TiO2–MnOx–Pt Hybrid Multiheterojunction Film Photocatalyst with

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TiO2−MnOx−Pt Hybrid Multiheterojunction Film Photocatalyst with Enhanced Photocatalytic CO2‑Reduction Activity Aiyun Meng,† Liuyang Zhang,† Bei Cheng,† and Jiaguo Yu*,†,‡ †

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, P. R. China ‡ Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia ABSTRACT: Photocatalytic CO2 conversion into solar fuels has an alluring prospect. However, the rapid recombination of photogenerated electron−hole pairs for TiO2-based photocatalyst hinders its wide application. To alleviate this bottleneck, a ternary hybrid TiO2−MnOx−Pt composite is excogitated. Taking advantage of the surface junction between {001} and {101} facets, MnOx nanosheets and Pt nanoparticles are selectively deposited on each facet by a facile photodeposition method. This design accomplishes the formation of two heterojunctions: p−n junction between MnOx and TiO2 {001} facet and metal−semiconductor junction between Pt and TiO2 {101} facet. Both of them, together with the surface heterojunction between {001} and {101} facets, are contributive to the spatial separation of the photogenerated electron−hole pairs. Thanks to their cooperative and synergistic effect, the as-prepared composite photocatalyst exhibits a promoted yield of CH4 and CH3OH, which is over threefold of pristine TiO2 nanosheets films. The conjecture of the mechanism that selective formation of multijunction structure maximizes the separation and transfer efficiency of photogenerated charge carriers is proved by the photoelectrochemical analysis. This work not only successfully achieves an efficient multijunction photocatalyst by ingenious design but also provides insight into the mechanism of the performance enhancement. KEYWORDS: film, TiO2 nanosheet, exposed (001) facet, heterojunction, photocatalytic CO2 reduction flow from the n-type semiconductor to the adjacent p-type semiconductor to balance the Fermi levels. A built-in electric field that directs from n-type semiconductor to p-type semiconductor can be established, which benefits the migration of photogenerated electrons and holes. When an n-type semiconductor is coupled with a metal with their work functions satisfying Ws < Wm, a Schottky barrier can be formed to expedite the migration of electrons from the semiconductor to the metal. Previous studies have already testified the promoted charge transfer and enhanced photocatalytic activities of photocatalysts by heterojunction engineering. For example, Xiong and his co-workers designed a Cu2O−Pd composite structure, and the accelerated charge separation by the m−s junction between p-type Cu2O and noble metal Pd led to the improved photocatalytic conversion efficiency.49 As yet, however, single heterojunction accomplishes so little in facilitating the charge separation. Therefore, the construction of multijunctions is necessary to synergistically improve the interfacial electron−hole separation and migration. Wang et al. reported a novel ternary Ag/BiVO4/Co3O4 hybrid photo-

1. INTRODUCTION Solar-driven photocatalysts show promise in hydrogen or chemical fuel evolution from water splitting or CO 2 reduction.1−11 However, because of the robust chemical bonds in the CO2 molecules and the complex conversion pathway involving multielectrons, the photocatalytic conversion rate of CO2 is still poor, restricting its practical application.12,13 The main problems lie in its low conversion efficiency, poor selectivity of carbon species products, and competition of H2production reaction.14,15 Therefore, it is desirable to develop photocatalysts for CO2 reduction with fascinating activity and largely suppress other competing reactions. In this context, great effort has been invested in modifying TiO2, NiO, ZnO, gC3N4, layered double hydroxides, and metal−organic framework (MOF).16−23 Among, anatase TiO2 with nontoxicity, good chemical stability, and low cost becomes eminent.24−29 The knotty problem that restricts the widespread application of TiO2 is the quick recombination of photogenerated electrons and holes. Several strategies including heterojunction construction and cocatalyst deposition have been proposed to suppress the recombination.20,30−36 The heterojunctions in the former one are composed of various heterojunction types, including conventional type-II heterojunction, metal−semiconductor (m−s) junction, p−n junction, direct Z-scheme heterojunction, and surface heterojunction, which are feasible to separate the photogenerated charge carriers.37−48 For instance, when a p−n junction is formed, the electrons can © XXXX American Chemical Society

Special Issue: Artificial Photosynthesis: Harnessing Materials and Interfaces for Sustainable Fuels Received: February 10, 2018 Accepted: April 25, 2018

A

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oven for 12 h at 180 °C. After the reaction, the FTO glass was taken out, washed, and dried for further use. The obtained TiO2/FTO was denoted as T. Selective Deposition of Platinum and/or Manganese Oxide. The selective deposition of platinum and manganese oxide alone, as well as platinum and manganese oxide together, was performed by similar photodeposition method. Typically, two pieces of FTO glass coated with TiO2 nanosheet (T) were placed at the bottom of Pyrex beaker (100 mL in volume), and then 50 mL of aqueous solution containing metal precursors was added into the beaker. The solution was irradiated under a 350 W xenon lamp at room temperature. After photodeposition, the sample was washed with deionized water and was finally dried at 80 °C overnight. For the deposition of platinum, 0.02 g L−1 H2PtCl6 solution with a volume of 50 mL was used as the precursor solution. The TiO2 substrate was irradiated for 1 h, and the obtained TiO2−Pt sample was denoted as TP. The deposition of manganese oxide was conducted in a manganese acetate solution (0.2 g L−1, 50 mL) under irradiation for 3 h; the obtained TiO2−MnOx sample was denoted as TM. The simultaneous deposition of platinum and manganese oxide was performed by a two-step procedure. Manganese oxide was first deposited onto TiO2 in a manganous acetate solution (0.2 g L−1, 50 mL). After 3 h of irradiation, the sample was washed and transferred into H2PtCl6 aqueous solution (0.02 g L−1, 50 mL). After the photodeposition process, the sample was washed with deionized water and dried at 80 °C. The obtained TiO2− MnOx−Pt sample was denoted as TMP. 2.2. Thin-Film Characterization. X-ray diffraction (XRD) pattern was obtained on an X-ray diffractometer (Rigaku) with Cu Kα irradiation (λ = 0.151 48 nm). Morphologies of samples were recorded using field emission scanning electron microscope (FESEM; JSM7500F, JEOL). The X-ray photoelectron spectroscopy (XPS) was performed on VG ESCALAB 210 electron spectrometer. The C 1s peak of the surface adventitious carbon at 284.8 eV was used as calibration reference. Light absorption was measured using a UV−vis spectrometer (Shimadzu, UV-2550). 2.3. Photocatalytic CO2 Reduction Measurement. The photocatalytic CO2 reduction activities of samples were measured in a 200 mL homemade Pyrex reactor. A 350 W xenon lamp was used as the light source. In a typical experiment, four pieces of FTO glass deposited with photocatalysts were placed onto the bottom of the reactor. CO2 and H2O vapors were in situ generated by the reaction between NaHCO3 (0.084 g) and H2SO4 (2 M, 0.3 mL). Before irradiation, the reactor was sealed and fluxed with N2 gas to ensure the anaerobic condition in the photocatalytic system. After illumination, 1 mL of mixed gas was pumped from the reactor and analyzed by a gas chromatograph (PGC-80, PANNA instrument) equipped with a flame ionized detector (FID) and methanator. Control experiment was conducted in the absence of CO2 or light source to assess the influence of CO2 and light irradiation during the photocatalytic system. 2.4. Photoelectrochemical Measurement. The photoelectrochemical (PEC) performances of the samples were conducted on a CHI 760E electrochemical workstation (Shanghai Chenhua Instrument). The FTO glass coated with samples, Pt foil, and Ag/AgCl electrode were used as the working electrode, reference electrode, and counter electrode, respectively. 1.0 M KOH aqueous solution was employed as electrolyte. The light source was a light-emitting diode (LED) light (3 W, 365 nm, Shenzhen Lamplic Technology Co. Ltd.).

catalyst by designing an m−s junction and a p−n junction on BiVO4 surface, respectively. The interplay of two heterojunctions resulted in cooperative enhancement of photocatalytic performance compared with single junctions.50 Notwithstanding, the achieved charge separation efficiency is still moderate, because the constructed multiheterojunctions are randomly distributed on the surface of photocatalysts. Confronted by this problem, it is intriguing to control the location of the heterojunction formation. It is reported that surface (or facet) heterojunction can be formed between {101} and {001} facets of anatase TiO2 due to their varied electronic structures, inflicting spatially separation of electrons and holes, which accumulate on the {101} and {001} facets, respectively.51,52 Inspired by this discovery, the separated electrons and holes on the surface of photocatalyst can guide the formation of multijunctions. The design motif and rationale are as follows. TiO2 is an n-type semiconductor, and thereafter a p−n junction on hole-rich {001} facets of TiO2 can be designed to promote the migration of holes from TiO2 to the p-type semiconductor, thus impeding the recombination of electron− hole pairs. Simultaneously, noble metal can be selectively photodeposited on the electron-rich {101} facets of TiO2 to form a metal−semiconductor junction, and consequently the Schottky barrier can play a significant role in facilitating the electron transfer from TiO2 to noble metal and concurrently inhibit the surface recombination of electron−hole pairs. Meanwhile, noble metals can serve as the reduction reaction active sites and catalyze the reduction process. In view of the high efficiency of platinum contributed by its low overpotential and large work function, it is selected as the component of m−s junction. Besides, to circumvent this obstacle and difficulty in separating TiO2 in its powdered form from the liquid-phase reaction system, immobilizing thin-film TiO2 onto various substrates emerges as an alternative.53,54 Despite the reduced surface area, the obtained TiO2 thin film is easy to handle. Herein, to realize the as-stated design, first, anatase TiO2 nanosheet film with coexposed {001} and {101} facets was prepared on transparent fluorine-doped tin oxide (FTO) conducting glass via a facile hydrothermal process. The coexposed hole-rich {001} facets and electron-rich {101} facets of TiO2 formed a surface heterojunction. Afterward, platinum and manganese oxide were selectively photodeposited on {101} and {001} facets, respectively, thus constructing the m−s junction and p−n junction on the TiO2 nanosheet film. The photocatalytic CO2 reduction performances of the composite photocatalysts were evaluated as the indicators of the photoactivity. As expected, the result demonstrated that the combination of three junctions is crucial in jointly expediting the migration of charge carriers and improving the photocatalytic activities. This work provides a new insight into constructing the multijunction photocatalyst and probing its working mechanism.

3. RESULTS AND DISCUSSION 3.1. Morphology and Component. XRD measurement was performed to investigate the crystalline structures of pure TiO2 (T), TiO2−Pt (TP), TiO2−MnOx (TM), and TiO2− MnOx−Pt (TMP), and the overall results are shown in Figure 1. The XRD pattern of FTO glass was also provided for comparison, in consistency with the previous report.55 By comparing the XRD pattern of FTO and sample T, the prepared TiO2 was confirmed to be anatase phase (JCPDS, No. 21-1272). The diffraction peaks of Pt and MnOx were hard to

2. EXPERIMENTAL SECTION 2.1. Preparation. Preparation of TiO2 Nanosheets Film. FTO glass was cleaned ultrasonically in water, ethanol, and acetone for 20 min, respectively. In a typical procedure, a mixed solution containing 15 mL of water and 15 mL of hydrochloric acid was stirred for 10 min in a Teflon-lined autoclave, and then 0.5 mL of tetrabutyl titanate and 0.25 g of ammonium hexafluorotitanate were added into the mixed solution with stirring for another 10 min. Subsequently, two pieces of FTO glass were placed at the bottom of autoclave with the conductive side facing up. The stainless steel autoclave was then put into a vacuum B

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of bare TiO2 were exposed (Figure 2a). Previous studies have revealed that the top/bottom flat square facets are the {001} facets and that the isosceles trapezoidal sidewalls are the {101} facets.53,54 Their dimensions were estimated to be ca. 1 μm long and 120 nm thick. The FESEM image under high magnification (inset in Figure 2a) showed that the angle between two planes was 68°, which was exactly the theoretical value for the angle between {001} and {101} facets of anatase TiO2. The percentage of {001} facets was calculated as 76%, based on the following equation.51 S001% =

S001 cos θ = b2 S001 + S101 cos θ + 2 − 1

(1)

a

Herein, θ is the theoretical value (68.3°) for the angle between the {001} and {101} facets of anatase, and a and b denote the lengths of the side of the square {001} “truncation” facets and the side of the bipyramid, respectively. S001 and S101 denote the surface area of {001} and {101} facets, respectively. After photodeposition in the manganese acetate aqueous solution, it was demonstrated in Figure 2c that MnOx nanoflakes were selectively loaded on the hole-rich {001} facet of TiO2; while {101} facet of TiO2 remained smooth. The corresponding energy-dispersive X-ray spectroscopy (EDS) spectrum proved the existence of Mn element (Figure 2d). Similarly, after photodeposition in chloroplatinic acid aqueous solution, Pt nanoparticles were selectively distributed on the electron-rich {101} facets of TiO2, also evidenced by the EDS spectrum (Figure 2e,f). No nanoparticles were discernible on the {001} facet of TiO2. After a two-step photo-oxidation and photoreduction deposition procedure, MnOx and Pt were simultaneously decorated on {001} and {101} facets of TiO2, respectively (Figure 2g,h). The schematic illustration and the related chemical reactions of selective deposition of MnOx nanoflakes and Pt nanoparticles are illustrated in Figure 3 and

Figure 1. XRD patterns of FTO glass, sample T, TM, TP, and TMP.

observe probably due to their low amounts. Moreover, obviously, the peak intensities of TP, TM, and TMP decreased compared with that of pure TiO2, owing to the shielding effect of the deposited MnOx and Pt on TiO2 over the incident and diffracted X-ray.52 Figure 2 shows the morphologies of bare TiO2 sheets and TiO 2 sheets deposited with MnOx nanoflakes and Pt nanoparticles. It was observed that two major smooth facets

Figure 3. Schematic diagram of selective photodeposition process of MnOx nanoflakes and Pt nanoparticles on anatase TiO2 {001} and {101} facets.

eqs 2−4. When TiO2 nanosheets were irradiated by UV light, the photogenerated electrons and holes were excited and transferred to {101} and {001} facets, respectively, due to the surface heterojunction of TiO2. Then Mn2+ can be oxidized into MnOx by the holes on {001} facets, while Pt4+ can be reduced into metallic Pt by the electrons on {101} facets. Finally, MnOx and Pt were selectively deposited on the {001} and {101} facets of TiO2 nanosheet film. TiO2 + hν → TiO2 + h+ + e− Mn

2+

+

−

+ 2(x − 1)h + xOH → MnOx + xH

Pt4 + + 4e− → Pt

(2) +

(3) (4)

To confirm the oxidation and reduction states of Mn and Pt species, XPS spectra were recorded. As shown in Figure 4a, Mn 2p peaks were observed from the survey spectrum of sample TM. The high-resolution spectrum of Mn 2p (Figure 4d) presented two peaks located at 641.9 and 653.5 eV, designated

Figure 2. Morphologies and corresponding EDS spectra of (a, b) asprepared T, (c, d) TM, (e, f) TP, and (g, h) TMP. C

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Figure 4. XPS spectra of four kinds of photocatalysts (T, TM, TP, and TMP): (a) survey spectra, high resolution of (b) Ti 2p, (c) O 1s, (d) Mn 2p, and (e) Pt 4f spectra.

Figure 5. (a) UV−vis DRS and corresponding colors of samples T, TM, TP, and TMP. (b) Tauc plot of sample TMP for determining the band gaps.

two-step selective deposition, it was observed that Mn 2p and Pt 4f peaks in sample TMP coexisted, located at the same position with that of TM and TP, demonstrating that both MnOx and Pt were successfully deposited on {001} and {101} facets of TiO2 nanosheets films, respectively. Thus, the trijunction (surface junction between {001} and {101} facets, m−s junction between Pt nanoparticles and TiO2 nanosheets, and p−n junction between n-type TiO2 and p-type MnOx) was successfully constructed on the surface of TiO2. The formed multi-heterojunctions at the interface of photocatalysts were

to Mn 2p3/2 and Mn 2p1/2, respectively. The spin−orbit splitting value of Mn 2p was 11.6 eV. The Mn species can be labeled as MnOx. This was attributed to the oxidation of Mn2+ by the photogenerated holes accumulated on {001} facets. Similarly, Pt 4f peaks can be easily found in the survey spectrum of sample TP (Figure 4a). The high-resolution spectrum of Pt 4f (Figure 4e) exhibited two peaks centered at 71.1 and 74.5 eV, which were assigned to Pt 4f7/2 and Pt 4f5/2, respectively, substantiating the existence of metallic platinum.56 This was due to the assembled photogenerated electrons on {101} facets that can reduce PtCl62− into Pt metallic nanoparticles. After the D

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Figure 6. (a) Photocatalytic CO2 reduction performance of T, TM, TP, and TMP films under irradiation for 3 h. (b) Cycling stability of photocatalytic CO2 reduction activity of TMP.

Figure 7. PEC performance of all the samples. (a) LSV curves under illumination of an LED light (365 nm), (b) I−t curves measured at applied voltage of 0.5 V under illumination of an LED light (365 nm), and (c) steady-state photocurrent density curves under continuous illumination (LED light, 365 nm).

A layer of photocatalyst on top of FTO glass will enhance the reflectance intensity and the light harvesting of bare FTO glass. Thus, in our case, the original baseline in the bandgap calculation does not apply to the TiO2-deposited FTO, and this issue has already been addressed in literature.59 To eliminate the interference of the increased light scattering, the intrinsic absorption edges of samples were determined as the point of intersection of two tangent lines rather than the x-intercept. The obtained bandgap value was 1.7 eV, falling into the range of the band gap of MnOx.60 When Pt nanoparticles were deposited on TiO2 nanosheets, the light absorption was slightly enhanced. After the simultaneous deposition of MnOx and Pt, the visible absorption in the range of 400−700 nm was improved. The enhanced light harvesting was favorable for promoting the photocatalytic CO2 reduction activity.

beneficial for accelerating the charge separation and improving the photocatalytic performance. 3.2. Optical Absorption Property. The optical absorption properties were obtained by the UV−vis reflectance spectroscopy. Figure 5a shows the UV−vis diffuse reflection spectra (DRS) and corresponding colors of four samples. For bare TiO2, an absorption onset at ca. 390 nm was observed, which corresponded to the bandgap energy of intrinsic anatase TiO2, that is, 3.2 eV. After MnOx nanoflakes were deposited, an obvious absorption increase in the wavelength of 400−700 nm emerged, indicating the enhanced absorption in the visible range. Moreover, an absorption edge at ∼730 nm was also observed. According to the fact that MnOx is an indirect band gap semiconductor, the corresponding Kubelka−Munk function was used to calculate the band gap energy (Eg) of MnOx.57,58 Tauc plot of sample TMP was shown in Figure 5b. E

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ACS Applied Materials & Interfaces 3.3. Photocatalytic CO2-Reduction Performance. The CO2 photoreduction activities of sample T, TM, TP, and TMP nanosheet films were evaluated. Considering that the film photocatalysts were difficult to detach from FTO glass, we investigated the photocatalytic activities of photocatalysts in film form. Therefore, we use the unit μmol/m2 rather than the frequently used μmol/g. Figure 6a shows the yields of CH4 and CH3OH for four samples after irradiation for 3 h. For sample T, the yields of CH4 and CH3OH after 3 h of irradiation were 28 and 31 μmol m−2, respectively. After MnOx nanoflakes were deposited on TiO2 {001} facets, the yield of the CH4 and CH3OH exhibited a small increment. After Pt nanoparticles were deposited on TiO2 {101} facets, the CH4 yield rose to 85 μmol m−2, even over tripled than that of bare TiO2 nanosheets films. Meanwhile, the yield of CH3OH also showed an increase. When both MnOx and Pt were loaded on the two facets of TiO2 surface, the yields of two products further increased. The different growth rate indicated that the photocatalytic reactions on the four photocatalysts followed complex and distinguishable pathways for CH4 and CH3OH formation. To inspect the photocatalytic stability of sample TMP, the cycling activity of TMP was measured and plotted in Figure 6b. A linear relationship between the yields of products and the irradiation time was manifested. And the highest value for CH4 and CH3OH yields reached 104 and 91 μmol m−2, respectively. 3.4. Photoelectrochemical Performance. The PEC performance of all the prepared samples was evaluated in 1.0 M KOH electrolyte. Figure 7a shows the linear sweep voltammetry (LSV) curves of T, TM, TP, and TMP photoanodes under irradiation. The onset potentials at the current density of 1 mA cm−2 for T, TM, TP, and TMP were −0.69, −0.67, −0.43, and −0.41 V, respectively. The trend indicated that the addition of MnOx and Pt was beneficial for the catalytic reaction. Notably, it can be observed that Pt was more efficient than MnOx in the onset potential decrement, which is reasonable, because Pt is recognized as the best electrocatalyst and cocatalyst. Photocurrent−time (I−t) curves of these samples are shown in Figure 7b. A hike in photocurrent was witnessed when the light was turned on. When the light was turned off, the photocurrent dropped steeply to nearly zero. The photocurrent response to the on− off cycles was reproducible over the chopped light illumination. The photocurrent density of TMP (0.16 mA/cm2) was ∼1.1, 2.7, and 3.5 times higher than that of sample TP (0.15 mA/ cm 2 ), TM (0.06 mA/cm 2 ), and T (0.045 mA/cm 2 ), respectively, demonstrating the promoted charge migration efficiency contributed by the selective coloading of Pt and MnOx. The photo-irradiation stability of samples was also investigated (Figure 7c). Under UV light illumination, a quick drop of photocurrent density was observed initially. Afterward, the photocurrent densities of four samples leveled off, suggesting the remarkable stability of the prepared sample as photonaodes. 3.5. M-S Measurement. Mott−Schottky (M-S) plots of samples were measured in 1.0 M KOH aqueous solution as electrolyte. In Figure 8, as for samples T and TP, the slopes of the tangent line were always positive in the whole range of potential, suggesting that the as-obtained TiO2 photocatalyst was an n-type semiconductor. However, for sample TM and TMP, positive (R1) and negative (R2) slopes coexisted, indicating that the capacitance response in R1 was controlled by an n-type semiconductor, while R2 was controlled by a ptype semiconductor.61,62 Therefore, it was deduced that the

Figure 8. M-S plots of T, TM, TP, and TMP in 1.0 M KOH electrolyte at a frequency of 1 kHz.

prepared MnOx nanoflake was of p-type. Thus, a p−n junction between TiO2 and MnOx was formed. Furthermore, the qualitative analysis of slopes of M-S plots showed that the introduction of Pt and MnOx led to a smaller slope, indicating a higher charge carrier density. Quantitative calculation of the slopes of M-S plots was also conducted to investigate the charge carrier densities of samples.63,64 Nd = (2/εε0e0)[d(1/C 2)/d V ]−1

(5)

where Nd is the donor density; ε represents the dielectric constant of TiO2; ε0 represents permittivity of vacuum; e0 represents electron charge; and V means the applied bias at the electrode. For TiO2, the value of ε is 170. The calculated charge densities of T, TM, TP, and TMP were 5.52 × 1017, 7.61 × 1017, 8.7 × 1018, and 2.1 × 1019 cm−3, respectively. For sample TM, the enhanced charge density was due to the accelerating electron−hole transfer across the p−n junction. For sample TP, the m−s junction between Pt and TiO2 was formed, and the electrons would flow from TiO2 to Pt owing to the larger work function of Pt, and thus suppressed the recombination of electrons and holes on TiO2, leading to a higher electron density. For sample TMP, the p−n junction on TiO2 {001} facets and m−s junction on TiO2 {101} facets collaboratively facilitated the migration of electrons and holes. The improved charge separation and migration were deemed responsible for the enhancement of PEC and photocatalytic performance. 3.6. Photocatalytic Mechanism. On the basis of the above results, the charge separation and migration process of ternary TiO2−MnOx−Pt heterojunction structure was proposed in Figure 9. The relative conduction band (CB) and valence band (VB) positions of anatase TiO2, MnOx, and Pt were illustrated in Figure 9a. According to the previous density functional theory (DFT) calculations, there was a slight difference between the valence band and conduction band for {101} and {001} facets of anatase TiO2 nanosheets.51 The CB bottom and VB top of {101} facets are lower than those of {001} facets. This difference demonstrates the feasibility of charge migration. The bandgap energy of MnOx obtained from the UV−vis spectrum was ca. 1.7 eV. The positions of CB and VB of MnOx at the point of zero charge were reckoned using the following empirical equations:50 1 ECB = χ − E0 − Eg (6) 2 E VB = ECB + Eg F

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Figure 9. Schematic diagram of proposed photocatalytic CO2 reduction mechanism of sample TMP. The relative band energy positions of TiO2, Pt, and MnOx (a) before contact and (b) after contact and under irradiation.

where χ is the bulk electronegativity of the MnOx (5.95 eV);65 E0 is the energy of free electrons versus the normal hydrogen electrode (ca. 4.5 eV); and Eg is the band gap of MnOx. The calculated results showed that the CB bottom and VB top of MnOx were located at 0.6 and 2.3 eV versus normal hydrogen electrode (NHE), respectively. Considering that acid gas (CO2 and H2O) was adsorbed on the surface of photocatalyst films when evaluating the photocatalytic activities, we assume TiO2− MnOx−Pt heterojunction worked at pH = 6. For TiO2, its band edge calculated from Equation 6 did not have to convert, because the point of zero zeta potential (pzzp) of TiO2 is pH = 5.8, which can be approximately considered as pH = 6.66 For MnOx, its isoelectric point is pH = 4.67 We assume the pzzp of MnOx is also pH = 4, because there are no additional potentialdetermining ions except for OH− and H+. For MnOx, eq 6 can be modified into ECB = χ − E0 −

1 Eg − ΔpH 2

were excited from TiO2 VB to CB, leaving the holes in the VB. After that, the photoinduced holes and electrons migrated to {001} and {101} facets of TiO2 nanosheets, respectively, in virtue of the surface heterojunction.51 On {001} facets, the accumulated holes further transferred to MnOx, the driving force of built-in electric field derived from the as-formed p−n junction, while the photogenerated electrons tended to migrate oppositely. In the meantime, on {101} facets, the abundant electrons would migrate toward Pt nanoparticles under the effect of the Schottky barrier formed between n-type TiO2 and Pt. Then Pt nanoparticles served as the reactive sites, and henceforth the accumulated electrons on Pt nanoparticles participated in the conversion of CO2 into fuels. As a result, the photogenerated charge carriers can be spatially separated by the cross-coupling effect of surface heterojunction between {001} and {101} facets, p−n junction between p-type MnOx and ntype TiO2 {001} facet, and m−s junction between metallic Pt and TiO2 {101} facet. The collective contribution of three heterojunctions suppresses the recombination of charge carriers and improves the photocatalytic performance.

(8)

Herein, ΔpH is the potential drop across the Helmholtz layer due to specific adsorption of OH− and H+ ions. Finally, the calculated CB bottom and VB top of MnOx are 0.48 and 2.18 eV, respectively, which were shown in Figure 9a. The Fermi level of MnOx, located near the VB, was lower than that of TiO2. When the p-type MnOx was deposited on the {001} facet of n-type anatase TiO2 nanosheet, a p−n junction at the interface of MnOx and TiO2 {001} facet was formed. When TiO2 and MnOx integrated, the Fermi level of TiO2 decreased, whereas the Fermi level of MnOx ascended until their Fermi levels reached equilibrium. Moreover, the whole energy band of MnOx shifted upward, while that of TiO2 shifted downward. On the one hand, during the dynamic process of electron migration, the negative charges accumulated on the MnOx side at the interface between TiO2 and MnOx, while the positive charges aggregated on the other side (TiO2), and a built-in electric field from TiO2 to MnOx was formed. On the other hand, the Fermi level of Pt located at 1.1 eV versus NHE, which was lower than that of TiO2.52 Therefore, when Pt nanoparticles were intimately attached on the {101} facets of TiO2, m−s junctions at the interface of Pt and TiO2 {101} facets were formed. The electron transfer from TiO2 to Pt was expedited by the Schottky barrier. When light irradiation was introduced in the ternary TiO2− MnOx−Pt photocatalytic system, the photogenerated electrons

4. CONCLUSIONS In summary, a ternary TiO2−MnOx−Pt hybrid photocatalyst was fabricated by a facile hydrothermal and photodeposition method. The as-prepared composite photocatalyst exhibited improved yields of CH4 and CH3OH, which is over threefold of bare TiO2 nanosheets films. The enhanced activity stems from three different junctions amalgamated in a sole composite for synergistic activity enhancement. Namely, the coexposed holerich {001} facets and electron-rich {101} facets of TiO2 formed a surface heterojunction, which separated the photoexcited electrons and holes onto different facets as the formation sites for the p−n junction and m−s junction, respectively. The constructed multijunctions on the surface of the well-faceted TiO2 nanosheets expedited the separation and transfer of photogenerated charge carriers. This was verified by the photocurrent−time curves and Mott−Schottky plots that the photocurrent response and charge carrier density of the TiO2− MnO x −Pt photocatalyst had a dramatic enhancement compared with bare TiO2. Such a multijunction design protocol may be applied to the fabrication of other catalysts. This work sheds light on the construction and comprehension of multijunction photocatalysts. G

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AUTHOR INFORMATION

Corresponding Author

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

Jiaguo Yu: 0000-0001-9308-2882 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (51320105001, 21573170, U1705251, and 21433007), the Natural Science Foundation of Hubei Province (2015CFA001), Innovative Research Funds of SKLWUT (2017-ZD-4), and the Fundamental Research Funds for the Central Universities (2016-YB-005).

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