Article pubs.acs.org/JPCC
Titania-Supported Palladium/Strontium Nanoparticles (Pd/SrNPs@P25) for Photocatalytic H2 Production from Water Splitting E. Hussain,†,‡ I. Majeed,† M. Amtiaz Nadeem,§ A. Badshah,† Yuxiang Chen,‡ M. Arif Nadeem,*,† and Rongchao Jin*,‡ †
Department of Chemistry, Lab 27, Quaid-i-Azam University, Islamabad 45320, Pakistan Department of Chemistry & Department of Environmental Sciences, Quaid-i-Azam University, Islamabad 45320, Pakistan ‡ Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States §
S Supporting Information *
ABSTRACT: Here we demonstrate a new strategy of boosting the photocatalytic activity of titania (P25) for photocatalytic H2 production from the water splitting reaction by depositing palladium/strontium nanoparticles, forming Pd/Sr-NPs@P25. The Pd/Sr-NPs are in situ prepared on the surface of P25. The effects of Pd and Sr in the photocatalytic reactions are further revealed. Strontium in the form of strontium oxide promotes electron transfer from the semiconductor surface to palladium nanoparticles by increasing the Fermi level of the P25 support. The structural and morphological characterizations of the Pd/Sr-NPs nanocomposite are carried out using UV−vis DRS, XRD, TEM, and XPS techniques, based upon which the mechanistic insights are discussed.
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surface properties of semiconductor via fluorination and addition of electron acceptors and sacrificial reagents in the reaction medium.9,10 To overcome the second issue (i.e., too large bandgap), modification of the band structure has been carried out via different strategies such as coupling with a narrow band structure, codoping with foreign ions,11 metal/ nonmetal ion incorporation, surface sensitization by organic dyes, and noble metal depositing/loading. Some studies have revealed that metal nanoparticles (MNPs) deposited on TiO2 can withdraw electrons out of TiO2, thus changing the original charge-carrier concentration. Metal nanoparticles on the TiO2 surface can also increase the photocatalytic activity owing to the faster interfacial electron transfer from the semiconductor to metal nanoparticles. These factors greatly reduce the electron/hole recombination, resulting in efficient separation of electron−hole pairs and thus enhancing the rate of photocatalytic reaction.12 For example, extremely high catalytic activity has been reported for H2 generation from alkaline formaldehyde at room temperature (H2 generation rate ∼250 mL g−1 min−1) by ultrathin anatase TiO2 nanosheets with exposed (001) facets modified by only 2 wt % of quantum-sized Pd dots.13 From both sustainable energy and environmental friendliness perspectives, hydrogen is considered as a promising alternative owing to its high caloric value.14−16 In addition to bandgap
INTRODUCTION To solve the problems of energy crisis and global warming, the most viable and attractive way is to convert solar energy into chemical energy (e.g., in the form of hydrogen, H2), which reduces the consumption of fossil fuels and hence CO2 emission (global warming). Realization of this goal highly depends on the improvement of the catalytic efficiency of photocatalytic systems. Several visible light responsive photocatalysts with narrow band gaps have been developed, with tailored energy band structures for excellent performance in photocatalytic H2 evolution, but still having limitations such as insufficient stability and hypertoxicity.1−3 Among the photocatalysts, TiO2 is the most important one for its multiple characteristics including long-term stability (especially in aqueous environment), low toxicity, low cost, favorable band edge positions, high energy conversion efficiency, easy preparation of various morphologies, and good recyclability. But the major disadvantages of TiO2 limiting the photocatalytic performance are (i) the fast recombination of photoexcited electrons and holes (e−/h+) and (ii) the too large bandgap (3.2 eV, i.e., only active under ultraviolet light). Various modifications (e.g., loading of noble metals, doping with nonmetal elements, and formation of composites) have been conducted on the TiO2 to promote interfacial redox processes by enhancing the separation of photogenerated electrons and holes.4−6 The influences of factors such as the preparation methods, metal dispersion on titania, and formation of heterojunctions are of importance.7,8 The fast recombination issue can also be leveraged by changing the © XXXX American Chemical Society
Received: May 9, 2016 Revised: July 13, 2016
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DOI: 10.1021/acs.jpcc.6b04695 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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NPs@P25 photocatalysts were calcined and activated at 350 °C for 2 h at a heating rate of 3 °C min−1 in air atmosphere and labeled as Pd1.8-Sr0.2@P25, Pd1.4-Sr0.6@P25, Pd0.8-Sr1.2@P25, Pd0.4-Sr1.6@P25, Pd0.2-Sr1.8@P25, and Pd2@P25. The subscripts indicate the nominal wt % loading of each metal. The catalyst (5.0 mg) was placed in a reactor containing pure water plus a sacrificial reagent. The catalytic activities were monitored by an online GC equipped with a thermal conductivity detector (TCD). Photocatalyst Characterization. Powder XRD patterns were collected on a Siemens D5000 diffractometer equipped with a curved graphite filter monochromator. The X-ray source is Cu Kα (λ = 1.5418 Å, 40 mA, 40 kV). The 2θ range is from 20° to 70° (step 0.05°, scan rate 2° min−1). The anatase and rutile crystallite sizes (τ) were determined using the Scherrer equation (τ = 0.9λ/(2 cos θ)) and line widths of the anatase (101) reflection at 2θ of 25.3° and rutile (110) reflection at 2θ of 27.4°, respectively. UV−vis DRS absorbance spectra of the powder catalysts were collected over the wavelength range of 200−900 nm on a Thermo Fisher Scientific UV−vis spectrophotometer equipped with praying mantis diffuse reflectance accessory. TEM analysis was performed using a Philips CM12/STEM electron microscope, PW 6030 (120 kV). XPS data was collected using a Kratos Axis UltraDLD equipped with a hemispherical electron energy analyzer and an analysis chamber at base pressure ∼1 × 10−9 Torr. Samples were excited using monochromatic Al Kα X-ray (1486.69 eV) with the X-ray source operating at 150 W. Samples were gently pressed into thin pellets of ∼0.1 mm thickness for the analyses. A charge neutralization system was used to alleviate the sample’s charging problem during the analysis. Survey scans were collected at pass energy of 80 eV over the binding energy range 1200−0 eV, while core level scans were collected with pass energy of 20 eV. The spectra were calibrated against the C 1s signal at 285.0 eV from adventitious hydrocarbons. Raman spectroscopy analysis was carried out on a StellarNet RamanHR spectrometer. The excitation wavelength was 785 nm with a power of ca. 3 mW on the sample. The scanning range was kept between 200 cm−1 and 1000 cm−1 in air, and the resolution was ca. 4 cm−1. To obtain comparable data, all the solid samples were measured in the same glass vial at room temperature. Hydrogen Production Experiments. Photocatalytic hydrogen production over all photocatalysts was carried out in a Pyrex reactor (140 mL) for UV lamp. Photocatalyst (5 mg) was loaded in the reactor containing 25 mL of specified reaction mixtures (ethanol 5% and water 95%). Prior to the start of each photocatalytic experiment, the reactor was continuously bubbled with nitrogen at a flow rate of 10 mL min−1 for 30 min to remove dissolved and headspace oxygen in the reactor. A Spectroline model SB-100P/F lamp (100 W, 365 nm) at a distance of 10 cm from the reactor was used for UV light irradiation of the catalysts. The photon flux measured at the sample was ca. 6.5 mW cm−2 (comparable to UV flux in day light). Hydrogen generation was monitored by taking gas head space samples (0.5 mL) at different time intervals and injecting them into the gas chromatograph (Shimadzu GC 2014) equipped with a TCD detector and molecular sieve capillary column (length = 25 mm; i.d. = 0.32 mm; average thickness 0.50 um). H2 produced through photoreaction was quantified against an internal calibration curve. The photocatalytic tests for each sample were repeated at least three times. The rate of gas (H2) evolution with units such as mmol g−1 as well as mmol
engineering to extend the optical absorption of TiO2 into the visible light regime, it is also of equal importance to optimize the photogenerated electron/hole separation process on the TiO2 surface. As discussed above, metal/TiO2 nanocomposites could effectively reduce the photogenerated electron/hole recombination because metals can serve as electron sinks to promote interfacial electron transfer17−21 and to provide additional sites for hydrogen atoms to form H2.17 The surface properties of the TiO2 photocatalyst can be altered by morphology and other factors. Generally speaking, the electronic levels and energies of the solid state photocatalyst (e.g., the Fermi level, vacuum level, and work function) are of critical importance in the events of charge carrier injection and transport. The electronic levels and energies can be engineered by tailoring the composition, structure, and morphology of the solid.22 These considerations motivate us to design a new palladium nanoparticle/TiO2 photocatalyst by incorporating strontium (Sr) into the Pd/TiO2 system. In this work, we have successfully synthesized Pd/Sr-NPs@ P25 with an average diameter of 2−3 nm by an impregnation method. The Pd/Sr-NPs are in situ prepared on the surface of P25. The important effects of Pd and Sr in the photocatalytic reactions are revealed. Specifically, by the in situ method, the Pd/Sr-NPs are uniformly deposited on the surface of the support P25. These uniformly deposited Pd/Sr-NPs on the P25 surface promote the photocatalytic activity of the Pd/Sr-NPs@ P25 photocatalysts. The mechanism for the enhanced activity lies in that the introduction of strontium oxide (SrO) raises the Fermi level of the TiO2 support, leading to enhanced electron transfer from the support to the noble metal active sites.
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EXPERIMENTAL SECTION Chemicals. In this study, the chemicals are Degussa P25 (Evonik Germany) with a surface area of ca. 50 m2 g−1 including two phases (anatase (80%) and rutile (20%)), palladium(II) acetate 99.98% (Sigma-Aldrich CAS Number 3375-313), strontium chloride hexahydrate 99% (Sigma-Aldrich CAS Number 10025-70-4), sodium borohydride 99% (SigmaAldrich CAS Number 16940-66-2), ethanol absolute 99.8% (Sigma-Aldrich CAS Number 64-17-5), and distilled water (99.99%, PAEC PK). Catalyst Preparation. For the preparation of Pd/Sr-NPs@ P25 photocatalysts, titanium dioxide (P25 Degussa) was used as the support. Most of our catalysts are monometallic and bimetallic composites of the above metals. Photocatalysts were prepared and tuned with different ratios of metal concentrations. For the preparation of the Pd-Sr series of photocatalysts, typically 100 mg of TiO2 (P25) was dispersed in 25 mL of distilled water (note: the distilled water was also purged with high purity N2 for 15 min to minimize the dissolved oxygen contents) to make a homogeneous slurry/solution in a 250 mL 2-neck round-bottom flask, and then the solution was left under stirring for ∼2 h. After that, the specified amounts of palladium acetate and strontium chloride solutions were added dropwise into the flask with continuous stirring to give the desired metal loading. After that, a 50 mL solution of NaBH4 (10 mg) was added dropwise to reduce the above dispersed metal under continuous stirring at room temperature. The solution was sonicated for 20 min, and then the catalyst was collected by filtration. This was followed by washing with water, until the AgNO3 test no longer showed the presence of residual chloride (Cl−). The products were filtered and washed several times with distilled water and dried at 80 °C. Finally, the Pd/SrB
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RESULTS AND DISCUSSION Catalyst Characterization. The powder XRD pattern of most active photocatalyst Pd1.8-Sr0.2@P25 is shown in Figure 1.
Figure 2. UV−vis (DRS) absorption spectra of Pd/Sr-NPs@P25 photocatalysts of different wt % deposited TiO2 (P25) as indicated.
absorption edge of metal-deposited photocatalysts was found to shift toward the visible region, indicating good contact between metal nanoparticles (M-NPs) and P25 support. Morphology and Crystal Structure. Figure 3 shows typical TEM images of various Pd/Sr-NPs@P25 photocatalysts, from which one can see that Pd nanoparticles of ca. 3−5 nm are highly dispersed on the surface of P25 for all the samples, including samples A (Pd1.8-Sr0.2@P25), B (Pd1.4-Sr0.6@P25), C (Pd0.8-Sr1.2@P25), D (Pd0.4-Sr1.6@P25), E (Pd0.2-Sr1.8@P25), and F (Pd2@P25). Of note, metal Sr nanoparticles are not stable and readily converted to SrO on the P25 surface; the mean size of SrO particles (or SrO-coated TiO2) is about 14− 25 nm. Figures 3A and 3B provide evidence for higher concentration of metal nanoparticles due to higher wt % loading of Pd metal as compared to strontium, and this is according to the synthesis procedure where Pd loading was kept at 1.8% and 1.4% and Sr was kept 0.2% and 0.6%, respectively. TEM micrographs of Figures 3C and 3D show relatively low concentrations of Pd nanoparticles as compared to Figures 3A to 3B because here Pd metal loading is 0.8% and 0.4% and Sr metal loading is 1.2% and 1.6%, respectively. In comparison to Figures 3A to 3D, the TEM micrograph of Figure 3F (2%Pd@P25) shows a poor distribution of Pd nanoparticles due to the absence of Sr. These results highlight the significant role of strontium in metal nanoparticle dispersion on the oxide support (TiO2) during the synthesis. From the XRD analysis (Figure 1), it is seen that the samples are mixed phases of anatase and rutile of TiO2. Besides the peaks of TiO2, no other peaks are found, implying very small particles (hence weak diffraction) and low loading of Pd/Sr particles on the P25. The as-synthesized Pd/Sr-NPs@P25 catalysts can be easily dispersed in aqueous solutions for photocatalytic hydrogen production applications. The photocatalytic activity is largely influenced by the chemical state of the deposited metal. Therefore, we performed X-ray photoelectron spectroscopy (XPS) to determine the chemical composition and oxidation state of the as-prepared Pd/Sr-NPs@P25 photocatalysts. In Figure 4C, the two peaks at binding energies of 457.81 and 464.4 eV correspond to Ti 2p3/2 and Ti 2p1/2 peaks for pure anatase TiO2 (P25), and the peak at binding energy of 529.04 eV confirmed the O 1s in metal oxide (Figure 4D). These results indicate the presence of Ti4+ in the samples. The presence of Pd NPs is indicated by two peaks at
Figure 1. Powder XRD pattern of Pd1.8-Sr0.2@P25 (calcined). Characteristic anatase and rutile peaks are labeled A and R, respectively.
The XRD pattern for Pd1.8-Sr0.2@P25 photocatalyst is dominated by peaks of anatase and rutile (typical components of the P25 support). No deformation of TiO2 crystal lattice by Pd−Sr deposition occurred. The absence of any obvious change in the diffraction peak positions of anatase and rutile suggests that neither is the deposited strontium and palladium incorporated into the TiO2 lattice nor is there any phase transition from anatase to rutile or vice versa. The anatase and rutile crystallite sizes were determined to be ∼20 nm and ∼26 nm, respectively, from the powder XRD data using the Scherrer equation and line widths of the anatase (101) reflection at 2θ = 25.3° and rutile (110) reflection at 2θ = 27.4°. These sizes of anatase and rutile are typical of P25. Moreover, neither the characteristic peaks of SrO nor that of Pd was found, which can be ascribed to the low metal loading and high dispersion of metal on the surface of TiO2. Optical Absorption Studies. The UV−vis diffuse reflectance spectra (UV−vis DRS) of the catalysts were measured (Figure 2). TiO2 showed almost zero absorption in the visible region; however, the deposition of Pd/Sr-NPs led to an increase of absorption in the visible region due to metal and defects, thus indicating the shifting of Fermi level. It has been reported that Pd particles smaller than 10 nm are only able to absorb in the UV region.23 However, our study shows that the optical absorption of Pd nanoparticles less than 10 nm extends into the visible region (Figure S1). Figure 2 clearly illustrates the broad absorption between 410 and 650 nm observed in Pd−Sr@P25 with different ratios of palladium to strontium, which should be responsible for enhancing the photocatalytic activity. The average particle size estimated by using the Scherrer formula (d = 0.9λ/(2 cos θ)) is in good agreement with the TEM investigations. All catalysts showed intense absorption below 400 nm, which corresponds to the intrinsic band gap absorption of TiO2 P25 support (Eg = 3.15 eV). The C
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Figure 3. TEM images and corresponding metal particle size distributions of Pd/Sr-NPs@P25 photocatalysts: (A) Pd1.8-Sr0.2@P25, (B) Pd1.4-Sr0.6@ P25, (C) Pd0.8-Sr1.2@P25, (D) Pd0.4-Sr1.6@P25, (E) Pd0.2-Sr1.8@P25, and (F) Pd2@P25, respectively. The particle size ranges are about 2−6 nm for Pd and 14−25 nm for SrO.
Figure 4. Core level XPS spectra of A) Pd 3d, (B) Sr 3d, (C) Ti 2p, and (D) O 1s of photocatalyst (Pd0.2-Sr1.8@P25).
that strontium is present in the Sr2+ oxidation state (Figure
binding energies of 335.58 and 340.92 eV, which are from Pd 3d5/2 and Pd 3d3/2, respectively (Figure 4 A). The XPS analysis confirmed the predominant metallic form of palladium. XPS spectra of Sr 3d5/2 at 133.55 eV and 3d3/2 at 135.32 eV verified
4B).18,24−27 Thus, it indicates that the as-prepared photocatalysts of Pd/Sr-NPs@P25 possess well-stabilized Pd 0 D
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The Journal of Physical Chemistry C nanoparticles, which is important for the high photocatalytic activity (vide inf ra). The Raman spectra of the samples are shown in Figure 5. Raman analysis shows no obvious shift due to attribution of low
Table 1. Comparison of Photocatalytic Activities of AsPrepared Pd/Sr-NPs@P25 Photocatalysts under UV Irritationa H2 production photocatalyst
b
Pd0.2-Sr1.8@P25 Pd0.4-Sr1.6@P25 Pd0.8-Sr1.2@P25 Pd1.4-Sr0.6@P25 Pd1.8-Sr0.2@P25 Pd2@P25
Pd/Sr (wt % ratio)
mmol g−1
mmol g−1 h−1c
0.2:1.8 0.4:1.6 0.8:1.2 1.4:0.6 1.8:0.2 2:0
91.3 97.9 112.5 133.4 146.9 105.5
15.2 16.3 18.7 22.2 24.5 17.5
a Spectroline model SB-100P/F lamp, 100 W, 365 nm. bAll photocatalysts have overall 98 wt % of TiO2 and were calcined at 300 °C for 2 h. c5% sacrificial reagent (ethanol) and 5 mg of photocatalyst were fixed during the photo reaction.
the activity of Pd1.8-Sr0.2@P25 photocatalyst. In this study, strontium in the form of SrO plays three key roles: (i) it improve the crystallinity of TiO2 (P25) in the synthetic procedure, (ii) strontium enhances the dispersion of Pd NPs on the surface of TiO2 (evidenced by TEM and XRD results), and (iii) strontium oxide suppresses the recombination rate of the e−/h+ pair on the surface of semiconductor and facilitates the flow of electrons toward the reduction centers. The performance of Pd1.8-Sr0.2@P25 has been compared with that of some other metal-supported photocatalysts reported in the literature (Table 2). Mechanism of Photocatalytic Water Splitting. The Gibbs free energy of splitting water is 237.2 kJ mol−1, and the occurrence of this reaction is essentially determined by the band structure of the photocatalyst.36−38 For the half-reaction of water reduction, the conduction band (CB) potential should be more negative than the H+/H2 reduction potential (i.e., 0.0 V vs the normal hydrogen electrode (NHE) at pH = 0). Similarly, for the oxidation half-reaction of water by holes (h+), the valence band (VB) edge must be more positive than the O2/H2O potential (1.23 V vs NHE, pH = 0). Thus, the theoretical minimal band gap (Eg) is 1.23 eV for water splitting. It is well-known that the particle size36 changes the Eg and other physical and chemical properties. It should be noted that water splitting does not always generate O2 together with H2; for example, when a sacrificial reagent (electron donor) is present, the photogenerated holes are consumed by the sacrificial reagent at the semiconductor surface.37,38 In our current work, loading dual cocatalysts (Pd/Sr-NPs) onto P25 is to lower the activation energy barriers for the two half-reactions and thus increase the performance of hydrogen production. On the basis of the previous reports on photocatalytic reduction of water, we propose the mechanism of catalytic water reduction over Pd/Sr-NPs@P25 under UV light, as shown in Figure 7. In this study, high photocatalytic activity of hydrogen production indicates that the reverse reaction between H2 and O2 has been suppressed significantly due to the formation of a Schottky barrier. Yamaguchi et al. found that the hydrogen evolution rate can be significantly increased by coating with a liquid electrolyte layer that can prevent the back reaction on the support surface.39 In our case, the reduced back reaction is attributed to the reduced overpotential and formation of Schottky barrier by Pd/Sr-NPs on the surface of semiconductor (P25).
Figure 5. Raman spectra of P25 and Pd0.2-Sr1.8@P25, Pd1.4-Sr0.6@P25, Pd1.8-Sr0.2@P25, and Pd2@P25.
metal loading. Three peaks were detected at 412 cm−1 (B1g), 520 cm−1 (A1g + B1g), and 642 cm−1 (Eg) that are attributed to the pristine anatase as well as rutile phase of the P25 (Degussa). The coexistence of rutile TiO2 (P42/mnm, D144h) is evident from the weak shoulder ∼451 cm−1 (Eg). These results match with the XRD analyses where the samples are with 80% anatase and 20% rutile phase.23,28 H2 Production Activities. Photocatalytic H2 production on all photocatalysts under UV irradiation was evaluated using 5% ethanol sacrificial agent, and the total reaction time for photocatalytic activities was optimized and fixed (6 h for all photocatalysts).29 No hydrogen production was observed in the absence of either photocatalyst or irradiation, suggesting that hydrogen was produced only by photocatalytic reactions. Due to large overpotential of Degussa P25, no noticeable activity was observed, because rapid recombination of electrons and holes occurs in P25. The effect of metal loading was studied by increasing the Pd loading from 0.2 to 1.8 wt % while keeping the Pd/Sr overall loading at 2 wt %. The hydrogen production rate was found to increase with increasing Pd wt % in the catalyst, with Pd1.8Sr0.2@P25 being the most active photocatalyst. Specifically, the hydrogen production activity of Pd0.2-Sr1.8@P25, Pd0.4-Sr1.6@ P25, Pd0.8-Sr1.2@P25, Pd1.4-Sr0.6@P25, and Pd1.8-Sr0.2@P25 is 91.32 mmol g−1, 97.93 mmol g−1, 112.50 mmol g−1, 133.39 mmol g−1, and 146.94 mmol g−1, respectively (see Table 1 and Figure 6). It is worth noting that the hydrogen production activity of Pd2@P25 photocatalyst is 105.17 mmol g−1 and this activity is less than the activity of Pd0.8-Sr1.2@P25, albeit the former has an even higher loading of Pd. This distinct contrast highlights the important role of strontium in the as-prepared photocatalysts. To further confirm the effect of Sr addition, the H2 production experiments were conducted on a series of monometallic Pd1@P25, Pd1.8@P25, Pd0.5@P25, Pd2@P25, and Sr0.2@P25 photocatalysts, and results are compared with our best performing Pd1.8-Sr0.2@P25 photocatalyst (Figure S2), which clearly indicates the synergistic role of SrO in enhancing E
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Figure 6. Comparison of photocatalytic activity results of Pd1.8-Sr0.2@P25, Pd1.4-Sr0.6@P25, Pd0.8-Sr1.2@P25, Pd0.4-Sr1.6@P25, Pd0.2-Sr1.8@P25, and Pd2@P25. Calcined at 300 °C for 2 h; sacrificial reagent, 5% ethanol in water mixture; reaction time, 6 h.
Table 2. Comparison of Quantum Efficiency (QE) of Pd1.8-Sr0.2@P25 with Other Metal-Supported P25 Photocatalysts Reported in the Literature catalyst
metal loading
sacrificial reagent
H2 production (mmol g−1 h−1)
QE (%)
ref
Pd−Sr@P25 Au/TiO2 Au/P25 CuO/P25 Ni(OH)2/P25 Cu(OH)2/P25 Pt/N-TiO2
2 wt % 0.25 wt % 0.4 wt % 10 wt % 0.23 (mol %) 0.29 (mol %) 1 wt %
5 vol % ethanol−water mixture 25 vol % methanol + 0.02 M EDTA 25 vol % methanol 10 vol % ethanol 20 vol % methanol 0.09 M ethylene glycol 25 vol % ethanol
24.5 1.25 0.36 0.20 0.30 3.41 2.25
18.5 7.5 4.14 5.1 12.4 13.9 12.3
present study 30 31 32 33 34 35
Figure 7. Water splitting on the Pd@P25 photocatalyst.
Electron Transfer from Support to Pd Cocatalyst. The work function of Pd is 5.55 eV, which is larger than that of TiO2 (4.2 eV). In this study, it is proposed that a Schottky barrier can form at the metal/P25 interface. In the Pd/Sr-
NPs@P25 photocatalyst, the Schottky barrier will drive electron migration from P25 to the Pd metal until a thermodynamic equilibrium is reached, at which time the Fermi levels (EF) of the semiconductor and the metal are F
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electrons from the P25 surface to active sites of the Pd metal cocatalyst, hence increasing the hydrogen production by creating more feasibility for water reduction on the semiconductor surface. The increased photocatalytic activities for hydrogen production induced by SrO can be the main consequence of the electronic effect between Pd and the support, as also clearly indicated by the UV−vis (DRS), XPS, and activity results. Increasing Pd addition in the presence of Sr causes a significant decrease in activation energy of the water reduction with the asprepared photocatalyst.45−47 Because the conduction bands for n-type semiconductors are close to their flat band potentials,42 the measured potentials of supports confirm the enhancement in the Fermi energy (EF) of alkaline earth metal oxide modified TiO2.42 To gain detailed insight into the essence of the electronic promotion, the optical absorption onsets are shown in Figure 2 and photocatalytic activities in Table 1. In Figure 2, one can see a systematic shift in the visible region with the increase in the wt % loading of cocatalyst. Role of the Sacrificial Reagent. Electron acceptors consume the electrons from the conduction band of TiO2, whereas electron donors consume the holes in the valence band and thus increase the lifetime of the electrons available to drive the water splitting reaction on the catalyst surface.47,48 Many studies have revealed that the rate of hydrogen production from water is extremely slow due to the high rate of recombination of photogenerated charge carriers and the rapid reverse reaction (H2 + O2 → H2O).49 Sacrificial reagents are employed to consume one type of charge carrier at a higher rate, thus leaving the other types of carriers to react with H2O. Electron acceptors such as Ag+, Fe3+, and Ce4+ are used to improve the O2 evolution rate from the oxidization of water on the catalyst surface. Electron donors such as methanol, ethanol, triethanolamine, lactic acid, CN−, EDTA,50 Na2S, Na2SO3,51 S2−/SO32−, Ce4+/Ce3+, and IO3−/I− and some biomass-derived carbohydrates are used for hydrogen production from water reduction. Methanol and ethanol are better choices for water splitting reactions because they directly quench the holes from the valence band of the TiO2 and alkoxide ion is also believed to inject electrons directly into the conduction band of TiO2. For longer chain alcohols, diffusion is restricted due to the bulkier structure and the competing dehydration in the presence of Pd.52 In the current research, we use ethanol as sacrificial reagent for photocatalytic reactions. The overall process of the ethanol oxidation at semiconductor surface can be described.53−56 The use of alcohol as a sacrificial electron donor has great potential because of the possibility to obtain them from renewable resources and hydrogen is generated not only from water but also from alcohol.57−59
aligned. Under the action of UV light, the photoinduced electrons can shift the EF of P25 to form a new quasi-Fermi level (EF*).36 In the meantime, the prior thermodynamic equilibrium state for electron transfer is altered, thus the electrons can now migrate from the P25 to the Pd metal to produce H2 at the active surfaces. The Schottky barrier at the Pd metal/P25 interface decreases the recombination rate of e−/ h+ pairs by making a continuum electron flow toward the active sites (sinks) of Pd metal. A noble metal with a larger work function relative to the semiconductor can result in a stronger Schottky barrier effect, and therefore a higher activity for hydrogen evolution. 40−42 Figure 8 shows a schematic
Figure 8. Schematic illustration of electron transfer between Pd and SrO-assisted P25 over the well-dispersed Pd/Sr-NPs@P25.
illustration of electron transfer between Pd and SrO-assisted P25. Here, Pd has a higher work function (i.e., 5.55 eV) than that of P25 or P25-SrO (4.2 eV), and electrons can transfer from the support to Pd until a dynamic equilibrium is reached, followed by generation of a depletion layer, i.e., Schottky barrier (see the energy levels42 in Table S1). It has been observed in situ that SrO can raise the Fermi level of TiO2 (from EF to EF*) and the conduction band level (from ECB to E CB *). Consequently, more electrons of P25-SrO will migrate across the interface to Pd-NPs where water is reduced to H2, resulting in enhanced hydrogen production on Pd sites. Yang et al. found that the electron-donating ability of TiO2/ alkaline earth metal oxide enhances with the elevation of its Fermi level.42 It has been noted that the Pd/Sr@P25 photocatalyst contributes higher photocatalytic activity for hydrogen production, because the SrO nanoparticles play a significant role to promote charges (e−) from TiO2 to Pd0 nanoparticles dispersed on the surface of TiO2. The charge transfer to Pd-NPs can be confirmed from the activities of hydrogen production at Pd sites because the hydrogen production activity of Pd1.8-Sr0.2@P25 is higher than that of Pd2@P25, indicating the important role of strontium. This indicates that the introduction of alkaline earth metal oxide to TiO2 can increase the surface electron densities of metal sites. Thus, the charge transfer from the support to the Pd cocatalyst is considered to be an important factor for the enhanced hydrogen production activity of Pd/Sr-NPs on TiO2 support.42,43 XPS can analyze the charge transfer between metal and the support.44 Pd/Sr-NPs supported on P25 are more negatively shifted than the Pd-NPs supported on P25 without Sr, which can be attributed to electron transfer from TiO2 (P25) to Pd-NPs by the additional contribution of SrO on the support. Moreover, a more negative potential for SrO/TiO2 means a significant electronic effect of SrO in promoting
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CONCLUSION Overall, the Pd/Sr-NPs@P25 photocatalysts exhibit much higher photocatalytic activity for hydrogen production than the Pd@P25 photocatalyst due to the major effect of strontium oxide, which is attributed to the electronic promotion from the support’s surface to metal active sites by increasing the Fermi energy of the support. The photocatalyst discussed in this work provides a new design of nanomaterials for photocatalytic hydrogen production with high efficiency. Photocatalysts that can function in visible light would have major applications in solar energy utilization. The method of preparation, loading amount, particle size, and chemical state of the metal are important factors for improving the photocatalytic activity of G
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TiO2 in hydrogen production. Although there are still many challenges ahead in the area, nanomaterial research holds promise in the eventual transition into a renewable solar and hydrogen based economy.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b04695. UV−vis DRS spectra and comparison of photocatalytic activities (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. Phone: (+92) 5190642062. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work was financially supported by Higher Education Commission (HEC) of Pakistan (No. 20-2704/NRPU/R&D/ HEC/12). R.J. is thankful for financial support from the Air Force Office of Scientific Research under AFOSR Award No. FA9550-15-1-9999 (FA9550-15-1-0154). E.H. acknowledges the IRSIP scholarship of Higher Education Commission (HEC) of Pakistan for financial support while carrying out the research at Carnegie Mellon University. The synthetic work and H2 production activities were done in Catalysis and nanomaterial research Lab 27, Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan. Characterization of samples was done at Carnegie Mellon University.
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REFERENCES
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DOI: 10.1021/acs.jpcc.6b04695 J. Phys. Chem. C XXXX, XXX, XXX−XXX
