MetalâOrganic Framework-Templated Synthesis of Bifunctional N

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Metal-organic framework-templated synthesis of bifunctional N-doped TiO2–carbon nanotablets via solid-state thermolysis Yifan Gu, Kuan Cheng, Yi-nan Wu, Ying Wang, Catherine Morlay, and Fengting Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01716 • Publication Date (Web): 26 Sep 2016 Downloaded from http://pubs.acs.org on September 30, 2016

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ACS Sustainable Chemistry & Engineering

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Metal-organic framework-templated synthesis of

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bifunctional N-doped TiO2–carbon nanotablets via

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solid-state thermolysis

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Yifan Gu, † Kuan Cheng, † Yi-nan Wu,* † Ying Wang,* † Catherine Morlay, ϕ and Fengting Li †

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†

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and Resource Reuse, Shanghai Key Lab of Chemical Assessment and Sustainability, Department

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of Chemistry, Tongji University, Siping Rd 1239, 200092 Shanghai, China.

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ϕ

College of Environmental Science and Engineering, State Key Laboratory of Pollution Control

Univ Lyon, Université Claude Bernard Lyon 1, INSA-Lyon, CNRS, MATEIS (UMR 5510), F-

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69621, LYON, France.

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*Corresponding Author

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Dr. Yi-nan Wu, Phone: +86 21-65980567; E-mail address: [email protected]

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Dr. Ying Wang, Phone: +86 13918374165; E-mail address: [email protected]

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ACS Paragon Plus Environment

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ABSTRACT. Porous nitrogen-doped TiO2–carbon hybrid nanotablets were prepared via one-step

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solid-state thermolysis of amino-functionalized titanium metal-organic framework NH2-MIL-

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125(Ti). Amorphous, anatase, rutile, or mixture phases of TiO2 were obtained controllably by

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manipulating pyrolysis temperature. The anionic N- in NH2-MIL-125(Ti)’s 3D structure formed

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as pyridinic nitrogen and pyrrolic nitrogen into the graphene layer. Meanwhile, a programmed

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evolution of the porous structure of resultant composites, i.e., microporous, hierarchically micro-

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/mesoporous, and mesoporous, was presented systematically. The morphology and specific

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nano-porous structure of the products are conferred by the metal-organic framework template.

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The resultant composites with hierarcical meso/miciroporous structures showed highly improved

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CO2 uptake ability compared with commercial P25 TiO2, g-C3N4 and 3D graphene. TiO2

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nanoparticles are well dispersed in the porous nitrogen-doped carbon matrix, endowing the

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obtained composites with effective photocatalytic activity. The nitrogen-doped TiO2–carbon

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nanotablets with hierarchically micro/mesoporous structure and anatase/rutile heterostructure

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exhibited the best photocatalytic performance with excellent adsorption capacity toward organic

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dyes. Results also indicated that the optimized composite possessed excellent long-term stability

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and regeneration ability. Benefiting from their versatile pore structure, heteroatoms doping, and

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semiconductor incorporation, the nitrogen-doped TiO2–carbon composites derived from metal-

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organic frameworks could find various potential applications, especially for sustainable

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chemistry and engineering.

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KEYWORDS. Metal-organic frameworks, Thermolysis, TiO2 heterostructure, CO2 adsorption,

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pollutants degradation

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Introduction

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Porous metal oxides (PMO) have been widely used in energy conversion and storage, catalysis,

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adsorption, and separation of pollutants for sustainable development because of their unique

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chemical, physical, electronic, and optical properties, as well as high surface area and

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crystallinity.1-3 Various strategies, such as sol–gel

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deposition 8, reinforced crystallization 9-10, and templating

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synthesis. Among these strategies, soft or hard templating has attracted considerable interest

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because appropriate template selection allows the facile tunability of pore diameter, wall

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thickness, and pore periodicity.13-16 Several recent studies have focused on carbon/metal oxide

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hybrid materials because of their robust chemical stability, enhanced conductivity, and highly

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dispersed characteristic compared with pure metal oxides. Such hybrid materials have a wide

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array of promising applications in sustainable chemistry and engineering.17

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Metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) are a novel class of

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nanoporous materials that consist of metal ions/clusters and coordinated organic linkers.18-20 The

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diverse structures, large surface area, large pore volume, tunable pore size, and adjustable

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chemical properties of MOFs or PCPs allow these materials to have versatile applications.21-24

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Although the high cost of MOF limits its application in industrial-scale, many efforts have

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already been well devoted to reduce the cost for some widely used MOFs.25-26 Considering the

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abundant metal and organic moieties in MOFs, previous works have successfully prepared pure

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metal oxides or highly porous carbon through the selective removal of the metal or organic

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components of MOFs as the sacrificial templates by nanocasting or decomposition.27-33

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Moreover, nanostructured PMO–carbon hybrid materials, which exhibit promising applications

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on catalysis

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4-5

, spray pyrolysis 11-12

, electrochemistry 34, and energy conversion

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6-7

, chemical vapor

have been developed for PMO

, can be obtained via controlled


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thermal conversion using a suitable MOF template with a unique thermal behavior. TiO2 is a

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known functional inorganic semiconductor with excellent performance under UV light in solving

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particular environmental problems faced today.36-37 Titania–carbon hybird materials with

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enhanced photocatalytic activities make this expectation even more notable.38 In a pioneer work

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on the carbothermal reduction of Ti-modified IRMOF-3, an organic titanate precursor was

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incorporated into a Zn-based MOF matrix.39 This process led to the formation of TiO2

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nanoparticles supported on amorphous carbon. However, the synthesized nanoparticles

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possessed a low Ti content of only 4.3 wt%. Typical Ti-based MOF, MIL-125(Ti) was first

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synthesized by Dan-Hardi et al.

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researches prepared porous TiO2 or TiO2–carbon composites through the calcination of MIL-

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125(Ti) for application in electrode, photocatalysis and oxidation.41-44 Introducing heteroatoms

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such as nitrogen and sulfur into carbon matrices or metal/metal oxide composites can improve

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physicochemical stability, adsorptive ability, and catalytic activity.45 It can be easily realized by

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direct decomposition of MOFs with designable organic linkers with specific functional groups or

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elements such as amidogen, sulfydryl and et al. Thus the transformation of specific components

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in organic linkers during the thermal conversion of MOFs process needs to be further studied.

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with a high BET surface area of 1550 m2 g−1. Several

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Scheme 1. Schematic of the preparation of TiO2–carbon nanotablets via solid-state thermolysis of MOF templates.

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NH2-MIL-125(Ti) is isostructural with MIL-125(Ti) by replacing H2BDC with NH2BDC in the

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framework.46 Compared with MIL-125(Ti), NH2-MIL-125(Ti) has a higher CO2 adsorption and

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visible optical response, as well as obviously different pyrolysis behaviour.47 In the present paper,

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we report the fabrication of porous nitrogen-doped TiO2–carbon hybrid nanotablets via one-step

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solid-state thermolysis of amino-functionalized titanium metal-organic framework NH2-MIL-

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125(Ti) (Scheme 1). Systematic observation of the phase transformation and porosity transition

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of the MOF template during thermal decomposition reveals a simultaneously programmed

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evolution of the porous structure and the TiO2 phases and N types on the resultant composites.

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The resultant composites with hierarchical meso/microporous structures showed remarkable CO2

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uptake ability compared with commercial P25 TiO2, g-C3N4 and 3D graphene. When evaluated

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as

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micro/mesoporous structure and anatase/rutile heterostructure exhibit the best photocatalytic

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performance with excellent adsorption capacity toward organic dyes, long-term stability, and

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regenerating ability.

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Experimental Section

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Reagents and chemicals

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Titanium tetraisopropanolate (TTIP) and 2-amino-1,4-benzenedicarboxylic acid (NH2-H2BDC)

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were purchased from Alfa Aesar Co., Ltd. CH3OH (MeOH) and dimethyl formamide (DMF)

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were provided by J&K Scientific Co., Ltd. Commercial P25, anatase, rutile and graphite oxide

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were provided from Aldrich. All reagents were used without further purification. Deionized

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water was used throughout this work.

photocatalysts,

the

nitrogen-doped

TiO2–carbon

nanotablets

with

hierarchically


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Preparation of NH2-MIL-125(Ti)

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NH2-MIL-125(Ti) was synthesized at 150 °C under microwave irradiation.47 Approximately 0.13

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g of NH2-H2BDC was dissolved with 3.0 mL of MeOH and 3.0 mL of DMF. Afterward, 100 µL

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of TTIP was added with further stirring for 5 min at room temperature. The reactants were sealed

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and placed in a mono-microwave system (Preekem NOVA-2S, maximum power of 300 W) and

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heated for 2 h at 150 °C. Yellow products were formed, filtered, and then washed with DMF for

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several times. The products were placed in methanol, which was decanted and replenished thrice.

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The final products were activated at 150 °C under vacuum for 24 h.

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Preparation of TiO2 and TiO2–carbon nanotablets

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TiO2 nanotablets were obtained as follows: 0.50 g of the prepared NH2-MIL-125(Ti) was

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annealed in air at targeted temperature (500 °C, 600 °C, 700 °C, and 800 °C) for 5 h with a

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ramping speed of 10 °C min−1 from room temperature, achieving samples of NMIL-125(Ti)-A5,

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-A6, -A7, and -A8, respectively. Porous TiO2–carbon nanotablets were prepared as follows: 0.50

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g of the prepared NH2-MIL-125(Ti) was heated in continuous N2 flow at targeted temperature

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(600 °C, 700 °C, 800 °C, and 900 °C) for 5 h with a ramping speed of 10°C min−1 from room

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temperature, achieving samples of NMIL-125(Ti)-N6, -N7, -N8, and –N9, respectively. Upon

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cooling, the final products were collected and stored under ambient conditions.

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Adsorptivity, photocatalytic activity and regeneration behavior measurement

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The adsorptivity activities of prepared TiO2 and TiO2-carbon nanotablets were evaluated by

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adsorption of CO2. CO2 isotherms were measured using a Micromeritics ASAP 2020 analyzer at

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273 K. The adsorption–photodecomposition performance of prepared TiO2 and TiO2-carbon


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nanotablets were evaluated by decolorzion of organic dyes. 10 mg of prepared sample was put

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into 50 ml of Rhodamine B (RhB) and Methyl orange (MO) aqueous solution (30 ppm). Before

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the photocatalytic test was initiated, the suspension was stirred in dark for 150 min to ensure the

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establishment of adsorption equilibrium. 300 W Hg lamp was used as the source of excitation

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for photocatalytic activities test. The distance between the light sources and the beaker

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containing reaction mixture was fixed at 10 cm. The regeneration behavior test was carried by

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repeating the above process. After each cycle, the photocatalysts were collected by centrifugation

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and dried at 60 °C. The concentrations of dyes were monitored by measuring the absorption

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intensity at its maximum absorbance wavelength (λ=467 nm for MO and λ=552 nm of RhB)

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using a UV-vis spectrophotometer (UV-1200, Shimadzu).

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Characterization

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Scanning electron microscopy (SEM) was performed using a FEI Quanta 400 FEG scanning

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electron microscope and a Phenom G2 PRO microscope equipped with an energy dispersive X-

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ray spectrometer system. Transmission electron microscopy (TEM) was performed under a FEI

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Tecnai G2 F20 S-Twin high-resolution transmission electronic microscope at an acceleration

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voltage of 200 kV. Powder X-ray diffraction (PXRD) was performed on a Bruker D8 Advance

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X-Ray powder diffractometer (40 kV, 40 mA, CuKα1 radiation of λ = 1.54059 Å) at room

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temperature. Quantitative determinations of the phase compositions of anatase/rutile mixtures

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were obtained by means of the software X’Pert High Score 3.0, PANalytical B.V using the

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International Center for Diffraction Data (2004) database. Raman spectroscopy was performed

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on a Renishaw Raman scope using a 532 nm He−Ne laser. The nitrogen adsorption and

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desorption isotherms were measured using a Micromeritics ASAP 2020 analyzer at 77 K. The

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Brunauer–Emmett–Teller (BET) method was employed to calculate the specific surface areas.


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Pore size distribution was calculated from non-local density functional theory (NLDFT) models,

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assuming cylindrical pore geometry. The physicochemical parameters of the products were

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defined as follows: the micropore surface area (Smicro, m2 g−1) was calculated using the t-Plot

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method, the total pore volume (Vtotal, cm3 g−1) was measured at P/P0=0.995, and the micropore

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volume was determined using the t-Plot method. Thermogravimetry (TG) measurement was

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performed on a TA SDT-Q600 analyzer. The samples were heated from room temperature to

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900 °C with a ramp of 10 °C min-1. X-ray photoelectron spectroscopic (XPS) experiments were

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carried out on a RBD upgraded PHI-5000C ESCA system (Perkin Elmer) with Mg Kα radiation

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(hν=1253.6 eV) or Al Kα radiation (hν=1486.6 eV). Binding energies were calibrated by using

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the containment carbon (C1s = 284.6eV). The data analysis was carried out by using the RBD

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AugerScan 3.21 software provided by RBD Enterprises. The diffuse reflection spectra were

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obtained using a Shimadzu UV-2550 UV–vis spectrometer. BaSO4 was used as a reference.

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Results and Discussion

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As the sacrificing template, amino-functionalized MIL-125(Ti) was prepared under microwave

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irradiation as previously described. As shown in Figure 1a, the synthesized NH2-MIL-125(Ti)

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exhibited well-defined tablet-like morphology. Compared with conventionally prepared NH2-

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MIL-125(Ti) (ca., 5 µm), the NH2-MIL-125(Ti) solvothermally synthesized under microwave

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irradiation had a smaller particle size (ca., 1 µm) possibly because of the limited particle growth

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during the shortened reaction time by microwave heating.47 The crystal structure of NH2-MIL-

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125(Ti) was confirmed through PXRD and was identical with the simulated one (Figure 1b). The

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Langmuir surface areas (SLangmuir) and micropore volume (Vt) of the prepared NH2-MIL-125(Ti)

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were determined using N2 adsorption–desorption isotherms (Figure 1c). The typical Type-I-

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shaped isotherm observed indicated the open microporous structure of NH2-MIL-125(Ti), which


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possessed a SLangmuir and Vt of 1286 m2 g–1 and 0.56 cm3 g–1, respectively. The pore size based on

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the NLDFT model was narrow at 0.66 nm.

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Figure 1. (a) SEM image of NH2-MIL-125(Ti); (b) XRD patterns of NH2-MIL-125(Ti); (c)

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Nitrogen sorption isotherm and pore width distribution based on the NLDFT model of NH2-

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MIL-125(Ti); (d) TGA curves of NH2-MIL-125(Ti) in air and N2.

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The thermal behavior of NH2-MIL-125(Ti) both in air and nitrogen was analyzed through

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thermogravimetric analysis (TGA; figure 1d). The profiles of NH2-MIL-125(Ti) in air and

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nitrogen were similar before 400 °C. The first weight loss between 50 °C and 100 °C was

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subjected to the departure of the physically absorbed water and solvent. The dehydrated structure,

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which was stable up to 200 °C, eventually collapsed with ligand elimination. An obvious weight

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loss of NH2-MIL-125(Ti) in air was observed at approximately 380 °C, which corresponded to


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the initiative decomposition of the framework to TiO2. Such weight loss in nitrogen was also

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observed at the same temperature, but the effect was weaker than that in air. The similar structure

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of MIL-125(Ti) collapsed at approximately 400 °C.47 This result indicates that NH2-MIL-125(Ti)

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is easier to pyrolyze than MIL-125(Ti) because of the NH2 group in the organic link. No further

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weight loss in air was recorded when the temperature continued to increase above 500 °C. This

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finding reveals the completion of framework decomposition and the formation of titanium

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dioxide. However, this weight loss gradually decreased after carbonization at 500–900 °C. This

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phenomenon indicates a dynamic and continuous ligand-to-carbon process. In general, the total

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weight loss percentages of NH2-MIL-125(Ti) in air and nitrogen were 79.35 % and 49.80 %,

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respectively. The discrepancy can be attributed to the additional carbon and nitrogen residues

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that remained after carbonization.

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Figure 2. XRD patterns of the thermal decomposition products of NH2-MIL-125(Ti) in air (a)

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and N2 (b) at different temperatures. (Green triangle: anatase-phase TiO2; red dot: rutile-phase

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TiO2).


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The thermal decomposition products of NH2-MIL-125(Ti) were further analyzed under different

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pyrolysis atmospheres and temperatures on the basis of the XRD results. The as-prepared NH2-

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MIL-125(Ti) was first annealed in air at 500 °C, 600 °C, 700 °C, and 800 °C to yield samples

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NMIL-125(Ti)-A5, -A6, -A7, and -A8, and then pyrolyzed in nitrogen at 600 °C, 700 °C, 800 °C,

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and 900 °C to produce samples NMIL-125(Ti)-N6, -N7, -N8, and -N9, respectively. Titania

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normally exists as three polymorphs, namely, anatase, brookite, and rutile. The anatase phase has

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considerably higher photocatalytic activity than the rutile phase, whereas the rutile phase is more

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thermodynamically stable than anatase phase.48 No fixed temperature is required for the phase

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changes between anatase and rutile to occur; the shift depends on the preparation conditions.

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Wide-angle PXRD analysis was primarily applied to monitor the phase transformation during the

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thermal decomposition of NH2-MIL-125(Ti) under different atmospheres and temperatures. As

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shown in Figure 2a, pure anatase phase TiO2 was obtained through the thermal decomposition of

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NH2-MIL-125(Ti) at 500 °C in air. The transformation from the anatase phase to the rutile phase

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clearly occurred at the following high temperatures. The rutile crystallite form first appeared

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after calcination at 600 °C. From 600 °C to 700 °C, the mixture of anatase and rutile phase TiO2

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was produced throughout. The mixed phase of TiO2 is expected to exhibit higher photocatalytic

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activity than either pure anatase or rutile phase alone. The anatase phase has a higher

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photocatalytic activity, whereas the smaller band gap of the rutile phase can extend the useful

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range of photoactivity into the visible region. In addition, the transfer of electrons to anatase

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lattice trapping sites allows holes that would have been lost during recombination to reach the

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surface, leading to the formation of catalytic hot spots at rutile/anatase interface.49 The peaks of

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the anatase phase disappeared when the calcination temperature was further increased to 800 °C.

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This finding implies that the transformation from the anatase phase to the rutile phase was


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completed and that the pure rutile phase TiO2 was obtained. Notably, unlike the calcination in air,

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a different phase transformation was exhibited during the thermal pyrolysis of NH2-MIL-125(Ti)

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in N2 (Figure 2b). Only amorphous TiO2 was obtained after carbonization at 600 °C; the mixed

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phase of TiO2 was obtained after calcination at the same temperature in air. Pure anatase phase

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TiO2 was obtained after calcination at 700 °C in N2. The transformation from the anatase phase

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to the rutile phase in N2 occurred at 800 °C, and the mixed phase of TiO2 was obtained. The

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ratios of anatase/rutile phase of TiO2 was 0.299, calculated by the software X’Pert High Score

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3.0, PANalytical B.V using the International Center for Diffraction Data (2004) database.

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Meanwhile, only pure rutile phase TiO2 was obtained in air. These results indicate that the

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transformation temperature is higher in N2 than in air. When the calcination temperature was

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further increased to 900 °C, all anatase phase TiO2 transformed to rutile phase TiO2; thus, pure

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rutile phase TiO2 was obtained.

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Raman spectra of TiO2–carbon composites were used to investigate the programmed evolution

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during the carbonization of NH2-MIL-125(Ti). First, phase changes of TiO2 at different

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temperatures were presented. The anatase structure was characterized by six Raman transitions

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(1A1g, 2B1g, and 3Eg). The rutile structure was characterized by five Raman transitions (A1g, B1g,

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B2g, and Eg). As shown in Figure S1, the Raman bands located at 150 (Eg), 400 (B1g), 504 (A1g),

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and 626 cm−1 (Eg) can be attributed to the characteristics of the anatase phase, whereas those

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located at 245 (multi-proton process), 446 (Eg), and 617 cm−1 (A1g) can be ascribed to the

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characteristics of the rutile phase. The transition from the anatase to rutile phase occurred with

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increasing carbonization temperature. This result is consistent with the XRD patterns. NMIL-

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125(Ti)-N8 possessed

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characteristic peaks at 1344 and 1609 cm−1 in the Raman spectra also revealed the graphitization

anatase/rutile heterostructure. Moreover, two additional broad


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degree of these TiO2–carbon composites on the basis of the relative ratios of G bands to D bands

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(ID/IG); the D bands originated from the defects in graphene, whereas the G bands can be

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attributed to the in-plane vibrations of the graphitic structure.50 The ID/IG values of NMIL-

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125(Ti)-N7,-N8, and -N9 were almost constant (0.98–0.99), indicating the presence of sp3-

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hybridized carbon atoms or N atoms as the defects in the graphene structure and the coexistence

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of developed graphene sheet. High-temperature carbonization with an increment from 700 °C to

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900 °C cannot improve graphitization and increase defects in the carbon component.

250 251

Figure 3. Nitrogen sorption isotherms and pore width distributions based on the NLDFT model

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of the thermal decomposition products of NH2-MIL-125(Ti) at 700 °C, 800 °C, and 900 °C in N2.

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Porous properties (surface area and pore size) were determined through N2 sorption measurement,

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shown in Figure 3. Compared with pure TiO2 obtained through the calcination of NH2-MIL-

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125(Ti) in air with a relatively low specific BET surface area of less than 20 m2 g−1, the TiO2–

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carbon composites prepared through pyrolysis in nitrogen presented a typical porous structure


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but varied with temperature. The sorption isotherm of NMIL-125(Ti)-N7 exhibited a typical

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Type-I behavior of microporous materials. The surface area calculated using the BET method

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and the micropore area determined using the t-plot method of NMIL-125(Ti)-N7 were 337 and

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243 m2 g−1, respectively, implying that the sample was mainly microporous. The pore size

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distribution centered at approximately 0.6 nm also confirmed such microporous structure, which

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is similar to that of the parent NH2-MIL-125(Ti). As the carbonization temperature was increased,

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NMIL-125(Ti)-N8 and –N9 decreased in BET surface area and micropore area while increased

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in pore size. In particular, the coexistence of steep increase in N2 sorption amount in low

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pressure (P/P0 < 0.1) and hysteresis loop observed at P/P0=0.6 indicate the configuration of

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hierarchically bimodal pore structure. The BET surface area of NMIL-125(Ti)-N8 was 274 m2

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g−1, with a micropore area of only 29 m2 g−1. The microporous size distribution that was centered

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at 0.6 nm remained, but additional dual mesoporous size distributions around 2 and 20 nm were

269

observed. This phenomenon indicates the occurrence of porosity transition from micropore to

270

mesopore. The small mesopores (pore diameter: 2–5 nm) possibly originated from the

271

association of several micropores in the carbon during the thermal conversion, whereas the large

272

mesopores (pore diameter: 5 – 20 nm) can be attributed to interparticle voids between fine TiO2

273

aggregates. The surface area considerably decreased when the carbonization temperature was

274

further increased to 900 °C. The BET surface area of NMIL-125(Ti)-N9 was only 45 m2 g−1,

275

with a type H3 hysteresis loop at a high relative pressure range of 0.8–1.0 on its nitrogen

276

sorption isotherm. Small micropores (< 1 nm) was absent. These results indicate the further

277

decomposition of the organic components. Similar to the calculation results derived from

278

NLDFT model, the sample NMIL-125(Ti)-N8 exhibited typical mesoporous structure with an

279

average mesopore size of 4 nm based on BJH model (Figure S2) which can be ascribed to the


14

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280

association of several micropores in the carbon during the thermal conversion. Meanwhile, the

281

sample NMIL-125(Ti)-N9 showed the presence of large mesopores (pore diameter: 10 – 60 nm

282

Figure S2) which can be attributed to interparticle voids between fine TiO2 aggregates. The

283

nitrogen sorption measurement results under a proper pyrolysis temperature in nitrogen imply

284

that the microporous structure was endowed by the parent NH2-MIL-125(Ti). Moreover, MOF

285

template and additional mesopores can be facilely introduced to form hierarchically micro-

286

/mesoporous structure with high surface area and low mass transfer resistance.

287 288

Figure 4. SEM and TEM images of the thermal decomposition products of NH2-MIL-125(Ti) at

289

700 °C (a, d), 800 °C (b, e), and 900 °C (c, f) in N2.

290

SEM and TEM were performed to observe the morphology and porous structures evolution of

291

the pyrolysis products of NH2-MIL-125(Ti) (Figure 4). In general, the nanotablet morphology of

292

the TiO2–carbon nanoparticles carbonized at different temperatures were similar to that of the

293

NH2-MIL-125(Ti) template. This result confirms the successful preparation of nanoporous TiO2–

294

carbon composites with well-defined architectures via solid-state thermolysis. Meanwhile, the


15


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295

surface roughness of the nanotablets increased and the size significantly reduced when the

296

pyrolysis temperature was increased. In accordance with the gas sorption characterization, the

297

carbonaceous matrix in NMIL-125(Ti)-N8 presented a nanoporous structure with very fine TiO2

298

nanoparticles (< 20 nm) embedded and mesoporous voids in the matrix with large TiO2

299

nanoparticles (~50 nm). The anatase/rutile junction of NMIL-125(Ti)-N8 was further confirmed

300

by the HRTEM image in Figure S3. The lattice spacing of 0.352 nm observed corresponds to the

301

[101] planes of anatase while the lattice spacing of 0.295 nm corresponds to the [001] planes of

302

rutile. Meanwhile, the carbonaceous structure was hardly observed and even large TiO2 particles

303

(~100 nm) can be found in NMIL-125(Ti)-N9. These characteristics explain why NMIL-125(Ti)-

304

N9 has the smallest porosity among the samples.

305 306

Figure 5. Surface fractal dimension analysis of the thermal decomposition products of NH2-

307

MIL-125(Ti) in N2.


16

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308

The degree of surface roughness or irregularities of nanoparticles can be studied by surface

309

fractal dimension, DSF (D), which is a dimensionless number between 2 and 3. D = 2 would

310

describe a perfectly smooth surface, which D = 3 describe a highly rough and irregular surface.51

311

By means of fractal analysis, the information on the surface roughness can be acquired as a key

312

parameter for the illustration of the properties such as activity, stability and efficiency. In this

313

work, D was recovered from the N2 sorption analysis data and based on the modified Frenkel–

314

Halsey–Hill (FHH) theory (Figure 5). The detailed calculation procedure was provided in the

315

supporting information. The D values were observed between 2.5 and 3.0 for all samples,

316

indicating a rough surface of the TiO2–carbon nanotablets via solid-state thermolysis (Figure 5).

317

The values of D for sample NMIL-125(Ti)-N7, -N8, and -N9 are 2.79, 2.60, and 2.58,

318

respectively. It can be seen that the D value decreased with the increase of the pyrolysis

319

temperature due to the stepwise elimination of the organic species and the fusion of small TiO2

320

nanocrystals into large grains. Interestingly, obvious positive correlation between D values and

321

corresponding surface area of micropore was observed, which probably suggests that with the

322

sample with developed microporous structure could exhibits a rougher surface than meso- and

323

macroporous one.

324

The XPS spectra of the thermal decomposition products of NH2-MIL-125(Ti) at 700 °C, 800 °C,

325

and 900 °C in N2 are shown in Figure S4 and S5. The wide-scan XPS spectra revealed the

326

presence of nitrogen-, carbon-, oxygen-, and titanium-related peaks (Figure S4a). The doublet

327

peak observed at 458.8 and 464.4 eV is characteristic of the TiO2 species in NMIL-125(Ti)-N7, -

328

N8, and -N9 (Figure S5d). To illustrate conversion of amino groups in NH2-MIL-125(Ti) during

329

the thermal decomposition process, detail analysis of core level binding energies of N 1s are

330

shown in Figure S5. For NMIL-125(Ti)-N7 and -N8, the nitrogen 1s core level showed a


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331

significant peak asymmetry and can be deconvoluted into two peaks at 398.1 and 400.3 eV. The

332

peak at 398.1 eV may be attributed to the anionic N- in NH2-MIL-125(Ti)’s 3D structure formed

333

as pyridinic nitrogen during thermal decomposition.52 In addition, the high binding energy peak

334

at 400.8 eV can be attributed to the pyrrolic nitrogen.52 However, for NMIL-125(Ti)-N9, the

335

nitrogen 1s core level only showed a single peak at 400.5 eV, indicating most nitrogen molecules

336

in solids were converted into pyrrolic nitrogen. If the N doped in the TiO2 lattice and TiO2

337

heterojunction, a peak at 455~456 ev contributed by Ti-N and obvious shift to lower energy in Ti

338

2p binding energy should be observed. However, in this research, no peak for Ti-N and no

339

obvious shift to lower energy were observed in Ti 2p binding energy in Figure S5d, indicating

340

that almost no N atom doped on anatase or rutile phase.53-54 As shown in Table S1, The XPS

341

results demonstrated that the nitrogen mass contents of NMIL-125(Ti)-N7, -N8, and -N9 were

342

1.37%, 1.20%, and 0.25%, respectively. After annealing at 900 °C, the mass ratio of Ti to N was

343

174.28. The mass ratio of Ti to C increased from 0.66 [NH2-MIL-125(Ti)] to 1.63 [NMIL-

344

125(Ti)-N7], 1.90 [NMIL-125(Ti)-N8], and 2.46 [NMIL-125(Ti)-N9]. Deconvoluted C 1s

345

spectra of (a) NMIL-125(Ti)-N7, N8, and N9 are shown in Figure S4. The main peak at 284.6

346

eV is corresponded to the graphite-like sp2 C, indicating most of the C atoms in the N-doped

347

graphene are arranged in a conjugated honeycomb lattice. The peaks observed around 289 eV is

348

characteristic of C-O. The pyridinic N that contributes to the π system with one p electron

349

corresponds to a tetrahedral nitrogen phase bonded to a sp3-hybridized carbon atom (N-sp3 C) at

350

about 286 eV.55 The pyrrolic N atoms with two p electrons in the π system corresponds to a

351

trigonal nitrogen phase bonded to a sp2-coordinated carbon atom (N-sp2 C) at about 287 eV.55 As

352

the carbonization temperature was increased from 700 oC to 800 oC, the ratio sp3-hybridized

353

carbon atom decreased from 13.2% to 10.7% (calculated by the area of peaks). As the


18

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354

carbonization temperature was further increased to 900 oC, the peak sp3-hybridized carbon atom

355

was disappeared indicating most nitrogen molecules in solids were converted into pyrrolic

356

nitrogen, which is in good accordance with analysis of core level binding energies of N 1s. The

357

above discussion clearly depicts that the N atom was substitutionally doped into the graphene

358

layer after the thermal decomposition of NH2-MIL-125(Ti) in N2. Meanwhile, the thermal

359

decomposition into TiO2 was identified for Ti 2p core levels.

360 361

Figure 6. CO2 sorption isotherms of thermal decomposition products of NH2-MIL-125(Ti) at

362

700 °C, 800 °C, 900 °C in N2, P25 TiO2 and C3N4.

363

CO2 uptake isotherms, at 273 K, of the pyrolysis products of NH2-MIL-125(Ti) are shown in

364

Figure 6 in comparison with commercial Degussa P25 and Graphitic Carbon Nitride g-C3N4. The

365

steepness of the slope in the low-pressure region of the CO2 adsorption isotherm indicates strong

366

adsorption of CO2 onto NMIL-125(Ti)-N7, NMIL-125(Ti)-N8 and, while that of the CO2

367

isotherms of P25 and g-C3N4 are quite linear and exhibit low uptake, signalling a weak

368

interaction between CO2 and the adsorbent. NMIL-125(Ti)-N8, which has the hierarchically


19


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Page 20 of 38

369

porous structure but moderate N content (1.20 wt%) and surface area (274 mg g−1), exhibit the

370

highest CO2 uptake capacity of 1.29 mmol g−1, followed by the microporous NMIL-125(Ti)-N7

371

with a surface area of 337 mg g−1 and CO2 uptake capacity of 1.17 mmol/g. The CO2 uptakes

372

normalized with respect to the surface areas for NMIL-125(Ti)-N7 and N8 are 0.0047 mmol m-2

373

and 0.0035 mmol m-2, which is similar to pioneer work reported by Aijaz et.al.56 NMIL-125(Ti)-

374

N9 with the lowest surface area of 45 mg g−1 and N content (0.25 wt %) displayed a lowest

375

uptake of 0.87 mmol g−1 amongst these solid-state thermolysis products. All these carbonization

376

materials show higher CO2 adsorption capacity than 3D graphene with BET surface area of 477

377

mg g−1 reported in literature (0.7 mmol g−1).57-58 The commercial Degussa P25, with mixed-

378

phase of anatase and rutile [similar to NMIL-125(Ti)-N8] and surface area of 50 mg g−1 [similar

379

to NMIL-125(Ti)-N9], shows a lower CO2 uptake capacity of 0.29 mmol/g. It is obviously that

380

the hierarchical meso/microporous structures as well as the crystal phase obtained at controlled

381

pyrolysis temperature determine the CO2 uptake ability. There is considerable interest in the

382

conversion of CO2 into fuels and other chemicals to help alleviate the environment impact of

383

greenhouse emissions and to complement the current technologies for carbon capture,

384

sequestration and storage. Considering TiO2 seems to be the most convenient candidate for

385

photocatalytic conversion of CO2, the high CO2 uptakes of hierarchical TiO2/N-doped porous

386

carbon composites will contribute to the enhanced CO2 conversion productivity.


20

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387 388

Figure 7. Adsorption and photocatalytic degradation toward (a) MO and (b) RhB with the

389

thermal decomposition products of NH2-MIL-125(Ti) at 700 °C, 800 °C, and 900 °C in N2 and

390

other related materials.

391

One of the key challenges for MOFs that needs to be overcome is to elevate their moisture and

392

chemical stability at applicable level.59-60 As moisture is omnipresent in target applications such

393

as post combustion CO2 capture or liquid phase catalysis, if MOFs are to ﬁnd use in these

394

processes, their instability toward water needs to be overcome.61 Nanostructured PMO–carbon

395

hybrid materials obtained from MOF template can both keep partial function of MOF and have

396

high moisture and chemical stability for further industrial applications. The adsorption–

397

photodecomposition performance of the TiO2–carbon nanotablets was measured through the

398

adsorption and degradation of MO and RhB in aqueous solution (Figure 7). Commercial P25

399

TiO2, anatase, rutile, g-C3N4 and graphite oxide materials were used as reference samples. The

400

adsorption behavior of the samples was measured in the dark in the first 150 min. For both MO

401

and RhB aqueous solutions, NMIL-125(Ti)-N8 and graphite oxide obviously decolorized the


21


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Page 22 of 38

402

solutions within the first 150 min. By contrast, anatase, rutile, g-C3N4, and NMIL-125(Ti)-N9

403

only absorbed similarly smaller amounts of MO and RhB due to the lower surface areas (Table

404

S2). The P25 used in this research is nano sized with surface area of 50 m2 g−1, which can absorb

405

some dye molecules in its outer space of crystals. Although NMIL-125(Ti)-N7 possesses the

406

largest BET surface area of 337 m2 g−1 with a micropore surface area of 243 m2 g−1, the major

407

micropore structure with narrow pore distribution at only 0.6 nm limited the diffusion of dye

408

molecules into the pores giving the adsorption capacity of MO (33.0 mg g−1) and RhB (22.95 mg

409

g−1) in aqueous solutions. The BET surface area of NMIL-125(Ti)-N8 was similar to that of

410

NMIL-125(Ti)-N7, but the former has larger pore size. The typical mesoporous structure with a

411

high surface area (274 m2 g−1) was the driving force for the adsorption of dye molecules (93.9

412

mg g−1 for MO and 59.3 mg g−1 for RhB). The significant differences in adsorption capacities

413

between MO and RhB may be caused by their space structures. MO is an azo dye with an

414

anionic iconicity size of 1.54 × 0.48 × 0.28 nm.62 The two benzene rings of MO are on a plane

415

without overlapping each other. By contrast, the anionic iconicity size of RhB is 1.56 × 1.35 ×

416

0.42 nm, which is larger than that of MO.62 In addition, the two benzene rings of RhB are on

417

different planes, leading to a stereospecific blockade that affects the adsorption capacities.

418

The photocatalytic degradation of MO and RhB by the TiO2–carbon nanotablets was measured

419

under UV-vis light irradiation after the adsorption equilibrium (Figure 7). The photocatalytic

420

activity of NMIL-125(Ti)-N8 was much higher than those of NMIL-125(Ti)-N7, N9 anatase,

421

rutile, g-C3N4 and graphite oxide materials. Room-temperature UV-vis absorption spectra is

422

shown in Figure S6. The NMIL-125(Ti)-N7 and –N8 samples showed a broad absorption from

423

UV light to visible light region. It is difficult to calculate the absorption edge of NMIL-125(Ti)-

424

N7 and –N8 due to the presence of large amounts of carbon residuals (Table S1), which affected


22

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425

the light absorption characteristics of TiO2. However, the presence of carbon can also benefit the

426

efficient separation pf electron-hole pairs.63 The NMIL-125(Ti)-N7 and –N8 also exhibited

427

strong absorption at ultraviolet part in the same way as commercial P25 TiO2. In addition,

428

NMIL-125(Ti)-N8 showed the highest intensity of optical absorption at ultraviolet part

429

comparing with NMIL-125(Ti)-N7 and -N9. As opposed to NMIL-125(Ti)-N7 (pure anatase

430

phase TiO2–carbon nanotablets) and -N9 (pure rutile phase TiO2–carbon nanotablets), NMIL-

431

125(Ti)-N8 possessed a heterojunction structure, i.e. mixture of anatase and rutile phase TiO2–

432

carbon nanotablets (the anatase/rutile ratio is 0.3). The transfer of electrons to anatase lattice

433

trapping sites allows holes that would have been lost during recombination to reach the surface,

434

leading to the formation of catalytic hot spots at the rutile/anatase interface.64 Thus,

435

photodegradation efficiency was obviously improved. In consequence, Degussa P25 is marketed

436

as a high performance mixed-phase titania photocatalyst and it is utilized as a reference material

437

here. This nanocrystalline material, formed by flame pyrolysis, consists of 80 wt% anatase and

438

20 wt% rutile. Due to that the ratio of anatase in NMIL-125(Ti)-N8 is much lower than that in

439

P25, NMIL-125(Ti)-N8 showed less photoactivity performance compared with P25 since anatase

440

has significantly higher photoactivity than rutile. As proof of concept, through appropriate

441

adjustment thermolysis conditions, the thermal decomposition products of NH2-MIL-125(Ti)

442

may achieve better photoactivity performance.

443

The NMIL-125(Ti)-N8 nanotablets showed both remarkable adsorptivity and photocatalytic

444

activity. Hence, this material was chosen for the recycling test toward RhB to evaluate stability

445

and regeneration efficiency. The adsorption–photodecomposition performance of NMIL-

446

125(Ti)-N8 during three consecutive cycles is shown in Figure S7. Almost no loss of RhB

447

decolorization activity was observed in NMIL-125(Ti)-N8 during the three cycles, indicating that


23


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Page 24 of 38

448

this material possesses excellent long-term stability. After each cycle, the NMIL-125(Ti)-N8

449

nanotablets were regenerated through the solid- liquid separation and dried at normal

450

temperature without additional high-temperature pore-activating processes. Compared with other

451

adsorbents that need additional pore-activating processes for reuse, NMIL-125(Ti)-N8

452

nanotablets which were composed of porous carbon can both adsorb and degrade organic

453

pollutants repeatedly through self-regeneration.

454

In summary, this work successfully prepared porous nitrogen-doped TiO2–carbon hybrid

455

nanotablets via one-step solid-state thermolysis of amino-functionalized titanium metal-organic

456

framework NH2-MIL-125(Ti). Compared with the pure TiO2 particles obtained through the

457

calcination of the MOF template in air, TiO2–carbon hybrid nanotablets inherited the

458

morphology of the template and kept the original nanoporous structure. The anionic N- in NH2-

459

MIL-125(Ti)’s 3D structure formed as pyridinic nitrogen and pyrrolic nitrogen into the graphene

460

layer. By adjusting the carbothermal treatment temperature, either TiO2 phase or porosity was

461

controllable to produce a series of TiO2-carbon compositions. The resultant composites with

462

hierarchical meso/microporous structures showed remarkable CO2 uptake ability compared with

463

commercial P25 TiO2, C3N4 and 3D graphene. Amongst the products, hierarchically micro-

464

/mesoporous carbon with anatase/rutile heterojunction embedded were successfully fabricated by

465

the pyrolysis of NH2-MIL-125(Ti) at 800 °C. Thanks to such hierarchical pore structure with

466

large surface area and heterostructure of photoactive TiO2, NMIL-125(Ti)-N8 showed a

467

remarkable adsorptive and photocatalytic activity toward organic dyes. Compared with other

468

adsorbents which call for additional pore-activation processes for regeneration, the prepared

469

nitrogen-doped TiO2–carbon hybrid nanotablets can both adsorb and degrade organic pollutants

470

repeatedly through a ‘self-regenerating’ process. Benefiting from their versatile pore structure,


24

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471

heteroatoms doping, and semiconductor incorporation, the nitrogen-doped TiO2–carbon

472

composites derived from metal-organic frameworks could find various potential applications,

473

especially for sustainable chemistry and engineering.

474

Supporting Information. XPS, Raman analysis results, Room-temperature UV-vis absorption

475

spectra and detail Calculation of surface fractal dimension (DSF) are given in supporting

476

information. This material is available free of charge via the Internet at http://pubs.acs.org.

477

AUTHOR INFORMATION

478

*Corresponding Author

479

Dr. Yi-nan Wu, Phone: +86 21-65980567; E-mail address: [email protected]

480

Dr. Ying Wang, Phone: +86 13918374165; E-mail address: [email protected]

481

Notes

482

The authors declare no competing financial interest.

483 484

ACKNOWLEDGMENT

485

This work was supported by the National Science Foundation of China (51203117, 21305046),

486

the Fundamental Research Funds for the Central Universities, program (2014KJ007) for Young

487

Excellent Talents in Tongji University, Foundation of State Key Laboratory of Pollution Control

488

and Resource Reuse (Tongji University) (PCRRY15007) and the Program (KY201402012) on

489

Demonstration and Capacity Building of Drinking Water Treatment in East-Africa by the

490

Ministry of Science and Technology of China (MOST) and Science & Technology Commission

491

of Shanghai Municipality (14DZ2261100). We thank Prof. Leiyu Feng from Tongji University

492

for providing synthesized g-C3N4 materials.


25


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Synopsis. Preparation of porous nitrogen-doped TiO2–carbon hybrid nanotablets via thermolysis of NH2-MIL-125(Ti) MOF toward efficient CO2 capture and degradation of pollutants.


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Scheme 1. Schematic of the preparation of TiO2–carbon nanotablets via solid-state thermolysis of MOF templates. Scheme 1 77x51mm (220 x 220 DPI)



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Figure 1. (a) SEM image of NH2-MIL-125(Ti); (b) XRD patterns of NH2-MIL-125(Ti); (c) Nitrogen sorption isotherm and pore width distribution based on the NLDFT model of NH2-MIL-125(Ti); (d) TGA curves of NH2-MIL-125(Ti) in air and N2. Figure 1 80x77mm (220 x 220 DPI)


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Figure 2. XRD patterns of the thermal decomposition products of NH2-MIL-125(Ti) in air (a) and N2 (b) at different temperatures. (Green triangle: anatase-phase TiO2; red dot: rutile-phase TiO2). Figure 2 88x62mm (220 x 220 DPI)



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Figure 3. Nitrogen sorption isotherms and pore width distributions based on the NLDFT model of the thermal decomposition products of NH2-MIL-125(Ti) at 700 °C, 800 °C, and 900 °C in N2. Figure 3 113x81mm (300 x 300 DPI)


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Figure 4. SEM and TEM images of the thermal decomposition products of NH2-MIL-125(Ti) at 700 °C (a, d), 800 °C (b, e), and 900 °C (c, f) in N2. Figure 4 88x58mm (220 x 220 DPI)



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Figure 5. Surface fractal dimension analysis of the thermal decomposition products of NH2-MIL-125(Ti) in N2. Figure 5 80x75mm (220 x 220 DPI)


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Figure 6. CO2 sorption isotherms of thermal decomposition products of NH2-MIL-125(Ti) at 700 °C, 800 °C, 900 °C in N2, P25 TiO2 and C3N4. Figure 6 78x60mm (220 x 220 DPI)



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Figure 7. Adsorption and photocatalytic degradation toward (a) MO and (b) RhB with the thermal decomposition products of NH2-MIL-125(Ti) at 700 °C, 800 °C, and 900 °C in N2 and other related materials. Figure 7 102x82mm (300 x 300 DPI)


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