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Article Cite This: ACS Omega 2019, 4, 1636−1644
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Biomimetic Polyimide-Supported Cuprous Oxide Photocatalytic Film with Tunable Hydrophobicity, Improved Thermal Stability, and Photocatalytic Activity toward CO2 Reduction I-Hsiang Tseng,*,† Li-Huan Kang,† Po-Ya Chang,† Mei-Hui Tsai,‡ Jui-Ming Yeh,§ and Ta-I Yang∥ †
Department of Chemical Engineering, Feng Chia University, Taichung 40724, Taiwan Department of Chemical and Materials Engineering, National Chin-Yi University of Technology, Taichung 41170, Taiwan § Department of Chemistry and ∥Department of Chemical Engineering, Chung-Yuan Christian University, Chungli 32023, Taiwan Downloaded via 5.188.216.67 on January 18, 2019 at 21:32:49 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: Flexible and thermally stable polyimide (PI) films containing a hierarchical surface structure were synthesized as substrates to support visiblelight active cuprous oxide for photocatalytic reduction of carbon dioxide for the first time. With the nanocasting technique, the surface structure on the leaves of Xanthosoma sagittifolium was successfully duplicated on PI films. Followed by the ion-exchange process and adequate thermal treatment, cuprous oxide nanoparticles were successfully immobilized on the artificial PI leaves and exhibited the capability to photoreduce carbon dioxide into carbon monoxide under visible-light illumination. With the selection of biomimetic structures and adjustment of fabrication parameters, the hydrophobicity and optical absorption edge of the photocatalytic film were tunable. An increase in hydrophobicity improved the yield of carbon monoxide. The introduction of a hierarchical structure on the surface and cuprous oxide within the matrix dramatically enhanced the thermal stability of the PI film. The flexible photocatalytic film is a promising material for the applications requiring high mechanical and thermal stability, such as industrial flue-gas treatments.
1. INTRODUCTION The inspiration from the nature continues to direct the development of novel materials and advanced systems for modern applications. Biomimetic designs motivated by delicate and precise structures or functions of natural species are considered to more efficiently solve complicated engineering issues as those designs have been optimized for survival.1−3 Artificial photosynthesis, which imitates the natural process of plants to convert light energy into chemical energy, is considered to be an energy-efficient strategy to reduce the concentration of atmospheric carbon dioxide.4−6 Novel photocatalysts or reaction systems have been developed for the higher efficiency of CO2 conversion.7−20 Among those unique designs, a few biomimetic or bioinspired strategies attracted our attention. For example, Zhou et al. designed a leaf-inspired photocatalyst by connecting a light-harvesting antenna, a molecular water oxidation center, and a CO2 adsorption/reduction center into nanolayered architectures for artificial photosynthesis.8 The improved yields of CO and H2 were attributed to the efficient charge transfer (CT) and increased CO2 adsorption on the artificial photosynthetic units. Kojima et al. fabricated a Ni complex as a photocatalyst, which is inspired by the natural CO2-fixation enzyme, CODH, for CO2 reduction.7 With the assistance of a photosensitizer and a sacrificial electron donor, their photocatalytic system showed high selectivity to produce CO under visible-light irradiation. © 2019 American Chemical Society
In addition to CO2 conversion, several biomimetic designs of photocatalysts were applied for photodegradation. Fan and Zhang et al. fabricated a biomimetic N-doped ZnO photocatalyst by the replication of macro- to nanostructures and functions of natural leaves.21 The artificial photocatalyst system showed improved capability to decompose methylene blue. Zhang et al. utilized the fluff of the chinar tree as a template to fabricate TiO2 photocatalysts with hierarchical structures and also demonstrated improved photoactivity for degrading rhodamine B (RhB).22 For most of the cases, high surface area and efficient CT were the main reasons and advantages to apply biomimetic structures on photocatalysts. In addition to the increase in surface area, the hierarchical structures of biomimetic materials provide the hydrophobicity of the surface.23−26 Lee and Yong combined lotus effect with solar water splitting to fabricate stable superhydrophobic surfaces under water.25 Recently, Yeh et al. fabricated electroactive polyimide (PI) coatings with natural leaf or petal-like surface structures for anticorrosive applications.27,28 Zhang et al. fabricated PI nanotube arrays by using the molding method. Also, the hydrophobicity of the PI surface can be tuned by altering the drying process.29 Xu et al. Received: November 20, 2018 Accepted: January 8, 2019 Published: January 18, 2019 1636
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papillose cells and prominent cuticular folds of X. sagittifolium leaves were observed from the scanning electron microscopy (SEM) images of tPI films shown in Figure 1a,b. The structure
fabricated an artificial cilia photocatalyst by growing TiO2 on a cobalt-assembled polydimethylsiloxane (PDMS) array.30 The magnet field-driven ciliary motion of the photocatalytic structure promoted the efficiency of mass transfer and UVlight absorption that assisted the photodegradation of RhB. Aromatic PIs exhibit superior mechanical and thermal properties that they have been applied in advanced microelectronics as substrates or interlayers, which must possess superior excellent stability under severe processing conditions. In addition, the easy formability of the film makes PI a promising material as flexible coatings or substrates in many circumstances. Recently, the introduction of a PI structure within photocatalysts has also been noticed to promote the absorption of visible lights.31−34 For photocatalytic reduction of CO2, the electrons excited from the valence band of a photocatalyst must transfer to nearby reactants before recombination with excited holes. Simultaneously, if the excited holes successfully transfer to another reactant, then the efficiency of the conversion will be improved. In most cases, H2O serves as the hydrogen source and the holes could react with H2O to complete the CO2 reduction. That is, the presence of H2O is essential; however, the competitive adsorption of two reactants, CO2 and H2O, on catalysts affects the transfer of excitons or the reaction pathway.11,35−37 The hydrophobicity of catalysts will influence their photoactivity. On the other hand, the immobilization of photocatalysts on substrates in more practical for easier recovery and reuse. Therefore, we fabricated a robust and flexible PI-based photocatalyst substrate to support the active species. Inspired by the lotus effect, we intentionally produced surface textures on PI-based photocatalytic films to tune their surface hydrophobicity. We have discovered that the bottom side of leaves of Xanthosoma sagittifolium plant is as waterrepellent as lotus leaves and their hierarchical structure can be more easily duplicated on the flexible PI films than lotus leaves. By tuning the fabrication parameters, we also successfully grafted cuprous oxide (Cu2O) nanoparticles on the biomimetic surface without wrecking the hierarchical surface structure. To the best of our knowledge, it is the first attempt to synthesize flexible PI films with a biomimetic surface structure to serve as the supports of the photocatalyst. A modified nanocasting technique28 was developed to duplicate the surface structure of X. sagittifolium on the PI film, followed by the ion-exchange technique38−42 to immobilize copper ions on polyamic acid (PAA) films. Finally, an appropriate thermal treatment converts copper ions into visible-light active Cu2O nanoparticles on PI. The parameters of both grafting and heattreatment processes were tuned to obtain photoactive Cu2O on PI and simultaneously maintain the mechanical strength and thermal stability of PI films. The “artificial PI leaves” exhibited photoactivity to convert gaseous CO2 under visiblelight illumination, which biomimicked the artificial photosynthesis process by natural leaves.
Figure 1. (a) Top-view and (b) cross-sectional SEM images of tPI films. (c) Top-view and (d) cross-sectional SEM images of Cu/tPI-10 films. (e) Top-view SEM image of Cu/itPI-10 films. EDS elemental mapping images: (e′) C, (e″) O, and (e‴) Cu-mapping results from the indicated parallelogram shown in (e). (c′) Cu-mapping result from the indicated square shown in (c). (f) Cross-sectional and (f′) top-view SEM images of Cu/tPI-30 films. Cross-sectional SEM images of (g) Cu/itPI-30 and (h) Cu/PI-10. Magnified SEM image from the papillae structure on (i) Cu/tPI-10, (j) Cu/tPI-30, (k) Cu/ itPI-10, and (l) Cu/itPI-30 films.
was consistent with the previous work and literature data.43,44 These images confirmed successful reproduction of the hierarchical structure on PI films by the PDMS nanocasting technique. The Cu/tPI-x series combined the imide ring opening and ion-exchange process. The KOH treatment on the PI film will lead to the imide ring cleavage producing carboxylate groups. We controlled this surface hydrolysis period to modify the top layers of films without scarifying its mechanical strength. The following immersion with the precursor solution of copper will conduct the loading of copper ions with the carboxylate anions through the ion-exchange process.38−42,45 Finally, the thermal treatment of the surface-modified film will reconstruct the imide ring through the imidization process and simultaneously oxidize the copper ions into Cu2O. The immersion period of CuAc-1, concentration and temperature of KOH(aq), and thermal treatment conditions were optimized to fabricate the Cu/tPI-x samples with detectable photocatalytic activity and adequate mechanical strength. The surface morphology and fracture surface of Cu/tPI-10 are presented in Figure 1c,d. The number and height of papillae on Cu/tPI-x were smaller than those on tPI as the KOH immersion hydrolysis process slightly
2. RESULTS AND DISCUSSION 2.1. Characteristics of Biomimetic PI Films. In this work, the huge elephant-ear-like leaves of X. sagittifolium obtained from famous Sun Moon Lake area were selected as the templates to duplicate their hierarchical structure on PI films. The introduction of a biomimetic surface texture not only enhances the capability of light trapping and reactant adsorption but provides the possibility to tune the hydrophobicity of the photocatalytic film. The characteristic 1637
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Figure 2. XPS depth profiling: the atomic concentration of C, N, O, and Cu within (a) Cu/tPI-10 and (b) Cu/itPI-10 films as a function of sputtering depth. XPS Cu 2p spectra of (c) Cu/tPI-10 and (d) Cu/itPI-10 films at various depths. (e) Deconvoluted XPS Cu 2p3/2 curves of the Cu/tPI-10 film at 27 nm depth. (f) XPS O 1s spectra of Cu/tPI-10 at various depths.
damaged the surface texture of PI films. Similarly, the cuticular folds, as shown in Figure 1d,e, were less prominent on Cu/tPIx, indicating a relatively smoother surface after imide ring cleavage and copper grafting. As shown in the cross-sectional images of both Cu/tPI-10 and Cu/itPI-10, Figure 1d,e, the dense PI matrix below the modified surface layer suggested that the fabrication parameters were well-tuned that the mechanical strength could be remained. Notably, the porous PI structure was observed from the previous work where films underwent prolonged immersion in KOH with high concentration, and consequently, those films exhibited poor formability. Another procedure inspired by Chang and Zeng46 was utilized to graft Cu species on the PAA dry film, followed by imidization to eliminate the imide ring cleavage process. The morphology of the obtained Cu/itPI-x samples was similar to that of Cu/tPI-x. However, the distribution of Cu species on the films derived from two methods was divergent. The surface morphology of Cu/itPI-10 was demonstrated in Figure 1e, and the energy-dispersive X-ray spectroscopy (EDS) mapping results of C, O, and Cu elements are illustrated in Figure 1e′−e‴. Without the imide ring opening by the alkaline
solution, the cuticular folds on Cu/itPI-10 were more obvious than that on Cu/tPI-10. The Cu-mapping images shown in Figure 1c′,e‴ indicated homogeneous distribution of Cu species on both films; however, the amounts of Cu species on Cu/tPI-30 were higher than that on Cu/itPI-10. The EDS results (Figure S1) revealed that the atomic percentage of Cu was 0.08% from Cu/itPI-10 and 1.34% from Cu/tPI-10, respectively, and 0.09% from Cu/itPI-30 and 2.02% from Cu/ tPI-30, respectively. The copper content from Cu/tPI-x series increased with the immersion period in Cu precursor solution. The cross-sectional images of Cu/tPI-30 and Cu/itPI-30 are exhibited in Figure 1f,g. The papillary structures were extremely covered by the Cu species with a prolonged period of Cu-ion exchange. The decrease in the surface roughness was also evidenced by the top-view images of tPI, Cu/tPI-10, and Cu/tPI-30 shown in Figure 1a,c,f′. For comparison, the crosssectional image of a reference sample (Cu/PI-10), which was prepared under the same procedure of Cu/tPI-10 but using plain PI films, is shown in Figure 1h. The modified layer containing Cu species on Cu/tPI-10 was less than 100 nm, which is similar to that on Cu/tPI-10 evidenced by the following X-ray photoelectron spectroscopy (XPS) results. The 1638
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Figure 3. XPS C 1s spectra of (a) Cu/tPI-10 and (b) Cu/itPI-10 films at various depths. The deconvoluted XPS C 1s curves of the (c) Cu/itPI-10 film at 27 nm depth. (d) FTIR spectra of PI, tPI, Cu/itPI-10, and Cu/tPI-10 films.
copper species anchored on PI films will later be confirmed as Cu2O nanoparticles. The magnified SEM images from the top of papillae structures on each film are displayed in Figure 1i−l. The dimensions of Cu2O grains on Cu/tPI-10 and Cu/tPI-30 were around 20−35 nm. The distribution of particles size was more uniform on Cu/tPI-10 than on Cu/tPI-30. As the lower content of Cu2O, it is hard to distinguish Cu2O particles on Cu/itPI-10. The surface roughness of the Cu/itPI-30 film was relatively high compared to others, and some Cu2O nanoparticles with the dimension of ca. 30 nm were distributed on Cu/itPI-30. Notably, the creation of those hierarchical structures on PI did not result in higher surface area measured by N2 adsorption. The Brunauer−Emmett−Teller (BET) surface area of the tPI film was less than 2 m2/g, which is comparable to that of plain PI film. The following XPS depth-profiling analysis was utilized to reveal the distribution of Cu in the thickness direction of films. Figure 2a,b plots the atomic concentration of C, N, O, and Cu within Cu/tPI-10 and Cu/itPI-10 films, respectively, as a function of sputtering depth. Most Cu species were distributed within the depth of 100 nm surface of Cu/tPI-x as the imide ring-opening reaction was tuned to occur within the topmost surface. The atomic concentration of Cu on the top surface of Cu/tPI-10 was as high as 40% and dramatically dropped to less than 3% at 200 nm depth. On the other hand, the concentrated Cu top layer was not observed from Cu/itPI-10. A relatively homogeneous distribution of Cu species was detected throughout the depth of Cu/itPI-10 as the carboxyl groups (−COOH) in PAA films can easily interact with Cu2+ ions through the electrostatic interaction. Therefore, the difference in atomic concentration of Cu within Cu/itPI-10 was negligible, and an average Cu concentration of 0.3 mol % was revealed from Figure 2b. The XPS Cu 2p spectra of two films at different depths are plotted in Figure 2c,d. The characteristic peaks centered at
932.3 and 952.2 eV were observed from Cu/tPI-10 at each depth suggested that Cu2O was the major Cu species on top layers of Cu/tPI-10.47,48 The characteristic Cu2O signals were also revealed from Cu/itPI-10, although relatively low intensity of peaks is shown in Figure 2d, which is consistent with depthprofiling results. The weak shake-up satellite structure from the Cu 2p spectra of Cu/tPI-10 at the depth less than 60 nm could be observed, indicating the presence of Cu2+ chemical states, such as CuO and Cu(OH)2 on the topmost layers of Cu/tPI10. The deconvoluted Cu 2p3/2 peaks of Cu/tPI-10 at 27 nm depth are shown in Figure 2e as an example to illustrate the composition of Cu species within Cu/tPI-10. The peaks located at 932.3 and 933.9 eV corresponded to the characteristic Cu+ and Cu2+ species, respectively. Moreover, from XPS O 1s spectra plotted in Figure 2f, the signals located from low to high binding energy suggested the presence of CuO, Cu2O, Cu(OH)2, or CO (from the PI matrix), and C−OH species on Cu/tPI-10 from top surface down to 138 nm depth.42,48,49 The XPS results indicated the successful tuning on the conditions of post-thermal treatment of Cu/tPIx films that Cu2O species are present on biomimetic PI films. Notably, the oxidation of the topmost surface was observed from Cu/tPI-10 that may lead to the delayed photoactivity as shown in the following results. The XPS C 1s spectra of Cu/tPI-10 and Cu/itPI-10 are plotted in Figure 3. The difference in the C 1s spectra between each layer of the Cu/itPI-10 film was negligible as shown in Figure 3b. The results further confirmed the well dispersion of Cu species within the PI matrix. The deconvoluted peaks shown in Figure 3c suggested the presence of C−C/CO, C−O/C−N, and CO signals at 284.5, 285.5, and 286.9 eV, respectively, which were correlated to the PI structures.50,51 On the other hand, the XPS C 1s spectra from Cu/tPI-30 were changed in intensity with the depth as shown in Figure 3a. As the Cu species were concentrated on the topmost layers of 1639
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was more homogeneous, such that the standard deviation of CA was smaller than that of Cu/tPI-30. In summary, the increased CA of the PI film was mainly achieved by the existence of hierarchical structures on the surface. If a designated leaf template was selected to modify the PI surface, the presence of Cu2O on the surface did not significantly affect CA. Notably, we applied another leaf template to produce a biomimetic surface with a higher hydrophilic feature, and the resultant film Cu/tPI_30* exhibited a CA of 65° in order to compare the activity toward CO2 conversion. The UV−vis spectra of a series of PI films are depicted in Figure 5. The absorption edge of tPI was around 543 nm,
Cu/tPI-30, relatively weak C 1s signals were obtained from surface layers. Similar C 1s spectra from the layers deeper than 100 nm were then revealed. The shapes of C 1s spectra from both Cu/tPI-10 and Cu/itPI-30 were identical, indicating undistinguishable PI structures between Cu/tPI-10 and Cu/ itPI-10 at specific surface layers. Notably, the intensity of C O signal from those PI matrices was relatively weaker than that from pristine PI, indicating the breakage of CO bonds during alkaline ring-opening and/or ion-exchange processes.51 The XPS results reflect the surface property; meanwhile, the Fourier transform infrared (FTIR) spectrum of each film confirms the characteristic functional groups for the bulk PI structure. As shown in Figure 3d, the shifting in the wavenumbers of representative signals was insignificant from reference and biomimetic PI films. However, the absorbance from tPI was obviously smaller than the reference PI because of the presence of a hierarchical structure. The effect of Cu2O grafting on the functional groups, or the imidization degree, of PI was negligible. Therefore, the characteristic absorption bands of symmetric and asymmetric CO stretching of the imide ring from all films were located at 1710 and 1773 cm−1, respectively. The signals located at 715 and 1364 cm−1 were attributed to C−N−C stretching and bending on the imide ring, respectively.52−56 The appearance of these functional groups on the imide ring and the disappearance of characteristic peaks (N−H and O−H) of PAA at 1503 and 3100−3500 cm−1 indicated the nearly complete imidization of the PI network on all biomimetic PI films under the current thermal curing condition. The hydrophobicity of each film was evaluated by probing the contact angle (CA) of water droplets on a series of PI films. The photograph and the average CA of each film are displayed in Figure 4. With the presence of a biomimetic structure, the
Figure 5. UV−vis spectra of PI, tPI, Cu/PI-30, Cu/tPI-10, Cu/itPI10, and Cu/itPI-30 films.
which is way longer than that of a reference PI film (473 nm). Accordingly, the band gap of PI was around 2.6 eV for the reference PI and 2.3 eV for tPI, respectively, which are consistent with reported values.32,33,57 The imidization degree of PI, the formation of heterojunction with PI, and even UV irradiation on PI affected the band gap.58 The hierarchical structures on PI may lead to the restraint of the PI matrix and consequently change the absorption edge.59 In addition, the biomimetic structure could promote its absorption in visible lights by reducing the reflection of lights.31 On the other hand, the presence of Cu2O on the surface layer can further extend the absorption edge to 550 nm. Even the reference Cu/PI-30 showed an absorption edge of 494 nm compared to 473 nm (reference PI). A series of tPI films containing Cu2O exhibited band gaps ranging from 2.1 to 2.3 eV, corresponding to the absorption edge from 576 to 535 nm. On the basis of the previous work, the indirect band gap of Cu2O crystals ranged from 1.95 to 2.00 eV depending on the shape and size of Cu2O.48 For Cu/tPI-10, the absorption edge at 535 nm was primarily contributed by Cu2O on the surface. Moreover, the presence of Cu2O within the PI matrix may lead to significant intra-/intermolecular CT between tPI backbones.60 The introduction of Cu2O particles in the top layers of PI films may promote the formation of CT centers. Consequently, the color of PI films with biomimetic structures or contained Cu2O was darker than that of the reference flat PI film as shown in the abstract graphic. That is, the tailing in the visible-light region may not only result from the CT effect between the PI networks but from the hybrid of Cu2O particles. The decomposition temperature (Td), which occurred at 5 wt % weight loss, was determined from thermogravimetric analysis (TGA) curves to evaluate the thermal stability of PI films. As shown in Figure 6, the Td value was 483 °C for the reference PI film and increased to 556 °C for tPI. The dramatic
Figure 4. Photographs of water droplets and their average CAs on (a) PI, (b) Cu/PI-30, (c) tPI, (d) Cu/itPI-10, (e) Cu/itPI-30, and (f) Cu/tPI-30 films.
CA significantly increased from 60° for a reference PI film to 122° for the tPI film. The reference Cu/PI-30 film exhibited a CA of 90°, indicating that the growth of Cu2O on the reference PI also increased the CA values. The average CA value was 120° and 125° for Cu/tPI-30 and Cu/itPI-30, respectively. According to XPS results, the chemical structures of carbon species on Cu/tPI-x and Cu/itPI-x were identical, such that the comparable hydrophobicity was obtained from both films. Notably, the distribution of Cu2O on top layers of biomimetic films (tPI), due to two grafting processes, was irrelevant to the change trend in CA values. However, the surface of Cu/itPI-30 1640
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Figure 6. TGA curves of PI, tPI, Cu/tPI-30, and Cu/itPI-30 films.
improvement in the Td value was also observed from both Cu/ itPI-30 and Cu/tPI-30, which are 523 and 542 °C, respectively. The presence of biomimetic structures on PI has been confirmed to improve its thermal stability because of enhanced hydrophobicity.61,62 The strong interaction between Cu2O and the PI matrix not only hindered the air from diffusion through the film but exhibited excellent thermal resistance.63 2.2. Photocatalytic Activity of Biomimetic PI Films. The photocatalytic activity of a series of PI films was evaluated by analyzing the gaseous products yielded from a batch CO2 photocatalytic reduction system. In this work, CO was the main gaseous product detected from the photocatalytic reduction of CO2 by the Cu2O-contained PI films. Other possible hydrocarbons, such as HCOOH, HCHO, CH3OH, and CH4, were not detected during photoreduction. On the other hand, an increase in the intensity of O2 signal in gas chromatography (GC) chromatogram was observed, confirming an accompanied photooxidation with photoreduction. However, the quantity of O2 cannot be correctly determined with irradiation time because of the parts per billion-level resolution of the pulsed-discharge helium ionization detector (PDHID) for hydrocarbons. The cut tops of N2 and O2 peaks cannot be avoided from the injection volume of 1 mL. Therefore, we only monitored the difference between the initial and final concentration of O2 as the increased amount cannot be precisely determined from each data point. Figure 7a plots the time course of CO yield from each sample under light-emitting diode (LED) lamp illumination. The absent of CO production from the system containing only CO2 but not the photocatalytic film confirmed the acceptable purity of CO2 feedstock. Moreover, the photodegradation of PI-based films under visible-light illumination could be ignored as trace CO was detected and leveled off after 10 h illumination. The CO yields from a series of PI films under the same Cu-precursor immersion time of 30 min were compared because they exhibited higher conversion rates than other films under other immersion times (10 or 60 min). After at least 4 h visible-light illumination, a reference film, Cu/PI-30, started to yield CO with an average rate of 0.5 nmol/h. With the presence of a biomimetic structure, Cu/itPI-30 exhibited doubled reaction rate and reduced the activation period (2 h). The shorter activation period was also observed from Cu/tPI-30; however, its stability of production was relatively worse than Cu/itPI-30. Without the presence of Cu2O on the surface, the blank tPI film could not convert CO2 into CO under the same reaction parameters. According to our previous work, the photoactivation period was also revealed from both dCu2O_gCN and sCu2O_gCN.48 After 12 h LED lamp illumination, the
Figure 7. (a) Time course of CO yield from each sample under LED lamp illumination. (b) Relationship between 12 h CO yield and CA of films with a fixed surface area of 5 cm2.
yield of CO via Cu/itPI-30 reached 10 nmol (or 100 nmol per gram of film), which is more efficient than the previous one using crystal Cu2O on gCN nanosheets under the same weight.48 Figure 7b demonstrates the relationship between average CO yields from 12 h illuminated films and their CAs. The Cu/ tPI_30* film with the smallest CA of 65° showed the lowest CO yield, followed by Cu/PI_30 with a CA of 90°. The production of CO was initially promoted by the increasing CA, or the hydrophobicity, of PI films. As the CA reached to 120°, the improved CO yield could be attributed to other factors. The rank of photoactivity from those films with a similar hydrophobicity (CA > 120°), from low to high, followed Cu/ tPI-30, Cu/itPI-10, and Cu/itPI-30. According to EDS results, similar Cu percentage of 0.10% from both Cu/itPI-10 and Cu/ itPI-30 and 2.01% from Cu/tPI-30 was revealed. XPS results also indicated more concentrated Cu2O surfaces on Cu/tPI series. However, the high concentration of Cu2O on films was irrelevant to high conversion of CO2. Recalling the UV−vis spectrum of each film, the red shift of the absorption edge implied higher capability to absorb visible lights, and consequently, the photocatalytic activity of films followed the trend of absorption edge. The higher capability to absorb visible lights combined with higher roughness is attributed to the higher activity of Cu/itPI-30.
3. CONCLUSIONS In this work, biomimetic PI films were successfully fabricated and incorporated with Cu2O nanoparticles to conduct photocatalytic conversion of CO2. Under very weak visiblelight illumination, gaseous CO2 was converted to CO at ambient condition. The presence of a hierarchical structure on the surface enhances hydrophobicity, thermal stability, and 1641
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4.3. Characterization. The infrared spectra of sample films were recorded by using a FTIR spectrophotometer (Nicolet Protégé-460) to identify the functional groups of PI films. The X-ray diffractometer (Bruker D8 SSS) using Cu Kα radiation (λ = 0.154178 nm) at 40 kV and 35 mA was employed to identify the crystallinity of samples. The UV−vis diffuse reflectance spectra of samples were acquired on a spectrophotometer (Jasco V-650) equipped with an integrating sphere (ISV-722). The morphology of composites was observed via a scanning electron microscope (JEOL JSM-7401F or JSM7100F) integrated with an energy-dispersive X-ray spectrometer (Oxford X-Max). The chemical composition of each film was investigated by an X-ray photoelectron spectroscope (ULVAC-PHI PHI5000 VersaProbe) using Al Kα radiation. The depth profiling was performed using Ar ions with an etching rate of 13.71 nm/min. The CA of each film presented in this study was an average of the CAs from five droplets at different locations of each film. The CA of water droplets on films was measured using a goniometer (DSA10-MK2) equipped with a camera to capture images of water droplets on the films at room temperature. TGA was carried out with a TGA-Q500 from TA Instruments at a heating rate of 20 °C/ min under nitrogen. A surface area and porosimetry analyzer (Micromeritics, ASAP2020) was utilized to determine the BET surface area of samples. 4.4. Measurement of Photocatalytic Activity. The photocatalytic reduction of CO2 was conducted in a batch reactor (100 mL) at ambient temperature. The film sample with a fixed dimension of 1 cm × 5 cm was placed in the center of the reactor with a LED bulb (Philips F6500, 14 W, 14 400 lux) illuminated through a quartz window from the top. The emission spectrum of this LED light bulb was recorded to confirm a visible-light-only irradiation. A small Petri dish containing about 2 mL of fresh water was placed next to the film inside the chamber. After purging the reactor with inert N2 for 30 min, the inlet gas was switched to high-purity CO2 (99.999%) for another 30 min. The outlet and inlet of the reactor were then closed and ready for the illumination of visible light on the sample film. A gas chromatograph (YL Instruments, YL6500) equipped with a highly sensitive PDHID was employed to analyze the products of photoreduction. A gas-tight syringe was applied to periodically withdraw 1 mL of gas sample from the reactor during the reaction. The carrier gas of GC was He, and two separation columns from Ohio Valley Specialty, including Porapak N for the detection of H2, O2, CO, and CH4, and Molecular Sieve 5A for CO2, H2O, and other C1−C2 hydrocarbons, were connected in series to simultaneously analyze all possible products from one injection.
absorption in the visible-light region. The enhanced hydrophobicity is attributed to the higher tendency of CO production. In addition, the homogeneous distribution of Cu2O within the sublayers further promoted the conversion of CO2. The high thermal degradation temperatures of PI films indicated the potential application of these photocatalytic films in severe conditions.
4. EXPERIMENTAL SECTION 4.1. Materials. The monomers, pyromellitic dianhydride (PMDA, electronic grades, CHRISKEV) and 4,4′-oxydianiline (ODA, purity: 97%, Sigma-Aldrich), were used as received. The solvent dimethylacetamide (DMAc, purity: 99.5%, Tedia) was dried overnight with molecular sieves before use. The twopart liquid component PDMS kit, SYLGARD 184 silicone elastomer, was purchased from Dow Corning. Ethanol (purity: 99%) from J.T. Baker, anhydrous copper(II) acetate (purity: 99%) from Acros, and potassium hydroxide (purity: 99%) from Sigma-Aldrich were used without purification. 4.2. Synthesis of PI Films with a Biomimetic Surface Structure. The surface texturing of PI films was conducted via a modified nanocasting technique.28 The two components of PDMS precursors were mixed and then poured into a container with fresh leaves of X. sagittifolium attached to the bottom. With degassing and curing process at 70 °C for 1.5 h, the soft PDMS negative template with the surface structure of leaves was obtained and can be detached from the container for the following structure duplication on PI films. The PI precursor, PAA solution, was obtained by dissolving equimolar PMDA and ODA in DMAc under N2. The viscous PAA solution was then poured onto the negative PDMS mold and cast by a stainless film applicator with a fixed gap of 2 mm. The PDMS mold was then detached from the PAA dry film after the following thermal treatment at 70 °C for 1.5 h. The dry PAA film was then step-heated at 150 and 170 °C for 1 h at each temperature and finally at 350 °C for 2 h in an aircirculating oven to complete the imidization process. The obtained biomimetic PI films were decoded tPI. The cleavage of the imide ring on tPI films was conducted through the immersion of tPI films in KOH solution at 50 °C for 5 min according to previous investigation. After being rinsed thoroughly with deionized water, the surface-modified tPI films were immersed in 0.1 M copper(II) acetate aqueous solution (CuAc-1) for 5−30 min to exchange K+ with Cu2+ ions. The rinsed copper ion-contained PI films were thermally treated under the same programming to 350 °C in the same oven. The resultant films were decoded Cu/tPI-x, where x indicates the period of ion exchange. For comparison, a reference PI film, decoded PI, was also fabricated by casting the same PAA precursor solution on the glass substrate to obtain flat PI films. The above process was also applied to anchor Cu2O nanoparticles on this flat PI film named Cu/PI-x films. Moreover, the second template from another X. sagittifolium leaf was fabricated and utilized to obtain Cu/ tPI-x* films with hydrophobicity different from Cu/tPI-x series. Another procedure was applied to immobile Cu species on tPI. The above PAA dry film was directly immersed in ethanol solution containing 0.1 M anhydrous copper(II) acetate (CuAc-2) for 5−30 min. After washing with pure ethanol solution, the Cu2+ ion-attached PAA films were placed in the above-mentioned oven for the same imidization process. The resultant film samples were decoded Cu/itPI-x.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b03247.
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Top-view SEM images and corresponding EDS results of Cu/tPI-x and Cu/itPI-x films (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +886-4-24517250 ext. 3666. Fax: +886-4-24510890 (I.-H.T.). 1642
DOI: 10.1021/acsomega.8b03247 ACS Omega 2019, 4, 1636−1644
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ORCID
I-Hsiang Tseng: 0000-0003-4690-9968 Jui-Ming Yeh: 0000-0003-2930-0405 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to acknowledge the financial support from the Ministry of Science and Technology (MOST) of Taiwan under the grant numbers MOST 104-2221-E-035-074MY3 and MOST 104-2622-E-035-018-CC3. The authors also appreciate the Precision Instrument Support Center of Feng Chia University in providing CA, TGA, and BET measurement facilities. The staff of Antique Assam Tea Farm located in Nantou County in Taiwan were appreciated for kindly providing the plants of X. sagittifolium.
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