Highly Efficient and Stable CO2 Reduction Photocatalyst with a

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Highly Efficient and Stable CO Reduction Photocatalyst with a Hierarchical Structure of Mesoporous TiO on 3D Graphene with Few-Layered MoS 2

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Hyunju Jung, Kyeong Min Cho, Kyoung Hwan Kim, HaeWook Yoo, Ahmed Al-Saggaf, Issam Gereige, and Hee-Tae Jung ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00002 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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ACS Sustainable Chemistry & Engineering

Highly Efficient and Stable CO2 Reduction Photocatalyst with a Hierarchical Structure of Mesoporous TiO2 on 3D Graphene with Few-Layered MoS2 Hyunju Jung †, Kyeong Min Cho †, Kyoung Hwan Kim †, Hae-Wook Yoo ‡, Ahmed Al-Saggaf§, Issam Gereige§, and Hee-Tae Jung *† †

Chemical and Biomolecular Engineering Department & KAIST Institute for Nanocentury,

Korea Advanced Institute for Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon, Korea ‡

The 4th R&D Institute, Agency for Defense Development, Yuseong-gu, Daejeon 305-600,

Korea §

Saudi Aramco, Research and Development Center, Dhahran 31311, Saudi Arabia

E-mail address: [email protected] Mailing address: National Research Lab., for Organic Opto-Electronic Materials, Department of Chemical and Biomolecular Eng. Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea

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Abstract The development of photocatalysts of CO2 reduction based on stable and earthabundant materials is essential for utilizing solar energy and storing it in chemical forms. Here we report the synthesis and characterization of a composite material consisting of a few layers of MoS2 on a hierarchical porous structure of mesoporous TiO2 and macroporous 3D graphene aerogel (TGM) as a high-performance, robust, noble-metal-free photocatalyst of CO2 reduction. The hierarchical structure contributed to the high photocatalytic catalyst performance, which was investigated by controlling the morphologies of the mesopores and macropores. By optimizing the relative amounts of each component and the configuration of the composite, a TGM system was fabricated. The resulting TGM showed a lower extent of charge recombination and a higher photocurrent density, and hence a higher CO photoconversion rate (92.33 µmol CO /g·h), than those of other composite combinations, i.e., bare TiO2, TiO2-graphene, TiO2-MoS2 and TiO2-graphene-multiple layered MoS2. Also, the role of each components and the underlying mechanism in the catalysis of the reaction by TGM were investigated. The long-term stability of the TGM composite was tested and compared with that of a TiO2-graphene-Ag composite. Over the course of 15 cycles, TGM composite retained its original conversion rate while the activity of the TiO2-graphene-Ag composite decreased. The hierarchical porous structure with mesoporous TiO2 and a few layers of MoS2 on macroporous 3D graphene is expected to have great potential as an affordable, robust, high-efficiency CO-selective photocatalyst of CO2 reduction.

Keywords: CO2 Reduction, 3D graphene, MoS2, Photocatalyst, Nanocomposite

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Introduction Artificial photosynthesis represents an ideal approach to reducing the ever-increasing atmospheric levels of carbon dioxide (CO2)1 resulting from emissions and providing future energy supplies by storing solar energy in chemical forms.2 A widely applicable artificial photosynthesis system should be based on inexpensive and earth-abundant materials and should display high efficiency and stability.3 To date, various photocatalysts based on semiconductors such as titanium dioxide (TiO2), cadmium sulfide (CdS) and zinc oxide (ZnO) have been widely studied to capture and convert CO2 into valuable organic fuels such as methane, carbon monoxide and methanol. A large input of energy is essential for breaking the C=O bonds of CO2 molecules adsorbed on the semiconductor surface in order to produce short-chain hydrocarbon fuels. Electron-hole pairs become generated in semiconductors exposed to photons with energy levels equal to or higher than their band gaps. While some photo-generated carriers separately migrate to the surface of the semiconductor to lead to the desired redox reactions, most of the carriers are dissipated through such charge recombination on the surface and in the bulk of the semiconductor. These recombination processes limit the efficiency of the photocatalytic reaction.4 In this regard, combining a photocatalyst with a secondary material such as a metal, other semiconductor, carbon material or biological enzyme improves the photocatalytic activity. These secondary co-catalytic materials are used to accept the photo-excited electrons and prevent the charge recombination acting as a sink for electrons and holes, and thus they improve catalytic conversion efficiency.5 In numerous cases, inclusion of a noble metal such as Pt, Ag, or Au as a co-catalyst in a semiconductor-based photocatalyst has been found to substantially enhance the catalytic performance.6 However, their high price and degradation in performance resulting from contamination of their surfaces have been obstacles to achieving an affordable and recyclable photocatalytic system.7 Therefore, the development of a scalable and recyclable photocatalyst is still required for future practical uses.8 3



As alternatives to the noble metals, layered transition metal disulfide materials (TMDs) such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2) have been drawing increasing attention because of their high stability, low price, and promising catalytic features.9 Asadi et al. showed the outstanding CO2 electro-conversion performance of MoS2, in particular showing a high current density and low overpotential compared with those of the noble metals.10 Recent studies indicated that S atoms on exposed edges of MoS2 have strong bonding energies, which increase the hydrogen evolution reaction activity of the surface.11 In addition, a previous study showed that the band gap of a layered MoS2 system depends on the number of layers, with a system comprised of a few layers showing a wider band gap than the bulk state, and hence a higher reactivity.12 Thus, if MoS2 is to be developed as a catalytic material for CO2 photo-conversion, its contribution to this process, and the dependence of its catalytic effect on its morphology, should be elucidated. In past studies, few-layered MoS2 has been mainly obtained by chemical vapor deposition,13 mechanical and chemical exfoliation,14 and thermal ablation. However, these approaches are characterized by low production yields, long and complicated processes, and loss of the pristine semiconductor character of MoS2 during Li intercalation, all of which hinder the extensive use of MoS2.15 The poor electric conductivity of MoS2 also restricts its co-catalytic activity.16 Its electrical conductivity and catalytic activity were improved by adding other conductive materials. In particular, graphene and reduced graphene oxide (rGO) show novel properties including excellent conductivity, robust mechanical and chemical strength, and a unique twodimensional (2D) structure, and have hence been combined with semiconductors to enhance their photocatalytic abilities.17-18 In the current work, we used a simple one-pot hydrothermal method to synthesize a highly efficient and stable CO2 photocatalyst with a hierarchical porous structure consisting of mesoporous TiO2 and a few layers of MoS2 on 3D graphene. The TiO2 used in this study is one of the most commonly used photocatalysts because of its high photoactivity,19 cost4


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effectiveness, high stability under photocatalytic conditions,20 and high oxidation power with respect to the water-splitting reaction.21 Moreover, mesopores on the surfaces of the TiO2 particles enlarge the reactive surface area.22 The self-assembled 3D graphene aerogel with irregular pores not only endowed the otherwise poorly conductive TiO2 and MoS2 with excellent conductivity, but also formed a macroporous structure offering efficient mass transport paths, robust mechanical/chemical stability, and a large surface area.18 The formation of a 3D structure from 2D graphene sheets may have reduced the channel length of the conducting path and shortened the diffusion length for photogenerated carriers.23 MoS2, a representative 2D semiconducting TMD material, was used as a co-catalyst due to its high robustness and layer-dependent activity. In particular, in this synthesis, two to three layers of MoS2 formed simultaneously with TiO2 and 3D graphene during a simple one-pot process. The performance of our photocatalyst, bearing a graphene electron-transfer channel and two semiconductors, apparently benefited from the synergic effects of the hierarchical porous structure and these three components. Structurally, the assembly offered a large surface area, and efficient mass transfer. Photochemically, the assembly dramatically decreased the charge recombination rate and enhanced the conversion efficiency.

Results and Discussion Our catalyst was fabricated simply by hybridizing TiO2 nanoparticles, graphene aerogel, and MoS2 nanosheets, denoted TGM, in a one-pot hydrothermal reaction (Figure 1). Graphene oxide (GO) was prepared by oxidizing graphite using a modified Hummers method.24 Titanium sulfate (TiO2 precursor, Kanto Chemical Co., Inc.), the MoS2 precursors (sodium molybdate and L-Cysteine, Sigma-Aldrich), and glucose were mixed with a graphene oxide (GO) solution, and the mixture was transferred to a Teflon-lined autoclave reactor. During thermal heating in the autoclave, the hierarchical structure formed without additional treatments. 3D graphene was generated through the self-assembly of physically 5



interconnected reduced GO (rGO) sheets. As the GOs were reduced under high temperatures25 or via chemical reductants, such as SH2 from L-cysteine,26 the hydrophilic groups in GO, such as the –OH or –COOH groups, detached and their hydrophobicity increased, leading to the formation of 3D graphene. At the same time, mesoporous TiO2 nanoparticles and few-layered MoS2 structures were uniformly grown in situ on the surface of the 3D graphene. Glucose played an important role in morphological control.27 During the hydrothermal reaction, the glucose formed amorphous carbon that inhibited the growth of TiO2 and MoS2 by attaching mainly to the (001) facet of TiO225 and the c-axis of MoS2.28 After the hydrothermal reaction, the hybrid composite was freeze-dried to retain the 3D porous structure during residue removal, followed by annealing in an Ar atmosphere to remove carbon residue from the glucose and to enhance the crystallinity of the MoS2 and TiO2. The nanostructure remained after thermal annealing. This one-pot synthesis has two significant advantages: the procedure is simple compared to other fabrication methods involving MoS2 delamination,29 and more importantly, the solution-based synthesis is amenable to mass production for future usages.

Figure 1. Fabrication process of TiO2-graphene-MoS2 composite through simple one-pot hydrothermal method.

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Scanning electron microscopy (SEM) images of TGM revealed that the 2D rGO sheets assembled into macroporous 3D structures with a pore size of 15–20 µm (Figure 2a). The transmission electron microscopy (TEM) image shown in Figure 2b reveals that the mesoporous TiO2 nanoparticles (with an average diameter of 15 nm) and the MoS2 sheets were uniformly distributed over the 3D graphene surface. The nitrogen physisorption isotherms shown in Figure S1 support the generation of mesoporous TiO2 particles on the 3D graphene, with a type IV hysteresis curve corresponding to porous materials 3–4 nm pore sizes, according to the IUPAC classification.30 Note that in the absence of glucose, the TiO2 particles were not well-dispersed over the 3D graphene, the mesoporous did not form on its surface, and the particle size distribution was broad (Figure S2). That was because the hydroxyl groups in glucose formed linkers that controlled the TiO2 crystal morphology by binding to both the TiO2 facets and to the graphene surface during the growth of TiO2. Thus, the presence of the glucose linkers between TiO2 and graphene enhanced the TiO2 dispersity and inhibited the growth of TiO2 at one face, thereby producing a mesoporous surface.27 High-magnification TEM images of the catalyst surface indicated the presence of a 0.35 nm interlayer spacing in the TiO2 particles, which corresponded to the (101) lattice spacing of the anatase TiO2 phase (Figure 2c). A few (mostly one or two) MoS2 sheets were clearly visible with a 0.64 nm interlayer spacing corresponding to the distance between (002) planes in the MoS2 layers,31 as shown in Figure 2c. The good dispersion of the MoS2 sheets and TiO2 particles across the 3D graphene surface was further clarified using energy dispersive X-ray spectroscopy (EDX) analysis (Figure 2d). EDX elemental mapping (C, S, Mo, and Ti) of the hybrid composite revealed the positions of MoS2 (Mo and S) and TiO2 (Ti) grains on the graphene (C) surface. These crystals were uniformly grown with a high dispersity across the graphene sheet. The crystals are presented as colored dots corresponding Mo and S at different positions on Ti. These characteristics

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indicate that the TGM consisting of mesoporous TiO2 and a few layered MoS2 sheets (nanostructure) on macroporous 3D graphene has a hierarchical porous structure. Nano-structure of our catalyst was further verified by X-ray diffraction (XRD) and Raman spectroscopy. The XRD diffraction peaks (Figure 2e) at 2θ values of 25.2°, 37.9°, 47.8°, 54.1°, 54.9°, 62.6°, 70.1°, and 75.1° are indexed to the (101), (004), (200), (105), (211), (204), and (215) plane of the anatase TiO2, respectively, confirming anatase phase TiO2 (■). The XRD diffraction peaks at 14.2°, 33.5°, and 59.3° are indexed to the (002), (100), and (110) planes of MoS2 (□) respectively. The peak intensity of (002) plane of MoS2 is much weaker than that of bulk MoS2, indicating a few layered MoS2 stacked along the (002) direction.28 The formation and chemical structure of 3D graphene were determined from the Raman spectral peaks at 1343 cm–1 and 1595 cm–1, which corresponded to the D and G bands of graphene (Figure 2f). The peaks at 155 cm–1 (Eg), 208 cm–1 (Eg), 399 cm–1 (B1g), 507 cm–1 (A1g + B1g), and 622 cm–1 (Eg) reconfirmed the presence of anatase phase TiO2. Note that the Raman spectra of MoS2 were not visible because the MoS2 spectra at 377 cm–1 and 402 cm–1 32 overlapped with the intense 399 cm–1 peak of TiO2 (Figure S3). The chemical structure of the 3D graphene was further identified by comparing the X-ray photoelectron spectroscopy (XPS) analyses of GO and TGM (Figure S4 and S5), in which GO was successfully reduced by removing oxygen involved bonds during the hydrothermal reaction. The ternary structure with anatase TiO2, few-layered MoS2, and 3D graphene could be fabricated via a simple one-pot reaction.

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Figure 2. . Morphology of the TiO2-graphene-MoS2 (TGM) catalyst. (a) the SEM image of macroporous TGM surface. (b) the TEM images of TGM with TiO2 nanoparticles with porous surface and MoS2 layers on graphene. The inserted low resolution TEM image shows highly dispersed TiO2 particles on catalyst surface. (c) HRTEM images of TGM indicating nano-sized pores on TiO2 particles, and lattice distance of TiO2 and MoS2 nanocrystals. (d) EDX mapping of elements; C, S, Mo, and Ti. (e) XRD pattern and (f) Raman shift of TGM.

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The roles of each component in our catalyst were investigated by measuring the charge transport properties of the four different hybrid composites: i) the single photocatalyst (bare TiO2), ii) the photocatalyst and an electron transfer channel material (TiO2 – 3D graphene: TG), iii) the photocatalyst and the co-catalyst (TiO2 – MoS2: TM), and iv) the photocatalyst and the co-catalyst with an electron transfer channel (TiO2 – 3D graphene – MoS2: TGM). Figure S6 shows each morphologies of the bare TiO2, TG and TM. The charge separation properties of the four different catalysts were characterized by collecting the photoluminescence (PL) spectra and photocurrent density measurements, as shown in Figure 3. The PL intensity of bare TiO2 was much higher than that of the other composites, indicating a high charge recombination rate at TiO2 surface (Figure 3a).33 Combining TiO2 with MoS2 (TM) formed a heterojunction and the excited electron-hole pairs was separated at the interface,34 resulting in a remarkable decrease in PL intensity. Incorporation of the 3D graphene into TiO2 (TG) and TiO2-MoS2 (TGM) also significantly reduced the PL intensities, indicating that the addition of the graphene carrier transfer channel resulted in the effective separation of photo-induced carriers due to the high conductivity of graphene.35 The low PL intensity of TGM compared to TG may have arisen from the fact that the MoS2 layer of TGM acted as a co-catalyst to reduce backward charge flow from graphene to TiO2 by presenting an alternative pathway for electrons to move from TiO2 to MoS2.36 Figure 3b plots the photocurrent density of the composites as a function of the Xe lamp irradiation time. As was observed with the PL results, the 3D graphene improved exciton separation and transfer to the electrode. The photocurrent density of TG was much higher than that of the bare TiO2. On the other hand, the photocurrent density of TM was lower than that of the bare TiO2, despite the low recombination rate in TM as compared to the bare TiO2 (Figure 3a). Since the sample of this article has a higher MoS2-TiO2 ratio (1:5 molar ratio) than other researches,36, 37 the less of contents of TiO2 per unit exposed area decreased the photocurrent density of TM. The highest current density was observed in TGM and was 10 and 1.5 times higher than the 10


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current densities measured in bare TiO2 and TG, respectively. Based on the PL and photocurrent data, the combination of the two semiconductors (TiO2 and MoS2) and the 3D graphene significantly improved charge transfer and delayed the recombination rate by precisely controlling the carrier flow from TiO2 via graphene to MoS2. This effect was verified by the CO2 conversion results.

Figure 3. Photocatalytic activities of the four different composites: TGM, TG, TM and bare TiO2. (a) PL spectroscopy. The TiO2 of the four catalysts shows the emission peak in a range of 450-470 nm appeared in the spectrum. (b) photocurrent density measured at 300 W Xe lamp illumination for 10 sec and covered for 10 sec, periodically. Each graph represents the TGM, TG, TM, and TiO2 as a red, blue, green, and black line respectively.

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CO2 conversion tests were performed in a stainless steel chamber with a quartz window and were monitored using gas chromatography, as shown in Figures 4 and 5. All samples exhibited a higher selectivity (ca. 97%) for CO2 reduction to yield CO as compared to other carbohydrates, such as methane or propane (Figure S7). The optimum ratio of TiO2 to graphene was found to be obtained using 1 ml of Ti(SO4)2 solution to 40 mg of GO (Figure S8). It can be inferred in Figure S8 that the excessive or small amount of graphene relative to the catalyst could abate the catalytic performance, which is explained in detail in the supporting data. The blank test was performed by filling the reactor containing the catalyst with only nitrogen, and no significant product was found. The CO formation rate in the presence of TGM was compared with the formation rate in the presence of controls, such as bare TiO2, TG, and TM samples (Figure 4a). The single photocatalyst (TiO2) provided an inefficient catalytic activity (6.37 µmol/g·h). The incorporation of an electron channel component (3D graphene) into TiO2 significantly enhanced the conversion rate (45.05 µmol/g·h). The introduction of only MoS2 onto TiO2 without providing an electron transfer channel could be used to determine the CO formation rate (4.49 µmol/g·h). Interestingly, the highest production yield was obtained in the presence of two photocatalysts connected via an electron channel material (TGM). An outstanding CO formation rate (92.33 µmol/g·h) was obtained and was 14.5 times the CO formation rate obtained on the bare TiO2 sample. The distinguished CO formation rate of TGM was attributed to two reasons: First, similar to the photocurrent investigation, the use of an electron channel material was important for enhancing the CO2 conversion rate because the conductive properties of graphene effectively separated photoinduced electron–hole pairs and improved charge transfer, which was demonstrated by comparing with the conversion rates of the bare TiO2 and TGM/TM. In the absence of an electron transfer channel, for instance, TM provided a low CO2 conversion rate as a result of the poor photocurrent and low redox potential of the TiO2-MoS2 heterojunction. 12


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Secondly, the presence of the few-layered MoS2 sheets as co-catalyst was attributed to the outstanding CO formation rate of TGM. The high CO2 conversion rate in the two photocatalysts and the presence of an electron channel, as compared to the conversion rate measured in a system comprising a single photocatalyst and electron channel, indicated that TGM provided distinct electron flow pathways. In TGM, the majority of photoinduced electrons moved through the TiO2–graphene–MoS2 in sequence (Figure 4b). The photoexcited electrons in the conduction band (CB) of TiO2 transferred through graphene to the edges of MoS2 in which adsorbed CO2 molecules were reduced to form CO,36 while the transfer of photoexcited electrons in TiO2 to MoS2 produces holes in the TiO2 valance band that oxidize water to form oxygen. The spatial separation of the photoinduced electrons and holes increased the potential of the oxidizing holes and reducing electrons and significantly improved charge transfer. In addition, the high concentration of electrons between the MoS2 layer and the graphene layer could exceedingly enhanced the electronic conductivity of composites,38 and therefore the co-catalytic activities of composites are improved significantly. The efficient reaction flow in TGM was constructed by thinning the MoS2 layers, which was demonstrated by investigating the catalytic performances of TGMs prepared in six different the MoS2–TiO2 ratios (MoS2:TiO2 molar ratio = 2:1, 1:1, 1:3, 1:5, 1:8, 1:10) with fixed the amounts of TiO2 precursor and graphene (Figure 4c). The low MoS2:TiO2 molar ratios (1:8 and 1:10) samples showed CO formation rates similar to that obtained from TG due to the absence of MoS2 on the composites. Samples containing MoS2 precursors in ratios less than 1:5 did not form MoS2 (Figure S9) due to the decrease of the probability of encountering between Mo and S elements in solution during MoS2 formation reaction. The 1:5 TGM sample, used for foregoing experiments, exhibited the highest activity toward photocatalytic CO2 reduction and provided 1-2 layers on MoS2. As in many reports, the band gap of MoS2 became lager as decreasing the layer numbers due to quantum confinement 13



effects39 to the point at which the band gap moved to negative potential.36, 40 Thus, the fewlayered MoS2 have more active edge which shorten the path of electron transfer and the unsaturated S atom was increased on exposed edges of nano sized MoS2, which enhanced its activity for CO2 reduction. On the other hand, the high MoS2:TiO2 molar ratios (2:1 and 1:1) yielded CO formation rates that were even lower than that of TG due to the unsuitable reduction potential of MoS2 for CO2 conversion. As the MoS2 content increased, the MoS2 sheets tended to grow thicker over 5–6 layers (Figure S10), the quantum confinement effects were diminished, and the MoS2 CB position was located lower than the CO2 reduction potential, as shown in Figure S11. The band gap and CB position in this system were unsuitable for catalyzing the reaction and hindered the catalytic activity, although the photoinduced charge transfered from a CB of TiO2 via graphene to a CB of MoS2. Surprisingly, our catalyst structure was found to be quite stable. Long-term stability is one of the most important issues in photocatalyst research. Natural leaves comprise highly efficient systems that are stable over the long term. By contrast, synthetic systems comprising metal-based CO2 photocatalysts, such as Pt, Ag, Au, Cu or their hybrids, have displayed poor stabilities, with a decline in the catalytic performance after even a few cycles due to surface oxidation by photoinduced holes and oxygen species41 as well as changes in the surface states. 42, 43

The stability was examined by measuring the CO formation rates in the presence of TGM,

and TG with Ag nanoparticles (Figure 4d). In this experiment the Ag was used as a CO2 reduction cocatalyst because it has been reported to provide the highest selectivity toward CO.44 The Ag nanoparticles were synthesized by photodeposition onto TG in a molar ratio equal to the MoS2:TG molar ratio in the TGM (Figure S12). The test results revealed that TGM remained stable even after 15 cycle tests without any weight loss, whereas the CO formation rates of TG with the Ag particles dramatically decreased over the reaction cycles. The Ag–TG system’s CO formation rate was far below the form rate obtained from TGM,

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except during the first and second cycle. The high stability of the TGM further supports the use of TGM as an affordable CO2 photocatalyst.

Figure 4. CO formation rate and stability of various hybrid composites. (a) CO formation rate of TGM, TG, TM and TiO2. (b) Possible CO2 conversion mechanism of the TGM. (c) CO formation rate as a function of MoS2:TiO2. Inset is TEM image of TGM with 2:1 ratio. (d) Stability tests of TGM (red bars) and TG with Ag particles (black bars) during 15 same reaction cycles.

The significance of hierarchical 3D structure on the catalyst performance was investigated by comparing the CO formation rates of TGM with three different structures (Figure 5): i) TGM with macroporous 3D graphene and mesoporous TiO2 (TGM). ii) TGM with macroporous 3D graphene and non-mesoporous TiO2 (non-meso TGM) (Figure S2b). iii) TGM with non-3D graphene and mesoporous TiO2, consisting of stacked two-dimensional graphene sheets (non-macro TGM). Although all samples were prepared using the same TGM fabrication procedure, the non-meso TGM was prepared without glucose to prevent the 15



formation of mesopores on the TiO2 surface, and the non-macro TGM omitted the freeze-dry step, thereby completely collapsing the macrostructure. Figure 5a-5c show the morphological structures of the samples. The TGM and non-meso TGM samples displayed similar macrostructures (Figure 5a–5b) with different morphologies and distributions of TiO2 and MoS2 particles on the graphene sheets (Figure S2). The non-macro TGM formed densely aggregated structures (Figure 5c). A Brunauer–Emmett–Teller (BET) analysis of the nitrogen adsorption/desorption isotherms indicated that the specific surface area (124.31 m2g–1) of TGM was much higher than that of the non-macro TGM (90.93 m2g–1). As a result, the CO formation rate (92.33 µmol/g·h) of the TGM was ca. 1.3 times that of the non-meso TGM (70.09 µmol/g·h), and ca. 3.4 times higher than that of the non-macro TGM (27.09 µmol/g·h) (Figure 5d). The formation rate of the non-meso TGM was lower than that of TGM, because the absence of mesopores on TiO2 surface decreased accessible surface area and shortened the electron diffusion length. In addition, the absence of glucose linker diminished co-catalytic activity by hindering the formation of few-layered MoS2 and caused the poor distribution of TiO2 and MoS2 particles on graphene sheets. Although both macroscopic and microscopic morphological control was attributed to improvement of CO yield, the data indicated that the macroporous structure contributed to a greater degree to the catalytic performance. That is because the 3D structure prevented the nature of aggregation phenomena of 2D materials,45 and enhanced light harvesting via multiple reflections from the randomly arranged porous structures,46 and improved the efficiency of mass flow to facilitate the diffusion and adsorption of CO2 across a large surface area.47 The photoinduced charges moved stereoscopically within the 3D graphene network and underwent efficient charge transfer compared to the electrons and holes confined within the 2D TiO2-MoS2 sheets. The hierarchical structure comprising the macroporous 3D graphene, the mesoporous TiO2, and MoS2 particles enhanced the catalyst performance in the context of CO2 conversion.

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Figure 5. SEM images of TGMs with differnet morphological sturctures. (a) the TGM having both macro- and micro structures. (b) the non-meso TGM having macroporous structure but mesoporous structure. (c) the non-macro TGM without 3D macroporous structure. (d) CO formation rates of the TGM, the non-meso TGM, and the non-macro TGM. Unit of the yield is µmol/g·h.

Conclusion In summary, we designed a highly efficient and stable photocatalyst that applied the hierarchical porous structure and effective conversion system to photoreduce CO2. The photocatalytic activity and CO2 conversion results revealed that the structure and photophysical properties of the system provided synergic improvements in the photocatalytic performances. The hierarchical structure increased the surface area, offering abundant reaction sites and efficient mass flow pathways at the randomly arranged porous backbone structure. The electron flow from TiO2 via graphene into the few-layered MoS2 could effectively lower the charge recombination rate and increase the potential for CO2 reduction. 17



The TiO2, graphene, and MoS2, all of which are stable and abundant materials, effectively contributed to the stability of the catalyst over many reaction cycles. Our noble metal-free photocatalyst offers a high CO selectivity of 97% and a high CO production yield (93.22 µmol/g·h) 14.5 times that of bare TiO2, and is resistant to photodegradation. This novel CO2 catalyst, TGM, shows promise as a scalable high-performance non-metal CO2 conversion catalyst.

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Methods Synthesis of Graphene Oxide: Natural graphite powder was oxidized to graphite oxide using the modified Hummers method. 24

A mixture of 1 g peroxidized graphite and 70 mL H2SO4 (98%, Sigma-Aldrich) was heated

in a 35 °C oil bath. Three and a half grams of KMnO4 (≥99.0%, Sigma-Aldrich) were then added slowly to prevent significantly increasing the temperature. After 4 hour stirring, the oil bath was replaced with an ice bath. Two hundred milliliters of D.I. water were poured very slowly over the mixture and 150 mL H2O2 were added. The suspension was filtered and redispersed in a HCl (10%, Sigma-Aldrich) solution. Finally, the solution was filtered overnight and dried in a vacuum oven.

Synthesis of the TGM: Graphene oxide (GO) was produced using the modified Hummers method24 and was dispersed to form a 20 mL aqueous solution (2 mg/mL). The solution was sonicated for 30 min after stirring for 5 min, and 48 mg Na2MoO4 (≥98%, Sigma-Aldrich), 60 mg L-cysteine (≥97%, Sigma-Aldrich), 240 mg Ti(SO4)2 (≥24%, Kanto Chemical Co., Inc) and 10 mg of glucose (≥99.5%, Sigma-Aldrich) were added with stirring. The mixture was transferred to a 100 mL Teflon-lined stainless steel autoclave and heated to 220 °C for 24 hr. A sponge-like black hydrogel compound was obtained after cooling under ambient conditions and was thoroughly rinsed in deionized water and ethanol. The sample was then freeze-dried and annealed at 400 °C for 3 hour under an Ar atmosphere to increase the crystallinity. The control samples, such as TG, TM, and bare TiO2, were synthesized using the same procedure with substitution of the appropriate precursors.

Synthesis of TG with Ag nanoparticles:

19



Ag nanoparticles were photodeposited onto the prepared TG samples. Twenty milligrams of TG were dispersed in 40 mL water. AgNO3 (≥99.0%, Sigma-Aldrich) was added with vigorous stirring in a molar ratio equal to that of MoS2 in TGM (MoS2: TiO2 = 1:5), with 50% TiO2 in TG. The samples were irradiated under 300 W Xe lamp illumination for 1 hr to deposit the Ag nanoparticles onto the TG surface. The black precipitate was washed with water and ethanol and then dried via freeze-drying.

Photocurrent Measurements: Photocurrent measurements were carried out on an electrochemical workstation using a conventional three-electrode system consisting of a Pt wire counter electrode, an Ag/AgCl reference electrode, and a catalyst paste on ITO glass as the working electrode. The electrolyte consisted of a 0.5 M Na2SO4 (≥99.0%, Sigma-Aldrich) solution. The catalyst pastes were fabricated by mixing the ground catalyst with a polymer binder (PVDF, 20 wt% of the paste, Sigma-Aldrich) in an NMP solution (99.5%, Sigma-Aldrich). The area and thickness of the catalyst paste applied to the ITO glass were controlled using the doctor-blade method. After baking for 3 hr in an 80 °C oven, the working electrode was exposed to 300 W Xe lamp illumination for 10 sec and covered for 10 sec, periodically.

CO2 photoconversion Tests: The photocatalytic CO2 conversion reactions were carried out in a stainless steel reactor with a quartz window positioned at the top of the reactor. The 300 W Xe lamp light source was positioned perpendicular to the reactor. The photocatalyst was placed on the stage of the reactor with 4 mL D.I. water. The chamber was then purged with CO2 gas for several hours at 40°C. After tightly sealing the reactor, the catalyst was exposed to light for 4 hr. A gas chromatography instrument (GC, Agilent 7890GC) was connected to the reactor and used to measure the composition of the gases. 20


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Characterizations: The prepared samples were characterized with a scanning electron microscope (SEM, Sirion FE-SEM, FEI), a transmission electron microscope (TEM, Philips Technical F30), an X-Ray diffractometer (RIGAKU, D/MAX-2500), an X-Ray photoelectron spectroscope (XPS, Thermo VG Scientific Sigma Probe), N2 adsorption was carried out at 77 K using a micromeritic ASAP 2000, Raman spectroscopy (ARAMIS), energy dispersive X-ray spectroscopy (EDX; GENENIS-4000).

Associated Content Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Additional SEM and TEM images, Raman, XPS data are included.

Author Information Corresponding Author * E-mail : [email protected] Present Addresses National Research Lab., for Organic Opto-Electronic Materials, Department of Chemical and Biomolecular Eng. Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea Notes The authors declare no competing financial interest

Acknowledgements This work was funded by Saudi Aramco-KAIST CO2 Management Center. In addition, this research was supported by a grant from the National Research Foundation of Korea funded by the Ministry of Science, ICT, and Future Planning (grant no. 2015R1A2A1A05001844). 21



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For Table of Contents Use Only

Synopsis This research proposes a practical and sustainable approach to reduce CO2 in air with a high effective and stable noble metal-free photocatalyst.

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Highly Efficient and Stable CO2 Reduction Photocatalyst with a

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