Enhancing the Photocatalytic Hydrogen Evolution Activity of Mixed

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Enhancing the Photocatalytic Hydrogen Evolution Activity of Mixed-halide Perovskite CH3NH3PbBr3xIx Achieved by Bandgap Funneling of Charge Carriers Yaqiang Wu, Peng Wang, Zihan Guan, Junxue Liu, Zeyan Wang, Zhaoke Zheng, Shengye Jin, Ying Dai, Myung-Hwan Whangbo, and Baibiao Huang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02374 • Publication Date (Web): 27 Sep 2018 Downloaded from http://pubs.acs.org on September 27, 2018

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ACS Catalysis

Enhancing the Photocatalytic Hydrogen Evolution Activity of Mixed-Halide Perovskite CH3NH3PbBr3x Ix

Achieved by Bandgap Funneling of Charge

Carriers Yaqiang Wu,† Peng Wang,*,† Zihan Guan,† Junxue Liu,‡ Zeyan Wang,† Zhaoke Zheng,† Shengye Jin,‡ Ying Dai,# Myung-Hwan Whangbo,§ and Baibiao Huang*,† †

State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China

‡

State Key Laboratory of Molecular Reaction Dynamics and Collaborative Innovation Center of

Chemistry for Energy Materials (iChEM), Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China #

School of Physics, Shandong University, Jinan 250100, China

§

Department of Chemistry, North Carolina State University, Raleigh, NC 27695–8204, USA.

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ABSTRACT:

Powder samples of mixed halide perovskite MAPbBr3-xIx (MA = methyl ammonium ion, CH3NH3+) were prepared by employing a facile light-assisted halide-exchange method in aqueous halide solution at room temperature. It was found that the distribution of iodide ions in the MAPbBr3-xIx particles to be largest on the surface becoming lower on going into the interior so that they have a correct bandgap funnel structure needed for transferring photogenerated charge carriers from the interior to the surface. Consequently, the MAPbBr3-xIx/Pt powder sample (250 mg) exhibits an enhanced photocatalytic activity for a H2 evolution under visible light (100 mW cm-2, λ ≥ 420 nm) with the rate of 651.2 µmol h−1 and a solar-to-chemical conversion efficiency of 1.06%.

KEYWORDS: perovskite, photocatalysis, halide gradient, bandgap funneling, hydrogen evolution


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INTRODUCTION The metal halide perovskites ABX3 (A = CH3NH3+, CH(NH2)+, Cs+; B = Pb2+; X = Cl-, Br-, I-) have emerged as attractive candidates for light harvesting materials.1-3 Among these perovskites investigated so far, the organic lead halide perovskites continue to attract the most attention and extensive investigation.4 As a solar cell material, MAPbI3 (MA = CH3NH3+) has shown an outstanding efficiency, reaching the power conversion efficiencies over 21 %.5 In addition, MAPbI3 has also been examined for photoelectrochemical hydrogen evolution,6,7 light-emitting diodes,8 photodetectors9 and electrochemiluminescence.10 Recently, MAPbI3 was used as a photocatalyst for hydrogen generation from hydriodic acid HI,11 and decorating MAPbI3 with reduced graphene oxide led to an efficient and stable visible-light photocatalyst for hydrogen evolution.12 In general, methods employed to improve the performance of photocatalysts involve modifications of their surface or bulk structures, such as metal and nonmetal doping, dye sensitization, co-catalyst loading, and semiconductor combinations.13-16 So far, there has been no report on modifying the bulk structure of MAPbX3 to make it an efficient photocatalyst. Devices based on MAPbI3 rely on the long diffusion length of the carriers in MAPbI3. The diffusion lengths up to micrometers have been reported in both polycrystalline and single-crystal MAPbI3.17,18 As recently found for mixed halide perovskites MAPbBrxI3-x and CsPbBrxI3-x nanocrystals,19,20 the photogenerated charge carriers flow directionally from the bromide-rich region with wider bandgap to the iodide-rich region with narrower bandgap. This carrierfunneling is believed to facilitate the carrier extraction at the perovskite/electrode interfaces when applied in solar cells. The bandgap of MAPbBr3 is considerably larger than that of MAPbI3 (i.e., 2.33 vs. 1.57 eV).21-23 In the present work, we prepare powder samples of the mixed-halide perovskite MAPbBr3-xIx that have a bandgap-funnel structure favoring the flow of the


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photogenerated charge carriers from the interior to the surface and show that MAPbBr3-xIx samples have a superb photocatalytic activity for hydrogen evolution. So far, mixed-halide perovskite films have been obtained by blending various lead halide materials into mixture precursors,24,25 so the resulting films consist of uniformly-mixed perovskites. Several post-synthetic strategies were proposed to obtain partially converted halide perovskite films, but they are confined to film fabrication and cannot be applied to aqueoussolution synthesis.26,27 Here we report a new method to prepare, in aqueous solution, powder samples of mixed-halide perovskite MAPbBr3-xIx with correct iodide-concentration gradient, namely, the concentration of iodide ions increases on moving from the interior to the surface of each sample particle. We employed a novel and facile light-assisted halide exchange method, which is based on the anion diffusion in MAPbBr3 particles dispersed in aqueous mixed acid, HBr/HI, solution. The powder samples of MAPbBr3-xIx are superb visible-light photocatalysts for hydrogen evolution, and the bandgap-funneling of charge carriers induced by the iodineconcentration gradient plays a vital role in enhancing the photocatalytic activity as a result (Figure 1).


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Figure 1. Bandgap funnel structure of MAPbBr3-xIx near the surface in MAPbBr3-xIx/Pt enhancing the photocatalytic H2 evolution on the Pt particles loaded on the surface of MAPbBr3-xIx. The bandgap is narrowest on the surface of MAPbBr3-xIx and becomes wider on going into the interior..

RESULTS AND DISCUSSION Excess MAPbBr3 powders were dispersed in aqueous mixed-acid HBr/HI solution saturated with MAPbBr3 under vigorous stirring. The halide ions of perovskite materials are chemically “softer” as compared to other materials at room temperature,28 suggesting that the halide ions are more easily exchangeable. Thus, in the mixed-acid solution saturated with MAPbBr3, halide ions will diffuse into the MAPbBr3 particles. After several hours of halide ion exchange reaction, the MAPbBr3 particles show a slight change in color,29 indicating the replacement of Br- ions with Iions on the surface of the particles. Notably, we found that light irradiation substantially accelerates this halide-exchange reaction, leading to the desired MAPbBr3-xIx samples (Figure S1). As shown in the inset of Figure 2a, after halide-exchange reaction under visible light (λ ≥ 420 nm), the color of the MAPbBr3 samples change from bright orange to dark brown, which provides a visual indication for the replacement of bromide ions by iodide ions. Powder X-ray


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diffraction (XRD) analysis (Figure 2a) reveals that the MAPbBr3-xIx particles precipitated from the saturated solution have a cubic structure, consistent with results reported by using other synthesis methods.30,31 The powder samples of MAPbBr3-xIx obtained from the halide exchange reaction under light irradiation maintain the cubic structure while the XRD peaks shift about 0.1 degree towards a lower angle and there are no characteristic peaks corresponding to MAPbI3. The XRD patterns suggest a partial replacement of the bromide ions in MAPbBr3 with iodide ions, while preserving the intrinsic cubic structure. Scanning electron microscope (SEM) images (Figure 2b, c) show that the surfaces of MAPbBr3 particles are relatively smooth but become rough with some nanocrystals precipitated after the halide exchange reaction. Similar results were also reported in an earlier study.26 The energy-dispersive x-ray spectroscopy (EDS) measurements of MAPbBr3-xIx indicate a uniform distribution of I atoms among the Br atoms in the surface region of the MAPbBr3-xIx particles, with the amount of the I atoms much smaller than that of the Br atoms (Figure 2d).


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Figure 2. (a) XRD profiles of MAPbBr3 and MAPbBr3-xIx. The insets show colors of the powder samples. SEM images of (b) MAPbBr3 and (c) MAPbBr3-xIx particles. The inset of (c) shows surface details of a single MAPbBr3-xIx particle. (d) SEM and EDS images of MAPbBr3-xIx taken for a local surface. The top left panel shows the SEM image, and the remaining panels images the EDS for Pb, Br and I elements.

The ion exchange in perovskite films depends on the diffusion of the exchanging species.28,32 Thus, we examine how the mixed HI/HBr acid solution affects the perovskite products MAPbBr3-xIx by performing a series of synthesis using different HI/HBr ratios. The XRD patterns (Figure S2) show a continuous shift towards a lower 2θ angel with increasing the HI/HBr volume ratio of the solution. Since I- is larger than Br- in size, this shows a larger amount of I substitution for Br as the mixed HI/HBr solution has more I- ions. The product synthesized using the HI/HBr volume ratio of 8% exhibits the maximum intensity in those XRD patterns and exhibits the greatest photocatalytic performance (Figure S3). Unless otherwise mentioned, the


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MAPbBr3-xIx sample discussed in the following refers to the product prepared by using the HI/HBr volume ratio of 8 %. Since the light irradiation is indispensable for the effective halide ion exchange reaction and does significantly accelerate this process, one may wonder how the ion-exchange rate affects the crystallinity of the resulting MAPbBr3-xIx samples hence influencing their photocatalytic performance. Thus, different batches of MAPbBr3-xIx samples were prepared under different intensities of light irradiation. As shown in Figure S4, the XRD intensities of the samples decrease with increasing the intensity of light irradiation, and so do there their photocatalytic activities (Figure S5). These results are reasonable because a much faster ion-exchange process will result in a weaker crystallinity and stronger defect densities. The MAPbBr3-xIx samples obtained under a weaker light irradiation, will have a higher crystallinity and a more ideal spatial distribution of the constituent elements. To verify the replacement of Br- ions by I- ions in the ion exchange reactions, we carried out X-ray photoelectron spectroscopy (XPS) measurement for MAPbBr3 and MAPbBr3-xIx samples. As shown in Figure 3a, the MAPbBr3 sample shows the Pb 4f5/2 and 4f7/2 peaks at 143.5 eV and 138.6 eV, respectively. Each of the two peaks is split into two subpeaks in MAPbBr3-xIx. The higher binding-energy (BE) subpeaks of MAPbBr3-xIx, at 143.8 and 138.9 eV, are assigned to the Pb-Br bonds of the Pb(Br, I)6 octahedra, and the lower BE subpeaks, at 142.9 and 138.0 eV, to the Pb-I bonds of the Pb(Br, I)6 octahedra. The Pb-Br bonds of MAPbBr3-xIx have a higher BE than do those of MAPbBr3. Similarly, the Pb-I bonds of MAPbBr3-xIx have a higher BE than do those of MAPbI3. These XPS results provide a distinct proof for the replacement of bromide ions by iodide ions.


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Figure 3. (a) High resolution XPS spectra of MAPbBr3, MAPbI3 and MAPbBr3-xIx for the Pb 4f5/2 (left peak) and 4f7/2 (right peak) states. (b) UV–vis diffuse reflectance spectra taken at several different halide exchange reaction times. (c) An SEM image of the cross section of a bisected MAPbBr3-xIx particle. The profile presented in cyan shows the variation of the I-content determined by EDX scanning along the yellow line profile. (d) An image of the EDX mapping for the I-element distribution of the MAPbBr3-xIx particle in the region marked with a white rectangle in Figure 3c. (e) XPS depth profiling for the valence


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band of MAPbBr3-xIx particles at four different depths, i.e., 0, 30 nm, 60 nm, 90 nm. (f) PL spectrum of MAPbBr3 and those of MAPbBr3-xIx taken at several different halide exchange reaction times.

To gain insight into the nature of the iodine distribution in the MAPbBr3-xIx particles, we first examine their UV-vis diffuse reflectance spectra recorded at several different reaction times of the halide exchange. As shown in Figure 3b, the pristine MAPbBr3 shows a typical absorption spectrum with absorption edge at ~570 nm, consistent with that reported for the single crystal MAPbBr3.30 As the halide exchange reaction goes on, the absorption band edge of MAPbBr3-xIx is continuously red-shifted, reaching the bandgap of 2.0 eV, which is smaller than that of MAPbBr3 (i.e., 2.33 eV) but still far greater than that of MAPbI3 (i.e., 1.57 eV).20,30,33 These observations indicate only a partial replacement of Br- ions in MAPbBr3 with I- ions. Furthermore, with increasing the reaction time, the absorption tails, covering between the entire red-light and near infrared regions, become stronger. This dependence of the absorption tails on the reaction time indicates an increasing amount of I- substitution and a broader distribution of Iin MAPbBr3-xIx, and is consistent with the observations from perovskite film materials.20 Since the exchange reaction begins from the surface of MAPbBr3 particles, which are in contact with the HI/HBr solution, it is expected that the amount of I- distribution in MAPbBr3-xIx particles should be largest in the surface region and become less on moving from the surface to the interior. Then, the bandgap of a MAPbBr3-xIx particle would be narrowest in the surface region and become wider on moving to the interior, since the bandgap is smaller for MAPbI3 than for MAPbBr3. To provide additional evidence for the above conclusion, we first carried out energy dispersive X-ray spectrometry (EDX) measurements for a cross section of a MAPbBr3-xIx particle. We employed the focused ion beam (FIB) technique to split the particle into exposed cross sections


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(see the Experiment Section for details). An image of the EDX mapping for the I-element distribution of the MAPbBr3-xIx particle in the region marked with a white rectangle in Figure 3c is presented in Figure 3d, which shows clearly that the content of the I-distribution is largest in the surface region of the MAPbBr3-xIx particle and becomes less with increasing the depth from the surface to the interior. The variation of the I-content, determined by EDX scanning, along the yellow line profile of Figure 3c is presented there as the profile in cyan. These results provide a strong support for the formation of the expected I-composition gradient. Furthermore, we characterized a continuous change in the bandgap of an MAPbBr3-xIx particle with changing the depth from the surface to the interior by performing XPS measurements for the valence bands at four different depths, i.e., 0, 30, 60 and nm. To carry out this XPS depth profiling, we etched MAPbBr3-xIx particles with argon ions to obtain the four samples (here “0 nm” sample means the sample with no etching). As summarized in Figure 3e, with the increasing depth from the particle surface, the valence band maximum (VBM) moves continuously towards the more negative direction from the vacuum energy level, indicating the formation of an expected bandgap funnel. To corroborate the above conclusion, we now analyze the photoluminescence (PL) spectra of MAPbBr3 and MAPbBr3-xIx. Figure 3f shows the PL spectra recorded for MAPbI3 and MAPbBr3-xIx samples after halide-exchange with different reaction times. Note that a PL results from the recombination of photogenerated electrons and holes, so it has the information about the bandgaps. The MAPbBr3 particles exhibit a characteristic peak centered at 550 nm (Figure 3f).30,34 As the halide exchange reaction proceeds, the ~550 nm peak becomes broadened, and a broad asymmetrical PL peak appears below ~760 nm, which is very close to the PL peak (~770 nm) of MAPbI3.35,36 This result indicates that the halide-exchange forms a I-rich region that is


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close to pure MAPbI3 in the MAPbBr3 perovskite particle at the very early stage of halideexchange reaction. However, such an I-rich region is almost unobservable in the UV-Vis spectra, implying that the I-rich region is present on the surface of particles with limited thickness. It is observed that the PL is not confined to the ~550 and ~760 nm regions but covers the whole wavelengths between ~550 and ~760 nm. Except for the very early stage of the halide exchange reaction, the PL intensity is lower in the shorter wavelength region, and stronger in the longer wavelength region. We note from the UV-vis diffuse spectra of MAPbBr3-xIx samples (Figure 3b) that the absorption band gap lies in the low wavelength region (i.e., the bromide-rich region below ~560 nm), but charge carriers are generated in the whole wavelength region studied. Nevertheless, the PL spectra (Figure 3f) shows that the charge carrier recombination occurs mainly in the iodide-rich region, showing the occurrence of charge carriers from the bromiderich to the iodide-rich region although radiative recombination of electron-hole pairs takes place during charge transfer process (from ~570 nm to 670 nm). As the reaction time exceeds 1 h, the PL intensity around ~550 nm continues to diminish to a relatively stable low level compared to those of the long wavelengths. These observations indicate that charge carriers are quickly transported from the bromide-rich region to the iodide-rich region, and hence the iodide-rich regions serve as the primary sites for charge-carrier recombination while the bromide-rich and halide-gradient regions play mainly the roles of absorbing light and transferring charge carriers. That is, MAPbBr3-xIx has a desired bandgap funnel structure.


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Figure 4. (a) Comparison of the H2 evolution activities without photocatalyst, with photocatalysts MAPbBr3, MAPbBr3/Pt, MAPbBr3-xIx and MAPbBr3-xIx/Pt. The inset shows a zoomed-in view of the results obtained for the first three experiments marked the by red circle. (b) H2 evolution activities of MAPbBr3-xIx/Pt during the first six consecutive cycles of experiments

A bandgap funnel structure plays a vital role in efficient light-emitting diodes.37 It is expected that a material with a bandgap funnel structure enhances photocatalytic reactions because, during photocatalytic reaction, the photogenerated electron-hole pairs can either migrate to the surface initiating photocatalytic redox reactions or recombine becoming ineffective for photocatalytic reaction. Thus, the activities for the photocatalytic H2 evolution of the perovskite samples were examined in aqueous mixed HI/HBr acid saturated with MAPbBr3-xIx. (We added hypophosphorous acid H3PO2 to the solution to selectively reduce the I3- ions produced in the photocatalysis process, because they darken the solution and interfere with the light absorption of the photocatalyst.10) Under visible light (λ ≥ 420 nm, 100 mW cm-2) irradiation, pristine MAPbBr3/Pt (250 mg) shows a poor photocatalytic activity for H2 evolution (2.8 µmol h−1), with some improvement (8.4 µmol h−1) after photo-loaded with Pt. Surprisingly, after the introduction of iodide-induced bandgap funnel, the MAPbBr3-xIx sample elevates the activity to a superb value of 255.3 µmol h−1. Photogenerated electron-hole pairs transferred to the photocatalyst


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surface can be separated by various means of surface modification. In our work, Pt particles are loaded on the surface of MAPbBr3-xIx samples to efficiently separate electron-hole pairs. Expectedly, the photocatalytic H2 evolution activity is further enhanced to 651.2 µmol h−1 after loaded with Pt, under the same measuring conditions (Figure 4a). The solar-to-chemical efficiency, which is equivalent to the efficiency of the light-driven HI splitting, of the mixed acid solution was determined without H3PO2 using the simulated sunlight (AM 1.5G 100 mW cm−2). In this case, the photocatalytic H2 evolution rate decreases with time (Figure S6) because the I3species interfere the light absorption by the photocatalyst. The initial solar-to-chemical efficiency is calculated to be 1.06 %. For details, see the Supporting Information and Experiment Section. The apparent quantum efficiency of MAPbBr3-xIx/Pt is determined to be 8.10 % under 450 nm light irradiation (Figure S7). MAPbBr3-xIx/Pt exhibits excellent stability without significant reduction in the activity of H2 evolution after 30 h of repeated experiments (5 h for each cycle for 6 cycles, Figure 4b). The XRD profiles (Figure S8) and SEM images (Figure 1c, Figure S9) of MAPbBr3-xIx before and after 30 h of photocatalytic reaction are compared in Supporting Information. There is neither discernible phase change in, nor structure failure, of the samples after 30 h of H2 evolution, that accompanies the surface roughening. This great stability can be understood in part because the photocatalytic reaction system is in dynamic equilibrium between the powder samples surface and solution, so that the continuous reconstruction can be accomplished at the interface of MAPbBr3-xIx powders and saturated solution through ion replacement during the whole reaction process.


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Figure 5. (a) PL decay spectra of MAPbBr3 and MAPbBr3-xIx powders collected at the indicated emission wavelengths under an excitation at 450 nm. The solid lines are the fits of the kinetics to a bi-exponential function with the average lifetimes τ. (b) Transient absorption spectra of MAPbBr3 before and after the 90-min of the ion-exchange reaction in the upper and lower panels, respectively. (c) Comparison of the ground state bleach recovery kinetics of MAPbBr3 after 0, 10, 60 and 90 min of the ion-exchange reaction. The solid lines are the fits of the kinetics to a bi-exponential function with the average lifetimes τ.

Our results described above strongly suggest that the photogenerated electrons in MAPbBr3-xIx particles are accumulated on their surface through a bandgap funneling, are rapidly transferred to the Pt particles loaded on surface and reduce H+ to H2. To confirm this suggestion, we carry out time-resolved PL measurements for the bromide-rich region (at 550 nm) and iodide-rich region (at 700 nm) using the 90-min exchange reaction time sample. The PL decays (Figure 5a) are


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fitted by a bi-exponential function, with the fitting parameters listed in Table S1. The averaged PL lifetime at 550 nm is 455.2 ps for MAPbBr3, and 234.5 ps for MAPbBr3-xIx, and is 713.0 ps at 700 nm for MAPbBr3-xIx. These results indicate that the photogenerated charge carriers in the bromine-rich region are rapidly transferred to the iodide-rich region, which causes the decrease of the lifetime corresponding to bromide-rich region at shorter wavelength. The continuous injection of charge carriers results in a longer lifetime in the iodide-rich region at longer wavelength, which is consistent with the PL results discussed above. We further investigate the change in the carrier dynamics induced by the halide-exchange reaction on the basis of the ultrafast transient absorption (TA) spectroscopy. Figure 5b compares the TA spectra of the perovskites before and after the 90 min of the halide-exchange reaction. Both samples exhibit the ground state bleach (GSB) signal at the bandgap due to the band filling by the photo-induced electrons and holes. The GSB of MAPbBr3-xIx is slightly red-shifted (by ∼10 nm) with respect to that of MAPbBr3, which is consistent with the result reported in CsPbBr3-xIx.20 However, the TA spectra of MAPbBr3-xIx is dominated by the signal from the Br-rich region. This indicates that the Br- ions in the inside of the perovskite particles are only partially replaced by I- ions, so the signal from the I-rich region is not captured by the TA measurements. We also compared the GSB recovery kinetics of MAPbI3 and MAPbBr3-xIx samples obtained at different reaction times of the halide-exchange. A faster recovery kinetics is found for a MAPbBr3-xIx sample obtained with longer reaction time (Figure 5c and Table S2). This result, consistent the PL measurements discussed above, indicates the carrier transport from the internal Br-rich region to the surface Irich region. For a longer halide-exchange reaction time results in a larger surface I-rich domain, thus leading to a faster carrier transport kinetics. As a result, during the photocatalytic process, the photogenerated charge carriers generated in the interior of the photocatalyst are rapidly


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transferred to the surface due to the bandgap funneling induced by the inhomogeneous halide-ion distribution. Subsequently, the accumulated electrons on the sample surface are extracted by the Pt particles loaded on the surface and get separated from the holes to reduce H+ to H2, leaving the holes to oxidize I- to I3- on the surface of the photocatalyst. CONCLUSIONS In summary, by using light-assisted halide exchange method in aqueous HBr/HI solution, we prepared MAPbBr3-xIx samples possessing a bandgap funnel structure. The MAPbBr3-xIx samples loaded with Pt particles on their surfaces have a superb photocatalytic H2 evolution activity under visible light irradiation due to an efficient bandgap funneling of the photogenerated charge carriers to the surface of MAPbBr3-xIx and then proceeds the electron-hole separation at the interface between MAPbBr3-xIx and Pt particles. The light-assisted halide exchange presented in this work is also applicable in introducing a bandgap funnel structure into other perovskite materials. EXPERIMENTAL SECTION Synthesis of methylammonium bromide (MABr). To prepare MABr, 15.5 mL methylammonium (33 wt% in ethanol) and 14 mL HBr acid (57 wt% in water) were mixed at 0 o

C and stirred for 2 h. The solvent was removed by rotary evaporation at 60 oC. The crude

product was recrystallized with ethanol/diethyl ether, and washed with diethyl ether. This process was repeated for three times. The obtained white powders were dried in a vacuum oven at 60 oC for 20 h.


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Preparation of MAPbBr3 and MAPbBr3-xIx powders and their saturated solutions. MAPbBr3 was precipitated from its saturated HBr acid solution. Excess MABr and PbBr2 were dissolved into HBr acid solution in molar ratio of 1:1 under vigorous stirring, the bright orange MAPbBr3 powder and its saturated solution were obtained through centrifugation. The obtained solution (40 mL) was mixed with different amounts of HI acid, and 10 mL hypophosphorous acid (H3PO2, 50 wt% in H2O) was added into this mixed HBr/HI acid to protect I- from being oxidazed during the photocatalysis reaction to be carried out later. Next, excess MAPbBr3 powders were added into the mixed solution, and ultrasonic-stirring was performed to saturate the solution. Then centrifugation was carried out to remove the excess powders and obtain the saturated mixed acid, which was preserved as the reaction solvent in the following stage. Then 250 mg MAPbBr3 powders were added into the saturated acid solution, followed by another 20 min stirring to get the mixture homogeneous. The suspension was exposed to visible light irradiation produced by a 300 W Xe lamp with a 420 nm cut-off filter for a desired duration of time, allowing the light-assisted halogen ion exchange reaction to proceed. The whole process was kept at 15 oC by a cooling water system. The MAPbBr3-xIx sample was centrifuged, dried, and preserved for further reaction and characterization. Fabrication of MAPbBr3-xIx/Pt. The MAPbBr3-xIx/Pt sample was fabricated using the lightloading method. 250 mg MAPbBr3-xIx and 8 mg H2PtCl6·6H2O were added to the pre-prepared perovskite-saturated solution with stirring to get the mixture homogeneous. The suspension was then exposed to visible light irradiation for 1 h under the same conditions with the light-assisted halide-ion exchange reaction process. The saturated solution with MAPbBr3-xIx/Pt was used for the H2 evolution measurement next.


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Characterization. Crystal structures of as-obtained products were characterized by XRD using a Bruker AXS D8 diffractometer with Cu Kα radiation. Fourier transform infrared (FTIR) spectra were obtained on a Bruker ALPHA-T spectrometer using KBr pellets. Morphologies and microstructures of the products were characterized by scanning electron microscopy (SEM) (Hitachi S-4800) equipped with an Energy Dispersive X-ray Spectrometer (EDS). The diffuse reflectance spectroscopy (DRS) of the products were recorded using a Shimadzu UV 2600 spectrophotometer equipped with an integrating sphere with 100% BaSO4 as a reflectance standard. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Thermo Fisher Scientific Escalab 250 spectrometer with monochromatized Al Kα excitation, which is equipped with an Ar+ cluster ion gun for the chemical depth profiling analysis, and C1s (284.6 eV) was used to calibrate the peak positions of various elements. The Ar+ ion sputtering was performed with a beam energy of 3.0 keV and a sputtering rate of 0.4 nm min−1 for 75 minutes for each depth. The cross-section of the perovskite particle was prepared and characterized using a focused ion beam (FIB) SEM (FEI Helios Nanolab600i). The photoluminescence (PL) spectra and time resolved PL decay spectra were measured using a home-built PL scan-imaging microscope coupled with a time-correlated single photon counting (TCSPC) module at 500 nm laser excitation at room temperature. The decay curves were fitted by a bi-exponential decay function to deconvolute the instrument response function. The setup for femtosecond transient absorption is based on a regenerative amplified Ti:sapphire laser system from Coherent (800 nm, 35 fs, 6 mJ per pulse, and 1 kHz repetition rate), nonlinear frequency mixing techniques, and the transient absorption spectrometer (TimeTech Spectra, femtoTA100). The 800 nm output pulse from the regenerative amplifier was split into two parts with a 50% beam splitter. The transmitted part was used to pump a TOPAS


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Optical Parametric Amplifier (OPA) which generates a wavelength-tunable laser pulse from 250 nm to 2.5 µm as pump beam. The reflected 800 nm beam was split again into two parts. One part with less than 10% was attenuated with a neutral density filter and focused into a 2 mm thick sapphire window to generate a white light continuum (WLC) from 420 nm to 800 nm used as the probe beam. The probe beam was focused with an Al parabolic reflector onto the sample. After the sample, the probe beam was collimated and then focused into a fiber-coupled spectrometer with CMOS sensors and detected at a frequency of 1 KHz. The intensity of the pump pulse used in the experiment was controlled by a variable neutral-density filter wheel. The delay between the pump and probe pulses was controlled by a motorized delay stage. The pump pulses were chopped by a synchronized chopper at 500 Hz and the absorbance change was calculated with two adjacent probe pulses (pump-blocked and pump-unblocked). Photocatalytic measurements. Photocatalytic H2 evolution in aqueous HBr/HI acid solution was carried out in a top-irradiation vessel connected to a glass-enclosed gas circulation system. A 300 W Xe-lamp (CEL-HXF300, Beijing CEAULight, China) with a 420 nm cut-off filter was used as a visible light source for the photocatalytic experiment. In a typical procedure, 250 mg of catalyst was added to 50 mL perovskite saturated solution (H3PO2 mixed) with constant stirring. The reaction temperature was maintained at 15 °C by a circulation of cooling water. The amount of evolved H2 was analyzed by a gas chromatograph (ShiweipxGC-7806) with Ar as the carrier gas. The cycling photocatalytic test to investigate the stability of the photocatalyt was performed every 5 h as a cycle, when the system was re-evacuated. The solar-to-chemical conversion efficiency, namely, the solar HI splitting efficiency in our photocatalytic system, is the ratio of solar light converted to break the chemical bonding of HI. It could be calculated by means of the standard redox potential of I- oxidation and the H+ reduction


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potential. To determine the solar-to-chemical efficiency, we measured the photocatalytic H2 evolution activity of the MAPbBr3-xIx/Pt perovskite sample (500 mg) in the mixed acid solution (HI:HBr = 3:40 in volume) without H3PO2. The solution was electrochemically reduced before testing the photocatalytic performance. As the light source, simulated sunlight produced by a 300W Xenon lamp (CEL-HXF300, Beijing CEAULight, China) coupled with an AM1.5 global filter was used with the light intensity calibrated to 1 sun (100 mW cm2). Our calculation for the HI splitting efficiency is based on the amount of evolved hydrogen. The standard hydrogen reduction potential is 0 V (versus RHE), and the I- oxidation potential to I3- is 0.53 V (versus RHE). The concentration of HI we used in the mixed acid is 0.53 mol L-1 while and the I3- ion concentration in the reduced HI solution is 1.22 × 10-4 mol L-1.11 Thus, the concentration of H+ is calculated to be 7.67 mol L-1, the redox potential could be determined from the Nernst equation: E (2H++2e- →H2) = 0 - 0.059/2×log(1/7.672) = 0.052 V (versus NHE) E (3I- → I3- +2e-) = 0.53+0.059/2×log(1.22×10-4/0.533) = 0.439 V (versus NHE) Thus, the total potential for the HI solution splitting in our mixed acid solution is calculated as 0.439 V - 0.052 V = 0.387 V. The solar HI splitting efficiency could be estimated from the following equation: Solar HI splitting efficiency = [Evolved H2 (mol)×6.02×1023×2×0.387 (eV)×1.6×10-19] / [Psol (W cm-2)×Area (cm2)×time (s)]×100 For instance, in the photocatalytic HI splitting process, 161.5 µmol H2 was generated in the first hour under 100 mW cm-2 light irradiation, with the light irradiation area of π cm2, so the solar HI splitting efficiency is calculated as 1.06%.


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The apparent quantum efficiency (QE) was measured under the same photocatalytic reaction conditions with irradiation light through a band-pass filter of 450 nm and a masked area of π cm2. The photon flux of the incident light was determined using a PL−MW2000 spectroradiometer (PerfectLight, China). The apparent QE was calculated from the ratio of the number of reacted electrons during the hydrogen evolution to the number of incident photons by using the expression:

QE =

2 ×the number of evolved hydrogen molecules ×100% the number of incident photons

ASSOCIATED CONTENT Supporting Information. Supplementary Figures S1−S8 and Tables S1−S2 (PDF). These materials are available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] ORCID Yaqiang Wu: 0000-0003-0459-5032 Peng Wang: 0000-0003-4250-8104


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M.-H. Whangbo: 0000-0002-2220-1124 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (51602179, 21832005, 21333006, 21573135, and 11374190), and the National Basic Research Program of China (973 program, 2013CB632401), PW thanks support from The Recruitment Program of Global Experts, China, and BBH acknowledges support from the Taishan Scholars Program of Shandong Province.

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Enhancing the Photocatalytic Hydrogen Evolution Activity of Mixed

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