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Insight into the impact of surface hydrothermal carbon layer on photocatalytic performance of ZnO nanowire
Applied Catalysis A, General 583 (2019) 117145
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Applied Catalysis A, General journal homepage: www.elsevier.com/locate/apcata
Insight into the impact of surface hydrothermal carbon layer on photocatalytic performance of ZnO nanowire ⁎
Peng Zhanga,c, Xiaoyan Yangb, Zhouzheng Jina, Jianzhou Guia, , Rui Tana, Jieshan Qiuc,
T ⁎
a
Tianjin Key Laboratory of Green Chemical Technology and Process Engineering, State Key Laboratory of Separation Membranes and Membrane Processes, School of Chemistry and Chemical Engineering, Tianjin Polytechnic University, Tianjin, 300387, China b School of Chemistry and Chemical Engineering, Shangqiu Normal University, Shangqiu, 476000, China c Liaoning Key Lab for Energy Materials and Chemical Engineering, State Key Lab of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian, 116024, China
A R T I C LE I N FO
A B S T R A C T
Keywords: ZnO Hydrothermal carbon Surface adsorption Photocatalysis
Due to the thorough interfacial contact and ignorable impediment for absorbing the incident light, the uniform hydrothermal carbon (HTC) coating is confirmed to efficiently enhance the photocatalytic performance of photocatalysts. To further explore the detailed effect of the HTC layer with various physical and chemical properties, a series of 1D ZnO@C nanocables are prepared via uniformly depositing HTC and high-temperature calcinating. The HTC layer thickness could be precisely tuned in a range of 3–8.5 nm by controlling the carbon source amount, while the calcination will decline the surface coating density of HTC and significantly increase its electroconductivity. The adsorption capacity of various ZnO@C nanocables is systematacially investigated for methylene blue (MB), as well as their photocatalytic performance for the MB-degradation. It is found that the surface adsorbability of ZnO@C nanocables generally increases with the HTC thickness, whereas 5 nm will maximally improve the photocatalytic activity of ZnO. Furthermore, the ZnO@C product calcinated at 600 °C simultaneously possesses the good surface adsorbability and high electron-transfer efficiency, thus showing the superior photocatalytic performance. Finally, a rational mechanism has been proposed to explain the improved effect of the coated HTC layer on the photocatalytic performance of primary ZnO nanowires.
1. Introduction In texile and fine chemical industries, the organic effluent has became a big environmental problem to threaten the water safety and human health. Among various wastewater treatment methods developed, photocatalysis attracts particular attentions due to its continuity, simplicity and high efficiency [1]. Many studies have reported that semiconductors could absorb the incident photons and excite the separation of photogenerated electron/hole pairs, showing good photocatalytic performance under the irradiation of UV or visible light [2,3]. Nevertheless, most of photoelectrons will instantaneously recombine with the photogenerated holes, which badly inhibits further improvement of the photocatalytic efficiency of conventional semiconductor materials. To effectively separate the photogenerated carriers, many ingenious hybridizing strategies [4–6] acquire a great development, involving various composite materials such as Ag nanowires [7], Au [8], Fe2O3 [9], and C3N4 [10]. In particular, carbon materials feature the high
⁎
capture rate for surface electrons, high specific surface area, low cost, and environmental compatibility, thus being considered as ideal candidates [11,12]. Of alternative carbon materials, hydrothermal carbon (HTC) [13–15] derived from monosaccharide or polysaccharide has recently attracted tremendous interests due to the controllable synthetic approach and good combinability with other compounds [16,17]. In addition, HTC is also verified to rapidly transfer photogenerated electrons and improve the photocatalytic activity of many semiconductor materials, such as TiO2 [18–20], ZnO [21], and CdS [22]. However, relevant researches are still insufficient to explore the optimized composite structure between HTC and semiconductors, because their interfacial area will strongly affect the catalytic property of photocatalysts [23]. In the previous work, our group one-step synthesized a rod-type ZnO@C photocatalyst with an thin HTC layer coated on the surface [24]. This HTC-wrapped core/shell structure seems to be an ideal combination between HTC and semiconductors, owning to their thorough interfacial contact and the ignorable impediment of the HTC layer for absorbing the incident light. It is also found that the thin HTC
Corresponding authors. E-mail addresses: [email protected] (J. Gui), [email protected] (J. Qiu).
https://doi.org/10.1016/j.apcata.2019.117145 Received 17 April 2019; Received in revised form 30 June 2019; Accepted 4 July 2019 Available online 05 July 2019 0926-860X/ © 2019 Published by Elsevier B.V.
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Fig. 1. Illustration of synthesis procedure of the ZnO@C nanocable.
Teflon-lined stainless steel autoclave again, which was maintained at 180 °C for 12 h. Afterward, the products were collected by filtering and washed with deionized water and absolute ethanol. After completely dried at 80 °C, the HTC-coated photocatalysts were finally obtained and denoted as x-ZnO@C, where x refers to the addition amount of glucose. Here, 0.05 g, 0.1 g, 0.2 g, and 0.4 g of glucose were adopted to synthesize corresponding HTC-coated photocatalysts. To prepare a series of 0.2-ZnO@C-T photocatalysts, the 0.2-ZnO@C sample was loaded into a tube furnace and calcinated at T °C for 2 h under the N2 atmosphere, with a heating rate of 5 °C/min.
layer can remarkably promote the photocatalytic performance of primary ZnO in two ways: On the one hand, as the electron reservoir, the HTC layer could effectively migrate surface electrons from ZnO, prolonging the lifetime of photogenerated charge carriers [25]; On the other hand, as the organic molecule trap, the strong adsorbability of HTC materials will greatly impel the surface enrichment of organic molecules on ZnO to accelerate the decomposition reaction [25,26]. Even so, two important issues are still pending and need to be resolved immediately: (1) How will the coated HTC layer thickness affects the adsorbability and catalytic performance of the ZnO@C photocatalyst? (2) It is demonstrated that the high-temperature calcination could significantly change the physical and chemical property of HTC, including the carbon structure, porosity, chemical composition, and electroconductivity [27–29]. Therefore, it is also highly desired to figure out the improved effect of calcined HTC layers with various physicochemical structures on the photocatalytic performance of the ZnO@C material. Herein, high-quality ZnO nanowires have been adopted as matrixes to prepare the cable-like ZnO@C composites with the core/shell structure. As shown in Fig. 1, the detailed synthesized process can be divided into two steps: (I) Using glucose as the carbon source, a thin HTC layer is formed and uniformly deposited on as-obtained ZnO nanowires under hydrothermal situations, yielding the HTC layer-coated ZnO@C nanocables. The HTC layer thickness could be precisely controlled in the range of 3–8.5 nm by adding the different amounts of glucose. (II) In the N2 atmosphere, the resultant ZnO@C nanocables are thermally treated to vary the chemical composition and physical property of the thin HTC layer, including the carbon structure, electroconductivity, and surface coating density. Subsequently, the photocatalytic performance of ZnO@C nanocables with various HTC layers are systematacially investigated for the degradation of methylene blue (MB) under UV light illumination. In terms of the experimental results and characterizations, a rational mechanism has also been proposed to explain the improved effect of the coated HTC layer on the photocatalytic performance of primary ZnO nanowires.
2.2. Characterization of photocatalysts Morphology and structure of various samples were observed by field-emission scanning electron microscopy (FESEM, Hitachi S4800) and transmission electron microscopy (TEM, Hitachi H7650). The diameter distribution of ZnO nanowires was obtained by collecting diameter sizes of 130 monomers. X-ray diffraction (XRD) was carried out using a Bruker D8 Advance apparatus with Cu Kα irradiation, operated at 40 kV and 100 mA. Fourier transform infrared (FTIR) spectra were measured by a Thermo Nicolet 6700 with KBr as the reference. Thermogravimetric analysis (TG) was performed in a Netzsch STA 449F3 in air, in which samples were heated from room temperature to 900 °C with a ramping rate of 10°/min. UV–vis diffuse reflectance spectra (DRS) were recorded in a range of 200–800 nm on a Shimadzu UV2700 with an integrating sphere attachment. Brunauer-EmmettTeller (BET) specific surface area was measured by Micromeritics ASAP 2020 at −196 °C, and the samples were degassed at 200 °C for 5 h under vacuum before the measurement. Photoluminescence (PL) spectra were measured using a Hitachi F-7000 with an excitation light of 355 nm. An electrochemical work station (CHI 760D, Chenhua Instrument Co. Ltd., Shanghai, China) was adopted to record the electrochemical impedance spectra (EIS) in 0.1 M Na2SO4 aqueous solution, using a standard three-electrodes system. The platinum wire electrode and saturated calomel electrode (SCE) were used as the counter electrode and reference electrode. Besides, 4 mg photocatalysts were ultrasonically dispersed in 2 mL ethanol for 10 min to yield a slurry, which was dipped into a 10 mm × 10 mm groove on an indium-tin oxide (ITO) glass. The photocatalyst/ITO was dried at 80 °C and heated at 200 °C for 8 h in N2, finally forming the work electrode. Under the irradiation of UV light, EIS were measured at 0.0 V and the Nyquist plots were recorded in the frequency range of 0.05–100 Hz with 5 mV of the sinusoidal AC perturbation.
2. Experimental section 2.1. Preparation of photocatalysts All the reagents involved were of analytical grade and used without any treatment. To synthesize ZnO nanowires, 0.2 g zinc chloride, 1.5 g sodium dodecyl sulfate (SDS) and 20 g sodium carbonate were added into 40 mL deionized water. After completely magnetic stirring, the white suspension obtained was transferred into a 50 mL Teflon-lined stainless steel autoclave, which was then heated at 140 °C for 12 h. When cooled down to the room temperature, the hydrothermal precipitates were collected by filtering, rinsed with deionized water and absolute ethanol for 3 times, and fully dried at 80 °C overnight. ZnO nanowires were successfully prepared and denoted as ZnO NW in this work. 0.2 g of the resultant ZnO nanowires were ultrasonically dispersed in 40 mL aqueous solution containing a certain amount of glucose, yielding a reaction mixture. This mixture was sealed in the 50 mL
2.3. Photocatalytic experiments UV-irradiated degradation of methylene blue (MB) aqueous solution was chosen as the probe reaction to evaluate the catalytic activity of various photocatalysts. The UV light was supplied by a 500 W highpressure Hg lamp with average light intensity of 150–200 mW/cm2. Typically, 20 mg catalysts were added into 40 mL MB aqueous solution (10 mg/L) to yield a reaction mixture, which was magnetically stirred in dark for 30 min to establish the adsorption/desorption equilibrium. When the UV light began to illuminate, about 4 mL suspension was 2
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that the four x-ZnO@C samples are covered by glucose-derived HTC materials on the surface [31,32]. To explore the detailed structure, four x-ZnO@C products have been characterized by TEM images, as shown in Fig. 4. From TEM images of the 0.2-ZnO@C (Fig. 4d and e), a HTC layer with thickness of ca. 5 nm can be revealed to uniformly coat on both top-end and middle section of the ZnO nanowire, forming a classic core/shell structure. Meanwhile, the uniform HTC coatings are also found on other x-ZnO@C composites (Fig. 4a–c), and their thicknesses are ca. 3, 4, and 8.5 nm for the 0.05ZnO@C, 0.1-ZnO@C, and 0.4-ZnO@C, respectively. Obviously, all resultant x-ZnO@C products exhibit the cable-like structure: the ZnO nanowire is uniformly deposited by a glucose-derived HTC layer on the surface, and its thickness gradually increases with the additive amount of glucose. The HTC content in x-ZnO@C nanocables can be qualitatively measured by TG in air, of which the detailed results have been shown in Fig. 5a. It is believed that slight mass losses below 200 °C are due to the desorption of water and gas on the surface, while obvious losses from 200 °C to 500 °C result from the removal of lattice water in ZnO nanowires and the combustion of surface HTC layers. Therefore, the weight percentage of HTC in the 0.05-ZnO@C, 0.1-ZnO@C, 0.2-ZnO@C and 0.4-ZnO@C can be estimated to be ca. 6.4%, 18.4%, 22.3%, and 45.3%, respectively. In UV–vis DRS of the ZnO NW and x-ZnO@C nanocables (Fig. 5b), not only a strong UV absorption (200–400 nm) attributed to the fundamental absorption edge of ZnO is observed, but also a visible absorption in the full range of 400–800 nm. Generally, it is considered that the visible absorption results from the surface HTC layer of xZnO@C nanocables, and its intensity increases with the thickness of HTC layer. This optical variation could be visually reflected by color change of five samples, as shown in insets of Fig. 5b. In spite of different visible-light absorptions, four x-ZnO@C nanocables show the equivalent UV absorption, indicating a neglected effect of HTC layer on the UV absorption of ZnO matrix. Besides promoting the visible-light absorption, the HTC layer is also certified to accelerate the separation of photogenerated hole/electron pairs on ZnO nanowires by PL spectra, as shown in Fig. S3. The strong emission peaks in ca. 380–400 nm mainly result from the recombination of photoinduced charges during the illumination process. In comparison to the ZnO NW, the weak emission peaks observed in x-ZnO@C nanocables manifest that the HTC layer could capture photogenerated electrons to increase the separation efficiency of hole/electron pairs on ZnO matrix. Particularly, the similar emission intensity is generally found in the 0.05-ZnO@C, 0.1-ZnO@C, and 0.2-ZnO@C, while greatly declines in the 0.4-ZnO@C. It is conjectured that the HTC layer with 3–5 nm contributes the constant electron-transfer efficiency, which will be enhanced when the HTC thickness exceeds 5 nm. The photocatalytic performance of the ZnO NW and x-ZnO@C nanocables could be evaluated by photodegrading MB solution under the UV light irradiation. Fig. 6a shows the absorbance variation of MB solution over different catalysts during reaction process, including adsorption in dark for 30 min and decomposition under the UV light for 60 min. For various x-ZnO@C products, their adsorption capacities gradually increase with the increasing thickness of surface HTC layer, as shown in Fig. 6. When exposed to the UV light, various catalysts exhibit the discrepant photocatalytic activities for the MB photodegradation, while the blank experiment proves that self-decomposition of MB is very slight. Similarly with the trend of MB adsorption, the photocatalytic activity of x-ZnO@C nanocables generally increases with the HTC content, as shown in Fig. 6b. However, the 0.4-ZnO@C with 8.5 nm HTC layer fails to show a superior catalytic activity than the 0.2ZnO@C with 5 nm HTC layer, although it possesses a larger adsorption amount of MB. It is concluded that the surface HTC coating can indeed promote the photocatalytic activity of ZnO matrix, and the ZnO@C sample with thicker HTC layer will exhibit the better photocatalytic performance. However, the HTC layer over 5 nm cannot facilitate the
sampled from the solution at interval of 10 min and centrifuged to remove the powder catalyst. The filtrate was measured by a UV–vis spectrophotometer (Thermo Evolution 300) at 664 nm that is the maximum absorption of MB. The degradation of MB was monitored by variations of the C/C0 ratio, where C is the absorbance of filtrates at different reaction times and C0 is the absorbance of the initial MB aqueous solution. To explore the photocatalytic mechanism, different scavengers were introduced into the standard MB photodegradation over the 0.2ZnO@C-600 catalyst. In this work, ethyl-enediaminetetraacetic acid disodium salt (EDTA-2Na), tert-butylalcohol (t-BuOH), silver nitrate (AgNO3), and benzoquinone (BQ) were used to trap holes (h+), hydroxyl radicals (%OH), electrons (e−), and superoxide radicals (%O2), respectively.
3. Results and discussions 3.1. Effect of HTC layer thickness on structure and photocatalytic performance of ZnO@C nanocables XRD is utilized to investigate the crystal structure of various hydrothermal samples, and corresponding patterns have been shown in Fig. 2. It is clearly observed similar featured peaks in all XRD patterns, which correspond to the wurtzite phase of ZnO (JCPDS no. 36-1451) [24], determining the main ZnO composition in the ZnO NW and xZnO@C products. However, no diffraction peak assigned to HTC could be found in Fig. 2. It is indicated that the crystallization of HTC is too poor to be detected by XRD. The detailed structure of various x-ZnO@C products will be discussed in this work behind. It is noteworthy that four x-ZnO@C composites have an equal XRD peak intensity with the ZnO NW, confirming that crystal structure of the ZnO NW do not be destroyed during the hydrothermal process. Representative SEM and TEM images (Fig. 3a and c) show the typical wire-shaped morphology and uniform structure in the ZnO NW. In terms of the statistical data, diameters of these ZnO nanowires are found to generally vary in a narrow range of 48–98 nm (inset, Fig. 3c). From magnified SEM and TEM images (Fig. 3b and d), the smooth surface could be revealed and suggests that the ZnO NW is a highquality nanowire-type product. After coated by the HTC layer, all of xZnO@C samples successfully maintain the intrinsic nanowire morphology, as shown in Fig. S1. Being consistent with XRD results, SEM images also confirm that the appearance and structure of the ZnO NW cannot be destroyed under the hydrothermal conditions. To verify the formation and deposition of HTC on x-ZnO@C products, their surface chemical property could be analyzed by FTIR (Fig. S2). As shown, a series of absorption bands at ca. 3500, 2900, 1700,1600, and 1400 cm−1 are clearly observed in four FTIR spectrums, which are assigned to the stretching vibrations of different functional groups, such as −OH, COO−, −COO−, and CeOeC. [30] Therefore, abundant oxygen-containing groups adequately demonstrate
Fig. 2. XRD patterns of ZnO NW and x-ZnO@C samples. 3
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Fig. 3. (a) Typical SEM, (b) magnified SEM, (c) typical TEM, and (d) magnified TEM images of ZnO NW; inset shows its diameter distribution.
disappeared from the 0.2-ZnO@C to 0.2-ZnO@C-900. Obviously, the high-temperature calcination can remove a large number of oxygencontaining groups on the surface. When the calcination temperature is as high as 900 °C, the 0.2-ZnO@C-900 shows little absorption bands in its FTIR spectrum, verifying that only trace functional groups could be retained in the calcined HTC layer. Similarly with x-ZnO@C samples, four 0.2-ZnO@C-T nanocables are found to possess the equivalent UV absorbance from the UV–vis DRS (Fig. 8b), indicating the calcinated HTC layer cannot impede UV absorption of ZnO matrix. However, the visible absorption of 0.2-ZnO@CT composites are seen to gradually decrease with the calcination temperature, accompanying the color variation of samples from black to gray (Fig. 8b). Previous works have reported that amounts of micropores would be formed inside HTC during the thermodynamic process. Therefore, in consideration of the constant HTC layer thickness, we can conjecture that many micropores are yielded inside the calcined HTC layer to decline the coating density, further resulting in the discolourization of samples. It should be noted that the HTC content in 0.2-ZnO@C-T nanocables is too low to form enough measurable micropores, whereas their BET surface areas are confirmed to be slightly increased, which are 0.27, 0.69, 2.98, 8.76 and 10.36 m2/g for the 0.2ZnO@C, 0.2-ZnO@C-600, 0.2-ZnO@C-700, 0.2-ZnO@C-800 and 0.2ZnO@C-900, respectively. After adsorbing MB, the saturated adsorption capacity of various 0.2-ZnO@C-T nanocables also exhibits a decreasing trend with the calcination temperature, as shown in Fig. 9. It is demonstrated that the surface adsorbability of ZnO@C nanocables is mainly affected by the coating density of the calcinated HTC layer, not the carbon structure. When exposed to UV light, five 0.2-ZnO@C-T samples show good photocatalytic activities for the degradation of MB, and their catalytic processes have been recorded in Fig. 9a. To reveal the relationship between physisorption and photocatalytic performance, Fig. 9b compares the equilibrium adsorption capacities and reaction constants of various 0.2-ZnO@C-T nanocables. It can be clearly observed that the photocatalyst with a larger adsorption capacity generally has an
further improvement of the catalytic activity of photocatalysts, probably due to that the overthick HTC layer will hinder the diffusion of reactants and products. In the present case, the 0.2-ZnO@C with 5 nm HTC layer is endowed with the best photocatalytic performance for the MB photodegradation, which can almost finish the full degradation of MB within 30 min (Figs. 6a and S3). 3.2. Effect of calcination temperature on structure and photocatalytic performance of ZnO@C nanocables Besides the HTC layer thickness, carbon structure also plays an important role in elevating the photocatalytic performance of ZnO@C composites. Previous works have revealed that the high-temperature treatment (above 550 °C) can dramatically change the physicochemical property of HTC materials, including the chemical composition, pore structure, electroconductivity, hydrophobicity and hydrophilicity [27,28,33]. Using the 0.2-ZnO@C as the precursor, a group of calcinated products can be prepared and thereafter denoted as 0.2-ZnO@CT, where T refers to the calcination temperature. In XRD patterns (Fig. S4), all 0.2-ZnO@C-T samples have featured peaks matched with the wurtzite phase of ZnO (JCPDS no. 36-1451) [24], indicating the consistent ZnO backbone during the high-temperature process. Moreover, it should be noted that the diffraction intensity of 0.2-ZnO@C-T samples generally has an escalating trend with the calcination temperature owning to the increasing crystallinity. For 0.2-ZnO@C-T nanocables, the thermal calcination has an ignorable effect on thickness of their surface HTC layers. From TEM images of the 0.2-ZnO@C-600 and 0.2-ZnO@C-900 (Fig. 7), it can be clearly seen that the calcined HTC layers with ca. 5 nm are uniformly coated on surface of both samples, which show an equal thickness with the original 0.2-ZnO@C. In spite of the identical thickness, the HTC layer on the 0.2-ZnO@C has graphitized under thermal conditions and transformed into carbon materials. From FTIR spectrums (Fig. 8a), it can be clearly observed that abundant absorption bands in a wide range of 500-4000 cm−1 have 4
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Fig. 4. TEM images of (a) 0.05-ZnO@C, (b) 0.1-ZnO@C, (c) 0.4-ZnO@C, and (d, e) 0.2-ZnO@C at top-end and middle section.
Fig. 5. (a) TG analysis and (b) UV–vis DRS of ZnO NW and x-ZnO@C samples; Insets are their digital photographs.
3.3. Photocatalytic mechanism of ZnO@C nanocables
enhanced photocatalytic activity, which is consistent with the above discussion of x-ZnO@C samples. However, compared with the 0.2ZnO@C, the 0.2-ZnO@C-600 exceptionally have the superior photocatalytic performance, despite it has a lower adsorbability for MB (Fig. 9b). Consequently, when the surface adsorption varies in a narrow range, the photocatalytic performance of ZnO@C nanocables is highly depending on the HTC layer structure.
From experimental information above, it is concluded that the photocatalytic performance of ZnO@C nanocables is greatly influenced by their physisorption capacity and HTC layer structure. To figure out the effect of HTC layer structure, two products with the similar saturated adsorption capacity have been chosen to further analyze their optical and electrochemical property, i. e. the 0.1-ZnO@C and 0.2ZnO@C-600, both of which can adsorb ca. 30% MB (40 mL, 10 mg/L) in 5
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Fig. 6. (a) Photodegradation of MB solution over ZnO NW and x-ZnO@C samples under UV light irradiation, and (b) their corresponding adsorption ratios and reaction constants.
Fig. 7. TEM images of (a) 0.2-ZnO@C-600 and (b) 0.2-ZnO@C-900.
Fig. 8. (a) FTIR spectrums and (b) UV–vis DRS of 0.2-ZnO@C-T samples. Insets are their digital photographs.
photogenerated electrons compared with the primary HTC layer in the 0.1-ZnO@C, although the HTC layer could also transfer surface electrons [25]. In previous works, it is believed that HTC with poor electroconductivity could be transformed into porous carbon materials with high electroconductivity during the calcination process [35,36]. We conjecture that the increasing electroconductivity of the calcinated HTC layer probably contribute to improve the electron-transferred efficiency of the 0.2-ZnO@C-600, thus resulting in its superior photocatalytic performance. To investigate the electrochemical properties, the 0.1-ZnO@C and 0.2-ZnO@C-600 have been characterized by EIS under the UV-light irradiation, and their Nyquist plots are shown in Fig. 10b. In comparison with the 0.1-ZnO@C, a depressed circular arc radius could be clearly seen in the 0.2-ZnO@C-600, suggesting a high electron diffusion
this experiment, as shown in Figs. 6 and 9. Fig. 10 compares PL spectra of the 0.1-ZnO@C and 0.2-ZnO@C-600 at an excitation wavelength of 355 nm at the room temperature. As shown, both a strong UV emission at ca. 380 nm and series of visible emission in 410–480 nm could be observed, which are attributed to the near band edge transition and surface defects of samples, respectively [34]. Particularly, the emission intensity of the 0.2-ZnO@C-600 is weaker than that of the 0.1-ZnO@C, including UV and visible emission. It is indicated that the 0.2-ZnO@C600 has a slow recombining rate of photogenerated carriers and less surface defects. Thus, higher separation efficiency of electron/hole pairs endows the 0.2-ZnO@C-600 an improved photocatalytic performance, of which reaction constants of the 0.2-ZnO@C-600 and 0.1ZnO@C are 0.051 and 0.045 min−1 (Figs. 6 and 9). Obviously, the calcinated HTC layer in the 0.2-ZnO@C-600 can effectively transfer the 6
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Fig. 9. (a) Photodegradation of MB solution over 0.2-ZnO@C-T samples under UV light irradiation, and (b) their corresponding adsorption ratios and reaction constants.
Fig. 10. (a) PL spectra and (b) Nyquist plots of 0.1-ZnO@C and 0.2-ZnO@C-600.
Fig. 11. Photocatalytic kinetics plots for MB degradation over 0.2-ZnO@C-600 in presence of no scavenger and various scavengers under the UV irradiation.
Fig. 12. Photocatalytic mechanism of ZnO@C nanocable for degradation of organic pollutants under UV light irradiation.
rate. It is confirmed that the calcinated HTC layer could capture the photogenerated surface electrons faster than the original HTC layer, which matches well with the PL result above. To detect the photocatalytic reactive species of the ZnO@C nanocable, series of trapping experiments have been performed, in which EDTA-2Na, t-BuOH, AgNO3, and BQ are chosen to capture holes (h+), hydroxyl radicals (%OH), electrons (e−), and superoxide radicals (%O2), respectively. From the fitting kinetics plots (Fig. 11), it is revealed that h+ and %OH scavengers can greatly reduce the photocatalytic performance of the 0.2-ZnO@C-600. Whereas, the photodecomposed process of MB is slightly suspended in the presence of e− and %O2 scavengers. Therefore, it is certified that in the photocatalytic reaction of the ZnO@C nanocable, h+ and %OH are the main oxidative species for the
removal of MB, although e− and %O2 are also found to degrade a small amount of organic dyes. Consequently, a possible photocatalytic mechanism has been proposed to explain the effect of HTC layer, as shown in Fig. 12. During the illumination of UV light, the ZnO matrix is excited to produce conduction band electrons (e−) and valence band holes (h+) [37]. Nevertheless, h+ will instantaneously recombine with e− (reaction b) to yield the low quantum efficiency of rough ZnO. On the surface of photocatalysts, the photogenerated h+ can react with OH− to obtain % OH (reaction d), thus both h+ and %OH will oxidize organics into nontoxic small molecules (reaction c and e). On the other hand, the photogenerated e− could be trapped by dissolved oxygen in aqueous 7
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solution, forming superoxide radicals (%O2) with the decomposed capacity for organic molecules (reaction f and g) [38]. The corresponding reactions could be expressed by equation a-g:
the surface HTC layer has also been proposed, which will possibly stimulate the design and application of more photocatalysts with the similar core/shell structure.
ZnO + hv → h+ + e−
Acknowledgements
(a)
−
h + e → ZnO + hv
(b)
organic + h+ → small molecules
(c)
h+ + OH− → %OH
(d)
organic + %OH → small molecules
(e)
+
−
%
e + O2 → O2
This work was financially supported by National Natural Science Foundation of China (No. 21576211 and 21706190), Natural Science Foundation of Tianjin City (No. 18JCQNJC76300) and Innovative Research Team in the University of Tianjin (No. TD13-5031). Appendix A. Supplementary data
(f)
organic + %O2 → small molecules +
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcata.2019.117145.
(g) −
Obviously, organic, h and e simultaneously participate in all photodegradation reactions, thus their concentrations are very important for accelerating the photocatalytic process. In the present study, the HTC layer can dually improve the photocatalytic performance of the ZnO@C nanocable in two aspects: 1) Due to the great adsorption capacity for organic molecules, the HTC layer is found to significantly elevate the surface concentration of organics on ZnO nanowires. 2) A rapid electron-migration would be verified from ZnO to the HTC layer under the UV-light irradiation (Fig. 12), which successfully suspends the recombination of h+ and e−. Therefore, after coated with the HTC layer, surface concentrations of h+, e− and organic on ZnO nanowires ([h+]s, [e−]s and [organic]s) simultaneously acquire a remarkable enhancement to inevitably accelerate series of degradation reactions (reaction c–g). Furthermore, the trapping experiments confirm that h+ and %OH play as the main catalytic active species of the ZnO@C nanocable, while e− and %O2 only contribute to the slight decomposition of toxic organics. Although the x-ZnO@C nanocable with thicker HTC layer theoretically have a superior photocatalytic activity, the 0.4-ZnO@C with 8.5 nm HTC layer fails to further improve its activity compared with the 0.2-ZnO@C with 5 nm, revealing that 5 nm is the optimal thickness of the surface HTC layer. This result indicates that the excessively thick HTC layer (over 5 nm) probably suspends the migration and diffusion of reactants and products. In addition, the 0.2-ZnO@C-600 has the equal adsorption capacity with the 0.1-ZnO@C, whereas its photocatalytic activity is higher. Obviously, due to the increase of the intrinsic electroconductivity, the calcinated HTC layer has a further increasing electron-transferred rate compared with the original HTC layer.
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4. Conclusion In summary, using ZnO nanowires as the matrix, the ZnO@C nanocable coated with a uniform HTC layer is successfully synthesized via a hydrothermal method, followed the high-temperature calcination. Particularly, the thickness, electroconductivity and coating density of the coated HTC layer could be tuned by the additive amount of glucose and the calcination temperature. In terms of the adsorption for MB molecules and the photocatalytic activity of the MB degradation, it is found that the surface adsorbability of the x-ZnO@C samples gradually increases with the HTC layer thickness, whereas the 0.2-ZnO@C with 5 nm shows the best catalytic activity, indicating that the overthick HTC layer will probably hinder the diffusion of reactants and products. Although the calcination can reduce the coating density of the HTC layer to decrease its surface adsorption, the transformation of carbon structure significantly facilitates the increase of its electroconductivity. Thus, the calcined HTC layer has the higher electron-transfer efficiency than the original HTC layer, further improving the photocatalytic performance of the ZnO@C nanocable. As a result, the 0.2-ZnO@C-600 exhibits the highest photocatalytic performance among all ZnO@C products due to both the good surface adsorbability and the rapid electron migration. Finally, a reasonable photocatalytic mechanism of 8
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Applied Catalysis A, General journal homepage: www.elsevier.com/locate/apcata
Insight into the impact of surface hydrothermal carbon layer on photocatalytic performance of ZnO nanowire ⁎
Peng Zhanga,c, Xiaoyan Yangb, Zhouzheng Jina, Jianzhou Guia, , Rui Tana, Jieshan Qiuc,
T ⁎
a
Tianjin Key Laboratory of Green Chemical Technology and Process Engineering, State Key Laboratory of Separation Membranes and Membrane Processes, School of Chemistry and Chemical Engineering, Tianjin Polytechnic University, Tianjin, 300387, China b School of Chemistry and Chemical Engineering, Shangqiu Normal University, Shangqiu, 476000, China c Liaoning Key Lab for Energy Materials and Chemical Engineering, State Key Lab of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian, 116024, China
A R T I C LE I N FO
A B S T R A C T
Keywords: ZnO Hydrothermal carbon Surface adsorption Photocatalysis
Due to the thorough interfacial contact and ignorable impediment for absorbing the incident light, the uniform hydrothermal carbon (HTC) coating is confirmed to efficiently enhance the photocatalytic performance of photocatalysts. To further explore the detailed effect of the HTC layer with various physical and chemical properties, a series of 1D ZnO@C nanocables are prepared via uniformly depositing HTC and high-temperature calcinating. The HTC layer thickness could be precisely tuned in a range of 3–8.5 nm by controlling the carbon source amount, while the calcination will decline the surface coating density of HTC and significantly increase its electroconductivity. The adsorption capacity of various ZnO@C nanocables is systematacially investigated for methylene blue (MB), as well as their photocatalytic performance for the MB-degradation. It is found that the surface adsorbability of ZnO@C nanocables generally increases with the HTC thickness, whereas 5 nm will maximally improve the photocatalytic activity of ZnO. Furthermore, the ZnO@C product calcinated at 600 °C simultaneously possesses the good surface adsorbability and high electron-transfer efficiency, thus showing the superior photocatalytic performance. Finally, a rational mechanism has been proposed to explain the improved effect of the coated HTC layer on the photocatalytic performance of primary ZnO nanowires.
1. Introduction In texile and fine chemical industries, the organic effluent has became a big environmental problem to threaten the water safety and human health. Among various wastewater treatment methods developed, photocatalysis attracts particular attentions due to its continuity, simplicity and high efficiency [1]. Many studies have reported that semiconductors could absorb the incident photons and excite the separation of photogenerated electron/hole pairs, showing good photocatalytic performance under the irradiation of UV or visible light [2,3]. Nevertheless, most of photoelectrons will instantaneously recombine with the photogenerated holes, which badly inhibits further improvement of the photocatalytic efficiency of conventional semiconductor materials. To effectively separate the photogenerated carriers, many ingenious hybridizing strategies [4–6] acquire a great development, involving various composite materials such as Ag nanowires [7], Au [8], Fe2O3 [9], and C3N4 [10]. In particular, carbon materials feature the high
⁎
capture rate for surface electrons, high specific surface area, low cost, and environmental compatibility, thus being considered as ideal candidates [11,12]. Of alternative carbon materials, hydrothermal carbon (HTC) [13–15] derived from monosaccharide or polysaccharide has recently attracted tremendous interests due to the controllable synthetic approach and good combinability with other compounds [16,17]. In addition, HTC is also verified to rapidly transfer photogenerated electrons and improve the photocatalytic activity of many semiconductor materials, such as TiO2 [18–20], ZnO [21], and CdS [22]. However, relevant researches are still insufficient to explore the optimized composite structure between HTC and semiconductors, because their interfacial area will strongly affect the catalytic property of photocatalysts [23]. In the previous work, our group one-step synthesized a rod-type ZnO@C photocatalyst with an thin HTC layer coated on the surface [24]. This HTC-wrapped core/shell structure seems to be an ideal combination between HTC and semiconductors, owning to their thorough interfacial contact and the ignorable impediment of the HTC layer for absorbing the incident light. It is also found that the thin HTC
Corresponding authors. E-mail addresses: [email protected] (J. Gui), [email protected] (J. Qiu).
https://doi.org/10.1016/j.apcata.2019.117145 Received 17 April 2019; Received in revised form 30 June 2019; Accepted 4 July 2019 Available online 05 July 2019 0926-860X/ © 2019 Published by Elsevier B.V.
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Fig. 1. Illustration of synthesis procedure of the ZnO@C nanocable.
Teflon-lined stainless steel autoclave again, which was maintained at 180 °C for 12 h. Afterward, the products were collected by filtering and washed with deionized water and absolute ethanol. After completely dried at 80 °C, the HTC-coated photocatalysts were finally obtained and denoted as x-ZnO@C, where x refers to the addition amount of glucose. Here, 0.05 g, 0.1 g, 0.2 g, and 0.4 g of glucose were adopted to synthesize corresponding HTC-coated photocatalysts. To prepare a series of 0.2-ZnO@C-T photocatalysts, the 0.2-ZnO@C sample was loaded into a tube furnace and calcinated at T °C for 2 h under the N2 atmosphere, with a heating rate of 5 °C/min.
layer can remarkably promote the photocatalytic performance of primary ZnO in two ways: On the one hand, as the electron reservoir, the HTC layer could effectively migrate surface electrons from ZnO, prolonging the lifetime of photogenerated charge carriers [25]; On the other hand, as the organic molecule trap, the strong adsorbability of HTC materials will greatly impel the surface enrichment of organic molecules on ZnO to accelerate the decomposition reaction [25,26]. Even so, two important issues are still pending and need to be resolved immediately: (1) How will the coated HTC layer thickness affects the adsorbability and catalytic performance of the ZnO@C photocatalyst? (2) It is demonstrated that the high-temperature calcination could significantly change the physical and chemical property of HTC, including the carbon structure, porosity, chemical composition, and electroconductivity [27–29]. Therefore, it is also highly desired to figure out the improved effect of calcined HTC layers with various physicochemical structures on the photocatalytic performance of the ZnO@C material. Herein, high-quality ZnO nanowires have been adopted as matrixes to prepare the cable-like ZnO@C composites with the core/shell structure. As shown in Fig. 1, the detailed synthesized process can be divided into two steps: (I) Using glucose as the carbon source, a thin HTC layer is formed and uniformly deposited on as-obtained ZnO nanowires under hydrothermal situations, yielding the HTC layer-coated ZnO@C nanocables. The HTC layer thickness could be precisely controlled in the range of 3–8.5 nm by adding the different amounts of glucose. (II) In the N2 atmosphere, the resultant ZnO@C nanocables are thermally treated to vary the chemical composition and physical property of the thin HTC layer, including the carbon structure, electroconductivity, and surface coating density. Subsequently, the photocatalytic performance of ZnO@C nanocables with various HTC layers are systematacially investigated for the degradation of methylene blue (MB) under UV light illumination. In terms of the experimental results and characterizations, a rational mechanism has also been proposed to explain the improved effect of the coated HTC layer on the photocatalytic performance of primary ZnO nanowires.
2.2. Characterization of photocatalysts Morphology and structure of various samples were observed by field-emission scanning electron microscopy (FESEM, Hitachi S4800) and transmission electron microscopy (TEM, Hitachi H7650). The diameter distribution of ZnO nanowires was obtained by collecting diameter sizes of 130 monomers. X-ray diffraction (XRD) was carried out using a Bruker D8 Advance apparatus with Cu Kα irradiation, operated at 40 kV and 100 mA. Fourier transform infrared (FTIR) spectra were measured by a Thermo Nicolet 6700 with KBr as the reference. Thermogravimetric analysis (TG) was performed in a Netzsch STA 449F3 in air, in which samples were heated from room temperature to 900 °C with a ramping rate of 10°/min. UV–vis diffuse reflectance spectra (DRS) were recorded in a range of 200–800 nm on a Shimadzu UV2700 with an integrating sphere attachment. Brunauer-EmmettTeller (BET) specific surface area was measured by Micromeritics ASAP 2020 at −196 °C, and the samples were degassed at 200 °C for 5 h under vacuum before the measurement. Photoluminescence (PL) spectra were measured using a Hitachi F-7000 with an excitation light of 355 nm. An electrochemical work station (CHI 760D, Chenhua Instrument Co. Ltd., Shanghai, China) was adopted to record the electrochemical impedance spectra (EIS) in 0.1 M Na2SO4 aqueous solution, using a standard three-electrodes system. The platinum wire electrode and saturated calomel electrode (SCE) were used as the counter electrode and reference electrode. Besides, 4 mg photocatalysts were ultrasonically dispersed in 2 mL ethanol for 10 min to yield a slurry, which was dipped into a 10 mm × 10 mm groove on an indium-tin oxide (ITO) glass. The photocatalyst/ITO was dried at 80 °C and heated at 200 °C for 8 h in N2, finally forming the work electrode. Under the irradiation of UV light, EIS were measured at 0.0 V and the Nyquist plots were recorded in the frequency range of 0.05–100 Hz with 5 mV of the sinusoidal AC perturbation.
2. Experimental section 2.1. Preparation of photocatalysts All the reagents involved were of analytical grade and used without any treatment. To synthesize ZnO nanowires, 0.2 g zinc chloride, 1.5 g sodium dodecyl sulfate (SDS) and 20 g sodium carbonate were added into 40 mL deionized water. After completely magnetic stirring, the white suspension obtained was transferred into a 50 mL Teflon-lined stainless steel autoclave, which was then heated at 140 °C for 12 h. When cooled down to the room temperature, the hydrothermal precipitates were collected by filtering, rinsed with deionized water and absolute ethanol for 3 times, and fully dried at 80 °C overnight. ZnO nanowires were successfully prepared and denoted as ZnO NW in this work. 0.2 g of the resultant ZnO nanowires were ultrasonically dispersed in 40 mL aqueous solution containing a certain amount of glucose, yielding a reaction mixture. This mixture was sealed in the 50 mL
2.3. Photocatalytic experiments UV-irradiated degradation of methylene blue (MB) aqueous solution was chosen as the probe reaction to evaluate the catalytic activity of various photocatalysts. The UV light was supplied by a 500 W highpressure Hg lamp with average light intensity of 150–200 mW/cm2. Typically, 20 mg catalysts were added into 40 mL MB aqueous solution (10 mg/L) to yield a reaction mixture, which was magnetically stirred in dark for 30 min to establish the adsorption/desorption equilibrium. When the UV light began to illuminate, about 4 mL suspension was 2
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that the four x-ZnO@C samples are covered by glucose-derived HTC materials on the surface [31,32]. To explore the detailed structure, four x-ZnO@C products have been characterized by TEM images, as shown in Fig. 4. From TEM images of the 0.2-ZnO@C (Fig. 4d and e), a HTC layer with thickness of ca. 5 nm can be revealed to uniformly coat on both top-end and middle section of the ZnO nanowire, forming a classic core/shell structure. Meanwhile, the uniform HTC coatings are also found on other x-ZnO@C composites (Fig. 4a–c), and their thicknesses are ca. 3, 4, and 8.5 nm for the 0.05ZnO@C, 0.1-ZnO@C, and 0.4-ZnO@C, respectively. Obviously, all resultant x-ZnO@C products exhibit the cable-like structure: the ZnO nanowire is uniformly deposited by a glucose-derived HTC layer on the surface, and its thickness gradually increases with the additive amount of glucose. The HTC content in x-ZnO@C nanocables can be qualitatively measured by TG in air, of which the detailed results have been shown in Fig. 5a. It is believed that slight mass losses below 200 °C are due to the desorption of water and gas on the surface, while obvious losses from 200 °C to 500 °C result from the removal of lattice water in ZnO nanowires and the combustion of surface HTC layers. Therefore, the weight percentage of HTC in the 0.05-ZnO@C, 0.1-ZnO@C, 0.2-ZnO@C and 0.4-ZnO@C can be estimated to be ca. 6.4%, 18.4%, 22.3%, and 45.3%, respectively. In UV–vis DRS of the ZnO NW and x-ZnO@C nanocables (Fig. 5b), not only a strong UV absorption (200–400 nm) attributed to the fundamental absorption edge of ZnO is observed, but also a visible absorption in the full range of 400–800 nm. Generally, it is considered that the visible absorption results from the surface HTC layer of xZnO@C nanocables, and its intensity increases with the thickness of HTC layer. This optical variation could be visually reflected by color change of five samples, as shown in insets of Fig. 5b. In spite of different visible-light absorptions, four x-ZnO@C nanocables show the equivalent UV absorption, indicating a neglected effect of HTC layer on the UV absorption of ZnO matrix. Besides promoting the visible-light absorption, the HTC layer is also certified to accelerate the separation of photogenerated hole/electron pairs on ZnO nanowires by PL spectra, as shown in Fig. S3. The strong emission peaks in ca. 380–400 nm mainly result from the recombination of photoinduced charges during the illumination process. In comparison to the ZnO NW, the weak emission peaks observed in x-ZnO@C nanocables manifest that the HTC layer could capture photogenerated electrons to increase the separation efficiency of hole/electron pairs on ZnO matrix. Particularly, the similar emission intensity is generally found in the 0.05-ZnO@C, 0.1-ZnO@C, and 0.2-ZnO@C, while greatly declines in the 0.4-ZnO@C. It is conjectured that the HTC layer with 3–5 nm contributes the constant electron-transfer efficiency, which will be enhanced when the HTC thickness exceeds 5 nm. The photocatalytic performance of the ZnO NW and x-ZnO@C nanocables could be evaluated by photodegrading MB solution under the UV light irradiation. Fig. 6a shows the absorbance variation of MB solution over different catalysts during reaction process, including adsorption in dark for 30 min and decomposition under the UV light for 60 min. For various x-ZnO@C products, their adsorption capacities gradually increase with the increasing thickness of surface HTC layer, as shown in Fig. 6. When exposed to the UV light, various catalysts exhibit the discrepant photocatalytic activities for the MB photodegradation, while the blank experiment proves that self-decomposition of MB is very slight. Similarly with the trend of MB adsorption, the photocatalytic activity of x-ZnO@C nanocables generally increases with the HTC content, as shown in Fig. 6b. However, the 0.4-ZnO@C with 8.5 nm HTC layer fails to show a superior catalytic activity than the 0.2ZnO@C with 5 nm HTC layer, although it possesses a larger adsorption amount of MB. It is concluded that the surface HTC coating can indeed promote the photocatalytic activity of ZnO matrix, and the ZnO@C sample with thicker HTC layer will exhibit the better photocatalytic performance. However, the HTC layer over 5 nm cannot facilitate the
sampled from the solution at interval of 10 min and centrifuged to remove the powder catalyst. The filtrate was measured by a UV–vis spectrophotometer (Thermo Evolution 300) at 664 nm that is the maximum absorption of MB. The degradation of MB was monitored by variations of the C/C0 ratio, where C is the absorbance of filtrates at different reaction times and C0 is the absorbance of the initial MB aqueous solution. To explore the photocatalytic mechanism, different scavengers were introduced into the standard MB photodegradation over the 0.2ZnO@C-600 catalyst. In this work, ethyl-enediaminetetraacetic acid disodium salt (EDTA-2Na), tert-butylalcohol (t-BuOH), silver nitrate (AgNO3), and benzoquinone (BQ) were used to trap holes (h+), hydroxyl radicals (%OH), electrons (e−), and superoxide radicals (%O2), respectively.
3. Results and discussions 3.1. Effect of HTC layer thickness on structure and photocatalytic performance of ZnO@C nanocables XRD is utilized to investigate the crystal structure of various hydrothermal samples, and corresponding patterns have been shown in Fig. 2. It is clearly observed similar featured peaks in all XRD patterns, which correspond to the wurtzite phase of ZnO (JCPDS no. 36-1451) [24], determining the main ZnO composition in the ZnO NW and xZnO@C products. However, no diffraction peak assigned to HTC could be found in Fig. 2. It is indicated that the crystallization of HTC is too poor to be detected by XRD. The detailed structure of various x-ZnO@C products will be discussed in this work behind. It is noteworthy that four x-ZnO@C composites have an equal XRD peak intensity with the ZnO NW, confirming that crystal structure of the ZnO NW do not be destroyed during the hydrothermal process. Representative SEM and TEM images (Fig. 3a and c) show the typical wire-shaped morphology and uniform structure in the ZnO NW. In terms of the statistical data, diameters of these ZnO nanowires are found to generally vary in a narrow range of 48–98 nm (inset, Fig. 3c). From magnified SEM and TEM images (Fig. 3b and d), the smooth surface could be revealed and suggests that the ZnO NW is a highquality nanowire-type product. After coated by the HTC layer, all of xZnO@C samples successfully maintain the intrinsic nanowire morphology, as shown in Fig. S1. Being consistent with XRD results, SEM images also confirm that the appearance and structure of the ZnO NW cannot be destroyed under the hydrothermal conditions. To verify the formation and deposition of HTC on x-ZnO@C products, their surface chemical property could be analyzed by FTIR (Fig. S2). As shown, a series of absorption bands at ca. 3500, 2900, 1700,1600, and 1400 cm−1 are clearly observed in four FTIR spectrums, which are assigned to the stretching vibrations of different functional groups, such as −OH, COO−, −COO−, and CeOeC. [30] Therefore, abundant oxygen-containing groups adequately demonstrate
Fig. 2. XRD patterns of ZnO NW and x-ZnO@C samples. 3
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Fig. 3. (a) Typical SEM, (b) magnified SEM, (c) typical TEM, and (d) magnified TEM images of ZnO NW; inset shows its diameter distribution.
disappeared from the 0.2-ZnO@C to 0.2-ZnO@C-900. Obviously, the high-temperature calcination can remove a large number of oxygencontaining groups on the surface. When the calcination temperature is as high as 900 °C, the 0.2-ZnO@C-900 shows little absorption bands in its FTIR spectrum, verifying that only trace functional groups could be retained in the calcined HTC layer. Similarly with x-ZnO@C samples, four 0.2-ZnO@C-T nanocables are found to possess the equivalent UV absorbance from the UV–vis DRS (Fig. 8b), indicating the calcinated HTC layer cannot impede UV absorption of ZnO matrix. However, the visible absorption of 0.2-ZnO@CT composites are seen to gradually decrease with the calcination temperature, accompanying the color variation of samples from black to gray (Fig. 8b). Previous works have reported that amounts of micropores would be formed inside HTC during the thermodynamic process. Therefore, in consideration of the constant HTC layer thickness, we can conjecture that many micropores are yielded inside the calcined HTC layer to decline the coating density, further resulting in the discolourization of samples. It should be noted that the HTC content in 0.2-ZnO@C-T nanocables is too low to form enough measurable micropores, whereas their BET surface areas are confirmed to be slightly increased, which are 0.27, 0.69, 2.98, 8.76 and 10.36 m2/g for the 0.2ZnO@C, 0.2-ZnO@C-600, 0.2-ZnO@C-700, 0.2-ZnO@C-800 and 0.2ZnO@C-900, respectively. After adsorbing MB, the saturated adsorption capacity of various 0.2-ZnO@C-T nanocables also exhibits a decreasing trend with the calcination temperature, as shown in Fig. 9. It is demonstrated that the surface adsorbability of ZnO@C nanocables is mainly affected by the coating density of the calcinated HTC layer, not the carbon structure. When exposed to UV light, five 0.2-ZnO@C-T samples show good photocatalytic activities for the degradation of MB, and their catalytic processes have been recorded in Fig. 9a. To reveal the relationship between physisorption and photocatalytic performance, Fig. 9b compares the equilibrium adsorption capacities and reaction constants of various 0.2-ZnO@C-T nanocables. It can be clearly observed that the photocatalyst with a larger adsorption capacity generally has an
further improvement of the catalytic activity of photocatalysts, probably due to that the overthick HTC layer will hinder the diffusion of reactants and products. In the present case, the 0.2-ZnO@C with 5 nm HTC layer is endowed with the best photocatalytic performance for the MB photodegradation, which can almost finish the full degradation of MB within 30 min (Figs. 6a and S3). 3.2. Effect of calcination temperature on structure and photocatalytic performance of ZnO@C nanocables Besides the HTC layer thickness, carbon structure also plays an important role in elevating the photocatalytic performance of ZnO@C composites. Previous works have revealed that the high-temperature treatment (above 550 °C) can dramatically change the physicochemical property of HTC materials, including the chemical composition, pore structure, electroconductivity, hydrophobicity and hydrophilicity [27,28,33]. Using the 0.2-ZnO@C as the precursor, a group of calcinated products can be prepared and thereafter denoted as 0.2-ZnO@CT, where T refers to the calcination temperature. In XRD patterns (Fig. S4), all 0.2-ZnO@C-T samples have featured peaks matched with the wurtzite phase of ZnO (JCPDS no. 36-1451) [24], indicating the consistent ZnO backbone during the high-temperature process. Moreover, it should be noted that the diffraction intensity of 0.2-ZnO@C-T samples generally has an escalating trend with the calcination temperature owning to the increasing crystallinity. For 0.2-ZnO@C-T nanocables, the thermal calcination has an ignorable effect on thickness of their surface HTC layers. From TEM images of the 0.2-ZnO@C-600 and 0.2-ZnO@C-900 (Fig. 7), it can be clearly seen that the calcined HTC layers with ca. 5 nm are uniformly coated on surface of both samples, which show an equal thickness with the original 0.2-ZnO@C. In spite of the identical thickness, the HTC layer on the 0.2-ZnO@C has graphitized under thermal conditions and transformed into carbon materials. From FTIR spectrums (Fig. 8a), it can be clearly observed that abundant absorption bands in a wide range of 500-4000 cm−1 have 4
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Fig. 4. TEM images of (a) 0.05-ZnO@C, (b) 0.1-ZnO@C, (c) 0.4-ZnO@C, and (d, e) 0.2-ZnO@C at top-end and middle section.
Fig. 5. (a) TG analysis and (b) UV–vis DRS of ZnO NW and x-ZnO@C samples; Insets are their digital photographs.
3.3. Photocatalytic mechanism of ZnO@C nanocables
enhanced photocatalytic activity, which is consistent with the above discussion of x-ZnO@C samples. However, compared with the 0.2ZnO@C, the 0.2-ZnO@C-600 exceptionally have the superior photocatalytic performance, despite it has a lower adsorbability for MB (Fig. 9b). Consequently, when the surface adsorption varies in a narrow range, the photocatalytic performance of ZnO@C nanocables is highly depending on the HTC layer structure.
From experimental information above, it is concluded that the photocatalytic performance of ZnO@C nanocables is greatly influenced by their physisorption capacity and HTC layer structure. To figure out the effect of HTC layer structure, two products with the similar saturated adsorption capacity have been chosen to further analyze their optical and electrochemical property, i. e. the 0.1-ZnO@C and 0.2ZnO@C-600, both of which can adsorb ca. 30% MB (40 mL, 10 mg/L) in 5
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Fig. 6. (a) Photodegradation of MB solution over ZnO NW and x-ZnO@C samples under UV light irradiation, and (b) their corresponding adsorption ratios and reaction constants.
Fig. 7. TEM images of (a) 0.2-ZnO@C-600 and (b) 0.2-ZnO@C-900.
Fig. 8. (a) FTIR spectrums and (b) UV–vis DRS of 0.2-ZnO@C-T samples. Insets are their digital photographs.
photogenerated electrons compared with the primary HTC layer in the 0.1-ZnO@C, although the HTC layer could also transfer surface electrons [25]. In previous works, it is believed that HTC with poor electroconductivity could be transformed into porous carbon materials with high electroconductivity during the calcination process [35,36]. We conjecture that the increasing electroconductivity of the calcinated HTC layer probably contribute to improve the electron-transferred efficiency of the 0.2-ZnO@C-600, thus resulting in its superior photocatalytic performance. To investigate the electrochemical properties, the 0.1-ZnO@C and 0.2-ZnO@C-600 have been characterized by EIS under the UV-light irradiation, and their Nyquist plots are shown in Fig. 10b. In comparison with the 0.1-ZnO@C, a depressed circular arc radius could be clearly seen in the 0.2-ZnO@C-600, suggesting a high electron diffusion
this experiment, as shown in Figs. 6 and 9. Fig. 10 compares PL spectra of the 0.1-ZnO@C and 0.2-ZnO@C-600 at an excitation wavelength of 355 nm at the room temperature. As shown, both a strong UV emission at ca. 380 nm and series of visible emission in 410–480 nm could be observed, which are attributed to the near band edge transition and surface defects of samples, respectively [34]. Particularly, the emission intensity of the 0.2-ZnO@C-600 is weaker than that of the 0.1-ZnO@C, including UV and visible emission. It is indicated that the 0.2-ZnO@C600 has a slow recombining rate of photogenerated carriers and less surface defects. Thus, higher separation efficiency of electron/hole pairs endows the 0.2-ZnO@C-600 an improved photocatalytic performance, of which reaction constants of the 0.2-ZnO@C-600 and 0.1ZnO@C are 0.051 and 0.045 min−1 (Figs. 6 and 9). Obviously, the calcinated HTC layer in the 0.2-ZnO@C-600 can effectively transfer the 6
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Fig. 9. (a) Photodegradation of MB solution over 0.2-ZnO@C-T samples under UV light irradiation, and (b) their corresponding adsorption ratios and reaction constants.
Fig. 10. (a) PL spectra and (b) Nyquist plots of 0.1-ZnO@C and 0.2-ZnO@C-600.
Fig. 11. Photocatalytic kinetics plots for MB degradation over 0.2-ZnO@C-600 in presence of no scavenger and various scavengers under the UV irradiation.
Fig. 12. Photocatalytic mechanism of ZnO@C nanocable for degradation of organic pollutants under UV light irradiation.
rate. It is confirmed that the calcinated HTC layer could capture the photogenerated surface electrons faster than the original HTC layer, which matches well with the PL result above. To detect the photocatalytic reactive species of the ZnO@C nanocable, series of trapping experiments have been performed, in which EDTA-2Na, t-BuOH, AgNO3, and BQ are chosen to capture holes (h+), hydroxyl radicals (%OH), electrons (e−), and superoxide radicals (%O2), respectively. From the fitting kinetics plots (Fig. 11), it is revealed that h+ and %OH scavengers can greatly reduce the photocatalytic performance of the 0.2-ZnO@C-600. Whereas, the photodecomposed process of MB is slightly suspended in the presence of e− and %O2 scavengers. Therefore, it is certified that in the photocatalytic reaction of the ZnO@C nanocable, h+ and %OH are the main oxidative species for the
removal of MB, although e− and %O2 are also found to degrade a small amount of organic dyes. Consequently, a possible photocatalytic mechanism has been proposed to explain the effect of HTC layer, as shown in Fig. 12. During the illumination of UV light, the ZnO matrix is excited to produce conduction band electrons (e−) and valence band holes (h+) [37]. Nevertheless, h+ will instantaneously recombine with e− (reaction b) to yield the low quantum efficiency of rough ZnO. On the surface of photocatalysts, the photogenerated h+ can react with OH− to obtain % OH (reaction d), thus both h+ and %OH will oxidize organics into nontoxic small molecules (reaction c and e). On the other hand, the photogenerated e− could be trapped by dissolved oxygen in aqueous 7
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solution, forming superoxide radicals (%O2) with the decomposed capacity for organic molecules (reaction f and g) [38]. The corresponding reactions could be expressed by equation a-g:
the surface HTC layer has also been proposed, which will possibly stimulate the design and application of more photocatalysts with the similar core/shell structure.
ZnO + hv → h+ + e−
Acknowledgements
(a)
−
h + e → ZnO + hv
(b)
organic + h+ → small molecules
(c)
h+ + OH− → %OH
(d)
organic + %OH → small molecules
(e)
+
−
%
e + O2 → O2
This work was financially supported by National Natural Science Foundation of China (No. 21576211 and 21706190), Natural Science Foundation of Tianjin City (No. 18JCQNJC76300) and Innovative Research Team in the University of Tianjin (No. TD13-5031). Appendix A. Supplementary data
(f)
organic + %O2 → small molecules +
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcata.2019.117145.
(g) −
Obviously, organic, h and e simultaneously participate in all photodegradation reactions, thus their concentrations are very important for accelerating the photocatalytic process. In the present study, the HTC layer can dually improve the photocatalytic performance of the ZnO@C nanocable in two aspects: 1) Due to the great adsorption capacity for organic molecules, the HTC layer is found to significantly elevate the surface concentration of organics on ZnO nanowires. 2) A rapid electron-migration would be verified from ZnO to the HTC layer under the UV-light irradiation (Fig. 12), which successfully suspends the recombination of h+ and e−. Therefore, after coated with the HTC layer, surface concentrations of h+, e− and organic on ZnO nanowires ([h+]s, [e−]s and [organic]s) simultaneously acquire a remarkable enhancement to inevitably accelerate series of degradation reactions (reaction c–g). Furthermore, the trapping experiments confirm that h+ and %OH play as the main catalytic active species of the ZnO@C nanocable, while e− and %O2 only contribute to the slight decomposition of toxic organics. Although the x-ZnO@C nanocable with thicker HTC layer theoretically have a superior photocatalytic activity, the 0.4-ZnO@C with 8.5 nm HTC layer fails to further improve its activity compared with the 0.2-ZnO@C with 5 nm, revealing that 5 nm is the optimal thickness of the surface HTC layer. This result indicates that the excessively thick HTC layer (over 5 nm) probably suspends the migration and diffusion of reactants and products. In addition, the 0.2-ZnO@C-600 has the equal adsorption capacity with the 0.1-ZnO@C, whereas its photocatalytic activity is higher. Obviously, due to the increase of the intrinsic electroconductivity, the calcinated HTC layer has a further increasing electron-transferred rate compared with the original HTC layer.
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4. Conclusion In summary, using ZnO nanowires as the matrix, the ZnO@C nanocable coated with a uniform HTC layer is successfully synthesized via a hydrothermal method, followed the high-temperature calcination. Particularly, the thickness, electroconductivity and coating density of the coated HTC layer could be tuned by the additive amount of glucose and the calcination temperature. In terms of the adsorption for MB molecules and the photocatalytic activity of the MB degradation, it is found that the surface adsorbability of the x-ZnO@C samples gradually increases with the HTC layer thickness, whereas the 0.2-ZnO@C with 5 nm shows the best catalytic activity, indicating that the overthick HTC layer will probably hinder the diffusion of reactants and products. Although the calcination can reduce the coating density of the HTC layer to decrease its surface adsorption, the transformation of carbon structure significantly facilitates the increase of its electroconductivity. Thus, the calcined HTC layer has the higher electron-transfer efficiency than the original HTC layer, further improving the photocatalytic performance of the ZnO@C nanocable. As a result, the 0.2-ZnO@C-600 exhibits the highest photocatalytic performance among all ZnO@C products due to both the good surface adsorbability and the rapid electron migration. Finally, a reasonable photocatalytic mechanism of 8