Research Article pubs.acs.org/journal/ascecg
External Water-Free Approach toward TiO2 Nanoparticles Embedded in Biomass-Derived Nitrogen-Doped Carbon Siyang Gao, Yong Yan, and Ge Chen* Beijing Key Laboratory for Green Catalysis and Separation, College of Environmental & Energy Engineering, Beijing University of Technology, Pingleyuan 100, 100124 Beijing, P.R. China S Supporting Information *
ABSTRACT: Biomass-derived nitrogen-doped carbon (BDNDC) is attracting increasing attention as a sustainable approach for potential application in energy conversion and storage. The use of BDNDC as a matrix to embed metal oxide nanoparticles has seldom been reported, even though such composites may possess the merits of both components to suit a broad range of applications. Inspired by the recipes used to bake cakes, we demonstrate here an external water-free synthesis of TiO2 nanoparticles embedded in a BDNDC matrix, in which a foam of chicken egg white and Ti(SO4)2 is obtained simply by beating with a domestic electric whisk and subsequently annealed under a reducing atmosphere at different temperatures. Anatase− BDNDC, anatase/rutile−BDNDC and TiO2(B)/rutile−BDNDC samples are obtained at 550, 720, and 880 °C, respectively. Moreover, the obtained TiO2−BDNDC composites can be used as anode material for lithium-ion batteries and also exhibit high performance. This study lays the groundwork for the benign synthesis of other metal oxide−BDNDC or metal−BDNDC composites that show promise for use in alternative clean energy technologies and environmental science. KEYWORDS: TiO2, Biomass, Nitrogen-doped carbon, Egg white, Lithium-ion battery
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INTRODUCTION Recently, nitrogen-doped carbon (NDC) materials derived from inexpensive, abundant, and renewable natural biomass for energy applications have received much attention because they are environmentally friendly and sustainable.1−5 On the other hand, metal oxide nanoparticles have been an appealing member of the broad family of functional materials for a long time and are particularly attractive for use in applications such as energy storage and conversion, catalysis, electronics, optics, and sensing.6−10 Therefore, biomass-derived nitrogen-doped carbon (BDNDC) embedded with metal oxide nanoparticles is expected to possess the advantages of both components and may find use in not just energy applications but also many other technologically important applications.11,12 Titanium dioxide (TiO2) is an important metal oxide with a variety of uses because of its unique physicochemical properties as well as environmental safety.13,14 For example, TiO2 has been intensively investigated as a photocatalyst for water splitting and environmental pollutant removal.15,16 TiO2 also shows potential as an anode material for next-generation lithium-ion batteries.17−21 Recently, the use of nanostructured TiO2 as © XXXX American Chemical Society
electrodes for supercapacitors has received much attention because of its nontoxicity, low cost, and high chemical stability.22,23 However, TiO2 has some drawbacks such as light absorption being limited to the ultraviolet (UV) region,24 poor electronic and ionic conductivity,25 and relatively low surface area, which have seriously hindered its use in practical applications. The integration of TiO2 with nanocarbons such as carbon nanotubes, graphene, and amorphous carbon derived from organic molecules has showed superior performance to pure TiO2 in aforementioned applications.26−29 Also, the carbon- and nitrogen-doped TiO2 nanoparticles showed enhanced performance.30−32 Thus, it is anticipated that TiO2−BDNDC composites would exhibit the advantages of both species and be effective in applications such as lithium-ion batteries, visible light-driven photocatalysis, and supercapacitors. Received: August 20, 2015 Revised: December 31, 2015
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DOI: 10.1021/acssuschemeng.5b00904 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
were measured at 77 K with an Autosorb system (Quanta Chrome, U.S.A.). Electron paramagnetic resonance (EPR) spectra were recorded at 77 K using a JES-FA200, JEOL spectrometer. Electrochemical Measurements. The electrode performance of TiO2−BDNDC composites are measured by a half-cell testing. A total of 2032 coin cells were assembled in an argon-filled glovebox. The slurry of electrode material containing 70 wt % active materials, 20 wt % acetylene black, and 10 wt % polyvinylidene fluoride (Aldrich) was pasted onto stainless steel foil. Pure lithium foil was used as the counter electrode, and glass fiber (GF/D, Whatman) was used as a separator. The electrolyte was 1 M LiPF6 in a 50:50 w/w mixture of ethylene carbonate and diethyl carbonate. For comparison, the electrode performance of pure TiO2 (commercial P25) was measured under the same conditions. The galvanostatic measurements were measured by a battery tester (Neware, Shenzhen, China). Cyclic voltammograms (CVs) and electrochemical impedance spectroscopy (EIS) were investigated using a VMP3 multifunctional electrochemical analysis instrument (Bio-Logic, France).
The co-blending of TiO2 nanoparticles with aqueous biomass to obtain TiO2−BDNDC composites seems to be a simple, direct approach; however, there are three factors that should be considered. First, TiO2 nanoparticles need to be prepared beforehand. Second, many biomass sources such as hair, fermented rice, seaweed, and wheat straw require complicated pretreatment to form an aqueous solution.33−36 Third, TiO2 nanoparticles tend to agglomerate at high loading, which leads to uneven dispersion in the BDNDC matrix. Thus, a facile approach to combine BDNDC with TiO2 nanoparticles uniformly is highly desirable but challenging. Compared with typical wet chemistry methods, solid-state synthesis is simple (blending and heating), water free, and scalable. However, this approach is rarely used to prepare nanomaterials because the high temperatures typically required often dramatically accelerate crystal growth. Inspired by the recipe used to bake cakes, here we demonstrate an external water-free approach toward the synthesis of TiO2 nanoparticles embedded in BDNDC using chicken egg white and Ti(SO4)2 as precursors. It is well known that proteins are a large part of the egg white and could be used as a precursor for nitrogen-doped carbon. The developed process is extremely facile, free from external water, and scalable. A foam of chicken egg white and Ti(SO4)2 is obtained using a domestic electric whisk to mix egg white and Ti(SO4)2 powder. In this process, titanium ions are introduced into the biopolymer matrix uniformly through the formation of coordination bonds. Then, the foam is solidified by heating at 200 °C for 1 h, followed by annealing under a reducing atmosphere for 4 h at various temperatures, and the proteins in the egg white could be prolyzed to nitrogen-doped carbon after the heat treatment. As a result, the foam is transformed into TiO2 nanoparticles embedded in a BDNDC matrix with good dispersion. Anatase−BDNDC, anatase/rutile−BDNDC, and TiO2(B)/rutile−BDNDC composites are obtained at 550, 720, and 880 °C, respectively. No external water or other chemicals are needed in the whole synthetic procedure, which is very attractive from the perspectives of environmental safety and large-scale production. Moreover, the obtained TiO2−BDNDC composites show high anode performance in lithium-ion batteries. This work opens the door to prepare metal oxide− BDNDC and metal−BDNDC composites with promise for applications such as energy storage and conversion, catalysis, electronics, and sensing.
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RESULTS AND DISCUSSION The procedure used to prepare TiO2−BDNDC composites is outlined in Figure 1, which shows photographs of the initial
Figure 1. Outline of the synthetic procedure used to prepare TiO2− BDNDC composites.
chicken egg white and Ti(SO4)2 powder, foam, and final TiO2− BDNDC composite. Figure 2 depicts XRD patterns of the samples annealed at 550, 720, and 880 °C. While the XRD pattern of S-550 exhibited peaks consistent with typical anatase phase, a broad background from 20° to 35° suggested the
EXPERIMENTAL SECTION
Synthesis of TiO2−BDNDC Composites. Ground Ti(SO4)2 (35g) was slowly added to chicken egg white (120 g) in a stainless steel pot. The mixture was stirred with a domestic electric whisk (Chang Di J-8844, 150 W, purchased from tmall.com) for 30 min. (Note: the stainless steel pot should be completely free of water.) The resulting foam was heated at 200 °C for 1 h and then annealed under H2 (5%)/Ar (95%) atmosphere for 4 h at 550, 720, and 880 °C to give samples named S-550, S-720, and S-880, respectively. Material Characterization. X-ray diffraction (XRD) patterns of samples were measured by a Bruker D8 Advance (German) using Cu Kα radiation with a voltage of 40 kV and current of 40 mA. Thermogravimetric (TG) analyses were recorded on a Seiko Instruments 6300 TG-DTA device. Scanning electron microscopy (SEM) images were obtained on a Hitachi S-4300 microscope. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy images (HRTEM) were captured with a Tecnai F20 microscope at an accelerating voltage of 200 kV. Xray photoelectron spectroscopy (XPS) was performed on a PHI Quantera spectrometer. Nitrogen adsorption−desorption isotherms
Figure 2. XRD patterns of the three TiO2−BDNDC composite samples. B
DOI: 10.1021/acssuschemeng.5b00904 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering existence of amorphous carbon. Some rutile phase was observed in S-720 as well as the major anatase phase. When the annealing temperature was further increased to 880 °C, the anatase phase disappeared, and TiO2(B)/rutile mixed phases were observed. A peak was also observed at 2θ = 12.6°, which indicated the existence of hydrogen titanate.37 The presence of the TiO2(B) phase was attributed to the phase transformation of anatase to TiO2(B). TiO2(B) is a well-known metastable phase of TiO2. However, the reported syntheses of TiO2(B) nanocrystals are often complicated and time consuming; in addition, corrosive chemicals and harsh reaction conditions are required.37−39 Thus, the developed method forming TiO2(B)/ rutile mixed phases in some cases is worth noting because it might provide an alternative approach to TiO2(B) nanocrystals. This result is also interesting because the phase transformation from anatase to TiO2(B) is rarely observed at high temperature. An explanation for this might be the effect of the BDNDC matrix and reducing atmosphere; however, a comprehensive investigation of the anatase to TiO2(B) transformation occurring in S-880 is outside the scope of this work. The amount of BDNDC in each sample was characterized by measuring their weight loss in air up to 800 °C. The TGA curves (Figure S1) showed weight losses of approximately 42.7%, 25.6%, and 17.1% at 800 °C for S-550, S-720, and S-880, respectively. Major weight losses were observed between 340 and 630 °C, which were attributed to the loss of carbon. It is clear that the BDNDC content of the composites decreases with increasing annealing temperature. SEM images of the ground samples (Figure S2) revealed bulk particles with sizes of tens of microns; however, for S-880, many tiny particles (about tens of nanometers) were observed on the surface of the larger particles at high magnification. Figure 3 shows TEM images of the samples. S-550 contained very small nanoparticles (
