Engineering Carbon Materials from the Hydrothermal Carbonization Process of Biomass, Notas de estudo de Engenharia Elétrica
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Engineering Carbon Materials from the Hydrothermal Carbonization Process of Biomass, Notas de estudo de Engenharia Elétrica

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Engineering Carbon Materials from the Hydrothermal Carbonization Process of Biomass

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Engineering Carbon Materials from the Hydrothermal Carbonization Process of Biomass

By Bo Hu, Kan Wang, Liheng Wu, Shu-Hong Yu,* Markus Antonietti, and

Maria-Magdalena Titirici*

Energy shortage, environmental crisis, and developing customer demands

have driven people to find facile, low-cost, environmentally friendly, and

nontoxic routes to produce novel functional materials that can be com-

mercialized in the near future. Amongst various techniques, the hydrothermal

carbonization (HTC) process of biomass (either of isolated carbohydrates or

crude plants) is a promising candidate for the synthesis of novel carbon-based

materials with a wide variety of potential applications. In this Review, we will

discuss various synthetic routes towards such novel carbon-based materials

or composites via the HTC process of biomass. Furthermore, factors that

influence the carbonization process will be analyzed and the special chemical/

physical properties of the final products will be discussed. Despite the lack of

a clear mechanism, these novel carbonaceous materials have already shown

promising applications in many fields such as carbon fixation, water puri-

fication, fuel cell catalysis, energy storage, CO2 sequestration, bioimaging,

drug delivery, and gas sensors. Some of the most promising examples will

also be discussed here, demonstrating that the HTC process can rationally

design a rich family of carbonaceous and hybrid functional carbon materials

with important applications in a sustainable fashion.

1. Introduction

Synthesis and application of carbon materials have a long history and carbon black, fabricated from fuel-rich partial combustion, has been used for ink, pigments, and tattoos for more than 3000 years.[1] Starting with the discovery of fullerenes[2] and carbon nanotubes,[3] the material science related to valuable carbon materials has become a hot area, motivated by its potential applications in carbon fixation, catalyst supports, adsorbents, gas

[*] Prof. Dr. S. H. Yu, Dr. B. Hu, K. Wang, L. H. Wu Division of Nanomaterials and Chemistry Hefei National Laboratory for Physical Sciences at Microscale Department of Chemistry University of Science and Technology of China Hefei, Anhui 230026 (P. R. China) E-mail: shyu@ustc.edu.cn

Dr. M.-M. Titirici, Prof. Dr. M. Antonietti Department of Colloid Chemistry Max Planck Institute of Colloids and Interfaces MPI Research Campus Golm 14424 Potsdam (Germany) E-mail: magdalena.titirici@mpikg.mpg.de

DOI: 10.1002/adma.200902812

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storage, electrode, carbon fuel cells and cell biology.[4–8] Many synthetic methods, such as carbonization, high-voltage-arc electricity, laser ablation, or hydrothermal carbonization have been reported for the preparation of amorphous, carbonaceous, porous, or crystalline carbon materials with different size, shape, and chemical compo- sitions.[9–13] In this Review, we will focus on a more sustainable approach, which relies on low specific energy input and replaces fossil-fuel-based starting products with biomass.

Biomass is a qualified carbon raw material for the synthesis of valuable carbon materials because it is available in high quality (e.g., as pure saccharose) and huge amount, and is a environmental friendly renewable resource. An illustration of its potential is the production of bioethanol, which has emerged as a new fuel for vehicles (usually by mixing gasoline with alcohol). In the United States, more than 7 billion gallons bioalcohol were produced in

2007. In Brazil, almost all the light automobiles are running on the blend of gasoline and bioalcohol, and similar scales can be easily envisaged formaterials, appropriate carbon products assumed. Even more abundant, waste biomass derived from agricultural resides and forest byproducts has drawn little attention as a raw material, since only simple combustion has been used to elevate the value of waste biomass. Carbon materials fabricated from waste biomass have shown promising applications as sorption materials, hydrogen storage, biochemicals, and others.[14–17] The problem is that there is still no general and satisfactory process for the production of valuable carbon materials from crude biomass to date.

In this respect, a hydrothermal carbonization (HTC) process might have the opportunity to turn into a powerful technique for the synthesis of valuable carbonmaterials from biomass, especially crude biomass (Scheme 1). According to different experimental conditions and reaction mechanisms, two HTC processes can be classified. Based on the pyrolysis of biomass, a high-temperature HTC process is apt to synthesize carbon nanotubes, graphite, and activated carbon materials under high temperature and high pressure.[18,19] A low-temperature HTC process is carried out up to 250 8C, employing several chemical transformation cascades, and is a more environmentally friendly route.[20,21] Various carbonac- eous materials with different sizes, shapes, and surface functional groups have been synthesized by this process. Furthermore, these

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Shu-Hong Yu received his B.Sc. degree at Hefei University of Technology and his Ph.D. from the University of Science and Technology of China (USTC). He joined Prof. Masahiro Yoshi- mura’s laboratory, Tokyo Institute of Technology, as a postdoctoral Fellow. Afterwards, he was as an Alexander-von-Humboldt Research Fellow in the Max Planck Institute of Colloids and

Interfaces, Germany, working with Prof. Markus Antonietti and PD Dr. habil. Helmut Cölfen. He joined the Department of Chemistry USTC as a full Professor in 2002, and was appointed the Cheung Kong Professorship in 2006 by the ChineseMinistry of Education. He is now leading the Division of Nanomaterials and Chemistry at the Hefei National Laboratory for Physical Sciences at Microscale, USTC. His research focuses on hydrothermal carbon, bioinspired self-assembly of new nanostructured materials, and hybrids with high performances.

Maria-Magdalena Titirici received her basic academic education in organic chemistry and material physics in Bucharest, Romania. After Ph.D. work with B. Selergren in Mainz and Dortmund, she joined the Max Planck Institute of Colloids and Interfaces where she is

currently head of the research group ‘‘Sustainable Functional Carbonaceous and Polymeric Materials’’. Her current projects and interests include self-assembly of nanostructured materials, molecular electronics, energy and hydrogen storage, CO2 sequestration agents, chromatographic stationary phases, and drug-delivery systems.

Bo Hu received his B.S. degree from Hefei University of Technology in 2002 and his Ph. D in Inorganic Chemistry from University of Science and Technology of China in 2008 under the supervision of Prof. Shu-Hong Yu. He is interested in the carbonaceous nanostructured materials and other inorganic nanoparticles as well as the self-assembly process for nanodevices.

Scheme 1. Schematic illustration of the hydrothermal carbonization (HTC) process as a powerful technique for the synthesis of valuable carbon materials from biomass and the potential applications of the as-produced carbon materials.

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carbonaceousmaterials can combine with other components, such as noble metal nanoparticles to form composites with special chemical and/or physical properties.

In this article, we will review the concept and history of the HTC process at both high and low temperature. The promising potential of the HTC process for the preparation of carbon materials from biomass will be demonstrated. Finally, we will briefly present some examples of the application of carbonaceous materials from the HTC process in fields such as environment, catalysis, energy storage, biology, and sensors.

2. Hydrothermal Carbonization: A New Way Towards Carbon Materials

2.1. Concept and History

Hydrothermal conditions, i.e., application of an aqueousmedium over 100 8C and 0.1 MPa, are widely found in nature, because many minerals form under these circumstances.[22] Since the pioneering work from 1960s to 1980s, the hydrothermal process has been widely used for the synthesis of a vast range of solid-state compounds such as oxides, sulfides, halides,[22–25] molecular zeolites, and other microporous phases.[25] Nowadays, the hydrothermal process has become an important technique for the synthesis of various kinds of inorganic materials, such as functional oxide[26] and non-oxide nanomaterials[27] with specific shapes and sizes, as well as for the synthesis of new solids.[28,29]

For the synthesis of valuable carbonmaterials, the educts of the HTC process usually include carbohydrates, organic molecules, and waste biomass. The treatment of carbon materials under hydrothermal conditions increases or changes solubility, melts

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crystalline parts, accelerates the physical and chemical interaction between reagents and the solvent, facilitates ionic and acid/base reactions, and finally leads to the precipitation/formation of the carbonaceous structures.

Although there is rarely a clear classification, the HTC process could be classified into two main parts by applied temperature.

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Figure 1. a) Transmission electron microscopy (TEM) image of MWNTs. Reproduced with permission from [40]. Copyright 2001, American Chemi- cal Society. b) TEM image of a chain of connected carbon cells. Reproduced with permission from [52]. Copyright 2001, Elsevier Publishing Group. c) Scanning electron microscopy (SEM) images of spherical carbon particles. Reproduced with permission from [54]. Copyright 2009, American Chemical Society. d) SEM images of olivary carbon particles. Reproduced with permission from [51]. Copyright 2005, American Chemical Society.

The high-temperature HTC process proceeds between 300 and 800 8C and is therefore clearly beyond the stability of standard organic compounds. Reactive gases and carbon fragments are to be expected from ‘‘thermolysis’’, which enable the synthesis of carbon nanotubes, graphitic carbon materials, and activated carbon materials.[19,30] The low-temperature HTC process per- forms below 300 8C, and functional carbonaceous materials can be produced according to dehydration and polymerization schemes known from ordinary organic chemistry. For the coalification of biomass, the low-temperature HTC process is presumably close to natural coalification[6] but, of course, at highly accelerated speed, decreasing the reaction time from some hundred million years to the time-scale of hours. In addition, it is a spontaneous, exothermic process, with the vast majority of the carbon of the starting products also found in the final product (the ‘‘carbon efficiency’’, a sustainability issue, is close to 1).

HTC in material synthesis is a 100-year-old technique, with increasing interest originating from the charcoal formation.[6]

Bergius first described the hydrothermal transformation of cellulose into coal-like materials in 1913.[31] Then, detailed investigations were focused on the biomass source,[32] the formation process,[33]

and the identification of the final coal composition.[34]

Since the discovery of carbon nanotubes in 1991,[3] the high-temperature HTC process has been developed quickly. At the beginning of the new century, a renaissance in the low-temperature HTC process appeared with the reports on the synthesis of uniform carbonaceous particles from sugar or glucose.[20,21] In the past few years, lots of functional carbonaceous materials from biomass have been produced via the HTC process and these materials have shown great potential in many fields.[5]

Nowadays, with the gradual acknowledgement of hydrothermal process and carbonization mechanism,[35] the HTC has been widely used to smartly design novel carbon and carbonaceous materials from biomass with important applications.[5,6]

2.2. Hydrothermal Carbonization at High Temperature

2.2.1. Carbon Nanotubes

The HTC process at high temperature is a powerful method for the fabrication of well-crystallized multi-walled carbon nanotubes (MWNTs). Some novel synthesis routes have been reported for the production of MWNTs.[36–39] In particular, Yoshimura co-workers reported a hydrothermal processing for the synthesis of high-quality MWNTs from amorphous carbon without the use of a metal catalyst at a temperature of 800 8C and a pressure of 100 MPa (Fig. 1a).[40] The stability and evolution of single-walled carbon nanotubes (SWNTs) has been investigated during hydrothermal treatment at temperature between 200 and 800 8C and a pressure of 100 MPa.[41,42] The SWNTs were stable even after mild and short-term treatment, and could transform to short MWNTs and graphitic nanoparticles in a high-temperture and high-pressure water system. The HTC process could also effectively modify the surface of MWNTs, such as the production of hydroxyl-group modified MWNTs.[43]

2.2.2. Three-Dimensional Carbon Structures

The HTC process at high temperature could be used to prepare carbon films and materials with high flexibility.[44,45] A nice case

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was the formation of carbon films on carbides under hydro- thermal conditions at 300–800 8C.[44] This simple route enabled to coat the surface of SiC fibers, powders, platelets, and single crystals with carbon films with controllable thickness from nanometer to micrometer. The mechanism is that the surface of the substrate transformed into carbon films as follow:

SiCxOy þ nH2O ! SiO2 þ xCþ nH2

Monodispersed carbon microspheres,[46–49] ellipsoidal carbon microparticles,[50] olivary carbon particles,[51] nanocells,[52] and graphite tubes[53] have been successfully synthesized from carbohydrates[47,49] and organic molecules[46,50,51,52,53] via the high-temperature HTC process (Fig. 1b–d). For the synthesis of uniform, pure, paramagnetic carbon particles (Fig. 1c), Pol and Thiyagarajan[54] have carefully studied the process by measuring the in situ autogenuous pressure and dissociated chemical species as a functions of temperature during the thermolysis of mesitylene. These two parameters are important for the production scale-up of these spherical carbon particles.

Further, the high-temperature HTC process was an effective technique for the fabrication of activated carbon materi- als.[18,30,55,56] For instance, Salvador et al. have reported the production of activated carbon materials from oak wood and anthracite by the high-temperature HTC process with a broad distribution of micropores and some mesopores.[19] Compared to steam activation, the HTC process has higher gasification rate and better penetration power into the pore structure of the char. The HTC process can quickly and greatly change the porosity of the oak char, widening the micropore structure homogeneously (Fig. 2). It was also shown that this must be carbon from biomass. For anthracite char, the HTC process has shown only little effect on the pore development.

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Figure 2. SEM images of oak wood and oak char. a) Oak char gasified with supercritical water at different burnoffs and b) oak char gasified with steam at different burnoffs. The scale bar was 20mm. Reproduced with permission from [19]. Copyright 2005, American Chemical Society.

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2.3. Hydrothermal Carbonization at Low Temperature:

Synthesis of Highly Reactive Carbonaceous Nanostructures

2.3.1. Morphology-Controlled Synthesis of Carbonaceous

Nanostructures

The HTC process at low temperature is apt to generate monodispersed colloidal carbonaceous spheres, as shown in Figure 3, from the carbohydrate sources such as sugar,[20]

Figure 3. a) SEM image of monodispersed hard carbon spherules. Repro- duced with permission from [20]. Copyright 2001, Elsevier Publishing Group. b) TEM image of carbon spheres. Reproduced from [21]. c) SEM images of carbonaceous materials. Reproduced with permission from [65]. Copyright 2009, American Chemical Society. d) TEM image of hollow spheres. Reproduced from [63].

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glucose,[21] cyclodextrins,[57] fructose,[58]

sucrose,[59] cellulose,[60] and starch.[59] The formation of these materials includes the processes of dehydration, condensation, poly- merization, and aromatization.[61] Compared with other routes, the HTC process has a couple of advantages, including low toxicolo- gical impact of materials and processes, the use of renewable resources, facile instrumen- tation and techniques, and a high energy and atom economy.[20,21,35] Catalysts, especially metal ions,[4,62,63,64] can effectively accelerate the HTC process of carbohydrates and direct the synthesis of various carbonaceous materi- als. Yu and co-workers.[63] have reported that iron ions and iron oxide nanoparticles could accelerate the HTC process of forming the hollow carbonaceous spheres from starch (Fig. 3d). When the controlled-dehydration products of carbohydrates are partially replaced by organic monomers, a new type of hybrid between carbon and polymer latex can be produced by copolymerization and

cycloaddition reaction. These latexes have not only the surface properties of the polymers, but also the structural, mechanical, thermal, and electric properties of the carbon framework.[65]

Titirici and co-workers have reported the production of carboxylate-rich carbonaceous materials in the presence of acrylic acid by the one-step HTC process of glucose (Fig. 3c).[65]

In the HTC process of carbohydrates, the formation process and the final material structures are rather complicated and a clear scheme has not been reported. This is mainly due to the formation of a multitude of furan-type dehydrated intermediates from carbohydrates, the complexity of the chemistry, and the lack of a satisfactory technique for the final structure discrimination, which could allow the identification of all-carbon sites with higher resolution.[35,58] For example, the dehydration and fragmentation of glucose can give rise to different soluble products, such as furfural-like compounds (5-hydroxymethylfurfural, furfural, 5-methylfurfural), organic acids and aldehydes (acetic, lactic, propenoic, levulinic, and formic acids), and phenols.[61,66–68] Then, polymerization or condensation reactions do occur forming the final forming the final carbonaceous material,[61,69] which have been identified to occure at least along three lines simultaneously, namely aldol-condensation, cycloaddition reactions, and a hydro- xymethyl-mediated furan resin condensation.[70]

Among the HTC process of diverse biomass (glucose, xylose, maltose, sucrose, amylopectin, starch), hexose-based carbon sources mainly produce 5-hydroxymethylfurfural (HMF) as the reaction-driving dehydration products, while pentosemainly works via the (more reactive) furfural.[35] A LaMer model[71,72] has been proposed for the explanation of the growth of the carbonaceous materials. This assumes that the final carbonaceous materials display a type of core–shell structure composed of a hydrophobic core and a stabilizing hydrophilic shell that is less dehydrated and contains a large number of reactive oxygen functional groups (hydroxyl/phenolic, carbonyl, or carboxylic).[21,61]

The as-synthesized carbonaceous materials usually have intrinsic porous structures with controllable morphology and

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Figure 4. a) SEM image of carbon nanofibers. Reproduced with permission from [74]. Copyright 2006, American Chemical Society. b) TEM image of hollow carbon materials. The scale bar corresponds to 200nm. Reproduced from [75]. Copyright 2007, American Chemical Society. c,d) SEM and TEM image of carbonaceous polymer nanotubes. Reproduced from [79].

Figure 5. a) TEM image of the hydrophilic nanoparticles. b) Absorption spectrum of the hydrophilic nanoparticles in water. Inset: the corresponding normalized fluorescence spectra at different excitation wavelengths. Repro- duced from [80]. c) TEM image of the submicrotubes. d) SEM images of these tubes. The inset shows a TEM image of the end group of the tube structure. Reproduced with permission from [81]. Copyright 2008, American Chemical Society.

surface functionality. For example, the monodispersed carbonac- eous spheres after heat treatment (at 250 8C) have uniform nanoporous structures and specific Brunauer–Emmett–Teller (BET) surface area of 400m2 g1. Surface modification of these porous carbonaceous materials (e.g., a high concentration of hydroxyl/phenolic, carbonyl, and carboxylic groups[61]) leads to a high reactivity, which broaden their application in environment, catalyst, electrochemistry, and drug delivery.[73] It is worthy to note that carbonaceous materials with different functionality could be produced by a general route of mixing the carbohydrates with other small organic monomers via the HTC process.[65]

Coupling either hard- or soft-templating effects with the HTC process has shown powerful capability in controlling the synthesis of various carbonaceous nanostructures with special morphology. For example, using ultrathin and ultralong Te nanowires as templates, well-defined ultralong carbonaceous nanofibers can be synthesized from glucose by the HTC process (Fig. 4a).[74] Adjusting the reaction time or the ratio of the tellurium and glucose can effectively control the diameters of carbonaceous nanofibers. Hollow carbonaceous spheres with porous walls could be produced from glucose by the HTC process using appropriately functionalized porous silica particles as templates (Fig. 4b).[75] The efficient deposition of a glucose-derived carbon precursor could be advanced by the electrostatic attraction between the positively charged silica and negatively charged carbon precursors.[75] Soft templates, such as sodium dodecyl benzenesulfonate (SDBS), could induce the template synthesis and assembly process to generate carbonaceous nanowires.[76]

Using the anionic surfactant sodium dodecyl sulfate (SDS) as a soft template, the HTC process has been developed for preparing hollow carbonaceous capsules with a reactive surface layer and tunable void size and shell thickness.[77,78]

Well-aligned and open-ended carbon nanotubes can be synthesized by using anodic aluminum oxide (AAO) films as

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templates in the HTC process and subsequent carbonization at high temperature (Fig. 4c and 4d).[79] This synthesis method shows obvious advantages over conventional approaches, such as arc discharge, laser ablation, and chemical vapor deposition (CVD), since it requires no catalysts and uses inexpensive raw materials. After being synthesized, both the inner and outer surface of the CNTs were decorated with Pt nanoparticles via the incipient wet method using NaBH4 as the reductant, resulting in shell–core–shell-like nanotube composites (Pt–CNT–Pt).[79]

These hybrid structures exhibited superior catalytic performance compared to a commercial carbon-black-supported Pt electrode and could be used as the anode catalyst in direct-methanol fuel cells (DMFCs).[79]

Owing to the low toxicity, low cost, and high stability, functional carbon nanoparticles have been recognized as benign substitutes for conventional quantum dots based on metallic elements. Various chemical methods have been reported for the controlled synthesis of carbon nanoparticles. However, due to the size dependence of their photoluminescence efficiency, most of the synthesized dots show no efficient visible emission. Bourlinos et al.[80] reported different chemical routes, which were utilized to produce carbon dots that are photoluminescent in the visible region with an average size of less than 10 nm through thermal carbonization of citrate and 4-animoantipyrine precursors, respectively (Fig. 5a and 5b). Functionalized hydrophilic nanoparticles were synthesized via the one-step hydrothermal decomposition of 2-(2-aminoethoxy)-ethanol ammonium citrate salt. As to the hydrophilic quantum dots, organic coronas were covalently tethered to the core by the resulting amide linkage (–NHCO–). The quantum dots exhibited visible-range emission when excited at different wavelengths. As the excitation wavelength increased, the emission-band max- imum of the hydrophilic nanoparticles in water shifted to longer wavelengths.

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Figure 6. a,b) SEM and TEM images of nanocables with encapsulated, pentagonal-shaped silver nanowires. Reproduced from [4]. c,d) SEM and TEM images of Ag@phenol formaldehyde resin core–shell nanospheres. Reproduced from [117].

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The HTC process can be combined with other methods for the production of novel carbonmaterials. Zhan and Yu[81] presented a novel solvothermal treatment of glucose in the presence of a suitable amount of pyridine followed by a successful self- assembly process to produce carbon-rich composite sub- microtubes (Fig. 5c and 5d). The carbon nanoparticles were formed via the pyridine thermal treatment and the surfaces of the nanoparticles were tethered by organic functional groups. After the solvothermal treatment, the obtained black solution contain- ing the carbon nanoparticles was diluted with distilled water and then the self-assembly proceeded gradually. The functional hydroxyl and pyridyl groups tethered to the surface of the sub-microtubes were confirmed by Fourier-transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) analysis, which played a vital role in the self-assembly of the nanoparticles in the mixed solution of water and pyridine.[81]

Driven by the different affinities between the functional groups and the solvents, the nanoparticles adjusted their mutual position automatically and assembled themselves forming the tubular structures, just like the self-assembly of surfactant molecules to micelles. It is worth noting that the the carbon content of the sub-microtubes was only about 40wt%, indicating that these structures were just carbon-rich composites, not conventional carbon tubes.

The as-synthesized carbonaceous materials with various shape, size, chemical composition, and surface functional groups have shown novel and interesting intrinsic properties, which have been widely studied. These carbonaceous spheres, after calcina- tions, have shown interesting electric properties and have been used as an anode material for lithium ion batteries, showing excellent specific capacitance, area capacitance, cyclic perfor- mance, volumetric capacitance, and reversible capacity.[20,82–86]

The materials have been investigated as a counter electrode for dye-sensitized solar cells[87] and even as an efficient fuel for indirect-carbon fuel cells.[78,88] The surface functional groups and porous structures could greatly enhance the efficiency of carbonaceous spheres for removing heavy metals from aqueous solutions.[62] Carbonaceous spheres also possess an unexpected intrinsic fluorescence, which makes them valuable as marker particles.[89]

2.3.2. Carbon-Based Nanocomposites: Encapsulation and In situ

Efficient Loading with Metal Nanoparticles

The HTC process of clean carbohydrates or organic monomers also provides a favorable reaction environment for twinned, only slightly coupled reaction schemes. When other reagents were added into the HTC process of carbohydrates or organic monomers, novel carbon-encapsulated core–shell composites, nanocables, and hybrids were successfully synthesized by one-step processes, such as one-pot syntheses of Ag@C[4,64,90–94]

and Cu@C[95] nanocables (Fig. 6a and 6b). Other similar core–shell structures have also been synthesized.[96–101]

Controlling the reagents and reaction conditions effectively adjusted the diameter, length, and thickness of carbonaceous coating. The individual Ag@C nanocables showed excellent conductivity, which is ideal for constructive interconnects in nanoscale devices.[102] Other carbon-encapsulated core–shell composites have also been synthesized in a similar one-step

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HTC process by mixing metal or metal oxide nanoparticles, such as Ag,[103,104] Pd,[105] Se,[96,106] Fe3O4,

[107] and SnO2, with a carbon source.[108] For example, Pd@C core–shell nanoparticles were found to be selective catalysts for the batch partial hydrogenation of hydroxyl aromatic derivatives.[105] Such one-step HTC process can also be used to synthesize hybrid materials with more complex structures and specific properties.[109–115] For example, using theHTC process core–shell Pt@C nanoparticles embedded in mesoporous carbon were successfully synthesized, which had shown excellent stability and high catalytic activity for metha- nol-tolerant oxygen electroreduction.[116] In addition, well-defined monodisperse Ag@phenol formaldehyde resin (PFR) core–shell spheres can be synthesized by a facile one-step method (Fig. 6c and 6d).[117]

In the HTC process, carbonaceous materials effectively and uniformly encapsulated preformed nanoparticles, but also arranged the nanoparticles to controlled superstructures through- out encapsulation (Fig. 7a and 7b), thus, interesting nanocom- posites were formed, or the properties of nanoparticels were improved.[118–124] Such nanocomposites show unique chemical and/or physical properties due to their special structures and components and may find applications in various fields such as catalysis, fuel cells, drug delivery, and bio-imaging.[5] For example, for the carbon-decorated FePt nanoparticles, carbonac- eous shell could not only offer a protective coating for FePt nanoparticles but also provide low coercivity and small magnetic interference from neighboring carbon-coated particles.[125]

Carbon-coated Fe3O4 nanospindles could serve as a superior anode material for lithium ion batteries with high reversible capacity, high coulombic efficiency in the first cycle, enhanced cycling performance, and high rate capability compared with bare hematite spindles and commercial magnetite particles.[126]

Carbon coating on Si nanodots can be facilely realized by the HTC process and further heat treatment will result in the

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Figure 7. a) Bare a-Fe2O3 composites in a fully lithiated state. Reproduced from [126]. b) TEM image of glucose-treated FePt nanoparticles. Repro- duced from [125]. c) TEM image of carbon spheres loaded with Ag nanoparticles. Reproduced from [21]. d) TEM image of carbon nanofibers loaded with Au nanoparticles. Reproduced from [132].

Figure 8. a) Schematic representation of the formation of metal oxide hollow spheres by using carbonaceous microspheres as templates. Repro- duced from [146]. b,c) SEM and TEM images of the SnO2 hollow spheres. Reproduced from [146]. d,e) TEM images of TiO2 nanotubes. Reproduced with permission from [169]. Copyright 2009, Royal Society of Chemistry. f,g) SEM and TEM images of tin-nanoparticle-encapsulated elastic hollow carbon spheres. Reproduced from [130].

formation of Si@SiOx/C nanocomposites with a thin layer of SiOx and carbon.

[124] Such nanocomposites have displayed significantly improved lithium-storage performance due to the generation of a passivated layer.[127,128]

Further treatment of such core–shell composites can result in more complex structures, especially in hollow structures with tunable void space, as shown in the synthesis of the SnO2@C double-shell hollow spheres by removing silica from the original silica@SnO2@carbon structures.

[129] Such SnO2@C double- shell structures were further used as nanoreactors to synthesize tin nanoparticles encapsulated in elastic hollow carbon spheres, while heated at 700 8C for 4 h under N2 atmosphere.

[130] Also, these materials were tested as promising anode materials for high-performance lithium ion batteries, due to their elasticity and the uptake of mechanical stresses.

Using carbonaceous capsules as picoliter containers, novel nanocomposites consisting of inorganic nanoparticles confined within a hollow mesoporous carbon shell had been successfully produced.[131] The as-synthesized core–shell materials contain a significant number of filled inorganic nanoparticles, large surface areas, high pore volumes, and the porosity arises from accessible pores of 2–2.5 nm.[131]

It was already stated above that the as-prepared porous carbonaceous spheres are also a favorable support for the inorganic nanoparticles. On one hand, the surface functional groups, such as hydroxyl, carbonyl, and carboxylic groups could in situ reduce noble metal ions into noble metal nanoparticles loaded on the carbon[21,132] as shown in Figure 7c and 7d. On the other hand, the surface functionality acts as a primer or binder to stabilize those hybrid structures. Metal,[133,134] bimetal,[135,136] or metal oxide[137,138] nanoparticles have been successfully deposited on the surface of carbonaceous materials. Again, electrochemical

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performance of such hybrids is very favorable, as shown in lithium ion batteries[139,140] and methanol electro-oxidation.[59,141–144]

2.3.3. Carbonaceous Materials as Sacrificed Template: Synthesis of a

Family of Functional Inorganic Hollow and Complex Nanostructures

Because of the facile removal of the carbonaceous materials fabricated by the HTC process, the as-synthesized carbonaceous spheres have been used as sacrificed templates for fabricating hollow spheres, which can be applied in catalysis, sensing, chemical/biological separation, and lithium ion batteries.[20,21,35]

The general synthesis method mainly includes the adsorption of metal ions from solution to the surface layer of carbonaceous spheres and subsequent removal of the carbonaceous cores via calcinations.[145–147] Based on this method, some hollow spheres were successfully synthesized (Fig. 8a–c), such as Ga2O3,

[146,148]

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Figure 9. a,b) TEM image of the hollow carbon spheres with mesoporous carbon shell. Reproduced from [8].

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GaN,[148] WO3, [149] SiC,[150] ZnO,[151,152] Mn-doped ZnO,[153]

SnO2, [154] NiO,[155,156] CoO,[146] In2O3,

[157] TiO2, [158,159] SiO2,

[160]

CuO,[147] MgO,[145] CeO2, [145] MnO2,

[161] Mn3O4, [146] Al2O3,

[146]

ZrO2, [162] Cr2O3,

[146] layered double hydroxide (LDH),[163]

Y2O3:Eu, [164] La2O3,

[146] Y2O3, [146] Lu2O3,

[146] MFe2O4 (M¼Zn, Co, Ni, and Cd),[165] Fe2O3,

[166] and Bi2WO6. [167] It is worth

mentioning that the sizes and structures of different metal oxide hollow spheres are predominantly determined by the templates. These as-synthesized hollow spheres showed high potential applications in the area of gas sensitivity or catalysis. Other kinds of templates have also been used for the synthesis of metal oxide hollow spheres via sacrificial-core techniques, such as colloidal nanoparticles (e.g., Au, Ag, or CdS) and sub-micrometer polystyrene spheres, but these templates are confined to the synthesis of a few particular compositions such as SiO2, TiO2, SnO2, ZrO2, and Fe3O4 and cannot be applied as widely as the HTC carbon templates.

Similarly, carbonaceous nanofibres also have been used as template for the synthesis of hollow metal oxide fibres (Fig. 8d and e).[168,169] This method can even be extend to the synthesis of uniform ternary oxide nanotubes such as BaTiO3.

[169] In particular, compared to carbon nanotubes, the carbonaceous fibers by the HTC process have higher surface reactivity, making them more suitable for templating production of a variaty of metal oxide nanotubes.[165]

The HTC-generated carbonaceous materials can also sacrifice themselves to template some special core–shell-like structures. For instance, synthesis of tin nanoparticles encapsulated within elastic hollow carbon spheres (TNHCs) has been reported (Fig. 8f and g).[130] SnO2 shells were first deposited on SiO2 spheres via the hydrolysis of Na2SnO3. After etching the SiO2 cores by sodium hydroxide solution, carbon shells were coated on the surface of the SnO2 hollow spheres via HTC of glucose. Then the samples were heat-treated at 700 8C for 4 h under N2 atmosphere, resulting in the TNHCs.

Li et al.[170] prepared Fe3O4@TiO2 by coating the Fe3O4 core with a glucose-derived carbonaceous layer via HTC, then absorbing the Ti-based precursor onto the surface, and finally calcining the sample under nitrogen. The Ti-precursor was converted to a titania shell, while the carbon layer was simultaneously removed by oxidative heating. This colloidal material was applied for the simple and fast enrichment of phosphopeptides via direct matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry. Using a quite similar procedure, a ‘‘jingle-bell’’ structure was obtained with a ferrite hollow sphere and a novel metal core, such as Ag@NiFe2O4, Ag@CoFe2O4, Ag@MgFe2O4, and Ag@ZnFe2O4.

[171] Further study on the templating effects of carbonaceous structures still needs to be carried out in more detail in order to realize the goal of controlled templating synthesis.

2.3.4. Porous Carbon Materials

With respect to the synthesis of porous carbonaceous materials, the HTC process not only proceeds under facile, low-temperature conditions but also produces porous carbonaceous materials with controllable morphology and surface functional groups as well. The carbonaceous materials collected directly after hydrothermal

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carbonization have the characteristic to possess only a small number of micropores and, therefore, a small surface area (as compared to activated carbon). However, by further carbonization at higher temperatures the surface areas can reach up to 400m2 g1 due to an increase in microporosity.[20] A disadvantage is that, by increasing the temperature, the surface functionality is partly lost and so is the possibility of further functionalization. This is why a variety of techniques were applied to increase the surface area in the as-synthesized HTC material. For instance, if hydrothermal carbonization of carbohydrates takes place in the presence of various templates or additives, interesting pore systems can be imprinted.

The first mesoporous hydrothermal carbons were produced by performing the hydrothermal carbonization in the presence of nanostructured silica templates (Fig. 9).[8] Thus, it was found that it is important to match the polarity of the template surface with the one of the carbon precursor. For mesoporous template, mesoporous carbon shells are obtained after removal of the template, with the whole carbonaceous structure composed of ca. 8–16-nm globular carbon nanoparticles. Furthermore, the moderately hydrophobic silica templates filled with 60wt% carbon precursor resulted in mesoporous carbonaceous micro- spheres, whereas application of 30wt% carbon precursor gave only small carbon spherules (6–10 nm in size), owing to the lack of interconnectivity between particles in the coating. This nicely illustrates that the carbon-coating process operates ‘‘patch-wise’’ via stable colloidal intermediates. For nonporous templates, hollow carbonaceous spheres with a robust carbon coating can be generally obtained.

After finding the optimal hydrothermal carbon replication conditions for mesoporous silica, a replica of an ordered mesoporous silica (SBA-15) has also been successfully produced (Fig. 10).[73] These ordered mesoporous hydrophilic carbon materials have rich polar functional groups (such as COOH, OH, C¼O) located at the surface of the pore. Furthermore, these residing functional groups could also be successfully converted into amino groups by a simple reaction of the hydroxyl surface groups with 3-chloropropylamine.

Also, porous carbonaceous materials were successfully synthesized by the HTC process using the uniform silica particles with the diameter of 100 nm as pore-forming templates.[172] Surface modification of porous carbonaceous material with oxygen-containing groups at the surface could extend their potential applications.[73]

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Figure 10. a) TEM image of the carbon replica obtained by 100% pore filling of a SBA-15 template. b) TEM image of the carbon replica obtained after 25% pore filling of a SBA-15 template. Reproduced with permission from [73]. Copyright 2007, Royal Society of Chemistry.

Figure 11. a) SEM image of the ‘‘soft’’ biomass of pine needles before the HTC process; the inset shows an SEM image of after HTC process. b) SEM image of ‘‘hard’’ biomass of oak leaf after the HTC process treatment. Reproduced with permission from [7]. Copyright 2007, American Chemical Society. c) SEM image of the coexistence of carbon spheres and a microstructured biological tissue. d) SEM image of carbon scaffold repli- cating of the nonsoluble carhydrates in rice. Reproduced from [63].

3. Hydrothermal Carbonization of Biomass: a Promising Strategy for Carbon Materials from Sustainable Resources

Facing increasing crude oil prices and the unclear availability of fossil resources on the longer run, it is an attractive option to create high-end materials from regrowing resources. Especially considering the pair biomass and related carbon products, this is a more than feasible option. As compared to biofuels or fermentation processes, which have already arrived in society, much less attention has been paid to the chemical conversion of crude feedstock, especially biowaste such as sawdust, rice husk, corn cobs, and grass. With an advanced materials end, these raw materials and their conversions can be considered as ‘‘sleeping gold’’.[173]

Hydrothermal conversion method represents one of the promising chemical routes to treat these raw materials because of the intrinsic advantages such as benign environment, versatile chemistry, enhanced reaction rate, and economic cost.[173–175]

Hydrothermal conversion of the crude biomass can be again divided into two domains according to the conditions and products: i) hydrothermal gasification in supercritical/subcritical water (the solid carbonaceous materials are converted into flammable gas mixture with or without the help of catalysts)[172]

and ii) hydrothermal carbonization (regarded as hydrothermal liquefaction) in hot compressed water.

For the hydrothermal carbonization, both solid carbonaceous material and water-soluble organic liquid can be generated. The bio-oil can be extracted by ether, ethyl acetate, and the gas chromatography mass spectrometry (GC-MS) showed that it consists of various commercially usable chemicals, e.g., carboxylic acids, ketones, and phenol derivatives.[177–183] To increase the yield of oil, alkaline salts (KOH, K2CO3, NaOH, Na2CO3, etc.) are usually added as catalysts. The solid carbonaceous materials are sometimes also classified as ‘‘biochar’’, which is defined as ‘‘fine-grained charcoal, high in organic carbon and largely resistant to decomposition’’ by the International Biochar Initiative Scientific Advisory Committee. The structure and composition of biochars are quite different depending on different biochar production techniques. Compared with slow, fast, or flash pyrolysis, the biochar from HTC process has the highest char yield, although less resistance to decomposition. Besides, the HTC process is both energy economic and atom economic:[7] i) it releases one third of the

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combustion energy throughout dehydration; ii) the wet condition avoids the pre-drying process; iii) the carbon efficiency is close to 1 after adequate reaction time under proper condition.

For the HTC process, the initial structure and composition of the biomass material can influence greatly the size, shape, and surface structure of the final biochar.[7] When sugar beets, chips, pine cones, pine needles, oak leafs, and orange peels were used as starting materials via a HTC process at 200 8C for 16 h, two kinds of carbonaceous materials were obtained. ‘‘Soft’’ plant tissues without an extended crystalline cellulose scaffold could lose their original structure, forming globular carbonaceous nanoparticles with very small sizes and interstitial porosity (Fig. 11a and b). ‘‘Hard’’ plant tissues with structural, crystalline cellulose scaffolds could preserve outer shape and large-scale structural features on the macro- and microscale (Fig. 11c). Mass loss caused by dehydration towards the carbon scaffold, however, results in significant structural changes on the nanometer scale, usually resulting in a spongelike, bicontinuous carbonaceous network with a well-defined mesoporous structure (Fig. 11d). In addition, the surface of carbonaceous nanoparticles and mesoporous carbonaceous materials were highly hydrophilic and water-wettable, owing to the presence of polar groups immobilized at the surface.

A catalyst is another factor than control the structure and properties of the final carbonization products. Recently, Yu and co-workers[63] reported that iron ions and iron oxide nanoparticles could effectively catalyze the hydrothermal carbonization of starch and rice grains under mild conditions (<200 8C) and had a significant influence on the formation of carbon nanomaterials with different shapes. In the presence of Fe2þ ions, both hollow

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and massive carbon microspheres could be obtained. In contrast, the presence of Fe2O3 nanoparticles led to the formation of very fine and ropelike carbon nanostructures.

Besides catalysis, temperature, pressure, and reaction time can all influence the final solid carbonaceous materials.[184,185] A study on the transformation from sawdust into synthetic coke showed that the solid yield, caloric power, degree of deoxygena- tion, and the degree of structural change varied depending on a combination of temperature, pressure, and reaction time.[185]

In recent years, people gradually rediscovered the importance of biochar technology, especially in some environmental applications. First, the biochar can be used as a soil amendment to improve the soil (‘‘conditioner’’). Biochar for instance can increase the cation exchange capacity and therefore retain Kþ, Mg2þ, Ca2þ, etc. By porosity and capollarity, water binding can be highly improved. Also the N, P elements from the original biomass can be temporarily locked into the final carbonaceous solid. Biochar from HTC process contains a high ratio of oxygen groups, and hence is a good absorbent for heavy metal ions. Zhang et al.[186] reported that biochar from pinewood and rice husk via a HTC process at 300 8C (named P300 and R300, respectively) could remove the lead ion from aqueous solution. The maximum absorption ability was 4.25 and 2.40mg g1 for P300 and R300 at 318K, respectively. The pH value, ion concentration, and adsorption temperature all influenced the adsorption capacity of the P300 and R300. Further study indicated that the adsorption followed pseudo-second-order kinetics and the Langmuir model. Of similar importance, biochar serves as a stable carbon pool that withdraws the carbon from the atmosphere,[7,187] thus potentially treating parts of the ‘‘green- house effect’’. A recent study showed that biochar from glucose and yeast via the HTC process could be added into soil as a decadal carbon pool as well as a fertilizer.[187] The stability of the biochar was influenced by the chemical composition and the type of soil. Besides, the soil micro-organisms could utilize both types of biochar as a new carbon source.

4. Applications of HTC Carbon Materials

4.1. Environment Applications

As HTC operates with biomass, all chemistry and technology on the base of HTC products are essentially sustainable, close to ‘‘CO2-neutral’’ operations. If these products are used in long- lasting applications, the products are even—on the lifetime of the products—‘‘CO2-negative’’, as they effectively sequester

Table 1. Comparison of Pb2þ- and Cd2þ-adsorption capacities of different ad Chemical Society.

Adsorbent Pb2þ(mg g1)

10AcA-C 351.4

Leonardite (low-rank coal) 250.7

HNO3 oxidized carbon nanotube 97.08

Algae 331.52

Amberlite IR-120 synthetic sulfonated resin 19.6

Carbon aerogel 35.0

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atmospheric CO2 via the detour of biomass in form of carbon products. (Note that any open biomass is rapidly degraded by microbe or fungi, for example, and turned into CO2 again.)

It is a brave thought, but given climate changes and the role of CO2 therein, it is in principle possible to develop a chemical ‘‘CO2 disposal’’ industry for sequestering the atmospheric CO2, even that from past industrialization. Biomass represents the biggest carbon converter with the highest efficiency to collect and bind CO2 away from the atmosphere and any type of carbonization can transfer this biomass into less degradable coal compounds.

Thus, using the HTC process to convert biomass into coal rapidly, could represent a most efficient tool for CO2 sequestra- tion.[6] First, the desirable acceleration of the coalification of biomass by a factor of 106–109makes it a technically attractive and realistic ‘‘artificial’’ instrument for fixing the carbon of biomass on large scales. Second, it is the most efficient strategy for carbon fixation, with ‘‘carbon efficiency’’ close to 1.

Water pollution now becomes a worldwide problem that threatens human health. Therein, heavy metal ions, pesticides, and drug residues are most hazardous. Recently, scientists began to test carbonaceous materials derived from biomass via HTC process as specific sorbents for those purposes, for their surfaces contain many functional groups to act as potential sorption sites.

Su and co-workers anchored glucose-derived carbon nanosheres (CSPs) onto the activated carbon host (obtained by calcining bulk activated carbon at 400 8C), obtaining a new composite, the so-called nanoarchitecured activated carbon (NAC).[188] The effect of the increase of surface functional groups of the product is confirmed by temperature program desorption (TPD), XPS, and FTIR. This functional-group-rich NAC showed much better adsorption capability for ions than raw activated carbon (AC). For example, the NAC has a maximum adsorption capacity of CrO4

2 up to 180mol g1 or 0.81mol m2, while that of raw AC is only 32mol g1 or 0.03molm2. A similar improvement was also found for the Fe3þ-adsorption capacity.

Another interesting method to prepare functional-group-rich carbonaceous material via HTCmethod is reported by Titirici and co-workers.[65] The authors hybridize biomass carbon with up to 10wt% of an organic monomer (e.g., acrylic acid) into a aqueous carbohydrate (glucose) solution. During the hydrothermal process, the glucose decomposes into HMF, then undergoes cycloaddition with acrylic acid and polycondenses into derivatized CSPs. Compared with the product from pure glucose, the surface of the resulting product surface here has much more carboxylate groups that can bind to metal ions easily. The sorption-capacity experiments showed that the resulting material has excellent ability for Pb- and Cd-ion removal (Table 1). The sorption capacity

sorbents. Adapted with permission from [65]. Copyright 2009, American

Cd2þ(mg g1) Conditions

88.8 pH¼ 6; room temperature 50.6 pH¼ 5.4–5.6; room temperature 10.86 pH¼ 5; room temperature

134.88 pH¼ 5; room temperatue 201.1 pH¼ 4–8; room temperature 15.0 pH¼ 4.5; 37 8C

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Table 2. Catalytic activity of differently supported Pd for the hydrofenation of phenol.[a] Adapted with permission from [105]. Copyright 2008, Royal Society of Chemisty.

Selectivity [%]

Catalyst Time [h] Conversion [%] Cyclohexanol Cyclohexanone

Pd@hydrophilic-C 10 60 — >99

Pd@hydrophilic-C 20 >99 5 95

Pd@hydrophilic-C 72 >99 50 50

Pd@hydrophilic-C [b] 20 45 30 70

10% Pd@C 20 100 100 0

10% Pd@C 1 100 100 0

10% Pd@Al2 O3 20 100 100 0

[a] In a typical reaction, 50mg of catalyst were added to 100mg of phenol and the

mixture was heated to 100 8C under 1Mpa of hydrogen pressure. [b] Reference test in cyclohexane

increases as the the ratio of acrylic acid increases. The capacity of a sample with 10wt% acrylic acid lead and cadminum ions at environmental pH around 6 is up to 351.4mg g1 and 88.8mg g1, respectively. This value is really high and well beyond that of most sorption materials, including technical ion-exchange resins. The method here opens a new door to produce carbonaceous materials with specific functionalities.

4.2. Catalytic Applications

It is well known that the performance of heterogeneous catalysts is highly affected by their supporting materials. It was a straightforward trial to explore biomass-derived HTC-carbon materials and colloids as supports for metal particles for different catalytic applications because of their high surface area and functionality. Liu et al. reported the synthesis of Pt-/Pd- loaded carbon microspheres (CMS) as catalysts for DMFCs—a promis- ing, inexpensive liquid energy source in the future.[143] The carbon spheres are obtained by a gentle HTC process and subsequent calcination at 600 8C. From the cyclic voltammogams, the as-prepared Pt@CMS and Pd@CMS show higher catalytic activity for methanol oxidation at alkaline media than commercial Pt (Pd)/carbon black. Similarly, several groups have demonstrated that other carbon materials from HTC loaded with Pt nanoparticles show better performance for methanol oxida- tion.[59,136,141,142,144] However, all these catalysts are still tradi- tional ‘‘simple’’ structures and, therefore, could not solve the problem of methanol diffusion that both poisons the catalyst and leads to a mixed potential.

A recent report describes an encouraging step on developing methanol-tolerant cathodic catalyst made of mesoporous carbon with highly distributed core–shell Pt@C nanoparticles in the nanochannels (Pt@C/MC).[116] The product generated from the HTC process shows excellent stability and high catalytic activity for methanol-tolerant oxygen electroreduction. The high stability is speculatively caused by a thin mesoporous carbon film on the Pt nanoparticles that enables the oxygen to touch the activity sites and keeps the methanol away.

Recently, Yu and co-workers have synthesized so-called ‘‘hybrid fleece’’ structures, constituted of uniform carbonaceous nanofi- bers embedded with noble-metal nanoparticles by using the HTC process.[132] Importantly, the resulting hybrid nanostructures perform as efficient catalysts for the 100% conversion of CO to CO2 at low temperature, which was attributed to the special nano- and microstructure, the presence of binary carbon–metal contacts, and their highly specific metal surface area. It must be noted that the second run of catalysis tests showed a better catalysis performance than the first run. This phenomenon indicates that the truly active species are formed during the first heating and oxidation cycle. Also the catalytic activity depends sensitively on the type of metal loading, among which the Pt system has the most efficiency. From combination with modern Au oxidation catalysis, however we still expect large room for further improvement.

Another report covered the selective catalytic property of Pd-nanoparticle@hydrophilic-carbon spheres via a low- temperature HTC process of furfural and palladium salts (Table 2).[105] The product can selectively hydrogenate phenol

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(or the derivaives) into cyclohexanone (or the derivatives) with a optimal selectivity and good conversion. In contrast, the commercial charcoal and alumina supported palladium con- verted under the same conditions phenol completely into cyclohexanol. This selection is probably based on the fact the hydrophobic cyclohexanone (or the derivatives) is less able to bind to the more H-bridge accepting carbon surface and, thereby, selectively depleted from carbon spheres. An alternative explanation is capillary pressure effects. In fact, as the reaction time increased, a higher percent of further hydrogenation could be found.

Similar to the HTC materials prepared in the presence of acrylic acid monomer, Titirici and co-workers[189] produced a mesoporous imidazolium-containing material which was then successfully used in heterogenous catalysis for the aromatization of unsaturated six rings (especially Diels–Alder condensation products) and Knoevenagel and Aldol condensations.

4.3. Electric Applications

Lithium ion batteries are widely used in laptops, cameras, cell phones and other electronic portables due to the relative high energy density. However, for advanced applications such rechargeable batteries with higher power density, improved rate capability and longer cycle life are eagerly demanded and become a new challenge to scientists. The conventional anode material— graphite—has high coulumbic efficiency (the recoverable part of energy) but a only moderate capacity (372mA h g1).[127] As a result, materials with higher theoretical capacities such as silicon (Li4.4Si¼ 4200mA h g1),[127] tin-based chemicals (790mA h g1 for SnO2, and 990mA h g

1 for Sn)[121] and transition metal oxides (500–1000mA h g1)[120] are explored as nanostructures.

However, these promising candidates have two common drawbacks:[126,127,128,129] namely their low conductivity and large volume change during the Liþ-insertion/extraction cycle. The second problem causes the disintegration of the material and the vanishing of the starting optimized nanostructure, therefore leading to a very poor cycling performance. To solve these

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Figure 12. a) TEM images of the Si@SiOx–C nanocomposite. b) Cycling and rate performance of pure-Si-nanoparticle and Si@SiOx–C-nano- composite electrodes cycled in vinylene carbonate (VC)-free and VC-containing 1M LiPF6 in EC/DMC solutions (solid symbols: charge; empty symbols: discharge). Reproduced from [127].

12

problems, scientists have developed some strategies, which, interestingly, are quite similar to each other. A first approach is to apply hollow or mesoporous nanostructures as an electrode, for the empty space serves as a good cushion for the volume change. A second approach is to coat a flexible carbon layer onto the particle surface to both increase the conductivity and absorb stress by volume change. The third approach is usually complemented (or even substituted) by a sealing layer of a stable but deformable solid–electrolyte interface (SEI) on the carbon/ particle composite to resist the harsh deformation (although this may cause a large irreversible capacities in the first few cycles).

Compared with other complicated carbon-coating methods such as CVD or physical vapor depositions (PVD), the HTC process is a muchmore facile ‘‘green’’ method that, in addition, is liquid-based and therefore cheap and scalable. A Si@SiOx–C nanocomposite has been synthesized via HTC process (followed by a low-temperature carbonization step) and indeed showed superior lithium-storage capacity (Fig. 12).[127] The synthesized nanocomposite has a core–shell structure (ca. 50 nm in size) with a silica core wrapped by a coating layer of SiOx and C. This composite, electrochemically tested in the presence of vinylene carbonate containing electrolyte, shows a high electronic capacity as well as a good cycling performance (1100mA h g1 even after 60 cycles at a current density of 150mA h g1 in the voltage range 0.05–1V). Even at high current density (1000mA g1), the reversible capacity can maintain at 600mA h g1. Another

Figure 13. A) Confocal microscopy images of core–shell particles with a size of 500 nm. B) Fluorescence microscopy images of H1299 cells. The intracellular fluorescent nanoparticles are shown. C) Cytotoxicity profiles of nonfluorescent control cells and fluorescent positive cells (error bars are the standard deviation (SD) of triplicates). Reproduced from [117].

important candidate for a Li-anode material is Sn or SnO2.

[111] Recently, SnO2@carbon hollow nanospheres have been synthesized by using the HTC process,[129] and the product shows a stable capacity of 520mA h g1 at a current density of 0.32C (C¼ 625mA h g1) after 100 cycles. After the current density was increased to 4.8 C and then reduced back for another 200 cycles, the capacity still main- tained at as high as 500mA h g1. The decay of capacity at initial cycles is caused by the irreversible formation of a SEI layer. In addition, Wan and co-workers[130] synthesized a tin-nanoparticle-encapsulated elastic hollow carbon spheres in a similar strategy but heated the SnO2/C at up to 700 8C to reduce the SnO2 layer to Sn particles. The product has a capacity higher than 550mA h g1 after 100 cycles at voltage range of 5mV–3V.

Using the hydrothermal treatment, Titirici et al.[145] also reported the hydrothermal carbonization of glucose in the presence of pre-formed SnO2 nanoparticles followed by subsequent removal of the carbon. Thus, mesoporous SnO2 micrometer-sized spheres composed of small SnO2-aggregated nanopar- ticles were produced and tested as anode materials in lithium ion batteries, showing high specific capacities (960mA h g1) and good cycling performances. The micrometer- sized spheres enable easy handling in terms of separation or film formation in comparison with their nanometer-sized constituents.

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Wan and co-workers[126] also reported an interesting carbon- coated magnetic nanospindle (Fe3O4-Carbon) with a capacity up to 530mA h g1 after 80 cycles at a relatively high current density while the capacities of bare a-Fe2O3 and Fe3O4 fading to 105 and 152mA h g1, respectively.

4.4. Biological Applications

Recently, people began to explore the favorable bioapplications of HTC carbon materials such as bioimaging and drug delivery. Yu

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et al. synthesized green-luminescent monodisperse Ag@phenol PFR core–shell spheres in a facile one-step method.[117] By changing the concentration or the molar ratio of the starting materials, different architectures with tunable size can be obtained, such as monocore–shell, multicore–shell, and eccentric core–shell.

These nanospheres give strong green emission under excitation at 340 nm. The fluorescent imaging showed that the particles could be internalized into living human lung-cancer cells and exhibited almost no cytotoxic effects on the cells (Fig. 13). Further modification of the surface of the polymer shell may allow for core–shell spheres that are more specifically targeted to cells or deliver drugs or genes in a tissue of specific fashion.

Kundu and co-workers[89] reported that the intrinsically fluorescent surface functional CSPs derived from the hydro- thermal treatment of glucose can deliver a membrane- impermeable molecule into the mammalian cell nuclei and further modulate gene expression in vivo. The confocal laser sanning microscopy (CLSM) monitoring showed that the CSPs entered the nucleus within 3 h, with a further increasing amount over time. The fluorescent images and Ramman spectrum also showed that the CSPs could cross the blood–brain barrier (a separation of circulating blood and cerebrospinal fluid) and

Figure 14. Response curves to a) alcohol, b) acetone, c) CS2, and d) NH3 at 4 Chemical Society.

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penetrate into the brain. Thus, the CSPs are potential carriers to deliver drugs against brain tumors. The in vivo experiment further confirmed that the CSPs are nontoxic to animals and can be consumed over several days. CSPs can deliver a membrane- impermeable molecule, CTPB (N-(4-chloro-3-trifluoromethyl- phenyl)-2-ethoxybenzamide), the only histone acetyltransferases (HATs) activator, into the cell nuclei.[89]

4.5. Sacrificial-Templating Synthesis of Hollow Structures for

Photocatalysts and Sensors

The chemical and physical properties of nanomaterials are tightly related to their morphologies. Recently, porous hollow spheres draw an increasing attention for their low density, high surface area, and good permeability, which make these hollow materials good candidates for catalysis, sensing or other applications. A commonly used method is to precipitate a layer of desired inorganic material onto the template surface followed by removal of the core, as discussed in Section 2.3.3. CSPs via HTC process have been used as such templates, offering a whole range of advantages, e.g., nontoxicity, low cost, and intrinsic surface functionality.[159]

00 8C. Reproduced with permission from [149]. Copyright 2004, American

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Using this method, several hollow nanometer-sized photo- catalysts have been fabricated with excellent catalytic efficiency. For example, Yuan and co-workers[159] synthesized titania hollow spheres that can decompose methylene blue under UV irradiation with a apparent rate constant almost 6 times as high as that of commercial P25 titania (0.26min1 and 0.045min1, respectively). Xu and co-workers[167] reported a Bi2WO6 nanocage consisting of small nanoparticles with size of ca. 50–80 nm by only refluxing the precursor with carbon spheres in ethylene glycol. The Bi2WO6 nanocages shows a high efficiency for the photodegradation Rhodamine-B, which is 10 times than that of commercial TiO2.

Porous hollow inorganic metal oxide can also be used as gas sensors using the HTC process resulting carbon spheres as template. When the ionic oxygen species (O2, O) fixed on the surface of metal oxide react with the reducing gas molecules, the trapped electrons release back to the crystal grains, the potential barriers at the grain boundary decrease, and the resistance is dramatically reduced.[154,157] For the porous hollow structures, the ionic oxygen can absorb onto both inner and outer surfaces and the detected gas molecules can penetrate through and react with them freely, thus increasing the sensitivity. Li et al. reported a WO3 hollow-sphere structure that showed certain selective response to the organic gas,[149] as shown in Figure 14. Using glucose-derived carbon spheres as template, Liu and co-workers[157] synthesized porous In2O3 hollow nanospheres that had a satisfactory response for ethanol, methanol, and other organic gases even at a very low concentration. Besides, the sensors have good recovery ability. After every measurement, the response curves return back to the baseline very quickly. Wu and co-workers[154] reported on a hollow SnO2 sphere structure that was very sensitive to (C2H5)3N and ethanol (7.1 for 1 ppb (C2H5)3N at 150 8C and 2.7 ppb for ethanol at 250 8C, respectively) with a short response time, while the sensor based on SnO2 nanoparticles alone showed much less response.

5. Conclusions and Perspectives

In summary, carbons and carbonaceous materials synthesized by hydrothermal carbonization (HTC) from carbohydrates, organic molecules, and biomass have been discussed. The different experimental conditions and reaction mechanisms between the high-temperature and the low-temperature HTC process result in different kinds of carbon and carbonaceous material. Carbon materials from the high-temperature HTC process have higher carbon content and some of the structures exhibit graphitic structures. The low-temperature HTC process, as an environ- mentally friendly route, has been widely used for the production of carbonaceous materials, which have the typical CHO structures that are rich in surface functional groups, micro- porosity, or mesoporosity but also possess great reactivity. Many parameters can influence the low-temperature HTC process and the final products, e.g., metal ions can speed up the reaction, or templates can give rise to various morphologies. These carbonaceous materials can also be used to form nanocomposites with other materials and further serve as building blocks of hierarchical structures. Importantly, the as-synthesized carbon materials have already shown very broad practical applications in

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a variety of areas, such as environmental, catalytic, electronic, sensing, and biological applications. It is believed that the HTC process will play an increasingly important role in the fabrication of nanostructured carbon materials in the future.

Although theHTC process has achieved great success in rather short time, there still remain some challenges and questions that should draw scientific attention: Can we find a proper catalyst to lower the reaction temperature, especially for the high- temperature HTC process? How to digest real waste biomass by the HTC to produce functional carbonaceous materials? What is the detailed chemical mechanism of low-temperature HTC and how can we rationally design the HTC process to control the detailed components of the carbonaceous materials? Solving these challenges and problems in the future will further facilitate and strengthen the capability for rational design of a variety of carbon materials and extended practical applications.

Acknowledgements

We acknowledge the funding support from the National Basic Research Program of China (2010CB934700), the National Natural Science Foundation of China (NSFC, Nos. 50732006, 20671085), the Partner Group Program of the Chinese Academy of Sciences, and the Max Planck Society. The authors thank Mr. Xue-Wei Xu and Mr. Lei Wang for their assistance during manuscript preparation. This article is part of a Special Issue on Carbon Materials.

Received: August 16, 2009

Revised: September 20, 2009

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