Letter pubs.acs.org/journal/ascecg
Cellulose Nanocrystal-Templated Synthesis of Mesoporous TiO2 with Dominantly Exposed (001) Facets for Efficient Catalysis Juan Xue, Fei Song,* Xue-Wu Yin, Ze-Lian Zhang, Ye Liu, Xiu-Li Wang, and Yu-Zhong Wang* Center for Degradable and Flame-Retardant Polymeric Materials (ERCPM-MoE), College of Chemistry, State Key Laboratory of Polymer Materials Engineering, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), Sichuan University, 29 Wangjiang Road, Chengdu 610064, China S Supporting Information *
ABSTRACT: The mesoporous structure and high exposure of the (001) facet are of great importance to the photocatalytic performance of TiO2. In this Letter, we report using cellulose nanocrystal (CNC) as a sacrificial template to develop mesoporous TiO2 with dominantly exposed (001) facets, for which CNC can provide confined space for the controlled crystal growth of TiO2 and create mesopores after being removed. Owing to the photoluminescence up-conversion, furthermore, carbon quantum dot (C-dot) is introduced to realize visible light catalytic property of TiO2. In particular, the TiO2/C-dot composite with an extremely low content of carbon dot exhibits high catalytic performance, for which the mechanism is discussed. These results indicate such biotemplating method offers the potential to develop more mesoporous nanomaterials with desirable structures. KEYWORDS: TiO2, Cellulose nanocrystal, Carbon quantum dot, Photocatalytic activity
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INTRODUCTION As an important semiconductor photocatalyst with applications in water splitting, pollutants treatment, and photovoltaics, TiO2 is of particular interest in recent decades because of its chemical stability, high corrosion resistance, and low-price.1−6 Generally, several factors are responsible for the photocatalytic performance of TiO2, including band gap, lifetime of photogenerated electron, specific surface area, etc. Among them, the adequate exposure of (001) facet plays an extraordinary role for the high catalytic activity because of its high surface energy (0.90 J· m−2),7,8 unique surface atomic structures, and electronic properties.9 To date, different approaches have been exploited to develop high active anatase TiO2 with dominant (001) facets. For example, Bhaumik et al. 9 synthesized TiO 2 nanoparticles with exposed (001) facet by using ionic liquid ([bmim][Cl]) as a porogen for water splitting. Dionysiou et al.10 synthesized nanocrystalline TiO2 particles with (001) facet via an alkoxide sol−gel method. Sinha et al.11 prepared TiO2 nanocrystals with (001) facets consisting of indium oxide nanocluster via a hydrothermal method for photocatalysis under visible light. Yang et al.12 used a silica-templated method to prepare TiO2 nanosheets with dominant (001) facets. In contrast, using biomaterials as templates to fabricate nanostructured TiO2 is considered as an ideal and mild strategy because the templates can be easily removed without attacking targeting materials. Some natural biotemplates, including alginate,13 butterfly wings,14 petal,15 and caltrop-stem,16 have already been successfully exploited to prepare porous TiO2. Among these biotemplates, the most abundant renewable © XXXX American Chemical Society
biomass, cellulose, can provide hierarchical and skeleton structure for construction of nanomaterials.17−21 As wellknown, acid-catalyzed hydrolysis of bulk cellulose can produce rod-like cellulose nanocrystal (CNC) with diameters of 5−15 nm and lengths of 100−300 nm.22 Although CNC has been widely recognized as an ideal sacrificial template to develop functional materials,23,24 the progress of directly CNCtemplated preparation of mesoporous TiO2 is still very limited. Lately, Exarhos et al.25 used an air-calcination method to generate porous crystalline TiO2 by using cellulose nanocrystal as a template. More importantly, a transparent TiO2 porous film has been developed on substrates with the assistance of CNC for the application as electrodes in photovoltaic devices. MacLachlan’s group26 used CNC as an intermediate template to fabricate chiral nematic mesoporous silica films, which was then employed as a hard template to prepare anatase TiO2. However, the importance of (001) facet has not received enough attention in these works. To date, how to develop mesoporous TiO2 with dominantly exposed (001) facets as well as superior catalytic performance directly by CNC templates has not been reported yet. In this Letter, we are aware of the self-organization of CNC into chiral nematic and liquid crystalline phase in water, which is able to produce iridescent films with retained helicoidal chiral nematic order and layered structure. The special structure can Received: February 4, 2017 Revised: March 22, 2017 Published: April 5, 2017 A
DOI: 10.1021/acssuschemeng.7b00341 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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m2/g. From the pore size distribution curve (Figure S3), the average pore size of as-prepared TiO2 is 18.1 nm, which well matches the size of CNC. Therefore, using CNC as the template is confirmed effective to develop mesoporous TiO2. Hereafter, C-dot is prepared via the alkali oxidation method according to a previous report by Ostrikov et al.,19 of which the diameter is detected at 4 ± 1 nm (Figure S4). Its lattice spacing calculated as 3.16 Å is in good agreement with the (002) plane of graphitic carbon. As well-known, C-dot with graphitic carbon structure owns photoluminescence up-conversion, which is in favor of the enhancement of photocatalytic performance of TiO2. Hence, FTIR, XPS, and Raman measurements are used to understand the structure of C-dots synthesized in this work. As shown in Figure S5, some chemical groups such as −OH, CC, CO and CC are determined. In addition, XPS spectra of C-dot (Figure S6) show that there are three main peaks associated with carbon atoms locating at 284.87 eV (C C sp2), 286.46 eV (CO), and 288.19 eV (CO). The G peak at 1650 cm−1 (Figure S7) confirms the existence of the inplane vibration of sp2 carbon atoms. These results indicate that the C-dot prepared herein is graphitic carbon in majority and contains oxygen atoms in the forms of carboxyl and hydroxyl groups. Because C-dots have excitation-dependent emission behaviors, a transition of emission within the band from cyan to green is detected herein when changing the excitation wavelength from 400 to 480 nm (Figure S8). As for the morphology of the TiO2/C-dot nanocomposite, interestingly, a large number nanoparticles with the diameter of 5.3 ± 0.5 nm are observed assembling together (Figure 1c). Within the composite, mesopores with the size of 20.2 ± 3.4 nm are detected, indicating the introduction of C-dot has not destroyed the mesoporous structure of TiO2. As shown in Figure 1d, a lattice spacing of 0.354 nm is determined, attributing to the traditional (101) facet of the anatase TiO2, for which the corresponding XRD peak is also detected at 25.3° (Figure S9). The (001) facet of anatase TiO2 with the lattice spacing of 0.235 nm is also observed from HRTEM image (Figure 1d). However, its corresponding XRD peak cannot be detected because of the systemic extinction when h + k + l = odd. In addition, no diffraction peak corresponding to carbon is observed owing to the extremely low amount of C-dot. The percentage of the exposed (001) facet is calculated as 64%, which is quite close to the optimal value according to the DFT equation.28 For the fabrication of the mesoporous TiO2 with dominantly exposed (001) facet, the CNC template has two main roles: (1) A suitable narrow confined space is provided in the case of sufficient concentration of CNC (higher than the critical concentration of isotropic-chiral nematic phase transition) for the crystal growth of TiO2. Together with the gentle calcination, the confined space can restrict the fast growth of crystals, resulting in the formation of TiO2 nanoparticles as well as the exposure of (001) facet. (2) As a sacrificial template, CNC is removed after the preparation of TiO2. This gives rise to the formation of mesopores with the matched size of CNC. Hence, the two key characteristics, mesoporous structure and high exposure of (001) facet, can be simply acquired by the biotemplating method. As determined from the nitrogen sorption isotherm (Figure S10), the adsorption−desorption isotherm and the BET surface area (196 m2/g) of the composite remain nearly unchanged compared with the neat anatase TiO2. As calculated, the porosity of TiO2/C-dot is 72.9%, which is higher compared with some analogical reports.4 The high surface area as well as
provide skeleton for the formation of mesoporous TiO2, and the confined space as well as a programmed gentle calcination can induce the high exposure of (001) facet. To the best of our knowledge, this is the first example of the CNC-templated development of mesoporous TiO2 with dominantly exposed (001) facets. Furthermore, to modulate the electronic structure and realize the visible light catalytic activity of TiO2, carbon quantum dot (C-dot) is introduced to construct TiO2/C-dot composites. Unexpectedly, the presence of an extremely low content of C-dot can bring high efficient catalytic performance to TiO2, for which the mechanism is discussed.
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RESULTS AND DISCUSSION Prior to the development of nano TiO2, uniform CNCs should be first prepared. As shown in Figure 1a, the diameter and
Figure 1. (a) TEM image of CNC obtained after hydrolysis of cellulose. (b) SEM image of the CNC film prepared at the CNC concentration of 8.7 wt %. (c) TEM image of mesoporous TiO2/C-dot nanocomposite obtained after removing CNC template. Selected HRTEM images of TiO2/C-dot nanocomposite: (d) TiO2 with different lattice spacings, (e) TiO2 and C-dot.
length of CNC which is obtained by the hydrolysis of degreasing cotton are 18 ± 4 nm and 133 ± 18 nm, respectively. The critical concentration, where CNC dispersions start to exhibit lyotropic chiral nematic behavior, is detected at 7.8 wt %. As a result, a CNC film with parallel layered structure (Figure 1b) is developed when the CNC concentration of filmforming solution is set above the critical concentration. Afterward, CNC is employed as a sacrificial template to prepare mesoporous TiO2. To understand the chemical structure of as-prepared TiO2, XRD measurement is conducted (Figure S1). The main component of TiO2 is anatase according to the temperature-dependent percentage content of rutile R(T) calculated from the empirical relationship reported by Depero et al.27 R(T ) = 0.679[I(R)/(I(R) + I(A)] + 0.312[I(R)/(I(R) + I(A)]2 (1)
,where IA and IR are the intensities of anatase and rutile reflection, respectively. Furthermore, its pore structure is investigated by nitrogen gas adsorption/desorption analysis. As shown in Figure S2, the adsorption−desorption can be classified as type V isotherm with H2 hysteresis loop according to IUPAC, suggesting the existence of “bottle-neck”-shaped pores. The corresponding BET surface area is calculated as 184 B
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Figure 2. XPS spectra of TiO2/C-dot nanocomposite (content of C-dot, 0.10 mg/mL). (a) XPS full survey, (b) Ti 2p, (c) C 1s, and (d) O 1s spectra.
Figure 3. (a) Photocatalytic degradation of Rh B under visible light. Normalized concentration change versus irradiation time for the TiO2/C-dot nanocomposites with different C-dot amounts. (b) Reusability of TiO2/C-dot nanocomposite with the C-dot amount of 0.10 mg/mL.
The photocatalytic activity of TiO2/C-dot nanocomposites is evaluated by determining the photodegradation behavior of rhodamine B. As shown in Figure 3, pure TiO2 exhibits no photocatalytic activity for the degradation of rhodamine B because of lacking light absorption in the visible region (Figure S11). In comparison, a particularly high activity is achieved once the introduction of C-dot into TiO2 nanoparticles because of the up-converted fluorescence of C-dot. The catalyst with Cdot amount of 0.1 mg/mL can degrade 96% of rhodamine B within 120 min (Figure S12). Especially, different from the previous findings that the photocatalytic efficiency of TiO2 increased first but declined subsequently with the increase in the C-dot amount,29,30 in this work, only a much lower amount of C-dot (0.05 mg/mL) is required for a remarkably enhanced photocatalytic efficiency. Furthermore, the efficiency is decreased gradually as the C-dot amount increased (Figure 3a). From the relationship between the photocatalytic efficiency and the concentration of C-dot, the catalytic performance can be divided into two regions. A preliminary critical concentration of C-dot amount is regarded at 0.5 mg/mL, below and above which obviously different photocatalytic behaviors are
the high porosity is also beneficial to the photocatalytic performance of catalyst. As for the morphology of the TiO2/C-dot composite, interestingly, C-dot is detected very close to anatase TiO2 (Figure 1d). To understand further the interaction between the C-dot and TiO2, XPS measurement is conducted. Compared with the C 1s spectrum of pure C-dot (Figure S6b), the nanocomposite has some additional characteristic signals (Figure 2a). The peaks at 458.4 and 464.3 eV are attributed to the Ti 2p3/2 and Ti 2p1/2 of TiO2 (Figure 2b). In addition, the peaks detected at 284.5, 286.0, 286.3, and 288.5 eV indicate the existence of CC, CO, OCO, and CO bonds (Figure 2c). The appearance of the new peaks illustrates that a OCO bond is newly formed after the incorporation of Cdot. From Figure 2d, TiO, CO, and COH bonds are affirmed because of the determination of peaks at 529.9, 530.5, and 532.4 eV. On the basis of the above results, we confirm that there is no TiC bonding between Ti and C atoms. Therefore, the newly formed OCO is resulted from the replacement of Ti atoms of TiO2 nanostructure with C atoms of C-dot. C
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actually, not much C-dot filler is needed for the up-converted photoluminescence to TiO2; the high exposed (001) facet and porosity are key factors for the high photocatalytic efficiency; formation of OCO bond should not be neglected as for the catalytic performance.
observed from that at this concentration. To illustrate better the importance of (001) facet for TiO2, furthermore, a kind of commercially available TiO2 (Degussa P25) without (001) facet, of which the structure is confirmed by XRD and HRTEM (Figure S13), is used as a control. After the introduction of Cdot, the as-prepared P25/C-dot composite shows very low photocatalytic activity under exposure to visible light (Figure S14). Furthermore, another control, TiO2 with (001) facet, is prepared by hydrothermal method according to a previous report,31 whose crystalline structure and morphology are also confirmed by XRD and HRTEM (Figure S15). Compared with the CNC-templated synthesized TiO2/C-dot composite, the hydrothermally prepared one exhibits weaker catalytic performance (Figure S16), attributing to the absence of mesoporous structure. The results clearly confirm the importance of the (001) facet and mesoporous structure for the photocatalytic performance of TiO2. As well-known, reusability is also an important issue for catalysts. Figure 3b illustrates the catalytic performance of the nanocomposite at different reusing cycles. Within the 6-cycle measurement, its relative activity is maintained above 95%. As indicated in previous reports,6 besides the up-converted property of C quantum dot, a high content of C quantum dot can compete absorption photogenerated electrons to form transient photocurrent response, which instead makes the photocatalytic activity of TiO2 decreased. However, a different but interesting phenomenon is found herein that a monotonous relationship exists between photocatalytic efficiency and C-dot amount. To find the reason, we further perform XPS measurement on the nanocomposites with C-dot amounts of 0.50 and 1.0 mg/mL. As calculated from Figure 2, S17, and S18, the content of OCO bond decreases with the increase in the C-dot amount. As investigated by the density functional theory calculations,18,32 two possible substitutional carbon-doped structures of anatase TiO2 are recommended when Ti was replaced with C: CO4 and CO6. For the former, the substitutional C is four-coordinated, presenting a tetrahedral geometry where the CO bond is shorter than the TiO bond; while for the latter, the CO6 structure turns to be an axially flattened octahedron where the two apical CO bonds become significantly shorter than the TiO bond. Cdoped anatase TiO2 would show a high catalytic activity if a linear OCO unit resembling carbon dioxide were formed.32 Owing to the high content of OCO bond formed at the low amount of C-dot (less than 0.5 mg/mL), consequently, the substitutional carbon-doped structure of anatase TiO2/C-dot nanocomposite prefers to be octahedron (Figure 4); whereas at the relatively high amount of C-dot (higher than 0.5 mg/mL), the structure tends to be tetrahedron. Therefore, some conclusions can be made here:
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CONCLUSION In summary, we propose a layer-structured CNC templating method to develop mesoporous TiO2 nanoparticles with dominantly exposed (001) facets. With the help of upconverted photoluminescence of C-dot, TiO2 can degrade rhodamine B effectively upon exposure to visible light. Despite at an extremely low amount of C-dot, high catalytic activity is realized because of the formation of linear OCO units in TiO2/C-dot composites. Such a green preparation strategy and results are beneficial for the construction of functional materials with desirable morphologies and high performance.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00341. Experimental section, FTIR, XRD, XPS, Raman, TEM, HRTEM, nitrogen adsorption/desorption isotherms, and fluorescence measurements (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*F. Song. E-mail:
[email protected]. *Y.-Z. Wang. E-mail:
[email protected]. ORCID
Fei Song: 0000-0001-5229-4379 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 51403136, 51421061, and J1103315), the Research Fund for the Doctoral Program of Ministry of Education of China (Grant 20130181120067), the Program for Changjiang Scholars and Innovative Research Teams in University of China (IRT 1026), and the Fundamental Research Funds for the Central Universities of China (2015SCU04A22).
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