Applied Surface Science 502 (2020) 144187
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Constructing efficient polyimide(PI)/Ag aerogel photocatalyst by ethanol supercritical drying technique for hydrogen evolution
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Xinfu Zhaoa, Xibin Yia, , Xinqiang Wangb, Wei Chua, Sipeng Guoa, Jing Zhanga, Benxue Liua, Xiaochan Liua a Shandong Provincial Key Laboratory of Special Silicone-Containing Materials, Advanced Materials Institute, QiLu University of Technology (Shandong Academy of Sciences), Jinan 250014, PR China b State Key Laboratory of Crystal Materials and Institute of Crystal Materials, Shandong University, Jinan 250100, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Polyimide(PI)/Ag Aerogel Photocatalyst Ethanol supercritical drying H2 production
As one kind of promising photocatalyst, polyimide (PI) still suffers from poor light absorption, low charge transfer efficiency and small specific surface area. Herein, PI/Ag aerogel photocatalysts were designed by the solgel method and ethanol supercritical drying technique. As expected, Ag element in PI/Ag aerogel is in two forms, one is introduced into the imide ring of PI in the form of AgeC bond, and another is the dopant Ag atoms. The former kind of Ag element can weaken the planar hydrogen bonding and improve the charge transfer efficiency, and the latter can form LSPR effect and promote the photoelectron-holes separation. Apart from that, the high specific surface area of the aerogel photocatalyst also favors the photocatalysis. All these effects boost the photocatalytic activity of the PI/Ag aerogel photocatalysts. The average rate of H2 production is up to 166.1 μmol h−1 g−1 for PI/Ag-1 aerogel, which is ca. 8 times as much as that of PI aerogel. The work supplies an efficient approach for constructing effective aerogel photocatalysts for H2 production.
1. Introduction Hydrogen production by photocatalytic water splitting is one of the most promising strategies for solving environmental pollution and energy shortage [1,2]. To meet the needs of practical application, much work needs to be done to design efficient photocatalysts for H2 evolution from water under solar light. Actually, the energy conversion efficiency of most photocatalysts is still limited by the poor light absorption, low charge transfer efficiency and small specific surface area [3–5]. Conjugated polymer, such as graphitic carbon nitride (g-C3N4) and polyimide (PI), as one kind of promising photocatalyst has been explored due to their excellent light absorption capability and charge transfer property [6–8]. Recently, our group synthesized PI-based photocatalysts used for degradation of antibiotics [9,10]. However, PIbased photocatalysts still suffer from the inherent photo-carrier transfer property of polymer, and show low photocatalytic efficiency. PI structure contains many N and O elements, many hydrogen bonds may be formed to block electrons conduction across the plane [11,12]. Furthermore, more photoelectron-holes would be recombined due to the low conductivity of the polymer. Much work shows that carrier mobility and sunlight absorption of conjugated polymers can be improved by weakening the planar hydrogen bonding [13,14]. Many methods were ⁎
used to weaken the hydrogen bonding, such as element doping, coupling with semiconductors and noble metals [15,16]. Among them, Ag element was usually selected to couple with PI polymers due to its low electronegativity and suitable radius to partially replace N element in the PI framework [17]. However, it has little influence on the electronic property of PI by depositing noble metals on the surface of PI. Therefore, introducing Ag into the imide ring of PI is the key to weaken the hydrogen bonding. Ethanol supercritical drying technique is one of commonly used methods for preparing special texture and nanomaterials. The texture, fractal and crystal phase can be changed effectively by this method [18,19]. At the same time, the thermal stability and photocatalytic activity of the catalysts also can be enhanced after the drying of supercritical ethanol [20]. Thus, introducing of Ag into the structure of PI might be achieved by the ethanol supercritical drying technique. Specific surface area is also vital for enhancing the photocatalytic activity. Wang et al. proposed that the smaller specific surface area was the main case that inducing the smaller H2 production rate of PI than that of g-C3N4 [21]. Most PI photocatalysts were prepared by a facile calcination approach, and the specific surface areas are in the range of 4.3–16.8 m2/g [21]. Whereas large specific surface area is one of the advantages of aerogel photocatalyst. Apart from that, the porous
Corresponding author. E-mail address:
[email protected] (X. Yi).
https://doi.org/10.1016/j.apsusc.2019.144187 Received 15 July 2019; Received in revised form 20 September 2019; Accepted 25 September 2019 Available online 17 October 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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reference. The pore properties of samples were characterized by a nitrogen adsorption-desorption apparatus (Micromeritics, ASAP2460). Photoluminescence (PL) spectra were measured by a Cary Eclipse Fluorescence Spectrophotometer (Agilent Technologies) with a excited wavelength of 260 nm.
structure of aerogel can reduce the reflection of light on the catalyst surface, prolong the propagation time and increase the photon utilization ratio of the catalyst [22,23]. Thus, the absorption efficiency of visible light could be improved for the aerogel photocatalyst. Herein, for the first time, PI/Ag aerogel photocatalysts with different Ag content were prepared for H2 production by ethanol supercritical drying technique. The high temperature and high pressure can accelerate the introducing of Ag element into the imide framework of PI. The formed Ag-C bonds can weaken the hydrogen bonding, and further enhance the photocatalytic activity. The effect of Ag content and the form of Ag element in the samples for the photocatalytic H2 evolution were discussed in detail.
3. Results and discussion 3.1. Structure and morphology analysis In our work, 4,4′-oxybisbenzenamine (ODA) was used as one of the precursor, N,N-dimethylformamide (DMF) was used as solvent. Reddish brown suspension appeared when AgNO3 was added to the ODA solution. It can be inferred that a small part of ODA can mix together with AgNO3 to form Ag+-ODA structure, which can be attributed to the interaction between Ag+ in AgNO3 and -NH2 in ODA [24]. Then 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), acetic anhydride and pyridine were added to achieve chemical imidization reaction. After aging, the obtaind PI/Ag+ gel was dried by supercritical ethanol at ca. 536 K and ca. 9.0 MPa for 2 h. The drying and reduction of PI/ Ag+ gel both happened in the ethanol supercritical drying process due to the reducing power of ethanol. Thus, the yellow aerogel was obtained. PI, PI/Ag-0.5, PI/Ag-1, PI/Ag-3, PI/Ag-5 and PI/Ag-10 aerogel photocatalysts were synthesized by tuning the amount of AgNO3. Compared with some other methods [25], the synthesis process of PI/ Ag aerogel in our work is simple, and the form of Ag element is different due to the different reduction way of Ag+. The morphology of PI and PI/Ag-1 aerogel photocatalysts were investigated by SEM and TEM. Fig. 1a shows that PI aerogel has porous framework composed of tangled fibrous PI aggregates. For PI/Ag-1 aerogel, there are some PI crisp fragments and Ag nanoparticles, apart from a part of tangled fibrous aggregates (Fig. 1b). As reported in some literature [26,27], the formation of PI structure includes two stages of crystallization and phase separation. When phase separation precedes and crystallization follows, PI structure of tangled fibrous aggregates is obtained. When crystallization precedes and phase separation follows, PI structure of crisp fragments will be formed. For PI/Ag-1 aerogel, it can be inferred that due to the interaction between Ag+ and ODA, crystallization occurred first in DMF solution when BPDA was added by flocculation of PI chains. Then the PI-rich and DMF-rich phases separation occurred with the increase of PI molecular weight. Thus, there are some PI crisp fragments appeared for the PI/Ag-1 aerogel. The SEM image of PI/Ag-1(freeze-dried) aerogel (Fig. S1) shows that the sample is also composed of PI structure with crisp fragments. Notably, there are no Ag nanoparticles can be seen in the image of PI/Ag-1(freeze-dried) aerogel. From Fig. 1c, Ag nanoparticles can be seen uniformly distributed in the PI/Ag-1 structure. The HRTEM image in the left corner of Fig. 1c shows that the lattice distance of the nanoparticles is 2.31 Å, which can be assigned to the (1 1 1) plane of Ag. The mapping images of PI/Ag-1 aerogel in Fig. 1d exhibit that C, O, N and Ag elements well distributed in the PI/Ag-1 aerogel. The XRD patterns of PI, PI/Ag-0.5, PI/Ag-1, PI/Ag-3, PI/Ag-5 and PI/Ag-10 aerogels are shown in Fig. 2a. All the samples exhibit the characteristic peaks of PI from 10–40° [28,29]. But the patterns show that the peaks located at 19.3°, 21.5° and 24.0° have a slightly blue shift with the addition of Ag element, which may be caused by the introducing of Ag into the 3D framework of PI. Furthermore, the crystalline of the samples are decreased due to the introducing of Ag element. That may be caused by that the formed bond lengths between Ag, C, and N are different due to their distinct ionic radius [30]. The peaks intensity of Ag (JCPDS no. 87-0717) enhanced with increasing content of Ag. Notably, no Ag signal peaks can be seen in the XRD pattern of PI/ Ag-1(freeze-dried) aerogel (Fig. S2), indicating that no Ag element was reduced in the freeze drying process. Thus, it can be inferred that the Ag nanoparticles of PI/Ag-1 aerogel was reduced in the ethanol supercritical drying. Fig. 2b shows the adsorption-desorption isotherms of
2. Experiment 2.1. Materials 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), 4,4′-oxybisbenzenamine (ODA) and N,N-dimethylformamide (DMF) were purchased from Alfa-Aesar. Silver nitrate (AgNO3), pyridine, acetic anhydride, ethanol and HNO3 (68%) were purchased from Sinopharm Chemical Co. Ltd (China). All reagents were used without further purification. 2.2. Synthesis of PI/Ag aerogel photocatalysts The synthesis of PI/Ag-1 aerogel is described as follows. 0.79 g ODA was added to 12.5 ml DMF solution and ultrasonicated for 1 h, then 0.032 g AgNO3 was added into the mixture under stirring. Immediately, reddish brown suspension appeared. After stirring for 30 min, 1.20 g BPDA was added and dissolved into the solution. After that, acetic anhydride (3.10 ml, 130 mmol) and pyridine (2.60 ml, 130 mmol) were added to the mixture above quickly. The gel was formed in 20 min and aged in ethanol. Subsequently, the solvent was exchanged in 24 h intervals with ethanol for six times. Then PI/Ag-1 aerogel was dried by supercritical ethanol at ca. 536 K and ca. 9.0 MPa for 2 h with a heating rate of 10 °C/min. PI, PI/Ag-0.5, PI/Ag-3, PI/Ag-5 and PI/Ag-10 aerogel photocatalysts were prepared as the method above, except that the content of AgNO3 was changed to 0.0 g, 0.016 g, 0.094 g, 0.16 g and 0.32 g, respectively. For comparison, PI/Ag-1 (freeze-dried) aerogel was synthesized as the method above, except that the sample was freeze-dried for 72 h. 2.3. Photocatalytic test Typically, 40 mg sample was added into a Pyrex glass container with 40 ml mixed solution of water/methanol (3:1 by volume) with methanol as a hole scavenger at ambient temperature. A 500 W Xe arc lamp with a cutoff filter (λ ≥ 420 nm) was selected as the light source. The evolved H2 was analyzed by an online gas chromatograph (GC7920, Ar as a carrier gas). 2.4. Characterization The morphology of the samples was acquired on a scanning electron microscope (SEM, ZEISS SUPRA55) operated at 5 kV. TEM image was acquired on a high-resolution TEM (HRTEM, GEOL-2010) with an accelerating voltage of 200 kV. Powder X-ray diffraction (PXRD) patterns were collected on an X-ray diffractometer (Rigaku D/Max 2200 PC) with a graphite monochromator and Cu Kα radiation (λ = 0.15148 nm). The element compositions of the samples were tested by Vario EL III organic elemental analyzer (Germany Elmentar Company). The Fourier transform infrared (FTIR) spectra were measured by a Nicolet 5DX-FTIR spectrometer with a KBr pellet as the support. UV–vis spectrometer (Perkin-Elmer Lambda-35) was used to measure the UV–vis diffuse reflectance spectra with BaSO4 as the 2
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Fig. 1. SEM image of PI (a), and SEM image (b), TEM image (c), EDS mapping images (d) of PI/Ag-1 aerogel.
the samples. The specific surface areas of PI, PI/Ag-0.5, PI/Ag-1, PI/Ag3, PI/Ag-5 and PI/Ag-10 aerogels are 141, 121, 97, 88, 86 and 69 m2/g, respectively. The surface area decreases with the addition of Ag element. In principle, the large surface area of sample is favorable for the photocatalytic reaction. The characteristic structure and chemical banding in PI/Ag aerogel were analyzed by Fourier transform infrared spectroscopy (FTIR) spectra. It can be seen from Fig. 3 that all the characteristic absorption bands of PI can be observed. The bands located at 1680 and 750 cm−1 can be assigned to the stretching and bending vibrations of imide carbonyl groups [31]. Notably, the band intensity of PI/Ag-1 aerogel centered at 1680 cm−1 is much stronger than that of PI. That might be caused by the presence of C]O species on the surface of Ag during the reduction of Ag+ [32]. The several bands between 1250 and 1600 cm−1 can be assigned to the bands of aromatic carbon nitride heterocycles [33]. Especially, the band located at 1382 cm−1 can be attributed to the stretching vibration of CeNeC in the imide ring.
Fig. 3. FTIR spectra of PI and PI/Ag-1 aerogels.
Fig. 2. XRD patterns (a) and adsorption-desorption isotherms (b) of PI, PI/Ag-0.5, PI/Ag-1, PI/Ag-3, PI/Ag-5 and PI/Ag-10 aerogels. 3
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Table 1 Elemental content of PI/Ag-1, HNO3-treated PI/Ag-1, PI/Ag-1(freeze-dried) and PI aerogels. Sample
Sample Weight (mg)
Atomic content of C (wt %)
Atomic content of N (wt %)
Atomic content of O (wt %)
Atomic content of H (wt %)
Atomic content of Ag (wt %)a
PI/Ag-1 HNO3-treated PI/Ag-1 PI/Ag-1(freeze-dried) PI
2.402 2.510 2.433 2.411
73.552 73.683 71.464 72.130
4.922 4.930 7.579 7.650
17.824 17.863 17.325 17.486
2.785 2.791 2.707 2.732
0.917 0.733 0.924 0
a
The atomic content of Ag was calculated by subtracting the atomic content of C, N, O, H from the total amount.
nanosized clusters [39]. In addition, the intensity of peak increases with the increase of Ag content for the aerogels, suggesting that the conjugate system was extended due to the introducing of Ag into PI structure.
Whereas the bands intensity of PI/Ag-1 aerogel are weaker than that of PI at this region, indicating that the carbon nitride heterocycles are damaged by the substitution of N atoms with Ag. The bands around 3200 cm−1 is the amines and their intermolecular hydrogen bands [31]. The intensity of these bands for PI/Ag-1 aerogel become weaker and wider compared with that of PI, suggesting that the introducing of Ag element into PI can promote the decrease of amines and intermolecular hydrogen bonds, which can boost the photocatalytic activity, theoretically. To certify the chemical states of Ag element in the PI/Ag-1 aerogel, adequate HNO3 solution (0.1 ml, 68%) was used to react with PI/Ag-1 aerogel (2.01 g), thus Ag atom can be oxidized. Adequate deionized water was used to wash the reaction product. After that, the sample was dried in a vacuum drying oven and the elements content were measured by Vario EL III organic elemental analyzer. It can be seen from Table 1 that the content of Ag element in PI/Ag-1 aerogel decreased after being treated by adequate HNO3 solution. Thus, there are Ag nanoparticles present in PI/Ag-1 aerogel. Anecdotally, the ratios of C, O and H elements are near the same for PI/Ag-1 and PI/Ag-1(freeze-dried) aerogels, but the content of N element decreased much for PI/Ag-1 aerogel. It can be induced that part of N element in PI/Ag-1 aerogel may be replaced by Ag element, which is introduced into the framework of PI and cannot be oxidized by HNO3. Therefore, the interaction between PI and Ag in PI/Ag-1 aerogel is not only simple physical contact but also strong chemical bond. Notably, the ratio of C, O, N and H elements are near the same in PI/Ag-1(freeze-dried) aerogel and PI, indicating that the Ag element is not introduced into the structure of PI for PI/Ag1(freeze-dried) aerogel. From the work certified by others, it can be certified that introduction of Ag into the structure of PI will weaken the planar hydrogen bonding of PI [17]. In principle, the charge transfer and sunlight absorption of photocatalyst will be influenced due to the presence of Ag-C structure and plasmonics Ag species. The interaction between PI and Ag was further discussed by XPS and EPR. It can be seen from Fig. 4a that the C 1s spectrum of PI/Ag-1 aerogel can be divided into two peaks located at 284.7 and 288.2 eV, which could be ascribed to the adventitious carbon and sp2-hybridized carbon atoms, respectively [34,35]. Notably, the peak of PI/Ag-1 aerogel located at 288.2 eV has a red shift, indicating that the chemical state of C atoms in PI is influenced due to the coupling with Ag. Furthermore, the peak of N atoms located at around 400.3 eV can be corresponded to the N atoms in the imide of PI [36], but the peak of N atoms in PI/Ag-1 aerogel shifts to 400.2 eV, implying that some N atoms have a changed chemical state in PI/Ag-1 aerogel (Fig. 4b). Fig. 4c shows that the two bands of Ag 3d of PI/Ag-1 aerogel can be divided into two peaks located at 367.7, 368.3 and 373.8, 374.5 eV, respectively. The peaks located at around 368.3 and 374.5 eV can be attributed to the Ag0 species, whereas the peaks centered at 367.7 and 373.8 eV correspond to AgeC bonds [37,38]. The molar ratio of Ag+:Ag in PI/Ag-1 aerogel calculated by the area of peaks is 4.1:1, which is near the value (4.0:1) certified by the elemental analysis (Table 1). The EPR spectra were further measured to certify the interaction of PI and Ag at room temperature. Fig. 4d shows that the Lorentzian lines center at a g value of 2.003 in the magnetic field from 3300 to 3450 G. The sharp peak signal can be attributed to an unpaired electron on the carbon atoms of aromatic rings within π-bonded
3.2. Optical and electronic properties The UV–vis diffuse reflectance spectra of PI, PI/Ag-0.5, PI/Ag-1, PI/ Ag-3, PI/Ag-5 and PI/Ag-10 aerogels are shown in Fig. 5a. The absorption intensity of samples improves with the increase of Ag content at the visible light region, which is up to the peak when the Ag content reached to 1 wt%. That is because the photo-induced electrons can be captured by Ag nanoparticles, and the recombination of electrons and holes is reduced at first due to the presence of Ag [40]. Then the absorption intensity decreases after the content of Ag beyond 1 wt%. That can be assigned to that excess Ag particles covered on the surface of PI will decrease the light absorption of PI [41]. The band gaps of the samples were calculated according to the Kubelka-Munk equation, and the band gaps of PI, PI/Ag-0.5, PI/Ag-1, PI/Ag-3, PI/Ag-5 and PI/Ag-10 aerogels were 2.69, 2.41, 2.05, 2.02, 2.00 and 1.85 eV, respectively (Fig. S3). From the VBXPS spectra of the samples, it can be seen that the energy level of valence band (VB) lowers with the increase of Ag content from 1.01 eV for PI to 1.65 eV for PI/Ag-10 aerogel. Therefore, the conduct band (CB) and VB edge positions of PI/Ag are both lowered (Fig. 5b and c). That may be caused by that the photo-generated electrons of PI can be transferred into the Ag nanoparticles, and the holes are left in the PI. Thus, the recombination of photo-induced carriers are suppressed. Although the downshift of the CB potential for PI/Ag may cause the lower reduction ability of electrons, its narrowed band gap can enhance the visible light absorption and improve the photocatalytic activity. PL technique is effective in evaluating the separation efficiency of photo-carriers. The PL spectra (Fig. 5d) show that the intensity decreases with the increase of Ag content, indicating that the recombination of charge carriers are efficiently suppressed for PI/Ag-10 aerogel, which is beneficial for the photocatalysis [42]. Notably, the sharp peak signal shifts to the longer wavelength with the increase of Ag content. That may be caused by that the π-conjugation system is extended due to the introduction of Ag into the PI structure, and the mobility of the charge carriers is accelerated [43,44]. Consequently, the two forms of Ag element both play key roles in the decrease of photocarriers recombination. To evaluate the separation efficiency of electron-hole pairs, Electrochemical impedance spectra (EIS) measurements were also carried out. As shown in Fig. 6a, the circular diameters of the EIS Nyquist plots for PI-based aerogels decrease with the increase of Ag content. A smaller circular diameter means a lower charge transfer resistance. It can be ascribed to that the presence of Ag element has an active role on the charge transfer effect. Notably, the circular diameter of PI/Ag1(freeze-dried) aerogel is smaller than that of PI aerogel, but much larger than the other PI/Ag aerogel. The high charge transfer resistance of PI/Ag-1(freeze-dried) aerogel compared with that of PI/Ag-1 aerogel is caused by the lack of Ag nanoparticles. Furthermore, the lifetimes of photo-induced electron (τ) of samples were calculated according to the following equation from Bode Phase plots in Fig. 6b [45,46]. 4
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Fig. 4. XPS spectra of C 1s (a) and N 1s (b) of PI and PI/Ag-1 aerogels, Ag 3d spectra of PI/Ag-1 aerogel (c), and electron paramagnetic resonance (EPR) spectra of PI, PI/Ag-0.5, PI/Ag-1, PI/Ag-3, PI/Ag-5 and PI/Ag-10 aerogels (d).
Fig. 5. Diffuse reflectance spectra (a), VBXPS spectra (b), band edge positions scheme (c) and photoluminescence (PL) spectra (d) of PI, PI/Ag-0.5, PI/Ag-1, PI/Ag-3, PI/Ag-5 and PI/Ag-10 aerogels.
τ = 1/(2Πfmax )
aerogel photocatalyst indicates that the recombination rate of photoinduced electrons and holes is the lowest, and that is consist with the results of PL and EIS measurements. Thus, it can be induced that the Ag element in PI/Ag-1 aerogel can highly enhance the charge transfer kinetics of the aerogel.
(fmax is the maximum frequence) The electron lifetimes in PI, PI/Ag1(freeze-dried) and PI/Ag-1 aerogels are 1.08 * 10−4 s, 1.48 * 10−4 s and 2.25 * 10−4 s, respectively. The longer electron lifetime in PI/Ag-1 5
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Fig. 6. Nyquist plots (a) and Bode phase plots (b) of PI-based photocatalysts.
Fig. 7. Time course of H2 evolution on PI, PI/Ag-0.5, PI/Ag-1, PI/Ag-3, PI/Ag-5, PI/Ag-10 and PI/Ag-1(freeze-dried) aerogels (a), and cycle runs for the photocatalytic H2 production over PI/Ag-1 aerogel (b) under a 500 W Xe arc lamp.
Characterizations by a series of techniques certified that the specific surface areas of PI/Ag-1 (97 m2/g) and PI/Ag-1(freeze-dried) (98 m2/g) are near the same (Fig. S5a), the optical absorption intensity in the visible light region of PI/Ag-1 is much stronger than that of PI/Ag1(freeze-dried) (Fig. S5b), and the PL intensity of PI/Ag-1 aerogel is much weaker than that of PI/Ag-1(freeze-dried) (Fig. S5c). That can be inferred that the form of Ag element in PI/Ag-1 aerogel is more favor to the improvement of H2 production amount. A series of characterizations above revealed that Ag element in PI/Ag aerogel is in two forms. One kind of Ag element is introduced into the imide ring of PI in the form of AgeC bond, which can extend the conjugate system, weaken the planar hydrogen bonding and improve the charge transfer efficiency. Another is the dopant Ag nanoparticles. The photo-generated electrons of the excited PI under visible light irradiation can be transferred into Ag nanoparticles by overcoming the Schottky barrier due to the low Fermi level of Ag [47]. The charge transfer continues until equilibrium is built between PI and Ag nanoparticles. Then an electric field is built near the interface of PI and Ag nanoparticles, which can promote the separation of photo-generated charge carriers [48–50]. Furthermore, the average rates of H2 production were calculated, which were 20.8, 84.3 and 166.1 μmol h−1 g−1 for PI, PI/Ag-1(freeze-dried) and PI/Ag-1 aerogels, respectively, indicating that the Ag element introduced into the framework of PI and the Ag nanoparticles coupling with PI in PI/Ag-1 aerogel both play vital roles in improving the photocatalytic activity. The average rate of H2 production for PI/Ag-1 aerogel is ca. 8 times as much as that of PI aerogel, though the value (166.1 μmol h−1 g−1) is not high enough when compared with some other semiconductors [51–55]. The stability of a photocatalyst is vital for its potential applications. Hence, to survey the stability of the PI/Ag1 aerogel, the photocatalytic performance of PI/Ag-1 aerogel was investigated. In the cycling experiments, the plots (Fig. 7b) show that the H2 production amount decreases little, indicating that the aerogel photocatalyst is stable. The SEM images (Fig. S6) of PI/Ag-1 aerogel before and after the cycling experiments are near the same, which also
3.3. Photocatalytic property Photocatalytic activities of PI, PI/Ag-0.5, PI/Ag-1, PI/Ag-3, PI/Ag-5, PI/Ag-10 and PI/Ag-1 (freeze-dried) aerogels for H2 production were measured by using methanol as a hole sacrificial reagent. As shown in Fig. 7a, the photocatalytic activity of H2 evolution over the samples is in the order: PI/Ag-1 ˃ PI/Ag-3 ˃ PI/Ag-5 ˃ PI/Ag-0.5 ˃ PI/Ag-10 ˃ PI aerogels. The H2 evolution rates over PI/Ag aerogels all higher than that of PI aerogel, indicating the presence of Ag plays a positive role in the photocatalytic activity. Take PI/Ag-1 as example, different bandpass filters were used to certify the wavelength dependence of the activity. As shown in Fig. S4, the H2 evolution rate over PI/Ag-1 aerogel matches well with its optical absorption spectrum, indicating the photocatalytic activity is mainly influenced by the optical absorption of the photocatalyst [44]. The amount of Ag on PI shows a pronounced optimum effect on the H2 evolution activity for PI/Ag aerogels. The H2 production amount increases with the addition of Ag content and reaches to the strongest value of 996 μmoL g−1 for PI/Ag-1 aerogel at 6 h. Then the H2 production amount decreases with the further increase of Ag content. The trend of H2 evolution rates over the PI/Ag aerogels is consist with the change of optical absorption intensity of the samples (Fig. 5a). Thus, the photocatalytic activity over the PI/Ag aerogels may be driven by that appropriate amount of Ag nanoparticles can reduce the recombination of electron-holes, whereas excess Ag particles covered on the surface of PI will decrease the light absorption of PI, thus the H2 evolution rate decreases. Notably, the H2 production amount of PI/Ag-1(freeze-dried) aerogel is little higher than that of PI, though the specific surface area of PI/Ag-1(freeze-dried) aerogel is smaller (Fig. S5a). That may be caused by that the Ag+ in the PI/Ag-1(freeze-dried) aerogel also can enhance the absorption intensity of visible light and reduce the recombination of photo-carriers, which can be certified by the diffuse reflectance spectra (Fig. S5b) and the PL spectra (Fig. S5c). Furthermore, the photocatalytic H2 production amount of PI/Ag-1 aerogel is much higher that that of PI/Ag-1(freeze-dried) aerogel. 6
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Fig. 8. Proposed photocatalytic mechanism for H2 production over PI/Ag-1 aerogel photocatalyst.
exhibit the stability of the photocatalyst. Based on the results above, a possible mechanism for photocatalytic H2 production is proposed (Fig. 8). Firstly, the PI/Ag-1 aerogel generates many electron-holes due to the narrowed band gap and the LSPR of Ag under visible light irradiation. Secondly, the photo-induced electrons can be rapidly captured by the dopant Ag nanoparticles. For one thing, Ag nanoparticles can collect electrons around them due to the strong electron effects of LSPR excitation. For another, the introducing of Ag into the PI structure can enhance the carrier mobility efficiency. Thirdly, the photo-generated holes left at the VB of PI can oxide methanol into carbon dioxide and water. Thus, the photo-carriers are effectively separated and more electrons can be used to reduce H2O into H2, the photocatalytic activity is boosted. 4. Conclusion In summary, a new kind of PI/Ag aerogel was prepared by chemical imidization and ethanol supercritical drying technique, which can accelerate the introduction of Ag into the PI structure. The Ag element in PI/Ag aerogel is in two forms, one is introduced into the framework of PI, and another is dopant Ag nanoparticles. The photocatalytic activity for H2 production over PI/Ag aerogel is improved much due to the enhanced carrier mobility efficiency, optical absorption and large specific area of the photocatalyst. This work provides a new way to design PI-based photocatalysts. Declaration of Competing Interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled. Acknowledgements This work is supported by the China Postdoctoral Science Foundation (2019M652452), Natural Science Foundation of Shandong Province (Nos. ZR2019BEM015, ZR2017BEM009, ZR2019MEM034, ZR2019PB029), National Natural Science Foundation of China (21603125), Youth Science Funds of Shandong Academy of Sciences 7
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