Converting Carbohydrates to Carbon-based Photocatalysts for

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Converting Carbohydrates to Carbon-based Photocatalysts for Environmental Treatment Zhuofeng Hu, Zhurui Shen, and Jimmy Chai Mei Yu Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 16 May 2017 Downloaded from http://pubs.acs.org on May 16, 2017

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Environmental Science & Technology

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Converting Carbohydrates to Carbon-based Photocatalysts for

2

Environmental Treatment

3

Zhuofeng Hu,a Zhurui Shen,a, b, * Jimmy C. Yu,a, *

4

a

5

Territories, Hong Kong, 999077, PR China, E-mail: [email protected]

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b

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Education, School of Materials Science and Engineering, Tianjin University, Tianjin,

8

300072, PR China, E-mail: [email protected].

Department of Chemistry, The Chinese University of Hong Kong, Shatin, New

Key Laboratory of Advanced Ceramics and Machining Technology, Ministry of

9 10 11 12 13 14 15 16 17 18 19

*Corresponding authors:

20

Jimmy C. Yu: Tel: +852-3943-6268, Fax: +852-2603-5057, E-mail:

21

[email protected].

22

Zhurui Shen: E-mail: [email protected].

ACS Paragon Plus Environment


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ABSTRACT

24

Carbohydrates in biomass can be converted to semiconductive hydrothermal

25

carbonation carbon (HTCC), a material that contains plenty of sp2-hybridization

26

structures. Under solar light illumination, HTCC generates photoexcited electrons,

27

holes and hydroxyl radicals. These species can be used for photocatalytic treatment

28

such as water disinfection and degradation of organic pollutants. The photocatalytic

29

activity of HTCC can be significantly enhanced by iodine doping. The enhancement

30

mechanism is investigated by density functional theoretical calculations and

31

electrochemical measurements. The iodine dopants twist and optimize the structures

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of the sp2-hybridization in HTCC, thereby favoring photon-induced excitation.

33

Moreover, the iodine dopants facilitate the charge transfer between different

34

sp2-hybridization structures, thus increasing the conductivity and activity of the

35

HTCC. An added benefit is that the I-doped HTCC exhibits lower cytotoxic effect

36

than the pure HTCC. In addition to monosaccharides (glucose), disaccharides (sucrose)

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and polysaccharides (starch), we have also transformed crops (e.g., rice), plants (e.g.

38

grass), and even agricultural waste (e.g. straw) and animal waste (e.g. cow dung). The

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conversion of carbohydrates to HTCC may be considered as a “Trash to Treasure”

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approach. We believe this discovery will attract a lot of attention from researchers

41

involved in environmental catalysis, waste recycling and pollution treatment.

42 43

Key words: Carbohydrate; Hydrothermal carbonation carbon (HTCC); Iodine doping;

44

Disinfection; Environmental remediation.

45


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INTRODUCTION

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Carbohydrates are common natural products, which are largely synthesized by

48

plants through photosynthesis (billion tons per year). Consisting of carbon, oxygen

49

and hydrogen, they are abundant, environmental-benign, and easily accessible. They

50

have H:O atomic ratios of 2:1, just like that in water.1 Depending on the

51

polymerization level, carbohydrates are mainly sorted into: monosaccharides (e.g.

52

glucose), disaccharides (e.g. sucrose), and polysaccharides (e.g. starch). Also,

53

carbohydrates form a major fraction of natural biomass such as some crops and grass.

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Moreover, many agricultural wastes (e.g. straw) and animal wastes (e.g. cow dung)

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contain large quantity of carbohydrates. In developing countries, these wastes are

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often burned as fuels and release plenty of harmful particulates and greenhouse gas.

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Converting these carbohydrates to high value-added functional materials seems to be

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a sensible approach for environmental sustainability

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Hydrothermal carbonation (HTC) treatment of carbohydrates has attracted growing

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attention since its discovery in 1913 by Bergius (using cellulose).2 Generally,

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hydrothermal carbonation carbon (HTCC) can be obtained in a sealed container with

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aqueous solution or suspension of carbohydrate biomass at 180-350 oC after several to

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tens of hours of reaction.3 The HTC treatment of carbohydrates is very attractive for

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the following reasons: 1) The HTC process is exothermic rather than endothermic.4 2)

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The HTC process is considered as a “wet pyrolysis” process. Unlike dry pyrolysis of

66

biomass where lots of energy is used to remove moisture, it is much less

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energy-consuming.2 3) Transformation of carbohydrate into HTCC rather than

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burning it to emit CO2 helps to mitigate greenhouse gas release,5 4) The HTCC has a

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relative low cost and nearly zero net emission2, 6 5) The HTCC is ready to be grafted

70

with organic functional groups on its surface. For example, Qi et al. have prepared

71

HTCC materials (from cellulose) with plenty of carboxylic groups on the surface.7

72

They were used as an efficient adsorbent for heavy metals8 and water soluble ionic

73

liquid, 1-butyl-3-methyl-imidazolium chloride ([BMIM][Cl]).9 Therefore, HTCC has

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been widely used in hazardous chemicals cleanup, catalysts support, and porous



75

carbon fabrication.6, 7, 9, 10 However, most of these applications just utilize the high

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surface area and rich functional groups of HTCC. The inner chemical structure of the

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HTCC is usually overlooked, and the full potential of HTCC has yet to be realized.

78

Recently, our group found that the HTCC derived from glucose was composed of

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skeletons of light-absorbing and semiconducting sp2-hybridization units.11 These units

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can generate photoexcited electrons and holes under illumination. The highly active

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radicals thus formed can oxidize toxic organic pollutants and deactivate bacteria in

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water.12-17 Unlike metal containing semiconductors that will release harmful metal

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cations, HTCC would not cause secondary pollution. However, these sp2-hybrided

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units are discrete, leading to poor charge transfer and conductivity in large particles.

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Therefore, HTCC by itself is not a good photocatalyst. It must be combined with other

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metal oxide semiconductors in practical applications.11, 18

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Herein, we report a simple iodine (I-) doping approach to enhance the

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photocatalytic activity of HTCC. Iodide is widely distributed in the environment, and

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it is essential for human health.19 The I-doped HTCC particles exhibit significantly

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higher activity in disinfection and organic pollutant degradation under visible-light

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illumination. The photocatalytic mechanisms for HTCC and iodine (I)-doped HTCC

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are deduced from theoretical simulations and spectroscopic measurements. The

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theoretical simulations show that the iodine dopants twist the sp2-hybridization

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structures of the HTCC, thereby favoring more photon-induced excitation. Also, the

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iodine dopants facilitate the charge transfer between different sp2-hybridization

96

structures, thereby increasing the conductivity and photocatalytic activity of the

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HTCC. Moreover, we have developed a series of I-doped HTCC from different kinds

98

of carbohydrate-based materials including sucrose, starch, rice, grass, straw and

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animal waste. These render the practical application of I-doped HTCC possible in the

100

field of environmental application.

101 102

EXPERIMENTAL SECTION

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Preparation of I-doped HTCC. 0.5 g of elemental iodine was dissolved into 20 mL

104

of absolute ethanol. With the addition of 2.0 g glucose, the solution was transferred to


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a 25 mL Teflon-lined stainless steel autoclave and heated at 180 oC for 4 hours. The

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product was collected via filtration and dried in a vacuum oven overnight. Besides

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monosaccharide, disaccharide of sucrose, polysaccharide of starch, grain of rice and

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grass were also used. For gram-scale preparation, the elemental iodine was increased

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to 25 g, while the glucose was increased to 100 g.

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Preparation of iodine-free HTCC. 2.0 g glucose was dissolved into 20 mL of

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distilled water to form a homogeneous solution. Subsequently, the solution was

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transferred to a 25 mL Teflon-lined stainless steel autoclave and heated at 180 oC for

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10 hours. After the reaction, the product was taken out and washed with distilled

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water before being dried in a vacuum oven. For gram-scale preparation, the amount of

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glucose was increased to 100 g.

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Characterization. Scanning electron microscopy (SEM) was performed on a FEI

117

Quanta 400 microscope X-ray diffraction (XRD) was performed on a Rigaku

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SmartLab X-ray diffractometer using a Cu Kα source irradiation (λ=1.5406 Å). Small

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angle X-Ray scattering measurement was performed on a Xenocs-SAXS/WAXS

120

system with X-ray wavelength of 1.5418 Å. X-Ray photoelectron spectroscopy (XPS)

121

was performed using a Sengyang SKL-12 spectrometer equipped with a VG CLAM 4

122

MCD electron energy analyzer and twin anode Mg Kα radiation (1253.6 eV). 13C

123

solid-state magic angle spinning (MAS) nuclear magnetic resonance (NMR)

124

experiments have been acquired on Bruker Avance 300 MHz (7 T) spectrometer using

125

the 4 mm zirconia rotors as sample holders spinning at MAS rate vMAS=14 kHz.

126

Brunauer–Emmett–Teller (BET) surface area was measured by a Surface Area and

127

Porosity Analyzer (ASAP 2460). Raman spectrum was performed on a DXR

128

Microscope (Thermo Electron Corporation) with a resolution of 1.496 cm-1 and

129

excitation wavelength of 532 nm. The slit is 50 µm and the power is 5 µW.

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Photocatalytic degradation of Rhodamine B (RhB). In all the degradation

131

experiments, 10 mg photocatalysts were suspended in 30 mL aqueous solution of RhB

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(20 ppm). The suspension was firstly stirred in the dark to reach an

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adsorption/desorption equilibrium. Then the suspension was illuminated by a 300 W

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halogen tungsten lamp equipped with a UV cutoff filter (λ> 420 nm). The



135

concentration of RhB was measured by a UV-Vis spectrophotometer at an interval of

136

1 hour.

137

The photocatalytic activity was evaluated using a time profiles of C/C0, where C is

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the concentration of the dyes at the irradiation time t, and C0 the concentration at the

139

equilibrium point before irradiation, respectively. The apparent rate constant for the

140

photodegradation of RhB was calculated by the equation of k=In(C0/C)/t.12

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Photocatalytic disinfection performance. The photocatalytic disinfection was

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performed by using a 300 W xenon lamp (PLS-SXE-300, Beijing Perfect Light Co.

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Ltd., Beijing) with a UV cutoff filter (λ> 420 nm). The visible-light intensity was

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measured by a light meter (LI-COR, USA) and was fixed at 200 mW cm-2. All glass

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apparatuses used were cleaned at 120 °C for 30 min to ensure sterility. The bacterial

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cells was incubated in 10% nutrient broth solution at 30 °C for 18 h with shaking, and

147

then washed with sterilized saline. The final photocatalyst concentration and cell

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density were adjusted to 200 mg L-1 and about 1×107 colony forming units per

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milliliter (cfu mL-1), respectively. The reaction temperature was 25 °C and the

150

reaction mixture was stirred with a magnetic stirrer during the experiment. Before and

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after the photocatalytic oxidation (PCO) treatment, an aliquot of the reaction solution

152

was sampled and immediately diluted with sterilized saline, and an appropriate

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dilution of the sample was spread on nutrient agar and incubated at 30 °C for 24 h.

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The number of colonies was counted to calculate the number of viable cells.

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Scavenger solutions included 1 M isopropanol for •OH radicals, 1 M K2Cr2O7for

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electrons and 1 M sodium oxalate for holes.

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Detection and measurements of photogenerated •OH radicals. 5 mg of

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photocatalysts were ultrasonically dispersed in 25 mL 1×10-3 M coumarin solution.

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The suspension was stirred under visible light. At an interval of 2 hour, 2 mL of the

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suspension was extracted and centrifuged. Then, 1 mL of the clear solution was

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collected for the PL measurement. The generated 7-hydroxycoumarin was monitored

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by fluorescence analysis with an excitation wavelength of 332 nm.20

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Photoelectrochemical measurements. The powder sample was fabricated into

164

electrode on a FTO glass by electrophoretic deposition. All the photoelectrochemical


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measurements

were

performed

in

a

three-electrode

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saturated-potassium-chloride silver chloride electrode (Ag/AgCl) as a reference

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electrode, a platinum foil (1.0×1.0 cm-2) as a counter electrode, and the HTCC

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electrode as a working electrode. The electrolyte was 0.1 M Na2SO4. A 300 W Xenon

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arc lamp coupled with an AM 1.5G global filter (100 mWcm-2) and UV cut-off filter

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(λ> 420 nm) were used as radiation source. Linear sweeps and transient photocurrent

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were measured by a CHI 660D electrochemical workstation.

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Density functional theoretical calculations. VASP computational package was used

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here for all the calculations.21 We applied projector-augmented-wave method with

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Perdew–Burke–Ernzerhof GGA functional.22-24 Electronic convergence limit was set

175

to be 1×10−5 eV. Optimization of atomic coordinates was considered to be converged

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if Hellmann–Feynman force was smaller than 1×10−2 eV Å−1. We applied Monkhorst–

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Pack scheme 2×2×2 for k-point selection. The Brillouin zone was sampled using

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2×2×2 Monkhorst-Pack

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RESULTS AND DISCUSSION

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Morphology, structure and chemical nature of I-doped HTCC. HTCC is formed

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via the cross-linking and intermolecular dehydration of glucose molecules under

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hydrothermal condition.25 Iodine can be incorporated into the HTCC to form I-doped

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HTCC with brownish black color and tens of grams yield (Figure 1a-c). The I-doped

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HTCC particles are irregular 2-4 µm microparticles (Figure 1b), and are confirmed

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amorphous by the absence of noticeable diffraction peaks in the XRD pattern (Figure

186

1d). Besides, SAXS is also performed to study their microstructures (Figure S1),

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Results show that: i) the I-doped HTCC displays much stronger scattering contrast

188

than that of HTCC, suggesting higher heterogeneity of electron density caused by

189

iodine doping. ii) both peaks decay smoothly all the way to the baseline, suggesting

190

the absence of nanoscale assembly and pores.26 BET adsorption–desorption isotherms

191

of I-doped HTCC (Figure S2a) exhibit a typical IV isotherm (based on IUPAC

192

classiﬁcation).27 The rapid increase below the relative pressure of 0.1 indicate the

193

existence of lots of micropores, while the hysteretic loop between 0.4 to 0.6 suggests


cell

with

a


194

the presence of the mesopores, as confirmed by the pore distribution curve (Figure

195

S2b).27 The BET surface area is c.a. 254 m²/g.

196

FT-IR spectra were recorded to examine the chemical structure of the samples.

197

Characteristic IR peaks for polyfuran were found as shown in Figure 1e. The wide

198

bands at 1610 cm-1, 1385 cm-1 and the shoulder bands at 1440 cm-1 and 960 cm-1 can

199

be assigned to the vibrational modes of the furan monomer.28, 29 The band at 1517

200

cm-1 is ascribed to the C=C stretching of furan ring. Moreover, the band at 798 cm-1 is

201

due to the α, α’-coupling of the carbon backbone, originating from the linear structure

202

in polyfuran (Scheme S1). Besides, the bands at 1190 cm-1 and 1010 cm-1 are ascribed

203

to the C-H bending and stretching modes, and the bands at 870 cm-1 and 630 cm-1 are

204

assigned to the out-of-plane bending of C-H modes in the furan rings.28, 29 The bands

205

at 2940 cm-1 and 1700 cm-1 are ascribed to the aliphatic C-H stretching mode and

206

C=O stretching, respectively. These suggest that some furan rings are open in the

207

polymer, which is not surprising for a large polyfuran structure. The O-H stretching

208

peak at 3410 cm-1 clearly indicates the adsorption of water molecules on the catalyst.

209

With the addition of iodine, the peaks related to polyfuran remain, suggesting the

210

doping of iodine does not destroy the backbone structure of the HTCC.

211

In the

13

C Solid-state CP-MAS NMR spectra of both HTCC and I-doped

212

HTCCC (Figure 1f), the peaks at 152 ppm and 115–127 ppm correspond to O-C=C

213

and C=C-C of the furan rings, and they are denoted as the polyfuran domains in the

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spectrum.30, 31 The wide peak centred at 40 ppm can be ascribed to the aliphatic C-H

215

groups. The peaks at 175 ppm and 205 ppm can be assigned to the –COOH and –C=O

216

groups, respectively. These three peaks (40 ppm, 175 ppm and 205 ppm) suggest there

217

are some open rings domains in the polyfuran, which is consistent with the result of

218

FT-IR spectra. While the Raman spectrum of I-doped HTCC (Figure S3) further

219

shows five bands at ca. 1600 (v1), 1550 (v2), 1275 (v3), 1024 (v4), and 965 cm-1 (v5),

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which is close to the Raman spectra of previously reported polyfuran.32 The elemental

221

analysis (EA) show that the weight percentage of C, H, O and I in HTCC are 63.6 %,

222

4.55 %, 28.1 % and 3.70 % respectively. The weight ratio of C, H, O are close to the

223

theoretical values of pure polyfuran (C: 72.73 %, H: 3.03 %, O: 24.24 %).


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In the XPS I 3d spectra (Figure 1g,h), the signals of iodine can hardly be observed

225

in the iodine-free HTCC, while two peaks related to I 3d 5/2 and I 3d 3/2 can be seen

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in the I 3d XPS spectra of I-doped HTCC, where the I 3d 5/2 peak with binding

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energy of 619.2 eV suggests the iodine should exist as I-.33-35 In the reaction, iodine is

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reduced to iodide while glucose is oxidized to other species such as carboxylic acid

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during the conversion process. Besides, the O 1s and C1s spectra are also studied and

230

the chemical state of HTCC and I-doped HTCC are further analyzed (Figure S4,

231

Section S1). Also, the depth profile XPS spectra (Figure S5, Table S1, Section S2)

232

suggest that the iodine dopants locate homogeneously in the I-doped HTCC.

233 234

Band structure and semiconductive nature of I-doped HTCC. The hybridized π

235

electrons in the sp2-hybridized structures in HTCC can be excited easily by absorbing

236

visible light photons.36 As shown in the ultraviolet-visible (UV-vis) spectrum of

237

I-doped HTCC (Figure 2a), an intrinsic semiconductor-like absorption is observed in

238

a wide region. The bandgap of the I-doped HTCC is then estimated to be about 1.0 eV

239

accordingly (Figure 2b). Iodine doping leads to a band-to-band absorption red shift,

240

suggesting a homogeneous distribution of iodine dopants,

241

the Argon ion sputtering XPS analysis. The red shift indicates narrowing of the

242

bandgap. This can be attributed to the twist structure of HTCC induced by iodine

243

doping, which will be demonstrated in the Vasp theoretical calculation below.

37

which is consistent with

244

The band structure of the I-doped HTCC is then estimated by Mott-Schotty plot

245

(Figure S6a)38, 39 and valance XPS spectrum (Figure S6b). Calculation detail can be

246

found in Section S3 and Section S4 in SI

247

We find that he Fermi level, conduction band (CB) and valence band (VB) of the

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I-doped HTCC is at 0.6 V, 0.26 V and 1.2 V vs. RHE. Similarly, the band structure of

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iodine free HTCC is also calculated as the reference (Figure S6c, d). The Fermi level

250

is at 0.15 V vs. RHE, the CB is at -0.24 V vs. RHE (0.15 – 0.39 V), the VB is at 1.15

251

V vs. RHE, and its bandgap is about 1.39 eV. It is shown that the insertation of iodine

252

caused a narrowing of the bandgap of I-doped HTCC, which is benefical to light

253

absorption.



254

The generation of photoexcited carrier is proved by the photocurrent measurement

255

(Figure 2c).40 Appreciable photocurrent is produced after 0.65 V and raised with

256

increasing positive bias under chopped visible light illumination (Figure 2c). The

257

onset potential (0.65 V) is very close to the flatband potential measured by the

258

Mott-Schotty plot. Meanwhile, repeatable photocurrent is produced in the chopped

259

transient photocurrent measurement at 1.23 V vs. RHE (Insert in Figure 2c).

260

The generation of active oxidative species (mainly •OH radical) is monitored using

261

coumarin as a fluorescence probe (Figure S7).20 Non-fluorescent Coumarin capture

262

•OH radical to produce fluorescent 7-hydroxycoumarin with an emission peak at 460

263

nm upon 315 nm excitation. As shown in Figure 2d, the generation of •OH radical

264

from I-doped HTCC is confirmed by a rising peak at 460 nm. The band structure and

265

excitation process of I-doped HTCC and HTCC are finally schematically shown in

266

Figure 2e.

267 268

Environmental treatment. After exploring its semiconductive nature and electronic

269

property, the I-doped HTCC is applied to photocatalytic disinfection. E. coil K-12,

270

which was chosen as a representative microorganism (Figure 3a-c). During the 3-hour

271

control experiment in dark, the population of bacterial remains constant with HTCC

272

whether iodine is doped or not (Figure 3a). Under visible light illumination, the

273

iodine-free HTCC exhibits very weak activity, whereas the I-doped HTCC exhibit

274

appreciable activity (Figure 3a).

275

The roles of photoexcited holes and electrons were determined by the addition of

276

scavenger chemicals (Figure 3c). Results show that photoexcited holes and electrons

277

in the VB and CB were signiﬁcantly involved in the reaction, respectively, confirmed

278

by the obvious inhibition of bacterial inactivation after the addition of corresponding

279

scavengers. Besides, the inhibition effect with OH radicals is more obvious than that

280

of hole and electron scavenger. This suggests the OH radicals play more important

281

role in the photocatalytic disinfection process.

282

The I-doped HTCC can also be used for degrading a representative pollutant

283

RhB.16 As shown in Figure 3d, RhB is stable under the illumination of visible light


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without photocatalysts. With HTCC, the degradation of RhB is very slow (k=0.0003

285

min-1). While for I-doped HTCC, the degradation of RhB becomes much faster

286

(k=0.0077 min-1, Figure 3d and Figure S8a). Moreover, I-doped HTCC also exhibits

287

appreciable activity (a 3.3-fold improvement) when compared with a well-known

288

metal free photocatalyst-graphic carbon nitride (g-C3N4) (k=0.0018 min-1, Figure S8b).

289

It is also comparable to another state-of-the-art photocatalyst CdS (k=0.0097 min-1,

290

Figure S8b). Na+, K+ and Fe2+/Fe3+ cations (1 wt%) are also added during

291

photocatalysis, and results show that they have no effect on the photocatalytic activity

292

(Figure S9).

293

These results indicate that I-doped HTCC has great potential in photocatalytic

294

disinfection of bacteria and degradation or organic pollutant, which will render its

295

practical application in environmental remediation. Moreover, it even shows good

296

stability even after irradiation by UV light for 10 h (Figure S10)

297 298

In addition, the adsorption ability (using Cd2+) of I-doped HTCC is summarized in Figure S11 and Section S5.

299 300

Function of iodine dopants ： density functional theoretical calculation and

301

spectroscopic analysis. It is known that halogen doping can influence the electronic

302

property of materials.41, 42 Herein, the function of iodine dopants will be studied by

303

using Vasp theoretical calculation, XPS spectra and electrochemical measurements

304

Since the semiconducting property of HTCC is mainly originating from polyfuran,

305

we use a polyfuran structure contain 42 units of furan rings for density functional

306

theory (DFT) calculation (Figure 4).43 The insertion energy (Einsert) is calculated to be

307

-2.09 eV, suggesting the insertion of iodine is energetically favorable (Section S6 and

308

Figure S12 in SI). Also, after the insertion of iodine, the structure is greatly disturbed

309

and the six furan units in a furan chain no longer in a plane (Figure 4a, c) The

310

structure distortion induced by iodine insertion greatly favors the electronic structure

311

of the HTCC. With iodine doped, the obvious discrete DOS peak becomes continuous

312

because the molecular conformation of polyfuran changes from plane to twist due to

313

the insertion of large iodine atoms (Figure S13a, b). Excitations are more possible to



314

occur between different positions. Also, the insertion of iodine causes a shrink of

315

bandgap from 1.5 eV to 1.0 eV, which is in agreement with our experimental results.

316

The VB edge rises toward high energy level and the CB edge drops toward low

317

energy level. The band structure (Figure S13c, d) shows similar result with that of

318

DOS. The highest occupied state is set to 0 eV. Obvious discrete bands can be found

319

in iodine-free HTCC, while they become continuous after insertion of iodine. This

320

suggests excitation can occur at more positions. Also, the effective electron mass of

321

the I-doped HTCC (0.166 m0) is smaller than that of HTCC (0.353 m0) at the bottom

322

of the CB, suggesting higher charge transfer efficiency (Detail see Section S7 and

323

Figure S14 in SI).

324 325

Therefore, as discussed above, the bandgap of the HTCC should be due to the polyfuran structures

326

The charges around atoms are quantitatively calculated44 and listed in Table S2, In

327

the absence of iodine, O atoms accept electron from C and H atoms with 7.59e around,

328

as confirmed by the red region in parallel charge density map (Figure 5a, b). In the

329

presence of iodine, I atoms also accept electron from the furan chain of H and C

330

atoms with 7.14e around. It should be noted that the iodine would not reach 8.0 as that

331

reflected in the XPS (I-) due to the calculation method of Bader analysis, which is

332

similar with other report about iodine 45 and other materials.46

333

However, in the charge density map that is perpendicular to the polyfuran chain

334

(Figure 5c, d), an “empty” of electron can be observed between two polyfuran chains.

335

This suggests the electron transfer between the two polyfuran chains is difficult.

336

However, with iodine, an obvious electron cloud around the iodine can be found. The

337

electron can distribute on the polyfuran and on the iodine atom between them.

338

Therefore, electron can transfer between different polyfuran chains and can lead to

339

faster charge transfer. This can also be confirmed by the band-decomposed charge

340

density iossurface (Figure S15 and Section S8).

341

Besides, the enhanced charge transfer in the I-doped HTCC can be confirmed by its

342

higher conductivity measured by Electrochemical impedance spectroscopy (Figure

343

S16). Results of such experiment are fitted via an equivalent circuit Rs(RctCPE) and


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344

summarized in Table S3 (the insert in Figure S16). As shown in Figure S16 and Table

345

S3, charge transfer resistance of I-doped HTCC decreases compared with its

346

iodine-free counterpart both in the dark and under illumination.

347

Overall, according to the investigations above, we propose an enhanced mechanism

348

for the I-doped HTCC. After absorbing photons, photoexcited charge carriers are

349

generated in the sp2-hybridized structures (polyfuran chain), and they will transfer

350

from one polyfuran chain to another. In the absence of iodine, the charge carriers are

351

difficult to transfer among different polyfuran chains. Therefore, they recombine

352

rapidly, thus resulting in low activity of iodine-free HTCC (Figure 3). The insertion of

353

iodine twisted the structure of the HTCC, which increase the excitation possibility.

354

Also, iodine will facilitate charge transfer because they will serve as a charge transfer

355

bridge among different polyfuran chains (Figure 5). Therefore, more charge carriers

356

can transfer to the surface to generate more active species like OH radicals (Figure 2d)

357

and lead to higher activity of I-doped HTCC.

358 359

Preparation of I-doped HTCC using versatile sources of biomass. Carbohydrates

360

are a large family. Besides glucose, we also use other carbohydrate and

361

carbohydrate-containing biomass to prepare I-doped HTCC, including bi-saccharide

362

of sucrose, polysaccharide of starch, crops of rice and plants of grass (Figure 6).

363

Furthermore, we also attempt to use agricultural waste of straw and animal waste of

364

cow dung, which were traditionally burned as fuel and release green house and PM2.5

365

particles. They all show appreciable activity in disinfection and organic dye

366

degradation (Figure 3b, d). The disinfection curve of I-doped HTCC prepared with

367

glucose, sucrose and starch show a shoulder at the beginning, which means the

368

disinfection rate at that time slot is not very fast.12,

369

descending sequence of glucose>sucrose>starch. Therefore, the activity of the

370

I-doped HTCC enhances reversely with the polymerization of the raw materials. This

371

is because the carbohydrates with higher degree of polymerization is more difficult to

372

be converted into HTCC (especially the polyfuran), as supported by the NMR and

373

FT-IR spectra (Figure S17, Section S9).48

47

Their activity follows a

374

Besides, the I-doped HTCC prepared with rice and grass also have a different linear

375

kinetic mode of disinfection, which can be ascribed to their multi-components



Page 14 of 30

376

(carbohydrates, cellulose and fat) and their more complicated formation process of

377

HTCC.

378 379

Cytotoxic Effect of I-doped HTCC and HTCC. In this work, we test the cytotoxic

380

effect of I-doped HTCC on representative Human Umbilical Vein Endothelial Cells

381

(HUVEC) by using the generated-used WST-1 assay (Detail see Experimental section

382

and Section S10).49, 50 As shown in Figure 7, higher absorbance corresponds to higher

383

viability of the cell and lower cytotoxic effect of the sample. Obviously, both I-doped

384

HTCC and HTCC show very low cytotoxic effect at low concentration of 5 ugml-1.

385

The I-doped HTCC maintains the low cytotoxicity while the pure HTCC exhibits

386

some cytotoxicity at a high concentration of 200 ugml-1, which is the concentration of

387

the samples in our photocatalytic treatment. This suggests that the I-doped HTCC do

388

not has apparent cytotoxicity toward human body, which will be further studied in our

389

lab.

390 391

Environmental Implication. Herein, converting carbohydrates into effective

392

photocatalysts opens a new route for their applications in environmental treatment. As

393

they are dominant constituents in many agricultural waste and animal waste, this

394

transformation may become a “Trash to Treasure” strategy from the perspective of

395

environmental science. Comparing with burning, the transformation of agricultural

396

waste (e.g. straw) and animal waste (e.g. cow dung) into HTCC are beneficial to

397

reducing greenhouse gas release and PM2.5 emission (Figure S18). It is highly

398

possible that other carbohydrate based materials can be transformed by using similar

399

strategy. Also, introduction of halogen atoms into the spacing between the

400

sp2-hybridized structures are helpful for the charge transform inside the polymer. Thus,

401

using iodine as dopant may also provide a new strategy for the practical application of

402

other

403

environmental treatment. This discovery will attract a lot of attention from researchers

404

involved in environmental catalysis, waste recycling and pollution treatment.

405

Acknowledgements.

polymers

with

discrete

sp2-hybridized

structures


for

photocatalytic

Page 15 of 30


406

The work described in this paper was partially supported by the grants from the

407

Research Grants Council of the Hong Kong Special Administrative Region, China.

408

(Project No. T23-407/13-N and Project No. 14304315). This work was also supported

409

by the National Natural Science Foundation of China (Ref. No. 21303118) and a grant

410

from the Vice-Chancellor's One-off Discretionary Fund of The Chinese University of

411

Hong Kong (Project No. VCF2014016). The theoretical calculation is supported by

412

National supercomputer center in GuangZhou and National supercomputing center in

413

Shenzhen (Shenzhen cloud computing center).

414 415 416

ASSOCIATED CONTENT

417

* Supporting Information

418

Additional information about SAXS patterns, Nitrogen adsorption/desorption

419

isotherms, analysis of FT-IR spectra, Raman spectrum, analysis of C1s and O 1s XPS

420

spectra, depth profile XPS spectra of I from the I-doped HTCC, calculation of band

421

structure, calculation of carrier density, calculation of insertion energy of I doping into

422

HTCC, density of States (DOS), calculation of effective electron mass,

423

band-decomposed charge density iossurface, Nyquist plots and cytotoxic effect of

424

samples and other additional experiments and related details.

425 426

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427

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TiO2 nanoparticles for improved activity of CO2 photoreduction with water

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Visible light responsive iodine-doped TiO2 for photocatalytic reduction of CO2

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572

Nanosheets

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573

Heterojunction Photocatalyst for Bacterial Inactivation under Visible-Light.

574

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579

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584

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585

Part. Fibre. Toxicol. 2015, 12, 36.


Page 23 of 30


(g)

(h)

586 587

Figure 1. (a) A photo of I-doped HTCC, SEM images of (b) I-doped HTCC and (c)

588

pure HTCC, (d) XRD patterns, (e) FT-IR spectra and (f) 13C Solid-state NMR of

589

I-doped HTCC and pure HTCC. I 3d XPS spectra of (g) I-doped HTCC and (h) pure

590

HTCC.

591



(a)

0.8

I-doped HTCC HTCC

0.6

(ahv)

2

0.4

1.0 eV

0.2

1.3 eV 500

1000

1500

0.0 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0

2000

Wavelength / nm

hv / eV

(c)

(d)

2

0.0010

2500

2

Light off

0.0008

0.0006

0.00060 0.00055 0.00050 0.00045 0.00040

Light on

0.00035 0

0.0004

20

0.6

60

80

Light off

0.0002

0.0000

40

Time / s

Light on

Absorbance / a.u.

0.00065 Current density mA/cm

Current density mA/cm

(b)

2

/ eV nm

-2

Absorbance / a.u.

I-doped HTC carbon HTCC

Page 24 of 30

I-doped HTCC (6 h) I-doped HTCC (4 h) I-doped HTCC (2 h) HTCC (4 h) Blank

2000 1500 1000 500 0

0.8

1.0

Potential /V vs. RHE

1.2

350 400 450 500 550 600 650

Wavelength / nm

(e)

592 593

Figure 2. (a) UV-Vis diffuse-reflectance spectra and (b) Converted Kubelka–Munk

594

plot of HTCC and I-doped HTCC particles. (c) Anodic scan of iodine-doped HTCC,

595

insert is transient photocurrent measurement at 1.23 V vs. RHE under visible light

596

illumination. (d) Time-dependent Fluorescence spectra of 1×10-3 aqueous solution of

597

coumarin containing 5 mg I-doped HTCC under visible illumination (332 nm

598

excitation). (e) Band diagram of the I-doped HTCC and HTCC.


Page 25 of 30


(b)

(a)

7 6 5 4 3 2

HTCC dark control HTCC visible light I-doped HTCC dark control I-doped HTCC visible

1 0 -1

Cell density / cfu/mL


8

7 6 5 4 3 2 1 0 -1

599

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Time / h

(c)

(d) 1.0 0.8

0

0.6

C/C


0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Time / h

8 7 6 5 4 3 2 1 0 -1

In the dark I-doped HTCC (straw) I-doped HTCC (starch) I-doped HTCC (cow dung) I-doped HTCC (sucrose) I-doped HTCC (glucose) I-doped HTCC (sucrose) I-doped HTCC (grass) I-doped HTCC (rice)

with OH radical scavenger with hole scavenger with electron scavenger without scavenger

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Time / h

No catalyst HTCC without I doped I-doped HTCC(cow dung) I-doped HTCC(starch) I-doped HTCC(straw) I-doped HTCC(sucrose) I-doped HTCC (grass)

0.4 0.2 0.0

0

50

100

I-doped HTCC(rice) I-doped HTCC (glucose)

150

200

250

Time / min

600

Figure 3. (a) Disinfection of E. coli K-12 by HTCC prepared from glucose without

601

and with iodine doped. (b) Disinfection of E. coli K-12 by I-doped HTCC prepared

602

from different kinds of carbohydrates. (c) Disinfection of E. coli K-12 by I-doped

603

HTCC prepared from glucose in the presence of different kinds of scavengers. (d)

604

Temporal course of photodegradation of RhB on iodine-doped HTCC prepared with

605

different kinds of carbohydrates under visible light irradiation.

606



(a)

(b)

(d)

(c)

607 608

Figure 4. (a) Side view and (c) front view of iodine free HTCC. (b) side view and (d)

609

front view of iodine free I doped HTCC.

610


Page 26 of 30

Page 27 of 30


(a)

(b)

(c)

(d)

611 612

Figure 5. Charge density map of plane parallel with polyfuran chains of (a) HTCC

613

and (b) I-doped HTCC and perpendicular to these (c) HTCC and (d) I-doped HTCC.

614



615 616

Figure 6. Different kinds of carbohydrate based materials and their corresponding

617

I-doped HTCC products.

618


Page 28 of 30

Page 29 of 30


Absorbtion a.u.

2.0

low sample concentration High sample concentration -1 -1 (5 ugml ) (200 ugml )

1.5

1.0

0.5 Blank

0.0 Blank Control

619

I-doped HTCC Control I-doped HTCC HTCC HTCC

620

Figure 7. The cytotoxic effects of HTCC and I-doped HTCC on Human Umbilical

621

Vein Endothelial Cells (HUVEC).

622



623

TOC (Graphical Abstract)

624


Page 30 of 30

Converting Carbohydrates to Carbon-based Photocatalysts for

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