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NH2-UiO-66 hybridized heterojunction for effective photocatalytic hexavalent chromium reduction
Journal of Colloid and Interface Science 555 (2019) 342–351
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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Regular Article
Construction of ternary Ag/AgCl/NH2-UiO-66 hybridized heterojunction for effective photocatalytic hexavalent chromium reduction Zhiguang Zhang a,⇑, Siqi Wang a, Mingjun Bao a, Jiawen Ren a, Sihang Pei a, Shijun Yu a, Jun Ke b,⇑ a b
School of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian 116029, PR China School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan 430073, PR China
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 8 June 2019 Revised 24 July 2019 Accepted 30 July 2019 Available online 31 July 2019 Keywords: Metal-organic frameworks Heterojunction Photo-reduction Cr (VI) removal Visible light response
a b s t r a c t An Ag/AgCl/NH2-UiO-66 hybridized photocatalyst was successfully constructed via facile solvothermal with UV reduction method for efficient photocatalytic Cr (VI) reduction. The photoelectrochemical data indicate that compared with the UiO-66, the charge separation and transfer efficiency of Ag/AgCl/NH2UiO-66 heterojunction is significantly enhanced due to the introduction of amine functionalization and formation of inorganic-organic hybrid. The surface plasmon resonance (SPR) effect deriving from Ag nanoparticles (NPs) largely extends photo-response range whilst the separation efficiency of photogenerated electrons and holes is improved significantly. The synthesized Ag/AgCl/NH2-UiO-66 hybrid system shows ameliorated structural stability and superior photocatalytic activity for Cr (VI) reduction under visible light irradiation, which is 1.7 times higher than that of the bare UiO-66. Furthermore, the possible mechanism of Cr (VI) reduction is proposed by analyzing electron transfer path in the ternary Ag/AgCl/NH2-UiO-66 hybridized system. Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction Heavy metal contamination in natural water, including hexavalent chromium (Cr (VI)) has been gradually becoming a serious environmental problem, which is produced in metal processing, leather tanning, and mining [1–4]. Once heavy metals enter ⇑ Corresponding authors. E-mail addresses: [email protected] (Z. Zhang), [email protected] (J. Ke). https://doi.org/10.1016/j.jcis.2019.07.103 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.
ecosystem, living things including human being have to face serious threats due to their high solubility, strong toxicity and bioaccumulation effect [5]. Therefore, converting heavy metal from high toxicity form to low even non toxicity is one of efficient strategies to mitigate heavy metal environment contamination [6–8]. Recently, semiconductor photocatalysis technique has been demonstrated as an effective and ‘‘green” route to reducing high toxic Cr (VI) to low toxic Cr (III). In general, the semiconductor photocatalysts are irradiated by solar incident light and produce
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electrons and holes. The excited electrons migrate rapidly to conduction band (CB) of the semiconductor and holes still stay at valence band (VB). Meanwhile, the hot electrons at CB have strong reductive ability to reduce adsorbed Cr (VI) on the surface of photocatalysts to Cr (III) [9–12]. Among numerous semiconductor photocatalysts, TiO2 is widely used in pollutant elimination and energy catalysis because of its low cost, remarkable chemical and photochemical properties [13–16]. Unfortunately, the wide band gap (3.2 eV) of TiO2 only absorbs UV light accounting for 5–8% solar light, indicating that the utilization efficiency of sunlight is extremely low [17–19]. Therefore, developing a novel and efficient photocatalysts is urgent need to enlarge practical applications of photocatalysis technology in future [20]. Metal-organic frameworks (MOFs) is a new type of organic porous materials possessing infinite 3D frameworks consisting of metal-oxo clusters and organic linkers blocks [21], which is endowed with desirable topology, structural flexibility, and high surface areas [22–24]. Recently, the MOFs-based photocatalysts have been arising extensive interests. Owing to their semiconductor-like behaviors, the organic linkers in MOFs can act as unique antennas to harvest solar light, which can activate their metal sites via the ligands to metal clusters charge transition (LMCT). The photoexcited electrons and holes could be used to trigger a series of catalytic oxidation and reduction reactions [25–27]. Furthermore, compared with traditional photocatalysts, band gap energies of MOFs-based photocatalysts mainly depends on the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), which can be easily tuned by varying inorganic units and organic linkers [28,29]. Besides, more reported works reveal that amine-functionalized MOFsbased photocatalysts exhibit enhanced visible light response, originating from the metal-oxo cluster direct excitation and charges transferring from excited organic linker to metal-oxo clusters. For example, Ye et al. [30] and Wu et al. [31] separately demonstrated that NH2-MIL-88B(Fe) and NH2-MIL-68(In) possessed extra absorption abilities in visible light region, which present higher photocatalytic efficiency of Cr (VI) reduction than those of without amino group modification under visible light irradiation. Meanwhile, our former work demonstrated that the NH2-MIL-101(Fe) possessed strong visible light response ability and superb photoxidation performance for gaseous toluene, where the corresponding conversion efficiency is up to 79.4% equivalent to the commercial photocatalysts [32]. Additionally, NH2-UiO-66 as a good choice is chosen owing to its good structural stability in liquid reaction. Wu and co-workers reported that UiO-66(ANH2) exhibited good photocatalytic activity in selective oxidation of alcohol [33]. However, the potential applications of amine-functionalized MOFs are still limited due to the unavoidable recombination of photogenerated electrons and holes. Loading precious metals is one of effective ways to suppress the rapid recombination of photoinduced electrons and holes in photocatalytic process. For instance, plasmonic Ag/AgCl not only strongly absorbs visible light owing to surface plasmon resonances (SPR) effect of metallic Ag but also enforces separation of photogenerated electrons and holes [34,35], where the holes formed at Ag nanoparticles can rapidly move to AgCl and produce Cl radicals, thereby enhancing the photocatalytic oxidation performance [36]. Based on the above analysis, constructing an excellent photocatalyst consisting of Ag/AgCl and modifying of amino group can realize enhancement of photocatalytic activity of UiO-66 for Cr (VI) reduction under the visible light. In this paper, we prepared a ternary Ag/AgCl/NH2-UiO-66 hybridized photocatalyst by a facile solvothermal coupled with UV photoreduction method, which exhibits enhanced Cr (VI) reduction photocatalytic performance in comparison with bare UiO-66. The photocatalytic mechanism of Cr (VI) reduction over the ternary Ag/AgCl/NH2-UiO-66
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hybridized system is proposed and discussed. The aminefunctionalized modification and Ag/AgCl introduction largely promote the visible light range and ability and significantly enhance separation efficiency of photoinduced charge carriers. This work could give us a new path to constructing a ternary Ag/AgCl/NH2UiO-66 hybridized photocatalyst for efficient photocatalytic activities with high stability.
2. Experimental 2.1. Synthesis of UiO-66 and NH2-UiO-66 The UiO-66 sample was synthesized through a facile solvothermal method [37]. In typical synthesis process, ZrCl4 (1.5 mmol) and H2BDC (1.5 mmol) were dissolved in DMF (75 mL). Then, the mixture was transferred into a 120 mL Teflon-lined stainless steel autoclave, heated to 120 °C, and maintained for 48 h. After cooled to room temperature naturally, the suspension was centrifuged and washed by using DMF and methanol, respectively. The sample was dried at 70 °C for 5 h, and the white UiO-66 sample was gained. The NH2-UiO-66 sample was synthesized through the above similar process except that H2BDC was replaced with H2ATA as a starting material. 2.2. Synthesis of Ag/AgCl/UiO-66 and Ag/AgCl/NH2-UiO-66 The Ag/AgCl/UiO-66 hybrid system was prepared by ultraviolet light reduction [38]. Firstly, a certain amount of AgNO3 was dissolved in EtOH. 0.1 g of UiO-66 sample was added into the transparent solution and vigorously stirred for 2 h, followed by UV light irradiation for 2 h. The suspension was centrifuged and washed with ethanol repeatedly, dried at 70 °C to obtain Ag/AgCl/UiO-66 sample. The Ag/AgCl/NH2-UiO-66 was prepared through the similar process except that NH2-UiO-66 was used as a starting material instead of UiO-66. The synthesis process of Ag/AgCl/NH2-UiO-66 is described in Fig. 1. 2.3. Characterization The characterization and instruments could be found in Supporting Information. 2.4. Photocatalytic activity A certain amount of prepared photocatalyst was added to the Cr (VI) solution (20 mg/L) at room temperature, and the pH value of reaction solution was varied to 2 by adding HCl solution. After that, the mixed solution was stirred in the darkness. And the samples were taken and measured in intervals of 20 min, confirming that the adsorption-desorption equilibrium has achieved before irradiation. At 40 min the difference of Cr (VI) concentration is negligible, indicating that the adsorption-desorption equilibrium reaches. Then the suspension solution was irradiated by a 300 W Xe lamp with a 420 nm filter. During the photocatalytic process, a certain amount of the solution was taken out at regular intervals. The supernatant was centrifuged and filtered using a 0.22-lm syringe filter to remove the photocatalyst particles before analysis. Absorbance of DPC-Cr (VI) complex at 540 nm was measured by using UV–vis spectrophotometer [39,40]. At the same wavelength, the concentration of Cr (VI) was calculated according to BeerLambert Law [41]. The corresponding Cr (VI) concentration can be calculated, and the corresponding adsorption-reduction efficiency of Cr (VI) was calculated following the formula:
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Fig. 1. Schematic of the synthesis processes of Ag/AgCl/NH2-UiO-66.
Adsorption reduction efficiency ð%Þ ¼ C=C 0 100 Here, C: concentration of Cr (VI); C0: initial concentration of Cr (VI). And the concentration of total Cr was determined by inductively coupled plasma (ICP). 3. Results and discussion 3.1. Characterizations The XRD patterns of the prepared samples are displayed in Fig. 2a. It is observed that the strong diffraction peaks at 7.4°, 8.6°, 25.8°, 31.8°, and 43.5° et al. can be attributed to characteristic diffraction peaks of UiO-66, which is well matched with the reported literature [42], indicating that the UiO-66 MOFs is successfully synthesized. For the amine-functionalized UiO-66, it is found that the characteristic diffraction peak is the similar with the UiO-66 sample, inferring that introduction of amino group has slightly influence on crystal frame structure of UiO-66 [43]. After coupling with inorganic Ag/AgCl, new diffraction peaks appear at 27.9°, 32.2°, 46.26°, 54.9° and 57.6° are detected in Ag/ AgCl/UiO-66 and Ag/AgCl/NH2-UiO-66 hybridized samples, which corresponds to (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) crystal planes of AgCl (JCPDS#31-1238), respectively [18]. Besides, the
characteristic peak at 38.2° and 44.6° are attributed to the (1 1 1) and (2 0 0) crystal plane of Ag (JCPDS#04-0783) [38]. These results demonstrate that Ag/AgCl inorganic component has been successfully introduced into the UiO-66 and NH2-UiO-66 organic components. Meanwhile, it is detectable that the characteristic peaks assigned to UiO-66 and its shift slightly, inferring that the formation of Ag/AgCl affect UiO-66 and NH2-UiO-66 crystal structure, which illustrates that the Ag/AgCl nanoparticles are deposited on the surface of UiO-66 and NH2-UiO-66 frame. The FT-IR spectra of the above synthesized samples are displayed in Fig. 2b. It is observed that, by comparing NH2-UiO-66 with UiO-66, new absorption peaks of NAH and CAN vibrational stretching at 3468 and 1259 cm1 appeared which are ascribed to the addition of amino groups [33]. In contrast, other characteristic peaks have insignificant changes, which is agreement with the results of XRD. After introduction of Ag/AgCl, the FT-IR spectra of hybridized samples are almost similar with bare UiO-66 and NH2-UiO-66 samples while no characteristic peaks corresponding to inorganic Ag/AgCl are detected, which may be attributed to low loading amount and weak infrared signal of Ag/AgCl in hybridized system. Furthermore, the results indicate that the formation of Ag/AgCl has no obviously influence on the structure of the original MOFs and the Cl ion mainly derives from the residue of ZrCl4 starting material.
Fig. 2. (a) XRD patterns and (b) FT-IR spectra of UiO-66, Ag/AgCl/UiO-66, NH2-UiO-66 and Ag/AgCl/NH2-UiO-66.
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To investigate nanostructure of Ag/AgCl in the hybridized samples, SEM and TEM were conducted, as shown in Fig. 3. For the prepared Ag/AgCl/NH2-UiO-66 sample in Fig. 3a, where it is observed that the NH2-UiO-66 is an octahedron nanostructure with about 300 nm of size. Furthermore, the black nanosized Ag/AgCl particles with about 20 nm of diameter are found and dispersed on the surface of NH2-UiO-66, as shown in Fig. 3b. The EDX analysis of the Ag/AgCl/NH2-UiO-66 composites exhibit that Ag and Cl elements exist, which further testifies the formation of AgCl component in the hybrid system in Fig. 3c. Meanwhile, C, N, O, and Zr elements are also detected in the Ag/AgCl/NH2-UiO-66 hybridized sample, which indicates the element composition of NH2-UiO-66 MOFs. Furthermore, HR-TEM image in Fig. 3d displays 0.23 nm and 0.15 nm lattice fringes assigned to the Ag (1 1 1) facet and AgCl (2 2 0) facet, respectively, which further demonstrates that the Ag/AgCl inorganic component had been formed on the NH2-UiO66 MOFs, resulting in construction of ternary hybridized system. The chemical states of the as-prepared Ag/AgCl/NH2-UiO-66 system were explored by means of XPS tool. In Fig. 4a, the survey spectrum clearly displays that the hybridized sample includes C, N, O, Zr, Ag and Cl elements, which is consistent with the results of EDX analysis. In case of N 1s XPS spectrum, as the shown in Fig. 4b, the strong binding energy peak at 399.0 eV is ascribed to the N1s in the ANH2 group due to amine functionalization. For the O1s XPS spectrum, the binding peak is fitted to two characteristic peaks at 531.8 and 530.9 eV in Fig. 4c, which are assigned to ACOOH of H2ATA and the ZrAO bonds in NH2-UiO-66 MOFs, respectively [44]. Fig. 4d shows the C1s binding energies at 288.2, 285.6 and 284.4 eV, which are typical values for the OAC@O on the benzene ring, the CAN bind in the ANH2 and the ACOOH group of the H2ATA linkers, respectively. In Fig. 4e, the characteristic peaks at 184.9, 182.5 eV are attributed to Zr 3d5/2 and Zr 3d3/2, indicating the existence of Zr4+ in the hybridized system [45].
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Meanwhile, the XPS of Ag 3d is displayed in Fig. 4f, where the two broad peaks are fitted into four secondary peaks because of two Ag species in the hybridized system. Two peaks at 367.9 and 373.9 eV are assigned to the Ag 3d5/2 and Ag 3d3/2 peaks of Ag+ in AgCl with 6.0 eV of peak spacing whilst other two peaks at 368.8 and 374.8 eV belong to the Ag 3d5/2 and Ag 3d3/2 peaks of Ag0 with the same peak spacing [46]. These results verify that there are two Ag forms in Ag/AgCl/NH2-UiO-66, including Ag+ and Ag0, which is agreement with the results of XRD and HR-TEM [47]. In addition, the XPS spectrum of Cl element is displayed in Fig. S1. The binding energies at 199.7 and 198.3 eV are assigned to Cl 2p1/2 and Cl 2p3/2, respectively. In comparison with the standard spectrum, the presence of chlorine in the prepared Ag/AgCl/NH2UiO-66 is Cl ion [48]. Actually, the UiO-66 crystal does not contain Cl element theoretically, while ZrCl4 is used as Zr source in our method. Moreover, the results of XRD, EDX and XPS indicate that the Cl element in form of AgCl crystal phase is clearly detected in Ag/AgCl/NH2-UiO-66 samples. Therefore, it can be speculated that Cl ions mainly derive from the Cl residuals due to the incomplete coordination on the surface of NH2-UiO-66 crystal, which results in partial Cl chelating with Zr ions. According to our previous work, when NH2-UiO-66 samples are added into the beforehand AgNO3 solution, since the solubility product constant (Ksp) of AgCl is small (about 1.8 1010 mol2 L2), AgCl nanoparticles are formed preferentially and adhered to NH2-UiO-66 surface or pores [49]. Furthermore, under ultraviolet light irradiation, part of Ag+ ions on the surface of AgCl nanoparticles are reduced to Ag0. As a result, the ternary Ag/AgCl/NH2-UiO-66 heterojunction is synthesized. In Fig. 5a, UV–vis absorption spectra of the prepared samples are displayed. In comparison with the bare UiO-66, a new absorption peak at 360–370 nm for amine functionalization NH2-UiO-66 is clearly found. We elucidate that the introduction of amino groups on organic linker significantly affects the absorption ability
Fig. 3. (a) SEM images; (b) TEM images; (c) EDX analysis; (d) HR-TEM images of Ag/AgCl/NH2-UiO-66.
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Fig. 4. XPS spectra of Ag/AgCl/NH2-UiO-66: (a) Survey; and high resolution spectra of (b) N 1s, (c) O 1s, (d) C 1s, (e) Zr 3d and (f) Ag 3d.
Fig. 5. (a) UV–vis absorption spectra and (b) band gap measurement of UiO-66, Ag/AgCl/UiO-66, NH2-UiO-66 and Ag/AgCl/NH2-UiO-66.
to light ranging from 300 to 450 nm. There is a pair of lone electrons in amino group in 2-aminoterephthalic acid linker, which has strong influence on electronegativity of organic ligands via pp conjugation [50]. Therefore, the same absorption band is still observed in Ag/AgCl/NH2-UiO-66 sample by comparing with the Ag/AgCl/UiO-66 sample. In addition, the obvious absorption tail at the onset of 650 nm appears, which is attributed to the strong SPR effect of Ag nanoparticles when depositing Ag/AgCl component on the surface of NH2-UiO-66. The phenomenon is also seen clearly in Ag/AgCl/UiO-66 sample in comparison with the bare UiO-66 sample. Thereby, it is concluded that both introduction of Ag/AgCl and modification of amine improve the photo-response ability to the UV–visible light. Band gap energies (Eg) of these prepared samples are calculated according to the following equation [51]:
ðahv Þ ¼ A hv Eg 2
where h means the Planck’s constant, v denotes light frequency, a is absorption coefficient, A means constant, Eg is the band gap energy. These estimated band gaps energy are 3.98, 3.82, 2.91 and 2.83 eV, which are corresponding to the bare UiO-66, Ag/AgCl/UiO-66, NH2UiO-66, and Ag/AgCl/NH2-UiO-66, respectively. Compared with 3.98 eV of UiO-66 band gap energy, the band gap of NH2-UiO-66 is largely reduced, which means the added amino group possesses significant visible light response ability. Meanwhile, the introduction of Ag/AgCl also reduces the band gap energy and increases the absorption range at low photo energy due to the SPR effect. These results infer that the inorganic-organic Ag/AgCl/NH2-UiO-66 sample has the minimum band gap energy, which means stronger
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solar light utilization efficiency and more photoexcited charge carriers in comparison with other samples, thereby facilitating to enhance photocatalytic performance. To inspect the photo-induced electrons-holes separation features, the transient photocurrent response technique is used to observe the photo-produced charge carriers for the photocatalytic behavior of obtained photocatalysts. Fig. S2 (a) displays the transient photocurrent responses of UiO-66, Ag/AgCl/UiO-66, NH2-UiO-66 and Ag/AgCl/NH2-UiO-66 under visible light irradiation. It can be seen that the photocurrent quickly decreases to zero when the light is switched off, indicating the recombination of photo-generated charge carriers. The transient photocurrent responses of all samples are reversible and relatively stable at light-on and light-off. And Ag/AgCl/NH2-UiO-66 presents higher photocurrent density response than others, which suggests the addition of NH2 and Ag/AgCl improves the separation of photogenerated electrons and holes effectively and inhibits the recombination of photo-induced electron-hole pairs. The separation and transfer efficiency of photo-induced charge carries also could be evaluated by the electrochemical impedance spectra (EIS) under visible light irradiation [52,53]. Fig. S2 (b) shows the typical Nyquist plots of UiO-66, Ag/AgCl/UiO-66, NH2-UiO-66 and Ag/AgCl/NH2-UiO-66 under visible light irradiation. It can be observed that the semicircle radius of Ag/AgCl/NH2-UiO-66 is smaller than others. Because the smaller semicircle radius shows more efficient transfer of photo-induced charge carries, Ag/AgCl/ NH2-UiO-66 can inhibit the recombination of photo-induced charge carries. The results are consistent with that of photocurrent responses. The time resolved photoluminence (TRPL) spectra of UiO-66, Ag/AgCl/UiO-66, NH2-UiO-66 and Ag/AgCl/NH2-UiO-66 were measured and the decay curves in Fig. S3 are fitted by exponentials to obtain the decay time. The average lifetimes of charge carriers of UiO-66, Ag/AgCl/UiO-66, NH2-UiO-66 and Ag/AgCl/NH2-UiO-66 are 0.79, 0.92, 0.89 and 2.98 ns, respectively. The above electrochemical analysis results are consistent with the DRS results. 3.2. Photocatalytic activity Photocatalytic activities of these prepared samples were evaluated through Cr (VI) reduction in the presence of DPC. In Fig. 6a, by comparing photoreduction activities of these synthesized photocatalysts, it is observable that the Ag/AgCl/NH2-UiO-66 hybridized sample has the best removal efficiency of 74.2% within 150 min under visible light irradiation, which is 1.2, 1.4, and 1.7 times higher than those of Ag/AgCl/UiO-66, NH2-UiO-66, and UiO-66
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samples. These results demonstrate that the addition of Ag/AgCl inorganic component and amino group result in the obvious increasing of removal efficiency. Meanwhile, it is found that the prepared samples possess to some degree adsorption ability to Cr (VI) under dark, which is attributed to porous nanostructure of UiO-66 MOFs while the amine functionalized one display better adsorption performance of Cr (VI) by comparing the bare NH2-UiO-66 and UiO-66, as shown in Fig. 6a. Furthermore, the photoreduction proportion of Cr (VI) is also increased due to the introduction of NH2 groups. It infers that NH2 groups act as adsorptive center even further photoreduction center. By comparing Ag/AgCl/UiO-66 with UiO-66, the photocatalytic reduction efficiency of Cr (VI) is further enhanced except the increasing adsorption removal efficiency, which demonstrates that the addition of Ag/AgCl significantly improves the photocatalytic reduction process due to the SPR effect. In Fig. 6b and Table S1, the kinetic constant k fitted by first-order kinetic reaction and the fitting coefficient value R2 are displayed. The results show that the reduction kinetic is better fitted by the pseudo-first order kinetic reaction. The pseudo-first order model as expressed by equation below, which is generally used for photocatalytic degradation process if the initial concentration of pollutant is low [54]:
ln ðC0 =CÞ ¼ kt where C0 and C are the concentrations of the pollutants in solution at time 0 and t, respectively, and k is the pseudo-first order rate constant. It is shown that the rate constants of Ag/AgCl/UiO-66 and NH2-UiO-66 are 1.8 and 1.4 times higher than that of bare UiO-66, which demonstrates that the introduction of Ag/AgCl and amino definitely facilitates the photoactivity of Cr (VI) reduction. On the other hand, the kinetic constant k of Ag/AgCl/UiO-66 is 2.7 times higher than that of bare UiO-66, which exhibits the synergistic effect between Ag/AgCl and amino in enhancement of photoactivity. We elucidate that the introduction of amino improve the transfer of photoexcited electrons between organic linker and metal-oxo clusters. In Fig. 7, effects of different factors on the photocatalytic of Cr (VI) reduction over Ag/AgCl/NH2-UiO-66 were investigated, including Ag/AgCl loadings, photocatalyst concentrations, h+ trapping agents and concentration. In Fig. 7a, it is revealed that when the loading amount of AgNO3 reached to 6 wt%, the removal efficiency of Cr (VI) is the best, while the addition of AgNO3 is not always beneficial for enhancement of photocatalytic reduction efficiency. When the addition of AgNO3 reaches to 9 wt%, the removal efficiency of Cr (VI) is decreased, which may be ascribed to excess Ag/AgCl formation covering on the surface of NH2-UiO-66, thereby
Fig. 6. (a) Photocatalytic performance of these prepared photocatalyst for Cr (VI) reduction; (b) the fitting of the pseudo-first-order linear line for the photocatalytic reaction process. Reaction conditions: 0.25 g/L photocatalyst, 50 mL of 20 mg/L Cr (VI), room temperature, pH = 2.
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Fig. 7. Photocatalytic activities of Cr (VI) reduction over the Ag/AgCl/NH2-UiO-66 sample with different Ag/AgCl loadings (a), photocatalysts concentrations (b), capture agents (c), and addition amounts of ethanol (d).
reducing the porosity and photocatalytic reduction efficiency. As shown in Fig. 7b, the heterojunction system has the best photocatalytic performance, up to 74%, when the concentration of Ag/AgCl/NH2-UiO-66 sample is 0.25 g/L. Nevertheless, when the concentration of the ternary heterojunction reaches to 0.35 g/L, the photocatalytic activity decreased to 63% despite the adsorption removal of Cr (VI) has no significant change. The reasons are that excess photocatalyst hampers the absorption efficiency of solar and deteriorates the separation of photogenerated holes and electrons due to collision contact between excess photocatalyst nanoparticles. Therefore, the excess photocatalysts is harmful for the photocatalytic performance and optimization of reaction condition is necessary [55]. Their fitting of the pseudo-first-order linear line for the photocatalytic reaction process [56], and the reaction rate constants (k) and the correlation coefficient value (R2) of Cr (VI) photocatalytic reduction under the different conditions could be found in Supporting Information (Fig. S4 and Table S2). Usually, the excited photocatalysts can produce reductive electrons and oxidative holes simultaneously, which can react with different reactants at final CB and VB, respectively, through the corresponding half reaction. In this work, Cr (VI) reduction by utilizing photoinduced reductive electrons so that the oxidative holes could be scavenged by holes capture agents, which can facilitate to suppress recombination of holes and electrons. In Fig. 7c, it is observed that EDTA-2Na and citric acid display apparent suppression to photocatalytic performance whereas EtOH and formic acid exhibit improvement feature clearly. It elucidate that EtOH and formic acid can efficiently scavenger the photo-induced holes over Ag/AgCl/NH2-UiO-66, limiting the recombination of photogenerated electrons and holes, thereby enhancing photoreduction activ-
ity. Moreover, it is found that EtOH is a better choice in inhibiting recombination of photogenerated electrons and holes. In contrast, in the presence of EDTA-2Na, the carboxylic groups in EDTA-2Na can preferentially link with amino group in NH2-UiO-66 via strong hydrogen bond interaction, competitive suppressing electrostatic adsorption between Cr (VI) and charged amino group, thereby leading to the decrease of adsorption removal efficiency. When the citric acid is added, the adsorption of Cr (VI) is enhanced while the photocatalytic reduction efficiency is significantly undermined. We speculate that the citric acid chelates with Zr ions, limiting photoinduced electron transfer from amino group to Zr-O sites, thus hampering photocatalytic reduction process. Owing to the holes consumption by EtOH, more electrons can be used to reduce Cr (VI). When 4 mL of EtOH is added into 0.25 g/L Ag/AgCl/NH2UiO-66 (6 wt%) hybridized sample, the photoreduction efficiency of Cr (VI) achieves to 93.7%, as described in Fig. 7d, while more addition of EtOH has slight increasing in removal efficiency. Under optimal conditions, the concentration of total Cr and Cr (VI) before and after photocatalytic reaction are measured by ICP and DPC method, and the specific data are shown in Table S4. The reduction of total Cr may be due to adsorption in dark. The adsorption rates are different, which calculated by the two methods. It may be because that, during the reaction, some Cr (VI) adsorbed was released and reduced to Cr (III). Furthermore, the performance of some typical MOFs or their composite photocatalysts for Cr (VI) reduction under visible light irradiation is shown in Table S3. In order to further explore the stability and reusability of catalysts, the Ag/AgCl/NH2-UiO-66 was collected after used and purified with DMF and EtOH several times. Actually, Ag0 is prepared by UV-assisted reduction method, existing in the form of solid. Moreover, during the photocatalytic process, the light conditions
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Fig. 8. (a) XRD patterns of Ag/AgCl/NH2-UiO-66 before and after the catalytic reaction; (b) repeated experiments of Ag/AgCl/NH2-UiO-66 for the photocatalytic reduction of Cr (VI).
Fig. 9. Possible mechanism of Cr (VI) reduction in water with Ag/AgCl/NH2-UiO-66.
are similar to the UV reduction process, so that the Ag+ ions can easily be converted to Ag0 despite the reaction system is at strong acid condition. Fig. 8a shows XRD patterns of Ag/AgCl/NH2-UiO-66 hybridized system before and after Cr (VI) reduction reaction. It is observed that the crystal structure of the ternary Ag/AgCl/NH2UiO-66 remains stable in spite of 3 cycles. The used composite has the similar photocatalytic performance as the fresh one, as described in Fig. 8b, which demonstrates that Ag/AgCl/NH2-UiO66 has better photostability than traditional Ag-based photocatalysts suffering from photocorrosion easily. Based on the above results, the photocatalytic mechanism of Cr (VI) reduction over Ag/AgCl/NH2-UiO-66 is proposed, as described in Fig. 9. The photocatalytic mechanism of Cr (VI) reduction over Ag/AgCl/NH2-UiO-66 is proposed, as described in Fig. 9. Under visible light irradiation, the valance band (VB) electrons of NH2-UiO66 can be excited to the conduction band (CB) to generate the electron-hole pairs [57]. Subsequently, the photoexcited electrons transfer to the Zr-O cluster via a carboxylic acid bond based on ligand-to-metal charge transition model [58]. The hot electrons at Zr-O clusters rapidly react with Cr (VI) and produce the reduced Cr (III). And the holes could participate in oxidation reactions. On the other hand, the introduced Ag0 nanoparticles owing to the strong SPR effect can efficiently absorb visible light and produce photoexcited electrons and holes [59]. The electrons rapid move to the surface of NH2-UiO-66, subsequently, trapped by the Cr (VI) [60]. Meanwhile, the electrons and holes may recombine in the bulk and surface of the photocatalysts. Therefore, holes trapped experiment was carried out to demonstrate that capture agents
such as EtOH could be oxidized to the corresponding oxidation product. It is concluded that the amine functionalization and Ag/ AgCl introduction not only enhance absorption ability to visible light but also promote the separation efficiency of photo induced electrons and holes. Thereby, the photocatalytic reduction efficiency of Cr (VI) is significantly improved in comparison with the bare UiO-66.
4. Conclusions In summary, an plasmonic Ag/AgCl/NH2-UiO-66 hybridized photocatalyst is successfully prepared by solvothermal route with UV reduction method. The Ag/AgCl/NH2-UiO-66 hybrid system exhibits efficient visible-light response and enhanced photocatalytic Cr (VI) reduction compared with the bare Ag/AgCl/UiO-66, NH2-UiO-66, and UiO-66. The photoelectrochemical data indicate that compared with the UiO-66, the charge separation and transfer efficiency of Ag/AgCl/NH2-UiO-66 heterojunction is significantly enhanced due to the introduction of amine functionalization and formation of inorganic-organic hybrid. Moreover, the photocatalytic reduction efficiency of Cr (VI) is 1.7 times higher than that of the bare UiO-66. When the hot electrons are produced in Ag/AgCl due to SPR effect, they rapidly transfer to the NH2-UiO-66 by means of LMCT model, which efficiently suppresses the recombination of electrons with holes staying at the Ag/AgCl. This work shows that the Ag/AgCl/NH2-UiO-66 ternary hybridized system could provide a new path to construct efficient photocatalysts for immediate
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applications in wastewater treatment and energy harvesting by direct sunlight irradiation.
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Acknowledgements This work was supported financially by the Key Project of Natural Science Foundation of Liaoning Province of China (No. 20170540578), Scientific Research Foundation of Education Department of Liaoning Province of China (No. LQ2019023) and Undergraduate Training Programs for Innovation and Entrepreneurship of Liaoning Province of China (No. 201710165000305). We also acknowledge the support of the National Natural Science Foundation of China (No. 21501138), the Natural Science Foundation of Hubei Province of China (2015CFB177). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.07.103.
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Regular Article
Construction of ternary Ag/AgCl/NH2-UiO-66 hybridized heterojunction for effective photocatalytic hexavalent chromium reduction Zhiguang Zhang a,⇑, Siqi Wang a, Mingjun Bao a, Jiawen Ren a, Sihang Pei a, Shijun Yu a, Jun Ke b,⇑ a b
School of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian 116029, PR China School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan 430073, PR China
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 8 June 2019 Revised 24 July 2019 Accepted 30 July 2019 Available online 31 July 2019 Keywords: Metal-organic frameworks Heterojunction Photo-reduction Cr (VI) removal Visible light response
a b s t r a c t An Ag/AgCl/NH2-UiO-66 hybridized photocatalyst was successfully constructed via facile solvothermal with UV reduction method for efficient photocatalytic Cr (VI) reduction. The photoelectrochemical data indicate that compared with the UiO-66, the charge separation and transfer efficiency of Ag/AgCl/NH2UiO-66 heterojunction is significantly enhanced due to the introduction of amine functionalization and formation of inorganic-organic hybrid. The surface plasmon resonance (SPR) effect deriving from Ag nanoparticles (NPs) largely extends photo-response range whilst the separation efficiency of photogenerated electrons and holes is improved significantly. The synthesized Ag/AgCl/NH2-UiO-66 hybrid system shows ameliorated structural stability and superior photocatalytic activity for Cr (VI) reduction under visible light irradiation, which is 1.7 times higher than that of the bare UiO-66. Furthermore, the possible mechanism of Cr (VI) reduction is proposed by analyzing electron transfer path in the ternary Ag/AgCl/NH2-UiO-66 hybridized system. Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction Heavy metal contamination in natural water, including hexavalent chromium (Cr (VI)) has been gradually becoming a serious environmental problem, which is produced in metal processing, leather tanning, and mining [1–4]. Once heavy metals enter ⇑ Corresponding authors. E-mail addresses: [email protected] (Z. Zhang), [email protected] (J. Ke). https://doi.org/10.1016/j.jcis.2019.07.103 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.
ecosystem, living things including human being have to face serious threats due to their high solubility, strong toxicity and bioaccumulation effect [5]. Therefore, converting heavy metal from high toxicity form to low even non toxicity is one of efficient strategies to mitigate heavy metal environment contamination [6–8]. Recently, semiconductor photocatalysis technique has been demonstrated as an effective and ‘‘green” route to reducing high toxic Cr (VI) to low toxic Cr (III). In general, the semiconductor photocatalysts are irradiated by solar incident light and produce
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electrons and holes. The excited electrons migrate rapidly to conduction band (CB) of the semiconductor and holes still stay at valence band (VB). Meanwhile, the hot electrons at CB have strong reductive ability to reduce adsorbed Cr (VI) on the surface of photocatalysts to Cr (III) [9–12]. Among numerous semiconductor photocatalysts, TiO2 is widely used in pollutant elimination and energy catalysis because of its low cost, remarkable chemical and photochemical properties [13–16]. Unfortunately, the wide band gap (3.2 eV) of TiO2 only absorbs UV light accounting for 5–8% solar light, indicating that the utilization efficiency of sunlight is extremely low [17–19]. Therefore, developing a novel and efficient photocatalysts is urgent need to enlarge practical applications of photocatalysis technology in future [20]. Metal-organic frameworks (MOFs) is a new type of organic porous materials possessing infinite 3D frameworks consisting of metal-oxo clusters and organic linkers blocks [21], which is endowed with desirable topology, structural flexibility, and high surface areas [22–24]. Recently, the MOFs-based photocatalysts have been arising extensive interests. Owing to their semiconductor-like behaviors, the organic linkers in MOFs can act as unique antennas to harvest solar light, which can activate their metal sites via the ligands to metal clusters charge transition (LMCT). The photoexcited electrons and holes could be used to trigger a series of catalytic oxidation and reduction reactions [25–27]. Furthermore, compared with traditional photocatalysts, band gap energies of MOFs-based photocatalysts mainly depends on the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), which can be easily tuned by varying inorganic units and organic linkers [28,29]. Besides, more reported works reveal that amine-functionalized MOFsbased photocatalysts exhibit enhanced visible light response, originating from the metal-oxo cluster direct excitation and charges transferring from excited organic linker to metal-oxo clusters. For example, Ye et al. [30] and Wu et al. [31] separately demonstrated that NH2-MIL-88B(Fe) and NH2-MIL-68(In) possessed extra absorption abilities in visible light region, which present higher photocatalytic efficiency of Cr (VI) reduction than those of without amino group modification under visible light irradiation. Meanwhile, our former work demonstrated that the NH2-MIL-101(Fe) possessed strong visible light response ability and superb photoxidation performance for gaseous toluene, where the corresponding conversion efficiency is up to 79.4% equivalent to the commercial photocatalysts [32]. Additionally, NH2-UiO-66 as a good choice is chosen owing to its good structural stability in liquid reaction. Wu and co-workers reported that UiO-66(ANH2) exhibited good photocatalytic activity in selective oxidation of alcohol [33]. However, the potential applications of amine-functionalized MOFs are still limited due to the unavoidable recombination of photogenerated electrons and holes. Loading precious metals is one of effective ways to suppress the rapid recombination of photoinduced electrons and holes in photocatalytic process. For instance, plasmonic Ag/AgCl not only strongly absorbs visible light owing to surface plasmon resonances (SPR) effect of metallic Ag but also enforces separation of photogenerated electrons and holes [34,35], where the holes formed at Ag nanoparticles can rapidly move to AgCl and produce Cl radicals, thereby enhancing the photocatalytic oxidation performance [36]. Based on the above analysis, constructing an excellent photocatalyst consisting of Ag/AgCl and modifying of amino group can realize enhancement of photocatalytic activity of UiO-66 for Cr (VI) reduction under the visible light. In this paper, we prepared a ternary Ag/AgCl/NH2-UiO-66 hybridized photocatalyst by a facile solvothermal coupled with UV photoreduction method, which exhibits enhanced Cr (VI) reduction photocatalytic performance in comparison with bare UiO-66. The photocatalytic mechanism of Cr (VI) reduction over the ternary Ag/AgCl/NH2-UiO-66
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hybridized system is proposed and discussed. The aminefunctionalized modification and Ag/AgCl introduction largely promote the visible light range and ability and significantly enhance separation efficiency of photoinduced charge carriers. This work could give us a new path to constructing a ternary Ag/AgCl/NH2UiO-66 hybridized photocatalyst for efficient photocatalytic activities with high stability.
2. Experimental 2.1. Synthesis of UiO-66 and NH2-UiO-66 The UiO-66 sample was synthesized through a facile solvothermal method [37]. In typical synthesis process, ZrCl4 (1.5 mmol) and H2BDC (1.5 mmol) were dissolved in DMF (75 mL). Then, the mixture was transferred into a 120 mL Teflon-lined stainless steel autoclave, heated to 120 °C, and maintained for 48 h. After cooled to room temperature naturally, the suspension was centrifuged and washed by using DMF and methanol, respectively. The sample was dried at 70 °C for 5 h, and the white UiO-66 sample was gained. The NH2-UiO-66 sample was synthesized through the above similar process except that H2BDC was replaced with H2ATA as a starting material. 2.2. Synthesis of Ag/AgCl/UiO-66 and Ag/AgCl/NH2-UiO-66 The Ag/AgCl/UiO-66 hybrid system was prepared by ultraviolet light reduction [38]. Firstly, a certain amount of AgNO3 was dissolved in EtOH. 0.1 g of UiO-66 sample was added into the transparent solution and vigorously stirred for 2 h, followed by UV light irradiation for 2 h. The suspension was centrifuged and washed with ethanol repeatedly, dried at 70 °C to obtain Ag/AgCl/UiO-66 sample. The Ag/AgCl/NH2-UiO-66 was prepared through the similar process except that NH2-UiO-66 was used as a starting material instead of UiO-66. The synthesis process of Ag/AgCl/NH2-UiO-66 is described in Fig. 1. 2.3. Characterization The characterization and instruments could be found in Supporting Information. 2.4. Photocatalytic activity A certain amount of prepared photocatalyst was added to the Cr (VI) solution (20 mg/L) at room temperature, and the pH value of reaction solution was varied to 2 by adding HCl solution. After that, the mixed solution was stirred in the darkness. And the samples were taken and measured in intervals of 20 min, confirming that the adsorption-desorption equilibrium has achieved before irradiation. At 40 min the difference of Cr (VI) concentration is negligible, indicating that the adsorption-desorption equilibrium reaches. Then the suspension solution was irradiated by a 300 W Xe lamp with a 420 nm filter. During the photocatalytic process, a certain amount of the solution was taken out at regular intervals. The supernatant was centrifuged and filtered using a 0.22-lm syringe filter to remove the photocatalyst particles before analysis. Absorbance of DPC-Cr (VI) complex at 540 nm was measured by using UV–vis spectrophotometer [39,40]. At the same wavelength, the concentration of Cr (VI) was calculated according to BeerLambert Law [41]. The corresponding Cr (VI) concentration can be calculated, and the corresponding adsorption-reduction efficiency of Cr (VI) was calculated following the formula:
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Fig. 1. Schematic of the synthesis processes of Ag/AgCl/NH2-UiO-66.
Adsorption reduction efficiency ð%Þ ¼ C=C 0 100 Here, C: concentration of Cr (VI); C0: initial concentration of Cr (VI). And the concentration of total Cr was determined by inductively coupled plasma (ICP). 3. Results and discussion 3.1. Characterizations The XRD patterns of the prepared samples are displayed in Fig. 2a. It is observed that the strong diffraction peaks at 7.4°, 8.6°, 25.8°, 31.8°, and 43.5° et al. can be attributed to characteristic diffraction peaks of UiO-66, which is well matched with the reported literature [42], indicating that the UiO-66 MOFs is successfully synthesized. For the amine-functionalized UiO-66, it is found that the characteristic diffraction peak is the similar with the UiO-66 sample, inferring that introduction of amino group has slightly influence on crystal frame structure of UiO-66 [43]. After coupling with inorganic Ag/AgCl, new diffraction peaks appear at 27.9°, 32.2°, 46.26°, 54.9° and 57.6° are detected in Ag/ AgCl/UiO-66 and Ag/AgCl/NH2-UiO-66 hybridized samples, which corresponds to (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) crystal planes of AgCl (JCPDS#31-1238), respectively [18]. Besides, the
characteristic peak at 38.2° and 44.6° are attributed to the (1 1 1) and (2 0 0) crystal plane of Ag (JCPDS#04-0783) [38]. These results demonstrate that Ag/AgCl inorganic component has been successfully introduced into the UiO-66 and NH2-UiO-66 organic components. Meanwhile, it is detectable that the characteristic peaks assigned to UiO-66 and its shift slightly, inferring that the formation of Ag/AgCl affect UiO-66 and NH2-UiO-66 crystal structure, which illustrates that the Ag/AgCl nanoparticles are deposited on the surface of UiO-66 and NH2-UiO-66 frame. The FT-IR spectra of the above synthesized samples are displayed in Fig. 2b. It is observed that, by comparing NH2-UiO-66 with UiO-66, new absorption peaks of NAH and CAN vibrational stretching at 3468 and 1259 cm1 appeared which are ascribed to the addition of amino groups [33]. In contrast, other characteristic peaks have insignificant changes, which is agreement with the results of XRD. After introduction of Ag/AgCl, the FT-IR spectra of hybridized samples are almost similar with bare UiO-66 and NH2-UiO-66 samples while no characteristic peaks corresponding to inorganic Ag/AgCl are detected, which may be attributed to low loading amount and weak infrared signal of Ag/AgCl in hybridized system. Furthermore, the results indicate that the formation of Ag/AgCl has no obviously influence on the structure of the original MOFs and the Cl ion mainly derives from the residue of ZrCl4 starting material.
Fig. 2. (a) XRD patterns and (b) FT-IR spectra of UiO-66, Ag/AgCl/UiO-66, NH2-UiO-66 and Ag/AgCl/NH2-UiO-66.
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To investigate nanostructure of Ag/AgCl in the hybridized samples, SEM and TEM were conducted, as shown in Fig. 3. For the prepared Ag/AgCl/NH2-UiO-66 sample in Fig. 3a, where it is observed that the NH2-UiO-66 is an octahedron nanostructure with about 300 nm of size. Furthermore, the black nanosized Ag/AgCl particles with about 20 nm of diameter are found and dispersed on the surface of NH2-UiO-66, as shown in Fig. 3b. The EDX analysis of the Ag/AgCl/NH2-UiO-66 composites exhibit that Ag and Cl elements exist, which further testifies the formation of AgCl component in the hybrid system in Fig. 3c. Meanwhile, C, N, O, and Zr elements are also detected in the Ag/AgCl/NH2-UiO-66 hybridized sample, which indicates the element composition of NH2-UiO-66 MOFs. Furthermore, HR-TEM image in Fig. 3d displays 0.23 nm and 0.15 nm lattice fringes assigned to the Ag (1 1 1) facet and AgCl (2 2 0) facet, respectively, which further demonstrates that the Ag/AgCl inorganic component had been formed on the NH2-UiO66 MOFs, resulting in construction of ternary hybridized system. The chemical states of the as-prepared Ag/AgCl/NH2-UiO-66 system were explored by means of XPS tool. In Fig. 4a, the survey spectrum clearly displays that the hybridized sample includes C, N, O, Zr, Ag and Cl elements, which is consistent with the results of EDX analysis. In case of N 1s XPS spectrum, as the shown in Fig. 4b, the strong binding energy peak at 399.0 eV is ascribed to the N1s in the ANH2 group due to amine functionalization. For the O1s XPS spectrum, the binding peak is fitted to two characteristic peaks at 531.8 and 530.9 eV in Fig. 4c, which are assigned to ACOOH of H2ATA and the ZrAO bonds in NH2-UiO-66 MOFs, respectively [44]. Fig. 4d shows the C1s binding energies at 288.2, 285.6 and 284.4 eV, which are typical values for the OAC@O on the benzene ring, the CAN bind in the ANH2 and the ACOOH group of the H2ATA linkers, respectively. In Fig. 4e, the characteristic peaks at 184.9, 182.5 eV are attributed to Zr 3d5/2 and Zr 3d3/2, indicating the existence of Zr4+ in the hybridized system [45].
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Meanwhile, the XPS of Ag 3d is displayed in Fig. 4f, where the two broad peaks are fitted into four secondary peaks because of two Ag species in the hybridized system. Two peaks at 367.9 and 373.9 eV are assigned to the Ag 3d5/2 and Ag 3d3/2 peaks of Ag+ in AgCl with 6.0 eV of peak spacing whilst other two peaks at 368.8 and 374.8 eV belong to the Ag 3d5/2 and Ag 3d3/2 peaks of Ag0 with the same peak spacing [46]. These results verify that there are two Ag forms in Ag/AgCl/NH2-UiO-66, including Ag+ and Ag0, which is agreement with the results of XRD and HR-TEM [47]. In addition, the XPS spectrum of Cl element is displayed in Fig. S1. The binding energies at 199.7 and 198.3 eV are assigned to Cl 2p1/2 and Cl 2p3/2, respectively. In comparison with the standard spectrum, the presence of chlorine in the prepared Ag/AgCl/NH2UiO-66 is Cl ion [48]. Actually, the UiO-66 crystal does not contain Cl element theoretically, while ZrCl4 is used as Zr source in our method. Moreover, the results of XRD, EDX and XPS indicate that the Cl element in form of AgCl crystal phase is clearly detected in Ag/AgCl/NH2-UiO-66 samples. Therefore, it can be speculated that Cl ions mainly derive from the Cl residuals due to the incomplete coordination on the surface of NH2-UiO-66 crystal, which results in partial Cl chelating with Zr ions. According to our previous work, when NH2-UiO-66 samples are added into the beforehand AgNO3 solution, since the solubility product constant (Ksp) of AgCl is small (about 1.8 1010 mol2 L2), AgCl nanoparticles are formed preferentially and adhered to NH2-UiO-66 surface or pores [49]. Furthermore, under ultraviolet light irradiation, part of Ag+ ions on the surface of AgCl nanoparticles are reduced to Ag0. As a result, the ternary Ag/AgCl/NH2-UiO-66 heterojunction is synthesized. In Fig. 5a, UV–vis absorption spectra of the prepared samples are displayed. In comparison with the bare UiO-66, a new absorption peak at 360–370 nm for amine functionalization NH2-UiO-66 is clearly found. We elucidate that the introduction of amino groups on organic linker significantly affects the absorption ability
Fig. 3. (a) SEM images; (b) TEM images; (c) EDX analysis; (d) HR-TEM images of Ag/AgCl/NH2-UiO-66.
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Fig. 4. XPS spectra of Ag/AgCl/NH2-UiO-66: (a) Survey; and high resolution spectra of (b) N 1s, (c) O 1s, (d) C 1s, (e) Zr 3d and (f) Ag 3d.
Fig. 5. (a) UV–vis absorption spectra and (b) band gap measurement of UiO-66, Ag/AgCl/UiO-66, NH2-UiO-66 and Ag/AgCl/NH2-UiO-66.
to light ranging from 300 to 450 nm. There is a pair of lone electrons in amino group in 2-aminoterephthalic acid linker, which has strong influence on electronegativity of organic ligands via pp conjugation [50]. Therefore, the same absorption band is still observed in Ag/AgCl/NH2-UiO-66 sample by comparing with the Ag/AgCl/UiO-66 sample. In addition, the obvious absorption tail at the onset of 650 nm appears, which is attributed to the strong SPR effect of Ag nanoparticles when depositing Ag/AgCl component on the surface of NH2-UiO-66. The phenomenon is also seen clearly in Ag/AgCl/UiO-66 sample in comparison with the bare UiO-66 sample. Thereby, it is concluded that both introduction of Ag/AgCl and modification of amine improve the photo-response ability to the UV–visible light. Band gap energies (Eg) of these prepared samples are calculated according to the following equation [51]:
ðahv Þ ¼ A hv Eg 2
where h means the Planck’s constant, v denotes light frequency, a is absorption coefficient, A means constant, Eg is the band gap energy. These estimated band gaps energy are 3.98, 3.82, 2.91 and 2.83 eV, which are corresponding to the bare UiO-66, Ag/AgCl/UiO-66, NH2UiO-66, and Ag/AgCl/NH2-UiO-66, respectively. Compared with 3.98 eV of UiO-66 band gap energy, the band gap of NH2-UiO-66 is largely reduced, which means the added amino group possesses significant visible light response ability. Meanwhile, the introduction of Ag/AgCl also reduces the band gap energy and increases the absorption range at low photo energy due to the SPR effect. These results infer that the inorganic-organic Ag/AgCl/NH2-UiO-66 sample has the minimum band gap energy, which means stronger
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solar light utilization efficiency and more photoexcited charge carriers in comparison with other samples, thereby facilitating to enhance photocatalytic performance. To inspect the photo-induced electrons-holes separation features, the transient photocurrent response technique is used to observe the photo-produced charge carriers for the photocatalytic behavior of obtained photocatalysts. Fig. S2 (a) displays the transient photocurrent responses of UiO-66, Ag/AgCl/UiO-66, NH2-UiO-66 and Ag/AgCl/NH2-UiO-66 under visible light irradiation. It can be seen that the photocurrent quickly decreases to zero when the light is switched off, indicating the recombination of photo-generated charge carriers. The transient photocurrent responses of all samples are reversible and relatively stable at light-on and light-off. And Ag/AgCl/NH2-UiO-66 presents higher photocurrent density response than others, which suggests the addition of NH2 and Ag/AgCl improves the separation of photogenerated electrons and holes effectively and inhibits the recombination of photo-induced electron-hole pairs. The separation and transfer efficiency of photo-induced charge carries also could be evaluated by the electrochemical impedance spectra (EIS) under visible light irradiation [52,53]. Fig. S2 (b) shows the typical Nyquist plots of UiO-66, Ag/AgCl/UiO-66, NH2-UiO-66 and Ag/AgCl/NH2-UiO-66 under visible light irradiation. It can be observed that the semicircle radius of Ag/AgCl/NH2-UiO-66 is smaller than others. Because the smaller semicircle radius shows more efficient transfer of photo-induced charge carries, Ag/AgCl/ NH2-UiO-66 can inhibit the recombination of photo-induced charge carries. The results are consistent with that of photocurrent responses. The time resolved photoluminence (TRPL) spectra of UiO-66, Ag/AgCl/UiO-66, NH2-UiO-66 and Ag/AgCl/NH2-UiO-66 were measured and the decay curves in Fig. S3 are fitted by exponentials to obtain the decay time. The average lifetimes of charge carriers of UiO-66, Ag/AgCl/UiO-66, NH2-UiO-66 and Ag/AgCl/NH2-UiO-66 are 0.79, 0.92, 0.89 and 2.98 ns, respectively. The above electrochemical analysis results are consistent with the DRS results. 3.2. Photocatalytic activity Photocatalytic activities of these prepared samples were evaluated through Cr (VI) reduction in the presence of DPC. In Fig. 6a, by comparing photoreduction activities of these synthesized photocatalysts, it is observable that the Ag/AgCl/NH2-UiO-66 hybridized sample has the best removal efficiency of 74.2% within 150 min under visible light irradiation, which is 1.2, 1.4, and 1.7 times higher than those of Ag/AgCl/UiO-66, NH2-UiO-66, and UiO-66
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samples. These results demonstrate that the addition of Ag/AgCl inorganic component and amino group result in the obvious increasing of removal efficiency. Meanwhile, it is found that the prepared samples possess to some degree adsorption ability to Cr (VI) under dark, which is attributed to porous nanostructure of UiO-66 MOFs while the amine functionalized one display better adsorption performance of Cr (VI) by comparing the bare NH2-UiO-66 and UiO-66, as shown in Fig. 6a. Furthermore, the photoreduction proportion of Cr (VI) is also increased due to the introduction of NH2 groups. It infers that NH2 groups act as adsorptive center even further photoreduction center. By comparing Ag/AgCl/UiO-66 with UiO-66, the photocatalytic reduction efficiency of Cr (VI) is further enhanced except the increasing adsorption removal efficiency, which demonstrates that the addition of Ag/AgCl significantly improves the photocatalytic reduction process due to the SPR effect. In Fig. 6b and Table S1, the kinetic constant k fitted by first-order kinetic reaction and the fitting coefficient value R2 are displayed. The results show that the reduction kinetic is better fitted by the pseudo-first order kinetic reaction. The pseudo-first order model as expressed by equation below, which is generally used for photocatalytic degradation process if the initial concentration of pollutant is low [54]:
ln ðC0 =CÞ ¼ kt where C0 and C are the concentrations of the pollutants in solution at time 0 and t, respectively, and k is the pseudo-first order rate constant. It is shown that the rate constants of Ag/AgCl/UiO-66 and NH2-UiO-66 are 1.8 and 1.4 times higher than that of bare UiO-66, which demonstrates that the introduction of Ag/AgCl and amino definitely facilitates the photoactivity of Cr (VI) reduction. On the other hand, the kinetic constant k of Ag/AgCl/UiO-66 is 2.7 times higher than that of bare UiO-66, which exhibits the synergistic effect between Ag/AgCl and amino in enhancement of photoactivity. We elucidate that the introduction of amino improve the transfer of photoexcited electrons between organic linker and metal-oxo clusters. In Fig. 7, effects of different factors on the photocatalytic of Cr (VI) reduction over Ag/AgCl/NH2-UiO-66 were investigated, including Ag/AgCl loadings, photocatalyst concentrations, h+ trapping agents and concentration. In Fig. 7a, it is revealed that when the loading amount of AgNO3 reached to 6 wt%, the removal efficiency of Cr (VI) is the best, while the addition of AgNO3 is not always beneficial for enhancement of photocatalytic reduction efficiency. When the addition of AgNO3 reaches to 9 wt%, the removal efficiency of Cr (VI) is decreased, which may be ascribed to excess Ag/AgCl formation covering on the surface of NH2-UiO-66, thereby
Fig. 6. (a) Photocatalytic performance of these prepared photocatalyst for Cr (VI) reduction; (b) the fitting of the pseudo-first-order linear line for the photocatalytic reaction process. Reaction conditions: 0.25 g/L photocatalyst, 50 mL of 20 mg/L Cr (VI), room temperature, pH = 2.
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Fig. 7. Photocatalytic activities of Cr (VI) reduction over the Ag/AgCl/NH2-UiO-66 sample with different Ag/AgCl loadings (a), photocatalysts concentrations (b), capture agents (c), and addition amounts of ethanol (d).
reducing the porosity and photocatalytic reduction efficiency. As shown in Fig. 7b, the heterojunction system has the best photocatalytic performance, up to 74%, when the concentration of Ag/AgCl/NH2-UiO-66 sample is 0.25 g/L. Nevertheless, when the concentration of the ternary heterojunction reaches to 0.35 g/L, the photocatalytic activity decreased to 63% despite the adsorption removal of Cr (VI) has no significant change. The reasons are that excess photocatalyst hampers the absorption efficiency of solar and deteriorates the separation of photogenerated holes and electrons due to collision contact between excess photocatalyst nanoparticles. Therefore, the excess photocatalysts is harmful for the photocatalytic performance and optimization of reaction condition is necessary [55]. Their fitting of the pseudo-first-order linear line for the photocatalytic reaction process [56], and the reaction rate constants (k) and the correlation coefficient value (R2) of Cr (VI) photocatalytic reduction under the different conditions could be found in Supporting Information (Fig. S4 and Table S2). Usually, the excited photocatalysts can produce reductive electrons and oxidative holes simultaneously, which can react with different reactants at final CB and VB, respectively, through the corresponding half reaction. In this work, Cr (VI) reduction by utilizing photoinduced reductive electrons so that the oxidative holes could be scavenged by holes capture agents, which can facilitate to suppress recombination of holes and electrons. In Fig. 7c, it is observed that EDTA-2Na and citric acid display apparent suppression to photocatalytic performance whereas EtOH and formic acid exhibit improvement feature clearly. It elucidate that EtOH and formic acid can efficiently scavenger the photo-induced holes over Ag/AgCl/NH2-UiO-66, limiting the recombination of photogenerated electrons and holes, thereby enhancing photoreduction activ-
ity. Moreover, it is found that EtOH is a better choice in inhibiting recombination of photogenerated electrons and holes. In contrast, in the presence of EDTA-2Na, the carboxylic groups in EDTA-2Na can preferentially link with amino group in NH2-UiO-66 via strong hydrogen bond interaction, competitive suppressing electrostatic adsorption between Cr (VI) and charged amino group, thereby leading to the decrease of adsorption removal efficiency. When the citric acid is added, the adsorption of Cr (VI) is enhanced while the photocatalytic reduction efficiency is significantly undermined. We speculate that the citric acid chelates with Zr ions, limiting photoinduced electron transfer from amino group to Zr-O sites, thus hampering photocatalytic reduction process. Owing to the holes consumption by EtOH, more electrons can be used to reduce Cr (VI). When 4 mL of EtOH is added into 0.25 g/L Ag/AgCl/NH2UiO-66 (6 wt%) hybridized sample, the photoreduction efficiency of Cr (VI) achieves to 93.7%, as described in Fig. 7d, while more addition of EtOH has slight increasing in removal efficiency. Under optimal conditions, the concentration of total Cr and Cr (VI) before and after photocatalytic reaction are measured by ICP and DPC method, and the specific data are shown in Table S4. The reduction of total Cr may be due to adsorption in dark. The adsorption rates are different, which calculated by the two methods. It may be because that, during the reaction, some Cr (VI) adsorbed was released and reduced to Cr (III). Furthermore, the performance of some typical MOFs or their composite photocatalysts for Cr (VI) reduction under visible light irradiation is shown in Table S3. In order to further explore the stability and reusability of catalysts, the Ag/AgCl/NH2-UiO-66 was collected after used and purified with DMF and EtOH several times. Actually, Ag0 is prepared by UV-assisted reduction method, existing in the form of solid. Moreover, during the photocatalytic process, the light conditions
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Fig. 8. (a) XRD patterns of Ag/AgCl/NH2-UiO-66 before and after the catalytic reaction; (b) repeated experiments of Ag/AgCl/NH2-UiO-66 for the photocatalytic reduction of Cr (VI).
Fig. 9. Possible mechanism of Cr (VI) reduction in water with Ag/AgCl/NH2-UiO-66.
are similar to the UV reduction process, so that the Ag+ ions can easily be converted to Ag0 despite the reaction system is at strong acid condition. Fig. 8a shows XRD patterns of Ag/AgCl/NH2-UiO-66 hybridized system before and after Cr (VI) reduction reaction. It is observed that the crystal structure of the ternary Ag/AgCl/NH2UiO-66 remains stable in spite of 3 cycles. The used composite has the similar photocatalytic performance as the fresh one, as described in Fig. 8b, which demonstrates that Ag/AgCl/NH2-UiO66 has better photostability than traditional Ag-based photocatalysts suffering from photocorrosion easily. Based on the above results, the photocatalytic mechanism of Cr (VI) reduction over Ag/AgCl/NH2-UiO-66 is proposed, as described in Fig. 9. The photocatalytic mechanism of Cr (VI) reduction over Ag/AgCl/NH2-UiO-66 is proposed, as described in Fig. 9. Under visible light irradiation, the valance band (VB) electrons of NH2-UiO66 can be excited to the conduction band (CB) to generate the electron-hole pairs [57]. Subsequently, the photoexcited electrons transfer to the Zr-O cluster via a carboxylic acid bond based on ligand-to-metal charge transition model [58]. The hot electrons at Zr-O clusters rapidly react with Cr (VI) and produce the reduced Cr (III). And the holes could participate in oxidation reactions. On the other hand, the introduced Ag0 nanoparticles owing to the strong SPR effect can efficiently absorb visible light and produce photoexcited electrons and holes [59]. The electrons rapid move to the surface of NH2-UiO-66, subsequently, trapped by the Cr (VI) [60]. Meanwhile, the electrons and holes may recombine in the bulk and surface of the photocatalysts. Therefore, holes trapped experiment was carried out to demonstrate that capture agents
such as EtOH could be oxidized to the corresponding oxidation product. It is concluded that the amine functionalization and Ag/ AgCl introduction not only enhance absorption ability to visible light but also promote the separation efficiency of photo induced electrons and holes. Thereby, the photocatalytic reduction efficiency of Cr (VI) is significantly improved in comparison with the bare UiO-66.
4. Conclusions In summary, an plasmonic Ag/AgCl/NH2-UiO-66 hybridized photocatalyst is successfully prepared by solvothermal route with UV reduction method. The Ag/AgCl/NH2-UiO-66 hybrid system exhibits efficient visible-light response and enhanced photocatalytic Cr (VI) reduction compared with the bare Ag/AgCl/UiO-66, NH2-UiO-66, and UiO-66. The photoelectrochemical data indicate that compared with the UiO-66, the charge separation and transfer efficiency of Ag/AgCl/NH2-UiO-66 heterojunction is significantly enhanced due to the introduction of amine functionalization and formation of inorganic-organic hybrid. Moreover, the photocatalytic reduction efficiency of Cr (VI) is 1.7 times higher than that of the bare UiO-66. When the hot electrons are produced in Ag/AgCl due to SPR effect, they rapidly transfer to the NH2-UiO-66 by means of LMCT model, which efficiently suppresses the recombination of electrons with holes staying at the Ag/AgCl. This work shows that the Ag/AgCl/NH2-UiO-66 ternary hybridized system could provide a new path to construct efficient photocatalysts for immediate
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applications in wastewater treatment and energy harvesting by direct sunlight irradiation.
[15]
Acknowledgements This work was supported financially by the Key Project of Natural Science Foundation of Liaoning Province of China (No. 20170540578), Scientific Research Foundation of Education Department of Liaoning Province of China (No. LQ2019023) and Undergraduate Training Programs for Innovation and Entrepreneurship of Liaoning Province of China (No. 201710165000305). We also acknowledge the support of the National Natural Science Foundation of China (No. 21501138), the Natural Science Foundation of Hubei Province of China (2015CFB177). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.07.103.
[16]
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[18]
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