Nanoporous TiO 2 nanoparticle assemblies with mesoscale morphologies: nano-cabbage versus sea-anemone †

We report the novel synthesis of nanoporous TiO 2 nanoparticle ensembles with unique mesoscale morphologies. Constituent nanoparticles evolved into multifaceted assemblies, exhibiting excellent crystallinity and enhanced photocatalytic activity compared with commercial TiO 2 . Such materials could be exploited for applications, like organic pollutant degradation.

Existing energy generation and energy conversion challenges and potential applications in environmentally friendly processes, such as pollutant remediation, have driven contemporary research on novel materials that possess characteristics that could be exploited. Several factors, including crystalline phase, crystallite size, specic surface area, and porous structure, have important effects on the photocatalytic activity. [1][2][3] Thus, a key challenge in this eld is the achievement of control over the size and shape of nanostructures, which can strongly affect their structural characteristics and, hence, their physiochemical properties. Among the materials that are being investigated in photocatalysis, TiO 2 remains among the most widely studied because of its low-cost, ease in handling, resistance to photochemical and chemical erosion, non-toxicity, high activity, and durability. Further, TiO 2 is used in other energy devices, such as dye sensitized solar cells, lithium ion batteries, and sensors. [4][5][6][7][8][9][10][11][12][13] Notably, TiO 2 -based photocatalysis has potential for removing environmental pollutants in wastewater, polluted air, among others, an important device characteristic for water purication in the developing world. [14][15][16][17][18] Various chemical synthesis methods have been developed for fabricating the TiO 2 nanomaterials, including sol-gel, hydrolysis, precipitation and solvothermal methods. [19][20][21][22][23][24] For the synthesis of porous TiO 2 nanoparticle (NP) architectures, templates have been used, including hard templates (like porous silica or latex spheres, etc.) and so templates (like triblock copolymers). [25][26][27][28][29][30][31][32][33][34][35][36][37][38] Zhan and collaborators fabricated TiO 2 hollow bres with mesoporous structure by combining a sol-gel process with a two-capillary spinneret electrospinning technique, using a triblock copolymer as a pore-directing agent. 1 Peng and collaborators prepared mesoporous TiO 2 nanopowders with high specic surface areas and anatase phase by using cetyltrimethylammonium bromide (CTAB) as a surfactant-directing and pore-forming agent. 39 However, aer many of the aforementioned syntheses, the template must be removed from the sample to make the pores accessible. This has been achieved by thermal treatment (calcination). Calcination processes mostly lead to the partial or complete collapse of the porous structure during the template removal process and, thus, result in the decrease of the porosity. In addition, some templates cannot be removed completely by thermal treatment because of their tight binding to the materials. Recently, the synthesis of hierarchical structures of porous TiO 2 has been reported. In this and other similar cases, complicated non-scalable synthesis procedures were used. In each of these examples, the resultant TiO 2 materials were amorphous, and postsynthesis treatments were required to obtain crystalline nanostructures. [40][41][42][43][44] Thus, the development of simple, low-cost methods to synthesize such porous materials remains a challenge for scalable material preparation and applications. Herein, nanoporous TiO 2 nanostructures of tunable morphology have been synthesized by a surfactant-free, convenient, and low-cost solvothermal technique. The pore volume, Brunauer-Emmett-Teller (BET) surface area, and the pore diameter distributions of the nanostructures were tuned by adjusting synthesis parameters. The effects of BET surface area, pore volume, and pore size on the photocatalytic activity were studied through the photodegradation of methylene blue (MB) dye and compared favourably with commercially available P25 TiO 2 .
To investigate the inuence of the experimental parameters on the morphology and nanostructure of the TiO 2 , we tuned the synthesis parameters individually. The most obvious effect was observed as the duration of the solvothermal process was tuned. The structural development of the various nano-assemblies was assessed by transmission electron microscopy (TEM); representative images of the nanostructures are presented in Fig. 1 and 2. Fig. 1 depicts TEM images of the as-prepared TiO 2 nanoassemblies, synthesized during a 6 h solvothermal reaction. Under high magnication, these nanostructures appear to comprise multiple faceted platelets, assembled into an object that appears like bundled cabbage with diameter around 100 nm. Complementary X-ray diffraction (XRD) measurements of these nano-assemblies conrmed that the materials possessed good crystallinity (Fig. 1d). The diffraction peaks of the so called nano-cabbage assembly structures were indexed to the anatase phase of TiO 2 (JCPDS 21-1272). No diffraction peaks associated with rutile or brookite TiO 2 phases were observed, conrming the purity of the TiO 2 nano-assemblies. We estimated the average crystallite sizes of anatase NPs by using the Scherrer formula. The average crystallite sizes of TiO 2 were estimated from the (101) diffraction peak was 15 nm. Comparing with the TEM images, this could be size of single component in the nano-assembly.
We observed dramatic differences in the morphology of the nano-assemblies when shorter solvothermal synthesis times were employed. The mesostructure that was observed in the TEM images of the materials (Fig. 2), which corresponded to a shorter 2 h solvothermal synthesis reaction, were quite distinct from the aforementioned nano-cabbage samples. Collections of tendril-like structures and assemblies were observed, comprised of long, TiO 2 nanobers that branched outward from a central location. High resolution TEM micrographs of these structures, called sea-anemone nano-assemblies, are depicted in Fig. 2b and c. The primary particle dimensions were $10 nm in diameter and a few hundred nm in length; the overall nanoassembly size was $0.5 mm, as identied from the TEM images. Thus, the comparative TEM overviews suggest that the shorter solvothermal duration had a strong impact on the synthesis. The XRD pattern of this assembly (Fig. 2d) showed diffraction peaks that correspond to the planes of anatase TiO 2 . Average grain sizes of the products have been estimated from the full width at half-maximum of the respective XRD patterns with the Scherrer's formula, which was 12 nm. This diameter could be related to the thickness of single component in the nanoassembly which is in good agreement with TEM images. Both samples showed similar patterns (Fig. 1d and 2d) with peaks corresponding to the (101), (004), (200), (105), (211), (204) and (215) planes of anatase. XRD peaks are intense and narrower in case of cabbage nano-assembly, indicating the formation of greater TiO 2 crystallites and enhancement of crystallization. This conclusion also is supported by the lack of strong, denitive lattice fringes in the TEM images.
The morphologies of both nanostructures (sea-anemone and nano-cabbage) comprise the assembly of multiple nanoscale facets emerging from a centrally located nucleation and growth sites. In the case of the sea-anemone assembly, the shorter solvothermal synthesis time resulted in thin elongated structures, possessing high surface area and porosity. However, in the case of the nano-cabbage, the longer solvothermal synthesis yielded considerably thicker structures and lower porosity. The evolution of the shape and organization of the nanomaterials was attributed to Ostwald ripening. To conrm the Ostwald Fig. 1 (a-c) TEM micrographs of different magnifications and (d) XRD spectrum of our TiO 2 assembly that corresponded to a 6 h solvothermal synthesis. This nanostructure was characterized as having nano-cabbage morphology. ripening effect, a control experiment was conducted at longer solvothermal durations than previously employed, with all other reaction conditions kept constant (ESI, S1 †). No obvious effects on the morphology of nal product were observed. However, the net diameters of the particles increased with solvothermal duration. Also, the amount of water was varied, maintaining the other synthesis conditions; no substantial effect was observed on the resultant nanomaterials (ESI, S2 and S3 †). Yet, in the absence of water, small, cubic nanoparticles were formed (ESI, S4 †).
To compare the porosity of nal products, nitrogen adsorption-desorption experiments were conducted. Fig. 3 exhibit the nitrogen adsorption-desorption isotherms and the corresponding Barrett-Joyner-Halenda (BJH) pore diameter distribution measurements. Sea-anemone nanostructures exhibited well-dened adsorption characteristics at a high relative pressure (P/P 0 ); the associated curve reects a typical type-II isotherm with a small hysteresis loop (according to IUPAC nomenclature). The BJH pore size distribution obtained from the adsorption branch also revealed that the majority of pores in the sea-anemone assembly were $1.6 nm. For the nanocabbage assembly and the P25 TiO 2 NP, no obvious hysteresis loops were seen in the corresponding isotherms. The standard multipoint Brunauer-Emmett-Teller (BET) method was used to calculate the surface area of the samples. BET surface area of the sea-anemone, nano-cabbage and P25 are 264, 61 and 53 m 2 g À1 respectively. Notably, the specic surface areas of the seaanemone assembly were 5 times higher than that of P25. This implies that the catalytic performance of sea-anemone assembly may be superior to the other TiO 2 because of the larger quantity of adsorption sites for interacting species.
The photocatalytic properties of TiO 2 NPs have been studied in detail; 44-50 TiO 2 NPs and nanoparticle ensembles with large BET surface area and porous structure are expected to exhibit enhanced photocatalytic properties because the large specic surface area of the nanostructure possesses many active sites such that substances can be adsorbed in large quantities onto the TiO 2 surface. The complementary high porosity facilitates pollutant access to those catalytic sites, and the subsequent adsorption and decomposition. The photocatalytic activity of our nano-assemblies was determined by the decomposition rate of MB (measuring the amount of MB that remains within solutions that are exposed to UV illumination at regular intervals). For comparison, the results from the P25 are shown in Fig. 4a. UV light irradiation is needed for the photodecomposition of the MB organic dye, which led to the generation of excited electrons in the conduction band and holes in the valence band (see ESI †). The photogenerated charge carriers reacted with oxygen and water from the environment to produce active oxygen and radical species, which decomposed MB into CO 2 and H 2 O. Further, since this phenomenon took place at the material's surface, the accessibility and interaction of MB with the surface must be high and must be directly linked to the porosity.  The photodegradation kinetics of MB is described according to the pseudo-rst-order equation as given by eqn (1): in which C 0 is MB initial concentration, C is the residual concentration of MB at reaction time t, t is the reaction time, and K is the pseudo-rst-order rate constant. By plotting ln(C/ C 0 ) as a function of time t (Fig. 4b) and tting a linear regressions to that data, we obtained for each sample the K (min À1 ) constant from the slopes of the tted straight lines. K was 0.024 min À1 , 0.010 min À1 , and 0.012 min À1 for the sea-anemone assembly, the nano-cabbage assembly, and P25, respectively. The value for the sea-anemone assembly conrms that its activity for photodegradation of MB is the highest among the analyzed materials. Comparing the P25 with our as-synthesized nanoassemblies (nano-cabbage & sea-anemone), the sample with a higher BET surface area (sea-anemone) exhibited enhanced photocatalytic activity, even though P25 had a specic surface area, like the nano-cabbage assembly. This meant that samples with higher BET surface area enabled more MB molecules to contribute to the photocatalysis. In summary, nanoporous anatase TiO 2 assemblies were prepared through a facile, low cost, and reproducible solvothermal synthesis. TEM images showed the range of achievable morphologies through this approach, depending on the solvothermal duration under otherwise identical reaction conditions. The obtained sea-anemone and nano-cabbage morphologies possessed BET specic surface areas of 264 and 61 m 2 g À1 , respectively. Our photocatalytic measurements conrmed that the activity is more efficient when the specic surface area was higher, with pores completely accessible to water, oxygen, and adsorbate species. Importantly, the photocatalytic activities of nanoporous sea-anemone TiO 2 assembly were found to be much better than that of commercial P25 TiO 2 . These novel structures are promising candidates to replace traditional nanoparticle-based catalysis schemes. The new synthesis approach could be extended to the fabrication of other transition metal-based mesoscale, nanoporous materials.