes, such as (001) and (111), are expected to be highly active in the photocatalysis reaction, and contribute to the high performance of the composites.¹⁹ Considering the high adsorption capacity and the enhanced photocatalytic performance, the TiO₂/rGO composites derived from the organo Ti–O clusters pillared GOF compound are promising as synergetic photocatalysts for the removal of trace organic contaminates in water environment.

In summary, a new organo Ti–O clusters pillared GOF is synthesized by reaction of organo-Ti reagent with highly hydrated GO in anhydrous ethanol. The structure of this GOF compound is analysed from XRD, TEM, ED, TG, and ¹³C-NMR data, based on which the intercalation process is proposed. We develop a facile route to transform this GOF into composites made by TiO₂ nanoplatelets and rGO layers, which show both high adsorption capacity and enhanced photocatalytic performance. Considering the large variety of organo-metal reagents, this study provides a new strategy to design novel organo-metal pillared GOFs and to take advantage of the synergetic integrity properties of their functional nanocomposite derivatives.

Acknowledgements

Authors thank water project of AIST for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra12312c

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