LDH Architecture: Tailored Structure for Visible

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3D Yolk@Shell TiO2-x/LDH Architecture: Tailored Structure for Visible Light CO2 Conversion Abolfazl Ziarati, Alireza Badiei, Rossella Grillo, and Thomas Burgi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17232 • Publication Date (Web): 16 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

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ACS Applied Materials & Interfaces

3D

Yolk@Shell

Tailored

Structure

TiO2-x/LDH

Architecture:

for

Light

Visible

CO2

Conversion Abolfazl Ziarati,†, ‡ Alireza Badiei, †,* Rossella Grillo, ‡ Thomas Burgi ‡,* †School

of Chemistry, College of Science, University of Tehran, Tehran 1417614418, Iran

E-mail: [email protected] ‡Department

of Physical Chemistry, University of Geneva, 30 Quai Ernest-Ansermet, 1211

Geneva 4, Switzerland. E-mail: [email protected] Keywords: CO2 conversion, Solar fuel, Nanoarchitectures, Photocatalysis, Oxygen vacancy

Abstract CO2 photo-conversion into hydrocarbon solar fuels by engineered semiconductors is considered as a feasible plan to address global energy requirements in times of global warming. In this regard, three dimensional yolk@shell hydrogenated TiO2/Co-Al layered double hydroxide (3D Y@S TiO2-x/LDH) architecture was successfully assembled by sequential solvothermal, hydrogen treatment and hydrothermal preparation steps. This architecture revealed a high efficiency for the photo-reduction of CO2 to solar fuels, without a noble metal co-catalyst. The time dependent experiment indicated that the production of CH3OH was almost selective until 2h (up to 251 µmol/gcat. h.), whereas the CH4 was produced gradually by increasing the time of reaction to 12h (up to 63 µmol/gcat. h.). This significant efficiency can be ascribed to the engineering of 3D Y@S TiO2-x/LDH architecture with considerable CO2 sorption ability in mesoporous yolk@shell structure, and LDH interlayer 1 ACS Paragon Plus Environment

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spaces. Also, oxygen vacancies in TiO2-x could provide excess sites for sorption, activation and conversion of CO2. Furthermore, the generated Ti3+ ions in the Y@S TiO2 structure as well as connecting of structure with LDH plates, can facilitate the charge separation and decrease the band gap of nanoarchitecture to the visible region.

Introduction The world’s increasing population and prompt growth of economy and industry comes along with huge amount of fossil fuel combustion. The resulting increase of carbon dioxide (CO2) concentration on earth contributes to the greenhouse effect. Different strategies are proposed at mitigating CO2 emissions including carbon recycling used in energy supply.1-2 Comparable with the photosynthesis in nature, photo conversion of CO2 to hydrocarbon solar fuels through extracting protons and electrons from water molecules can be the promising method to realize a sustainable carbon-neutral society.3-4 The main challenge in this respect is production of appropriate, stable, and effective photocatalytic materials. 5-6 Most reported semiconductors are not suitable for the CO2 conversion in visible light due to insignificant efficiency. Moreover, the rapid charge recombination rate created by undesirable band position critically limits their utilization, that is an important restriction to be resolved as well.3,

7

Hence, to supply the requirements of CO2 conversion, the expansion of novel

photocatalysts that efficiently use the solar energy spectrum and with an enhanced charge separation performance for maximum CO2 conversion remains a tremendous challenge.8 In order to improve the CO2 conversion efficiency, an applicable method is the construct of engineered hybrid semiconductors with high performance of visible light utilization.9-14 Lately, study on the advanced semiconductors is considered to multi-dimensional materials because of their excellent photoactivity and high surface area.15-17 Layered double hydroxides (LDHs), have attracted great attention as photocatalysts due to their activity in visible region and high quantum yield.18-19 Nevertheless, pure LDHs commonly suffer from difficult adjust over 2 ACS Paragon Plus Environment

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structure, morphology and architecture, which limits the efficiency of photo-conversion and charge separation performance.20 Given that titanium dioxide (TiO2) can employ as a proper acceptor for photo-generated electrons,21 the combining of LDHs with TiO2 would provide an appropriate method to enhance their photocatalytic efficiency and charge separation ability. In this regard, Lee et al. have prepared P25@CoAl layered double hydroxide through hydrothermal synthesis in one step with good photo-activity for the CO2 reduction to CO.22 Han et al. have synthesized TiO2@LDH core–shell nanospheres for O2 generation with considerable light harvesting efficiency.23 All of the designed photocatalysts presented higher activity than as-synthesized LDHs or TiO2. However, since the morphology and physicochemical factors of photocatalysts have tremendous effects on their efficiencies,24-26 we have proposed a kind of TiO2/LDH heterostructure with the yolk@shell architecture contain oxygen vacancy. Yolk@shell (Y@S) architectures, as a modern class of advanced functional structures, are a specific generation of core-shell structure with particular void space between inner core and outer shell. These architectures feature high light-reflecting potential and reduced diffusion resistance which along with an adjustable availability can offer a variety of opportunities for photocatalysis. Moreover, the void inner space may employ as nanoreactor and let the effective diffusion of solvents, substrates and products via the porous surface.27-28 This specialized architecture can be consequently choose to develop the photocatalytic performance particularly for titania based compounds.29-30 To further improve this special morphology, we have proposed a kind of Y@S structure contain Ti3+ species. The reduced TiO2 oxygen vacancy not only increases the visible light activities and charge separations, but also provides excess electrons and adsorption sites for CO2 conversion.31-33 The creation of defect sites on TiO2 leads to the preparation of Ti3+ centres, which could form donor levels in the electronic structure of TiO2 that can transfer to adsorbed CO2 to further reduction. In addition, oxygen vacancies can effect on the type of final products of CO2 photo-reduction, 3 ACS Paragon Plus Environment


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causing the tendency of defect sites to adsorb some intermediates to repair their defects. For example, it has been revealed that methanol formation can help to repair the defect sites on the structure, while the produced hydrogen during water oxidation can help to regenerate the defect sites and, thus, retain the photocatalytic cycle. For these causes, there is a tremendous attention in the progress of advanced structures based TiO2 with defect sites and operating their exclusive properties for photocatalytic CO2 conversion reactions.34-35 Therefore, based on these premises and following research efforts from our groups,36-38 herein we present an unique approach for the rational design of 3D Y@S TiO2-X/LDH architecture with decoration of Y@S TiO2-X by 2D LDH planes. This designed architecture containing oxygen vacancies in connecting with LDH plates is expected favour nanophotoreactor for CO2 reduction to highly valuable solar fuels (CH3OH/CH4) due to high gas absorption ability in mesoporous yolk@shell structure, defect sites and interlayer LDH spaces as well as reduced band gap and improved light absorption to the visible region.

Results and discussion Preparation and characterization of 3D Y@S TiO2-x/LDH architectures Figure 1 shows overall flowchart of procedure and the corresponding FESEM, TEM and HRTEM images of the architectures at each production step. Figure 1 shows the preparation process contains the following main steps: I) one-pot solvothermal preparation of Y@S TiO2 in presence of polyethylene glycol as soft template, II) high temperature hydrogen treatment procedure to prepare Y@S-TiO2-x structure and III) hydrothermal decorating of Co-Al LDH on the surface of Y@S-TiO2-x structure to achieve 3D Y@S TiO2-x/LDH architecture.

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Figure 1. Overall flowchart for fabrication of 3D Y@S TiO2-x/LDH architecture (steps I-III); corresponding FESEM (a-f), TEM (g-i) and HRTEM (j-l) images of the 3D Y@S TiO2-x/LDH fabrication procedure. The scale bars are 5 µm (a-c), 1 µm (d-i) and 5 nm (j-l). Figure 1(a-f) show FESEM images at different steps of the 3D Y@S TiO2-x/LDH architecture preparation. Figure 1a clearly shows that the produced Y@S-TiO2 structures have uniform spherical shapes ~ 2.8 µm. From cracked Y@S-TiO2 structures (Figure 1d), the hollow space between outer shell and inner core can be visualized to confirm of Y@S structure. As shown in Figure 1b and 1e, spherical structures of TiO2 Y@S are conserved without any changes after hydrogen treatment. The unique 3D flower like structures of the Y@S TiO2-x/LDH architecture after hydrothermal LDH decorating are illustrated in Figure 1c and 1f, from which a number of 3D flower-like spheres can be seen evenly distributed. Figure 1 (g-i) show TEM images for each step of 3D Y@S TiO2-x/LDH architecture preparation. As shown in 5 ACS Paragon Plus Environment


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Figure 1g, the Y@S structures of TiO2 sphere contain hollow space between core and shell. Figure 1h indicates the TEM image of Y@S-TiO2 after hydrogen treatment, which shows that the Y@S structure is completely preserved after treatment process. Interestingly, from TEM image of the 3D Y@S TiO2-x/LDH (Figure 1i), the flower-like structure after decorating of Y@S TiO2-x with ultrathin LDH plates is clearly obvious. HRTEM images of 3D Y@S TiO2-x/LDH architecture in each preparation steps are presented in Figure 1 (j-l). Y@S-TiO2 (Figure 1j) and Y@S-TiO2-x (Figure 1k) display that both samples are crystalline and the lattice spacing between (101) TiO2 planes is 0.35 nm (red arrows). Moreover, a narrow disordered surface layer encircling the crystalline structure is clear (white line) in Figure 1k, that may be ascribed to the generation of Ti3+ species near the surface of Y@S-TiO2-x structure resulting hydrogen treatment.39 The HRTEM image of 3D Y@S TiO2-x/LDH architecture also show same lattice space (0.35 nm) and a thin disordered surface layer due to hydrogenation process. Moreover, the lattice spacing 0.27 nm can be attributed to (012) planes of Co-Al LDH (pink arrows).22 This result evidences intimate contact between the reduced TiO2 nanocrystals and CoAl-LDH nanosheets, the two materials being identified by their typical lattice fringes (Figure 1l). To attain insight into the formation of the 3D Y@S TiO2-x/LDH architecture, samples were collected at different times during the hydrothermal decorating of Co-Al LDH on the surface of Y@S-TiO2-x structure. As shown in Figure 2a, the surface of primary Y@S-TiO2-x sample is smooth. After the decorating of Co-Al LDH process for 2h, a rough structure is clear (Figure 2b). By extending of decorating time, numerous nanoplatelets come into formation on the Y@S-TiO2 structure. After 12h, a well-developed 3D Y@S TiO2-x/LDH architecture was formed (Figure 2d), whereas with prolonging the time of reaction to 12 h, the flower-like architecture started to agglomerate and partially disappeared (Figure 2e).


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Figure 2. Time-dependent experiments for the hydrothermal decorating of Co-Al LDH on the surface of Y@S-TiO2-x structure (a-e). The scale bars are 1 µm. To better study the interaction of LDH with TiO2-X, XPS analysis was performed for the 3D Y@S TiO2-x/LDH nanoarchitecture. The spin orbit components of Ti 2p3/2 and Ti 2p1/2 peaks can be detected in the high resolution spectrum of 3D Y@S TiO2-x/LDH structure (Figure 3a). After deconvolution, four peaks at 464.6 eV, 463.3 eV 458.8 eV, and 457.6 eV the Ti 2p peaks appeared. Two peaks at 464.6 eV and 458.8 are attributed to the Ti4+ (2p1/2 and 2p3/2), while the peaks at 463.3 eV and 457.6 are assigned to the 2p1/2 and 2p3/2 core levels of Ti3+, respectively.40

This clearly reveals the presence of Ti3+ ions in 3D Y@S TiO2-x/LDH

architecture. On the other hand, the binding energies at 797.3 eV and 781.3 are due to Co 2p1/2 and Co 2p3/2, respectively (Figure 3b). The appearance of satellite peaks at 803.3 eV and 787.4 indicates the existence of a high-spin divalent state of Co2+ in this structure.23

Figure 3. XPS Ti 2p (a) and Co 2p (b) spectra of 3D Y@S TiO2-x/LDH architecture. Figure 4a depicts the XRD patterns of Y@S TiO2, Y@S TiO2-x and 3D Y@S TiO2-x/LDH structures. For Y@S TiO2, the typical diffraction arises from anatase TiO2 (JCPDS 89–4921). The sharp anatase (101) reflection of Y@S TiO2-x compared to that of Y@S TiO2 may be ascribed to increased crystallinity due to the hydrogen treatment in high temperature. 7 ACS Paragon Plus Environment


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Moreover, the diffraction reflections of Y@S TiO2-x show a slight move to a higher diffraction angle, indicating the decrease of the interplanar space of the crystalline phase, that proves structural modifications in TiO2 in the hydrogen treatment.41 For 3D Y@S TiO2-x/LDH, some more reflections are observed compared to Y@S TiO2-x. These diffraction peaks can be attributed to the hydrotalcite phase with CO32− in the interlayer region (JCPDS 51–0045) which demonstrates the integration of both TiO2-x and LDH and the crystallinity of both phases. The absorption characteristic of a semiconductor material is of crucial importance its photocatalytic efficiency. In this regard, diffuse reflectance spectroscopy (DRS) in the UV– vis region were carried out. As shown in Figure 4b, no notable absorption is detected in Y@S TiO2 spectrum in the visible region, while for hydrogen treated Y@S TiO2-x sample, a considerable response ability in the range of 400-700 nm (visible region) is evident that is due to the creation of Ti3+ species on the structure. Accordingly, the UV–vis spectrum of 3D Y@S TiO2-x/LDH architecture exhibits a significant absorption extending to the visible light range. The observed strong response of visible light indicates the capability of 3D Y@S TiO2-x/LDH architecture as a visible-light-active photocatalyst. From the plot of (αhν)2 vs. hν the band gap energy of the 3D Y@S TiO2-x/LDH architecture is calculated to 2.64 eV (Figure S2, Supporting Information). The photoluminescence (PL) analysis is practical to recognize the performance of photoexcited electrons-holes in semiconductors because of PL emission consequences of the charge recombination in photocatalysts. Figure S3 (Supporting Information) display the PL spectra of Y@S TiO2, Y@S TiO2-x and 3D Y@S TiO2-x/LDH samples in the wavelength range of 350– 700 nm with excitation at 320 nm. The PL intensities of the Y@S TiO2-x decrease compared with Y@S-TiO2. This observation indicates that the recombination rate of light-generated electrons- holes have been inhibited noticeably in Y@S TiO2-x structure, because of the creation of defect sites during the hydrogen treatment. Moreover, the PL intensities of the 3D Y@S TiO2-x/LDH architecture are weaker compared to both of Y@S TiO2 and Y@S TiO2-x 8 ACS Paragon Plus Environment

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structures. The presence of oxygen vacancies as well as the TiO2-x – LDH interface connecting the structure can serve as electron capture traps, and consequently isolate the electrons-holes and decrease the recombination rate considerably. Valence band XPS was used to determine the valence band position of the 3D Y@S TiO2x/LDH

architecture. As shown in figure S4, this value was determined to 1.73 V and can be

used together with the optical band gap energy to determine the corresponding conduction band potential of -0.91 V.

Figure 4. a) XRD patterns of Y@S TiO2, Y@S TiO2-x and 3D Y@S TiO2-x/LDH; b) DRS spectra of Y@S TiO2, Y@S TiO2-x and 3D Y@S TiO2-x/LDH; c) N2-adsorption/desorption isotherms of P25, LDH, P25/LDH, H-P25/LDH, Y@S-TiO2 and 3D Y@S TiO2-x/LDH; d) CO2 adsorption isotherms of P25, Y@S-TiO2, Y@S-TiO2-x and 3D Y@S TiO2-x/LDH. BET surface area and pore characteristics of the photocatalyst are two important properties that affect its performance. These properties were studied by N2-adsorption/desorption measurements (Figure 4c). Unlike P25, LDH, P25/LDH and hydrogen treated P25/LDH (HP25/LDH) which is not mesoporous, in Y@S TiO2, Y@S TiO2-x and 3D Y@S TiO2-x/LDH architectures, typical IV isotherms with hysteresis loops are observed, showing the presence of mesopores in these samples. 3D Y@S TiO2-x/LDH architecture displays considerable hysteresis loop without any limiting adsorption at high P/P0 region, indicating the creation of 9 ACS Paragon Plus Environment


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more porosity by the stacking of LDH platelets with 3D arranging on Y@S TiO2-X structure. Specifically, the 3D Y@S TiO2-x/LDH architecture possess the highest specific surface area (293 m2.g–1), much larger than other samples. Furthermore, as shown in Figure S5 pore size distribution analysis displays narrow pore distribution in the range 4−8 nm for the 3D Y@S TiO2-x/LDH architecture. The high surface area and mesoporous feature of 3D Y@S TiO2x/LDH

architecture facilitates the exposure of active sites and promotes their photocatalytic

activity. Figure 4d, shows the CO2 adsorption analysis for P25, Y@S TiO2, Y@S TiO2-x and 3D Y@S TiO2-x/LDH structures. With the exception of non-porous P25 structure, the other samples show a rapid rise of CO2 adsorption with increasing CO2 partial pressure. The large increase in CO2 uptake revealed by the Y@S TiO2 may be ascribed to their hollow space and porosity as shown above by N2-adsorption/desorption analysis. Interestingly, Y@S TiO2-x sample exhibits a larger CO2 adsorption capacity compared to Y@S TiO2 , that can be referred to the production of additional adsorption sites at defects.31, x/LDH

42

Remarkably, the 3D Y@S TiO2-

architecture shows much larger CO2 adsorption in comparison to the other samples.

This superior absorption ability can be attributed to the 3D decorating of LDH plates on the mesoporous Y@S TiO2-x structure that can absorb CO2 molecules not only in hollow spaces and oxygen vacancies, but also in the interlayer space of LDH plates.43 This great adsorption ability of 3D Y@S TiO2-x/LDH architecture could facilitate the photocatalytic CO2 conversion because of the large availability of CO2 molecules on the catalyst sites. Photocatalytic activity The engineered 3D Y@S architecture contain both LDH plates and hydrogen treated titania with high visible response ability and great CO2 absorption potency, as well as nanophotoreactor like void inner space, which makes 3D Y@S TiO2-x/LDH architecture a unique structure for photocatalytic CO2 reduction applications. P25, P25/LDH, H-P25/LDH 10 ACS Paragon Plus Environment

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and Y@S TiO2/LDH composites were also evaluated here for comparison (Figure 4). The CO2 photo-reduction efficiency of the samples were investigated by CO2 bubbling in water under visible light irradiation. Control experiments were carried out in the absence of either photocatalysts or photo irradiations and no determinate quantities of any hydrocarbon compounds were measured. Figure 5 displays the photocatalytic CH3OH production rates of the samples after 1 hour. P25 exhibited the poorest photoactivity for CH3OH production, presumably reflecting non visible light activity and fast charge carrier recombination. Both the P25/LDH and Y@S TiO2/LDH composites exhibited better photocatalytic activity compared to P25, which evidences a synergy between the TiO2 and CoAl-LDH semiconductor components. Interestingly, with HP25/LDH, the CH3OH production was increased remarkably, indicating the importance of hydrogen treatment on CO2 photoreduction. The 3D Y@S TiO2-x/LDH architecture prepared by 3D decoration of LDH on the surface of hydrogen treated Y@S TiO2, exhibited an outstanding CH3OH production from CO2 photoreduction (251 µmol/gcat. h.). This corresponds to a more than 20-fold increase compared to that observed for both of P25 and LDH and an increase of ca. 16% compared to H-P25/LDH structure (216 µmol/g cat. h.).

Figure 5. Comparison of photocatalytic CH3OH production rates of P25, P25/LDH, HP25/LDH, Y@S TiO2/LDH and 3D Y@S TiO2-x/LDH samples in the CO2 photoreduction reaction. 11 ACS Paragon Plus Environment


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The observed considerable CO2 photo-conversion efficiency of 3D Y@S TiO2-x/LDH architecture under visible illumination can be ascribed to seven features of the structure: 1) the hydroxyl ions in LDH scavenge the photoinduced holes and convert into hydroxyl radicals which enhances CO2 reduction;22 2) the alkali nature of LDH which may led to higher CO2 dissolution in water without using NaOH and therefore preventing side effects; 3) the interlayer space of LDH, which led to enhanced CO2 sorption capacity of structure; 18, 43 4) the difference in band gaps of TiO2 core, hydrogenated shell and decorated LDH plates, which promotes charge separation; 5) the possible multi-scattering/reflection of light on the 3D surface and inside the structure by the inner core enhances light adsorption;44-45 6) hollow inner space beside high porosity of the 3D Y@S morphology, which is beneficial for the transport and sorption properties of CO2; and 7) the presence of oxygen vacancies on the Y@S-TiO2 structure, which not only reduces the band gap of titania to the visible region but which also provides adsorption sites with excess electrons for CO2 and H2O at the defect sites.31, 42 To provide better insights into the photoreduction of CO2 in presence of 3D Y@S TiO2-x/LDH architecture, time-dependent photocatalytic reaction has been performed. As shown in Figure 6, the CH3OH was the main product until 2h, while considerable amount of other CO2 photoreduction products such as CH4, HCOOH and HCHO were not detected using the GC-FID method. By extending the time of experiment, the CH4 also produced and increased gradually. So that after 12h, the amounts of CH3OH and CH4 increased to 726 µmol/gcat. and 453 µmol/gcat., respectively. These results indicate that the engineered 3D Y@S TiO2-x/LDH architecture can employ as an effective photocatalyst for selective production CH3OH (by control the time of reaction) or maximize production of solar fuel (CH3OH/CH4) under visible light irradiation.


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Figure 6. Time-dependent photocatalytic experiment of CO2 using 3D Y@S TiO2-x/LDH architecture. To rationalize the effect of the Co-Al LDH on the surface of Y@S TiO2-X, a mechanism was proposed for the photocatalytic CO2 reduction under visible light irradiations (Figure S6, Supporting Information). As calculated using UV-vis analysis and valance band potential, the minimum conduction bands energy potential of 3D Y@S TiO2-x/LDH architecture is -0.91 V. This number is negative than the reported standard potential of the CO2/CH3OH and CO2/CH4 redox couples (-0.38 V and -0.24 V vs. NHE, respectively). Furthermore, the calculated valence band energy potential of the structure (1.73 V) is positive than the standard potential of the O2/H2O redox couple (1.23 V vs. NHE). Accordingly, the excited electrons in the conduction band (CB) drive the reduction of CO2 into CH3OH and CH4. Meantime, the photoexcited holes transfer from valence band of TiO2-x to valence band of LDH, and oxidize water to produce required H+ and O2 for CO2 conversion reaction (Figure S6, Supporting Information). In this architecture, the connection of LDH plates with Y@S TiO2 contains numerous oxygen vacancies that could promote CO2 adsorption, activation and conversion. Especially sorption of CO2 gas in the LDH interlayer space as well as strong interaction of CO2 and oxygen vacancy allow efficient electron transfer from the catalyst to the captured CO2 species. Therefore, besides the advantageous of LDH connection with TiO2 for this photocatalytic reaction, the existence of oxygen vacancies may play a significant role in CH3OH/CH4 formation. So that, as previously revealed by theoretical calculations, the 13 ACS Paragon Plus Environment


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intermediates of CO2 reduction reaction can adsorb at the oxygen vacancy and CH3OH and CH4 may be produced from •CH3 that is produced by the breaking of C-O bond of CH3O adsorbed at the defect sites.34 To examine the recyclability of the Y@S TiO2-x/LDH architecture, a photostability experiment was performed. The photocatalyst was reused in 7 cycles (Figure S7, Supporting Information). The structure was centrifuged, washed with water and then dried in nitrogen gas flow, in each run. The production of methanol remained almost constant. After seven cycles the decrease of methanol production was less than 11%. After the photostability test, the recovered 3D Y@S TiO2-x/LDH sample was examined by SEM and XRD analysis to probe stability. As indicated in Figure S7, no considerable change was detected in structural and chemical features of fresh and recovered 3D Y@S TiO2-x/LDH catalyst, indicating that designed architecture is stable under the photocatalytic reactions. In order to investigate the productivity of the designed 3D Y@S TiO2-x/LDH architecture, we compared the results of the photocatalytic CO2 conversion to CH3OH with recently studied (Table S1). This photocatalyst shows higher methanol production than most of the presented structures. Furthermore, the high stability in term of recyclability and not employment of noble metal elements in this architecture produces this structure as a stable and inexpensive photocatalyst for photocatalytic CO2 reduction reaction.

Conclusions In summary, engineered 3D Y@S TiO2-x/LDH architecture was prepared using a three-step method and applied as powerful photocatalytic structure. The preparation involves the solvothermal step to produce TiO2 with Y@S structures, followed by hydrogenation process to create Ti3+ ions in the Y@S TiO2 structure and finally hydrothermal decoration of 2D CoAl-LDH plates on the surface of prepared structure. The engineered 3D Y@S TiO2-x/LDH 14 ACS Paragon Plus Environment

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nanocomposite considerably enhanced the CO2 photo-reduction efficiency in solar fuels production, compared to P25, P25/LDH, H-P25/LDH and Y@S TiO2/LDH photocatalysts. The advanced morphology of Y@S TiO2 containing oxygen vacancies in connecting with LDH plates increases the CO2 reduction by high gas absorption in defect sites and interlayer spaces as well as by reducing the band gap and providing improved visible light response. The suggested approach led to outstanding activity and selectivity in reduction of CO2 to MeOH by control the time of reaction. Our insight for the architectural engineering of this structure can serve as inspiration for the rational design of advanced photocatalysts, which can pave the way to further improve the energy conversion efficiency.

Methods Synthesis of Y@S TiO2 microspheres: The Y@S TiO2 architecture was synthesized according to our previous research.36 Synthesis of Y@S TiO2-x structures: The prepared Y@S TiO2 microspheres were heated under H2/Ar (1:1) atmosphere (200 psi flow rate) at 500 °C with the heating rate 3 °C/min for 3 h. Next, the prepared structure was cooled under H2/Ar atmosphere to 25 °C (obtained material named Y@S TiO2-x). The procedure was repeated with P25 (instead of Y@S TiO2) to yield hydrogenated P25 (H-P25). 3D Y@S TiO2-x/LDH architecture: The Y@S TiO2-x microspheres (0.15 g) were dispersed in a solution containing Co(NO3)2·6H2O (3 mmol), Al(NO3)3·9H2O (1 mmol), urea (10 mmol), NH4F (4 mmol) and water (15 mL) using sonication bath for 10 min. The obtained solution was moved to a Teflon-lined stainless steel autoclave at 100 °C for 10 h. Then, the temperature of autoclave was reduced naturally to the 25 °C and the sample was separated by centrifugation, washed with water and ethanol several times and dried under vacuum at 50 °C overnight. For comparison, the process was repeated with P25, H-P25 and Y@S TiO2 (instead of Y@S TiO2-x) to produce P25/LDH, H-P25-LDH and Y@S TiO2/LDH, respectively. 15 ACS Paragon Plus Environment


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Photocatalytic experiments: The photocatalytic of CO2 conversion reaction was carried out in a 500 mL cylindrical glass reactor with a 300 W Xenon lamp (Lelesil innovative system), equipped with magnetic stirrer and chiller to control the temperature (15 °C) during the reactions (Figure S1). Prior to reaction 400 mL of milli-Q water was put into the photoreactor and pure CO2 (99.9%) was bubbled in the reactor solution for half hour to ensure that dissolved O2 was removed. Next, 400 mg of photocatalyst was transferred into reactor, and the magnetic stirrer and xenon lamp were turned on to start the photoreaction. The reaction was studied in two liquid phase and gas phase modes for measuring CH3OH and CH4, respectively. For the liquid phase investigation, the CO2 was continuously bubbled throughout the process. The liquid samples were continuously taken from the reactor at given intervals (1 h) for quantitative analysis based on the external standard using a gas chromatograph (Bruker SCION 456-GC) with a flame ionization detector (FID) after centrifugation. In the gas phase examination, the system was sealed after catalyst loading and gaseous samples were continuously taken from the photoreactor and analysed using GC-FID. Supporting Information Supporting Information contains details of the Materials and apparatus used, comparing the efficiency of different photocatalyst, schematic of photo-reactor, plot of(αhν)2 vs. hν, pore volume distribution, valence-band XPS spectrum and PL spectra of some of structures, as well as the reusability study of 3D Y@S TiO2-x/LDH architecture. This material is available free of charge via the Internet at http://pubs.acs.org/.

Acknowledgements The authors thank University of Tehran and University of Geneva for supporting this work. AZ would like to thank Jafar Afshani for his help in Photoluminescence analysis. AZ acknowledges Ali Banitalebi for BET measurements (Laboratory of Inorganic Chemistry,


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University of Tehran). AZ thanks Dr. Celine Besnard (Laboratory of Crystallography, University of Geneva) for XRD analysis.

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ToC figure


LDH Architecture: Tailored Structure for Visible

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