Boosting the Photoelectrochemical Water Oxidation at Hematite

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Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Boosting the Photoelectrochemical Water Oxidation at Hematite Photoanode by Innovating a Hierarchical Ball-on-Wire-Array Structure Mingyang Li,† Yunyi Qiu,§ Weitao Qiu,† Songtao Tang,† Shuang Xiao,‡ Shenglan Ma,† Gangfeng Ouyang,† Yexiang Tong,*,† and Shihe Yang*,‡

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†

MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, The Key Lab of Low-Carbon Chemistry and Energy Conservation of Guangdong Province, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, China ‡ Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 999077, China § Guangzhou First High School, Guangzhou 510163, China S Supporting Information *

ABSTRACT: Hematite nanoarchitecture offers opportunities to simultaneously enhance the light harvesting and charge transport, which are the key to boosting the photoactivity for photoelectrochemical (PEC) water oxidation. However, it still remains a challenge to efficiently design and synthesize the nanostructures realizing that in hematite photoanodes. To address this problem, herein we develop an ingenious hierarchical ball-on-wire-array (BWA) structure that can simultaneously improve light harvesting and carrier transport, via a serial growth strategy that is able to feasibly decorate hematite nanowire arrays with in situ generated tunable nanoballs. The simulation and experimental results prove that the nanoballs provide additional light trapping for the augmented light scattering and absorption, and synchronously the orientational nanowire arrays enhance the charge transport in the hierarchical nanostructure. As a result, the hematite BWA photoanode coated with a nickel−ironbased oxygen evolution catalyst (OEC) achieves an outstanding photocurrent density up to 2.3 mA cm−2 at 1.23 V vs RHE. This work provides significant insights on how the nanoarchitecture of hematite photoanode is correlated to its PEC performance. KEYWORDS: hematite, photoanode, hierarchical structure, morphology engineering, dopant engineering, light trapping, water oxidation

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To circumvent the limitations, the simultaneous augmentation on the carrier transport and light harvesting become the prerequisite to substantially enhance the light availability and photoactivity of hematite photoanodes.13,14 For example, Qiu et al. had deliberately designed hematite nanocone and nanospike arrays by template-assisted pattern method to enhance the photoactivity.15 The ingenious arrays not only improved the light harvesting via trapping more light but enhanced the charge transfer along the growth axis. The nanophotonic structures which spontaneously own an excellent carrier transport property had exhibited a significant PEC improvement.14 But the vulnerable template and deliberate preparation process prevented their further modifications. To simplify the tedious process, Au nanospheres had been grafted to hematite nanorods and also brought out improvements in PEC water oxidation.16 Although the light availability was

s an essential process for solar energy conversion and sustainable energy exploitation, photoelectrochemcial (PEC) water splitting has been intensively investigated over the past few decades for its convenience, low cost, and tremendous practical potential.1,2 However, the unsatisfactory energy conversion efficiency severely hinders its worldwide application.3,4 Substantially boosting the efficiency is the inevitable and essential way to extend this promising green technology.5,6 Thus, the modifications and improvements around photoanodes have been extensively investigated for decades. It is because the sluggish water oxidation over their surface is the rate-limiting step shackling the entire water splitting processes.7−9 Among the various improvement strategies, how to significantly enhance the light availability, is the hard-core solution to circumvent the limitations.10 As a prominent candidate photoanode material for its extraordinary earth abundance and working reliability, hematite photoanode also faces the same concerns. What is worse, the severe tradeoff between the long absorption depth of photons and short diffusion length of minority carriers in hematite extremely hampers its PEC improvement.11,12 © XXXX American Chemical Society

Received: August 1, 2018 Accepted: October 1, 2018 Published: October 1, 2018 A

DOI: 10.1021/acsaem.8b01266 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

Figure 1. (a) Schematic illustration of the preparation of hierarchical hematite nanostructures. (b, c) SEM images of ET-Fe2O3 BWA-10. (d) Section SEM image of ET-Fe2O3 BWA-10. (e) EDS mapping image of ET-Fe2O3 BWA-10.

indeed improved because of the plasmon resonance effect and inherent light scattering in the Au−hematite heterostructure, the metal−semiconductor interface had resulted in too many recombination sites and led to the improvement being undistinguishable.16,17 Enhancing the light harvesting in nanowire/nanorod arrays, which have the inherent advantage of facilitating charge transfer, possesses the tremendous potentials to break away from the dilemma.18−20 Yet rarely any efficient and facile methods have been reported on achieving hematite nanostructures that can combine both the factors of carrier transport and light harvesting up to now.12 Herein, we developed a novel serial growth procedure in which decorating the hematite nanowire arrays with in situ generated tunable nanoballs and forming an ingenious ball-on-wire-array (BWA) hierarchical structure to boost the PEC water oxidation of hematite. Taking advantage of the novel charge transfer in nanowire arrays and the improved light harvesting deriving from nanoballs, the hierarchical hematite BWA structure simultaneously possesses the notable light availability and carrier transport. By further elaborately coating the oxygen evolution catalyst (OEC), the optimized BWA hematite photoanode scored a significant enhancement in PEC performance. Figure 1a illustrated the preparation processes synthesizing the hierarchical nanostructure, which was assembled by nanowires and nanoballs on a FTO substrate. Akaganeite (βFeOOH) nanowires (Supporting Information Figure S1) were first grown on the FTO through a hydrothermal method under 95 °C for 1 h. Instead of cooling the autoclave, it was instantly transferred to a relatively higher temperature oven (200 °C) for another 10 min to in situ graft nanoballs to nanowires (Figure S2). The obtained hierarchical sample was denoted as

FeOOH BWA-10. As displayed, a large number of nanoballs with about 500 nm diameter intimately grew on 650 nm height nanowire arrays. Time-dependent growth experiments showed that the amount and size of nanoballs increased as the subsequent time increased, and eventually they became a dense spheres layer when the time rose to 30 min (Figure S3). Obviously, the serial growth led to a structural modification on the FeOOH film. The obtained Raman and XRD spectra also proved that, along with the time increase, the crystal gradually transformed from akaganeite to hematite (Figure S4). We could ascribe the structural variation to serial growth and Ostwald ripening, which is a crystal growth process that involves coarsening and recrystallization.21 Larger crystals with smaller surface to volume ratio are favored over the energetically less stable and smaller crystallites during the course of ripening.22 The novel hierarchical nanostructure consisting of nanowires and nanoballs would harvest vastly more light than the mono nanowire arrays according to Mie theory.17 To take full advantage of the ingenious nanostructures and avoid the morphology distortion under the high temperature activation, the morphology and doping engineering were applied to retain the intriguing nanostructure and further improve the charge transport.23 Thus, the additional silica encapsulation and tin-ion doping were followed on FeOOH BWA-10 that suffered from 550 °C annealing. After further annealing the engineered sample in air at 800 °C, the silica shell was selectively etched by a fluorous mixture. The obtained sample was identified as hematite and denoted as ETFe2O3 BWA-10 (Figure S5). As displayed in Figure 1b,c, not only the length and diameter of ET-Fe2O3 BWA-10 were maintained after the high temperature calcination when compared to FeOOH BWA-10 but the intimate contact B

DOI: 10.1021/acsaem.8b01266 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Figure 2. (a, b) TEM images of ET-Fe2O3 BWA-10. (c) HRTEM image of ET-Fe2O3 BWA-10. Inset: FFT pattern of the selected square area. Scale bar is 1 1/nm. (d−f) EDS mapping images of Fe-K, Sn-K and O-K signals obtained from a nanowire in ET-Fe2O3 BWA-10.

Figure 3. Optical properties of ET-Fe2O3 NWs and ET-Fe2O3 BWA-10. (a) Experimentally characterized and simulated UV−vis absorption spectra of ET-Fe2O3 NWs and ET-Fe2O3 BWA-10. FDTD simulation results of the distribution of electric field under the 550 nm wavelength illumination for (b) ET-Fe2O3 NWs and (c) ET-Fe2O3 BWA-10. The color index at the right position of each figure reflects the magnitude of |E| at that point.

between nanowires and nanoballs was also well reserved. TEM images (Figure 2a) confirmed that the ingenuity of the morphology was protected as well. Moreover, Figure 2b elucidated the nanoball was comprised of little crystals, corresponding to the Ostwald ripening taking place during synthesizing the nanoballs. The distinguished diffraction spots in the nanowire (inset in Figure 2c) indicated the nanowire had a very high crystallinity, which had been expected to accelerate the charge transfer in its bulk.24 The EDS mapping image on the section of ET-Fe2O3 BWA-10 (Figure 1d and Figure S6) was collected to detect the distribution of the elements. Figure 1e proved that the tin signal was uniformly distributed along the ET-Fe2O3 BWA-10 growth axis, indicating the uniformity of Sn4+ dopant in the sample, which was consistent with the TEM (Figure 2e) and XPS (Figure S7) results. Notably, the hematite BWA possessed an intriguing hierarchitecture and uniform tin-ion doping. It also further confirmed the availability of the morphology and doping engineering strategy.23

To examine the light harvesting in the hierarchical nanostructure, UV−vis spectroscopy and optical absorption simulations of samples were performed. Hematite nanostructure without the serial growth and just consisting of a mononanowire arrays was measured and compared as well. The well reserved hematite nanowire arrays which had experienced the morphology and doping engineering were denoted as ETFe2O3 NWs (Figure S8). Figure 3a showed the measured (solid lines) and calculated (dashed lines) absorption for ETFe2O3 NWs and ET-Fe2O3 BWA-10 photoanodes. In the experimental curves, ET-Fe2O3 BWA-10 had shown a stronger absorption over the entire wavelength compared to the mononanowire arrays. Since the experimental result just indicated the overall absorption in the bulk, the origination of the light utilization in nanoarchitectures should be analyzed with other methods, such as the computational calculation and incident photon-to-current conversion efficiency (IPCE). The computational calculation was first performed. The experimental geometry of hematite photoanodes as in Figure 1 was simulated via the finite difference time domain (FDTD) C

DOI: 10.1021/acsaem.8b01266 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Figure 4. (a) LSVs of T-Fe2O3 BWA-10, ET-Fe2O3 NWs, and ET-Fe2O3 BWA-10 collected at 20 mV s−1 in 1.0 M aqueous KOH under one sun illumination (100 mW cm−2) and in dark. (b) IPCE spectra of T-Fe2O3 BWA-10, ET-Fe2O3 NWs, and ET-Fe2O3 BWA-10 collected at 1.23 V vs RHE. (c) Mott−Schottky plots and (d) EIS spectra of T-Fe2O3 BWA-10, ET-Fe2O3 NWs, and ET-Fe2O3 BWA-10. Inset: Equivalent circuit used to fit the EIS spectra.

overall photoactivity of the samples. Thus, although the experimental result had shown the stronger light absorption in ET-Fe2O3 BWA-15, -20, and -30 resulting from the increased dimensions of nanoballs, we still believe that ET-Fe2O3 BWA10 would display the best performance among the samples. It could own the more delicate balance of the light harvesting between nanoballs and nanowires. The PEC performance of ET-Fe2O3 BWA-10 and ET-Fe2O3 NWs was measured to evaluate the improvement from the light harvesting, and the experiments were conducted in a typical three-electrode electrochemical cell using a platinum sheet as a counter electrode and an Ag/AgCl (saturated KCl) as a reference electrode. Figure 4a compared the linear sweep voltammograms (LSVs) collected in 1.0 M KOH electrolyte in dark and under illumination. Significantly, ET-Fe2O3 BWA-10 showed an excellent photocurrent density of 1.82 mA cm−2 at 1.23 V vs RHE, in which the improvement was as high as 56% when compared to ET-Fe2O3 NWs (1.17 mA cm−2), representing a benchmark among the bare hematite photoanodes.4,25−27 To eliminate the possibility that the improvement comes from the thickness increasing, the ET-Fe2O3 BWA-10 was also compared to the hematite photoanode with longer nanowire arrays, which exhibited a comparable light absorption with that of ET-Fe2O3 BWA-10. As shown (Figure S10), ET-Fe2O3 BWA-10 still displayed a higher photoactivity, supporting our hypothesis that the spheres had a unique influence on boosting PEC performance of nanowire arrays. It is because although the light absorption was promoted for the increased thickness in the traditional nanowire arrays, the amount of electron−hole traps would rise spontaneously in the dense nanowire samples and the

solution. As shown in the calculated curves, ET-Fe2O3 BWA10 displayed a stronger absorption than ET-Fe2O3 NWs as well, especially when the wavelength was larger than 450 nm. For example, the improvement in ET-Fe2O3 BWA-10 reached up to 200% in comparison with that of ET-Fe2O3 NWs at the 550 nm wavelength. It is because in the hierarchical structure the diameter of nanoballs is close to the incident light wavelength leading to the light diffraction, and the electromagnetic wave will redistribute in the hierarchical area and produce a notable electric field enhancement.15 The enhancement originated from the light scattering and absorption has also been known as “light trapping” in nanophotonic structures. To further identify the light trapping sites, the electric field intensity distribution of the electromagnetic wave was simulated under the 550 nm illumination for the samples. As shown in Figure 3b, each nanowire displayed the comparable electric field intensity among the arrays. In the obvious contrast, the nanoball in ET-Fe2O3 BWA-10 possessed a substantially intensified electric field and the electric field on nanowires was improved as well (Figure 3c). The hierarchical structure had shown its unique property in the light trapping. Meanwhile the simulations on the control samples also revealed that, with the diameter of nanoballs increased, the electric field intensity on the bottom nanowire arrays had declined (Figure S9). The nanoballs on the upper layer could block the light utilization from the bottom nanowire arrays. Because the nanowire arrays possessed a very high crystallinity (Figure 2c), which would substantially facilitate the charge transfer compared to the nanoballs when under the illumination, the declined light utilization in nanowire arrays undoubtedly could hinder the charge transfer and impair the D

DOI: 10.1021/acsaem.8b01266 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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simulation and IPCE results confirmed that the nanosphere would result in the additional light trapping by augmenting the scattering and absorption. Finally the optimized ET-Fe2O3 BWA-10 yielded an outstanding photocurrent density of 2.3 mA cm−2 at 1.23 V vs RHE after the OEC coating, surpassing a substantial number of previous records. This present strategy paves a novel way for hematite photoanodes to design hierarchical structures archiving the excellent photoactivity. We also anticipate that this serial growth strategy could be extended to improve the PEC performance of other metal oxides.

photocurrent in nanowire arrays could reach a summit or even decease.23 The improvement from the light harvesting had manifested its utmost effectivity on enhancing the photoactivity of hematite photoanodes. FeOOH BWA-10 experiencing a tin doping process and then converting to hematite was also investigated and denoted as T-Fe2O3 BWA-10. As the most common observations,28 T-Fe2O3 BWA-10 had displayed a notably distorted morphology after the high temperature annealing (Figure S11). Its photoactivity was substantially lower than ET-Fe2O3 BWA-10, indicating that the ingenious nanostructure was imperative for the hematite photoanode. In addition, IPCE spectra were collected to further elucidate the light harvesting in the samples. As shown in Figure 4b, ETFe2O3 BWA-10 had a considerably higher efficiency than those of ET-Fe2O3 NWs and T-Fe2O3 BWA-10 over the entire range of wavelength between 300 and 600 nm. Remarkably, there existed three wide peaks around the wavelength of 420, 500, and 570 nm in ET-Fe2O3 BWA-10, which corresponded to the light trapping and electromagnetic redistribution observed in the modeling results. ET-Fe2 O 3 BWA-10 photoanode possessed a substantially efficient light utilization because of the intriguing hierarchitecture. Additionally, ET-Fe2O3 BWA10 had also shown a higher photoactivity than ET-Fe2O3 BWA-15, -20, and -30, confirming that ET-Fe2O3 BWA-10 displayed the more delicate balance of the light harvesting between nanoballs and nanowires (Figure S12). Apart from the promoted light harvesting, another essential factor influencing the high photoactivity is the efficient charge transport. To evaluate the charge transfer in the hematite photoanodes, electrochemical impedance spectroscopy (EIS) measurements were carried out on the samples. Figure 4c showed the Mott−Schottky plots generated based on the capacitances derived from the electrochemical impedance values obtained at each potential. The donor density of ETFe2O3 BWA-10 was calculated to be 6.89 × 1018 cm−3, which was comparable to that of ET-Fe2O3 NWs (6.45 × 1018 cm−3) and an order of magnitude higher than that of T-Fe2O3 BWA10 (8.22 × 1017 cm−3). The higher donor density was expected to improve the electric conductivity of hematite photoanode and therefore reduce the electrical resistance in the hierarchical nanostructures.29 Nyquist plots were collected from the EIS data as well. As shown in Figure 4d, ET-Fe2O3 BWA-10 displayed a lowest charge resistance (Rct) of 185.7 Ω compared to ET-Fe2O3 NWs (532.3 Ω) and T-Fe2O3 BWA-10 (967.5). It indicated that the doping engineering and nanowire array retaining had succeeded in enhancing the charge transfer, and ET-Fe2O3 BWA-10 had possessed the most efficient charge transfer kinetics for its unique array structure. Finally, we had further optimized the hierarchical structure via coating the OEC on its surface to facilitate the oxygen evolution kinetics. The coated NiFe(OH)x OEC significantly increased the photocurrent density to 2.3 mA cm−2 at 1.23 V vs RHE and exhibited excellent photostability and ABPE (Figure S13), surpassing a number of reported results.4,26,27,30 In summary, we have demonstrated a serial growth strategy that can feasibly decorate hematite nanowire arrays with in situ generated nanoballs to boost the photoactivity of hematite photoanodes. It was discovered that the dimension size of nanoballs was mediated by the subsequent time based on the serial growth and Ostwald ripening during the hydrothermal growth. After further morphology and doping engineering, the hierarchical nanostructure has realized an excellent light harvesting and simultaneous charge transport. The FDTD

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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/acsaem.8b01266.

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Synthetic and analytical methods, SEM images, and UV−visible, XRD, Raman, and XPS spectra, as well as photoelectrochemical and electrochemical data (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.T.). *E-mail: [email protected] (S.Y.). ORCID

Shuang Xiao: 0000-0001-8171-0714 Gangfeng Ouyang: 0000-0002-0797-6036 Yexiang Tong: 0000-0003-4344-443X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program of China (Grant No. 2016YFA0202604), the Natural Science Foundation of China (Grant Nos. 21476271, 21461162003, and 21773315), the Science and Technology Plan Project of Guangdong Province (Grant Nos. 2015B010118002 and 2018A030310300), NSFC/Hong Kong RGC Research Scheme (Grant No. N_HKUST610/14), Shenzhen Peacock Plan (2016), the Fundamental Research Funds for the Central Universities (Grant No. 17lgjc36), and the Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program.

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REFERENCES

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DOI: 10.1021/acsaem.8b01266 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX