Bidirectional Light-Driven Ion Transport through Porphyrin Metal–Organic Framework-Based van der Waals Heterostructures via pH-Induced Band Alignment Inversion

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE3 Oct 2022Bidirectional Light-Driven Ion Transport through Porphyrin Metal–Organic Framework-Based van der Waals Heterostructures via pH-Induced Band Alignment Inversion Yuhui Zhang, Linfeng Yang, Yating Yang, Wei Li, Biying Liu, Xiaoyan Jin, Min Zhou, Run Long, Lei Jiang and Wei Guo Yuhui Zhang CAS Key Laboratory of Bio-Inspired Materials and Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing University of Chinese Academy of Sciences, Beijing Google Scholar More articles by this author , Linfeng Yang CAS Key Laboratory of Bio-Inspired Materials and Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing University of Chinese Academy of Sciences, Beijing Google Scholar More articles by this author , Yating Yang College of Chemistry, Key Laboratory of Theoretical and Computational Photochemistry of Ministry of Education, Beijing Normal University, Beijing Google Scholar More articles by this author , Wei Li CAS Key Laboratory of Bio-Inspired Materials and Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing University of Chinese Academy of Sciences, Beijing Google Scholar More articles by this author , Biying Liu CAS Key Laboratory of Bio-Inspired Materials and Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing University of Chinese Academy of Sciences, Beijing Google Scholar More articles by this author , Xiaoyan Jin CAS Key Laboratory of Bio-Inspired Materials and Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing University of Chinese Academy of Sciences, Beijing Google Scholar More articles by this author , Min Zhou CAS Key Laboratory of Bio-Inspired Materials and Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing University of Chinese Academy of Sciences, Beijing Google Scholar More articles by this author , Run Long College of Chemistry, Key Laboratory of Theoretical and Computational Photochemistry of Ministry of Education, Beijing Normal University, Beijing Google Scholar More articles by this author , Lei Jiang CAS Key Laboratory of Bio-Inspired Materials and Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing Google Scholar More articles by this author and Wei Guo *Corresponding author: E-mail Address: [email protected] CAS Key Laboratory of Bio-Inspired Materials and Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202101588 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Heterogeneous two-dimensional layered membranes reconstructed from natural or synthetic van der Waals materials enable novel ion transport mechanisms by coupling with the chemical and optoelectronic properties of the layered constituents. Here, we report a light-driven and pH-dependent bidirectional ion transport phenomenon through porphyrin metal–organic framework (PMOF) and transition metal dichalcogenides-based multilayer van der Waals heterostructures with sub-nanometer ionic channels. In acidic solutions, we find generation of a net ionic flow through the PMOF-WS2 multilayers upon visible light illumination. Surprisingly, in alkaline solutions, the light-driven ionic flow can be switched to the opposite direction. The driving mechanism is generally understood as a photovoltaic effect due to type II band alignment of PMOF and WS2. Copper ion substitution in porphyrin centers is incomplete; and at high pH, deprotonation of the non-copper-occupied porphyrin units gives rise to a band alignment inversion that explains the unexpected ionic photocurrent reversion. We demonstrate a light-powered and pH-stimulated ionic synapse as an application of this bidirectional driving mechanism. Using pH as a chemical modulation factor for postsynthetic band alignment engineering and even band alignment inversion should lead to further exploration of light-induced transport properties, and we can anticipate photonic–ionic circuits for neuromorphic computing and artificial photosynthesis. Download figure Download PowerPoint Introduction Controlled ion transport under biological or artificial confinement is fundamentally involved in a wide range of disciplines, such as cellular electrophysiology, electrochemistry, chemical analysis, and membrane-based technology.1–5 Particularly, with the advance in liquid processing strategies, natural or synthetic van der Waals materials are used to construct macroscopic assemblies that further reduce the characteristic size of artificial ionic channels from the nanometer scale down to even the angstrom scale6,7; and this leads to a variety of iontronics-related applications, for example, precise ionic or molecular sieving, electrochemical energy storage, and biomimetic energy conversion.8,9 In addition to conventional two-dimensional (2D) crystals exfoliated from naturally occurring layered minerals,10 other families of synthetic 2D materials, including zeolites,11,12 transition metal carbides and nitrides,13,14 and metal–organic frameworks,15,16 emerge as a source for assembling functional membranes. Under light stimuli, the rich chemistry and fascinating optoelectronic properties of the layered constituents enable efficient charge transfer therein,17,18 which provides new possibilities for driving ion transport in the interlayer space.19,20 The porphyrin-based metal–organic framework (PMOF) combines the merits of both porphyrins and metal–organic frameworks, such as strong light absorption in the whole visible spectrum, fast energy transfer dynamics, high porosity, and ordered structure; and thus finds application in, for example, biomimetic catalysis, gas adsorption, optoelectronic devices, and energy storage.21,22 In the synthesis of PMOF, it is generally accepted that transition-metal ions with four coordination numbers, such as Zn2+ and Cu2+, are inclined to enter the free-base porphyrin center in a square-planar geometry, due to suitable electronic configuration.23 Based on such understanding, only metalated porphyrin centers are considered by default in corresponding PMOFs (see refs 24–26 for example). In fact, however, whether the porphyrin center can be fully substituted by the metal ions, or to what degree, remains ambiguous. The function of the non-metal-substituted porphyrin centers is largely unexplored. Currently, light-driven ion transport in artificial systems is unidirectional; that is, once the material is synthesized, the transport direction would be predetermined, depending on the relative energy level or chemical affinity of separate parts.27–29 In situ engineering of the band alignment may provide new opportunities for tailoring charge transfer properties at the heterogeneous interface; it also alters the ion transport direction. However, most of existing modulation approaches are presynthetic, such as tuning the chemical composition or thickness of the layered components.30,31 Apart from these, a postsynthetic modulation strategy that allows dynamic control of the transport direction remains a great challenge. To this end, we report a light-driven and pH-dependent bidirectional ion transport phenomenon through PMOF and transition metal dichalcogenides-based multilayer van der Waals heterostructure. Continuous one-dimensional channels and cascading lamellar channels both with sub-1-nm size in reassembled 2D-PMOF and 2D-WS2 multilayers, respectively, constitute the ionic and fluidic transport pathway across the membrane. Upon visible light illumination, we find generation of a net ionic flow through the PMOF-WS2 membrane in acidic or weak alkaline environment. Surprisingly, under alkaline pH, the light-driven ionic flow can be switched to the opposite direction. By experimentally determining the band structure, we find PMOF and WS2 form type II band alignment, and more intriguingly, a band alignment inversion is discovered under high pH. A synergistic effect between surface potential difference and band-alignment-induced charge separation explains the ionic photocurrent reversion (IPR). Based on analysis of molecular spectra and density functional theory (DFT) calculations, we unveil the incomplete central coordination of porphyrin with copper ions results in the coexistence of copper-occupied and non-copper-occupied porphyrin centers in PMOF. Under high pH, deprotonation of the non-copper-occupied porphyrin units becomes a reason for band alignment inversion. As an application, we develop a light-powered and pH-stimulated artificial synapse following the transport mechanism. Experimental Methods Materials 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (TCPP) was purchased from Tokyo Chemical Industry (Shanghai). Cu(II) tetra(4-carboxyphenyl)porphyrin was purchased from Frontier Scientific (USA). Copper(II) nitrate hydrate, benzoic acid, and N,N-Dimethylformamide (DMF) were purchased from Beijing Innochem Science & Technology (Beijing). WS2 nanosheets were purchased from Nanjing MKNANO (Nanjing). Other chemicals were analytical grade and used without purification. Deionized water (18.2 MΩ cm, Milli-Q) was used for preparing ionic solutions. Synthesis of 2D-PMOF 2.0 mL of 1 mM TCPP suspension in DMF and 1.0 mL of 10 mM Cu(NO3)2 aqueous solution were added to a 100 mL round-bottom flask. Afterward, benzoic acid dissolved in DMF (0.27 g, 10 mL) was added to form a homogeneous solution. The suspension was kept at 90 °C for 4 h with stirring. After reaction, the product was collected by centrifugation and washed with ethanol several times. Then, PMOF nanosheets were redispersed in ethanol for further use. Membrane fabrication The PMOF-WS2 membrane was fabricated via a two-step vacuum filtration of 2D-WS2 and 2D-PMOF nanosheets ( Supporting Information Figure S6).32 First, 10 mL 2D-WS2 aqueous dispersion (0.2 mg mL−1) was filtrated through a polycarbonate filter (25 mm diameter, 0.2 μm nominal pore size, Millipore). The resulting WS2 membrane was suction-dried overnight. Afterward, 10 mL 2D-PMOF ethanol dispersion (0.03 mg/mL) was filtrated onto the WS2 membrane. Then, the PMOF-WS2 membrane was suction-dried for 48 h. Characterization Surface morphology and thickness of individual 2D-PMOF nanosheets were characterized by atomic force microscopy (AFM) (Dimension ICON, Bruker, USA). Fourier transform infrared spectroscopy (FT-IR) characterization was carried out on a Scientific Nicolet iS5 FT-IR spectrometer (Thermo Fisher Scientific, USA). Zeta potential measurements were conducted on a Malvern Zetasizer NanoZS90 (UK). X-ray photoelectron spectroscopy (XPS) was acquired with a K-Alpha spectrometer (Thermo Scientific, USA). UV–vis absorption spectrum was recorded on a Cary 5000 UV–vis spectrometer (Agilent, USA). Surface contact angle measurements were conducted on an OCA20 platform (DataPhysics, Germany). Microstructures of the membrane were characterized by a field-emission SEM (ZEISS Gemini 300). X-ray diffraction (XRD) characterization was carried out on a Bruker D8 Focus diffractometer with a Cu-K radiation source. Grazing incident XRD (GIXRD) was conducted on a Rigaku Smart Lab X-ray diffractometer (Japan) using Cu-K radiation in a 2θ range from 2.5° to 45° with a scanning rate of 5° min−1. The structural model of PMOF was constructed using Materials Studio software (Accelrys, USA) as referred to in the literature.33,34 GIXRD patterns were calculated with this software. The Brunauer–Emmett–Teller surface area was measured by nitrogen adsorption–desorption on an Autosorb-IQ analyzer (Quantachrome, USA). UV photoelectron spectroscopy was acquired using a Kratos Axis Ultra DLD spectrometer (Germany). The UV–vis diffuse reflection spectrum was recorded on a Cary 7000 UV–vis spectrometer (Agilent). Ionic photocurrent measurement A piece of PMOF-WS2 membrane was mounted in a two-compartment electrochemical cell with each reservoir opened to a light source through a glass-sealed window (1 cm × 1 cm).20 The membrane area was about 0.2 mm2. To ensure full hydration of the ionic channels, the membranes were immersed in water for about 1 day.35 Two light-emitting diode (LED) lamps were used as the light source, and the light intensity was calibrated before the tests. Ionic current recordings were carried out with a Keithley 2636B source meter through Ag/AgCl electrodes. The electrodes were placed 1 cm away from the testing membrane, and were out of the light path. They were separately protected with a light-proof shield. The illumination duration was 30 s each time for the time trace recording. The pH of the ionic solution was adjusted by adding concentrated HCl or KOH solutions, and it was calibrated by a pH meter before each test. DFT calculation Details of the calculation method and model parameters can be found in the Supporting Information, part 14. Results and Discussion The 2D-PMOF nanosheets were synthesized by a solution reaction of tetrakis(4-carboxyphenyl)porphyrin and Cu(NO3)2·3H2O with benzoic acid as adjuvant.36 The TCPP molecules were coordinated with Cu2(COO)4 paddle-wheel metal nodes, forming MOF structure. AFM characterization shows their mean thickness was 1.82 ± 0.16 nm, corresponding to 4–5 reticulate layers,24 and the lateral size was 1.68 ± 0.72 μm ( Supporting Information Figure S1). FT-IR spectroscopic analysis confirmed the formation of MOF structure ( Supporting Information Figure S2). The N–H in-plane vibration at 964 cm−1 decreased, and a new absorption peak appeared at 999 cm−1, suggesting that the hydrogen proton was substituted by a copper ion. Also, the N–H stretching vibration at 3315 cm−1 almost disappeared in the MOF structure, further confirming the copper ion substitution. Meanwhile, the weakened C=O stretching vibration at 1700 cm−1 indicated the formation of Cu2(COO)4 paddle-wheel metal nodes.37 XPS survey spectrum confirmed the presence of C, N, O, and Cu elements ( Supporting Information Figure S3a). The Cu 2p spectrum suggested a valence state of +2 for copper ions ( Supporting Information Figure S3b). The weakened C–OH peak with respect to the C=O peak in O 1s spectrum also suggested the coordination of carboxyl groups with copper ions ( Supporting Information Figure S3c).38 Powder XRD analysis further confirmed its crystalline structure ( Supporting Information Figure S4) in agreement with the literature.39,40 The WS2 nanosheets were prepared with a lithium intercalation method.41 AFM characterizations show their lateral size mostly ranged from 0.3 to 0.8 μm, and the thickness was about 1.2–1.3 nm ( Supporting Information Figure S5). The heterostructured membrane was fabricated by sequential filtration of 2D-WS2 and 2D-PMOF colloidal solutions (see the Experimental Methods and Supporting Information Figure S6). The PMOF-WS2 membrane is flexible, and the two sides can be distinguished from their apparent color (Figure 1a). The WS2 side is hydrophilic (CA = 49.1° ± 1.2°), while it is more hydrophobic on the PMOF side (CA = 85.5° ± 3.4°). On the one hand, PMOF contains organic components in its molecular structure. On the other hand, from zeta potential measurements, the WS2 nanosheets carry more negative charges than the PMOF (Figure 1b). The as-prepared PMOF-WS2 membranes exhibit excellent stability in water and under a wide range of pH without additional pretreatment ( Supporting Information Figure S7). Figure 1 | Characterization of PMOF-WS2 membrane. (a) Scheme and photograph of the heterogeneous PMOF-WS2 membrane. The atomic structures are plotted following the experimental characterizations, showing the stacking mode and interlayer distance. Contact angle measurements show surface wettability on each side. (b) Zeta potential measurements show surface charge property of PMOF and WS2 nanosheets at different pH. (c) SEM observation on membrane cross section exhibits laminar microstructure along the entire thickness. EDS mapping of W and Cu elements show a clear interface between the two parts. (d) Out-of-plane XRD patterns of homogeneous WS2 and PMOF membranes, and heterogeneous PMOF-WS2 membrane. All diffraction peaks in the homogeneous membranes can be found in the heterogeneous membrane. (e) In-plane XRD pattern of PMOF membrane (up) and the simulation result (down) referring to the AB stacking mode in (a). (f) XPS analysis of N 1s spectra (hollow point) suggests coexistence of Cu-TCPP (blue) and H-TCPP (green) in the MOF structure. Download figure Download PowerPoint Scanning electron microscopic (SEM) characterization shows laminar microstructure along the cross section of the PMOF-WS2 membrane (Figure 1c). Further energy-dispersive X-ray spectrometry (EDS) mapping of W and Cu elements identified a clear boundary between the two parts (indicated by white arrows in the figure). The thickness of PMOF and WS2 multilayers is about 540 and 680 nm, respectively. The XRD pattern of the PMOF-WS2 membrane shows a combination of the diffraction peaks found in homogeneous WS2 and PMOF stacks (Figure 1d). The WS2 part exhibits a (001) diffraction peak centered at 2θ = 7.5°, corresponding to an interlayer distance of 11.8 Å. Considering the thickness of its triple atomic structure (about 3.2 Å),42 the effective height of interlayer lamellar channels in WS2 part is about 8.6 Å. For the PMOF part, the out-of-plane XRD pattern shows a (002) peak centered at 2θ = 19.4° (Figure 1d; it also appears in powder diffraction, Supporting Information Figure S4), corresponding to a lattice distance of 9.0 Å. However, the in-plane GIXRD pattern (Figure 1e), as well as the simulation results, indicates the PMOF layers should adopt an AB stacking mode, that is, the interlayer distance between neighboring reticulate layers should be only 4.5 Å (Figure 1a). Considering thickness of the reticulate layer (about 2.7 Å),37 the interlayer space cannot be the pathway for water and ion transport. To solve this problem, nitrogen gas adsorption–desorption isotherms were employed to understand the pore structure in the PMOF part. The sorption curves exhibit a combination of type I and II isotherm. Rapid gas uptake at low pressure indicates the presence of micropores, while slow gas uptake at moderate pressure and capillary condensation suggest the existence of larger mesopores ( Supporting Information Figure S8a).43 The specific surface area of PMOF membranes is about 548.2 m2 g−1. Pore size distribution clearly shows the majority of micropores possesses an effective size of about 8.4 Å ( Supporting Information Figure S8b). The simulated GIXRD pattern suggests these micropores are formed by misplaced reticulate layers for 1/4 of unit cell (Figures 1a and 1e). In addition, there is also a small amount of mesopores with mean size of about 16 and 40 Å ( Supporting Information Figure S8b). From the literature, we believe these larger pores belong to the unavoidable few-nm-sized pinholes during deposition of high aspect ratio 2D crystals.44,45 To unveil the chemical composition of the PMOF part, first, UV–vis absorption spectra show a prominent Q-band absorption at 545 nm that indicates the copper-occupied porphyrin units (termed Cu-TCPP, Supporting Information Figure S9). Nevertheless, the weak Q-band absorptions at 516, 592, and 650 nm have not fully disappeared. This evidence suggests there is still a considerable amount of non-copper-occupied porphyrin units (termed H-TCPP) in the framework. Then, we used XPS N 1s spectra to quantify the relative content of Cu-TCPP and H-TCPP (Figure 1f). The peak area of the fitted iminic nitrogen spectrum (–C=N–, contributed to by both Cu-TCPP and H-TCPP) accounts for nearly 82% of the total N 1s spectrum, whereas a relatively weak pyrrolic nitrogen spectrum (–NH–, contributed to merely by H-TCPP) accounts for the remaining 18%. For H-TCPP, it should contribute equally to either –C=N– or –NH– peaks, because the peak area of –C=N– in free-base TCPP is approximately equal to that of the –NH– peak.46 Based on these considerations, the proportion of Cu-TCPP should be about 64%, while the H-TCPP accounts for the remaining 36%. By controlling the amount of benzoic acid in synthesis process, the relative content of Cu-TCPP and H-TCPP can be further modulated ( Supporting Information Figure S10). To investigate transmembrane ion transport properties, a piece of PMOF-WS2 membrane was mounted in between two compartments of an electrochemical cell ( Supporting Information Figure S11a). 5 mL of unbuffered KCl solution (10−3 M) was filled in each reservoir. Ag/AgCl electrodes were used to record ionic current. Under a scanning voltage, a rectified current–voltage response was found through the PMOF-WS2 membrane ( Supporting Information Figure S11b). The ionic current rectification (ICR) ratio approached 2.3 at ±1.0 V. In contrast, homogeneous PMOF and WS2 membranes exhibited nonrectified current–voltage response ( Supporting Information Figures S11c and S11d). The PMOF-WS2 membranes showed nearly perfect cation selectivity under a wide range of pH with cation transference numbers (t+) over 0.9 ( Supporting Information Table S1 and Figure S12). The high ionic conducting state under positive bias represents a preferential cation transport from PMOF to WS2 ( Supporting Information Figure S11b). To achieve ion transport tests under illumination, the electrodes were protected by light-proof shields to avoid light pollution.20 The illumination was separately conducted from two sides of the membrane (Figure 2a). The wavelength of the incident light was centered at 470 nm. Electrolyte solution in the two reservoirs was 10−1 M KCl (pH ∼ 5.4). Figure 2 | pH-dependent bidirectional ion transport. (a) Scheme of the experiment. The two magnified views illustrate the structure of ionic channels in the two parts. (b) At low pH (for example, pH 5.4), upon illumination, a negative ionic photocurrent (Iph) is generated through the heterogeneous membrane, and its direction is from WS2 to PMOF. (c) Once the experiment was conducted in alkaline solutions (e.g., pH 11.8), the ionic photocurrent goes in the opposite direction, from PMOF to WS2. (d) Light intensity dependence of ionic photocurrent at pH 5.4 (shorted as pH 5, triangles) and pH 11.8 (shorted as pH 12, circles). (e) Reversibility of the opposite ionic photocurrent under low and high pH. In (b), (c), and (e), the light intensity was 94.1 mW cm−2, and its duration was 30 s. All tests were conducted in 10−1 M KCl solution. Download figure Download PowerPoint With equivalent ionic concentration on both sides, upon illumination (94.1 mW cm−2), we observed an immediate ionic photocurrent (Iph, Figure 2b) and photovoltage response (Vph, Supporting Information Figure S13a) through the PMOF-WS2 membrane, without externally applied electrical bias. The ionic photocurrent rose from zero to about 122.3 nA within 20 s, and its direction went from WS2 to PMOF, suggesting a net cation transport in that direction. Likewise, the photovoltage reached its peak value of 9.6 mV in the meantime, reflecting a light-induced ion transport motive force. The observations are consistent with other light-powered ion transport systems reported by our group47,48 as well as others.27,28 As control experiments, no clear ionic photoresponse was found with the polymeric nanoporous membrane or the homogeneous PMOF or WS2 membranes ( Supporting Information Figure S14). Surprisingly, once the experiment was conducted in an alkaline environment, for example at pH 11.8, we observed the opposite ionic photocurrent (Figure 2c) and photovoltage ( Supporting Information Figure S13b), indicating a reversed ion transport direction, from PMOF to WS2. The magnitude of ionic photocurrent measured at low pH was generally higher than that at high pH under identical light intensity (Figure 2d). More importantly, the bidirectional ionic photoresponse was fully reversible by simply switching the pH (Figure 2e). The excellent reversibility also confirms the stability of the hybrid membrane under high or low pH. Furthermore, we conducted parallel ionic photocurrent tests under a series of continuously changed pH. As shown in Figure 3a and Supporting Information Figure S15, the reverse of ionic photocurrent takes place at about pH 10. To explore which part of the hybrid membrane dominates the unexpected IPR, we adjusted the pH from only one side of the membrane. As shown in Figure 3b, we kept the pH on the WS2 side at 5, while shifting the pH on the PMOF side from 3 to 12. Similar IPR is observed under high pH with the transition point near pH 10. In contrast, once the pH change occurs on the WS2 side (Figure 3c), the ionic photocurrent goes in unchanged direction with merely its magnitude rising with the pH. This increment can be attributed to the enhanced surface charge density at high pH (Figure 1b), in accord with our recent findings on transition metal dichalcogenides-based heterogeneous membrane.29 These results unveil the dominant role that PMOF plays for IPR. Figure 3 | PMOF-dominated IPR. (a) With symmetric pH on two sides, the IPR takes place at about pH 10. To determine which part of the heterogeneous membrane is the dominating factor, we adjusted pH on only one side of the membrane (b and c). (b) First, we fixed the pH on WS2 side to about 5 while changing the pH on PMOF side from about 3 to 12. Similar IPR can be observed with the transition point at about pH 10. (c) Then, we fixed the pH on the PMOF side to about 5 while changing the pH on WS2 side from about 3 to 12. In this case, the generated ionic photocurrent remains negative, that is, no IPR is observed. This evidence suggests that the IPR effect is dominated by the PMOF part. Download figure Download PowerPoint To understand the driving mechanism, we directly probed the energy band structure of PMOF and WS2 under different pH by UV photoelectron spectroscopy and UV–vis diffuse reflection spectroscopy ( Supporting Information, part 12). The results are summarized in Figure 4a. PMOF and WS2 form a type II semiconductor heterojunction. Under low pH, the conduction band minimum (CBM) was present in PMOF, and the valence band maximum (VBM) stayed in WS2. Upon illumination, photoexcited electrons in WS2 jump to PMOF; while holes generated in PMOF transfer to WS2 due to type II band alignment (Figure 4b).49 The separation of photoexcited charge carriers establishes a photovoltaic motive force between the two parts that drives the ion transport from WS2 to PMOF. Figure 4 | Band alignment inversion under high pH. (a) CBM and VBM of the PMOF and WS2 membranes are experimentally determined. In each pH condition, PMOF and WS2 form a type II band alignment. For WS2, its CBM and VBM are insensitive to pH. But for PMOF, its CBM and VBM move upward with the increasing pH. Under higher pH over 10, a clear band alignment inversion is observed. (b) Since WS2 is more negative than PMOF, at pH below 8, surface potential difference between the two parts facilitates the band-alignment-induced charge separation (top), showing a positive synergistic effect. Thus, the ionic photocurrent increases with surface potential difference (bottom). (c) In contrast, after the band alignment inversion, surface potential difference impedes the band-alignment-induced charge separation (top), showing a negative synergistic effect. Thus, the ionic photocurrent shows a declining trend with surface potential difference (bottom). Download figure Download PowerPoint With the increase of pH, both CBM and VBM of the PMOF move toward a more positive level (Figure 4a). In contrast, the energy level of WS2 is not sensitive to pH change. In this case, a band alignment inversion takes place at high pH. Under illumination, photoexcited charge carriers move in the opposite direction (Figure 4c), establishing a reversed photovoltaic driving force, that acc