Dependence of exposed facet of Pd on photocatalytic H2-production

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Dependence of exposed facet of Pd on photocatalytic H2-production activity Shaowen Cao, Han Li, Yao Li, Bicheng Zhu, and Jiaguo Yu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00259 • Publication Date (Web): 16 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018

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ACS Sustainable Chemistry & Engineering

Dependence of exposed facet of Pd on photocatalytic H2production activity Shaowen Cao,† Han Li,† Yao Li,† Bicheng Zhu,† and Jiaguo Yu*,†,‡ †

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing,

Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, P. R. China ‡

Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia

*Corresponding author. E-mail: [email protected] (J. Yu) KEYWORDS: Hydrogen production, Photocatalysis, Cocatalysts, Palladium, Interface interaction Abstract: Structure-sensitive activity of metal cocatalysts in photocatalysis has been one of the most important topics in recent years. Herein, we construct a model photocatalytic system of shape-controlled Pd/TiO2 hybrids to investigate the effect of surface atomic structure of Pd cocatalysts in photocatalytic H2 evolution reaction. Cubic and tetrahedral Pd nanocrystals are respectively in situ deposited on the surface of cubic anatase TiO2, establishing strong interface contact between the two indispensable components in photocatalytic H2 evolution reaction: photocatalyst (TiO2) and cocatalyst (Pd). Experimental results and theoretical calculations based on this Pd/TiO2 model system demonstrate that the cocatalytic performance of Pd(111) facets is more effective than that of Pd(100) facets in various reaction solution. This is due to the more efficient capture of photogenerated electrons from the conduction band of TiO2, as well as the more preferable adsorption of H, transformation of H into H2 molecule and desorption of H2 molecule in the HER process of Pd(111) surface. 1

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INTRODUCTION The increasing demand of renewable energy, has stimulated research focus on the renewable and sustainable energy conversion and storage, such as electrocatalytic and photocatalytic water splitting, solar cells, regenerative fuel cells, rechargeable metal– air batteries.1-6 Among them, photocatalytic water reduction is a promising strategy to convert solar energy into hydrogen energy over semiconductor photocatalysts.7-12 The efficiency of this reaction heavily relies on the transfer and utilization of photoexcited charge carriers from the photocatalysts.13-19 Coupling with highly efficient cocatalysts is desirable for improving this charge transport process in photocatalytic system.20-25 Especially, noble metal nanocrystals can serve as extremely effective cocatalysts by trapping photoexcited electrons from the photocatalysts and reducing the overpotential of water reduction reaction. In the past decades, noble metal nanocrystals such as Pt, Au, Pd have been widely used as efficient catalysts in diverse catalytic applications.26-29 And the size and shape of metal nanocrystals have shown great impact in catalytic reactions. Normally, decreasing the size of catalyst particles could enhance the catalytic performance due to the increased surface atoms, while different shapes with different exposed crystal facets are varied on atom densities and coordination environment, thereby affecting the catalytic activity.30,31 Well-defined cubic, tetrahedral, octahedral shapes etc. have been synthesized for the clearance of structure-sensitive activity/selectivity relationship of these noble metals towards a variety of catalytic reactions such as water-gas shift, oxygen reduction reaction, isomerization of unsaturated olefins, hydrogenation of 2


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aromatic molecules, etc.30-32 These reactions show strong dependence on the surface atomic structure of metal nanocrystals, which are enclosed by different facets. However, studies on photocatalytic water reduction mainly focus on the modification of semiconductor photocatalysts.33-40 For instance, decreasing the size of TiO2 could influence the bandgap and lead to a more negative conduction band to enhance the photocatalytic activity.41 Only a few attempts have been made to control the cocatalytic behavior of metal nanocrystals based on their structure sensitivity.42-44 The metal cocatalysts in the photocatalytic process are mainly prepared by impregnation-reduction or photo-deposition method, which is difficult to control the sizes and shapes. The resulting metal nanoparticles are not uniform with well-defined surface atomic structure. On the other hand, ex situ methods coupling photocatalysts with pre-prepared well-shaped metal nanocrystals are ineffective in establishing a strong metal-semiconductor interaction due to the poor interface contact. In addition to the afore-mentioned charge transport process, the availability of hydrogen evolution reaction (HER) on metal surface is another vital factor for the photocatalytic efficiency.45-48 Different facets on the nanocrystals may have a profound influence on the HER process because of their different atomic arrangement. To this end, singlefacet metal nanocrystals establishing close contact with well-defined semiconductor particles should be an ideal model system for investigating the facet effect of metal cocatalyst for photocatalytic water reduction. Herein, we construct the Pd/TiO2 hybrid system as the model system by in situ growth of cubic and tetrahedral Pd nanocrystals onto the surface of cubic anatase TiO2 3



photocatalysts. The resultant strong interface contact of Pd and TiO2 enables the investigation of cocatalytic behavior of Pd(100) and Pd(111) facets in photocatalytic water reduction. The comparison in the H2 production in the presence of various scavengers, exclusively demonstrates that the photocatalytic H2 evolution reaction more effectively occurs on the Pd(111) facet. Characterizations indicate that Pd(111) facet is a better electron trapping agent than the Pd(100) facet. Moreover, density functional theory (DFT) calculations for each step in the HER process of different facets elucidate that the Pd(111) facet is more preferable towards the adsorption of H, transformation of H into H2 molecule and desorption of H2 molecule.

EXPERIMENTAL SECTION Synthesis of cubic-TiO2. The cubic-TiO2 was synthesized via a solvothermal process.49 1 mL 1-butyl-3-methylimidazolium tetrafluoroborate ([bmin][BF4]), 2.5 mL deionized water, and 40 mL acetic acid (HAc) were well mixed. Then 1 mL tetrabutyl titanate (TBT) was added into the solution under magnetic stirring. The resultant mixture was transferred into a 50 mL Teflon-lined autoclave and heated at 180 °C for 24 h. The white precipitate was washed with water and ethanol for several times, and dried in an oven at 80 °C overnight. The as-prepared sample was denoted as TC. Synthesis of Pd/cubic-TiO2 hybrid photocatalysts. Different facet-selective capping agents were used to obtain different surface facets of Pd. The Br –and I–ions could act as a capping agent of the Pd{100} facets and PVP could serve as a stabilizer.50 The HCHO and Na2C2O4 was used as the reductant and {111}-facet selective agent.51 4


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The Pd/cubic-TiO2 hybrid photocatalysts were synthesized as follows. For the preparation of cubic Pd nanoparticles coupled with cubic-TiO2 photocatalyst, 32 mg TC, 4.4 mg Na2PdCl4, 89.3 mg KBr and 25.0 mg poly vinyl pyrrolidone (PVP, Mw = 300000) were dissolved in 5 mL deionized water by ultrasonication for 30 min. Then, 150 μL KI aqueous solution (0.01 M) was added to the aqueous suspension under stirring for 10 min. The pH of the mixed solution was adjusted to 3 with HCl solution (1 M). Subsequently the volume of the solution was added to 7.5 mL with DI water. Afterward, the mixture was transferred into a 50 mL Teflon-lined autoclave and heated at 150 °C for 3 h. After cooling down to room temperature, the product was washed with acetone, water and ethanol for several times and dried at 80 °C. Then, the resultant product was irradiated with an UV/Ozone cleaner for 30 min to generate nearly atomically clean surfaces. The obtained sample was denoted as PC-TC. For the preparation of tetrahedral Pd nanoparticles coupled with cubic-TiO2 photocatalyst, 32 mg TC, 4.4 mg Na2PdCl4, 50.3 mg Na2C2O4 and 8.3 mg PVP (Mw = 300000) were dissolved in 5 mL deionized water by ultrasonication for 30 min. Then, 200 μL formaldehyde solution was added to the aqueous suspension under stirring for 10 min. The pH of the mixed solution was adjusted to 3 with HCl solution (1 M). Subsequently the volume of the solution was added to 7.5 mL with DI water. Afterward, the mixture was transferred into a 50 mL Teflon-lined autoclave and heated at 150 °C for 2 h. After cooling down to room temperature, the product was washed with acetone, water and ethanol for several times and dried at 80 °C. Then, the resultant product was

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irradiated with an UV/Ozone cleaner for 30 min to generate nearly atomically clean surfaces. The obtained sample was denoted as PT-TC. Characterization. The morphology was analyzed by FESEM (JEOL JSM-7500F, Japan), TEM and HRTEM (JEOL JEM-2100, Japan), and spherical aberrationcorrected HRTEM (JEOL JEM-ARM200F, Japan). The crystal phases were determined by an X-ray diffractometer (Rigaku, Japan) with Cu Kα radiation at a scan rate (2θ) of 0.05°s1. The Fourier transform infrared spectra (FTIR) of the samples were recorded on an IR Affinity-1 FTIR spectrometer. The X-ray photoelectron spectroscopy (XPS) measurements were performed on an ultra-high-vacuum VG ESCALAB 210 electron spectrometer loaded with a multi-channel detector. All the binding energies were referenced to the C 1s peak at 285 eV of the adventitious carbon. The loading amount of Pd element in the as-prepared Pd-TiO2 hybrid photocatalysts was measured on a 4300DV inductively coupled plasma atomic emission spectrometry (ICP-AES).The optical properties were investigated by diffuse reflectance spectra (DRS) using a UV– visible spectrometer (UV-2600, Japan). The Brunauer–Emmett–Teller (BET) specific surface areas (SBET) were determined by a Micromeritics ASAP 2020 nitrogen adsorption apparatus (USA) according to a multipoint BET method using adsorption data in the relative pressure (P/P0) range of 0.05–0.25. Pore size distributions were obtained via the Barrett–Joyner–Halenda (BJH) method using the desorption data. The nitrogen adsorption volume was used to calculate the pore volume and average pore size. Time-resolved photoluminescence (TRPL) decay spectra were measured using a

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FLS920 fluorescence lifetime spectrophotometer (Edinburgh Instruments, UK). The excitation wavelength was 290 nm. Photoelectrochemical measurement. Electrochemical impedance spectroscopy (EIS) was measured at the open circuit voltage (0.5 V), using a CHI 660D electrochemical analyzer (electrochemical workstation, Chenhua Instrument, Shanghai, China) with a conventional three-electrode configuration. The as-prepared sample, a Pt wire and Ag/AgCl (saturating KCl) were used as working electrode, counter electrode and reference electrode, respectively. Na2SO4 (0.5 M) aqueous solution was utilized as the electrolyte, and a low-power LED (3 W, 420 nm, Shenzhen LAMPLIC Science Co, Ltd, China) was used as the light source. The as-obtained samples (50 mg) were ground with silver-epoxy adhesive and ethanol to form a slurry. The slurry was then coated on a 2 × 1.2 cm2 F-doped SnO2-coated glass (FTO glass) substrate by the doctor-blade method, and then dried in air for 3 h. Photocatalytic H2 evolution reaction. The performance of photocatalytic hydrogen production was evaluated in a 100 mL three-neck flask with three sealed openings at room temperature and atmospheric pressure. A 350 W Xe lamp (XD350, Changzhou Siyu, China) was used as the light source to trigger the photocatalytic reaction. In a typical photocatalytic experiment, 50 mg of the as-prepared photocatalyst was dispersed in 80 mL aqueous solution containing different kinds of sacrificial agents (10 vol.% triethanolamine, 10 vol.% lactic acid, 25 vol.% CH3OH, respectively, as the electron donor) by sonication and stirring. Then N2 was purged through the mixture for 30 min to ensure an anaerobic condition in the reactor. Under irradiation, 0.4 mL gas 7



was extracted every hour from the reactor and was injected into the gas chromatograph (GC-14C, Shimadzu, Japan, 5 Å molecular sieve column) to detect the produced hydrogen amount using N2 as the carrier gas. Computational methods. Density functional theory (DFT) calculation was carried out with the Perdew–Burke–Ernzerhof (PBE) functional within the generalized gradient approximation using the CASTEP code. The cutoff energy and Monkhorst– Pack k-point mesh were set as 450 eV and 4 ×4 ×1, respectively. Pd{100} and Pd{111} models were built by cleaving the bulk Pd unit cell along {100} and {111} directions, respectively. Both these two models are composed of five layers and each layer contains four Pd atoms. To simulate surface, the bottom two layers were fixed and a vacuum space of 20 Å was constructed above the Pd20 structures. These models were geometrically optimized and then the work function and energy of the optimized models were calculated. During the geometry optimization, the convergence tolerances were set as 0.03 eV/Å for maximum force and 1.0 × 10–5 eV/atom for energy.

RESULTS AND DISCUSSION Structure analysis of photocatalysts. Well-defined anatase TiO2 nanoparticles with cubic morphology and a uniform edge length of ~100 nm (See FESEM image in Figure 1) were successfully prepared via a solvothermal method. A single TiO 2 cube (TC) contains four lateral {100} facets and two top {001} facets. The percentages of exposed {100} and {001} facets are calculated to be about 75% and 25%, respectively. Using anatase TiO2 cubes as the substrates, we in situ grew cubic and tetrahedral Pd 8


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nanocrystals via a simple hydrothermal process. FTIR measurements was used to confirm the removal of PVP from the surface of the samples. As shown in Figure 2, the resultant product treated with an UV/Ozone cleaner did not show obvious peaks of PVP, indicating the successful removal of PVP form the sample surface. Figure 3 shows that the Pd nanocrystals are well distributed on the surface of TiO2 in both cubic-Pd/cubicTiO2 (PC-TC) and tetrahedral-Pd/cubic-TiO2 (PT-TC) hybrid structures. The HRTEM images indicate that uniform Pd nanocubes and tetrahedrons with edge length of ~14 nm have been successfully obtained. These well-shaped nanocrystals can be indexed as face-centered cubic (fcc) structures and are enclosed by {100} and {111} facets, as evidenced by the corresponding spacing of lattice fringes of 0.196 and 0.225 nm, respectively. In addition, the indexed lattice spacing of 0.385 or 0.382 nm matches well with the {100} planes of anatase TiO2. The results indicate that Pd nanoparticles tend to be reduced on {100} facets of TiO2. This is because the electrophilic character and the coordinated environment of Ti cations on the {100} facets favour the reduction of metal ions.52 Good crystalline contact between Pd and TiO2 can be clearly observed from the HRTEM images of both PC-TC and PT-TC hybrids. The real contents of Pd element measured by ICP-AES are 2.73 and 2.81 wt% for PC-TC and PT-TC hybrids, respectively (See Table 1).

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Figure 1. FESEM image of the TC sample.

Figure 2. FTIR spectra of the as-prepared samples.

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Figure 3. TEM and HRTEM images of (a) the PC-TC and (b) PT-TC samples.

Table 1. Physical properties of the samples. Pd loading amount Sample

SBET

Vpore

dpore

(m2 g1)

(cm3 g1)

(nm)

Colour (wt%)

TC

0

White

15

0.04

10

PT-TC

2.81

Gray

13

0.03

10

PC-TC

2.73

Gray

13

0.03

10

Crystal phase and chemical state of photocatalysts. Figure 4 shows the XRD patterns of the cubic TiO2 and Pd/TiO2 hybrids. The XRD peaks of anatase TiO2 (JCPDS No. 04-0477) can be observed for all photocatalyst samples, without any other TiO2 phase. The diffraction peaks at 25.20o, 36.90o, 37.83o, 38.49o, 47.89o, 53.90o,

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54.91o, 61.96o, 62.64o, 68.68o, 70.12o, 75.00o and 75.97o correspond to (101), (103), (004), (112), (200), (105), (211), (213), (204), (116), (220), (215) and (301) planes of anatase TiO2. The other peaks in the patterns of Pd/TiO2 hybrids can be assigned to metallic fcc Pd (JCPDS No. 05-0681), verifying the good crystallization of Pd cocatalyst. The chemical states are also investigated by XPS analysis (Figure 5). The strong photoelectron peaks appear at the binding energies of 459 eV (Ti 2p), 531 eV (O 1s) and 285 eV (C 1s) for all samples in the XPS survey spectra (Figure 5a). Compared with TC, the PT-TC and PC-TC samples both have an additional weak peak at the binding energy of 336 eV belonging to Pd 3d. The C 1s peak is attributed to the surface adventitious carbon from the instrument. It is noted that four peaks can be deconvoluted in the Pd 3d high-resolution spectra of PT-TC and PC-TC samples (Figure 5b). The two strong peaks at 335.3 and 340.6 eV are attributed to Pd 3d5/2 and Pd 3d3/2 of metallic Pd.53 The other two shoulder peaks at 336.9 and 342.1 eV belong to the Pd 3d5/2 and Pd 3d3/2 of Pd2+,54 which is caused by the surface O2 dissociation due to the long-term storage of the sample before XPS characterization.55 Furthermore, the O 1s signal in Figure 5c can be deconvoluted into two peaks. The peaks at 530.4 and 531.7 eV can be ascribed to the lattice oxygen associated with Ti-O-Ti linkages in TC and the hydroxyl group (OH) of adsorbed water molecules, respectively.56 The high-resolution XPS spectra of Ti 2p in Figure 5d exhibit two characteristic peaks at 459.2 and 464.9 eV belonging to Ti 2p3/2 and Ti 2p1/2, respectively, which are identical to Ti4+ of TiO2.57

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Figure 4. XRD patterns of TC, PC-TC and PT-TC.

Figure 5. XPS survey spectra (a) and high-resolution XPS spectra of Pd 3d (b), O 1s (c), Ti 2p (d) for the TC, PT-TC and PC-TC samples. 13



Strong interaction between pristine photocatalyst and cocatalyst. The intimate contact is crucial for achieving an efficient synergetic effect between photocatalyst and cocatalyst, to promote the transfer of photogenerated charge carriers from photocatalyst to cocatalyst, further to enhance the solar-to-hydrogen conversion.58-62 To elucidate the close interface contact of Pd/TiO2 hybrids, aberration-corrected HRTEM observation was conducted for the PT-TC sample. As shown in Figure 6a, the lattices of Pd and TiO2 are in firm contact, with the interfacial portion of the Pd nanocrystals partially buried into the surface layer of TiO2. This result reveals that strong interface contact has been established between Pd and TiO2 during the in situ growth process. Moreover, the UV–vis absorption spectra of all photocatalysts were measured to clarify the interface contact between Pd and TiO2. As compared to pure TiO2, the optical absorption of Pd/TiO2 hybrids in the visible region is obviously stronger, which is because the loaded Pd nanocrystals change the color of samples into deep gray. Importantly, a distinct peak at 459 or 485 nm was observed for PC-TC or PT-TC sample (see Figure 6b). This peak can be identified as the surface plasmon resonance (SPR) effect of Pd nanocrystals. The different SPR absorption peaks of PT-TC and PC-TC samples should be caused by the different geometric asymmetries which result in different distinct dipole resonance.63 Note that SPR of freestanding Pd nanoparticles is normally located in the UV region.64,65 However, significant red shift of SPR occurs for the Pd/TiO2 hybrids. This is because the TiO2 substrates increase the local dielectric constant near the Pd nanocrystals and delocalize the surface electrons of Pd upon plasmon excitation, resulting in electron deficiency and shifting the absorption band to 14


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longer wavelength.66,67 Such phenomenon further demonstrates the strong interface contact between Pd and TiO2. Hence, a strong metal−semiconductor interaction was established, leading to the heterojunction formation at the Pd/TiO2 interface, which can effectively capture the photogenerated electrons and holes, and subsequently retard the recombination of electron-hole pairs through the photocatalytic reaction.68-70

Figure 6. (a) Spherical aberration-corrected HRTEM image of PT-TC sample; (b) UVvis DRS of TC, PT-TC and PC-TC samples.

Photocatalytic hydrogen production performance of photocatalysts. Inspired by the precise synthesis of Pd/TiO2 hybrids with well-defined shapes and strong interface contact, their photocatalytic H2-production activities were investigated in the presence of various scavengers. Control experiments indicate that no noticeable H2 was produced over cubic TiO2 without the deposition of Pd nanocrystals, suggesting that the cocatalytic behavior of Pd is critical in this photocatalytic reaction. Figure 7 shows the comparison of the photocatalytic H2 production for Pd/TiO2 hybrids with different Pd shapes. Through a 4-run cycling test of 16 hours, both Pd/TiO2 hybrids exhibit quite stable photocatalytic performance for H2 production using different sacrificial agents 15



including methanol (25 vol.%), triethanolamine (10 vol.%) and lactic acid (10 vol.%). In particular, the H2-production rate of PT-TC sample is 1.52 times higher than that of the PC-TC sample in all experiments. The results show that the photocatalytic H2 evolution is extremely sensitive to the morphologies of Pd nanocrystals, which does not relies on the reaction solution. In addition, the photocatalytic activity under visible light irradiation (>420 nm) was evaluated using the same light source with the addition of a 420 nm cut-off filter, to examine the SPR effect. As shown in Figure 8, both PT-TC and PC-TC samples show observable H2 production under visible light irradiation in 10 vol.% lactic acid solution. This result indicates the SPR-induced photocatalytic behavior of the supported Pd nanoparticles, as TiO2 cannot be excited in this condition.

Figure 7. Photocatalytic hydrogen production activities of TC, PT-TC and PC-TC samples with different sacrificial agents of (a) 25 vol.% methanol solution, (b) 10 vol.% triethanolamine (TEOA) solution and (c) 10 vol.% lactic acid solution. 16


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Figure 8. Photocatalytic hydrogen production activities of PT-TC and PC-TC in 10 vol.% lactic acid solution under visible light irradiation (>420 nm).

Essence of structure-dependent cocatalytic behavior of Pd. To pursue the essence of such structure-dependent cocatalytic behavior of Pd nanocrystals, a series of characterization and theoretical calculation have been performed. We first investigated the BET surface areas and pore structures of all photocatalyst samples. As shown in Figure 9, all the nitrogen adsorptiondesorption isotherms are type IV isotherms with H3 hysteresis loops according to Brunauer-Deming-Deming-Teller (BDDT) classification.71,72 The pore size distribution (inset) shows a wide range from 2 to 140 nm, indicating the presence of mesopores and macropores. It is found that the deposition of Pd nanocrystals slightly decreases the BET surface area of TiO2, probably due to the coverage of TiO2 surface pores by Pd nanocrystals. However, there is no difference of BET surface areas and pore structures between PT-TC and PC-TC samples (Table 1). Hence, the structure-dependent photocatalytic H2 production is not related to such factors.

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Figure 9. Nitrogen adsorptiondesorption isotherms and the corresponding pore size distribution curves (inset) of the TC, PT-TC and PC-TC samples.

Figure 10. (a) Schematic illustration of the mechanism for enhanced H2 production in Pd/TiO2 hybrids.

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Figure 11. (a) EIS Nyquist plots of the TC, PC-TC and PT-TC samples; (b) TRPL decay curves of the PC-TC and PT-TC samples; (c) schematic illustration of photogenerated electron transfer from the conduction band of TiO2 to Pd(100) and Pd(111) surface.

Now the question comes: why Pd nanocrystals with different facets cause such a difference in the photocatalytic H2 production over TiO2? As discussed above, it is believed the strong interface interaction between Pd and TiO2 favors the interface charge transfer. Under irradiation, the Pd nanocrystals (even Pd2+ on the shallow surface) can capture photoexcited electrons from the surface of TiO2 through the metalsemiconductor heterojunction. As shown in Figure 10, the electrons transferred to the 19



Pd surface can react with the surface-adsorbed H+ to produce H2; meanwhile the sacrificial agent can consume the hole left behind in the valence band of TiO2. By this way, the electron-hole separation can be greatly improved in the presence of Pd nanocrystals, finally leading to the enhancement of photocatalytic H2 production. As a powerful method, electrochemical impedance spectra (EIS) analysis can be applied to study the charge transfer process. The EIS Nyquist plots of the TC, PC-TC and PT-TC samples are shown in Figure 11a. A Randles electrical equivalent-circuit model (inset of Figure 11a) was used to simulate the EIS test system, in which the Rp stands for charge-transfer resistance of the working electrode and Rs represents the total Ohmic resistance of the electrolyte solution.73 Hence, the size of the arc radius in the Nyquist plots corresponds to the electron transfer properties of the samples (Rp). Smaller radius indicates lower charge transfer resistance, Namely, more efficient charge transfer.74-76 It can be seen that both Pd/TiO2 hybrids show a smaller radius than that of TiO2 sample, confirming that the deposition of Pd nanocrystals promotes the charge transfer process.77,78 Importantly, the PT-TC sample shows the smallest radius, indicating that the introduction of Pd(111) nanocrystals is more effective than that of Pd(100) nanocrystals to capture the electrons from the conduction band of TiO2. To further probe the charge transfer dynamics affected by the different Pd facets, time-resolved photoluminescence (TRPL) decay spectra were investigated for Pd/TiO2 hybrids (Figure 11b). A bi-exponential fitting is applied to analyze the luminescence decay curves, which suggests that the decay lifetime is contributed by radiative process (τ1) and non-radiative process (τ2).79-81 It is noteworthy that both lifetime components of 20


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PT-TC sample are shorter that of PC-TC sample (inset of Figure 11b) and moreover, the average lifetime (τav) of PT-TC is shorter than that of PC-TC, indicating the more efficient electron transfer rate from TiO2 to Pd(111) surface as compared to that from TiO2 to Pd(100) surface. We further compare the work functions of different Pd surface by DFT calculations and find that, the computational work function of Pd(100) surface (4.89 eV) is smaller than that of Pd(111) surface (5.04 eV), in consistent with the previous calculated and experimental values.82,83 Therefore, it is rational that the tetrahedral Pd with {111} facets are more effective than the cubic Pd with {100} facets, to capture photogenerated electrons from the conduction band of TiO2 (Figure 11c).

Figure 12. Side views of the models in the H2 evolution process. (a,e) Initial Pd(100) and Pd(111) surfaces. (b,f) Pd(100) and Pd(111) surfaces adsorbed with two H atoms. (c,g) H2 molecule-adsorbed Pd(100) and Pd(111) surfaces. (d,h) Fresh Pd(100) and Pd(111) surfaces after the desorption of generated H2 molecule. The spheres with large and small radii are Pd and H atoms, respectively. 21



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Figure 13. Energy variation in the H2 evolution process on Pd(100) facets and Pd(111) facets. The total energy of initial Pd surface and two isolated H atoms is set to zero. The parenthesized numbers mark the three reaction steps. TS is short for transition state.

Table 2. Calculated Eads, Ebar and Edes. Facet

Eads (eV)

Ebar (eV)

Edes (eV)

Pd (100)

–6.57

0.034

0.29

Pd (111)

–6.70

0.018

0.16

In addition to the above-mentioned charge transfer between Pd and TiO2, the availability of hydrogen evolution reaction (HER) on Pd surface is another vital factor for the photocatalytic reaction. We then calculated the percentage of surface atoms of cubic Pd and tetrahedral Pd nanoparticles according to the well-known method84 reported in the year of 1969, which are 8.22% and 20.98%, respectively. In addition, the percentage of edge and corner atoms of cubic Pd and tetrahedral Pd nanoparticles

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are 0.23% and 1.22%, respectively. Considering the 1.52 times performance difference of H2 production between PT-TC and PC-TC samples, it is believed that the under-coordinated surface atoms not only the edge and corner atoms are the reactive sites. Therefore, we employ DFT calculations for each step in the HER process of different Pd surfaces. The HER occurred on Pd surface consists of three steps: Pd + 2H+ + 2e– → Pd–2H

(1)

Pd–2H → Pd–H2

(2)

Pd–H2 → Pd + H2

(3)

At the first step, with the help of photogenerated electrons, the Pd surface reacts with two H+ and then Pd surface with two adsorbed H atoms is formed. The second step is the transformation from the two adsorbed H atoms to adsorbed H2 molecule. At last, the H2 molecule desorbs from the Pd surface and fresh Pd surface reappears. This H2 evolution process is universal for both Pd(100) facets and Pd(111) facets as shown in Figure 12. The energy variation in the H2 evolution process on Pd(100) facets and Pd(111) facets was examined by DFT calculation (Figure 13). The energy change in step (1) and (3) are defined as adsorption energy (Eads) of H atoms and desorption energy (Edes) of H2 molecule, respectively. In general, Eads and Edes are negative and positive values, respectively. Moreover, a more negative Eads corresponds to a thermodynamically more favorable adsorption process, and a smaller Edes means that the desorption of H2 molecule from Pd surface is easier. As for the reaction step (2), the generation of H2 molecule needs to overcome an energy barrier (Ebar) which determines the reaction rate. 23



The calculated Eads, Ebar and Edes are listed in Table 2. It is found that the Pd(111) surface has more negative Eads, smaller Ebar and Edes. This indicates that the Pd(111) surface has stronger adsorption ability of H atoms than Pd(100) surface and moreover, the transformation from H atoms to H2 molecule and the desorption of H2 molecule on Pd(111) surface are more easier than those on Pd(100) surface. All these theoretical aspects strongly reveal that The HER process is more favorable on the Pd(111) surface than on the Pd(100) surface.

CONCLUSIONS In summary, in situ growth of cubic and tetrahedral Pd nanocrystals onto the surface of cubic anatase TiO2 has been achieved, to form Pd/TiO2 hybrid photocatalysts with strong interface contact between the catalyst component and photocatalyst component. The Pd/TiO2 hybrids are thus employed as the model system to elucidate the effect of exposed facet of Pd nanocrystals on enhancing the performance of photocatalytic H2 evolution. The results demonstrate that the Pd(111) facet is more effective than Pd(100) facet to cocatalyze the photocatalytic H2 evolution reaction over TiO2, unaffected by the reaction solution. This is because the Pd(111) surface exhibits the better electron capture ability and more preferable adsorption of H, transformation of H into H2 molecule and desorption of H2 molecule during the HER process, in comparison with Pd(100) surface. It is noteworthy that such structure-dependent cocatalytic behavior of Pd nanocrystals are also observed in the photocatalytic H2 production over CdS and graphitic carbon nitride in our undergoing experiments. The finding in this work 24


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provides a rational way for optimizing the performance of metal cocatalysts in photocatalytic reactions by tuning the exposed facets.

AUTHOR INFORMATION Corresponding author. *E-mail: [email protected] (J. Yu). Phone: 0086-27-87871029. Fax: 0086-2787879468. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (51472191, 21773179, 51320105001 and 21433007), the Natural Science Foundation of Hubei Province of China (2017CFA031 and 2015CFA001) and the Innovative Research Funds of SKLWUT (2017-ZD-4).

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TOC/Abstract graphic

Remarkable facet dependence of Pd on photocatalytic H2 evolution is elucidated by the Pd/TiO2 model system with strong interface interaction.

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Dependence of exposed facet of Pd on photocatalytic H2-production

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