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Mass Production, Enhanced Visible Light Photocatalytic Efficiency and Application of Modified ZnO Nanocrystals by Carbon Dots Xiao-Yan Zhang, Jin-Ku Liu, Jian-Dong Wang, and Xiao-Hong Yang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie504444w • Publication Date (Web): 27 Jan 2015 Downloaded from http://pubs.acs.org on February 1, 2015

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Industrial & Engineering Chemistry Research

Mass Production, Enhanced Visible Light Photocatalytic Efficiency and Application of Modified ZnO Nanocrystals by Carbon Dots Xiao-Yan Zhang1, Jin-Ku Liu1,*, Jian-Dong Wang1, Xiao-Hong Yang2

1

Key Laboratory for Advanced Materials, East China University of Science and Technology, Shanghai,

200237 P.R. China

2

Department of Chemistry, Chizhou University, Chizhou, 247000, P.R. China

*

Corresponding author; E-mail address: [email protected] 1

ACS Paragon Plus Environment


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Abstract: The carbon dots (C-ZnO NCs) modified zinc oxide with novel visible-light catalytic ability was mass produced by combustion method. During the formation process, the generated gas of reagents under high temperature can effectively break the product into small particles. Meanwhile, a certain amount of carbon dots have been formed and loaded on the surface of ZnO NCs. The results suggest that the carbon dots have a huge impact on the photocatalytic performance. Specifically, when the molar ratio of glycine to zinc nitrate was 2:1, the product was monodisperse particle with average diameter of about 70~80 nm and BET specific surface area of 11.88 m2·g-1, which exhibited the best catalytic efficiency. The degradation process of Rh. B can be completed within 15 minutes, with an increase of 433.3% compared with pure ZnO materials (80 min). The present study is not only significant for improving large-scale production of modified ZnO NCs but also for suggesting practical applications fields such as the environmental catalysts.

KEYWORDS: nanocrystals; mass production; visible light photocatalytic performance; water purification system.

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1 INTRODUCTION In recent years, nanomaterials have received more and more attention due to their unique optical, chemical, catalytic and electronic properties.1-3 For instance, the zinc oxide nanocrystals (ZnO NCs) are found to be a promising candidate for its use in different applications such as solar cell,4 the photoelectrochemical splitting of water,5 degradation of organic contaminantand,6 and etc.7,8 A number of

innovative methods

have been employed to improve the photocatalytic activity of ZnO NCs such as increasing the surface area,9 generating defect sites10 and surface modification with metals11,12 (Al, Cu, Ag) or non-metals (C, N, P).13 These methods serve as highly effective routes to improve products’ ability to use visible light, and consequently achieve greater photocatalytic capacity than pure ZnO NCs. The carbon-doped ZnO NCs have advantages such as stabilization, room temperature ferromagnetism and higher photocatalytic activity.14,15 Zhu, et al.16 adopted a one-step calcination method to obtain carbon-doped ZnO. They indicated that when the content of g-C3N4 in the nanocomposites was 50.7 w%, the products showed the best photocatalytic property. Bu, et al.17 successfully synthesized quasi-shell-core structured graphene-ZnO composite using a simple one-step precipitation method, which also showed high photocatalytic activity and excellent stability. They suggested that the catalytic particles have more potential applications due to their graphene coating on the surface. Ouyang, et al.18 prepared carbon-doped zinc oxide by hydrothermal reaction which showed a porous structure and enhanced photocatalytic activity.

However, , few of these studies have

considered/examined the important factors such as the photocatalytic regularity of ZnO 3



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NCs dependency on doped elements, the industrialization production and application of ZnO NCs with enhanced visible light photocatalytic activity. In this paper, the modified ZnO NCs by carbon dots (C-ZnO) were prepared by combustion method. The heat released by the combustion of glycine at high temperature was implemented to accelerate the diffusion of elements in the ZnO NCs. The gas could “break” the products to get smaller sizes. In other words, the strong exothermic reaction produced a large amount of gases and heat that not only prevented the reunion of NCs, but also resulted in looser products with an increasing number of ZnO crystal nucleus. At the same time, during the combustion of organic matters, since a certain amount of carbon dots has been loaded on the surface on ZnO NCs, the modified ZnO NCs by carbon dots (C-ZnO) can be obtained. Compared with other existing approaches including sol-gel method,17 hydrothermal,6 gas phase synthesis,19 this method can obtain unique products with smaller size, higher performance and other potential properties. Furthermore, the carbon dots modified ZnO NCs possessed better photocatalytic ability than the Al doped ZnO NCs20 and C doped ZnO NCs6. While the doped Al and C atoms changed the lattice structure of ZnO crystals ，the modified carbon dots can produce a newly-formed conduction band in the products, which contributes to an increased level of electron migration to the valence band and thereby enhances their photocatalytic activity. The time of the catalysis of C-ZnO NCs was shortened to 65 minutes and the catalytic efficiency increased by 433.3% compared with pure ZnO NCs. In addition, a feasible water purification unit employing the obtained C-ZnO NCs was introduced. By exploiting solar light, the experiment significantly reduced the cost of water purification and may further 4


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improve the utility value of novel nanomaterials in other potential areas.

2 EXPERIMENTAL SECTIONS 2.1 Synthesis of C-ZnO NCs All the reagents used in this research are analytical grade, which was used without further purification. The zinc nitrate was mixed with different mass of glycine which served as the carbon source by grinding in an agate mortar. The resulting mixture was added in mortar and ground fully to get a transparent liquid, and then poured into the crucible. The crucible was put into a drying oven at 160 ◦C for 4 h. Then crucible was put into a muffle and heated at about 200 ◦C until no smoke further emerged. Then it was heated at 600 ◦C in air for 2 h. Finally, the light C-ZnO NCs with different amount of carbon were obtained (shown in Scheme 1).

Scheme 1. Synthesis process of the C-ZnO NCs

2.2 Characterizations X-ray powder diffraction was measured on the Shimadzu XD-3A diffractometer to

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test the crystal structure. The transmission electron microscopy and high resolution electron microscopy (HREM, JEM-2100F) with an acceleration voltage of 200 kV (Hitachi-800) were used to analyze the microstructures and morphologies. UV–Vis spectroscopy (Shimadzu, UV-2600) was measured to explore the absorption properties. X-ray photoelectron spectroscopy (XPS, VG ESCALAB MKⅡ) and Philips S-4800 energy disperse spectroscopy (EDS) were measured to study the surface composition. The isothermal nitrogen adsorption-desorption analysis by Micromeritics ASAP 2400 was tested to study the specific surface area (BET). The fluorescence property was measured using fluorescence spectrophotometer (Cary Eclipse, Varian).

2.3 Photocatalytic Degradation Experiments The 0.1 g C-ZnO NCs were added into a 30 mL quartz batch reactor containing 30 mL 2×10-5 mol/L Rh. B aqueous solution and, respectively ,and then exposed under visible-light. The reaction system was stirred for 30 min in the dark to meet the adsorption-desorption equilibrium before irradiation. A 350 W Xe lamp with UV cut-off filters were used as light source (wavelength > 420 nm). Finally, the reactor was exposed to visible light under stirring. The same procedures were repeated under sunlight. The reactions were implemented during 11 am - 2 pm with a room temperature of 30 ◦C ± 3 ◦C. The UV-Vis absorption spectra of samples (liquid phase) obtained throughout the experiment were analyzed at room temperature by Shimadzu UV-2600 spectrometer.

3 RESULTS AND DISCUSSION 3.1 Morphologies and Structure 6


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Figure 1. TEM images of C-ZnO NCs with different molar ratio of glycine to zinc atoms: (a) 1:10; (b) 1:3; (c) 1:1; (d) 2:1; (e) 3:1.

All the C-ZnO NCs were prepared by combustion method. Figure 1 showed the TEM images of C-ZnO NCs with different molar ratio of glycine to zinc atoms. As can been seen， all products had sphere-like shape with different degrees of aggregation. Figures 1a and 1b showed that the products were united with the size about 120 nm in diameter, while 1c, 1d and 1e suggested that the crystallization and dispersal was 7



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increasing. This strong exothermic reaction released a large amount of gases such as N2, CO2, NO2 and water vapor that not only prevented the concentration, but also facilitated rapid heat diffusion made the products looser. A lot of bubbles in the drying process can be observed (shown in Scheme 1). Just as shown in Figures 1d and 1e, the modified products posessed an ideal dispersal and particle size of around 70~80 nm. Additionally, the TEM results indicated that the presence of carbon did not influence morphologies of ZnO, which proved that the carbon dots were confined to the surface of ZnO NCs, and didn't create other compounds which would change its morphology.

Figure 2. (a, b) HRTEM image and (c) EDS spectrum of the C-ZnO (the molar ratio of glycine to zinc atoms was 2:1).

The high resolution electron microscopy (HREM, JEM-2100F) with an acceleration voltage of 200 kV was adopted to analyze the surface of the C-ZnO NCs. Figure 2 showed a typical HRTEM image and EDS spectrum of the highly magnified C-ZnO NCs. As can be seen from the HRTEM image with higher magnification (Figure 2a), there were some dots on the surface, which implied that some amount of carbon dots had been successfully grown onto the surface of ZnO NCs.21 The atomic lattice spacing of ZnO 8


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NCs was 0.26 nm that matched well with the {002} face of ZnO wurtzite structure (JCPDS, No: 36-1451) and the amorphous carbon on the surface of ZnO NCs can be clearly observed (Figure 2b). The Figure 2c also showed that the structures were composed of only Zn, O, and C. These results suggested that the structures were mixed C materials and ZnO NCs. However, due to the limitation of the preparation method, the small number of carbon dots on the surface of the zinc oxide was not evenly distributed. In addition, the EDS spectrum was based on a semi-quantitative analysis. The average was derived from several selected regions. These findings will be confirmed by XPS analysis result.

Figure 3. XRD patterns of C-ZnO NCs with different molar ratio of glycine to zinc atoms: (a) pure ZnO NCs; (b) 1:3; (c) 1:1; (d)2:1; (e) 3:1.

Figure 3 showed the XRD patterns of C-ZnO NCs with different molar ratio of glycine to zinc atoms. All diffraction peaks were matched with the standard XRD patterns of hexagonal ZnO wurtzite structure (JCPDS, No: 36-1451) and the peaks of carbon dots can not be observed in XRD patterns of the composites, which demonstrated that the 9



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modified elements would not change the ZnO crystal lattice and the little carbon dots in these composites can not be detected (less than 5%). Moreover, we can infer that C-ZnO NCs had a preferred orientation along the (101) crystal plane and all products have a good crystallization.22

3.2 Optical and Conductive Properties

Figure 4. UV-Vis spectrum of C-ZnO NCs with different molar ratio of glycine to zinc atoms: (a) pure ZnO NCs; (b) 1:3; (c) 1:1; (d)2:1; (e) 3:1.

The UV-Vis absorption spectra of the C-ZnO NCs were shown in Figure 4. It was indicated that pure ZnO NCs (Figure 4a) had no absorption in the visible region, whereas the C-ZnO NCs had a significantly better performance in the range of 400-630 nm (Figure 4b-4e). The significant difference between pure ZnO and C-ZnO proved that carbon dots can modify and improve the performance of ZnO materials. It also showed that the optimal modified molar ratio of glycine to zinc atoms was 2:1. According to the formula (Eq.1),23

αhν = A(hν - Eg)n/2 10


(1)

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the absorption edge of 4a, 4b and 4c was 404.7 nm, 438.2 nm and 415 nm and their band energy gap were 3.06 eV, 2.83 eV and 2.99 eV, respectively. The band gap energy of C-ZnO NCs (the molar ratio of glycine to zinc nitrate was 2:1) was 2.83 eV, which was narrowed 0.13 eV compared to pure ZnO NCs.22 The enhanced light adsorption can raise the efficiency of the use of visible light, which can increase the production of electron-hole pairs under visible light irradiation, and then result in a higher photocatalytic activity. These findings can be following visible-light photocatalytic experiment.

Figure 5. The molecular fluorescence spectra of C-ZnO NCs with different molar ratio of glycine to zinc atoms: (a) pure ZnO NCs; (b) 1:3; (c) 1:1; (d)2:1; (e) 3:1.

Figure 5 were photoluminescence spectra of C-ZnO NCs at an excitation wavelength of 290 nm under the room temperature. A strong UV emission light at about 390 nm could be observed. The decreasing UV emission showed the change of recombined rate of photogenerated electrons and holes.24 It’s clear that with the increasing of glycine ( shown in Figure 5c, 5d, 5e), the emission intensity decreased, which implied that the defects on the surface was increasing, so the recombination rate of electrons and holes would 11



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decrease. Figure 5d indicated that when the molar ratio of glycine to zinc nitrate was 2:1, the hetero architectures exhibited the lowest emission intensity among them. It also indicated that the recombination of the photogenerated charge carrier was inhibited greatly in the C-ZnO NCs.25,26 The efficient charge separation could increase the lifetime of the charge carriers and enhance the efficiency of the interfacial charge transfer to adsorbed substrates, and then eventually contribute to a higher photocatalytic activity of the C-ZnO hetero architectures. The resistivity of the pure ZnO NCs and C-ZnO NCs were about 2.1×104 Ω·cm and 1.2×104·Ω·cm, respectively. This result also provided solid evidence that the ZnO NCs product was effectively modified by carbon dots.

3.3 XPS Analysis of C-ZnO NCs

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Figure 6. XPS spectrum of C-ZnO NCs ( the molar ratio of glycine to zinc atoms was 2:1): (a) all; (b) high-energy-resolution Zn 2p core-level spectra; (c) high-energy-resolution O 1s core-level spectra; (d) high-energy-resolution C 1s core-level spectra.

XPS studies were performed to investigate the states of elements of zinc oxide and modified carbon dots. Figure 6b, 6c and 6d presented the binding energy peaks of Zn, O, and C, respectively. As shown in Figure 6, due to the asymmetric broad peak in the C 1s region, we could conclude that chemical state of carbon in products can take various forms. The peaks of Zn2p at 1045.2 and 1022.0 eV (Figure 6b) assigned to Zn2p1/2 and Zn2p3/2 lines27, 28 proved the presence of Zn2+ in the sample. In the XPS spectrum of O 1s, the main peak at 530.8 eV was assigned to O2- ions in the Zn-O bonding, and the shoulder peak was related to -OH group absorbed onto the surface of the composite.29 The C 1s spectrum could be divided into three components. The 285.2 eV peak was signed to graphite of the sp2-hybridized carbon,30 whereas the 286.7 eV peak was attributed to C-O bond.31 The small peak observed around 289.2 eV was also found on ZnO samples, attributed to carboxyl-containing contamination, most likely from adsorbed atmospheric CO2.32-33 This results revealed that there were no glycine in the produced composite. The quantitative analysis of the XPS also showed that the real atomic ratios of Zn to C in the product structures was 25.4 : 7.8. The carbon atom concentration of C-ZnO NCs was estimated to be 13.3 at%. Thus, it can be observed that the ratio was consistent with the EDS analysis discussed above. The carbon dots were only about 1.8 weight % and therefore the carbon dots can not be observed in TEM and XRD results.

3.4 Photocatalytic Performance 13



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Figure 7. Photodegradability of Rh. B under visible light by (a), pure ZnO NCs; (b), C-ZnO NCs.

The degradation of Rh. B with different samples under visible-light was investigated. A 350 W Xe lamp was used as a visible-light source with UV light cut-off filters (λ > 380 nm). As indicated by the UV-Vis curves in Fig. 7b, the Rh. B with initial concentration of 2 × 10-5 mol/L was removed completely within 175 min, while the one with pure ZnO NCs ( Fig. 7a) was degraded only by 53.2% . The results suggest that the existence of carbon dots greatly improved its visible-light photocatalytic activity, which broaden its daily application.

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Figure 8. (C/C0)-t curves of degradation of Rh. B under sunlight by different C-ZnO NCs with the different molar ratio of glycine to zinc atoms.

Degradation curves of C-ZnO catalysts with different modified ratio of glycine in sunlight irradiation were shown in Fig. 8(11am-2pm, with the radiation intensity of about 220 W·m-2).34 The Rh. B aqueous solution was investigated as the model contaminant to estimate the photocatalytic activity of the C-ZnO NCs, and the results showed that the Rh. B with pure ZnO photocatalyst basically completed within 80 minutes. Along with the increase of glycine, the degradation time of Rh. B solution was decreased obviously. The results showed the C-ZnO NCs with molar ratio of glycine to zinc atoms of 2:1 had the best degradation ability, which could complete the degradation with just 15 minutes that the absorbance in 552 nm didn't descend any more. The chemical oxygen demand (COD) was used to test the content of organic residue in the dye solution which had been degraded to be colorless, and the result showed that the value of COD was zero, which indicated there was no organic residue present in the degraded solution. In other words, the Rh. B was completely degraded into CO2 and H2O under the catalysis of as-prepared sample. However, with the molar ratio increasing to 3:1, the degradation time was shortened to 23 minutes. So the glycine addition was one of the influencing factors of photocatalytic degradation performance of the prepared C-ZnO NCs. If the addition account of glycine was low enough to provide space to hold carbon dots, its degradation efficiency had upside potential. On the contrary, if the addition account of glycine was too much, a lot of thicker carbon dots adhesive layer appeared on the ZnO surface. After that the products would absorb large amount of incident light, 15



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which reduced the photoabsorption and the photocatalytic ability. Furthermore, excessive superficial carbon dots could be the recombination centers of photogenerated electrons and holes, which were bad to the photocatalytic efficiency.35 Compared with pure ZnO NCs, C-ZnO NCs' photocatalytic efficiency ( molar ratio of glycine to zinc oxide was 2:1, which was the best doping ratio) increased by 433.3% ( Eq.2 ). Increase efficiency = (T-T0)/T0 * 100%

(2)

T0 stands for the degradation time of the pure ZnO NCs, T stands for that of the samples.

Figure 9. Rh. B degradation curves with different added trapping agents. (a) methanol; (b) isopropyl alcohol.

In order to determine whether photoproduction electronics or holes is the major influencing factor, we carried out two parallel photocatalytic experiments: 1) isopropyl alcohol could catch photoproduction electronics; 2) methyl alcohol could catch holes. The results showed that the experiment with methyl alcohol had a faster degradation rate than the other36-39, which meant that the photoproduction electronics was the major factor influencing the photocatalytic process. This showed that the surface of the prepared NCs contained large quantities of electoronics. However, it is interesting to note that while the

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methyl alcohol also influenced the degradation rate (shown in Figure 9a), so the holes and electrons worked at the same time, the electronics were the major factor.

4 Photocatalytic mechanism of C-ZnO NCs under visible light

Figure 10. The photocatalytic mechanism of the C-ZnO NCs catalyst (molar ratio of glycine to zinc atoms was 2:1).

From the above results, it is clear that the carbon dots affected the UV-Vis light absorption and were good for the existence of holes, both of which contribute to the enhanced photocatalytic activity. The photocatalytic mechanism of carbon dots modified C-ZnO NCs was shown in Figure 10. The C-ZnO NCs showed better photocatalytic ability than pure ZnO materials, which could be explained by three possible reasons: (1) due to the carbon dots, the C-ZnO NCs could absorb the light easily so that more electronics on value band (VB) could move up to the conductive band (CB) easily.42 Then the photoproduction electronics transmitted to the surface to absorb O2 to form the superoxide radical anions (O2-). At the same time, the holes reacted with H2O to form the hydroxyl radicals (OH·), which eventually leads to the degradation of Rh. B.43-45; (2) the 17



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band gap was narrowed by a new-formed VB which resulted in more electronics movement from VB to CB; (3) the carbon dots on the surface of catalyst with high reducibility can prevent photocorrosion reaction (Eq. 3) of ZnO NCs, and therefore the carbon dots enhanced photocatalytic rate effectively. ZnO + 2 h+ = Zn2+ + 1/2O2

(3)

5 Application of C-ZnO NCs with the best photocatalytic efficiency

Figure 11. Continuous purification reactor under sunlight using C-ZnO NCs as photocatalysts (a, C-ZnO NCs; b, TEM image; c, size distribution; d, reactor diagram)

A purification system with three continuous photodegradation reactor using the solar light as free energy was shown in Figure 11. In the actual operation, 1.0 g C-ZnO NCs was added into three reactors respectively ( the effective reaction area was about 0.02 m2). Figure 11b and 11c illustrate the TEM device and particle size distribution, respectively. The Rh.B solution (2×10-5 g/L) was selected as the pollutant and the UV-Vis absorption

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spectra of starting and the ending solutions were tested, which can determine the photocatalytic efficiency.

The photodegradation process was taken between 11.00 am and 2.00 pm. The simulate effluent of Rh. B solution was degraded under sunlight continuously, and clear liquid was obtained from the initial red solution. With the flow velocity of 60 mL/min, the degradation rate of three reactors was about 37 %, 59 %, 100 %, respectively. We also used the chemical oxygen demand (COD) to test the content of organic residue in the dye solution which had been degraded to be colorless as shown in the degradation experiment in Figure 8. The results showed that the value of COD was zero, indicating that the degraded solution contained no organic residue. This water purification system has reduced cost of degradation by using sunlight and largely expanded application areas of C - ZnO NCs. Moreover, different organic effluents can be purified flexibly in the reactor by selecting effective photocatalysts.

6 CONCLUSION In the present study, a novel C-ZnO NCs was synthesized through a combustion reaction. In the synthesis process, an increasing amount of gas emissions prevented the agglomeration, leading to a decreased size of particles. The optimum molar ratio of glycine to zinc atoms was found to be 2:1. Due to the increased absorbance in visible region and the new VB, the degradation rate increased remarkably by 433.3% compared with pure ZnO under visible light. A mechanism for degrading Rh. B under visible light was proposed to show the effect of carbon dots. In addition, we manufactured a feasible

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water purification system with continuous photodegradation reactors at greatly reduced cost by optimizing natural sunlight and we also expanded the application areas. C-ZnO NCs could be used as a potential photocatalyst in utilizing sunlight effectively due to its advantages of reusable, cost-effective, visible-light-driven and environmental-friendly.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 21341007), Fundamental Research Funds for the Central Universities (Grant 222201313005) and State Key Laboratory of Pollution Control and Resource Reuse Foundation (Grant 13019).

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