Synthesis, Characterization, and Light-Induced Spatial Charge

Article Cite This: Chem. Mater. 2018, 30, 2084−2092

pubs.acs.org/cm

Synthesis, Characterization, and Light-Induced Spatial Charge Separation in Janus Graphene Oxide Alexander Holm,*,† Joonsuk Park,‡ Emmett D. Goodman,§ Jiaming Zhang,∥ Robert Sinclair,‡ Matteo Cargnello,§ and Curtis W. Frank*,† †

Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States § Department of Chemical Engineering and SUNCAT Center for Interface Science and Catalysis, Stanford University, Stanford, California 94305, United States ∥ Hewlett-Packard Laboratories, 1501 Page Mill Road, Palo Alto, California 94304, United States ‡

S Supporting Information *

ABSTRACT: Janus graphene oxides and Janus graphenes are materials with different functionalization on opposite faces of atomically thin carbon sheets. Owing to their monolayer nature, these Janus sheets show unique properties where the functionalization on one face can modulate the properties on the opposite face. However, few general procedures to create and characterize Janus graphene oxides or Janus graphenes have been reported, and as a consequence these intriguing materials remain largely unexplored. Here we report a general synthesis of Janus graphene oxide, where particles are deposited in situ from molecular precursors on opposite faces of monolayer graphene oxide (GO). We used a silicon wafer and a polymer film to successively expose and protect alternate graphene oxide faces for asymmetric deposition of Pt and TiO2 nanocrystals, thus producing Janus graphene oxide composites (Pt|GO|TiO2). We used electron microscopy of Janus graphene oxide cross-sections to conclusively show that Pt and TiO2 particles are placed on opposite faces of monolayer sheets. Furthermore, we demonstrate the utility of Janus graphene oxide asymmetric chemistry by showing that photogenerated electrons and holes accumulate on opposite faces of the atomically thin sheets. The general nature of the synthesis and characterization protocols enables both production and asymmetry verification of a wide range of Janus graphene oxides and therefore provides a general approach for spatial charge separation across two-dimensional structures and other potential applications.

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INTRODUCTION

unique class of materials is hampered by the lack of reliable and versatile methods to produce and characterize them. Previously, Janus GO has been fabricated by attachment of premade Au and ZnO particles onto opposite faces of GO monolayer sheets.24 Using current-sensitive atomic force microscopy (AFM), it was shown in dry experiments that the composite, while deposited on a gold-coated wafer, exhibited light-induced electron transfer from the ZnO particles to the GO sheet.24 To test the feasibility of separating light-induced charges on opposite faces of Janus GO, it must further be shown that electrons transfer from photoabsorber particles on one face to electron accepting particles on the opposite Janus GO face. In addition, many photoprocesses occur in liquid,29−33 and it is therefore of interest to demonstrate that Janus GO, dispersed in liquid, supports this electron transfer. Separating light-induced charges on opposite sides of thin twodimensional structures is fundamentally and practically im-

Janus particles are anisotropic composites that contain two distinct regions with different surface chemistries.1−3 These particles often show dramatically different properties compared to their isotropic counterparts, which makes them interesting for many applications such as stabilizers for Pickering emulsions,4−7 colloidal self-assembly,8−10 drug delivery,11,12 photoactivated micromotors,13−15 and photocatalysts.16−22 Janus graphenes and Janus graphene oxides constitute a new class of Janus particles where different functionalities are bound to opposite faces of single-layer graphene or graphene oxide (GO) sheets. Although only a few demonstrations of Janus graphene or Janus GO have been reported,23−28 these Janus sheets clearly show unique and attractive properties. As an example, de Leon and coauthors showed that Janus GO was substantially more amphiphilic than symmetrically functionalized sheets.28 Furthermore, Zhang and colleagues showed that the chemical identity of one face of Janus graphene impacted both the reactivity and wettability of the other face, which was attributed to the extremely thin nature of the material.23 Nevertheless, exploration of the properties of this © 2018 American Chemical Society

Received: January 8, 2018 Revised: February 25, 2018 Published: February 26, 2018 2084

DOI: 10.1021/acs.chemmater.8b00087 Chem. Mater. 2018, 30, 2084−2092

Article

Chemistry of Materials

Figure 1. Process to synthesize Pt|GO|TiO2 Janus GO. Schematic illustration of process: (a) GO deposition onto SiO2|Si wafer. (b) Growth of anatase TiO2. (c) Spin-coating of PMMA film. (d) HF etch to detach PMMA film. (e) Growth of Pt on unreacted GO faces. (f) Dispersion of Pt|GO|TiO2 Janus GO in solvent. Height-mode AFM images: (g) GO sheets on SiO2|Si wafer. (h) After TiO2 deposition. (i) After PMMA film deposition, where a scratch is imaged and used to measure film thickness. Phase-mode AFM images: (j) After flipping sheets onto PMMA film. (k) After asymmetric Pt deposition (outline of sheets added to guide the eye).

with the fresh, unreacted face exposed and the functionalized face protected, a second asymmetric functionalization can be made on the fresh face, thus producing Janus GO. In previous work, nanoparticles (ZnO and Au) were asymmetrically attached to GO monolayers using weak, noncovalent interactions between preformed particles and GO sheets.24 Taking inspiration from this previous work, we adapted the process for asymmetric growth of TiO2 and Pt nanoparticles in situ on opposite faces of monolayer GO. We used a silicon wafer with a thermally grown SiO2 layer (SiO2|Si) and a thin poly(methyl methacrylate) (PMMA) film to successively expose and protect GO faces for asymmetric particle depositions. The method is schematically illustrated in Figure 1a−f. To demonstrate the step-by-step fabrication, we followed specific sheets through the process from GO to Pt|GO|TiO2 Janus GO using AFM (Figure 1g−k). GO was deposited onto a SiO2|Si wafer by cationic-assisted Langmuir−Blodgett (LB) deposition.35,36 We deposited nonoverlapping GO monolayers with lateral sizes in the range ∼1−30 μm and with wafer surface coverage controlled by degree of compression in the LB-trough (Figure S2). To illustrate the production of Janus GO, we used wafers with low coverage (Figure 1g) so that single sheets could be followed, but we also fabricated Janus GO from wafers with higher GO coverages (Figure 2 and Figure S1), with equivalent

portant. As an example, this process is necessary for natural photosynthesis, where membrane-embedded photosystems I and II work in concert to generate light-induced charge separation across two-dimensional thylakoid membranes in chloroplasts.34 However, although light-induced charge separation across two-dimensional structures is critically important in natural systems, this property has scarcely been demonstrated in engineered systems.22 We surmised that if Janus GO could be synthesized with appropriate photoabsorber particles on one face, and electron accepting particles on the opposite face, then such Janus GO could support light-induced and unidirectional electron transfer between the two faces. Electrons and holes would thereby be separated on opposite faces of the twodimensional Janus sheets, providing an intriguing and novel way to separate and manipulate excited charges. Here, we report the synthesis of Janus GO with Pt and TiO2 particles grown in situ from molecular precursors on opposite faces of GO monolayers, thus forming Pt|GO|TiO2 Janus sheets. Successful asymmetric functionalization was suggested by following specific individual sheets through the whole functionalization process with atomic force microscopy (AFM). The asymmetric functionalization was then conclusively confirmed by studying cross-sections of monolayer sheets with electron microscopy. Finally, light-induced electron transfer and spatial separation of charges in Pt|GO|TiO2, dispersed in solution, were demonstrated by reductive deposition of Pd onto Pt. Our work shows that Janus GO can be produced by deposition of particles from molecular precursors, thus removing the need for molecular linkers that are often required with premade particles.24 In addition, the work demonstrates a powerful method (using electron microscopy) for direct confirmation of asymmetric functionalization in Janus GO, a task that was previously a major challenge.28 Finally, the work shows the feasibility of using Janus GO for spatial separation of charges in nanoscopic particles. Because the synthesis and characterization methods are general for Janus GO based on inorganic particles, the use of Janus GO to separate light-induced charges across two-dimensional structures may be extended to many systems.

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Figure 2. (a) Height-mode AFM image of typical GO deposition on SiO2|Si wafer. (b) The height profile shows GO sheets with uniform thickness of ∼1 nm, which confirms GO was deposited as monolayers.37

results. All deposited GO sheets show a uniform thickness of ∼1 nm as measured by AFM (Figure 2b), thus confirming that GO was completely exfoliated into monolayers.37 Following GO deposition, amorphous titania particles were directly grown onto exposed GO faces by a sol−gel method consisting of immersing the GO-loaded SiO2|Si wafers (GO| SiO2|Si) in a solution containing titanium butoxide. The amorphous titania layer was thereafter crystallized by steam

RESULTS AND DISCUSSION

Synthesis of Pt|GO|TiO2 Janus Graphene Oxide. When GO is deposited on a suitable flat substrate, only one face is available for chemical functionalization, while the other face is protected. Then, if the GO is transferred onto a second substrate 2085


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Chemistry of Materials treatment,38,39 thus forming TiO2|GO|SiO2|Si (Figure 1h). Particle size was controlled by reaction time in the titanium butoxide solution, and we produced samples with particle sizes of ∼2−6 and ∼5−15 nm (Figure 3a−d). Raman spectra of plain

grows preferentially on GO compared to the SiO2 surface, likely due to anchoring oxygen groups on the GO surface. Next, a ∼280 nm thick PMMA film was spin-coated onto the TiO2|GO|SiO2|Si substrate (Figure 1i). The polymer film was detached from the SiO2|Si wafer by etching the SiO2 layer in concentrated hydrofluoric acid (HF), thus producing GO|TiO2| PMMA with TiO2 functionalized GO faces protected and unfunctionalized faces exposed (Figure 1j). When performing this operation, we expect the flipped sheets to appear as mirror images under the AFM. Thus, because the micrographs of TiO2| GO|SiO2|Si before flipping (Figure 1h) and of GO|TiO2|PMMA after flipping (Figure 1j) are mirror images, the AFM characterization strongly suggests that GO monolayers were flipped. Pt nanoparticles were then photodeposited from an aqueous K2PtCl4 solution onto the exposed GO faces, thus producing Pt|GO|TiO2|PMMA (Figure 1k). The PMMA film wrinkled while in contact with the K2PtCl4(aq) solution, which prevented collection of phase-mode AFM images of the same quality as before Pt deposition. Even so, images after Pt deposition (Figure 1k) provide confirmation that the Pt|GO| TiO2 Janus sheets were in the same locations as before Pt deposition. Finally, the Pt|GO|TiO2 Janus GO was collected by dissolving the PMMA film in N-methyl-2-pyrrolidone (NMP), thus providing a colloidal dispersion of the composite. Structure of Pt|GO|TiO2 Janus Graphene Oxide. In order to structurally compare our Janus GO with conventional composites where Pt and TiO2 particles are randomly (symmetrically) deposited onto both faces of the GO sheets, we synthesized a symmetric counterpart (Pt-TiO2-GO) following procedures available in the literature.42 Note that Pt-TiO2-GO is the typical nomenclature for symmetric composites.43−46 We want to emphasize that no hierarchical information is contained in this nomenclature and that both Pt and TiO2 particles are randomly (symmetrically) distributed on both GO faces in PtTiO2-GO composites. Representative plan-view transmission electron microscopy (TEM) images reveal that Pt and TiO2 nanoparticles are densely packed on the GO sheets in both Pt| GO|TiO2 and Pt-TiO2-GO (Figure 4a,a′). In addition, plan-view images of Pt|GO|TiO2 (Figure 4a−d) confirm the AFM observation (Figure 3d) that TiO2 particles are ∼5−15 nm in size. High-resolution TEM (HRTEM) images (Figure 4b−d,b′− d′) confirm that both Pt (darker, smaller particles) and TiO2 (lighter, larger particles) are crystalline, thus corroborating the Raman analysis (Figure 3f). Plan-view TEM micrographs of Pt| GO|TiO2 and Pt-TiO2-GO look similar, which is expected, because GO is electron transparent,47,48 and it is not possible to distinguish on which face of the sheet the particles reside. While asymmetry in Pt|GO|TiO2 cannot be assessed by planview electron microscopy, it can be assessed by studying crosssections. We made cross-sections of individual Pt|GO|TiO2 and Pt-TiO2-GO monolayers using a focused ion beam (FIB) system (details in the Experimental Section and in Figure S5). Typical TEM images of Pt|GO|TiO2 and Pt-TiO2-GO cross- sections are presented in Figure 4e,e′, respectively. Typical scanning transmission electron microscopy (STEM) images of the crosssections are presented in Figure 4f,f′. Here, the Z-contrast yields darker particles identified as anatase TiO2, while brighter particles are Pt, an assignment also supported by STEM with energy-dispersive X-ray spectroscopy (EDS) mapping presented in Figure 4g,g′. During STEM-EDS mapping, beam damage gives rise to white dots in the STEM images (Figure 4g,g′) whichas demonstrated by the EDS mapsare not Pt particles. Further discussion about this artifact is given in the Supporting

Figure 3. AFM and Raman characterization of TiO2|GO|SiO2|Si. Height-mode AFM images of samples produced by reaction times of 1.5 h (a, b) or 5 h (c, d). The high-resolution images (b, d) were taken at positions marked by arrows in the low-resolution images (a, c). (e) Optical Microscopy image of the area imaged in panel c. (f) Raman spectra at location marked in panel e of plain SiO2|Si wafer (1), GO on wafer (2), after amorphous titania deposition (3), and after steam treatment (4). Raman maps of TiO2|GO|SiO2|Si after steam treatment: (g) GO, D-band (1200−1500 cm−1).28,40 (h) GO, G-band (1500−1700 cm−1).28,40 (i) Anatase TiO2 (120−185 cm−1).39

SiO2|Si wafer, GO on wafer, and the samples after amorphous titania deposition and after steam treatment are presented in Figure 3f. After GO deposition, the graphene oxide characteristic D (1200−1500 cm−1) and G (1500−1700 cm−1) bands appeared.28,40 After deposition of titania, no additional peaks were observed, suggesting that the titania was amorphous. However, after steam treatment, a peak developed at 151 cm−1 suggesting that as-grown amorphous titania crystallized into anatase TiO2.39 Height-mode AFM images clearly reveal that GO sheets were still present underneath the deposited particles (Figure 3a,c) which suggests that only a thin film of particles was deposited. The relatively small amount of TiO2 deposited may explain why only the strongest39,41 Raman peak at 151 cm−1 became observable after steam crystallization. Raman microscopy maps taken after steam crystallization (Figure 3g−i) further confirm the locations of GO on the wafer and suggest that TiO2 2086


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Figure 4. Structural characterization of asymmetric (left side, unprimed letters) and symmetric samples (right side, primed letters). (a, a′) Lowmagnification plan-view TEM images with enlarged sections (insets). (b, b′) Plan-view HRTEM images. (c, c′, d, d′) Enlarged sections showing lattice spacing of anatase TiO2 particles (black borders) and Pt particles (red borders). The sections are 3.3 nm × 3.3 nm. TEM (e, e′) and STEM (f, f′) images of the cross-sections. (g, g′) STEM images of cross-sections with associated EDS maps (left to right): Ti, Pt, Si, and overlay. (h, h′) Schematic interpretations of cross-section morphologies. Although the GO monolayer in the composites cannot be discerned in the electron microscopy characterization, due to the flexibility of GO monolayers, we expect the sheets to follow the particle contours as illustrated in panels h and h′. Further discussion regarding this point is given in the Supporting Information.

Figure 5. Asymmetry of Pd deposition onto Pt|GO|TiO2 from K2PdCl4 in 1:4 ethanol:water during light irradiation. (a) STEM image with associated EDS maps: Ti (b), Pt (c), Pd (d), and overlay (e).

and is presented in the Supporting Information, Figures S6 and S7. Light-Induced Electron Transfer in Pt|GO|TiO2 Janus Graphene Oxide. When anatase TiO2-GO suspensions are irradiated, electron−hole pairs form in the anatase TiO2 phase.49,50 Anatase TiO2 is an n-type semiconductor, and a space-charge layer develops at the TiO2−liquid interface, which drives holes to oxidation sites at the TiO2 surface.49,50 In contrast, electrons are injected from TiO2 into the GO sheet, thus reducing it.51,52 Furthermore, electrons move from GO to deposited Pt particles.53 Thus, if Pt particles act as reduction sites during light irradiation of Pt|GO|TiO2 suspensionsbut TiO2 particles do notthen it can be inferred that photogenerated electrons transfer from TiO2 to Pt on the opposite face of the Janus GO. Metal salts are commonly used to map locations of reduction sites in photocatalyst particles because they form small clusters or particles following electron injection.17,22,54 To map

Information. The monolayer nature of GO precludes it from being identified in the cross-sections, but its presence is demonstrated by our method of cross-section preparation (see the Experimental Section and Figure S5). In cross-sections of the symmetric sample Pt-TiO2-GO (Figure 4e′−g′), the particles appear randomly scattered across the composite, with Pt and TiO2 particles residing on both GO faces. In contrast, in Pt|GO|TiO2 Janus GO cross-sections (Figure 4e−g), all Pt particles reside below TiO2 particles. This observation suggests that no Pt deposition occurred on TiO2 in Pt|GO|TiO2, but only on the unfunctionalized GO face available for reaction (Figure 1j). Thus, we conclude that Pt particles reside on one GO face, and TiO2 particles reside on the opposite face of Pt|GO|TiO2. Morphological interpretations of Pt|GO| TiO2 and Pt-TiO2-GO cross-sections are given in Figure 4h,h′, respectively. Further electron microscopy characterization helps support the asymmetry argument for the Pt|GO|TiO2 sample 2087


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immersion in a base bath containing 8 L of isopropyl alcohol, 2 L of deionized (DI) water, and 500 g of KOH and then rinsed copiously in DI water, and finally in Milli-Q water before drying in a clean oven at 120 °C. GO Synthesis. GO was synthesized by modification of Hummers and Offeman’s method.75 A 2 g portion of graphite was stirred with 92 mL of sulfuric acid for 20 min. The mixture was cooled on an ice bath while 2 g of NaNO3 and 12 g of KMnO4 were added. The mixture was heated to 35 ±5 °C and stirred for 70 min; 160 mL of water was added (Caution! This reaction is highly exothermic), raising the temperature to 90 ± 5 °C, and the mixture was stirred for 30 min. Finally, 230 mL of water was added followed by dropwise addition of 30% hydrogen peroxide (∼15 mL) until no more gas evolution was observed. The warm reaction mixture was then filtered over vacuum, and the filter cake was washed with 100 mL of water and suspended in 250 mL of water by stirring. The pH in suspension was ∼3, and to facilitate exfoliation of graphitic oxide into GO monolayers, 1 M NaOH(aq) was added until pH ∼ 5 was reached. To remove very small and very large or unexfoliated sheets, the GO sample was purified by repeated centrifugation (Sorvall, ST8, Thermo Scientific, HIGHConic III fixed angle rotor) and sonication: (1) 4 × 8000 rpm, 20 min centrifugation. After each centrifugation step, the supernatant was discarded, and the sediment redispersed (vortexing) in water: (2) 1 × 8000 rpm, 20 min centrifugation. The supernatant was then discarded, and the sediment redispersed in water, pH set to ∼5 using 1 M NaOH(aq): (3) The sample was sonicated (Branson M1800) for 15 min: (4) 2 × 1000 rpm, 3 min centrifugation. After each centrifugation step, the supernatant was collected, and the sediment discarded: (5) 2 × 8000 rpm, 20 min centrifugation. After each centrifugation step, the supernatant was discarded, and the sediment redispersed in water: (6) 2 × 2500 rpm, 10 min centrifugation. The supernatant was collected and the sediment discarded. The purified suspension was diluted to 1.3 L of water; pH was adjusted to 10 using 1 M NaOH, and the suspension was stored in a separatory funnel for 8 days while sediment being formed was removed daily. This yields a ∼0.05 g L−1 suspension containing predominantly monolayers of GO in the size range ∼1−30 μm. Deposition of GO onto SiO2|Si Wafer (Producing GO|SiO2|Si). We modified a published protocol for GO deposition.35 To render SiO2| Si wafer pieces hydrophilic, they were immersed in a freshly prepared solution of ammonium hydroxide (30 mL, 30% in water) and hydrogen peroxide (30 mL, 30% in water) for 60 min, then rinsed copiously in water and dried under a nitrogen flow. For Langmuir−Blodgett (LB) depositions, a KSV 5000 Nima trough (Biolin Scientific) with dimensions 150 × 580 mm was used. Trough and barriers were washed with a soft brush and then rinsed, first with deionized (DI) water, then ethanol, and finally DI water again. The GO suspension described above was then added to the trough as subphase, and the temperature was set to 21 °C. The subphase was allowed to equilibrate for 60 min before the air−water interface was cleaned by aspirating from the surface with a pipet during compression of the trough (75 cm2 min−1). The SiO2|Si wafer piece (typically 22 mm × 88 mm) was then immersed into the subphase. A single-use paper Wilhelmy plate (Biolin Scientific) attached to a tensiometer was used to record surface pressure (Π) in the trough. Prior to use, the plate was immersed for 2 h in DI water and then rinsed in DI water. A cationic surfactant (15 μL, 7.2 mM DOTAP in chloroform) was then spread onto the subphase using a microsyringe (Hamilton). Before spreading, the syringe was rinsed with chloroform. The air−water interface was then allowed to equilibrate for 55 min before the first isotherm was collected. The trough surface was compressed (20 cm2 min−1) until Π was optimal for deposition of a desired film morphology (Figure S2). The air−water interface was then allowed to equilibrate with the subphase for 25 min before a deposition was made. LB films were deposited onto SiO2|Si wafer pieces by pulling the preimmersed substrates through the air−water interface at a speed of 2 mm min−1. Surfactant was removed from the deposited films by washing with acidified acetone (500 mL of acetone and 1 mL of 0.1 M aqueous HCl) and acidified ethanol (500 mL of ethanol and 1 mL of 0.1 M aqueous HCl). Asymmetric Deposition of TiO 2 onto GO on Wafer (Producing TiO2|GO|SiO2|Si). Dai et al. have developed a protocol

reduction sites in Pt|GO|TiO2, we used a solution of aqueous K2PdCl4 with Pd(II) being reduced to metallic Pd by the following reaction: [PdCl4]2 − (aq) + 2e− → Pd(s) + 4Cl−(aq)

(1)

Pt|GO|TiO2 Janus GO was dispersed in the aqueous solution containing the Pd salt, and the suspension was irradiated for 80 min using a Xe arc lamp. Representative plan-view STEM images with associated EDS maps of Pt|GO|TiO2 after light-induced Pd deposition are presented in Figure 5. In the STEM image (Figure 5a), brighter particles are predominantly composed of both platinum and palladium, as demonstrated by EDS maps (Figure 5c,d). Thus, Pd did not deposit directly onto TiO2 or the GO sheet, but instead onto Pt particles (Figure 5b−e). The colocalization of Pt and Pd strongly suggests that electrons, photogenerated in TiO2, efficiently transferred to the GO sheet and then to reduction sites on Pt before they could reach reduction sites at the TiO2− or GO−liquid interfaces. The flow of photogenerated electrons from TiO2 to Pt thus separates electrons (in Pt) from holes (in TiO2) on opposite faces of Pt| GO|TiO2 Janus GO. Additional STEM-EDS characterization showing Pd−Pt colocalization is presented in Figure S8.

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CONCLUSIONS We have reported the fabrication of Pt|GO|TiO2 Janus graphene oxide. We used electron microscopy of Pt|GO|TiO2 crosssections to conclusively show structural asymmetry with Pt nanocrystals on one face and TiO2 nanocrystals on the opposite face of monolayer graphene oxide. Confirming asymmetry in Janus graphene oxide or Janus graphene is challenging, and the method for direct confirmation of asymmetry described here is attractive for future research into particle-based Janus graphene oxide or Janus graphene. Furthermore, we showed that in Pt|GO| TiO2 Janus graphene oxidedispersed in liquidphotogenerated electrons transfer to Pt. Because photogenerated holes are injected into the liquid from TiO2,49,50 the transfer of electrons to Pt suggests that photogenerated electrons and holes are separated on opposite faces of the Pt|GO|TiO2 Janus graphene oxide. Finally, a large library of photoabsorbers can be deposited onto GO (TiO2,42,43,51,52,55−60 Fe2O3,61,62 WO3,63 BiVO4,64,65 CdS,66−68 ZnO,69,70 CuInZnS,71 La2Ti2O7,72 Cu2O,73,74 and many more). For this reason, we believe the Janus graphene oxide system presented here can be extended to many other combinations of materials, thus providing exciting novel composites for several different photoactive applications.

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EXPERIMENTAL SECTION

Chemicals and General Cleaning Procedures. Graphite powder was purchased from Bay Carbon (grade SP-1). SiO2|Si wafers with 500 nm thermally grown SiO2 were purchased from Pure Wafer (previously WRS materials). Ethanol, sodium nitrate (NaNO3), potassium permanganate (KMnO4), potassium tetrachloropalladate (K2PdCl4), titanium butoxide (Ti(BuO)4), hydrofluoric acid (HF, 48%), and poly(methyl methacrylate) (PMMA, 996 K) were purchased from Sigma-Aldrich. Methanol, ammonium hydroxide (NH4OH, 30% in water), hydrogen peroxide (H2O2, 30% in water), sulfuric acid (H2SO4), sodium hydroxide (NaOH), potassium hydroxide (KOH), chlorobenzene, chloroform, acetone, hexanes, isopropyl alcohol, and N-methylpyrrolidone (NMP) were purchased from Fisher Scientific. The 1,2di(9Z-octadecenoyl)-3-trimethylammonium-propane (chloride salt in chloroform; DOTAP) was purchased from Avanti Polar Lipids. Potassium tetrachloroplatinate (K2PtCl4) was purchased from Acros Organics. Milli-Q water was supplied by a Millipore system and used in all experiments. For all experiments, glassware was cleaned by 2088


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on the PMMA film containing Janus GO (i.e., on Pt|GO|TiO2|PMMA). The drop was aspirated after 5−10 s, which was enough time for the NMP to dissolve the PMMA top layer, thus dispersing the Janus GO in NMP. We call this method NMP drop transfer. Photodeposition of Pd onto Pt|GO|TiO2 Janus Graphene Oxide from Homogeneous Suspension. A glass vial containing a stir bar and 1.9 mL of a freshly prepared solution of K2PdCl4 (0.037 g L−1, 0.11 mM) in 1:4 v/v ethanol:water was placed in the photoreactor. A 10 μL portion of NMP solution containing Janus GO (prepared by drop transfer from 2 cm2 PMMA film) was then added to the K2PdCl4 solution. The photoreactor was thereafter placed under the light source (see above) and the headspace purged with argon. The suspension was irradiated for 80 min with stirring, and the sample was then drop-cast on a lacey carbon TEM grid. Symmetric Deposition of TiO2 onto GO (Producing TiO2-GO). The synthesis was adapted from published methods,42 and was carried out as described above for production of TiO2|GO|SiO2|Si, but with GO freely dispersed (0.03 g L−1) in reaction solution, rather than immobilized on a wafer. After reaction (5 h), the TiO2-GO sample was collected and washed, first with ethanol and then with water by repeated centrifugation at 8000 rpm. Finally, to crystallize amorphous titania, the sample was dispersed in 20 mL of water and transferred to a 45 mL Teflon-lined autoclave, which was heated to 180 °C for 13.5 h. The sample was then collected by centrifugation (8000 rpm) and dispersed in ethanol by sonication. Symmetric Photodeposition of Pt onto TiO2-GO (Producing Pt-TiO2-GO). The same protocol as for asymmetric photodeposition of Pt onto GO|TiO2 on PMMA film (producing Pt|GO|TiO2|PMMA) was used, but with TiO2-GO freely dispersed in reaction solution rather than immobilized on a PMMA film. In the photoreactor, TiO2-GO was dispersed in 55 mL of a freshly prepared solution of K2PtCl4 (0.045 g L−1, 0.11 mM) in 1:4 v/v ethanol:water; a stir bar was added, the photoreactor placed under the light source (see above), and the headspace purged with argon. The sample was irradiated for 1 h with stirring, and the Pt-TiO2-GO sample was collected by centrifugation and dispersed in ethanol. Preparation of Plan-View Lacey Carbon TEM (LC-TEM) Grids. A grid with Pt|GO|TiO2 Janus GO was prepared by NMP drop transfer from a Pt|GO|TiO2|PMMA film. After transfer, the grid was washed dropwise by 8 × 2 μL of NMP and 8 × 4 μL of acetone. Finally, the grid was immersed in acetone for 20 min and dried in a desiccator. A grid with symmetric Pt-TiO2-GO sample was prepared by drop-casting PtTiO2-GO dispersed in ethanol onto the grid. The grid was then dried in a desiccator. A grid with the Pt|GO|TiO2 Janus GO sample after photodeposition of Pd from homogeneous suspension was prepared by drop-casting the reaction solution onto the grid immediately after reaction. The grid was placed on a Kimwipe so that solution was efficiently sucked through the grid. All reaction solution (1.9 mL) was sucked through the grid in this fashion (using 50 μL aliquots). The LCTEM grid was then washed dropwise by 8 × 2 μL of 1:4 v/v ethanol:water, 8 × 2 μL of NMP, and 8 × 4 μL of acetone. Finally, the grid was immersed in acetone for 20 min and dried in a desiccator. Preparation of Pt|GO|TiO2 and Pt-TiO2-GO Cross-Sections for Electron Microscopy. After Janus GO synthesis, Pt|GO|TiO2 resides on a PMMA film (Pt|GO|TiO2|PMMA). The composite was transferred from the PMMA film to a SiO2|Si wafer by clamping the film to the wafer, with the composite facing the wafer. The PMMA film was removed in acetone vapor, thus producing TiO2|GO|Pt|SiO2|Si (method described schematically in Figure S5a,b). The TiO2|GO|Pt|SiO2|Si wafer piece was then immersed in acetone for 50 min to remove remaining PMMA. AFM was then used to identify a Pt|GO|TiO2 monolayer for cross-sectioning (Figure S5c). The height of a typical monolayer was ∼10 nm (Figure S5d) which correlates well with particle sizes measured by AFM after TiO2 deposition (Figure 3). In Figure S5c, patches with particles that are not attached to sheets are also observed, and these are likely TiO2 and/or Pt particles that deposited in the interstices between sheets during Pt|GO|TiO2 synthesis. Fortunately, identification of sheets was straightforward, because sheets were clearly visible in SEM, while particles that were not attached to sheets could not be seen (Figure S5e). By comparing AFM and SEM images (Figure S5c,e), we thus identified

for symmetric in situ growth of TiO2 nanocrystals onto GO.42 For asymmetric TiO2 deposition onto GO, we modified the protocol developed by Dai; instead of dispersing GO sheets freely in reaction solution, we immersed wafer pieces (22 × 88 mm) with predeposited GO. A precursor solution was prepared in a glovebox by mixing 0.18 mL of concentrated sulfuric acid and 12 mL of anhydrous ethanol, followed by 0.432 mL of titanium butoxide (Ti(BuO)4). The precursor solution can be prepared outside the glovebox, but we noticed that preparing it in the glovebox improves reproducibility. A second solution containing 141 mL of anhydrous ethanol and 9.4 mL of water was prepared in a flask. This solution was thoroughly mixed using magnetic stirring and a Teflon stir bar before the precursor solution was added, and the resulting solution was heated to reflux at 80 °C. After 20 min, the wafers with predeposited GO sheets were added to the reaction mixture. After the desired reaction time (1 1/2 h for ∼2−6 nm particles and 5 h for ∼5−15 nm particles), the wafers were taken out, dipped in two consecutive anhydrous ethanol baths, and then rinsed copiously in ethanol. The amorphous titania was further crystallized by hydrothermal steam treatment. First, 1 mL of water was added to a 45 mL Teflon-lined autoclave (Parr Instrument Company, model 4744); the wafer pieces were then added so that only a small fraction (bottom 2.5 mm) of the pieces was covered by liquid water. The autoclave was then heated to 180 °C for 13.5 h; the pieces were taken out, rinsed in ethanol and water, and dried under a nitrogen flow. Spin-Coating of PMMA Film onto TiO2|GO on Wafer To Produce PMMA|TiO2|GO|SiO2|Si. A solution of 4.8 wt % PMMA in chlorobenzene was prepared by dissolving 1.22 g of PMMA in 21.6 mL of chlorobenzene. A TiO2|GO|SiO2|Si wafer piece was attached to the spin-coater, and covered with a plastic (polypropylene, Nalgene) container. Inside this container, a saturated atmosphere of chlorobenzene was produced by introducing Kimwipes soaked in the solvent. Spin-coating solution was added through a septum in the Nalgene container to cover the wafer piece, and the spin-coater was immediately started (ramp rate, 3000 rpm s−1; spin rate, 2000 rpm; spin time, 60 s). The wafer was then baked at 170 °C for 5 min. This procedure produces a PMMA film of ∼280 nm thickness, as measured with height-mode AFM after making a scratch in the film (Figure 1i). HF Etch To Remove SiO2 Layer of PMMA|TiO2|GO|SiO2|Si and Expose Fresh GO Faces to Produce PMMA|TiO2|GO. Wafer dicing tape was attached to the PMMA film of PMMA|TiO2|GO|SiO2|Si (22 mm × 22 mm piece). To facilitate HF penetration in the SiO2 layer, PMMA was scraped from the edges of the wafer. The piece was then immersed in concentrated (48%) HF (200 mL) for 35 min, which removes the SiO2 layer and transfers the PMMA film to the dicing tape, thus exposing fresh GO faces while protecting TiO2 functionalized faces. (CAUTION! HF is extremely corrosive and toxic and should be handled with appropriate protective equipment.) The resulting tape| PMMA|TiO2|GO piece was washed by immersion in four beakers containing deionized water. Finally, the piece was attached to a support wafer, GO side facing up, with the aid of double-sided tape to facilitate AFM characterization and further processing. Asymmetric Photodeposition of Pt onto GO|TiO2 on PMMA Film To Produce Pt|GO|TiO2|PMMA. Our home-built quartz photoreactor was cleaned with hexanes, acetone, ethanol, and isopropyl alcohol. The PMMA|TiO2|GO (attached to support wafer) was placed on a glass stage inside the photoreactor, GO facing up. A Teflon stir bar and 55 mL of a freshly prepared solution of K2PtCl4 (0.045 g L−1, 0.11 mM) in 1:4 v/v ethanol:water were added, and the photoreactor was covered with a quartz lid and placed under the light source (150 W xenon arc lamp, Newport, Model 6255). A water filter to remove IR radiation was inserted into the light beam path, and the headspace of the photoreactor was purged with argon. The sample was irradiated for 1 h with stirring of the reaction solution. The sample (Pt|GO|TiO2|PMMA) was then washed by immersion in two consecutive water baths. The light intensity incident on the outer window (19 cm2) of the photoreactor was 177 mW cm−2. Collection and Redispersion of Pt|GO|TiO2 Janus Graphene Oxide by NMP Drop Transfer. Pt|GO|TiO2 Janus GO was transferred from the PMMA film into liquid dispersion by using N-methylpyrrolidone (NMP). Using a micropipette, a 1 μL NMP drop was placed 2089


Chemistry of Materials regions with Pt|GO|TiO2 monolayers and regions with particles not attached to any sheet. A protective layer of amorphous carbon was then deposited onto the selected Pt|GO|TiO2 monolayer (Figure S5f), and the cross-section was made (Figure S5g−j). Cross-sections of the PtTiO2-GO sample were made by first drop-casting the Pt-TiO2-GO sample onto SiO2|Si wafers from a dilute dispersion in ethanol. Crosssections were then made in the same fashion as described for the Pt|GO| TiO2 sample. All cross-sections were prepared by a focused ion beam (FIB, FEI Helios Nanolab 600i) system with the lift-out technique. Samples were thinned using a 30 kV and 5 kV Ga ion beam. Characterization Techniques. Atomic force microscopy height images were collected on a Veeco Multimode III system in tapping mode (Figures 1g,h and 3a−d). In addition, AFM height and phase images were collected on a Park NX-10 system in noncontact mode (Figures 1i−k and 2, and Figures S1, S2b,c, and S5c) . To follow one GO monolayer sheet from GO deposited on a SiO2|Si wafer to Pt|GO|TiO2 deposited on PMMA film (Figure 1), a mark was made by a wafer cutter in the SiO2|Si wafer, and locations of specific sheets were noted with respect to the mark. Raman point spectra and Raman microscopy maps were collected using a Horiba Xplora Plus confocal Raman microscope. Sufficiently low laser intensity must be used, as amorphous titania may crystallize under the laser otherwise. For point spectra, we used 100× objective, diffraction grating with 1200 gr mm−1, 100 μm slit, 300 μm pinhole, and a 532 nm laser, with filter set to pass 1% of laser light. We used 20 s acquisition time and 45 accumulations for each spectrum. These conditions did not lead to crystallization of amorphous titania, but generated sufficient signal to identify crystalline anatase after steam treatment. The settings for Raman maps were as follows: 3 × 3 μm pixels (laser spot size ∼0.5 μm), 100× objective, 532 nm laser with 10% filter, diffraction grating with 1200 gr mm−1, 100 μm slit, 300 μm pinhole, 1 s acquisition, 20 accumulations. Although this map condition crystallized amorphous titania, the point spectra confirmed that steam-treated titania was crystalline before mapping. To create maps, the spectrum in each pixel was normalized for intensity; baseline subtraction was applied, and peak areas were then used to create maps. TEM images were obtained by a spherical aberration corrected TEM (FEI Titan ETEM 80−300) operated at 80 kV. STEM-high- angle annular dark-field (HAADF)/STEM-EDS characterization was performed using a double aberration corrected TEM (FEI Titan Cubed Themis3) operated at 200 kV. It is equipped with CEOS DCOR image and probe correctors, highbrightness XFEG Schottky field emission gun, and a super-X detector for EDS. The beam with a convergence semiangle of 21 mrad with 0.5 nA was applied for STEM-EDS mapping.

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ACKNOWLEDGMENTS

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REFERENCES

A.H. acknowledges support from the Sweden−America Foundation and the Blanceflor Boncompagni Ludovisi, née Bildt Foundation and unrestricted funds. E.D.G. acknowledges support from the National Science Foundation Graduate Research Fellowship under Grant DGE-1656518. M.C. acknowledges support from the School of Engineering at Stanford University and from a Terman Faculty Fellowship. Part of this work was performed at the Stanford Nano Shared Facilities (SNSF, Stanford University), supported by the National Science Foundation under award ECCS-1542152 and at the nano@ Stanford laboratories, which are also supported by the National Science Foundation as part of the National Nanotechnology Coordinated Infrastructure under award ECCS-1542152.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b00087. Supporting discussion, characterization of Langmuir− Blodgett deposition of GO (isotherms and AFM images), additional electron microscopy characterization, characterization of cross-section preparation, additional characterization of Pd−Pt colocalization , and STEM image of cross-section after EDS mapping (PDF)

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*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Alexander Holm: 0000-0002-3660-4389 Matteo Cargnello: 0000-0002-7344-9031 Notes

The authors declare no competing financial interest. 2090


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Synthesis, Characterization, and Light-Induced Spatial Charge

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