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Applied Surface Science 257 (2011) 8686– 8691

Contents lists available at ScienceDirect

Applied Surface Science

jou rn al h om epa g e: www.elsev ier .com/ locate /apsusc

imple approach to carboxyl-rich materials through low-temperature heatreatment of hydrothermal carbon in air

hen Chena, Lijian Maa, Shuqiong Lia, Junxia Genga, Qiang Songa, Jun Liua, Chunli Wanga, Hang Wanga,uan Lia, Zhi Qinb, Shoujian Lia,∗

College of Chemistry, Sichuan University, Key Laboratory of Radiation Physics and Technology (Sichuan University), Ministry of Education, Chengdu 610064, PR ChinaInstitute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China

r t i c l e i n f o

rticle history:eceived 21 April 2011eceived in revised form 11 May 2011ccepted 11 May 2011vailable online 17 May 2011

eywords:

a b s t r a c t

It was found that a large number of oxygen-containing functional groups (OFGs) could be created on thesurface of hydrothermal carbon (HTC) by simply heating at lower temperature in air during the course ofour preliminary experiments which focused on oxidation pre-treatment of pristine HTC for the purposeof grafting functionalization. Especially carboxyl groups on HTC would increase significantly, from 0.53to 3.70 mmol/g after heat treatment at 300 ◦C. So, effects of heat-treatment on the OFGs on the carbonmicrosphere were deeply studied to confirm and explain the findings. Experiments involving different

ydrothermal carbonxidative functionalizationeat treatmentarboxyl groupsdsorption

materials (HTC, activated carbon and glucose) were performed under varying conditions (heating tem-perature and time, in air or in Ar atmosphere). A reaction mechanism for newly generating carboxylgroups on HTC surface during heat-treatment process was supposed based on the results from the sam-ple characterization using Boehm titrations, infrared spectra, X-ray photoelectron spectroscopy, energydispersive spectrometry and elemental analysis. In addition, the as heat-treated product has excellent

2+ an

sorption capability for Pb

. Introduction

Carbon materials have long been a hot topic in chemistry andaterial science for their physico-chemical stability, porosity, high

pecific surface area and environmental friendliness [1–3]. Up toow, carbon materials and their derivatives have been extensivelypplied in adsorption [4], catalysis [5,6], electrochemistry [7], andiomedicine [8], etc., which make urgent demand for the materialsith functional diversity and high guest-specificity. Thus, surfaceodification, as an effective method for extending carbon materials

eyond their existing structural and functional features, has beenrawing more attention in recent years.

Define abbreviations: the heat-treated samples were denoteds HTC-100, if heat-treated at 100 ◦C, HTC-300, if heat-treated at00 ◦C, and so on.

For the surface modification of carbon materials, chemical mod-fications, such as grafting [9,10], are more positive than physical

odifications, such as impregnation [11]. In order to obtain highoncentration of oxygen-containing functional groups (OFGs), such

s carboxyls, phenols and hydroxyls on sample surface, oxidationreatment is now a priority candidate [12,13]. These OFGs couldhen not only adsorb metal ions and some organic compounds

∗ Corresponding author. Tel.: +86 028 85412329; fax: +86 028 85412907.E-mail address: sjli000616@scu.edu.cn (S. Li).

169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2011.05.048

d Cd2+ ions.© 2011 Elsevier B.V. All rights reserved.

though the adsorption would generally be unselective, but also actas activity sites, which can be used to anchor and immobilize spe-cific functional groups on the material surface for the purpose ofenhancing efficiency of corresponding applications [14,15]. Nev-ertheless, for chemical oxidation, strong acids or strong oxidantscould usually cause collapse and/or corrosion to framework andsurface of carbon material [16]. Therefore, more attention has beenpaid to both development of novel carbon materials with activesurface rich in functional groups and exploitation of more ratio-nal approaches for surface modification in synthesis and surfacechemistry of carbon materials [3].

Hydrothermal carbon (HTC) is a kind of semi-carbonizedmaterial [17,18], which can be prepared from mono- and polysac-charides [19,20], rice, leaves [21] and even cyclodextrin [22] orpyrrole [23]. The synthesis of HTC has the following features: cheapcarbon sources, mild reaction conditions, and absolutely “green”as it involves no organic solvents, catalysts or surfactants [1]. It isespecially praiseworthy that the mild hydrothermal condition inthe synthesis procedure results in much more abundant OFGs onthe surface of HTC particles than that of other carbon materials[12]. Moreover, HTC particles usually present perfectly sphericalmorphologies as well as excellent hydrophilicity and dispersity.

Current researches on HTC are focused on the design and prepa-ration of the materials with different frameworks and variousapplications of raw HTC. However, there are few researches onfunctional groups on HTC [1,2,24–29]. Only Demir-Cakan et al.

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30] recently declared carboxylate-rich carbonaceous materialsbtained via one-pot hydrothermal carbonization of glucose withhe presence of acrylic acid. But they did not report the content ofhe carboxyl groups.

In order to anchor specific OFGs onto HTC for environmentalurposes, ordinary oxidation method had been examined in ourreliminary experiments to obtain more carboxyl groups as thective grafting sites. However, the amount of OFGs on the sam-le surface was not satisfactory after treatment with traditionalhemical oxidation methods, and moreover, HTC particles wouldissolve while HNO3 concentration was above 3 mol/L and tem-erature above 50 ◦C. After analysis of experimental data, it wasound that the amount of the OFGs, especially carboxyl groups,ncreased greatly after heat-treatment, and the degree of acquiredncrease was directly associated with heating temperature. There-ore, we performed deeper studies on oxidative functionalization ofTC employing different materials involving HTC, activated carbon

AC) and glucose under varying conditions of heating tempera-ures, time and atmospheres without using any strong acids orxidizing agents. Boehm titrations, scanning electron microscopeSEM), infrared spectra (FT-IR), X-ray photoelectron spectroscopyXPS), energy dispersive spectrometry (EDX) and elemental anal-sis were employed for characterization of samples. Moreover,orption capability of the as heat-treated product for heavy metalons was examined. Finally, a reaction mechanism for the oxidativeunctionalization process was proposed.

. Experiments

All chemicals and reagents used were of A.R. grade without fur-her purification and were purchased from Kelong Chemical Co.,hengdu.

.1. Choosing a precursor material

Glucose, sucrose, potato starch and microcrystalline celluloseere chosen as carbon precursors. Typically, 20.0 g of glucose were

dded in a 100 mL Teflon-lined stainless steel autoclave, which washen filled with deionized water up to 90% of the total volume. Aftertirring for 10 min, the autoclave was sealed and preheated in anven at a rate of 5 ◦C/min to 180 ◦C, maintained at the tempera-ure for 24 h, then cooled down naturally to room temperature.he resulting brown powders were washed with deionized waternd then ethanol till neutral, finally, dried in a vacuum oven at0 ◦C for 12 h. Other precursors were treated in the same way sep-rately. Based on the results of a comparison of carboxyl groupsontent on the different products determined by Boehm titrations31], as shown in Table 1, glucose was selected as the precursorn the following experiments, and the glucose-based product wasesignated as HTC.

.2. Preparation of samples

The brown powder samples, HTC, were further carbonized in auffle furnace for 5 h in air at a temperature range of approximately

able 1arboxyl group content of the products synthesized with different carbonrecursors.

Carbon precursor Carboxyl groups content (mmol/g)

Glucose 0.53 ± 0.08Sucrose 0.45 ± 0.04Potato starch 0.35 ± 0.11Microcrystalline cellulose –a

a Unpolymerized.

nce 257 (2011) 8686– 8691 8687

100–300 ◦C. The heat-treatment solids were washed thoroughlywith deionized water and subsequently ethanol until neutral, andthen dried in a vacuum oven at 50 ◦C for 12 h. The heat-treated sam-ples were denoted as HTC-100, if heat-treated at 100 ◦C, HTC-300,if heat-treated at 300 ◦C, and so on.

2.3. Comparison experiments

In order to investigate the mechanism of newly generatingcarboxyl groups on HTC surface during heat-treatment, differentmaterials and treatment methods were used in the following exper-iments. Sample AC-HNO3 was prepared via oxidization of AC by7 mol/L HNO3 solution at 70 ◦C for 6 h and subsequently washingwith deionized water and ethanol until neutral, and then dryingin a vacuum oven at 50 ◦C for 12 h; AC-300 and Ar-AC-300 wereobtained by heating AC in muffle furnace at 300 ◦C for 5 h in airor in Ar atmosphere, respectively; glucose-300 was prepared byheat treating pure glucose at 300 ◦C for 5 h; and Ar-HTC-300 wasobtained from HTC using the same procedure for Ar-AC-300.

2.4. Adsorption experiments of Cd2+ and Pb2+ ions

The adsorption of Cd2+ and Pb2+ ions from the dilute aqueoussolution was operated as the following procedure: (1) pH value ofaqueous metal nitrate solution (200 mg/L) was adjusted to around4 with 0.1 mol/L HCl or NaOH solution; (2) 0.025 g HTC or HTC-300was mixed with 50 mL of the metal nitrate solution in a conicalflask, then the flask was sealed and kept agitating in a shaking bathfor 12 h at room temperature and 120 strokes/min; (3) at the endof the adsorption period, the supernatant solution was retained foranalysis after centrifugation. The adsorption capacity Q (mg/g) ofthe metal ions was calculated using the following equation:

Q = (C0 − Ce)Vm

where C0 is the initial concentration of metal ions (mg/L), Ce is theequilibrium concentration of the ions (mg/L), V is the volume of thetesting solution (L), and m is the weight of sorbent (g).

2.5. Characterization

The content of OFGs on raw material and the resulting productswas examined by Boehm titrations [31]. Surface topography of thecarbon microspheres was observed by SEM (FEI Company, OR, USA).FT-IR was recorded with Perkin-Elmer IR-843 spectrometer (USA).XPS (XSAM800, England) was used to measure the elemental com-position and chemical state of the surface elements (mainly C and Oatoms) in the carbon microspheres. CHN content was determinedby the elemental-analysis device (CARLO 1106, Italy). Elementalcomposition of the carbon microspheres was determined by EDX(INCA PentaFEI, UK). Metal ions in the solution were analyzed byinductively coupled plasma atomic emission spectroscopy (ICP-AES, Thermo Elemental, USA).

3. Results and discussion

3.1. Characterization of samples

3.1.1. Weight-loss ratios of samples and amount of OFGs indifferent products

The weight-loss ratios and contents of the surface OFGs ofTHC after heat treatment at different temperatures are shown in

Tables 2 and 3, respectively. The weight-loss ratios became higherwith the increase of temperature, probably due to the volatilizationand decomposition of some incompletely polymerized monomers,oligomers and/or small molecules on HTC frameworks. However,

8688 Z. Chen et al. / Applied Surface Science 257 (2011) 8686– 8691

Table 2Weight-loss ratios of samples.

Sample Weight-loss ratioa (%)

HTC-100 9.0HTC-150 12.7HTC-200 16.3HTC-250 30.7HTC-300 50.8HTC-350 73.0HTC-400 99.7

a Weight-loss ratio was obtained by the ratio of the loss weight after heat-treatment to the weight of HTC.

Table 3Content of OFGs in different products determined by Boehm titrations.

Sample Carboxyl (mmol/g) Lactone (mmol/g) Phenol (mmol/g)

HTC 0.53 ± 0.08 1.94 ± 0.09 1.06 ± 0.02HTC-100 0.77 ± 0.01 3.05 ± 0.07 2.56 ± 0.07HTC-150 1.39 ± 0.01 3.06 ± 0.12 2.28 ± 0.04HTC-200 1.75 ± 0.03 2.69 ± 0.08 2.24 ± 0.08HTC-250 2.61 ± 0.02 2.70 ± 0.21 1.30 ± 0.19

aipmcatf

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Fig. 1. SEM images of HTC (a) and HTC-300 (b).

HTC-300 3.70 ± 0.07 2.90 ± 0.11 0.79 ± 0.06HTC-350 3.87 ± 0.03 2.82 ± 0.09 0.52 ± 0.05

s shown in Table 3, the amounts of OFGs increased with increas-ng calcination temperature, and it could be seen clearly that morehenolic groups increased after the treatment at about 100 ◦C butuch more carboxyl groups at higher temperatures. Taking into

onsideration the above mentioned two factors and also physicalnd chemical stability of HTC spheres, the calcination tempera-ure of 300 ◦C was chosen as optimal for the following oxidizingunctionalization of HTC.

.1.2. SEMSEM images of HTC and HTC-300 displayed typical morpholog-

cal changes of HTC before and after heat treatment. The carbonroducts were spherical particles with uniform diameters rang-

ng from 0.5 to 1.0 �m, while there were also a few bulky onesith diameters between 1.5 and 2 �m. Serious adhesions between

arbon microspheres could be observed in HTC sample as shownn Fig. 1a. But the adhesions decreased with increase of the heat-reatment temperature, leading to an increase in the spheres withutaway sections, and at heating temperature up to 300 ◦C, most ofhe carbon spherules exhibited a relatively perfect spherical mor-hology (Fig. 1b).

.1.3. FT-IRThe FT-IR spectra of the pristine and the resulting materi-

ls heat-treated at different temperatures are shown in Fig. 2.ll materials were rich in OFGs such as carboxyl, hydroxyls andsters. The characteristic adsorption bands for all the materi-ls belonged to C–OH stretching and –OH bending vibrations1156 cm−1), carboxyl anhydride groups (1270 cm−1), lactoneroups (1398 cm−1), C C stretching of aromatic (1620 cm−1), car-onyl groups (1720 cm−1), C–H stretching (2844 and 2925 cm−1)nd hydroxyl groups (3454 cm−1), respectively [32]. However, itas much more significant that only the intensity of the adsorp-

ion bands at 1720 cm−1, belonging to C O, increased graduallylong with the rise of the heat-treatment temperatures. However,here were not significant differences in band-intensity betweenhe characteristic bands for each of the sets of samples studied. Theesults provided more direct evidence that heat treatment had a

reat influence on the content of carbonyl groups on HTC. Accord-ng to the results of Boehm titrations, the newly increased carbonylroups after heat treatment, especially at hither temperature, wereainly in the form of carboxyl groups.

Fig. 2. FT-IR spectra of HTC (a), HTC-100 (b), HTC-150 (c), HTC-200 (d), HTC-250 (e)and HTC-300 (f).

ce Science 257 (2011) 8686– 8691 8689

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Table 4Carbon to oxygen atoms ratio by EDX.

Samples Cutaway sections (wt.%) Surface (wt.%)

C O C O

Z. Chen et al. / Applied Surfa

It was also worthy of note that there were almost no new typesf functional groups formed on HTC after the heat treatment.

.1.4. XPSFig. 3 shows the C 1s spectra of HTC and HTC-300. Four peaks in

ig. 3 were corresponded to the aliphatic/aromatic carbon groupsCHx/C–C/C C, 284.8 eV, peak I), hydroxyl groups (–OH, 286.1 eV,eak II), carbonyl groups ( C O, 288.1 eV, peak III) and carboxylroups, esters or lactones (–COOR, 289.4 eV, peak IV), respectively33–35]. Compared with the corresponding peaks of HTC in Fig. 3a,he peak III of HTC-300 in Fig. 3b was slightly increased and peakV was noticeably increased. Combining the results of Boehm titra-ions and FT-IR, the increasing intensity of the peak IV could alsoe attributed to the newly formed carboxyl groups after heat-reatment.

In addition, as it was reported that the oxygen in the core of HTCs probably in the form of stable groups (i.e., ether, quinone, pyrone,tc.), whereas the oxygen functionalities present on the shell mayonsist of more reactive/hydrophilic groups (i.e., hydroxyl, car-onyl, carboxyl, ester, etc.) [12]. The O/C ratios on the surface ofTC and HTC-300 calculated from XPS peak area were 0.23 and.29 (calculated by peak area), respectively. The result was in goodgreement with the data (0.34 and 0.44) in EDX analysis (Table 4).herefore it could be inferred that air played an important role inhe surface oxidation of HTC during heat-treatment process.

.1.5. EDXThe results from EDX analysis of the HTC and HTC-300 are shown

n Table 4, which indicated that the oxygen contents on the surfaces

Fig. 3. XPS spectra of C 1s of HTC (a) and HTC-300 (b).

HTC 76 24 74 26HTC-300 69 32 69 31

and cutaway sections of both carbon microspheres were enhancedsignificantly after heat-treatment in air.

3.1.6. CHN analysisThe elemental analyses of the heat-treated HTC samples and

their parent material are given in Table 5. It was shown that bothC% and H% of the samples decreased remarkably with the increaseof heat treatment temperature, and no nitrogen was found in anytested samples, indicating that there was no nitrogen mixed ordoped into the samples tested during the heating process. Sincethere were not any other elements besides C, H and O in all reagentsused in the sample preparation and washing procedures, it was rea-sonable to expect that the as-synthesized materials only consistedof oxygen as well as carbon and hydrogen. The oxygen contentin samples can be calculated by subtracting the sum of carbonand hydrogen from 100. According to the results from the calcu-lations, the content of oxygen in the tested samples was 31.96% forHTC, 32.54% for HTC-100, 35.63% for HTC-150, 38.60% for HTC-200,39.32% for HTC-250 and 41.07% for HTC-300, respectively, whichalso supported the results from Boehm titration, FT-IR spectra andXPS.

3.1.7. Adsorption capability of HTC-300 for heavy metal ionsIn order to assess the effectiveness of the oxidative functional-

ization of the HTC samples via the heat treatment approach usedin this work, adsorption capacities of HTC and HTC-300 for heavymetal ions (Pb2+ and Cd2+) separately, were examined. The resultsare shown in Fig. 4. The adsorption capacities of Pb2+ and Cd2+ ionson HTC-300 were 326.1 ± 3.0 mg/g and 150.7 ± 2.7 mg/g, 3 and 30times higher than that on HTC, respectively.

A comparison of adsorption capacities for the two heavy metalions between HTC-300 and other reported sorbents is shown inTable 6. The capability of the as-heat-treated carbon material isclearly superior to all other sorbents listed. Especially, the pH valueof adsorption medium used in the tests mentioned above was thelowest among others listed.

3.2. Mechanism presumed for the oxidative functionalization

Comparison experiments were carried out to investigate themechanism for newly generating carboxyl groups on the surfaceof HTC during the heat-treatment process. The results of Boehmtitrations for the different carbon samples in the experiments are

shown in Table 7. It could be seen that after heat treatment in air,the amount of carboxyl groups was increased obviously on both ACand HTC. However, carboxyl groups sharply decreased when HTCand AC were heated in Ar atmosphere. Comparison experiment was

Table 5Results of elemental analyses of the parent and heat-treated HTCs.

Sample C (wt.%) H (wt.%) N (wt.%)

HTC 63.73 4.31 –a

HTC-100 63.56 3.90 –a

HTC-150 61.48 2.89 –a

HTC-200 59.56 2.61 –a

HTC-250 58.30 2.38 –a

HTC-300 56.66 2.27 –a

a Under detection limit.

8690 Z. Chen et al. / Applied Surface Science 257 (2011) 8686– 8691

Fig. 4. The adsorption for Pb2+ and Cd2+ ions on carbonaceous materials.

Table 6Adsorption capacities of Pb2+ and Cd2+ on different sorbents.

Sorbent Pb2+ (mg/g) Cd2+ (mg/g) Conditions Ref.

HTC-300 326.1 150.7 pH 4; RTa This work10AcA-C 351.4 88.8 pH 6; RT [30]Leonardite (low rankcoal)

250.7 50.6 pH 5.5; RT [36]

HNO3 oxidized carbonnanotubes

97.1 10.7 pH 5; RT [37]

Algae 331.5 134.9 pH 5; RT [38]Amberlite IR-120synthetic sulfonatedresin

19.6 201.1 pH 4–8; RT [39]

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Table 7Comparison experiments results of Boehm titrations.

Sample Carboxyl content (mmol/g)

AC 0.15 ± 0.03AC-HNO3 0.45 ± 0.09HTC 0.53 ± 0.08AC-300 1.10 ± 0.04HTC-300 3.70 ± 0.07Glucose-300 1.45 ± 0.14Ar-AC-300 0.09 ± 0.06Ar-HTC-300 0.31 ± 0.02

Table 8Influence of heating time on the content of carboxyl groups.

Sample Content of carboxyl groups (mmol/g)

1 h 3 h 5 h

HTC-100 0.74 ± 0.01 0.76 ± 0.05 0.77 ± 0.01HTC-150 1.29 ± 0.01 1.30 ± 0.01 1.39 ± 0.02HTC-200 1.72 ± 0.03 1.74 ± 0.02 1.75 ± 0.06

Carbon aerogel 35.0 15.0 pH 4.5; 37 ◦C [40]

a Room temperature.

lso conducted using pure glucose. After heating at 300 ◦C, thougharboxyl groups was observed on glucose-300, its amount was far

ower than that on HTC-300. From these experimental facts, wean infer that (1) comparing with AC, more active sites located onTC material, which could transform into carboxyl groups easilyia heat treatment; (2) oxygen in air has great influence on such

Fig. 5. Mechanism assumpt

HTC-250 2.06 ± 0.02 2.48 ± 0.02 2.61 ± 0.02HTC-300 3.42 ± 0.07 3.65 ± 0.03 3.70 ± 0.02

a transformation; (3) the increase of carboxyl groups on HTC-300may not be due to incomplete carbonization of glucose.

In addition, the influence of heating time on the amount ofcarboxyl groups was also investigated. As shown in Table 8, the con-tent of carboxyl groups increased from 0.53 mmol/g to 3.42 mmol/gafter heating for 1 h, and only to 3.70 mmol/g for 5 h. It could bededuced that the formation of the increased carboxyl groups wasa relatively rapid chemical process and the heating time had littleinfluence on the carboxyl group content.

In summary, a mechanism of oxidative functionalization of HTCin heat treatment process is supposed and illustrated in Fig. 5. Theillustrating structure of HTC was derived from previous reports[12,41]. The C C band in HTC, especially the non-aromatic one,lost an electron when heated and subsequently converted into afree radical 2, which reacted with oxygen to form positive ion rad-

ical 3 that contained two oxygen atoms. Then, a free radical chainreaction was triggered between 3 and 1, resulting in a transitionstate 4 and a new 2 which was able to repeat the reaction above.

ion for the oxidation.

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y the cyclization reaction of 4, four-membered ring with two oxy-en atoms 5 was formed and decomposed rapidly into an aldehydend/or a ketone. Carboxyl groups thus formed via oxidation of alde-yde with the participation of oxygen and water in air during theooling procedure.

. Conclusions

Carboxyl-rich hydrothermal carbon materials can be obtainedy fast, mild and effective heat-treatment in air. The simplepproach could provide much more grafting sites on this kind ofaterials for further functionalization. In other words, it could

ffectively improve grafting ratio and efficiency of the function-lized products, which would exhibit wide promising applicationsn adsorption, separation, catalysis, ion exchanging, electrochem-stry, chromatography, drug delivery, gas storage and so on. Theroposed mechanism would be a contribution towards a deepernderstanding of the effective oxidative functionalization of HTCased on simple heat-treatment.

cknowledgments

The financial support from the National Natural Science Founda-ion of China (Grant 20571053 and 20871086), Chinese Academy ofciences (Grant KJCX2-YW-N50-3), the Institute of Chemical Engi-eering and Material, China Academy of Engineering Physics (GrantG2010040), and Radiation Chemistry & Radiation Chemistry Keyaboratory for Fundamental Sciences, Peking University (Grants010-05) are gratefully acknowledged.

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