Facile Synthesis of FeOOH Quantum Dots Modified ZnO Nanorods

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Facile synthesis of FeOOH quantum dots modified ZnO nanorods films via a metal-solating process Faqi Zhan, Yahui Yang, Wenhua Liu, Keke Wang, Wenzhang Li, and Jie Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00776 • Publication Date (Web): 02 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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ACS Sustainable Chemistry & Engineering

Facile synthesis of FeOOH quantum dots modified ZnO nanorods films via a metal-solating process Faqi Zhan†,‡, Yahui Yang§, Wenhua Liu†,‡, Keke Wang†,‡, Wenzhang Li†,‡*, Jie Li†,‡* †

School of Chemistry and Chemical Engineering, Central South University,

Changsha 410083 China ‡

Key Laboratory of Hunan Province for Metallurgy and Material Processing of

Rare Metals, Changsha, 410083, China §

College of Resources and Environment, Hunan Agricultural University,

Changsha 410128, China

*

Corresponding author. Tel.: +86 731 8887 9616; fax: +86 731 8887 9616.

E-mail addresses: [email protected], [email protected]

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Abstract：： In this study, we referenced the formation principle of rust in nature and the FeOOH quantum dots were prepared using a metal-solating process. The FeOOH QDs exhibited an average diameter of 3.5 nm with well crystallinity. Further, the asprepared FeOOH QDs were deposited on ZnO nanorods film as a cocatalyst for water oxidation. The crystal phase, microstructures and optical properties of the synthesized films were established through X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy and ultraviolet-visible absorption spectroscopy (UV-vis). Applied as a photoanode for solar water splitting, the FeOOH QDs/ZnO nanorods film exhibited a photocurrent density of 0.44 mA/cm2 at 1.23 V vs. RHE, which is 2.1 times higher than that of pure ZnO film. After the loading of FeOOH QDs, the ZnO photoanode showed about twotime higher surface charge injection efficiency and better long-term stability. The analysis of electrochemical measurements displayed that, as a cocatalyst of oxygen evolution reaction, FeOOH QDs resulted in a noticeable cathodic shift of photocurrent onset potential for water oxidation and a remarkable improvement of surface charge injection efficiency. In addition, the metal-solating method can be applied to preparing the other metal oxides QDs, such as WO3 and ZnO QDs.

Key words: ZnO nanorods; FeOOH QDs; metal oxides QDs; photoelectrochemical property

Introduction Exploitation of sustainable fuels through photoelectrochemical (PEC) water splitting based on the semiconductors materials has become a promising technique since the photocatalysis was discovered on titanium dioxide (TiO2) in 1972.1-3 In the

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past few decades, many semiconductors have been applied as photoelectrodes for solar water splitting,4-5 such as TiO2, WO3, ZnO, Fe2O3 and so on.6-8 Among these semiconductors, inexpensive and nontoxic zinc oxide (ZnO) has been an promising material for efficient PEC water oxidation owing to its good electrical properties, and easy fabrication methods.9 ZnO possesses a similar band structures to TiO2,10 but a 10-100 times higher electron mobility than that of TiO2,11 which makes ZnO be favorable for PEC water splitting.12 Actually, the reported PEC activity of the ZnO semiconductor is still inferior to the TiO2, limited by the low separation efficiency of photogenerated electron-hole pairs and slow kinetics of the photocatalytic water oxidation reaction. Recently, many strategies have been developed to overcome these challenges of ZnO photoanodes. On one hand, ZnO nanostructures with different lowdimensional morphologies have been synthesized, including nanorods, nanosheets and nanowires . Among these nanostructures, ZnO nanorods (NRs) structures are favorable for carrier separation and provide an especial surface morphology for water oxidation owing to their direct electrons transport pathways

and axial light

absorption.13-15 On the other hand, the modification with a cocatalyst accelerates the holes to inject into electrolyte and inhibits the photocorrosion of the semiconductor.1618

Choi et al deposited 10-30 nm Co-based catalysts nanoparticles on the ZnO surface,

which enhanced the anodic photocurrent of ZnO and also negatively shifted the photocurrent onset potential by 0.23 V.19 Recently, numbers of oxygen evolution catalysts (OECs) have been loaded on the surface of semiconductors including IrO2,20 RuO2,21 Co-Pi,22 CoOx,23 Ni2P 24 and NiO.25 The transition metal hydroxides, basing on Fe, Co, Ni and Mn elements, have been widely used as the OECs due to their excellent catalytic performances and low costs.26-29 Most recent papers state that the OER catalytic activity follows the

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sequence of Fe > Co > Ni > Mn, excluding the effect of tiny Fe dopant in NiOOH or CoOOH.30-34 Among them, FeOOH has become a popular and efficient earthabundant OEC. It has shown significantly improved O2 evolution kinetics in water oxidation reaction.35-36 Generally, FeOOH can be easily synthesized through various methods, including photodeposition,37 hydrothermal process,38 sol-gel process,39 and electrodeposition.40 Ding et al reported that amorphous FeOOH was photodeposited on WO3 film and the FeOOH OER catalyst not only increased the photocatalytic activity but also improved the photostability of WO3 anode.41 Ye et al designed Fe2O3 photoanodes modified with nanostructured FeOOH through photoelectrodeposition. Four-time increase of photocurrent density and an noticeable negative shift of the photocurrent onset potential were achieved.42 However, the FeOOH cocatalysts prepared by above methods are usually amorphous, with poor crystalline, and possess large size or thick layer, which may result in a poor electric conductivity. Thus, well crystalline FeOOH quantum dots (QDs) synthesized by an uncomplicated synthesis method may be an promising candidate for surface decoration of photoelectrodes. Here, well crystalline FeOOH quantum dots (QDs) were prepared using a metalsolating process. To our best knowledge, it is the first attempt to apply the metalsolating process to synthesize the FeOOH QDs for further deposition on photoanodes films as a cocatalyst for water oxidation. Importantly, the metal-solating method can be applied to preparing the other metal oxides QDs, such as WO3 and ZnO QDs. Then, the as-prepared FeOOH QDs were modified on the surface of ZnO nanorods films by a low-temperature chemical dip-coating method. The FeOOH QDs/ZnO nanorods photoanode exhibited a noticeable negative shift of photocurrent onset potential, increased charge injection efficiency and better PEC performance compared to the pure ZnO photoanode.

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Experimental Section Synthesis of ZnO nanorods films ZnO nanorods films grown on FTO substrates were synthesized through a chemical bath deposition (CBD) method according to a previous report.9 Firstly, a thin ZnO seed layer was grown on the FTO substrates using a pulse electrodeposition method in zinc nitrate hexahydrate and potassium chloride aqueous solution. The pulse parameters were chosen as follows: Emax= -100 mV, Emin= 0 mV, t1= 0.5 s, t2= 0.5 s. And the whole electrodeposition time was maintained as 1 min. After that, ZnO seed layer was washed with deionized (DI) water and annealed at 350 ℃ for 0.5 h. Then, ZnO nanorods were further grown on ZnO seed layer through chemical bath deposition (CBD). The process was performed in a 0.025 M aqueous solution containing zinc nitrate (Zn(NO3)2·6H2O) and hexamethylenetetramine (C6H12N4) at 85 ℃ for 10 h. Finally, ZnO NRs films were rinsed with DI water and annealed at 450 ℃ for 0.5 h.

Synthesis of FeOOH QDs decorated ZnO nanorods films FeOOH QDs: in a typical preparation, 20 mg/L of Fe powder (200 mesh) was ultrasonically dispersed into deionized water for 2 h with a ultrasonic power of 200 W. After stewing 3 days, 3 mL of the supernatant sol was added into 30 mL of N,NDimethyl formamide (DMF) and the mixture was refluxed for 8 h. Afterward, the FeOOH QDs solution was obtained by centrifuged for 30 min at 8000 rpm. FeOOH QDs/ZnO films: the prepared ZnO NRs films were soaked in the above DMF solution of FeOOH QDs for 24 h. After soaking, the as-prepared FeOOH QDs/ZnO films were dried at 120 ℃ for 1 h. The other WO3, ZnO QDs and QDs 5



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modified ZnO films were prepared by the same process as the FeOOH QDs and FeOOH QDs/ZnO films. The images of samples are shown in Fig. S1. Characterization The microscopic morphologies were observed using scanning electron microscope (SEM, Nova NanoSEM 230) equipped with X-ray energy dispersive spectrometer (EDS), high resolution transmission electron microscope (HRTEM, G2 F20) and high angle annular dark-field imaging (HAADF). The crystal phases of all films were investigated by X-ray diffraction (XRD, D/Max2250, Rigaku Corporation, Japan)

with

Cu

Kα

(λ=0.15406

nm)

radiation.

A diffused

reflectance

spectrophotometer (DR-UVS, Shimadzu 2450 spectrophotometer) was used to record the UV-vis absorption spectra. The surface composition and elemental valence state of samples were detected by X-ray photoelectron spectroscopy (XPS, K-Alpha 1063, Thermo Fisher Scientific). Photoelectrochemical measurements Photoelectrochemical (PEC) measurements of bare ZnO and FeOOH QDs/ZnO films were conducted by an electrochemical work station (Zennium, Zahner) with a typical three-electrode configuration; the samples were used as working electrode, platinum plate was the counter electrode and saturated Ag/AgCl electrode was the reference electrode. A phosphate buffer (PB, 0.1 M, pH=7) was used as the electrolyte. A 150 W Xenon lamp was used as the illumination source and adjusted to simulated 1 sun illumination (AM 1.5 G, 100 mW/cm2). All photoelectrodes were irradiated from the back side. The scan rate of the photocurrent curve (J-V) was 20 mV/s. The MottSchottky measurements were conducted with an AC frequency of 1 kHz, and the electrochemical impedance spectroscopy (EIS) was conducted at 1.23 V (vs. RHE) 6


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with an AC frequency range of 10 kHz~100 mHz. The EIS spectra data were analyzed by Z-View program (Scribner Associates Inc.). The incident photon-to-current conversion efficiency (IPCE) was performed with a bias of 1.23 V (vs. RHE) using a Xenon lamp (150 W, Oriel) equipped with a monochromator. The intensity modulated photocurrent spectroscopy (IMPS) were conducted with a Zahner CIMPS system. A blue light-emitting diode (LED) lamp illumination controlled by both 90% DC and 10% AC components of current was used as the light source. The electrode potentials versus the reversible hydrogen electrode (RHE) is transformed from the Ag/AgCl 0 0 electrode using the formula:43 ERHE = E Ag / AgCl + E Ag / AgCl + 0.059 pH , where E Ag / AgCl is

0.1976 V at 25 ℃. For 0.1 M PB solution (pH=7), ERHE = E Ag / AgCl + 0.61 V.

Results and Discussion Characterization of the synthesized photoanodes The shape and size of the synthesized FeOOH, ZnO and WO3 QDs were detected by TEM as shown in Fig. 1(a-f). All the QDs display relatively uniform sphere with an average diameter around 3.5 nm. The UV-vis light absorption spectra of the synthesized QDs solution were obtained as presented in Fig. 1g. All the QDs reveal a strong absorption in the UV region around 230 nm and a broad absorption in visible region. Matching with their optical band gap, the maximum light absorption edges of FeOOH QD, WO3 QD and ZnO QD are 600, 450 and 400 nm, separately.

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Fig. 1 TEM images and the corresponding size distribution of FeOOH QDs (a, d), ZnO QDs (b, e), WO3 QDs (c, f), and the UV-visible absorption spectra of the QDs solution (g).

The composition and crystal phase of the QDs/ZnO films were investigated by XRD and Raman. As illustrated in Fig. 2a, the XRD pattern indicate that the distinct diffraction peaks centered at 31.8°, 34.5° and 36.3° were referred to the (100), (002) and (101) planes of ZnO (JCPDS No. 75-0576). Neither observable shifts of the main characteristic diffraction peaks nor other QDs crystalline phases were found in all hybrid films. This is attributed to the low loading amount of QDs in the samples. To verify the presence of FeOOH QDs in the ZnO film, Raman spectra were conducted as shown in Fig. 2b. The five characteristic peaks located within 200 to 1000 nm are belong to the bands of ZnO. The peaks around 330 cm-1, 440 cm-1 and 560 cm-1 are interpreted as the (O-Zn-O) bending vibrations. Importantly, the peaks of FeOOH at 220, 378, 477, 584 and 631 cm-1 appear in the composites,44 which further demonstrate that FeOOH QDs were successfully deposited on the surface of ZnO 8


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nanorod film.

Fig. 2 XRD patterns (a) and Raman (b) of the samples

After immersing treatment with FeOOH QDs, the white pristine ZnO film is changed to light yellow in color. Fig. 3a and e show the SEM images of the uniformly ZnO nanorods vertically grow on the FTO substrate with an average diameter of 120 nm and smooth surface. The loading QDs make the surface of ZnO nanorods become rough, compared with the pristine ZnO (Fig. 3b-h). Seen from the TEM images in Fig. 4a-b, the clear and good crystallinity FeOOH QDs are evenly distributed on the surface of ZnO nanorods. Fig. 4c-f present the Zn, O and Fe elemental mapping, demonstrating that the FeOOH QDs are distributed on the ZnO nanorods. Fig. S2 shows the XPS spectra of FeOOH QDs/ZnO film, which also confirms the existence of FeOOH QDs.

Fig. 3 SEM images of ZnO (a, e); FeOOH QDs/ZnO (b, f); WO3 QDs/ZnO (c, g); and ZnO 9



QDs/ZnO films (d, h)

Fig. 4 TEM (a), HRTEM (b), and elemental mapping images (c-f) of FeOOH QDs/ZnO film.

The UV-vis diffuse reflectance spectroscopy was used to analyze the optical properties of QDs modified ZnO films. Fig. S3 shows that pure ZnO film exhibits strong light absorption within UV region (< 400 nm), matching with its optical band gap (3.2 eV). Compared with the bare ZnO film, the absorption edge of FeOOH QDs/ZnO film red shifts to ~550 nm and the visible light absorption is increased. However, the WO3 and ZnO QDs modified ZnO films exhibit no significant red shift, except for a little increased visible absorption. These indicate that WO3 and ZnO QDs cannot act as the photosensitizers for ZnO nanorod films. Photoelectrochemical properties of as-prepared photoanodes The effect of FeOOH QDs surface deposition on the photoelectrochemical properties of ZnO photoanode was investigated through testing the photocurrentpotential curves (J-V) under chopped 1 sun illumination (Fig. 5a). It is important to

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note that FeOOH is unstable for water oxidation below pH 7 due to water oxidation will acidify the unbuffer solution, especially at the surface of electrodes.45-46 Thus, we investigated the PEC properties of photoanodes in 0.1 M phosphate buffer solution (pH=7). Fig. 5a shows that the bare ZnO exhibited an onset potential at ~0.7 V (vs. RHE), and the photocurrent density reached to 0.21 mA/cm2 at 1.23 V (vs.RHE). Upon deposition of the ZnO nanorods surface with FeOOH QDs, much higher photocurrent density was obtained within the whole range of applied potential. At 1.23 V (vs. RHE), the photocurrent density of 0.44 mA/cm2 was achieved, which was a 110% improvement over the bare ZnO film. Importantly, the onset potential of photocurrent was negatively shifted to 0.4 V. A summary and comparison of literature with the similar catalyst system for PEC water oxidation was listed in Table S1. Compared with other ZnO-based photoanodes and FeOOH cocatalysts modified photoanodes, our FeOOH QDs/ZnO photoanode exhibited better improvement of PEC performance.

Fig. 5 Photocurrent density (a, b), and IPCE curves (c) of photoanodes.

To further study the photocurrent response of the photoanodes at different wavelength,

the

incident

photon-to-current

conversion

efficiency

(IPCE)

measurements were conducted at 1.23 V (vs. RHE). IPCE can be calculated by IPCE = (1240 I ) / ( λ J light ) × 100% ,47 where I (mA cm-2), λ (nm) and Jlight (mW cm-2) are the photocurrent density, the incident light wavelength and power density , separately. Seen from Fig. 5c, the bare ZnO film exhibited a maximum photo-

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response wavelength of 390 nm, while the FeOOH QDs/ZnO film was photoelectrochemically active up to 420 nm, consistent with the light absorption spectra (Fig. S3). This indicated that FeOOH QDs extended the optical absorption of ZnO film to some extent and increased the IPCE value of bare ZnO film from 20% to 38%. With regard to other QDs modification (WO3 QDs and ZnO QDs), they also improved the photocurrent density and IPCE value of ZnO photoanode (Fig. 5b and c). Fig. 6c presents the photo-stability test curves for 7200 s, the results indicate that the photocurrents of FeOOH QDs/ZnO and pure ZnO films rapidly decay in the first 1000 s, but after that the FeOOH QDs/ZnO film exhibits a better stability than pure ZnO film in the PEC system. As an amphoteric oxide, the stability of ZnO in solution needs to be further improved, such as modification with TiO2 protection layer.

Fig. 6 Mott-Schottky plots (a), OCP curves (b), and photo-stability test (c) of photoanodes.

To clarify the strong correlation between loading of electrocatalyst and the increased photocurrent density and IPCE values, Mott-Schottky measurement was carried out with a frequency of 1 kHz. As presented in Fig. 6a, both M-S plots show a positive slope, reflecting that both two photoanodes are n-type semiconductors and electrons are the majority carriers in FeOOH QDs/ZnO film. The carrier density can be evaluated from the Mott-Schottky plots by the following equation:48 N d = (2 / εε 0 q )  d (1/ C 2 ) / dV 

−1

(1)

where ε and ε0 are the dielectric constant of the semiconductor and the vacuum permittivity, Nd is the carrier density, q is the electron charge, and V is the applied 12


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potential , respectively. The electron density of bare ZnO film was calculated to be 4.5×1018 cm-3, while the Nd value of FeOOH QDs/ZnO film was 4.8×1018 cm-3. The FeOOH QDs did not increase the Nd of ZnO film obviously, consistent with other report.49 By extrapolating the X-intercepts of the linear fit to the Mott-Schottky plots, the flat band potential (Vfb) of photoanodes was evaluated. The Vfb of FeOOH QDs/ZnO photoanode shows a negative shift by 0.1 V. Additional evidence was provided from the open circuit potential (OCP) measurements. Without a net charge circulation, OCP measurements recorded electrodes surface equilibrium potentials under both dark and light conditions.50 The light OCP value means the quasi-Fermi level position of ZnO film under pseudo-equilibrium conditions.51 As shown in Fig. 6b, FeOOH QDs coating enables a larger photovoltage generation and more enough surface driving force for the water oxidation reaction. Analyzed in Fig. S4, one possible reason for the potential shift is that the surface states lead a potential drop within the Helmholtz layer, but this was eliminated by surface FeOOH QDs passivation.50 Another explanation is probably owing to the fast hole migration from ZnO to FeOOH QDs, thus changing the band alignment.52 Many reports have indicated that modification with cocatalyst would increase the band bending of semiconductor photoanodes.53-54 The cathodic shift of the flat band potential implied more driving-force for water splitting, larger carriers transfer rate and better separation of photogenerated carriers in the FeOOH QDs/ZnO film.55 Furthermore, the charge transport and transfer processes in photoanodes were characterized by electrochemical impedance spectroscopy (EIS) and intensitymodulated photocurrent spectroscopy (IMPS). Seen from the EIS results in Fig. 7a, the introduction of FeOOH QDs accelerated the water oxidation kinetics of ZnO film through reducing the hole migration resistance. Fig. 7a presents EIS plots of the bare

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ZnO and FeOOH QDs/ZnO films obtained at a 1.23 V bias. Each Nyqusit plot exhibits a single semicircle, indicating that the Faradaic charge transfer process on the photoelectrode surface is the rate-determining step for the PEC water oxidation process.48 The Nyquist plots can be fitted using the Randles equivalent circuit which consists of a series resistance (R1), a charge transfer resistance (R2) and a constant phase element (CPE1).56 By fitting the EIS plots using the Z-view program, the resistances R1, R2 were calculated to be 20.9 Ω, 3589 Ω for the bare ZnO film, and 22.4 Ω, 2229 Ω

for the FeOOH QDs/ZnO film, separately. This indicated that

FeOOH QDs/ZnO photoanode owned lower charge transfer resistance than ZnO film, and FeOOH QDs accelerated the conduction of holes across the ZnO/eletrolyte interface. Meantime, the efficient electron lifetime in photoelectrodes was obtained from Bode plots in Fig. 7b. The lifetime of electrons correlated with a time constant (τe) can be calculated by this equation: τe=1/(2πfmax).57 Larger τe values suggest that electrons possess longer lifetime, faster diffusion rate and lower recombination rate.58 The τe values for bare ZnO and FeOOH QDs/ZnO photoanodes are 3.8 ms and 6.4 ms. This suggests that the improved electron lifetime and a low recombination rate of photogenerated electron-hole pair were achieved for ZnO film with introducing FeOOH QDs. The IMPS is usually used to characterize the electron transport properties of PEC cells.59 The electron transport time (τd) is the average time that photogenerated electrons need to transfer from semiconductors to the FTO conductive glass and it can be calculated by the formula:60 τd=1/(2πfIMPS), where fIMPS is the frequency at the imaginary minimum. Seen from Fig. 8a, τd of the bare ZnO film and FeOOH QDs/ZnO film are 0.19 ms and 0.11 ms, separately. The results of IMPS also demonstrated that the FeOOH QDs accelerated the electron transit rate of ZnO film.

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Fig. 7 (a) EIS Nyquist plots and (b) Bode plots of photoanodes.

Usually, the advanced water oxidation kinetics after FeOOH QDs decoration comes from improved charge injection efficiency (ηinj) on the surface of the photoanode. To certify this suppose, we examined the photo-oxidation of a frequentlyused hole scavenger, Na2SO3, because the oxidation of sulfite ions is kinetically more attainable than water oxidation.37 The surface charge injection efficiency (ηinj) was evaluated by comparing the photocurrent of sulfite oxidation (JNa2SO3) with that of water oxidation (JH2O) (Fig. 8b). The surface charge injection efficiency (ηinj) was ascertained by the Equation:61 ηinj = J H 2 O / J Na 2 SO 3 .The calculated efficiencies presented in Fig. 8c indicate that the (ηinj) value for the FeOOH QDs/ZnO photoanode is about two-time higher than that of bare ZnO. The results illustrate that the increased photocurrent generation and negative shifted onset potential for FeOOH QDs/ZnO film comes from the increased surface charge injection efficiency.

Fig. 8 IMPS plots (a), photocurrent density (b) and charge injection efficiency (c) of photoanodes.

Based on previous papers, cyclic voltammetry (CV) was conducted to examine

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the electrochemical double layer capacitance of the photoelectrodes at non-Faradaic potentials region to evaluate the effective electrode surface areas (ECSA). Therefore, a sequence of CV measurements were accomplished at various scan rates (10, 20, 40, 60, and 80 mV/s) within -0.1~0.1 V vs. Ag/AgCl (Fig. 9a-d). By plotting the difference between the anodic and cathodic current densities (Janodic-Jcathodic) at -0.05 V vs. Ag/AgCl against the scan rate, a linear trend was observed (Fig. 9e). As we known, the slope of the fitting line is twice as much as the double layer capacitance (Cdl) value, which is proportional to the effective electrode surface area of the photoelectrodes. Hence, the ECSA of different electrodes can be compared with another based on their Cdl values. The Cdl values were calculated to be 16, 24, 38 and 53 mF/cm2 for pure ZnO, ZnO QDs/ZnO, WO3 QDs/ZnO and FeOOH QDs/ZnO films. The FeOOH QDs/ZnO film exhibited about 3.3 times higher ECSA than bare ZnO film, indicating that FeOOH QDs not only improved the charge transfer efficiency but also provided a large active area and more highly-active catalytic sites.

Fig. 9 CV curves of bare ZnO (a), ZnO QDs (b), WO3 QDs (c), FeOOH QDs (d) modified ZnO films and plots used for determination of Cdl (e).

In addition, the loading amounts of FeOOH QDs were studied to optimize the PEC performance of FeOOH QDs/ZnO films. After EDS detection, the loading 16


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amount of FeOOH QDs for above mentioned FeOOH QDs/ZnO photoanodes was 2.08 at.%. Seen from Fig. S5, with the increase of loading amounts (From 0.48 at.% to 7.38 at.%), the agglomeration of FeOOH QDs becomes more and more serious. Their corresponding photocurrent densities were shown in Fig. S6. At 1.23 V vs. RHE, the FeOOH QDs/ZnO (2.08%) photoanode exhibited an optimal current density of 0.48 mA/cm2, When the loading amounts increased to 4.89% and 7.38%, the photocurrent densities decreased to 0.36 mA/cm2 and 0.27 mA/cm2. This may be due to the agglomeration of FeOOH QDs, which hindered the absorption of photon and the charge transfer.

Conclusion In this study, we referenced the formation principle of rust in nature and the FeOOH QDs were prepared using a metal-solating process. The FeOOH quantum dots exhibited an average diameter of 3.5 nm with well crystallinity. Further, the asprepared FeOOH QDs were deposited on ZnO nanorods films as a cocatalyst for oxygen evolution reaction. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Raman spectroscopy and ultraviolet-visible absorption spectroscopy (UV-vis). The PEC properties were investigated by photocurrent density (J-V) measurement and incident photon to current conversion efficiency (IPCE). The FeOOH QDs/ZnO nanorods photoanode exhibited a 2.1 times higher photocurrent density than that of pure ZnO film. Through the analysis of Mott-Schottky (M-S), electrochemical impedance spectroscopy (EIS) and intensity modulated photocurrent spectroscopy (IMPS), the improvements in PEC performances were attributed to a remarkable cathodic shift of the flat-band potential through the passivation of surface states, and a

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remarkable improvement of surface charge injection efficiency compared to the pure ZnO photoanode. In addition, the metal-solating method can be also applied to preparing the other metal oxides QDs, such as WO3 and ZnO QDs.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:

AUTHOR INFORMATION Corresponding Authors * (W.L.) E-mail: [email protected] * (J.L.) E-mail: [email protected]

Acknowledgments This study was supported by the National Nature Science Foundation of China (21471054).

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TOC graphic

A promising metal-solating process prepared cocatalyst for sustainable energy conversion: Iron rust derived FeOOH QDs for solar water oxidation.

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Facile Synthesis of FeOOH Quantum Dots Modified ZnO Nanorods

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