Effect of reaction temperature on properties of carbon nanodots and their visible-light photocatalytic degradation of tetracyline

Abstract

Water-soluble carbon nanodots (CND_t) with diverse sizes, crystal structures, surface properties, and characteristic fluorescence spectra were synthesized by the hydrothermal carbonization of larch at different temperatures. The effects of reaction temperature on the diameter distribution, structural components, and fluorescence properties of the CND_t were investigated systematically. The synthesized CND_t were found to be monodispersed spherical polymer nanodots with a low degree of aromatization and abundant oxygen-containing groups on their surface. Increasing the reaction temperature decreased the average size of the nanodots from 20.35 to 6.48 nm, while their quantum yield increased from 13% to 18%. Broader and weaker UV characteristic peaks were detected when the reaction temperature was increased from 200 to 260 °C. All the CND_t samples exhibited excitation and emission-independent properties, and obvious blue shift of the excitation and emission peaks occurred at higher reaction temperatures owing to the smaller size and different surface properties obtained. The CND_t were used as a photosensitizer in a CND/TiO₂ system to effectively degrade tetracycline hydrochloride (TCH) under visible-light irradiation. The obvious blue shift exhibited by the smaller CND_t allowed the light to be fully used by the TiO₂, resulting in nearly 100% TCH degradation for CND₂₆₀/TiO₂.

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1. Introduction

Antibiotics are used extensively worldwide in medical treatment and agriculture but are often poorly absorbed by humans and animals; 50% to 80% of all antibiotics drain out and enter the aquatic environment. These residues may induce soil toxicity, development of antibiotic-resistant pathogens, and other problems that may threaten human health.^1,2 For example, tetracyclines are the second most common antibiotics in both production and usage, and have been successfully used for treating various infectious diseases. However, there is still a lack of effective, complete, and systematic technologies for the removal of pollutants like tetracyclines. Compared with other methods such as adsorption and microbial degradation, photocatalytic degradation is a simple and effective way of removing such macromolecular organic compounds in aqueous systems.^3,4 Photocatalysts can degrade macromolecules into non-toxic small molecules, inhibiting their toxic properties and restricting their transport in water thoroughly. Traditional semiconductors such as TiO₂ and SiO₂ are widely used for the photocatalytic removal of environmental pollution because they are good oxidants with high photocatalytic activity, chemical stability, and low toxicity. However, they can only effectively use a narrow spectral range of UV irradiation, which comprises 5% of incident sunlight.

The preparation and applications of quantum dots have attracted much attention because their quantum effects can give them unique properties. When particles are nanoscale, the size limit domain can give rise to size, quantum confinement, macroscopic quantum tunneling, and surface effects, resulting in chemical and physical properties different from those of macroscopic materials. Quantum dots have been widely used in the fields of bio-imaging, photocatalysis, medicine, and functional materials.^5–8 The emerging carbon nanodots successfully overcome the drawbacks of traditional semiconductor and metal quantum dots such as poor water-solubility, high toxicity, and incompatibility with biological environments, opening up new directions and broadening the scope in which quantum dots may be used. Carbon nanodots are a new kind of nanoscale particles consisting of carbon existing in the form of a conjugated system of SP² hybridized and unsaturated SP³ hybridized carbon atoms, which creates obvious responses to light, electricity and magnetism.^9,10 Therefore, the search for a simple and environmentally friendly method of preparing carbon nanodots has become a hot topic.

The northeast of China is rich in larch resources, and a huge amount of sawdust waste is produced and discarded every year. The exploitation of such waste plays a significant role in the development of renewable energy sources.^11,12 Hydrothermal carbonization has been proved to be an effective way to convert biomass into useful functional carbon materials. Through this process, biomass can be hydrolyzed to generate various value-added polymer precursors from which carbon materials with controllable shapes and sizes can be obtained.^13,14 Carbon nanodots with ordered crystal lattices and frequency up-conversion properties have been successfully prepared via hydrothermal carbonization of biomass.¹⁵ Our groups have previously synthesized water-soluble carbon nanodots with substantial fluorescence via hydrothermal carbonization of pentosan extracted from pulp refining, which exhibited good photocatalytic properties under visible light.¹⁶

Here, we report the synthesis of water-soluble carbon nanodots with diverse sizes, crystal structures, and optical properties, using larch as the carbon source. The particle size, quantum yield, and characteristic fluorescent peaks of the nanodots were controlled by adjusting the reaction temperature. The resulting CND_t were systematically investigated as a photosensitizer in a TiO₂-based photocatalytic system for the degradation of tetracycline hydrochloride.

2. Materials and methods

2.1 Hydrothermal carbonization of larch to produce CND_t

In a typical experiment, larch biomass (1.5 g) was dispersed in distilled water (40 mL) and ultrasonically agitated for 15 min. The suspension was added to a stainless steel autoclave of 55 mL in volume. The autoclave was placed in a furnace at 200 °C, 220 °C, 240 °C, and 260 °C for 12 h, and then allowed to cool to room temperature. The resulting brown suspension was collected and subjected to centrifugation at 12 000 rpm for 20 min. The light yellow supernatant containing the CND_t was subjected to dialysis (1000 Da molecular weight cutoff) for about 72 h before further characterization.

2.2 Preparation of the CND/TiO₂ system

CND/TiO₂ composites were synthesized using a sol–gel method.¹⁷ Tetrabutyl titanate (0.5 mL) was dissolved in ethanol (20 mL) to form aqueous solution A, while water (9 mL) and aqueous ammonia (5 mL, 28%) were mixed to form aqueous solution B. Solution B was added to A in a dropwise manner with continual stirring and the mixture was left to age for 12 h. The resulting precipitate was collected by centrifugation, dried at 80 °C for 6 h, and then calcined at 500 °C for 1 h to yield TiO₂. The obtained TiO₂ (0.1 g) was added to the CND_t solution (5 mL) with stirring over 10 min, followed by drying before further use.

2.3 Characterization

Transmission electron microscopy (TEM) images of the CND_t samples were obtained on a JEOL 2011 (FEI, Holland) operated at 200 kV. FT-IR spectra were recorded on a Fourier transform infrared (FT-IR) instrument (Perkin Elmer TV1900, Waltham, MA, USA) from 400–4000 cm⁻¹ at a resolution of 4 cm⁻¹. X-ray photoelectron spectra were recorded on a Physical Electronics PHI 5400 spectrometer (Physical Electronics, USA) using Al-Kα radiation (hν = 1486.6 eV). Binding energies were referenced to the C_1s line at 284.6 eV. Elemental analysis (C, H, O, and N) was carried out using an EA 3000 analyzer (EuroVector, Milan, Italy). UV-vis absorption spectra were obtained using a TU-1900 UV-vis spectrometer. Fluorescence spectroscopy was performed on a LS-55 fluorescence spectrophotometer (PerkinElmer, USA) equipped with a 120 W xenon lamp as the excitation source. The morphologies and sizes of the CND/TiO₂ composites were observed using environmental scanning electron microscopy (SEM; Quanta 200, FEI, Hillsboro, OR, USA).

2.4 Photocatalytic activity and quantum yield test

Photocatalytic activity tests were carried out in a cylindrical quartz photoreactor (275 mL) using TCH as the model compound and a 380 W Xe lamp (510 nm, Shanghai Hualun Lamp Factory, China) positioned inside the reactor as the light source. The photocatalyst powder (0.1 g) was added to 100 mL aqueous solution (50 mg L⁻¹). Before the photocatalytic degradation, the suspension was magnetically stirred in the dark for 20 min to establish TCH adsorption/desorption equilibrium. Aliquots of 5 mL were collected from the suspension and filtrated using a 0.22 μm filter membrane. The concentration of TCH in the reaction solution after illumination was determined spectrophotometrically using a TU-1900 UV-vis spectrometer at 357 nm.

The fluorescence quantum yields of the CND_t were calculated using 5 μg mL⁻¹ Rhodamine B in ethanol as a standard (quantum yield 90%). The absorbance of Rhodamine B at 543 nm was less than 0.05. The calculation was performed according to the following equation: Φ_u = Φ_s (Y_u/Y_s)(A_s/A_u), where u and s represent the CND_t and Rhodamine B, respectively, Φ is the quantum yield, and Y is the integrated fluorescence area. A is the absorption at fluorescence excitation.

3. Results and discussion

3.1 Morphology of the CND_t

Changing the reaction temperature had a direct effect on the average diameter of the obtained CND_t, as shown in Table 1. TEM images and histograms of the size distribution of CND₂₀₀ and CND₂₆₀ are presented in Fig. 1, and clearly show that the CND_t were well dispersed spherical particles with different ordered lattice structures. CND₂₀₀ exhibited a mean diameter of 20.35 nm (100 particles measured), and an obvious stripe-like hexagonal structure was detected on the surface. For CND₂₆₀, the average diameter was only 6.48 nm, and an ordered cubic structure was clearly seen. These results suggest that increased reaction temperature led to a decrease in size and change in lattice structure of the CND_t. As the hydrothermal process mainly involved dehydration, polymerization, aromatization and carbonization,¹⁸ various water-soluble organic products were also produced in the HTC process, including sugars and organic acids, maximum sugar and organic acids concentrations were obtained under low-temperature conditions but degraded considerably at higher temperatures,¹⁹ besides, higher reaction temperature favor carbonization,²⁰ which means reaction was more completely at higher temperature with more condensed carbon structure, leading to the size of carbon dots decreased as temperature increased.

Table 1 Average diameters and XPS analysis (wt%) of CND_t

Sample	Average diameter (nm)	C (wt%)	O (wt%)	N (wt%)	Others (wt%)	O/C (%)
CND200	20.35	77.31	20.17	0.21	2.31	26.09
CND220	16.4	78.35	19.80	0.16	1.69	25.22
CND240	11.9	77.92	20.37	0.41	1.30	26.14
CND260	6.48	72.38	25.85	0.27	1.50	35.71

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Fig. 1 TEM images and corresponding diameter distributions of CND₂₀₀ (a and c) and CND₂₆₀ (b and d).

3.2 Surface properties of the CND_t

FT-IR analysis (Fig. 2(a)) was used to reveal the functional groups on the surface of the raw material and CND_t samples. Larch wood is predominantly composed of cellulose, hemicelluloses, and lignin, which have large amounts of oxygen-containing surface groups such as hydroxyls, carbonyls, and carboxyls. The FT-IR peaks observed for CND₂₀₀ were similar to those of the raw materials, while the CND₂₂₀, CND₂₄₀ and CND₂₆₀ samples showed peaks with obvious changes, in accordance with previous studies showing that 200 °C is too low for the wood structure to be broken down completely.

Fig. 2 FT-IR spectra (a) and XPS survey spectrum (b), high-resolution C_1s peaks and the fitting curves (c) and high-resolution O_1s peaks and fitting curves (d).

The bands observed at 3300–3500 cm⁻¹ and 1028 cm⁻¹ were assigned to C–OH groups. Compared with those observed for the raw materials, the intensity of these peaks was much lower for the samples prepared at high temperature, because such groups are easily degraded to intermediate products during hydrothermal carbonization. The new peaks appearing at 1000–1460 cm⁻¹ corresponded to the C–O stretching vibration, confirming that hydroxyl, ester, and ether groups had been generated.²¹ The new peaks at 1625 and 875–750 cm⁻¹ were attributed to the stretching vibration of C=C and the out-of-plane bending vibration of aromatic C–H, respectively, confirming the formation of a large amount of aromatic structure.²² Finally, the adsorption band at 1700 cm⁻¹ corresponded to carboxylate groups. These results indicate that the structure of the larch wood had been completely destroyed to form carbon material containing aromatic structures rich in surface functional groups including hydroxyls, esters, ethers, carbonyls, and carboxylic groups.

The elemental compositions (measured by XPS) of the surface of the raw materials and CND_t samples are summarized in Table 1. The samples mainly contained C, O, and N elements, with trace amounts of Na, Si, Ca, and S. When the reaction temperature was increased from 200 °C to 260 °C, the content of C on the surface declined from 77.3% to 72.4%, while the O content increased from 20.2% to 25.9%. This further confirmed that the CND_t were condensed carbon materials with a large amount of oxygen-containing groups. In detail, the ratio of O and C exhibited an obvious rising trend with reaction temperature, different from that observed for previously obtained solid products.²⁰ This was caused by the increased number of molecules available for polymerization, confirming that the structure of the CND_t samples may have been different from those of CSs. The present CND_t may have been polymer nanodots rich in oxygen-containing groups with a low degree of aromatization.²³

XPS was conducted to analyze the chemical composition of CND₂₆₀. C_1s, O_1s, N_1s, and Na peaks were observed, as shown in Fig. 2(b). The XPS results indicated that these nanodots were mainly composed of C, O, and N, as well as a limited amount of Na element. The high C and O content confirmed that a condensed material with abundant oxygen-containing groups was produced at 260 °C. The C_1s spectrum was fit to four curves and attributed to aliphatic groups C–C (283.9 eV), C–N (285.2), C–O (286.1), and C=O/C=N (287.3).^18,22,24 The O_1s spectrum exhibited three peaks at 533.4, 534.3, and 535.5 eV, which were attributed to O–C=O, C–OH/C–O–C groups, and chemically adsorbed oxygen, respectively.²³ These results indicate the presence of an aromatic structure and abundant oxygen-containing groups on the surface of CND₂₆₀.

3.3 Optical properties of the CND_t

UV-vis and PL spectra were measured to investigate the optical properties of the CND_t samples. All obtained samples were water-soluble and formed light yellow aqueous solutions. The UV-vis absorption spectra exhibited a high and obvious peak around 281 nm, as shown in Fig. 3. This can be attributed to complex electron transition on the surface, the π conjugated aromatic system, and the n–π* transition of the carbonyl and other oxygen-containing groups.^25,26 It can be clearly seen that increased reaction temperatures led to the characteristic peak becoming broader and weaker, as the conjugate system is to reduce the total energy and increase the stability,²⁷ the structure of the CND_t prepared at low temperature was incomplete with lower stability, hence causing the conjugate peak to be more obvious. This result is consistent with the FT-IR analysis.

Fig. 3 UV-vis absorption spectrum of CND_t samples.

The emission spectrum of CND₂₀₀ at around 400–600 nm exhibited a peak at 440 nm upon excitation at 350 nm. A corresponding peak centered at 350 nm in the excitation spectrum appeared at an emission wavelength of 440 nm. With increasing hydrothermal temperature, both the excitation and emission peaks exhibited blue shift to lower wavelength, as shown in Fig. 4 and Table 2. It is well known that the PL properties of a material can be attributed to energy-level transition, surface energy traps, and the radiative recombination of electrons and holes,²⁸ as well as interactions between electrons and holes and the environment around them. Additionally, polycyclic aromatic compounds within the CNDs may have been responsible for their excellent PL properties.²⁹

Fig. 4 PL emission and excitation spectra of CND_t samples.

Table 2 Characteristic parameters of CND_t

Samples	Average diameter (nm)	Quantum yield (%)	Characteristics excitation wavelength (nm)	Characteristics emission wavelength (nm)	Photocatalytic degradation efficiency as photosensitizer (%)
CND200	20.35	13.4	350	440	89.7
CND220	16.4	14.9	330	420	93.6
CND240	11.9	16.6	320	380	95.9
CND260	6.48	18.1	280	365	99.0

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Size is an important factor that affects the excitation and emission wavelengths, and can give a material special characteristics.³⁰ Furthermore, changes in structural and surface properties such as the degree of aromatization and amount of oxygen-containing groups can cause changes in the energy-level difference between the first excited state and ground states, leading to changes in the energy of absorbed and emitted photons, showing as obvious blue shift.³¹

The PL spectra also indicated that the maximum emission and its intensity had a direct relationship with the excitation wavelength; this is a PL characteristic of C dots.³² For CND₂₀₀, upon changing the excitation wavelength from 260 to 420 nm, the emission peak red-shifted from 428 to 493 nm with a gradual decrease in intensity. Therefore, visible emission from CND₂₀₀ over the blue-to-red wavelength range was obtainable using the same sample under UV and visible excitation. For samples prepared at different temperatures, the excitation wavelength exhibited no obvious shift, while the main peak of the emission spectrum shifted from 440 nm to around 360 nm, in accordance with the analysis above (Fig. 5).

Fig. 5 Emission spectra of CND_t at different excitation wavelengths.

Using Rhodamine B in ethanol as a standard, the quantum yield of CND₂₀₀ was calculated to be about 13.4%, higher than that of most reported CNDs.^23,28 The quantum yield increased obviously to 18.1% for CND₂₆₀, confirming that increasing the reaction temperature caused an increase in quantum yield (as shown in Table 2).

3.4 pH sensitivity and chemical stability tests

Digital photos and emission spectra of CND₂₂₀ at different pH, adjusted using a sodium bicarbonate and sodium carbonate buffer solution, are shown in Fig. 6. All prepared samples (CND₂₀₀, CND₂₂₀, CND₂₄₀, and CND₂₆₀) exhibited color ranging from light yellow to dark yellow as the pH was varied from 4.0 to 10.0. The intensity of the emission peak decreased with increasing pH without a change in wavelength, as previously reported.³³ The abundant oxygen-containing groups on the surface imparted acid properties to the CNDs, so at high pH values an increasing negative charge density accumulated on the surface. This influenced the π–π* and n–π* electronic transitions, leading to the observed color changes and fluorescence quenching.

Fig. 6 PL spectra of CND₂₂₀ excited at 320 nm in solutions of different pH.

To investigate the chemical stability, FL quantum yields (QY) and optical absorption intensity (OAI) of CND_t synthesized for six weeks were tested as shown in Table 3. They all exhibit good stability on both quantum yields (QY) and optical absorption intensity even samples lasted for six weeks. Both the QY and fluorescence intensity keep almost constant, confirming the chemical stability of obtained CND_t.

Table 3 Quantum yields and optical absorption intensity parameters of CND_t synthesized for six weeks^a

	QY (%)	QYa (%)	OAI (a.u.)	OAIa (a.u.)
a QY = sample quantum yield, QYa = quantum yield of samples synthesized for six weeks, OAI = optical absorption intensity, OAIa = optical absorption intensity of samples synthesized for six weeks, optical absorption intensity was tested via irradiation by their respective characteristic excitation wavelength.
CND200	13.4	13.0	673.5	642.1
CND220	14.9	14.5	685.2	650.9
CND240	16.6	16.1	692.9	658.3
CND260	18.1	17.2	693.4	662.1

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The photochemical was tested by fluorescence intensity vs. time of carbon nanodots under continuous UV irradiation (using UV lamp with power of 30 W, wavelength of 254 nm, light intensity of 107 μW cm⁻²), the results show that under constant UV illumination for 5 h, the fluorescence intensity remained constant, almost no bleaching was observed, suggesting the emission was quite stable (Fig. 7).

Fig. 7 Photochemical stability of CND₂₆₀ under continuous UV irradiation.

The decay curve of synthesized CND_t were measured by HORIBA Fluoromax-4, from Fig. 8 it can be clearly seen the decay curve of CND₂₆₀ can be very well fitted to triple-exponential function, the lifetimes are τ₁ = 8.34, τ₂ = 2.59 and τ₃ = 1.26 ns, and the mean lifetime is calculated to be 5.48 ns, which is longer than previous reports;³⁴ based on the same calculation method, the life time was 3.23 ns, 4.32 ns, 5.06 ns and 5.48 ns for CND₂₀₀, CND₂₂₀, CND₂₄₀, and CND₂₆₀ respectively.

Fig. 8 Fluorescence decay curve and the fitting curve of CND₂₆₀ (a) decay curve comparison of CND_t.

3.5 Photocatalytic activity of CND/TiO₂

The CND_t samples investigated here were rich in oxygen-containing groups and have been proved to be easily attached in a stable manner to the surface of TiO₂.³⁵ The CND were combined with TiO₂ in a composite system, as shown in Fig. 9(a) and (b). The corresponding HRTEM image shows that CND₂₆₀ (lattice spacing 0.22 nm) were in close contact with the anatase TiO₂ nanoparticles with the (101) crystal plane exposed (lattice spacing 0.35 nm), confirming that CND_t were successfully attached to the surface of the TiO₂.

Fig. 9 (a) and (b) SEM and corresponding high-resolution TEM images of CND/TiO₂ composite, (c) TCH degradation percentage vs. time curves and (d) catalytic mechanism for CND/TiO₂ under visible-light irradiation.

The visible-light catalytic activities of the prepared CND_t/TiO₂ composites were evaluated using the decomposition of tetracycline hydrochloride (TCH) under visible-light irradiation, as shown in Fig. 9(c). Only 9.6% and 12.5% TCH reduction were obtained when pure CND₂₀₀ and TiO₂ were used as catalysts, respectively, and were mainly because of adsorption. Although the photocatalytic efficiency of P-25 was higher than pure CND₂₀₀ and TiO₂, the value was still low of only 21%. The CND_t/TiO₂ composites all gave efficient catalysis with high TCH degradation, indicating that interaction between the CNDs and TiO₂ occurred. The CND₂₀₀/TiO₂ composite exhibited a good photocatalytic activity and gave around 89.7% TCH degradation. The saturated adsorption value was low under the dark condition, confirming that the observed decrease in TCH concentration mainly derived from photocatalytic degradation. The degradation efficiency reached 93.6%, 95.9% and 99.0% for CND₂₂₀/TiO₂, CND₂₄₀/TiO₂ and CND₂₆₀/TiO₂, respectively, as shown in Table 2. Nearly 100% degradation of TCH was obtained for CND₂₆₀/TiO₂.

The proposed degradation mechanism is as follows (Fig. 9(d)): the CND_t with abundant oxygen-containing groups are sensitizer, when irradiation by visible light they can act as a photosensitizer and absorb photons. Photons with adequate energy can excite electrons in the HOMO, thus transferring them to the LUMO of the CND_t. Free electrons as well energy can then be transferred to the conduction band of TiO₂.³⁶ When the energy is equal or greater than the band-gap energy of TiO₂, conduction band electrons (e⁻) and valence band holes (h⁺) are generated. In this system, photogenerated holes could either recombine with electrons, react with OH⁻ or H₂O, oxidizing them to highly oxidizing species such as OH˙ and O₂˙ radicals, or oxidize the adsorbed TCH molecules.⁶ Such highly oxidizing species were responsible for the decomposition of TCH by TiO₂. Additionally, the photogenerated electrons could reduce adsorbed TCH or react with electron acceptors such as O₂ adsorbed on the TiO₂ surface or dissolved in water, reducing them to super-oxide radical anion O₂˙⁻, which can be transformed to OH˙ radicals by further reactions. In neutral conditions, reversible isomerization of asymmetric carbon atoms can occur, forming epimers of TCH. These epimers and their dehydration products are the main degradation products.³⁷

As discussed above, higher synthesis temperature led to changes in the size, structure, surface properties of the CND_t as well the amount of oxygen-containing groups on their surface.²¹ These changes can cause changes in the energy-level difference between the first excited state and ground states, leading to changes in the energy of absorbed and emitted photons, showing as obvious blue shift of emission and excitation spectra,³⁰ CND₂₆₀ exhibited smallest size with most abundant oxygen-containing groups, have strongest response to light irradiation, hence, CND₂₆₀/TiO₂ exhibited the highest photocatalytic activity.

4. Conclusion

Water-soluble carbon nano dots (CND_t) were synthesized via one-pot simple hydrothermal carbonization of lignocellulose-larch at different temperatures. The as-prepared CND_t were nanoscale spherical polymers with a low degree of aromatization and abundant oxygen-containing groups. They exhibited excellent photoluminescence (PL), excitation-dependence, and pH sensitivity, and their size, crystal structure, surface properties, and fluorescence characteristics could be varied by changing the reaction temperature from 200 °C to 260 °C. Increasing the reaction temperature decreased the average size and increased the fluorescence quantum yield of the CND_t, and was accompanied by variation in their ordered lattice structure from an obvious stripe-like hexagonal structure to a cubic structure. An obvious blue shift of the excitation and emission peaks was also observed, which arose from the differences in size and surface properties.

A CND/TiO₂ system was found to be an effective photocatalyst; CND_t loaded on the surface of TiO₂ acted as a photosensitizer. The spectral response of the composite system could hence be widened from the UV region to the visible-light region. Effective degradation of tetracycline hydrochloride (TCH) was achieved under visible-light irradiation. CND_t with smaller size, higher quantum yield and shorter excitation characteristic peak wavelength were found to be more beneficial as a photosensitizer in the CND/TiO₂ system; nearly 100% TCH degradation was obtained for CND₂₆₀/TiO₂.

Acknowledgements

This work was financially supported by grants from Special Fund for Forest Scientific Research in the Public Welfare (201504605), the Fundamental Research Funds of the Central Universities (2572014EB01), the National Natural Science Foundation of China (No. 31570567, 31500467), and Zhejiang Key level 1 Discipline of Forestry Engineering (2014lygcz017).

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