lignin-derived flower-like carbon composite with excellent photocatalytic activity and recyclability

Journal Pre-proof Fabricating ZnO/lignin-derived flower-like carbon composite with excellent photocatalytic activity and recyclability Binpeng Zhang, Dongjie Yang, Xueqing Qiu, Yong Qian, Huan Wang, Conghua Yi, Dongqiao Zhang PII:

S0008-6223(20)30184-6

DOI:

https://doi.org/10.1016/j.carbon.2020.02.038

Reference:

CARBON 15088

To appear in:

Carbon

Received Date: 29 November 2019 Revised Date:

2 February 2020

Accepted Date: 14 February 2020

Please cite this article as: B. Zhang, D. Yang, X. Qiu, Y. Qian, H. Wang, C. Yi, D. Zhang, Fabricating ZnO/lignin-derived flower-like carbon composite with excellent photocatalytic activity and recyclability, Carbon (2020), doi: https://doi.org/10.1016/j.carbon.2020.02.038. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Graphical Abstract

1

Fabricating ZnO/lignin-derived flower-like carbon composite with excellent

2

photocatalytic activity and recyclability

3

Binpeng Zhanga, Dongjie Yanga,b,∗, Xueqing Qiua,c,∗, Yong Qiana,c, Huan Wanga,

4

Conghua Yia, Dongqiao Zhanga

5

a

6

Research Center for Green Fine Chemicals, South China University of Technology,

7

381Wushan Road, Tianhe District, Guangzhou, 510641, China

8

b

9

Tianhe District, Guangzhou, 510641, China

School of Chemistry and Chemical Engineering, Guangdong Provincial Engineering

Key Laboratory of Fuel Cell Technology of Guangdong Province, 381Wushan Road,

10

c

11

Technology, 381Wushan Road, Tianhe District, Guangzhou, 510641, China

State Key Laboratory of Pulp and Paper Engineering, South China University of

12 13

Abstract

14

The application of ZnO nanoparticles as photocatalyst is significantly hampered by

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limited absorption range for solar spectrum, fast recombination of photogenerated carriers

16

and poor recyclability. The modification with carbon structures has attracted attention as

17

their advantageous performance in photocatalysis. Herein, we first report a lignin-derived

18

flower-like carbon (LFC), which is used to modify ZnO. The resulting composite

19

(ZnO/LFC) is composed of ZnO nanoparticles (~10 nm) embedded on a flower-like

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carbon consisting of two-dimensional corrugated nanosheets. Especially, LFC exhibits

21

stable three-dimensional structure and rich oxygen-containing functional groups. So ZnO

22

can be uniformly anchored on the LFC. Composite presents an extended optical ∗ Corresponding author. Tel: 86-20-87114722. Fax: +86-20-87114721. E-mail: [email protected] (D. Yang), [email protected] (X. Qiu). 1

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absorption and enhanced separation of photogenerated carriers due to the interface

24

electronic interaction between ZnO and LFC. The hierarchical flower-like structure

25

facilitates fast substance transfer and high light-harvesting efficiency. Therefore,

26

ZnO/LFC presented an excellent photocatalytic activity toward degradation of

27

sulfamethazine and hydrogen evolution, which were about 3.0 and 2.1 times that of pure

28

ZnO, respectively. Moreover, the recyclability of composite photocatalyst was also

29

significantly better than pure ZnO. This work not only provides a facile, low-cost and

30

scalable strategy to promote practical application of photocatalyst but also opens new

31

path toward the high-value utilization of industrial lignin.

32

Keywords: Photocatalysis; Zinc oxide; Lignin; Flower-like carbon; Composite

33

photocatalyst

34 35

1. Introduction

36

Photocatalysis technology is an effective approach to address the current environmental

37

pollution and energy crisis over the world, which degrades the toxic pollutants completely

38

and produces hydrogen (H2) from water splitting using solar energy [1-3]. Although many

39

novel photocatalysts have been prepared to provide convincing prospects for the practical

40

application in these related fields, ZnO is believed to be an excellent material for

41

wide-scale use because of its intrinsic features such as affordability, accessibility, and

42

nontoxicity [4, 5]. However, ZnO only utilizes UV light accounted for ~4% of sunlight,

43

and the charge carriers are easily recombined, which results in low photocatalytic

44

efficiency. Moreover, ZnO nanoparticles occur photocorrosion and agglomeration easily 2

45

during photocatalytic process, leading to poor recyclability [6]. All of the above

46

drawbacks limit the widespread application of ZnO as photocatalyst in energy and

47

environment fields.

48

Various strategies have been used to modify ZnO for improving the photocatalytic

49

performance. One of the most effective methods is coupling the photocatalyst with other

50

materials such as noble metals [7, 8], semiconductors [9, 10], carbon-based materials [11],

51

etc. Carbon, existing widely in the form of compound and simple substance in nature, is

52

typically applied to the modification of ZnO due to its characteristics of high chemical

53

and thermal stability, nontoxicity and abundance [12, 13]. Many carbon-based materials

54

such as graphene (GR), carbon nanotube (CNT) and graphite-like carbon present superior

55

electron conductivity and mobility due to abundant sp2-hybridized carbon atoms.

56

Therefore, they are often used to couple semiconductor photocatalyst as electron-transfer

57

bridges for promoting the separation of photogenerated electron-hole pairs [14, 15].

58

Moreover, the interface electronic interaction between photocatalyst and carbon-based

59

material can extend the range of optical absorption [16]. GR, as a two-dimensional (2D)

60

carbon-based material, is the most efficient for improving photocatalytic performance of

61

photocatalyst compared with other carbon-based materials because of excellent

62

adsorption property, high specific surface area and good transparency [17]. However, the

63

use of GR leads to high cost. Moreover, the GR need be oxidized firstly to improve its

64

dispersibility, and then reduced during the preparation of composite. 2D structure tend to

65

restack or aggregate through van der Waals interaction, and photocatalyst can hardly be

66

supported on GR uniformly, which leads to the limited effect of modification [18]. At the 3

67

same time, this 2D structure is unfavourable to the recyclability of catalyst. Above issues

68

limit the wide application of ZnO/GR composite photocatalyst.

69

Lignin, as the second most abundant biopolymer after cellulose in the nature, is a

70

renewable organic resource [19, 20]. Our early studies indicate lignin is composed of

71

massive aromatic skeleton, and carbon content can reach up to 60%. So lignin has a

72

great potential to prepare highly graphitized carbon-based material. In addition, there

73

are a variety of active functional groups in lignin, which presents a good flexibility

74

and designability as carbon precursor [21-24]. Industrial lignin is mainly derived from

75

pulping and papermaking and biomass refining process, which is produced more than

76

50 million tons per year [25, 26]. With the rapid development of cellulosic bioethanol

77

industry, enzymatic hydrolysis lignin (EHL) as a by-product in this process accounts

78

for more and more proportion of industrial lignin. Inexpensive and abundant EHL is

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an ideal carbon precursor for preparing highly graphitized carbon material with

80

special morphology.

81

Three-dimensional (3D) structure exhibits good stability compared with other

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nanostructure [27]. In recent years, 3D flower-like structure has drawn wide attention.

83

This structure assembled with nanosheets not only retains the advantages of 2D structure,

84

but also avoids stacking or agglomeration of nanosheets. In addition, the hierarchical

85

flower-like structure can facilitate fast substance transfer and multiple light reflections

86

[28]. Therefore, this structure can endow catalyst excellent photocatalytic activity and

87

recyclability. Liu et al. [29] compared 3D (MoS2)/2D (g-C3N4), 2D (MoS2)/2D (g-C3N4)

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and 0D (MoS2)/2D (g-C3N4), and found 3D (MoS2)/2D (g-C3N4) with flower-like 4

89

structure exhibited the highest photocatalytic H2 evolution rate and best recyclability.

90

According to these premises, carbon-based material with flower-like structure is

91

prepared using EHL initially, and then coupled with ZnO nanoparticles to obtain

92

composite photocatalyst, which is a more low-cost and effective method to improve the

93

shortages of ZnO. In order to get this desired material, preparation strategies should be

94

carefully considered. Generally, the carbon matrix with flower-like structural can be

95

fabricated using templates or structure-directing agent [30, 31]. Liang et al. [32] reported

96

a flower-like hierarchical carbon material prepared through hydrothermal reaction and

97

calcination by Ni(OH)2 as a structure inducer and glucose as a carbon precursor. But the

98

preparation process is complex, and the degree of disorder of carbon material is high.

99

Evaporation-induced self-assembly (EISA) is widely recognized as a simple and

100

powerful method for fabricating dissipative 1D, 2D or porous structure with controlled

101

dimensions [33, 34]. Precursors can self-assemble into desired morphology through the

102

organic or inorganic structure-directing agent during the solvent evaporation, while

103

effectively increasing the order degree. So EISA and carbonization are promising to

104

prepare ordered carbon material with flower-like morphology. Subsequently, ZnO in situ

105

grows on the surface of carbon material to obtain composite photocatalyst by

106

solvothermal method [35]. This two-step approach also could avoid carbon coating

107

that results in ineffective light absorption [36].

108

In this work, we firstly synthetized a lignin-based flower-like carbon (LFC)

109

assembled with 2D corrugated carbon nanosheets with Mg(OH)2 as a structure inducer

110

based on a facile EISA and carbonization process. The LFC presented high specific 5

111

surface area and a large number of oxygen-containing functional groups, which is

112

beneficial to anchor ZnO nanoparticles. To the best of our knowledge, it is the first

113

report about synthesizing lignin-based carbon (LC) with flower-like structure. After

114

coupling with LFC through solvothermal process, ZnO was supported uniformly on

115

the carbon nanosheets. Composite (ZnO/LFC) photocatalyst exhibited extended light

116

absorption and fast separation of photogenerated electron-hole pairs because of the

117

interface electronic interaction between ZnO and LFC. In addition, ZnO/LFC still

118

maintained the hierarchical flower-like structure, which is good for mass transfer and

119

light-harvesting. Hence, the composite showed much better photocatalytic efficiency

120

and recyclability for photodegradation of sulfamethazine and photocatalytic hydrogen

121

production compared with pure ZnO. This simple strategy could provide great

122

opportunities in developing low-cost and highly-active composite photocatalysts for

123

practical application in energy and environment fields.

124

2. Experiment

125

2.1. Chemicals

126

Enzymatic hydrolysis lignin (EHL) was obtained from the corn stalks bio-refinery

127

residue provided by Shandong Longlive Bio-Technology Co., Ltd. (Shandong

128

Province, China). The detail information of EHL was shown in Table S1. Nanometer

129

MgO with sheet structure was purchased from Aladdin, and its SEM pictures were

130

observed from Fig. S1. Graphene was obtained from Suzhou Tanfeng Graphene

131

Technology Co., Ltd. (Jiangsu Province, China). Analytically pure sulfamethazine

132

(SMT) was purchased from Aladdin. Zinc acetate dihydrate (Zn(CH3COO)2·2H2O), 6

133

potassium hydroxide (KOH), triethanolamine, ethylenediaminetetraacetic acid

134

disodium salt dehydrate (EDTA-2Na), p-benzoquinone (BQ) and tert-butyl alcohol

135

(TBA) were all purchased from Alfa Aesar. All chemicals were used without further

136

treatment.

137

2.2. Synthesis of the LFC

138

EHL and MgO with a certain proportion were dispersed in 40 mL of pure water under

139

vigorous stirring for 30 min. The mixture was subsequently kept stirring under certain

140

temperature to evaporate the water. The different mass ration of EHL to MgO (2:0.5, 2:1,

141

2:1.5), and evaporation temperature (80 °C, 100 °C, 120 °C) were studied for controlling

142

the morphology of composite. Dried sample was ground and transferred into a tube

143

furnace for calcination, which was carried out at 600 °C (heating rate of 5 °C/min) under

144

N2 flow for 2 h. Then the obtained sample was added into dilute hydrochloric acid

145

solution (1 mol/L) and kept stirring for 1 h to remove the templates. The LFC was

146

collected by filter after washing with pure water thoroughly. In addition, lignin-based

147

block-like carbon (LBC) was also prepared through the same procedures without

148

addition of MgO for comparison.

149

2.3. Synthesis of the ZnO/LFC

150

Briefly, Zn(CH3COO)2·2H2O (0.277 g) was dissolved in 25 mL of methanol,

151

followed by the addition of the prepared LFC. The mixture was stirred constantly at

152

60 °C for 1 h, then added dropwise with 15 mL of KOH methanol solution (0.17

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mmol/mL) and kept stirring at same temperature for another 3 h, followed by

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transferring into a 100 mL Teflon-lined stainless-steel autoclave and heating at 180 7

155

°C for 12 h. The resulting composite was collected through centrifugation, washed

156

thoroughly with ethanol and dried at 80 °C for 24 h. The ZnO/LFC composite was

157

obtained after annealing process (N2 atmosphere, 600 °C, 2 h). By adjusting the amount

158

of LFC, the composites with various carbon contents were obtained. Composite samples

159

are hereafter referred to as ZnO/LFC-X, with X indicating the addition amount of LFC.

160

In addition, the pure ZnO, ZnO/LC and ZnO/GR were also prepared using the same

161

method for comparative experiments.

162

2.4. Characterization

163

The morphologies of the samples were observed by scan electron microscope (SEM,

164

Merlin of Zeiss) and high-resolution transmission electron microscopy (TEM, JEOL

165

JEM-2100F, 200 kv). In addition, energy dispersive X-ray (EDX) spectrum and element

166

mapping pictures were also recorded. The thickness of the nanosheets was further

167

confirmed by atomic force microscopy (AFM, XE-100, Park Systems, Korea). The

168

crystal structures of obtained samples were recorded by X-ray diffraction (XRD, D8

169

Advance, Bruker, Germany). Automated surface area and pore size analyzer (model

170

Tristar II 3020, USA) was used to measure the specific surface area and pores

171

distribution of the lignin-derived carbon through the Brunauer–Emmett–Teller (BET)

172

method. X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher,

173

USA) was used to analyze the chemical state of elements in the samples. Raman test

174

was carried out on a raman spectrometer (model LabR AMA Ramis, France).

175

Thermogravimetric analysis (TGA) was performed by a thermal analyzer (model

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STA449C, Germany) under air atmosphere from 25 to 800 °C. The UV-vis diffuse 8

177

reflectance spectra (DRS) of as-prepared materials were acquired with a UV–vis

178

spectrophotometer (UV-2600, Shimadzu, Japan), and photoluminescence (PL)

179

emission spectra were measured using a fluorescence spectrometer (HITACHI, Japan).

180

Electron spin resonance (ESR) spectra were recorded through a JEPL mode

181

JES-FA200 spectrometer.

182

2.5. Photoelectrochemical measurements

183

The

photocurrent

and

electrochemical

impedance

spectroscopy

(EIS)

184

measurements were detected on the CHI 660E electrochemical workstation. A typical

185

three-electrode cell with Pt gauze and saturated calomel electrode as the counter and

186

reference electrode respectively was used. Meanwhile, 0.1 M Na2SO4 solution was

187

used as the electrolyte. Generally, working electrode was obtained through following

188

methods. 3 mg of sample was suspended in 3 mL ethanol to get the slurry, and then

189

spread onto indium-tin oxide glass electrode. After drying at 60 °C for 12 h, the

190

working electrode was prepared.

191

2.6. Evaluation of photocatalytic activities

192

The photocatalytic activities of the obtained materials were evaluated through the

193

photodegradation of sulfamethazine (SMT) and photocatalytic hydrogen production

194

from water splitting. For SMT photodegradation experiment, 20 mg of obtained

195

material was added into 50 mL SMT aqueous solution (20 mg/L), which was then

196

transferred

197

Zhongjiaojinyuan Co. Ltd, China). The mixture solution was stirred in the dark for 0.5 h

198

to achieve adsorption equilibrium. Subsequently, the photocatalytic degradation of SMT

into

photochemical

reaction

9

instrument

(CEL-LAB500E,

Beijing

199

was carried out with a 500 W Xe lamp as light source for simulating sunlight. At given

200

time intervals, 1 mL of mixture solution was taken out for analysis. The concentration of

201

SMT in the supernatants was measured by high performance liquid chromatography

202

(HPLC, Agilent ZORBAX, Agilent, USA) with isocratic elution of 0.1% formic

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acid/methanol (65/35, v/v), and the wavelength of the UV detector was 262 nm. The final

204

degradation percentage of SMT in aqueous solution was calculated according to the

205

decrease in the final (C) and initial (C0) concentrations. In addition, the intermediate

206

products were analyzed by high performance liquid chromatography mass spectrometry

207

(LC-MS, Agilent 6400, Triple Quad LC/MS, USA) with a C18 column (25 mm×2.1

208

mm×5 mm). For photocatalytic hydrogen production experiment, the 50 mg of

209

photocatalyst was added into water (180 mL) and triethanolamine (20 mL) mixed

210

solution, and then transferred into Pyrex reaction cell. In addition, Pt (1 wt%) was used

211

as co-catalyst through directly dissolving H2PtCl6 into above solution. Photocatalytic

212

reaction was carried out with 300 W Xe lamp for simulating sunlight. The mixed

213

solution was degassed with Ar to remove the dissolved oxygen and maintain

214

oxygen-deprived conditions. The produced H2 was analyzed through an online gas

215

chromatograph (GC, TECHCOMP, GC-7900).

216

3. Results and discussion

217

3.1. Modification strategy of ZnO nanoparticles

10

218 219

Fig. 1 Schematic illustration of the synthetic process of the ZnO/LFC composite photocatalyst.

220

The modification scheme of ZnO nanoparticles was briefly shown in Fig. 1. Firstly,

221

EHL and MgO were dispersed in aqueous solution, which was then heated for water

222

evaporation. The flower-like EHL/Mg(OH)2 composite assembled with composite

223

nanosheets was prepared through EISA process. Subsequently, the EHL/Mg(OH)2

224

composite was carbonized at 600 °C for 2 h, and then was washed using dilute

225

hydrochloric acid to obtain LFC with abundant carbon nanosheets. Finally, ZnO was

226

anchored onto carbon nanosheets by in-situ growth based on solvothermal process, and

227

ZnO/LFC composite photocatalyst was obtained.

228

3.1.1. Formation process and characterization of LFC

229

The synthesis of LFC is the key to modifying ZnO nanoparticles. So the detailed

230

formation process of LFC was thoroughly analyzed. As shown in Fig. 2a, EHL

231

aggregated together when the mixtures of MgO and EHL were added in water due to the

232

neutral surrounding. The flaky MgO was dispersed around EHL. The hydration reaction

233

of MgO occurred, and Mg(OH)2 with sheet structure was obtained after heating, which

234

resulted in the increase of pH in mixture solution [37, 38]. Thus, EHL was swelled, and

235

Mg(OH)2 was diffused into the three-dimension net structure of EHL. Moreover, there

236

were a large number of hydroxyl groups on the surface of Mg(OH)2 nanosheets and in

237

EHL [39, 40]. Therefore, EHL and Mg(OH)2 were coupled together through strong 11

238

hydrogen-bonding interaction. The two-dimensional composite nanosheets were formed

239

with the evaporation of water under heating. Eventually, the composite nanosheets

240

assembled together and formed flower-like structure after the removal of water, as shown

241

in Fig. 2c. There were a large number of interleaving nanosheets on the surface of

242

composite, which could be observed by the further enlargement. From Fig. 2b, the XRD

243

pattern of the composite revealed the appearance of characteristic diffraction peaks

244

belonging to Mg(OH)2 (JCPDS 86-0441), which indicated MgO was converted into

245

Mg(OH)2 after the evaporation of water. Then EHL/Mg(OH)2 composite was calcined

246

under inert gas atmosphere, and carbon nanosheets was formed after the carbonization of

247

EHL. As shown in Fig. 2d, the composite remained flower-like morphology with large

248

amount of nanosheets after calcination. The XRD pattern of composite after calcination

249

(Fig. 2b) was readily indexed to the MgO (JCPDS 75-0447), indicating that Mg(OH)2

250

was transformed into MgO again. Mg(OH)2 was decomposed into MgO and H2O in this

251

process, which could protect the oxygen-containing functional groups of EHL from

252

excessive pyrolysis. Consequently, the obtained LFC could retain large quantities of

253

oxygen-containing functional groups, which was beneficial to the following loading of

254

ZnO. The structure of LC/MgO composite was further investigated through TEM (Fig. 2e

255

and 2f). Many layered structures were observed in the curled sheets, which indicated that

256

sheet structure of LC/MgO composite was formed through the stacking of nanosheets. In

257

the small-angle XRD pattern (inset of Fig. 2f), the peak at 2θ=1.98° was determined,

258

which also indicated the existence of layered structures in LC/MgO composite. The

259

interlayer distances of MgO was about 4.4 nm according to Bragg equation. In addition, 12

260

elemental line scans of curled surface indicated sheet structure was consisted of MgO and

261

LC, and LC was between MgO nanosheets (Fig. 2g and 2h). These supported above

262

inference about the formation mechanism of carbon nanosheets. EHL was sandwiched

263

between Mg(OH)2 sheets during self-assembly process, and the carbon nanosheets was

264

prepared after carbonization. The EDX elemental maps of LC/MgO composite revealed

265

that the Mg and C were uniformly distributed (Fig. S2), suggesting the uniform

266

self-assembly of EHL and Mg(OH)2. After the removal of MgO, LC with flower-like

267

structure was obtained. Throughout the whole process, Mg(OH)2 ingeniously played

268

multifunctional roles: pH regulator, the inducer of flower-like hierarchical structure and

269

the protective agent of oxygen functional group.

270 271 272 273 274

Fig. 2 (a) The detail formation process of flower-like structure in LFC; (b) XRD patterns of mixture before and after

275

Importantly, the morphology of carbon-based material has great effect on its

calcination; (c) and (d) SEM images of the EHL/Mg(OH)2 and LC/MgO, respectively; the inset in (c) and (d) was the further enlargement of corresponding regions with white lines; (e-f) TEM images of LC/MgO; the inset in (f) was the small-angle XRD pattern; (h) elemental profiles of curved section of LC/MgO corresponding to (g).

13

276

performance. The mass ratios of EHL to MgO and heating temperature were the main

277

factors influencing the morphology of LC, which was investigated following. In particular,

278

the desired morphology was obtained with the mass ratio of 2:1 for EHL to MgO at 100

279

°C (2:1-100). As shown in Fig. S3, Fig. 3a and 3b, the LFC with the size about several

280

microns presented a flower-like structure overall, and the magnified observation further

281

exhibited that the flower-like structure was assembled with ultrathin porous nanosheets.

282

From the TEM pictures (Fig. 3c), the inner structural of LFC was observed further. LFC

283

had the loose core around with a large number of corrugated nanosheets. A

284

high-magnification observation (Fig. 3d) on edge region indicated there were many pores

285

in carbon nanosheets, which was well consistent with the morphology of the SEM

286

pictures. The carbon nanosheets on the surface of LFC were measured through atomic

287

force microscopy (AFM) analysis (Fig. S4). The results showed that the thickness of

288

carbon nanosheets was about 4 nm. Besides, undesired morphologies of LC obtained with

289

other mass ratios of EHL to MgO and heating temperature (2:0.5-100, 2:1.5-100, 2:1-80

290

and 2:1-120) were observed in the SEM and TEM pictures (Fig. S5 and S6), respectively.

291

At low temperature, the slow formation of Mg(OH)2 led to low pH value for mixture

292

solution before complete evaporation. Thus, the swelling of EHL was not obvious, and

293

the templates were attached on the surface of EHL, which only produced some folds on

294

the surface of LC. On the contrary, at high temperature, fast hydration of MgO induced

295

high pH value of solution and aggregation for Mg(OH)2, ultimately producing thick

296

carbon nanosheets. Low addition of MgO had similar effect and reason as low

297

temperature. With high addition of MgO, the intense formation of Mg(OH)2 caused thin 14

298

carbon nanosheets, which was easily destroyed during calcination.

299 300 301

Fig. 3 (a-b) SEM images and (c-d) TEM images of LFC; (e) Raman spectra of LBC and LFC; (f) XPS spectrum of C 1s

302

The LC with desired morphology was further investigated. From the Raman spectra

303

(Fig. 3e), the typical D peaks (1328 cm-1) and G peaks (1580 cm-1) were observed from

304

LFC and LBC, which ascribed to disordered carbon (sp3) and graphite carbon (sp2). The

305

intensity ratio of ID/IG for LFC was much lower than that for LBC, indicating more

306

graphite carbon in LFC. In addition, the 2D peak (2670 cm-1) of LFC was higher than that

307

of LBC, showing that the crystallinity degree of LFC was higher. Mg(OH)2 as a

308

structure-directing agent could improve the lignin’s disordered structure during the

309

evaporation of water, and the LFC with high graphitization degree and crystallinity

310

degree was obtained after carbonization. This ordered sp2-hybridized carbon structure

311

with less amorphous content was beneficial to the shuttle electrons [7]. So LFC has a

312

great promise for promoting the separation of photogenerated electron-hole pairs in ZnO.

313

From the typical XPS survey spectrum of LFC (Fig. S7), C and O were observed instead

314

of Mg, indicating the complete removal of structure-directing agent. The C 1s high

315

resolution spectrum of LFC (Fig. 3f) presented five peaks at 284.6, 285.5, 286.4, 288.6

316

and 288.9, corresponding to the sp2-bonded carbon, sp3-bonded carbon, C-O, C=O and

for the LFC; (g) N2 adsorption-desorption isotherms and (h) DFT pore size distribution of LBC and LFC.

15

317

O=C-O, respectively. The high intensity referred to sp2-bonded carbon indicated the high

318

degree of graphitization, which was consistent with the result of Raman. Moreover, the

319

content of oxygen reached 14.2% based on the analysis of XPS, illustrating the abundant

320

oxygen-containing functional groups in LFC. H2O from thermal decomposition of

321

template agent Mg(OH)2 could protect the oxygen-containing functional groups from

322

excessive pyrolysis. LFC containing abundant oxygen-containing functional groups was

323

easy to bind Zn2+, making ZnO nanoparticles well dispersed in carbon nanosheets with

324

intimate contact. The N2 adsorption-desorption isotherms were used to further investigate

325

the specific surface area and pore structure of prepared materials. As shown in Fig. 3g,

326

the isotherm for LFC presented a combined type I/IV pattern with a narrow H3 hysteretic

327

loop in the range of 0.5-1.0 P/P0, indicating the coexistence of micropores, mesopores and

328

macropores. The N2 adsorption-desorption capacity of LBC was very low, which showed

329

there was little pore structure. The BET surface area of LFC and LBC were 827 and 18

330

m2 g-1, respectively. Mg(OH)2 as a structure-directing agent could greatly increase

331

specific surface area of LFC, endowing LFC more active sites for coupling with ZnO.

332

From Fig. 3h, DFT pore size distribution directly indicated the hierarchical structure of

333

LFC, which was favorable to the mass transfer and multiple light reflections.

334

3.1.2. Formation process and characterization of ZnO/LFC

335

Subsequently, the LFC was used to modify ZnO. ZnO nanoparticles were coated on the

336

carbon nanosheets of LFC to form composite photocatalyst with in-situ growth method

337

based on solvent-thermal process. First of all, Zn2+ was tightly adsorbed on the surface of

338

LFC through chemical bonds in methanol under heating. Then Zn(OH)2 crystal nuclei 16

339

was gradually formed with the addition of KOH methanol solution. Finally, ZnO was

340

prepared and anchored in the carbon nanosheets of LFC during solvent-thermal process.

341

LFC itself has no photocatalytic activity, which is introduced as co-catalyst for improving

342

the photocatalytic performance of ZnO nanoparticles. The content of LFC in composite

343

can influence the photocatalytic performance. By adjusting the addition amount of LFC

344

(1%, 4%, 7% and 10%), the composite with various LFC content can be obtained. As a

345

typical sample, the composite with 4% LFC (ZnO/LFC-4%) was selected as

346

representative for further characterization. The actual content of LFC in composite was

347

about 6% by the thermal gravimetric analysis (Fig. S8). As shown in SEM images (Fig.

348

4a and 4b), the composite still presented a flower-like structure, and a large number of

349

ZnO nanoparticles were adhered to the carbon nanosheets. The TEM images of

350

composite (Fig. 4c) also confirmed the retaining of flower-like morphology with much

351

corrugated nanosheets. A high-magnification observation (Fig. 4d) on the edge of

352

composite showed that ZnO nanoparticles with a diameter of ~10 nm were well dispersed

353

in carbon nanosheets. Moreover, the d-spacing of lattice fringe was determined to be

354

0.281 nm through high-resolution TEM image (Fig. 4f), corresponding to the (100) plane

355

of ZnO. The uniform dispersion of elemental C, O, and Zn from EDX mapping images

356

(Fig. 4e) also confirmed the uniform distribution of ZnO nanoparticles in the carbon

357

nanosheets.

17

358 359

Fig. 4 (a-b) SEM images, (c-d) TEM images, (e) EDX mapping images and (f) HRTEM image of ZnO/LFC-4%.

360

The XRD patterns of LFC, ZnO and ZnO/LFC-4% were shown in Fig. 5a. There was

361

only a broad diffraction peak belonging to carbon structure in LFC, indicating the

362

removal of the structure-directing agent. The present diffraction peaks in ZnO and

363

composite patterns showed the existence of the wurtzite-type ZnO (JCPDS: 36-1451),

364

which illustrated that the crystal structure of ZnO was not influenced after coupling with

365

LFC. As shown in Fig. S9, there were no peaks belonging to other elements except for Zn,

366

O and C in XPS survey spectrum of ZnO/LFC-4%, suggesting the high purity of

367

as-prepared sample. In addition, the weak C 1s peak was observed in pure ZnO, which

368

was due to the pollution by carbon from air. From Fig. 5b, the O 1s spectrum of

369

ZnO/LFC-4% composite could be split into three peaks at 530.4, 531.7 and 532.6 eV,

370

which were in agreement with O-2 ions of Zn-O bonds, oxygen vacancies (Ovac)/Zn-O-C

371

bonds and C-O/C=O bonds, respectively. From Fig. 5c, Zn 2p spectrum of pure ZnO

372

presented two peaks at 1021.8 (Zn 2p3/2) and 1044.8 eV (Zn 2p1/2) with an energy

373

difference of 23.0 eV. Remarkably, the peaks of Zn 2p3/2 and Zn 2p1/2 for ZnO/LFC-4%

374

composite red-shifted compared with the pure ZnO, which indicated that electron transfer

375

occurred within ZnO/LFC composite, and a strong interaction was existed between LFC

376

and ZnO. The intimate contact could efficiently transfer the photogenerated electrons 18

377

from ZnO to carbon nanosheets [41].

378 379 380

Fig. 5 (a) XRD spectra of pure ZnO, ZnO/LFC-4% and LFC; (b) XPS spectrum of O 1s for ZnO/LFC-4%; (c) XPS

381

3.2. Photocatalytic properties

382

3.2.1. Photocatalytic degradation of SMT

spectra of Zn 2p for pure ZnO and ZnO/LFC-4%.

383

In order to test the photocatalytic activity of prepared photocatalysts, the

384

photodegradation of SMT was investigated. Firstly, we compared the effect of

385

modification by LC samples prepared with different conditions and GR. The composite

386

samples are prepared with the same addition amount of carbon-based material (4%). As

387

shown in Fig. S10, LC(2:1-100) presented the best effect. This result indicated LC with

388

optical flower-like morphology (LFC) is the most suitable carbon-based material for

389

modifying ZnO. Then the photocatalytic efficiency of ZnO/LFC composites with

390

different LFC content was investigated. As shown in Fig. 6a, ZnO/LFC-4% presented the

391

highest efficiency. More than 95% of SMT could be degraded after 3 h irradiation for

392

ZnO/LFC-4% under simulated sunlight, which was much higher than 63% for pure ZnO.

393

The photodegradation rate of SMT based on ZnO/LFC-4% was about 3.0 times that of

394

pure ZnO (Fig. 6b). Moreover, the photocatalytic activity of ZnO/LFC-4% for SMT was

395

also higher than other ZnO-based materials [42] or novel photocatalyst [43, 44] on the

396

same test condition. Impressively, ZnO/LFC-4% composite also presented excellent 19

397

recyclability in 5 consecutive cycles (Fig. 6c). But the degradation efficiency of pure ZnO

398

decreased significantly with cycle number for the photocorrosion and agglomeration

399

behavior [45]. For ZnO/LFC composite, the photocorrosion and agglomeration of ZnO

400

nanoparticles were efficiently inhibited by the hybridization with the LFC matrix

401

substrate, which was similar to the findings by Xu [46]. The HPLC spectra of SMT

402

before and after photocatalytic degradation were shown in Fig. 6d. The peak (3.7 min)

403

corresponding to SMT decreased obviously with increasing illumination time. The peaks

404

in 1.5~3.5 min range were also observed after illumination, which indicated the existence

405

of some reaction intermediates. This result showed SMT firstly degraded into

406

intermediate products and then further decomposed into H2O and CO2 during

407

photocatalytic reaction. LC-MS was used to identify the intermediates as shown in Fig.

408

S11, which mainly included N-(4,6-dimethylpyrimidin-2-yl) benzene-1,4-diamine

409

(m/z=215,

410

4,6-dimethylpyrimidin-2-amine (m/z =124, C). Therefore, a reasonable degradation route

411

was proposed to account for the intermediates observed with ZnO/LFC composite as

412

photocatalyst (Fig. 6e). First, A was formed through the elimination of SO2 in SMT. Then,

413

the C-N bond and benzene ring were destroyed by active species to form substance B and

414

C. Finally, intermediate products B and C thoroughly decomposed into nonpoisonous

415

innocent H2O and CO2 with the further oxidation of active species.

A),

4-aminocyclohexan-1-ol

20

(m/z

=115,

B),

and

416 417 418 419 420

Fig. 6 (a) The photocatalytic degradation of SMT plots and (b) kinetic rate plots with pure ZnO and ZnO/LFC composite; (c) Stability test of pure ZnO and ZnO/LFC-4% composite for SMT degradation in five consecutive cycles; (d) HPLC spectra of SMT after photocatalytic degradation; (e) The degradation pathway of SMT.

3.2.2. Photocatalytic hydrogen production

421

H2 evolution experiment was also used to test the photocatalytic activity of prepared

422

photocatalysts. Similarly, the effect of modification by LC samples and GR was

423

compared firstly. As the Fig. S12 shows, LC(2:1-100) presented the best effect. This

424

result was similar with the photocatalytic degradation experiment. Subsequently, the

425

photocatalytic efficiency of ZnO/LFC composites with different LFC content was

426

measured. As shown in Fig. 7a, the ZnO/LFC-4% composite presented best performance.

427

H2 production rate of ZnO/LFC-4% is 29.0 µmol h-1, which is about 2.1 times that of pure

428

ZnO (14.1 µmol h-1). Additionally, the hydrogen evolution rate of composite

429

photocatalyst was superior to that of other ZnO-based photocatalysts [47-49]. Apart from

430

the excellent photocatalytic H2 evolution performance, ZnO/LFC-4% composite also

431

exhibited a higher stability with 5 consecutive cycles compared with pure ZnO (Fig. 7b).

432

The hybridization with the LFC matrix can effectively suppress photocorrosion and

433

agglomeration of ZnO nanoparticles during photocatalytic reaction. Thus, the 21

434

435 436 437 438

recyclability of ZnO is improved obviously.

Fig. 7 (a) The photocatalytic hydrogen evolution curves with pure ZnO and ZnO/LFC composite; (b) Stability test of pure ZnO and ZnO/LFC-4% composite for hydrogen evolution in five consecutive cycles.

3.3. Possible photocatalytic mechanism

439

In order to identify the main active species during photodegradation process, quencher

440

benzoquinone (BQ), tertiary butanol (BuOH) and EDTA-2Na were selected as scavenges

441

of ·O2-, ·OH and h+, respectively (Fig. 8a). The addition of EDTA-2Na caused a

442

remarkable decrease of photocatalytic degradation of SMT, which indicated h+ was the

443

primary active specie during degradation process. Moreover, the addition of BQ and

444

BuOH also inhibited the photodegradation efficiency of SMT, suggesting that ·O2-

445

and ·OH were active participants during degradation process. EPR spin-trap using DMPO

446

was applied to investigate the reactive oxygen species of pure ZnO and ZnO/LFC-4%

447

under the irradiation (Fig. 8b and 8c). Neither of the signals of ·O2- or ·OH for ZnO and

448

ZnO/LFC-4% was observed without irradiation. Both the signals of ·O2- and ·OH from

449

ZnO and ZnO/LFC-4% were detected clearly under UV irradiation. In addition, the

450

signals of ·O2- and ·OH for composite were significantly stronger than those for

451

ZnO. ·O2- and ·OH were formed from photogenerated electrons and holes, respectively.

452

Thus, the result showed that composite had a better separation efficiency of 22

453

photogenerated carriers compared with pure ZnO. As shown in Fig. 8d, the defects over

454

ZnO and ZnO/LFC-4% were characterized with EPR. The field signals at g=1.995 were

455

clearly observed, which was belonging to the defects related to oxygen vacancies. It is

456

noteworthy that this signal in ZnO/LFC-4% was stronger than that in ZnO, indicating

457

more content of oxygen vacancies in composite. The hybrid structure might introduce a

458

large number of defects into ZnO. The optical properties of ZnO and ZnO/LFC-4% were

459

analyzed through UV-vis diffuse reflectance spectra as shown in Fig. 8e. Composite

460

presented an extended light absorption region compared with pure ZnO and the physical

461

mixture of LFC and ZnO in the same proportion. The enhancement of absorption in the

462

visible range may be due to the joint electronic system and abundant Ovac in composite.

463

Hence, the sun light harvesting ability of ZnO was enhanced after coupling with LFC. PL

464

spectra were used to evaluate the separation efficiency of photogenerated carriers (Fig.

465

8f). Pure ZnO exhibited an obviously high and wide peak compared with ZnO/LFC-4%,

466

demonstrating the existence of LFC in composite could suppress the recombination of

467

photogenerated electron-hole pairs. As shown in Fig. 8g, composite presented higher

468

photocurrent density compared with pure ZnO, indicating that the separation rate of

469

photogenerated electron-hole pairs increased because of the existence of LFC. Moreover,

470

the EIS Nyquist plot of prepared samples was shown in Fig. 8h. ZnO/LFC-4% exhibited

471

Nyquist arc with smaller radius, indicating the electron-transfer resistance of ZnO was

472

reduced after introducing of LFC.

23

473 474 475 476

Fig. 8 (a) Photocatalytic degradation of SMT with the addition of hole and radical scavenger; (b) Hydroxyl radical EPR

477

On the basis of the above results, the photocatalytic enhancement mechanism of

478

composite is proposed as shown in Fig. 9. ZnO nanoparticles can be uniformly anchored

479

on the carbon nanosheets of LFC. ZnO/LFC composite presented a hierarchical structure

480

which was similar to the 3D nanostructure of LFC. This structure could endow the

481

photocatalyst with the characteristics of fast substance transfer and enhanced

482

light-harvesting. In addition, the porous carbon nanosheets also provided large number of

483

adsorption sites for capturing reaction substrate. These all could improve the

484

photocatalytic efficiency of ZnO. More importantly, the interface electronic interaction

485

between LFC and ZnO resulted in the formation of “dyade” structure, which can extend

486

the light absorption range and promote photogenerated carriers separation of ZnO [16].

487

When the sunlight irradiating to composite, electrons (e-) were excited from valence band

488

(VB) to the conduction band (CB) of ZnO, and holes (h+) were left. Generally, most of e-

489

and h+ will recombine quickly without participating in any chemical reaction, leading to

490

low reactivity. Fortunately, the carbon nanosheets with abundant sp2-hybridized carbon

491

atoms were highly efficient in storing and shuttling e-. The photoexcited e- from ZnO was

spectra, (c) superoxide radical EPR spectra, (d) EPR spectra, (e) UV-Vis DRS spectra, (f) PL spectra, (g) transient photocurrent responses, (e) EIS Nyquist plots of pure ZnO and ZnO/LFC-4% composite.

24

492

transferred to carbon nanosheets due to the intimate contact between them. For

493

photodegradation process, the SMT was absorbed on the surface of ZnO/LFC and

494

oxidized directly by h+, which was the main degradation reaction. In addition, h+ could

495

also react with H2O or OH- to form ·OH, and the e- in carbon nanosheets reacted with O2

496

absorbed on its surface to produce ·O2-. The ·OH and ·O2- also participated in the

497

oxidation reaction of SMT. For photocatalytic hydrogen production process, the e- in

498

carbon nanosheets reacted with H+ through co-catalyst Pt nanoparticles to produce H2.

499

Integrating multiple vital merits, composite presented an excellent photocatalytic

500

efficiency for photodegradation of SMT and photocatalytic H2 evolution. In addition, the

501

composite photocatalyst can retain high catalytic activity after being used several times,

502

which is mainly due to the photocorrosion and aggregation of ZnO was inhibited after

503

hybridization with LFC [50].

504 505 506

Fig. 9 Photocatalytic mechanism for the degradation of SMT and H2 evolution over the ZnO/LFC composite.

4. Conclusion

507

In summary, lignin-based carbon with flower-like structure was fabricated by a facile

508

EISA and carbonization method, and then ZnO nanoparticles were anchored uniformly in

509

the LFC through in situ growth. Composite photocatalyst presented extended optical 25

510

absorption and enhanced separation of photogenerated carriers due to the interface

511

electronic interaction between ZnO and LFC. In addition, the novel flower-like structure

512

also has multiple advantages, such as hierarchical structure to provide high mass

513

transferring efficiency, enhanced light-harvesting and good stability. So ZnO/LFC

514

composite presented excellent photocatalytic efficiency and recyclability for the

515

photodegradation of SMT and photocatalytic H2 evolution compared with pure ZnO. This

516

study ingeniously designed and fabricated LFC to address the shortages of ZnO, offering

517

viable strategy for practical application of ZnO.

518

Conflicts of interest

519

The authors declare no competing financial interest.

520

Acknowledgments

521

This work was supported by National Key Research and Development Plan of China

522

(2018YFB1501503), National Natural Science Foundation of China (21436004,

523

21878114), and Guangdong Province Science Foundation (2018B030311052,

524

2017B090903003).

525

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526 527 528 529 530 531 532 533 534 535 536 537

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Highlights 1. Lignin-based carbon with flower-like structure was first prepared. 2. Flower-like carbon was obtained with a facile EISA and carbonization process. 3. ZnO nanoparticles were anchored uniformly onto LFC by in-situ growth method. 4. LFC extended optical absorption and enhanced separation of charge carriers of ZnO. 5. ZnO/LFC presented excellent photocatalytic efficiency and recyclability.

Author contributions Binpeng Zhang: Conceptualization, Methodology, Investigation, Writing - Original Draft.

Dongjie Yang: Resources, Validation, Supervision, Formal analysis, Visualization.

Xueqing Qiu: Resources, Validation, Formal analysis, Visualization.

Yong Qian: Resources, Writing - Review & Editing, Data Curation.

Huan Wang: Writing - Review & Editing, Data Curation.

Conghua Yi: Writing: Review & Editing.

Dongqiao Zhang: Writing: Review & Editing.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: