Article pubs.acs.org/IECR
Cite This: Ind. Eng. Chem. Res. 2019, 58, 10941−10950
A Novel Microchannel Synthesis Strategy for Continuous Fabrication of Nanosized γ‑CuI and Their Photocatalytic Performance Li You,†,‡,⊥ Jianxin Cao,†,‡,⊥ and Fei Liu*,†,§ †
School of Chemistry and Chemical Engineering, Guizhou University, Guiyang, Guizhou 550025, P. R. China Key Laboratory of Green Chemical and Clean Energy Technology, Guiyang, Guizhou 550025, P. R. China § Key Laboratory of Efficient Utilization of Mineral and Green Chemical Technology, Guiyang, Guizhou 550025, P. R. China Downloaded via BUFFALO STATE on July 30, 2019 at 16:26:30 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: A novel microchannel synthesis route for continuous synthesis of nanosized copper iodide (γ-CuI) using hydrated hydrazine as reducing agent was proposed. We continuously produced γ-CuI spherical nanoparticles in 99% yield and grain size of ca. 44−100 nm. Furthermore, the utilization of hydrated hydrazine was essential in precluding powder plugging the microchannel by self-generated N2. The microreaction process conditions, such as residence time, pH value, temperature, and reactant concentration, had great impact on the crystal phase and grain size. The optical properties analysis showed that red-shift in band age and emission intensity of photoluminescence spectra at 428 nm decreased with decreasing grain size. Moreover, the nanosized spherical γ-CuI exhibits enhanced photocatalytic performance in the photodegradation of methylene blue and methyl orange. als.15 However, the microchannel is prone to blockage by the particles produced, which hampers development and application in the nanopowder synthesis industry.16 In order to overcome this issue current strategies utilize an inert liquid/gas medium,17 modified microreactor,18 or external force enhancement,19 resulting in a more complex procedure. Herein, KI and Cu(NO3)2·3H2O were used as raw materials in the preparation of nanosized γ-CuI via a microchannel synthesis route. We found that selecting hydrazine hydrate as the reducing agent greatly improved the reaction efficiency by effectively controlling the pH of the system. The production of N2 in the hydrazine hydrate reduction reaction generated a gas−liquid segmentation flow in the microchannel, which effectively alleviated the blockage issue in the microchannel. By taking advantage of the higher mass transfer and continuous transport mode, it was possible to separate the precipitation and reduction process in γ-CuI synthesis. Hence, we optimized the microchannel process conditions, including residence time, pH value, temperature, and reactant concentration. γ-CuI nanoparticles were successfully prepared in a stable and continuously high yield by microchannel synthesis route, with considerably smaller particle size compared to traditional
1. INTRODUCTION Recently, γ-CuI has been of particular interest due to its wide band gap Eg = 3.1 eV, stable p-type conductivity at room temperature, and fast ionic conductivity at high temperature.1 Numerous studies have shown that the structure of materials has a significant relationship with its photocatalytic properties.2−8 For instance, Masoud Salavati-Niasari found that different structures of γ-CuI exhibit various photocatalytic activities.9,10 Therefore, it is significant to study the influence of the structure of γ-CuI on its photocatalytic performance. The synthesis of γ-CuI via the liquid phase precipitation strategy is often employed due to its mild synthesis condition and consists of a two-step process: Cu2+ reduction and γ-CuI precipitation.11,12 However, conventional liquid phase precipitation in a stirred batch reactor exhibits significantly inferior mixing performance with intermittent operation, which promotes the precipitation and reduction process to proceed simultaneously, thus hindering the continuous preparation of nanosized γ-CuI with homogeneous particle size. Presently, continuous formation of nanosized γ-CuI with homogeneous particle size is impossible. Thus, the realization of such a challenging prospect is of significant scientific and practical interest. The synthesis of nanomaterials using the microchannel strategy is often employed due to its superior high mass and heat transfer efficiency, instantaneous adjustment ability, and a small scale-up effect.13,14 In general, microchannel synthesis strategy can effectively control the particle size of nanomateri© 2019 American Chemical Society
Received: Revised: Accepted: Published: 10941
January 20, 2019 May 31, 2019 June 4, 2019 June 4, 2019 DOI: 10.1021/acs.iecr.9b00354 Ind. Eng. Chem. Res. 2019, 58, 10941−10950
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Industrial & Engineering Chemistry Research
Figure 1. Microreactor structure and experimental flow.
The yield of γ-CuI is defined as follows
precipitation methods. Moreover, these small-sized spherical γCuI nanoparticles showed enhanced optical properties.
Y=
2. MATERIALS AND METHODS All reagents used in these experiments, including Cu(NO3)2· 3H2O (95%), N2H4·H2O (98%), KI (99%), and KOH (95%), were analytically pure and purchased from Aladdin. γ-CuI was prepared using a microsieve dispersion reactor and traditional batch stirred reactor, respectively. A stainless steel rectangle microsieve plate with φ200 μm pore size was used as dispersion structure, and the geometrical dimensions of the stainless steel microchannel were 20 mm × 1 mm × 1 mm (length × width × height). The reactor outlet tube was a PTFE tube φ2 mm in diameter. The microreactor structure and experimental flow are shown in Figure 1. Preparation of γ-CuI by microsieve dispersion reactor: The molar ratio of the raw materials was Cu2+:I−:N2H4·H2O = 1:1:1. A certain amount of KOH was added to adjust the pH of the reaction system. In a typical experiment, 2.49 g of KI was dissolved in 30 mL of deionized water, and then 0.75 mL of hydrazine hydrate and 1.5 mL of 5 mol/L KOH solution were added thereto and stirred uniformly to obtain the mixed solution, which was employed as the dispersed phase (0.5 mol/ L). Cu(NO3)2·H2O (3.645 g) was dissolved in 30 mL of deionized water and then was stirred evenly, as continuous phase solution (0.5 mol/L). The two-phase solution was pumped into the microsieve dispersion reactor using a circulating pump (the ratio of the dispersed phase and continuous phase feed flow rate was 1:1, flow rate is 20 mL/ min), and the dispersed phase feed was injected from the dispersed feed inlet through the microsieve to the microchannel to mix with the continuous phase feed from the continuous feed inlet. Production was obtained at the outlet of the microreaction tube. Precipitates obtained by operating the microreactor for 1 min were washed one time by water and two times by ethanol, centrifuged for 3 min at 10 500 r/min after each wash, and then dried at 105 °C for 2 h. Preparation of γ-CuI by traditional batch stirred reactor: The molar ratio of the raw materials was Cu2+:I−:N2H4·H2O = 1:1:1. First 100 mL of 0.5 mol/L N2H4·H2O and KI mixed solution was dropped into a batch reactor containing 100 mL of 0.5 mol/L Cu(NO3)2·3H2O solution under stirring at 600 r/min (pH = 4.4, pH was adjusted using KOH solution) and continuously stirred for 30 min after the mixed solution was exhausted. Precipitates were washed one time by water and two times by ethanol, centrifuged at 10 500 r/min, and then dried at 105 °C for 2 h.
M1 × 100% M
(1)
where Y is the yield of γ-CuI and M1 (g) is the actual mass of γCuI prepared by running the microreactor for 1 min. M (g) is the theoretical mass of γ-CuI prepared by running the microreactor for 1 min. A digital microscope camera with fast photographing was used to observe the bubble generation in the microchannel. X-ray powder diffraction (XRD) patterns of the samples were recorded using a PANalytical X’Pert PRO X-ray diffractometer. Sample sizes and particle size distribution were measured using a FeiTitan 80° transmission electron microscope (TEM); the mean primary particle size and its distribution were quantified based on a statistical numberweighted method by surveying more than 200 particles on the TEM images through Digital Micrograph Software. The morphology of the samples was observed by Hitachi SU8010 scanning electron microscope (SEM). X-ray photoelectron spectra (XPS) of the samples were taken using a K-Alpha+ Xray photoelectron spectroscope manufactured by Thermo Fisher, USA (the binding energies obtained in the XPS analysis were corrected for specimen charging by referencing the C 1s to 284.80 eV). The surface characteristic functional groups of the sample were qualitatively characterized by iS50 FT-IR infrared spectrometer (FT-IR) manufactured by Thermo Fisher, USA. The UV−vis spectrum of the sample (wavelength range of 200−800 nm), methylene blue solution absorbance (wavelength of 664 nm), and methyl orange absorbance (wavelength of 464 nm) were measured by Hitachi UV3600Plus spectrophotometer. The fluorescence spectrum of the sample was analyzed by Hitachi Cary Eclipse fluorescence spectrometer (excitation wavelength of 370 nm). The Scherrer equation, eq 2, was used to determine the particle size of the crystals: D = 0.89λ /B cos θ
(2)
where D is the grain size, B is the full width at half-maximum (radian system), λ is the X-ray wavelength, which was maintained at 0.154 06 nm in our experiments, and θ is the Bragg angle; the diffraction peak of θ = 25.5° was used to calculate the particle size. The γ-CuI photocatalytic activity of various particle sizes was tested by monitoring the degradation of methylene blue (MB) and methyl orange (MO) under UV light irradiation. The photocatalytic reaction was carried out under UV light 10942
DOI: 10.1021/acs.iecr.9b00354 Ind. Eng. Chem. Res. 2019, 58, 10941−10950
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Figure 2. Preparation of γ-CuI particle size in different residence times: (a) 1, (b) 5, and (c) 10 s. (T = 20 °C, C = 0.5 mol/L, residence time = 1, 5, and 10 s, pH = 4.4.)
Figure 3. (a) XRD and (b) FT-IR patterns of samples synthesized at different pH values. (T = 20 °C, C = 0.5 mol/L, residence time = 5 s, pH = 3.2, 4.4, 5.8, and 6.9.)
3. RESULTS AND DISCUSSION 3.1. Effect of Residence Time in Microchannel on Particle Size. The residence time in the microchannel has a significant influence on the particle size. For our microreaction system, the residence time of the mixed reaction liquid in the microchannel was adjusted by changing the length of the exit microreactor tube. XRD analyses of the samples are shown in Figure S1. TEM images of γ-CuI prepared at different residence times in the microchannel are shown in Figure 2. XRD spectra show that the diffraction peak of the samples, which were synthesized at different residence times (1, 5, and 10 s), are consistent with the standard card (JCPDS: 06-0246, space group: F4̅3m (216)), indicating that synthesized γ-CuI was a cubic crystal. According to TEM images, γ-CuI possessed a larger particle size >100 nm when the residence time was 1 s. As the residence time extended to 5 and 10 s, the average particle size of γ-CuI was reduced to ca. 50 nm with a more homogeneous particle size. The rapid mass transfer characteristics of the microreactor can achieve effective mixing in a very short time period, resulting in a short Cu2+ reduction time. Hence, the reduction and precipitation process could be completely separate. The continuous transport mode of the microreactor channel promotes a homogeneous and stable nucleation process via control of the residence time of the reaction mixture in the microreaction tube. This characteristic embodies one of the
(the lamp power was 300 W, the liquid level was 10 cm from the light source, the current intensity was 20 A, and the reaction vessel was a 200 mL double-jacketed glass beaker). Fifty milligrams of γ-CuI was placed in a jacketed beaker containing 100 mL of MB or MO solution (20 mg/L). The glass beaker was subjected to water cooling to control the reaction temperature to room temperature. Under magnetic stirring (the suspension was stirred in the dark for 30 mines before UV light to ensure the adsorption/desorption equilibrium of dyes on the surface of γ-CuI particles), the suspension was taken out every 20 min and centrifuged to remove γ-CuI particles. The degradation process was monitored by measuring the absorbance of MB or MO in the solution at 664 or 464 nm by a UV−vis spectrophotometer. Dyes degradation rate is defined using eq 3 R=
A0 − A × 100% A0
(3)
where R is the degradation rate of dyes, A0 is the initial concentration of dyes, and A is the concentration of dyes after the degradation reaction. The degradation rate constant of dyes is defined as
−
dA = KA dt
(4)
K (s−1) is the degradation rate constant. 10943
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Figure 4. (a) XPS full spectra of samples synthesized at different pH values and (b) the N 1s peak of the sample synthesized at pH = 6.9
Figure 5. (a) Cu 2p peak of XPS spectrum of samples synthesized at different pH values and (b) the results of fitting the O 1s peak of the synthesized samples at pH = 5.8 and 6.9.
primary advantages of the microchannel synthesis strategy in solving the two-step reaction process. However, when a shorter residence time in the tube was employed, the precipitation process occurred in the beaker outside the reaction tube, which results in inhomogeneous and larger grain size. Conversely, adhesion of γ-CuI on the tube’s inner wall gradually increases with extended residence time, causing an increase in γ-CuI loss within the tube. Therefore, the optimum residence time in the tube is 5 s. 3.2. Effect of Reaction pH on Crystal Phase and Particle Size. The effect of different pH conditions on the crystal phase and particle size was investigated by adjusting the amount of KOH added to the dispersed phase. XRD, FT-IR, XPS, TEM, and particle size distribution analysis of the samples are shown in Figures 3−6. XRD spectra (Figure 3a) show that the diffraction peaks of the samples, synthesized at different pH values (3.2, 4.4, and 5.8), are consistent with the standard card, indicating that the synthesized γ-CuI was a cubic crystal. As pH increased, the diffraction peak of the samples tended to broaden, and a gradual decrease in intensity was observed. At pH = 6.9, the impurity diffraction peak appeared in XRD diffraction pattern of the sample. FT-IR spectra (Figure 3b) show that at pH = 6.9, the sample exhibited the corresponding N−H stretching vibration, NO
stretching vibration, and N−H bending vibration absorption peaks at 3271.69, 1574.47, and 1140.70 cm−1, respectively. This sample also displayed N 1s peak in XPS full spectrum (Figure 4a). The N 1s orbital binding energy was 400.64 eV (Figure 4b), which corresponded to the binding energy of N O and N−H.20,21 Therefore, we confirmed that the sample synthesized at pH = 6.9 contains NO and N−H groups. Cu 2p orbital binding energy in the samples gradually increases with increasing pH (Figure 5a), and they are within Cu+ binding energy range (931.0−931.8 eV).22 Using the obtained results, the O 1s peak of the synthesized samples under different pH conditions was fitted using Thermo advantage software. The generated spectra displayed a single peak at 3.2 and 4.4 pH values (Figure S2) and binding energy of 532.53 eV, which corresponds to CO compound state. At pH = 5.8, the spectrum shows a double peak (Figure 5b), with binding energies of 530.96 and 532.46 eV, which corresponds to the combined states of Cu−O−Cu and CO, respectively. At pH = 6.9, three decomposition peaks are observed (Figure 6b), with binding energies 530.80, 532.20, and 533.22 eV, corresponding to Cu−O−Cu, CO, and NO, respectively.22 Thus, based on previous XRD analysis, Cu2O was not present in the samples at pH = 3.2 and 4.4; however, at pH = 5.8 and 6.9, Cu2O was formed. The presence of Cu2O was not 10944
DOI: 10.1021/acs.iecr.9b00354 Ind. Eng. Chem. Res. 2019, 58, 10941−10950
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Figure 6. TEM images and size distribution histograms of γ-CuI synthesized at different pH values: (a and b) pH = 3.2 and (c and d) pH = 4.4. (T = 20 °C, C = 0.5 mol/L, residence time = 5 s.)
Figure 7. Particle size of γ-CuI synthesized at different temperatures (a) T = 20, 40, 60, and 80 °C, pH = 4.4, C = 0.5 mol/L, residence time = 5 s, and different concentrations (b) T = 20 °C, pH = 4.4, C = 0.01, 0.2, 0.5, and 1.0 mol/L, residence time = 5 s.
The corresponding mean particle sizes at pH = 3.2 and 4.4 are ca. 150 and 51 nm, respectively (Figure 6). From the histogram of the statistical distribution of particle size, particle size decreased and became homogeneous at optimal pH. This trend may extend to the reduction ability of hydrazine hydrate related to pH elevation, resulting in an increase in the amount of Cu+ and enhancement in instant nucleation of γ-CuI. In the case of pH = 4.4, γ-CuI mean particle size was ca. 51 nm, with 99% yield. The actual reactions in the microchannel are shown in Figure S3. N2 gas bubbles were generated in the reaction of hydrazine hydrate (Figure S3a), and a gas−liquid segmentation
detected in the XRD pattern at pH = 5.8 because the Cu2O content was below the detection limit of the instrument. It is well-known that hydrazine hydrate produces H+ during the reduction process, and H+ combines with N2H4 to form N2H5+, which seriously weakens the reducing ability of hydrazine hydrate.23 However, the addition of OH− to the reaction system effectively consumes H+, hence improving the reduction rate of Cu2+. The concentration of OH− increases with increasing pH, where excess OH− combines with Cu+ to form CuOH that decomposes to Cu2O. Therefore, the reaction pH should not exceed 4.4. 10945
DOI: 10.1021/acs.iecr.9b00354 Ind. Eng. Chem. Res. 2019, 58, 10941−10950
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Industrial & Engineering Chemistry Research flow was formed in the microchannel. The microreactor was operated in a continuous and stable manner for more than 6000 min, which indicates that N2 produced during the reduction reaction effectively alleviates blockage in the microchannel. On the contrary, under the same operating conditions, no bubbles were observed during the reaction with Na2S2O3 (Figure S3b), leading to undesirable blockage in the microchannel after 120 min of continuous operation. 3.3. Influence of Several Key Factors and Particle Size Control. In order to study the factors affecting the particle size of γ-CuI, the reaction temperature and reactant concentration were further investigated. The XRD patterns of all samples are shown in Figures S4−S5. The diffraction peaks of all samples were consistent with the standard card, indicating that those synthesized in various conditions are cubic γ-CuI. Synthesized γ-CuI particle size in different conditions is shown in Figure 7. A gradual decrease in particle size was found as the reaction temperature increased (Figure 7a); however, elevated production cost must be considered at such temperatures. More importantly, the viscosity of the precipitate in the tube increases with increasing reaction temperature, leading to blockage in the microchannel. As the concentration increases to 0.5 mol/L, γ-CuI particle size decreases, and above this concentration a slight increase in particle size occurs (Figure 7b). Similar to the classic nucleation−growth theorem,24 γ-CuI particle size via microchannel synthesis strategy is closely related to reactant concentration. Furthermore, the fluidity of the precipitate in the tube decreases with increasing reactant concentration; therefore, at high concentrations blockage of the microchannel was observed. In summary, a pure cubic phase γ-CuI with high yield (99%) and particle size of ca. 44−100 nm was continuously achieved at a temperature of 20 °C, residence time of 5 s, reactant concentration of 0.5 mol/L, and pH value of 4.4. 3.4. Comparison of γ-CuI Particle Size Prepared by Microchannel Synthesis Strategy and Traditional Liquid Phase Precipitation. In order to compare γ-CuI particle sizes using different strategies, γ-CuI was prepared under the same conditions using a microsieve dispersion reactor and batch stirred reactor. XRD patterns, TEM images, SEM images, EDS patterns, and particle size distributions of the samples are shown in Figures 8−10. The diffraction peaks of the
synthesized samples via both strategies are consistent with the values in the standard card (Figure 8), indicating that all are cubic γ-CuI. The XRD diffraction peak of γ-CuI, prepared using a microreactor, was broad and of weak intensity, which indicated that the particle size was relatively smaller than that of the batch stirred reactor. Compared to the conventional batch stirred reactor (ca. 418 nm), the particle size was significantly larger than those prepared in the microreactor (ca. 51 nm). Moreover, the size distribution of γ-CuI particles prepared by the microreactor was more homogeneous than by the conventional batch stirred reactor (Figure 9). The SEM images and EDS patterns of γ-CuI synthesized via different methods are shown in Figure 10. The morphology of γ-CuI synthesized via microchannel synthesis strategy is mainly spherical particles (Figure 10a), and the γ-CuI synthesized via traditional liquid phase precipitation is an irregular block (Figure 10c). The elements of the samples synthesized via different methods are only Cu and I. The atomic ratio of Cu:I is approximately 1:1, affirming the purity of the samples synthesized by different methods. The crystal nucleation process of cuprous iodide by different strategies is described in Supporting Information eqs 1−2 . In the two-step process, supersaturation (eq 2) is lower due to the significantly inferior mixing performance and intermittent operation in the stirred tank reactor, which results in an increase in crystal grains (eq 1). Conversely, as described in section 3.1, the rapid mass transfer characteristics of the microreactor can achieve effective material mixing in a very short time period to enhanced supersaturation. Continuous transport mode can effectively separate the reduction and precipitation reaction process and remove the product from the reactor in time to avoid secondary growth of the crystal,25 resulting in smaller and more homogeneous particle size. Therefore, compared to traditional liquid phase precipitation, microchannel synthesis strategy has advantage of high mass transfer and continuous transport mode in the preparation of nanosized γ-CuI with greater homogeneous particle size. Table 1 shows some reported particle sizes of γ-CuI prepared via different methods. The size of γ-CuI synthesized in this study was similar to the reported values. However, compared to the traditional batch synthesis process, microchannel synthesis strategy provides better potential applications for its advantages of continuous and stable synthesis process. 3.5. Photocatalytic Degradation of Organic Dyes. The UV−vis diffuse reflectance spectra and fluorescence spectrum of γ-CuI with different particle sizes are shown in Figure 11. Figure 11a shows the UV−vis diffuse reflectance spectra of γCuI with various particle sizes. The absorption peaks located at 433 and 425 nm in the spectrum of samples with ca. 51 nm and ca. 418 nm particle sizes, respectively. Optical band gaps of samples (Eg) may be estimated based on the optical absorption spectra using the Tauc equation (αhv)n = A(hv − Eg )
(5)
where hv is the photon energy and α is absorbent, A is a material constant, and n is 2 for direct transitions. The optical band gap is obtained by extrapolating the linear portion of the (αhv)n curve versus hv to zero.4,30−32 Figure 11a inset image shows the curve (αhv)n versus hv of γ-CuI with different particle sizes. The band gap of γ-CuI with particle sizes ca. 418 nm and ca. 51 nm is 2.92 and 2.86 eV, respectively.
Figure 8. XRD pattern of samples prepared in a traditional batch stirred reactor and microreactor. (T = 20 °C, C = 0.5 mol/L, pH = 4.4.) 10946
DOI: 10.1021/acs.iecr.9b00354 Ind. Eng. Chem. Res. 2019, 58, 10941−10950
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Figure 9. TEM images and particle size distribution of γ-CuI prepared by different reactors: (a and b) microchannel synthesis strategy and (c and d) traditional stirred batch reactor. (T = 20 °C, C = 0.5 mol/L, pH = 4.4.)
Figure 10. SEM images and EDS of samples synthesized by different methods: (a and b) microchannel synthesis strategy and (c and d) traditional liquid phase precipitation. (T = 20 °C, C = 0.5 mol/L, pH = 4.4.)
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DOI: 10.1021/acs.iecr.9b00354 Ind. Eng. Chem. Res. 2019, 58, 10941−10950
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Industrial & Engineering Chemistry Research Table 1. Different Preparation Methods and Particle Sizes of γ-CuI authors
raw materials
methods
morphology
mean size/nm
Salavati-Niasari26 Rabinal27 Afshar28 Li29 this work
Cu(NO3)2·H2O; LiI; watermelon juice; cherry juice; carrot juice I2; copper sheet; ethyl alcohol CuSO4; KI Cu(dmg)2; KI Cu(NO3)2·H2O; KI; N2H4·H2O
liquid phase precipitation ultrasound mechanochemical microemulsions microchannel synthesis strategy
flowerlike spherical spherical nanorods spherical
40−70 20−22 40−70 50−80 44−100
Figure 11. (a) UV−vis diffuse reflectance spectrum (inset: the plot of (αhv)2 vs hv) and (b) fluorescence spectrum of γ-CuI with different particle sizes.
Figure 12. (a) Degradation rate and (b) degradation kinetics of MB degraded by different particle size γ-CuI.
Figure 13. (a) Degradation rate and (b) degradation kinetics of MO degraded by different particle size γ-CuI. 10948
DOI: 10.1021/acs.iecr.9b00354 Ind. Eng. Chem. Res. 2019, 58, 10941−10950
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Industrial & Engineering Chemistry Research Fluorescence spectra of γ-CuI with various particle sizes are shown in Figure 11b. The fluorescence spectrum consisted of a strong peak at 428 nm that can be ascribed to a high-level transition in γ-CuI semiconductor crystallites. It has been reported that this kind of band edge luminescence arises from the recombination of excitons and/or shallowly trapped electron−hole pairs.33 The emission intensity of photoluminescence spectra at 428 nm decreased for as-synthesized γ-CuI powder with decreasing grain size. This may be due to a reduction in particle size, which diminishes electron−hole recombination.34 The γ-CuI photocatalytic activity of various particle sizes was tested by monitoring the degradation of MB and MO under UV light irradiation. The photodegradation curves and kinetic curves of MB and MO after 160 min are shown in Figures 12−13. Figure 12a exhibits decolorization of 46.53% and 92.30% MB degraded after 160 min for γ-CuI with particle size ca. 418 nm and ca. 51 nm. The corresponding photodegradation rate constant is 0.004 19 and 0.016 92 min−1, respectively (Figure 12b). Figure 13a shows decolorization of 83.54% and 96.87% MO degraded after 160 min for γ-CuI with particle size ca. 418 nm and ca. 51 nm. The corresponding photodegradation rate constant is 0.062 79 and 0.918 48 min−1, respectively (Figure 13b). Figure 14 shows scheme of photocatalytic degradation of various dyes over γ-CuI nanostructures under UV light
particle size, which diminishes electron−hole recombination and increases the migration rate of the electron pairs, which effectively enhances the degradation efficiency of organic dyes.35 Therefore, nanosized spherical γ-CuI synthesized via microchannel synthesis strategy exhibits greater photocatalytic activity than that of the micron-sized bulky γ-CuI prepared via traditional liquid phase precipitation.
4. CONCLUSION Herein, for the first time, nanosized γ-CuI particles synthesis based on microchannel synthesis strategy using hydrazine as the reducing agent was proposed. N2 produced by hydrazine hydrate in the reduction reaction promoted gas−liquid segmentation flow in the microchannel, which effectively alleviated the problem of microchannel blockage. A pure cubic phase γ-CuI was continuously achieved in 99% yield and ca. 44−100 nm particle size at a temperature of 20 °C, residence time of 5 s, reactant concentration of 0.5 mol/L, and pH value of 4.4. Compared to the traditional precipitation method, microchannel synthesis strategy has the advantage of high mass transfer and continuous transport mode for preparing γ-CuI, which results in higher yield and more homogeneous smaller particle size. Moreover, the nanosized γ-CuI exhibits better photocatalytic performance.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b00354. Part 1, supplementary map: XRD patterns of the samples synthesized at different residence times (Figure S1); O 1s peak of the samples XPS synthesized at pH = 3.2 and 4.4 (Figure S2); actual reaction in the microchannel: (a)N2H4·H2O was employed as reducing agent; (b)Na2S2O3 was employed as reducing agent (Figure S3); XRD patterns of the samples synthesized at different temperatures (Figure S4); XRD patterns of the samples synthesized at different reactant concentrations (Figure S5); experimental device of microchannel synthesis strategy (Figure S6); Part 2: two-step reaction nucleation process (PDF)
Figure 14. Reaction mechanism of photocatalytic degradation of various dyes over γ-CuI structures under UV light irradiation.
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irradiation. According to the scheme of photocatalytic degradation, the probable mechanism of the photocatalytic degradation of organic dyes can be summarized as follows:
AUTHOR INFORMATION
Corresponding Author
γ‐CuI + hv → γ‐CuI* + e + h
(6)
*Tel.: 0851-83604936. Fax: 0851-83604936. E-mail: ce.feiliu@ gzu.edu.cn.
h+ + H 2O → HO·
(7)
ORCID
e− + O2 → O2−·
(8)
Author Contributions
−
·
OH + O2
−·
+
Fei Liu: 0000-0003-3775-0035 ⊥
These authors contributed equally to this work and should be considered cofirst authors.
+ MB (or MO) → Degradation products (9)
Notes
As a result, under UV light irradiation, the photodegradation ability of γ-CuI synthesized via microchannel synthesis strategy with particle sizes ca. 51 nm was higher than that synthesized via traditional liquid phase precipitation with particle sizes ca. 418 nm. This may be due to two reasons: one is the morphology of γ-CuI. The regular spherical γ-CuI synthesized via microchannel synthesis strategy has a stronger adsorption capacity for organic dyes than bulky γ-CuI synthesized by traditional liquid phase precipitation. Another is a reduction in
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (21666007), Scientific and Technological Innovation Talents Team Project of Guizhou (20185607), One Hundred Person Project of Guizhou (20165655), Science and Technology Project of Guizhou (20175788 and 10949
DOI: 10.1021/acs.iecr.9b00354 Ind. Eng. Chem. Res. 2019, 58, 10941−10950
Article
Industrial & Engineering Chemistry Research
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20185781), and Graduate Research Fund of Guizhou (KYJJ2017021).
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DOI: 10.1021/acs.iecr.9b00354 Ind. Eng. Chem. Res. 2019, 58, 10941−10950
