Cubic-shape hematite decorated with plasmonic Ag-Au bimetals for enhanced photocatalysis under visible light irradiation

Cubic-shape hematite (C-Fe2O3) was facilely prepared by hydrothermal autoclave reaction of Fe3+ in the presence of 1,12-diaminododecane at 130 °C for 10 h. The surface of C-Fe2O3 was decorated with nanosilvers through the sonochemical reduction of Ag precursor (0.1–0.4 ml of 1.0 wt.% AgNO3), so-called C-Fe2O3@Ag. After then, the C-Fe2O3@Ag was plated with Au layer via galvanic-assisted reduction of Au precursor (0.04–0.14 ml of 1.0 wt.% HAuCl4), so-called C-Fe2O3@Ag-Au. Scanning electron microscopy demonstrated the formation of cubic-shape hematite deposited with plasmonic nanometals. X-ray photoelectron spectroscopy analysis confirmed the existence of Ag and Au crystals. Photocatalytic performance of the hematite samples was estimated towards the degradation of methylene blue (MB) under visible light. The C-Fe2O3@Ag (0.2 ml) exhibited the five-fold increase of photocatalytic activity to that of the pristine C-Fe2O3. Furthermore, Au-deposited C-Fe2O3@Ag (0.2 ml), i.e., C-Fe2O3@Ag-Au, exhibited the 200% increase of photocatalytic activity to that of the C-Fe2O3@Ag (0.2 ml), owing to the plasmonic coupling effect on the extended visible light absorbance and enhanced separation efficiency of electron-hole pairs on the hematite surface.


Introduction
At present, environmental pollution caused by organic toxic chemicals is getting more aggravated because of the rapid and reckless development of industrialization, i.e., air pollutant and industrial wastewater are significantly uprising [1,2]. In solving these serious problems, conventional biological and chemical technologies are quite restricted mainly due to the generation of secondary pollutants during their applications [3,4]. On the other hand, photocatalytic treatment using renewable sunlight is an environmental friendly and low-cost process that can effectively decompose organic materials into harmless components [5].
Among many semiconductors as alternative photocatalysts, Fe 2 O 3 can absorb broader wavelength of visible light because of low bandgap energy of 2.1 eV. In addition, it is chemically stable, non-toxic, and earth-abundant material as a promising sunlight-driven photocatalyst [15]. However, its photocatalyst activity is quite restricted due to the rapid recombination of electron-hole (e --h + ) pairs, very short diffusion length of hole carrier (2-4 nm), and low electrical conductivity [16,17]. Therefore, it is absolutely needed to improve the photocatalytic activity of Fe 2 O 3 using various synthetic routes controlling structural design (nanosheets, nanorods), heterostructure formation, and plasmonic nanometal deposition [18]. The heterostructure formation usually follow the combination strategy of two or three semiconductors that have different bandgap energies [19][20][21][22]. For instance, the heterostructure of Fe 2 O 3 -CdS improved the photocatalytic activity of pristine Fe 2 O 3 because of efficient separation of photo-generated charge carriers and extended visible light absorption [23][24][25][26].
As an alternative strategy, the deposition of plasmonic nanometals (Au, Ag, Pt, and Cu) can increase the separation efficiency of e --h + pairs on the hematite because the deposited nanometals can trap the photoexcited electrons temporarily and reduce the recombination of photo-generated carriers [27][28][29][30]. Bimetallic NPs are formed as the alloys of two different nanometals and/or core-shell nanostructures. Thus, bimetallic nanostructures can optimize surface plasmon energy resonance and provide additional degrees of freedom, consequently leading to enhanced optical, electronic and catalytic effects. Furthermore, plasmonic coupling effect is beneficial for enhancing charge separation and extending visible light absorption [31][32][33][34].
In this work, hydrothermal autoclave reaction was carried out to prepare cubic hematite particles (so-called C-Fe 2 O 3 ) at 130°C for 10 h. The C-Fe 2 O 3 was subsequently decorated with nanosilvers via an sonochemical reduction of Ag precursor (0.1-0.4 ml of 1.0 wt.% AgNO 3 ) [35]. Ag-deposited C-Fe 2 O 3 (C-Fe 2 O 3 @Ag) was further plated with Au layer, i.e., the formation of Ag-Au bimetals via galvanic-assisted plating of Au precursor, so-called C-Fe 2 O 3 @Ag-Au [36]. The C-Fe 2 O 3 @Ag-Au exhibited the significantly enhanced photocatalytic activity as compared to nanometal-free analogue, because of the plasmonic coupling effect on the visible light harvesting and separation efficiency of e --h + pairs. This work provides the simple and cost-effective synthetic route for plasmonic hematite as highly efficient photocatalyst under visible light.  1 v/v%) solution for 2 h. After then, the homogeneous mixture of 50 ml was put into a Teflon-lined reactor at 130°C for 10 h. The product was centrifuged at 7000 rpm for 20 min and washed with DI water to purify the sample. Finally, the C-Fe 2 O 3 product was dried overnight at 60°C [37].

Au-plated C-Fe 2 O 3 @Ag (C-Fe 2 O 3 @Ag-Au)
Ag-deposited C-Fe2O3 (C-Fe2O3@Ag) was dispersed in aqueous solution containing 66 mg of PVP and 100 mg of ascorbic acid (AA). After that, 1.8 ml of 0.1 M HCl was added to make acidic solution of pH 2-3. An aliquot (0.04 ml, 0.09 ml, 0.14 ml) of 1.0 wt% HAuCl4 was slowly injected into the acidic solution for 1 h under vigorously stirring at room temperature (RT) [39,40]. The final product, C-Fe2O3@Ag-Au, was purified by centrifugating at 7500 rpm for 20 min, followed by washing with DI water.

Photocatalytic tests
The photocatalytic activities of the samples (C-Fe2O3, C-Fe2O3@Ag and C-Fe2O3@Ag-Au) were estimated towards the degradation of methylene blue (MB) under visible light. The photocatalyst was dispersed in 30 ml of DI water containing 10 ppm MB. During photocatalytic reaction under visible light, 1.2 ml of solution was sampled at predetermined time and centrifuged at 10 000 rpm to remove the photocatalyst. Then, 0.5 ml of the supernatant was diluted with 3 ml of DI water to measure the absorbance at 664 nm using a UV-vis spectrophotometer.

Results and discussion
3.1. Synthesis and characterization of C-Fe 2 O 3 samples Scheme 1 shows the stepwise procedures for fabricating the final product. First, C-Fe2O3 was synthesized by the hydrothermal autoclave reaction of Fe 3+ in aqueous solution (containing 50 vol.% of ethanol). The sonochemical reduction of Ag + led to the deposition of nanosilvers on the C-Fe2O3. Then, the surface coverage of nanosilver was controlled by changing the loading volume of Ag precursor (1.0 wt.% AgNO 3 ). The Agdeposited C-Fe 2 O 3 was added to the aqueous solution containing PVP (capping agent) and L-Ascorbic acid. After the addition of Au precursor (1.0 wt.% of HAuCl 4 ), Au-Ag bimetals were finally formed on the C-Fe 2 O 3 via a galvanic-assisted plating of Au layer, referred to as C-Fe2O3@Ag-Au.

Ag
shows the scanning electron microscopy (SEM) image of Ag-deposited C-Fe2O3 samples prepared by the consecutive hydrothermal reaction and ultrasonic-assisted deposition method. Using the dynamic light scattering (DLS) instrument at the Smart Materials Research Center for IoT at Gachon University, the particle size of cubic-shape hematite was measured as 479.8±83.9 nm, which was well matched with the SEM images of C-Fe 2 O 3 (figure S1 is available online at stacks.iop.org/MRX/7/095014/mmedia). The surface morphology of C-Fe2O3 was retained after nanosilver deposition, and the size of Ag NPs was estimated as ∼30-40 nm. The surface coverage of nanosilver was increased with loading volumes of Ag precursor, and the size of Ag NPs was increased probably due to their aggregation at a high loading volume (0.4 ml of 1.0 wt.% AgNO 3 ).       precursor (0.04-0.14 ml of 1.0 wt.% HAuCl4). Au precursor (Au 3+ ) was directly plated on the surface of nanosilvers deposited on the C-Fe 2 O 3 , and the size of Ag-Au bimetal was increased with loading volume of Au precursor. The size of bimetallic Ag-Au was estimated as ∼50−70 nm. In addition, the surface morphology of Ag-Au bimetals became more roughened without significantly reducing the exposed surface area of C-Fe 2 O 3 . Figure 5(e) shows the UV-vis absorption spectra of as-prepared samples. The strong and broad absorbance band over 700-1000 nm was attributed to the wide range of light absorption by hematite materials. The Agdeposited Fe2O3 (C-Fe2O3@Ag) exhibited the weak absorbance band around ∼480 nm owing to the SPR of nanosilver, and the C-Fe2O3@Ag-Au exhibited the slight red-shift of absorbance band from 480 nm to 520 nm, due to the formation of plasmonic Ag-Au bimetals. Furthermore, the absorbance band of hematite at ∼850 nm was gradually shifted to a longer wavelength with the progressive deposition of nanometals (Ag and Au).
X-ray photoelectron spectroscopy (XPS) was performed to investigate the chemical state and elemental composition of Fe 2 O 3 @Ag-Au. According to the survey scan of XPS spectrum of figure 6(a), typical peaks of Fe 2 p, O 1 s, N 1 s, C 1 s and Ag 3d, Au 4f are clearly observed. The peaks of C1s and N 1s are attributed to diamine linker (1, 12-diaminododecane) incorporated into the hematite structure during the hydrothermal synthesis. Figure 6(b) shows two distinct peaks of Fe2p 3/2 at 709.58 eV and Fe2p 1/2 at 723.08 eV, and the separation of the 2p doublet is ∼13.5 eV. In addition, two satellites with binding energies of 718.08 and 732.78 eV are clearly  observed [41]. Figure 6(c) shows the O1s spectrum that is deconvoluted into three peaks. The peaks at 528.68 eV and 530.48 eV are attributed to lattice oxygen species of FeO and FeOH, respectively. The peak at 533.1 eV represents adsorbed water molecules [42][43][44]. Figure 6(d) shows the spectrum of Ag 3d, in which the peaks at 366.88 eV and 372.88 eV are assigned to Ag 3d 5/2 and Ag 3d 7/2 components, respectively [45]. Figure 6(e) shows the spectrum of Au 4f, in which the peaks at 82.88 eV and 86.58 eV are assigned to Au 4f 7/2 and Au 4f 5/2 components, respectively. When compared with the pure metals (368 eV for Ag 3d 5/2 and 84 eV for Au 4f 7/2 ), Ag-Au bimetals exhibited the negative shift of binding energies by 1.1 eV (366.88 eV for Ag 3d 5/2 and 82.88 eV for Au 4f 7/2 ), suggesting the plasmonic coupling effect by Ag-Au bimetals [45,46].
The photocatalytic activities of as-prepared samples were tested towards the degradation of MB dye under visible light irradiation. The C-Fe2O3@Ag-Au sample was prepared by plating the C-Fe2O3@Ag (0.2 ml) with different loading of Au precursor (0.04 and 0.14 ml of 1.0 wt% HAuCl4). Figure 7 compares the degradation efficiency of MB dye depending on the deposition of nanometals on the hematite. The C-Fe 2 O 3 @Ag-Au showed the higher photocatalytic activity than any other samples (C-Fe2O3 and C-Fe2O3@Ag). The reason may be that Ag-Au bimetals induce the plasmonic coupling effect, consequently leading to the enhanced photocatalytic activity. In the meantime, the C-Fe 2 O 3 @Ag-Au showed the similar degradation efficiency irrespective of loading amounts of Au (0.04 ml and 0.14 ml), indicating the direct deposition of Au layer on Ag NPs. The EDX analysis  did not show the significant difference of Au elements among the samples, which was ranged in 0.43−0.59% of atomic composition.
Prior to the photodegradation reaction of MB dye (10 ppm), the photocatalyst sample was first dispersed in the aqueous solution for 5 min in the dark. Figure 8 Figure 8(b) shows the Langmuir-Hinshelwood (L-H) kinetics for the degradation of MB dye under visible light. The experimental data were fitted by ln (C 0 /C)=kt, where k is the rate constant, and C 0 and C are the concentrations at time zero and t, respectively. The linear plot of ln(C 0 /C) versus time indicated that the photodegradation rate followed the pseudo-first-order kinetics. The rate constant of C-Fe 2 O 3 , C-Fe 2 O 3 @Ag, C-Fe 2 O 3 @Ag-Au were calculated as k=6.59´10 −4 , 3.04´10 −3 , 6.05´10 −3 , respectively. The C-Fe 2 O 3 @Ag exhibited five-fold increase of photocatalytic activity to that of the pristine C-Fe 2 O 3 , and the C-Fe 2 O 3 @Ag-Au exhibited 200% increase of photocatalytic activity to that of C-Fe 2 O 3 @Ag. The significantly enhanced photocatalytic activity was mainly attributed to the plasmonic coupling effect of Ag-Au bimetals, which could harvest wider range of visible-light and enhance the separation efficiency of photo-generated charge carriers.
PL emission spectra represent the excited state of the photocatalyst through the recombination process of photo-excited electrons and remained holes in the valence band. Thus, the intensity of PL emission indicates the degree of recombination rates of e --h + pairs. Figure 8(c) showed the increasing order of PL emission intensity under an excitation wavelength of 340 nm as follows: C-Fe 2 O 3 @Ag-Au<C-Fe 2 O 3 @Ag<C-Fe 2 O 3 . The C-Fe 2 O 3 @Ag-Au exhibited the lowest PL intensity (i.e., the lowest recombination rate), resulting in the highest photocatalytic activity among the samples. Figure 8(d) shows the time-evolution of change in absorbance at 360 nm during the photocatalytic degradation of colorless tetracycline hydrochloride (5 ppm) over Fe 2 O 3 @Ag-Au. The degradation efficiency of tetracycline hydrochloride under visible light was found to be 64.3%, which was lower than that of the MB dye (75.6%). Figure 8(e) shows that the photocatalytic efficiency was gradually decreased with cycling numbers, probably due to the slight loss of photocatalyst during a recovery process. However, the photocatalytic efficiency after 5 cycles was slightly decreased by 9.5%, indicating the sustainability of C-Fe 2 O 3 @Ag-Au as a visible-light photocatalyst.
The schematic diagram of photocatalytic mechanism of C-Fe 2 O 3 @Ag-Au was briefly described in figure 8(f). The hematite with low bandgap receives wide range of visible-light, and the activated photoelectrons are easily separated from e --h + pairs. Nonetheless, the hematite has poor electrical conductivity and very short length of hole diffusion, and the separated electrons are quickly recombining with the remained holes, leading to the decreased photocatalytic activity. On the other hand, plasmonic nanometals deposited on the hematite can play as a temporary reservoir of photo-generated electrons and reduce the recombination rates of e --h + pairs. In particular, Ag-Au bimetal shows the better photocatalytic performance than monometallic Ag, because the work function of Ag is higher than that of Au, thereby transferring more electrons from Fe 2 O 3 to Ag-Au bimetal. When compared with nanometal-free analogue, the C-Fe 2 O 3 @Ag-Au exhibited the more enhanced absorbance of visible light and electron trapping effects owing to the plasmonic coupling effect of Ag-Au bimetal [50][51][52]. The photo-generated charge carriers produced superoxide (·O 2 − ) and hydroxyl radicals (·OH) that mainly participated in the photodegradation of MB dye under visible light.

Conclusions
In this work, cubic hematite particles (C-Fe 2 O 3 ) were facilely prepared by hydrothermal autoclave reaction of Fe 3+ in the presence of DA-12 at 130°C for 10 h. After then, nanosilvers were directly deposited on the hematite surface via sonochemical reduction of Ag precursor. The C-Fe 2 O 3 @Ag was further plated with Au layer via galvanic-assisted reduction method, forming so-called C-Fe 2 O 3 @Ag-Au. SEM and UV-vis spectroscopy analysis clearly demonstrated the formation of cubic-shape hematite deposited with plasmonic nanometals. XRD patterns of the C-Fe 2 O 3 @Ag clearly exhibited the pronounced XRD peaks corresponding to hematite and deposited Ag. XPS analysis of C-Fe 2 O 3 @Ag-Au confirmed the co-existence of Ag and Au crystals on the hematite. As-prepared samples were subjected to the photocatalytic degradation of MB dye under visible light. The Fe 2 O 3 @Ag (0.2 ml) showed the higher photocatalytic activity than those of C-Fe 2 O 3 @Ag (0.1 ml, 0.3 ml, and 0.4 ml), due to the optimal deposition of nanosilvers on the hematite surface. Furthermore, the C-Fe 2 O 3 @Ag-Au exhibited the 200% increase of photocatalytic activity to that of C-Fe 2 O 3 @Ag, probably because of plasmonic coupling effect of Ag-Au bimetal on the extended absorption of visible light and increased separation efficiency of photo-generated electrons. Optical band gap of C-Fe 2 O 3 @Ag-Au was calculated as ∼1.93 eV (versus 2.0 eV of pristine C-Fe 2 O 3 ) from the diffuse reflectance spectrum, suggesting that the bandgap was decreased due to the SPR effect of noble metals. In addition, the reusability test confirmed the sustainability of C-Fe 2 O 3 @Ag-Au as a visible-light photocatalyst. This work provides the facile synthetic route for plasmonic cubic hematite with enhanced photocatalysis under visible light.