A novel preparation method for ZnO/γ-Al2O3 nanofibers with enhanced absorbability and improved photocatalytic water-treatment performance by Ag nanoparticles

Zhiqiang Cheng a, Shengzhe Zhao *a and Lihao Han *b
aCollege of Resources and Environment, Jilin Agriculture University, Changchun 130118, People's Republic of China. E-mail: ShengzheZhao2015@163.com
bJoint Center for Artificial Photosynthesis (JCAP), California Institute of Technology (CALTECH), Pasadena, CA 91125, USA. E-mail: hanlihao@caltech.edu

Received 28th December 2017 , Accepted 24th February 2018

First published on 27th February 2018

A novel method for synthesizing ZnO/γ-Al2O3 nanofibers by electrospinning and subsequent calcination is reported. The prepared nanofibers were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The ZnO/γ-Al2O3 nanofibers exhibited excellent capacity for adsorbing organics with a negative zeta potential such as methyl orange (95.8%) and heavy metal ions such as Cr(VI) in aqueous solution. The mechanism of adsorption was investigated, and the adsorption results were fitted using the Langmuir and Freundlich models. Once silver nanoparticles (Ag NPs) were decorated on the surface of the nanofibers by photoreduction, the Ag/ZnO/γ-Al2O3 nanofibers manifested efficient photocatalytic degradation of methyl orange under UV-light illumination. Results confirmed that our Ag/ZnO/γ-Al2O3 nanofibers are a promising adsorbent for the removal of methyl orange and Cr(VI) ions and the adsorbent can be sustainably reused.

1. Introduction

In recent years, organic dyes have been extravagantly used in cosmetic, plastics, printing, and textile industries, resulting in severe environmental problems. Among various classes of dye compounds, the azo dyes account for ∼80% of the total amount of industrial organic dyes.1–3 Long term concerns about environmental and human health require a strict discharge standard of pollutants for industrial and municipal sewage plants; thus, various wastewater treatment techniques have been developed, including filtration, ion-exchange, adsorption, electrocoagulation and catalytic oxidation.4–6 Among these methods, the application of absorbable photocatalysts is considered as one of the greenest technologies to degrade the organic dye contaminants in water during industrial production, due to their simple fabrication procedure, reusability, high efficiency and cost effectiveness (input energy mainly from solar irradiation).7

Zinc oxide (ZnO), a multifunctional metal oxide with a wide bandgap (3.37 eV), has been focused on for absorbable photocatalytic water treatment, as it is environmentally friendly and has high catalytic performance.8 However, the recombination of photogenerated electrons and hole pairs in traditional ZnO suspensions strikingly limits the photodegradation activity, directly leading to low quantum yields in the reactions.9–11 In addition, the high interfacial free energy of the material accelerates the aggregation of the ZnO powders, resulting in an undesirable reduction of the active surface area and suppressing the photodegradation efficiency.12–14 To tackle these problems, a number of methods have been developed, such as the fabrication of ZnO nanostructures integrated with metal or nonmetal elements, and coupling with other semiconductors.15 Various structures in the nanoscale, such as nanorods, nanoflowers, nanowires, nanofibers and nanoparticles, have been developed and applied to improve the charge separation efficiency and increase the number of active sites in the photocatalytic reactions.16–21

Alumina (Al2O3) materials generally have porous structures with a high surface-area-to-volume ratio, and are considered to be a promising carrier for other metal oxides as the tunable pore size makes them ideal for adsorbing a wide range of molecules in photocatalytic reactions.22–24 Al2O3 can be coupled quite well with ZnO, and the matched combination has a positive effect on the separation and transportation of photogenerated charges.25 The adsorption of organic pollutants by alumina is closer to ZnO active sites in the ZnO/γ-Al2O3 nanofibers than in the traditional ZnO powders, resulting in a more efficient reaction rate.26,27 Various synthesis methods such as solvothermal methods, sol–gel methods, chemical vapor deposition, and alloy-evaporation deposition have been explored by several research groups to fabricate ZnO/γ-Al2O3 nanofibers, but an easier, more cost-effective and practical fabrication method still needs to be explored.20

In this work, we report a novel synthesis method of ZnO/γ-Al2O3 nanofibers by electrospinning and subsequent calcination. ZnO/γ-Al2O3 nanofibers exhibited excellent capacity for adsorbing negative organics such as methyl orange (MO) and heavy metal ions such as Cr(VI) in aqueous solutions. To enhance the photocatalytic effect for water treatment, silver nanoparticles (Ag NPs) were decorated on the surface of the nanofibers by photoreduction and hydrothermal methods, respectively. The Ag/ZnO/γ-Al2O3 nanofibers exhibited efficient photocatalytic degradation of simulated pollutants under UV-light illumination.

2. Experimental

2.1 Chemicals

Zinc acetate [ZnAc2·2H2O powder, 99.0%], aluminum chloride (AlCl3 powder, 99.9%) and hexamethylenetetramine (HMTA powder, 99.0%) were purchased from Tianjin Zhiyuan Chemical Reagent, China. Polyvinylpyrrolidone (PVP powder, K-90) was provided by Bodi Reagent, Tianjin, China. N,N-Dimethylformamide (DMF solution, >99.9%) and silver nitrate (AgNO3 powder, 99.9%) were obtained from Beijing Chemical Works, China. Methyl orange (MO, 96.0%), methylene blue (MB, 97.0%), and rhodamine B (RhB, 98.0%) were acquired from Aladdin. All chemical reagents were of analytical grade and were used without further purification.

2.2 Synthesis of ZnO/γ-Al2O3 nanofibers

The synthesis procedure of the nanofibers is presented in Scheme 1. Firstly, the precursor solution for electrospinning was prepared by adding 0.8 g PVP to 5.0 mL DMF with magnetic stirring at 50 °C in a water bath. After the solution was cooled down to room temperature, 0.4 g ZnAc2·2H2O and 0.2 g AlCl3 were added into the solution with magnetic stirring (the Zn[thin space (1/6-em)]:[thin space (1/6-em)]Al ratio was optimized by mixing 0.4 g ZnAc2·2H2O with 0.1, 0.2 and 0.3 g AlCl3, respectively, and 0.2 g AlCl3 was found to be the optimal weight as shown in Fig. S1 and S2 in the ESI). The prepared precursor solution was then loaded into a syringe which was connected to a high voltage of 13 kV. The flowrate of the solution was controlled at 0.4 mL h−1, and the distance from the needle (anode) to the rotating acceptor (cathode) was set to 14 cm (Scheme 1). During the synthesis procedure, the relative humidity was controlled below 30% in a fume hood. A thin film of nanofibers was formed on the surface of the acceptor at room temperature, and then collected for calcination at 700 °C for 2 h (ramping rate: 2.2 °C min−1, duration: 5.2 h) under an air atmosphere. The organic compounds and solvents were completely evaporated during the thermal treatment, and uniform ZnO/γ-Al2O3 nanofibers were formed after cooling down.
image file: c7nr09683f-s1.tif
Scheme 1 The schematic of the synthesis process for Ag/ZnO/γ-Al2O3 nanofibers.

2.3 Decoration of Ag NPs onto the nanofibers

Two synthesis methods, photoreduction and hydrothermal, were developed for the decoration of Ag NPs onto the synthesized ZnO/γ-Al2O3 nanofibers. During the photoreduction procedure, 50 mg ZnO/γ-Al2O3 nanofibers were immersed in 50 mL AgNO3 solution (0.02 M) with magnetic stirring in the dark for 2 h. Subsequently, AgNO3 was decomposed into Ag NPs when the mixture was illuminated by a UV lamp for 5 min (Philips, 120 W, TL/05, 5 cm distance, spot size: 20 cm × 20 cm, main wavelength: 365 nm):
2AgNO3 = 2Ag + 2NO2↑ + O2(1)
(light illumination or heat irradiation).

After cleaning in DI water and drying in air, the Ag-decorated ZnO/γ-Al2O3 nanofibers were obtained.

The hydrothermal method was used as a comparison method for the decoration of Ag NPs. 50 mg ZnO/γ-Al2O3 nanofibers were transferred into a Teflon-lined autoclave containing 50 mL AgNO3 and HMTA (1[thin space (1/6-em)]:[thin space (1/6-em)]1 volume ratio) aqueous solution (0.01 M each). The Teflon-lined autoclave was kept at 120 °C for 2 hours, and AgNO3 was reduced into Ag NPs by HMTA. The nanofibers in the solution had a higher surface energy than that of the autoclave; therefore, Ag was crystallized and decorated onto the surface of nanofibers easily. Finally, the Ag/ZnO/γ-Al2O3 nanofibers were collected and washed in DI water.

2.4 Characterization

The morphologies of the nanofibers were studied by scanning electron microscopy (SEM, SHIMADZU S-550) equipped with energy dispersive X-ray spectroscopy (EDS) analysis, as well as high-resolution transmission electron microscopy (HR-TEM, Tecnai G2, operating voltage: 15 kV). The crystal structure, chemical composition and physical phase of the samples were characterized by X-ray powder diffraction (XRD) using a D/MAX 2250 V diffractometer (Rigaku, Japan, Cu Kα radiation (λ = 0.15418 nm) under 40 kV, 30 mA, scanning range: 20°–80°). X-ray photoelectron spectroscopy (XPS) was performed on a VG ESCALAB LKII instrument with a Mg-Kα-ADES ( = 1253.6 eV) source at a residual gas pressure of below 1028 Pa. Fourier-transform infrared spectroscopy (FTIR, SHIMDZU, 1.50SU1, Japan) was used to identify the vibration of the functional groups present in the material.

The zeta potential of the adsorbent at different pH was determined by using a BDL-B surface potential instrument (Shangli Instruments, Shanghai, China). The Brunauer–Emmett–Teller (BET) specific surface area of the nanofibers was investigated using nitrogen adsorption (Micromeritics, ASAP 2010). The samples were degassed at 200 °C before the BET measurements. UV-Vis spectroscopy (Shimadzu, UV-2550, scanning range from 200 to 800 nm) was employed to detect the absorbance of the simulated organic pollutants in the solution, and the corresponding concentrations of each pollutant were determined using the absorbance peak intensity according to the Beer–Lambert law.

2.5 Organic adsorption characterization

Methyl orange (MO, 20 mg L−1), methylene blue (MB, 20 mg L−1), and rhodamine B (RhB, 20 mg L−1) were used in this work as the simulated organic pollutants to characterize the adsorption performance of our nanofiber material. MO was chosen as it is considered as one of the most challenging organic pollutants to be removed in the environment. 40 mg of the as-prepared nanofibers were suspended in 40 mL organic pollutant solution in a quartz tube for each condition. The suspension was magnetically stirred in the dark, and a few milliliters of the solution were extracted from the mixture for centrifugation at time intervals of 20 min. The adsorption performance expressed in terms of adsorption percentage (%) and the adsorption capacity qe (mg g−1) were calculated using the following equations:24,25
Adsorption = (c0ce)/c0 × 100%(2)
qe = (c0ce) × V/W(3)
where c0 is the initial concentration of organic pollution, ce is its equilibrium concentration after adsorption, V is the volume of organic pollution solution (mL), and W is the weight of the synthesized adsorbent (mg).

2.6 Heavy metal ion adsorption

Potassium dichromate, K2Cr2O7, is one of the most common simulated heavy-metal-ion pollutants. Cr(VI) ions in K2Cr2O7 aqueous solution are chronically harmful to health; therefore, reducing Cr(VI) ions into non-toxic Cr(III) and Cr(IV) in waste water is strictly required by industry standards before discharging into the environment. The adsorption of heavy metal Cr(VI) by synthesized ZnO/γ-Al2O3 nanofibers (40 mg) was conducted in K2Cr2O7 solutions (0.1 mM, 40 mL) at different pH values, which were adjusted by adding 0.1 M HCl or 0.1 M NaOH. The concentration of heavy metal ions (Cr2O7)2− was analyzed using a liquid scintillation analyzer (Packard 3100 TR/AB), and the calculation of the adsorption ratio was the same as that for the organic pollutants.

2.7 Photocatalytic performance characterization

The photocatalytic activities of the nanofibers were studied using the degradation ratio of MO solution. After reaching the adsorption–desorption equilibrium between MO and nanofibers, the mixture was exposed to an ultraviolet radiation lamp (Philips, 120 W, TL/05, 5 cm distance, spot size: 20 cm × 20 cm, main wavelength: 365 nm). 2 mL solution was transferred from the mixture and centrifuged at given time intervals of 20 min. The concentration of MO in the solution was determined by UV-vis spectroscopy (Shimadzu, UV-2550) at 465 nm.

3. Results and discussion

3.1 Morphology of ZnO/γ-Al2O3 nanofibers

The morphology of the obtained samples is shown by the SEM images in Fig. 1A–C, and typical nano-sized fibrous structures can be observed. The average diameter of the ZnO/γ-Al2O3 nanofibers was 200 ± 20 nm, and the uniformity was estimated to be above 80% (Fig. 1A). After loading Ag NPs by photoreduction, the morphology of the photoreduced-Ag/ZnO/γ-Al2O3 nanofibers had no obvious change, as the size of the Ag NPs was significantly smaller than the diameter of the nanofibers (Fig. 1B). However, the Ag NPs deposited by the hydrothermal method were larger (100 ± 20 nm, Fig. 1C) than those deposited by photoreduction (20 ± 4 nm, Fig. 1E), as the reaction temperature was higher and the deposition time was longer during the hydrothermal procedure (120 °C vs. room temperature, 2 h vs. 5 min, details described in Section 2.3). EDS analysis was performed to characterize the elemental mapping of the photoreduced-Ag/ZnO/γ-Al2O3 nanofibers. As shown in Fig. S3 in the ESI, Zn and Ag elements were widely spread over the entire photoreduced-Ag/ZnO/γ-Al2O3 nanofibers, meaning that the Ag/ZnO/γ-Al2O3 nanofibers were successfully fabricated using the electrospinning method and photoreduction.
image file: c7nr09683f-f1.tif
Fig. 1 SEM images of the synthesized ZnO/γ-Al2O3 nanofibers (A), photoreduced-Ag/ZnO/γ-Al2O3 nanofibers (B), and hydrothermal-Ag/ZnO/γ-Al2O3 nanofibers (C); TEM images of photoreduced-Ag/ZnO/γ-Al2O3 nanofibers (D), and a single photoreduced-Ag/ZnO/γ-Al2O3 nanofiber showing the Ag NPs densely coated onto its surface (E); an HR-TEM image showing the lattices of the photoreduced-Ag/ZnO/γ-Al2O3 nanofibers and confirming the presence of Ag and ZnO (F).

The surface of the as-prepared ZnO nanofibers was smooth as presented in the HR-TEM image in Fig. S4. In contrast, the photoreduced-Ag/ZnO/γ-Al2O3 nanofibers have wormhole-like pores (Fig. 1D and E), which may originate from the decomposition of polyvinylpyrrolidone (PVP) in the precursor solution during the electrospinning process and the difference of nucleation internal stresses between the ZnO and γ-Al2O3 crystals during the calcination process. As shown by the black dots in Fig. 1E, Ag NPs were loaded on the surface of the porous nanofibers. Clear crystal lattices can be observed directly in Fig. 1F, with spacing distances of 0.203 nm and 0.284 nm, corresponding to Ag (200) and ZnO (100), respectively. During the hydrothermal process, the size control of Ag NPs coated onto the ZnO/γ-Al2O3 nanofibers was difficult. In contrast, Ag NPs coated onto the ZnO/γ-Al2O3 nanofibers by photoreduction have a good size distribution and uniformity. The photoreduced-Ag/ZnO/γ-Al2O3 nanofibers have a preferred microporous structure and more active surface area for adsorption and photodegradation reactions. Compared with other fabrication methods, our procedure has obvious advantages: (i) simple processing, (ii) cost-effectiveness, (iii) good repeatability, (iv) the thickness of the nanowires can be tuned accurately, and (v) the nanofibers form a thin flexible film before and after calcination, allowing good compatibility for other applications.

3.2 XRD patterns

X-Ray diffraction (XRD) analysis was performed to confirm the crystallization of the ZnO and Ag/ZnO/γ-Al2O3 nanofibers (Fig. 2A, the XRD patterns of ZnO/γ-Al2O3 nanofibers with different Zn/Al ratios are plotted in Fig. S1 in the ESI). The sharp and intense diffraction peaks indicate that the nanofibers were well-crystallized. The diffraction peaks located at 31.94°, 34.58°, 36.39°, 47.67° and 56.78° can be indexed to the (100), (002), (101), (102) and (110) diffraction planes of crystallized ZnO in the hexagonal wurtzite structure (JCPDS 36-1451), respectively. The intense peaks at 36.84°, 44.82° and 65.31° can be indexed to the (311), (400) and (440) diffraction planes of γ-Al2O3 (JCPDS 10-0425), respectively. The Ag peaks were not clearly observed in the XRD pattern, as the Ag amount was too low to be detected. In addition, the XRD peaks of the Ag/ZnO/γ-Al2O3 nanofibers look like the combination pattern of hexagonal wurtzite ZnO and γ-Al2O3, implying that Zn atoms in the ZnO lattices were not replaced by Ag atoms.23
image file: c7nr09683f-f2.tif
Fig. 2 XRD patterns of photoreduced-Ag/ZnO/γ-Al2O3 (red) and ZnO nanofibers (black) (A); XPS spectra of photoreduced-Ag/ZnO/γ-Al2O3 (red) and ZnO nanofibers (black) (B), Zn 2p peaks (C) and Ag 3d peaks (D). (C) and (D) are the zoomed-in range of (B).

3.3 XPS analysis

XPS was performed to analyze the oxidation, elemental and chemical states of the photoreduced-Ag/ZnO/γ-Al2O3 nanofibers, and the results are shown in Fig. 2B–D. Ag, Zn, Al, O, and C signals can be observed in Fig. 2B. The presence of the C peak in the XPS spectrum is ascribed to the adventitious carbon-based contaminant during the XPS characterization, as no C element could be found in the EDS results of Ag/ZnO/γ-Al2O3 nanofibers (details in Fig. S3). The binding energy values of Zn 2p3/2 and Zn 2p1/2 are also found to be 1020.9 and 1043.8 eV, respectively (Fig. 2C). The spin–orbit splitting of 22.9 eV represents that Zn mainly exists in the form of ZnO clusters. Fig. 2D presents the XPS spectrum of Ag in the Ag/ZnO/γ-Al2O3 nanofibers. The peaks of Ag 3d5/2 and Ag 3d3/2 appear at a binding energy of 367.7 eV and 373.8 eV, respectively. This binding energy difference of 6.1 eV confirms the presence of the metallic nature of Ag.13 Here, interestingly, the 3d5/2 peak of Ag in our work is found to be red-shifted to the lower binding energy, compared with the standard value (368.2 eV for bulk metallic Ag),12 indicating that the binding energy of metallic Ag was reduced by the interaction between Ag NPs and ZnO.15

3.4 Adsorption performance of organic pollutants and effects of polarity

Methyl orange (MO), methylene blue (MB), and rhodamine B (RhB) as simulated organic pollutants were introduced to test the adsorption performance of the as-prepared ZnO/γ-Al2O3 nanofibers under the same conditions, and the adsorption time was controlled for an hour. The adsorption of MO, MB and RhB by ZnO/γ-Al2O3, photoreduced-Ag/ZnO/γ-Al2O3 and hydrothermal-Ag/ZnO/γ-Al2O3 nanofibers under four different temperatures (15, 25, 35 and 45 °C) is plotted in Fig. 3. The adsorption by ZnO or γ-Al2O3 nanofibers is both close to 0% at these temperatures, so we have omitted these results.
image file: c7nr09683f-f3.tif
Fig. 3 The adsorption performance of common organic pollutants by as-prepared nanofibers under different temperatures: (A) ZnO/γ-Al2O3; (B) photoreduced-Ag/ZnO/γ-Al2O3; and (C) hydrothermal-Ag/ZnO/γ-Al2O3.

The bars in Fig. 3 show that the adsorption trends of the three samples were roughly the same, and the adsorption of each organic pollutant was slightly improved after the Ag NPs were decorated (Fig. 3B and Cvs.Fig. 3A). The reason for this small improvement might be that the AgNO3 solution infiltrated into the micropores between ZnO and γ-Al2O3, and the porosity of the nanofibers was enhanced when more Ag NPs were generated under UV illumination.

At each temperature, more MO was adsorbed compared to MB and RhB by the three types of nanofibers, confirming that the polarity is a primary factor to determine the adsorption of organic pollutants. The ZnO/γ-Al2O3 nanofibers have positive-polarity, leading to better adsorption for organic dyes with negative-polarity (such as MO) than the dyes with positive-polarity (such as MB and RhB). With the temperature enhancement, the adsorption of MO was significantly improved, due to the more active polar interaction between the adsorbent and adsorbate. The size of the organic dye molecule is considered to be the second key factor for the adsorption rate (molecular volume: RhB > MO > MB). Under the same polar conditions, MB has the smallest size among the three organic pollutants, and it is much easier for it to enter the porous tunnels of the nanofibers and get trapped. At this time adsorption is mainly determined by the fibrous porosity. Van der Waals forces are also an important factor affecting the adsorption effect. The RhB molecules have relatively larger relative molecular mass than the MB molecules, so the van der Waals force between RhB and the nanofibers are stronger than that between MB and the nanofibers. As they both have the same polarity as the adsorbent, the stronger van der Waals force results in less adsorption of RhB compared to MB. With increasing temperature, the van der Waals force between RhB and the nanofibers became weaker, resulting in a growing RhB adsorption effect (BET and FTIR results are shown in Fig. S5 and S6).

3.5 Heavy metal ion adsorption and the effect of pH

Besides the adsorption of organic pollutants, the positive polarity of ZnO/γ-Al2O3 nanofibers could also interact with heavy metal ions. Typical toxic heavy metal ion Cr(VI) from K2Cr2O7 solution was applied to test the removal performance of ZnO/γ-Al2O3 nanofibers. The zeta potential analysis of ZnO and ZnO/γ-Al2O3 nanofibers (40 mg respectively) at different pH values and the removal capacity of Cr(VI) ions (40 mL, 0.1 mM K2Cr2O7 aqueous solution) for 1 hour are shown in Fig. 4. The adsorptions are calculated to be 32.7%, 27.4%, 16.2%, and 11.3% under the corresponding pH conditions (pH 4, 6, 8 and 10), respectively.
image file: c7nr09683f-f4.tif
Fig. 4 Effects of pH on the zeta potential and adsorption of Cr(VI) (initial concentration: 0.1 mM) by ZnO/γ-Al2O3 nanofibers.

The zeta potentials of the adsorbents play an important role in the removal of simulated pollutants, by controlling the adsorption of simulated pollutants at the interface between the adsorbent and water.28,29 The zeta potentials of the adsorbents were mainly affected by the pH in the solution. In acidic solutions, the zeta potentials exhibited rising positive polarity with the increase of pH, resulting in strong intermolecular attraction to (Cr2O7)2− with negative polarity via van der Waals forces. The second factor is that the K2Cr2O7 solution behaved slightly acidic due to its hydrolysis into (CrO4)2− and H+; therefore, the number of negative-polarity ions increased. The synergistic effects of these two factors make the samples a strong adsorber of (Cr2O7)2−.

In aqueous medium, the ZnO/γ-Al2O3 nanofibers have a surface charge due to the adsorption of H+ or OH, and the polarity and potential of the surface charge are dependent on the pH value of the solution. The surface of ZnO/γ-Al2O3 nanofibers suspended in the solution is hydrated, forming M–OH groups and starting the following reactions (M is the cation in the oxide, such as Al3+ and Zn2+):

MO(surface) + H2O(solution) ⇌ M–OH(surface)(4)
M–OH(surface) + H+(solution) ⇌ M–OH2+(5)
M–OH(surface) + OH(solution) ⇌ M–O + H2O(6)

When the pH is below a certain value, the M–OH2+ group is formed on the surface of the material; hence, the particles are positively charged. When the pH value increases, the M–OH2+ group releases the protons. The surface group then becomes M–O and the particle is negatively charged. If a proton is migrated to the surface of the particle to form an M–OH matrix, the surface becomes electrically neutral.

3.6 Mechanism of MO adsorption

To describe how the adsorbate molecules interact with the adsorbent, the Langmuir and Freundlich isotherm equations were used to interpret the experimental adsorption data. The linear form of Langmuir and Freundlich isotherms can be described as:30,31
ce/qe = 1/KLqm + ce/qm(7)
log[thin space (1/6-em)]qe = log[thin space (1/6-em)]KF + log[thin space (1/6-em)]ce/n(8)
where qe is the equilibrium adsorption capacity of the adsorbent (mg g−1), Ce is the equilibrium concentration of the dye (mg L−1), qm is the maximum amount of MO dye adsorbed (mg g−1), KL is the Langmuir constant which is related to the strength of adsorption, KF is the constant related to the adsorption capacity of the adsorbent (mg1−n Ln g−1), and n is the constant related to the adsorption intensity and adsorption capacity. The adsorption isotherms are shown in Fig. 5. The data are analyzed by Langmuir and Freundlich equations and the results are summarized in Table 1. In all cases, the experimental data were better fitted using the Langmuir model since the coefficients (R2 ≥ 0.993) for the Langmuir plots were always greater than those obtained from the Freundlich model. The results indicate that the decrease of the MO concentration is a mixing effect of both adsorption and photodegradation.

image file: c7nr09683f-f5.tif
Fig. 5 Adsorption Langmuir (A) and Freundlich isotherms (B) of MO by different adsorbents: ZnO/γ-Al2O3 nanofibers, photoreduced-Ag/ZnO/γ-Al2O3 nanofibers and hydrothermal-Ag/ZnO/γ-Al2O3 nanofibers.
Table 1 Adsorption parameters of the Langmuir and Freundlich models for the adsorption of MO onto the ZnO/γ-Al2O3, photoreduced-Ag/ZnO/γ-Al2O3 and hydrothermal-Ag/ZnO/γ-Al2O3 nanofibers
  Langmuir model Freundlich model
q max K L R 2 K F 1/n R 2
ZnO/γ-Al2O3 227 0.01 0.999 15.8 0.389 0.978
Photoreduced-Ag/ZnO/γ-Al2O3 226 0.007 0.996 10.69 0.444 0.985
Hydrothermal-Ag/ZnO/γ-Al2O3 222 0.004 0.993 6.16 0.518 0.988

3.7 Photocatalytic activity and recycled photocatalytic degradation

To further confirm the adsorption properties and photocatalytic performance, we chose MO as the degradation material under UV illumination (λ = 365 nm). c0 and ct are the initial MO concentration (20 mg L−1 in this experiment, 40 mL) and time-dependent MO concentration, respectively. The absorptions of the solutions were measured by UV-Vis spectroscopy, and the typical MO solution has an intensive absorption peak at 465 nm (Fig. S7 in the ESI). The concentration of each solution was calculated using the Beer–Lambert law. The MO solution with 40 mg different nanofibers was magnetically stirred in the dark for the first 2 hours, and the concentrations of the MO solutions were all degraded from 100% by the four types of nanofibers. The ct/c0 ratio as a function of degradation time (at a time interval of 20 min) is presented in Fig. 6A. The MO degradation rate was the fastest by the photoreduced-Ag/ZnO/γ-Al2O3 nanofibers in the first hour, but almost half of the initial amount of the MO was adsorbed by the three samples with γ-Al2O3. In contrast, the ZnO nanofibers did not exhibit a remarkable adsorption effect and only 4.2% MO was degraded after 2 hours.
image file: c7nr09683f-f6.tif
Fig. 6 Adsorption degradation (A) and the subsequent photocatalytic degradation (B) of 40 mL MO solution (20 mg L−1 initially) using 40 mg as-prepared nanofibers in each condition; photos of the MO solutions by photoreduced-Ag/ZnO/γ-Al2O3 nanofibers during the reaction process (C); absorption curves of photoreduced-Ag/ZnO/γ-Al2O3 nanofibers under ultraviolet light illumination (data from B in a time range of 120–240 min) (D); and the MO degradation ratio by the photoreduced-Ag/ZnO/γ-Al2O3 nanofibers as a function of recycle number (E).

After the adsorption equilibrium was reached, the mixtures were illuminated by UV light for another 120 min to characterize the photocatalytic activity of the nanofibers (Fig. 6B). The photoreduced-Ag/ZnO/γ-Al2O3 nanofibers were able to photodegrade ∼95% of MO within 80 min illumination. In contrast, ∼87%, ∼77% and ∼38% of MO were photodegraded by hydrothermal-Ag/ZnO/γ-Al2O3, ZnO/γ-Al2O3 and ZnO nanofibers within the same 80 min UV illumination, respectively. More efficient MO degradation by the photoreduced-Ag/ZnO/γ-Al2O3 nanofibers was achieved, compared to the hydrothermal-Ag/ZnO/γ-Al2O3 nanofibers, indicating that smaller Ag NPs are more catalytically active for the reactions.

The MO degradation repeatability by the photoreduced-Ag/ZnO/γ-Al2O3 nanofibers was also investigated. 40 mg as-prepared photoreduced-Ag/ZnO/γ-Al2O3 nanofibers were used to adsorb MO molecules for 120 min in the dark, followed by another 120 min photodegradation of MO under UV illumination. This 240 min degradation procedure was defined as one cycle, and the ratio of the final MO concentration of each cycle and the initial MO concentration (20 mg L−1) is plotted. As shown in Fig. 6E, the nanofibers exhibited good repeatability, and the degradation efficiency could reach 84.2% of the initial value after being reused for four cycles (16 hours in total). The main reason for this minor decline in efficiency may be due to the photocorrosion of ZnO under intensive UV illumination.

3.8 Kinetics and mechanism of photocatalytic degradation

From the kinetics point of view, the photodegradation of MO can be considered as a pseudo-first-order reaction, and the degradation rate can be expressed as:23
ln[thin space (1/6-em)]c0/ct = kt(9)
where k is the degradation rate constant (min−1). The values of the rate constant have been determined from the slope of ln c0/ctversus t plots. As shown in Fig. 6D, the as-synthesized photoreduced-Ag/ZnO/γ-Al2O3 nanofibers show high photocatalytic activity with a rate constant (k) of 0.036 min−1, much higher than that of hydrothermal-Ag/ZnO/γ-Al2O3 nanofibers (0.021 min−1), ZnO/γ-Al2O3 nanofibers (0.015 min−1) and ZnO nanofibers (0.005 min−1). The higher photocatalytic activity of the photoreduced-Ag/ZnO/γ-Al2O3 nanofibers is mainly due to their larger surface area, providing more specific surface sites for the reaction and interaction between the catalysts and MO molecules.

The enhanced photocatalytic performance of the Ag/ZnO/γ-Al2O3 nanofibers can be further explained by the schematic diagram in Fig. 7. The zeta potential of the porous-structured ZnO nanofibers was enhanced after the integration with γ-Al2O3, making them adsorptive to the organic dyes with negative-polarity onto the nanofiber surface. A heterogeneous structure was formed at the interface of Ag NPs and ZnO. Under the UV illumination, the photogenerated electrons in the valence band of ZnO in the Ag/ZnO/γ-Al2O3 nanofibers were excited to the conduction band. The carrier separation occurred on the surface of ZnO after decorating an appropriate amount of Ag NPs, and the captured electrons reacted with O2, generating radical species such as ˙OH. The holes on the surface of ZnO reacted with ˙OH and produced reactive oxygen radicals, degrading MO molecules. The intermediate compounds were mostly unstable products containing C[double bond, length as m-dash]C and N[double bond, length as m-dash]N, and were photodegraded into CO2, NO3 and H2O ultimately. Therefore, the concentrations of the unstable intermediate compounds may have limited inference on the photodegradation effect for the organic molecules, and they were not plotted.

image file: c7nr09683f-f7.tif
Fig. 7 The schematic of adsorption and the photocatalytic process of Ag/ZnO/Al2O3 nanofibers.

4. Conclusions

In summary, a novel fabrication method of ZnO/γ-Al2O3 nanofibers using electrospinning and subsequent calcination was proposed. The prepared ZnO/γ-Al2O3 nanofibers exhibited excellent capacity for adsorbing negative organics such as methyl orange (95.8%) and heavy metal ions such as Cr(VI) in aqueous solutions. The mechanism of adsorption was investigated, and the adsorption results were fitted with the Langmuir model. Ag NPs were decorated on the surface of the nanofibers by photoreduction and hydrothermal methods, respectively. The photoreduction-Ag/ZnO/γ-Al2O3 nanofibers manifested more efficient photocatalytic degradation of MO than the hydrothermal-Ag/ZnO/γ-Al2O3 nanofibers under UV-light illumination. We believe that our fabrication process and the synthesized nanofibers provide a new avenue for the removal of organic pollutants and heavy metal ions efficiently and effectively. The synthesized adsorbent and the photocatalytic procedure are environmentally friendly, providing a promising cost-effective approach for large-scale applications.

Conflicts of interest

The authors declared that they have no conflicts of interest to this work.


This work was funded by the Jilin Province provincial industrial innovation special funds project (2018C041-2), the Jilin Provincial Department of Science and Technology Natural Science Foundation (20180101212JC) and the Changchun Science and Technology Project (17DY012). The authors would like to thank Dr Ian Sullivan (California Institute of Technology, USA) for polishing the manuscript.

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Electronic supplementary information (ESI) available. See DOI: 10.1039/c7nr09683f
These authors contributed equally to this work.

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