ALD-coated ultrathin Al₂O₃ film on BiVO₄ nanoparticles for efficient PEC water splitting

ALD-coated ultrathin Al 2 O 3 film on BiVO 4 nanoparticles for efficient PEC water splitting 1 2 first-author 2 2 3 2 2 2 corresp Corresponding author: nanowangkun@sinap.ac.cn Corresponding author: nanowangkun@sinap.ac.cn nanowangkun@sinap.ac.cn Department of Physics, College of Sciences, Shanghai University, Shanghai 200444, China Department of Physics, College of Sciences, Shanghai University, Shanghai 200444, China Division of Physical Biology & Bioimaging Center, Shanghai Synchrotron Radiation Facility, CAS Key Laboratory of Interfacial Physics and Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China Division of Physical Biology & Bioimaging Center, Shanghai Synchrotron Radiation Facility, CAS Key Laboratory of Interfacial Physics and Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China Chemistry Department, King Saud University, Riyadh 11451, Saudi Arabia Chemistry Department, King Saud University, Riyadh 11451, Saudi Arabia Bismuth vanadate (BiVO 4 ) is a promising semiconductor material for solar energy conversion via photoelectrochemical (PEC) water splitting, whereas its performance is limited by surface recombination due to trapping states. Herein, we developed a new method to passivate the trapping states on BiVO 4 surface using ultrathin aluminium oxide (Al 2 O 3 ) overlayer by atomic layer deposition (ALD). The coated ultrathin Al 2 O 3 film on BiVO 4 significantly enhanced photocurrent densities of the BiVO 4 anodes under standard illumination of AM 1.5 G (100 mW/cm 2 ). The electrochemical impedances and photoluminescence spectra were studied to confirm that the improved PEC water splitting performance of BiVO 4 was due to the decreased surface recombination state on BiVO 4 , which effectively enhanced the charge separation. Ultrathin Al 2 O 3 overlayer Photoelectrochemical water splitting Surface state Atomic layer deposition 1 Introduction 1 Photoelectrochemical (PEC) water splitting, offers the ability to convert solar energy into chemical energy by producing O 2 and H 2 gases [ 1 1 , 2 2 ] . BiVO 4 has become a promising n-type semiconductors for PEC water splitting since 1999 when it was reported by Kudo and coworkers as a photocatalyst for water oxidation [ 3 3 , 4 4 ] . Monoclinic scheelite BiVO 4 has high light absorption (bandgap, E g=2.4 eV), ample abundance, low toxicity, and appropriate valence band position for O 2 evolution [ 3 3 ] , but it suffers from poor electron transfer and collection in the photoelectrode, and sluggish water oxidation kinetics [ 3 3 - 5 5 3-5 ] . Efforts having been made to alleviate these restrictions include doping with nitrogen [ 6 6 ] , introducing catalysts such as Co-Pi [ 7 7 ] , p-type Co 3 O 4 [ 8 8 ] , NiOOH [ 9 9 ] , and constructing nanostructures or heterojunctions [ 10 10 - 12 12 10-12 ] . The use of passivation layers has been an effective way to improve charge separation and transfer at semiconductor-liquid interfaces [ 13 13 ] , for example, using Al 2 O 3 [ 14 14 ] or TiO 2 [ 15 15 , 16 16 ] and other nanomaterials [ 17 17 - 19 19 17-19 ] to passivate hematite, and NiO or CoO x to passivate BiVO 4 [ 20 20 , 21 21 ] . We developed a new method to passivate BiVO 4 , in which atomic layer deposition (ALD) was utilized to deposit an ultrathin Al 2 O 3 overlayer on BiVO 4 photoanodes, which perform much better in PEC water splitting. 2 Experimental 1 2.1 Preparation of BiVO 4 photoanode 2 Fluorine-doped tin oxide-coated glass substrates (FTO, TEC15, NSG) were seriatim cleaned with acetone, ethanol and deionized water. The mixed precursor (0.486 g bismuth nitrate pentahydrate, 1 mL acetylacetone, and 0.273 g vanndyl acetylacetonate, from Sinopharm or Sigma-Aldrich) were dissolved in 20 mL acetic acid. The mixed precursor of 10 μL was dropped on to a FTO glass (available area 10 mm×10 mm) by pipettor. After 30 min, the glass was heated in an air oven to 450 ° C for 2 min, and allowed to cool down in air. After repeating this procedure for 5 times, the sample was calcined in at 550 ° C for 4 h. As the glass cooled down, a thin yellow film could be seen on the FTO glass. The ALD process used deionized water and Al(CH 3 ) 3 (trimethylaluminum) as precursors, and film thickness of Al 2 O 3 could be controlled in angstrom scale. Typical pulse durations for trimethylaluminum and water were 0.05 s and 0.03 s, respectively. After ALD depositing, the samples were heated in a furnace at 380 ° C for 0.5 h to produce uniform and crystalline Al 2 O 3 layer. 2.2 Material Characterization 2 Morphologies of the samples were characterized with a scanning electron microscope (SEM JME2011, JEOL, Japan), and a high-resolution transmission electron microscope (HRTEM, FEI TECNAI G2 F20). TEM samples were prepared by scrapping the BiVO 4 film from the FTO glass with a new blade, and transferring the powder carefully into an EP tube containing methanol (HPLC class). X-ray photoelectron spectra (XPS) were measured with a PerkinElmer 1257 model, operating at an average base pressure of ~5.9×10 -9 Torr at 300 K with a non-monochromatized Al Kα line at 1486.6 eV and a hemispherical sector analyzer with resolution of 25 MeV. 2.3 Electrochemical Characterization 2 The BiVO 4 photoanodes were fabricated by sealing the BiVO 4 electrodes (including the edges) with epoxy resin except for a 0.25 cm 2 square (unsealed) left for photo-excitation. An external Cu wire was connected to the FTO surface using a 63/37 Sn/Pb solder (Youbang Soldering Company, Hangzhou, China). An Autolab electrochemical station (Metrohm AG, Switzerland) equipped with a Nova 1.8 software was employed to study electrochemical properties and stability of the as-fabricated photoanodes in a three-electrode electrochemical cell. The photoanodes were illuminated on the FTO side and tested. 2.4 PEC measurements 2 PEC activity of the samples was measured using line scan voltammetry in a standard three-electrode setup. BiVO 4 photoanode was regarded as the working electrode, a platinum plate as the counter electrode, and Ag/AgCl as the reference electrode. PBS (pH 7) was used as electrolyte in the measurements. The photocurrent density of each photoanode was recorded by the Autolab electrochemical station under the simulate light of 100 mW/cm 2 provided by a 500 W Xe lamp (Newport, Model SP 94023A) with an AM 1.5 G filter. The light intensity was standardized in advance using a calibrated silicon photodiode (Newport, 91150 V). The same lamp was used in photocurrent transient measurement. Electrochemical impedance spectra (EIS) were collected by the Autolab electrochemical workstation. 3 Results and Discussion 1 The BiVO 4 photoanodes were prepared by dropping mixed precursor on FTO substrates, BiVO 4 nanoparticles were formed after crystallization. Ultrathin Al 2 O 3 films were coated on BiVO 4 photoanodes via ALD process ( Fig. 1a Fig. 1a ). SEM images in Figs. 1(b) and 1(c) Figs. 1(b) and 1(c) show similar morphologies of BiVO 4 photoanodes before and after ALD process (5 cycles), indicating that ALD coating Al 2 O 3 had little effect on the morphology of BiVO 4 photoanode., TEM images in Figs. 1(d) and 1(e) Figs. 1(d) and 1(e) show that the BiVO 4 and BiVO 4 ·Al 2 O 3 particles have an interlunar spacing of 0.309 nm, being well consistent with the JCPDS 75-2480, and the pristine and coated nanostructures do not differ from each other. According to Ref.[ 14 14 ], the 5-cycle Al 2 O 3 is 0.75 nm thick (by spectroscopic ellipsometry on Si), and that is probably why the ALD treatment has little influence on the morphology and nanostructure of BiVO 4 . The electronic structure of BiVO 4 was checked by XPS ( Figs. 2a–2c Figs. 2a–2c ). The pure BiVO 4 sample exhibited typical spin-orbit split of Bi 4f 5/2 and Bi 4f 7/2 signals, and V 2p 1/2 and V 2p 3/2 signals, in agreement with the literature [ 22 22 , 23 23 ] . This confirms that the BiVO 4 sample prepared was identically the monoclinic scheelite BiVO 4 . For the Al 2 O 3 -coated BiVO 4 sample, as shown in Fig. 2(d) Fig. 2(d) , Al was detected, and a significant shift of O 1s peak can be seen in Fig. 2(e) Fig. 2(e) , indicating two crystal structures in the sample. All these were consistent with the fact that the Al 2 O 3 layer had covered the samples through ALD deposition. PEC performance of the BiVO 4 photoanodes before and after the ALD process was investigated. As shown in Fig. 3(a) Fig. 3(a) , the photocurrent densities of Al 2 O 3 -coated BiVO 4 photoanodes were significantly higher than the pristine. In particular, the photocurrent density of BiVO 4 ·Al 2 O 3 photoanode by 5 ALD cycles reached over 1.0 mA/cm 2 at 1.23 V vs. RHE under the standard illumination of AM 1.5 G (100 mW/cm 2 ), which was twice of the pristine one’s. It confirms that Al 2 O 3 overlayer deposition can strongly enhance the PEC performance of BiVO 4 photoanodes. Yet, when by further increasing the ALD cycles to 10 ALD cycles, the photocurrent density decreased, partly because of the large increased resistance in the Al 2 O 3 overlayer. In fact, the Al 2 O 3 -coating was optimized at 5 ALD cycles.. Photocurrent transient measurement was performed to assess the charge recombination at the sample/electrolyte interface. The measurement was taken under curtain bias potential, and the time-resolved photocurrent was recorded while the illumination was turned on and off, with the BiVO 4 ·Al 2 O 3 sample of 5 ALD cycles. As shown in Figs. 3(b) and 3(c) Figs. 3(b) and 3(c) , the BiVO 4 ·Al 2 O 3 sample at 0.5 and 0.9 V vs. RHE has much smaller current spikes than those of the pristine BiVO 4 photoanode. According to Ref.[ 14 14 ], the appearance of sharp spike is mainly because that photogenerated holes accumulate at the interface or oxidize the trap states on the semiconductor surface and in the bulk. A larger bias potential allows the holes to cross over the interface more easily to oxidize water. And the motion of holes reduces the anodic current. In our experiment, the anodic current spike at 0.5 V vs. RHE is much higher than that at 0.9 V vs. RHE. After the ALD treatment, the current spikes became smaller and much smoother. This suggests that the charge recombination on the surface of BiVO 4 photoanode may be passivated after depositing the ultrathin Al 2 O 3 overlayer. To better support this hypothesis, photoluminescence (PL) of the Al 2 O 3 -coated BiVO 4 sample (5 cycles) was studied. PL spectroscopy is an effective technique to study separation efficiency of the photogenerated carriers. The higher PL intensity indicates the bigger probability of charge recombination [ 24 24 ] . Fig. 4 Fig. 4 shows the PL quantum yield of the Al 2 O 3 -coated BiVO 4 photoanode was smaller than the pristine one, indicating that the charge recombination in BiVO 4 photoanode was effectively inhibited after the Al 2 O 3 coating [ 8 8 ] . Therefore, the ultrathin Al 2 O 3 overlayer can effectively passivate the charge recombination on the surface of BiVO 4 photoanodes. Also, electrochemical impedance spectroscopy (EIS) was performed with the samples covered with 5 ALD cycles Al 2 O 3 and the control (without Al 2 O 3 overlayer). Here we used an equivalent circuit consisted of 2 RC elements in series, accounting for semiconductor behavior and surface processes, as shown in Fig. 5(a) Fig. 5(a) [ 14 14 , 25 25 ] . In Fig. 5(b) Fig. 5(b) , space charge resistances ( R SC ) of the samples before and after ALD treatment are all stable around 10 3 Ω, but a remarkable decrease in charge transfer resistances ( R CT ) can be seen after ALD treatment ( Fig. 5c Fig. 5c ). The R CT decrease could lead to a reduction of overpotential, hence the increase of photocurrent we observed. Yet it will be much clearer to reveal the effect of Al 2 O 3 layer when we look into the change of Helmholtz capacitance ( C H ) or space charge capacitance ( C SC ) after ALD treatment (Figs. 6d and 6e). The C SC increased and C H decreased as a result of Al 2 O 3 -coating. As capacitance is the charge over voltage, so the C H decrease after the ALD treatment may be related to the change in voltage or charge distribution at the sample/electrolyte interface. In this experiment, the C H decrease is due to the decreased free charge density in surface trap states because of the passivation of surface states. Another reason is that the relatively high dielectric of Al 2 O 3 layer may improve the charge screening of anions in Helmholts layer [ 14 14 ] . On the other hand, as shown in Fig. 5(d) Fig. 5(d) , after ALD treatment, the space charge capacitance ( C SC ) increased significantly in the entire range of potential bias applied. According to Ref.[ 14 14 ], this can be explained as follows: in order to balance the applied voltage, charges are extract more from the space-charge layer than before due to the decrease of surface states, leading to charge increase in the space charge region [ 14 14 ] , i.e. the significant increase of C SC . Therefore, we can conclude that the Al 2 O 3 overlayer brought about a significant decrease in the charge density on the surface (surface states), and better performance of the BiVO 4 photoanode. 4 Conclusion 1 In summary, ALD coating ultrathin Al 2 O 3 overlayer can effectively enhance the PEC water splitting performance of BiVO 4 photoanodes. Particularly, 5 ALD cycles Al 2 O 3 deposition increased the photocurrent density of BiVO 4 photoanode from ~0.5 mA/cm 2 to ~1.0 mA/cm 2 , at 1.23 V vs. RHE under the standard illumination (100 mW/cm 2 ). In transient photocurrent measurement, the anodic current spikes of the sample became much smoother after depositing Al 2 O 3 overlayer. In photoluminescence spectra, the photoluminescence quantum yield of the Al 2 O 3 coated BiVO 4 photoanode was smaller than the pristine one. And in electrochemical impedance measurement, the charge transfer resistance and capacitance of Al 2 O 3 coated sample differed significantly from the pristine. 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DOI: 10.1016/j.electacta.2006.12.041 http://doi.org/10.1016/j.electacta.2006.12.041 Electronic properties of thermally formed thin iron oxide films 10.1007/s41365-016-0122-6 This work was supported by the National Natural Science Foundation of China (21473236, 21227804), the Shanghai Municipal Commission for Science and Technology (13NM1402300) and the Chinese Academy of Sciences. Ali Aldalbahi acknowledges the support by the Deanship of Scientific Research, College of Science Research Center at King Saud University. 2016-05-17 2016-06-22 2016-07-01 2016-08-25 2016-10-20 2021-04-11 © China Science Publishing & Media Ltd. (Science Press), Shanghai Institute of Applied Physics, the Chinese Academy of Sciences, ChineseNuclear Society and Springer Nature Singapore Pte Ltd. 2016 This is a post-peer-review, pre-copyedit version of an article published in Nuclear Science and Techniques. The final authenticated version is available online at: http://dx.doi.org/10.1007/s41365-016-0122-6 http://dx.doi.org/10.1007/s41365-016-0122-6 . 2016 Nuclear Science and Techniques 05 27