3.1.3. Deposition-Precipitation

Deposition-precipitation is also a facile approach to synthesize M/S nanocomposites. Typically, in the work of Wu et al., TiO2 nanoparticles were dispersed in an aqueous solution of chloroauric acid. Then NaOH solution was added into the suspension to adjust the pH value to the desired level. After stirring, the precipitates were filtered, washed and dried. Then, the as-obtained products were calcined to form the Au-decorated TiO2 nanoparticles [50].

#### 3.1.4. Chemical Vapor Deposition (CVD)

As is well known, CVD is a powerful approach in synthesizing nanostructure and is often utilized to prepare metal-decorated nanostructures. Typically, in the work of Shi et al., HAuCl4 was vaporized at a high temperature in a tube furnace. Then, the vapor was blown at the TiO2 nanorod arrays at a relatively low temperature with the carrier gas of nitrogen. Thus, the gold would deposit on the TiO2 nanorods, leading to the formation of Au-decorated TiO2 nanorod arrays [51].

#### *3.2. Core-Shell Structure*

The core-shell structure was first developed to improve the quantum yield of quantum dots in the 1990s [52]. After that, the core-shell structure attracted grea<sup>t</sup> interest from researchers and its application areas were greatly extended. In photocatalysis, M/S nanocomposite photocatalysts with core-shell structure occupy an important position due to their outstanding photocatalytic properties [53]. Besides, the enhanced charge transfer between metal and semiconductor, the core-shell structure can also hinder the aggregation of particles and protect the metal core from undesired corrosion or dissolution during the photocatalytic process.

The synthesis of core-shell M/S nanoparticles usually involves the coating of a semiconductor layer on metal nanoparticles. For example, in the work of Sudeep, photocatalytic Ag@TiO2 core-shell nanoparticles were synthesized through controlled hydrolysis of titanium-(triethanolaminato) isopropoxide (TTEAIP) on the surface of Ag nanoparticles. Briefly, TTEAIP and silver nitrate were added into 2-propanol followed by stirring. Then dimethyl formamide (DMF) was added to the solution. Next, the solution was heated and refluxed. During this process, silver nitrate was first reduced to Ag nanoparticles by DMF. Then, due to the interaction between Ag nanoparticles and triethanolamine ligands, TTEAIP hydrolyzed on the surface of Ag nanoparticles, leading to the formation of Ag@TiO2 core-shell nanoparticles [54].

#### *3.3. Yolk-Shell Structure*

The yolk-shell structure originates from the pioneer work of Xia's group as a variation of the core-shell structure [55]. Under the efforts of researchers, the yolk-shell structure has shown promising application in nanoreactors, drug delivery and lithium ion batteries [56]. Recently, some researchers have also investigated the application of yolk-shell structures in photocatalysis [57–59].

The synthesis of yolk-shell structures usually needs a sacrificial template. In the work of Li et al., gold nanoparticles were first coated with a layer of SiO2 by the hydrolysis of tetraethyl orthosilicate (TEOS). Next, the SiO2-coated gold nanoparticles were coated with a layer of TiO2 by the hydrolysis of tetrabutyl titanate (TBOT). Then, the as-prepared gold nanoparticles were coated with a layer of SiO2 again to protect the TiO2 layer during the calcination process. After calcination, the two SiO2 layers were removed by the etch of NaOH solution. Thus, the Au@TiO2 yolk-shell nanoparticles were prepared. The SiO2 layers served as the sacrificial template during the synthetic process [57].

#### *3.4. Janus Structure*

Janus particles, as first described by de Gennes in 1991, refer to the particles with anisotropic structure, which are composed of two distinct parts [60]. The dual nature of Janus particles endows themselves with fascinating properties, such as unique surface properties, controlled self-assembly behavior and response to multiple stimuli, etc. [61,62]. When applied in photocatalysis, Janus nanoparticles composed of metal and semiconductor can also exhibit remarkable photocatalytic properties.

Different from conventional structures, the synthesis of Janus structures needs delicate control over the combination of two distinct parts. As demonstrated by the work of Seh et al., Au/TiO2 Janus nanoparticles were synthesized through controlled hydrolysis of titanium diisopropoxide bis(acetylacetonate) (TAA) on the surface of gold nanoparticles. The reason for choosing TAA was that the hydrolysis rate of TAA was rather slow, which had a significant influence on the structure of the products. Thus, during the slow hydrolysis of TAA in the alkaline suspension of gold nanoparticles in isopropanol, TiO2 combined with gold nanoparticles to form the Au/TiO2 Janus nanoparticles [63].

## *3.5. Array Structure*

Due to the promising application in nanodevices, the array structure has been widely researched during the past few decades [64–66]. In photocatalysis, the array structure has also been utilized to improve the photocatalytic properties of M/S nanocomposites [67].

The synthesis of array structures is mainly achieved through a template-assisted deposition process. In the work of Wang et al., an anodic aluminum oxide (AAO) template with one side deposited with a layer of gold and connected to a piece of aluminum foil was utilized as the working electrode in an electrochemical cell. For the deposition of CdS, the working electrode was biased to −2.5 V vs. standard calomel electrode (SCE) in the electrolyte of sulfur and cadmium chloride dissolved in dimethyl sulfoxide (DMSO). For the deposition of Au, the working electrode was biased to –0.95 V vs. SCE in commercial Au Orotemp 24. After alternate deposition of CdS and Au, the AAO template was removed through chemical etching. Thus, photocatalytic Au/CdS nanorod arrays were obtained [67].

## *3.6. Multi-Junction Structure*

M/S nanocomposite photocatalysts with multi-junction structures are formed by sandwiching metal nanoparticles between two semiconductors. Thus, the electron transfer process in multi-junction M/S nanocomposite photocatalysts is very similar to that of Z-scheme photocatalytic systems in nature, which could endow them excellent photocatalytic properties [68–70].

Take the pioneer work of Tada et al. for example. The CdS/Au/TiO2 multi-junction nanoparticles as the all-solid-state Z-scheme photocatalytic system were synthesized through a two-step deposition process. In the first step, gold was deposited on the TiO2 nanoparticles through an impregnation process with chloroauric acid as the precursor. In the second step, cadmium sulfide was deposited on the Au/TiO2 nanoparticles through the UV-induced reaction of sulfur and cadmium perchlorate. Thus, CdS/Au/TiO2 multi-junction nanoparticles were prepared [71].

#### **4. Applications and Properties of M/S Nanocomposite Photocatalysts**

To date, the application areas of photocatalysis are mainly focused on environmental remediation, selective organic transformation, hydrogen evolution and disinfection.

#### *4.1. Environmental Remediation*

The photo-excited electrons and holes in semiconductor photocatalysts as well as the associated radicals possess high chemical activity, which can degrade pollutants to low- or non-hazardous substances [72–75]. As reported, most of the researches in photocatalytic environmental remediation were focused on degradation of organic pollutants such as Rhodamine B (RhB), Methylene blue (MB), methyl orange (MO) and so on. To the best of our knowledge, the only inorganic pollutant that has been involved in the research of photocatalytic environmental remediation is nitric oxide (NO). Table 1 lists some of the most representative M/S nanocomposite photocatalysts applied in environmental remediation [51,59,71,76–100].


**Table 1.** Metal/semiconductor (M/S) nanocomposite photocatalysts for environmental remediation.

Due to the facile synthesis, more than half of the M/S nanocomposite photocatalysts listed in Table 1 possess the conventional structure. In the work of Lin et al., through coupling ZnO nanofibers with Ag, the as-synthesized Ag/ZnO photocatalysts exhibited much better properties for photocatalytic degradation of RhB than pristine ZnO photocatalysts, which was revealed in Figure 5a [79]. As revealed in Figure 5b, the properties of Ag/ZnO photocatalysts did not increase linearly with the Ag loading content and reached a peak at the Ag loading content of 7.5 at%. In addition, the repeatability test results in Figure 5c manifested the attenuation of the properties of Ag/ZnO photocatalysts was negligible after three cycles. The excellent properties of Ag/ZnO photocatalysts originated from the enhanced charge separation at the Ag-ZnO interface, which was evidenced by the photoluminescence (PL) spectra in Figure 5d. Toward removal of NO, Li et al. synthesized Bi/A-BO photocatalysts which could remove NO effectively under visible light illumination [86]. The remarkable properties of Bi/A-BO photocatalysts originated from SPR of Bi which could absorb visible light effectively and enhance the charge separation in A-BO. Li et al. also utilized electron spin resonance (ESR) spectroscopy for in-situ investigation of the reactive species during the photocatalytic process. (For detailed information about utilizing ESR spectroscopy to detect radicals, these two reviews, [101,102], are recommended.) The results revealed superoxide radicals (O2−•) radicals were the major active species for photocatalytic NO oxidation.

For the M/S nanocomposite photocatalysts possessing the core-shell structure, their properties in photocatalytic environmental remediation were in strong relationship with the metal core and shell thickness. In the work of Zhang et al., the effect of Au, Pd and Pt cores on the photocatalytic activity of M@TiO2 (M = Au, Pd and Pt) core-shell nanoparticles for degradation of RhB was investigated [91]. As revealed in Figure 6, core-shell M@TiO2 (M = Au, Pd and Pt) exhibited different photocatalytic properties under the irradiation of UV light and visible light. Under irradiation of UV light, the photocatalytic properties of core-shell M@TiO2 (M = Au, Pd and Pt) followed the order P25 TiO2 > Pt@TiO2 > Pd@TiO2 > Au@TiO2, while under irradiation of visible light the photocatalytic properties of core-shell M@TiO2 (M = Au, Pd and Pt) followed the order Pd@TiO2 > Pt@TiO2 > Au@TiO2 > P25 TiO2. The results indicated the metal core had two impacts on the photocatalytic properties of core-shell M@TiO2 (M = Au, Pd and Pt), i.e., improving charge separation at the metal-TiO2 interface

and modulating light absorption of TiO2. In addition, Zhang et al. also demonstrated that the hydroxyl radicals (•OH) generated on core-shell M@TiO2 played a more important role than superoxide radicals and holes in degradation of RhB under irradiation of UV light, while under the irradiation of visible light holes generated on core-shell M@TiO2 played a predominant role in degradation of RhB. The effect of shell thickness on core-shell M/S nanocomposite photocatalysts was revealed by Kong et al. [93]. In their research, the properties of core-shell Au@Cu2O photocatalysts for degradation of MO varied with the Cu2O shell thickness in the order 127 nm > 96 nm > 197 nm > 250 nm > 158 nm. Kong et al. thought the nonlinear relationship between the properties of core-shell Au@Cu2O photocatalysts and the shell thickness could be attributed to several factors, i.e., surface area, light absorption and scattering, charge-carrier dynamics and core-shell interactions, which competed with each other.

**Figure 5.** (**a**) Kinetics of the photodegradation of RhB by Ag/ZnO nanoparticles; (**b**) The degradation rate constant versus the Ag loading content; (**c**) Photodegradation of RhB by Ag/ZnO nanoparticles for three cycles; (**d**) PL spectra of the Ag/ZnO nanofibers with various Ag loading contents ((a) pure ZnO, (b) 1 atom% Ag, (c) 3 atom% Ag, (d) 5 atom% Ag, (e) 7.5 atom% Ag, (f) 10 atom% Ag). Reproduced from [79], with copyright permission from American Chemical Society, 2009.

**Figure 6.** (**a**) Degradation of RhB by P25 and M@TiO2 (M = Au, Pd and Pt) under UV irradiation; (**b**) Degradation of RhB by P25 and M@TiO2 (M = Au, Pd and Pt) under visible light irradiation. Reproduced from [90], with copyright permission from American Chemical Society, 2011.

Compared with M/S nanocomposite photocatalysts possessing the conventional and core-shell structures, there is less research on M/S nanocomposite photocatalysts with the yolk-shell, Janus and multi-junction structures for environmental remediation. Wang et al. investigated the properties of yolk-shell Au@TiO2 photocatalysts for degradation of RhB [59]. In their research, they also introduced

reduced graphene oxide (r-GO) into the TiO2 shell. The results demonstrated the properties of yolk-shell Au@r-GO/TiO2 photocatalysts were better than yolk-shell Au@TiO2 photocatalysts, which indicated the charge separation in the TiO2 shell increased with the electron conductivity of the TiO2 shell. In the work of Yao et al., the properties of Janus Au/ZnO photocatalysts were investigated for degradation of MO [100]. The results demonstrated the properties of Janus Au/ZnO photocatalysts for degradation of MO were higher than that of pristine ZnO photocatalysts and increased with the size of Au nanoparticles under UV light illumination. In the pioneer work of Tada et al., multi-junction CdS/Au/TiO2 photocatalysts were synthesized as the all-solid-state Z-scheme photocatalysts for the degradation of MV [71]. As revealed in Figure 7a, multi-junction CdS/Au/TiO2 photocatalysts exhibited much higher properties than CdS/TiO2, Au/TiO2 and TiO2 photocatalysts, which revealed the charge separation caused by TiO2 → Au → CdS Z-scheme electron transfer was more efficient than that of the single- or double-component systems. The Z-scheme electron transfer was illustrated in Figure 7b. Besides, Tada et al. pointed out that the properties of all-solid-state Z-scheme photocatalysts could be further improved by modifying the energy band structures of the semiconductor components.

**Figure 7.** (**a**) Photocatalytic reduction of MV by TiO2, Au/TiO2, CdS/TiO2 and Au@CdS/TiO2 (CdS/Au/TiO2). Reproduced from [71], with copyright permission from Springer Nature, 2006; (**b**) Schematic diagram of Z-scheme electron transfer. Reproduced from [68], with copyright permission from John Wiley & Sons, 2014.

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#### *4.2. Selective Organic Transformation*

Unlike the non-selective mineralization process in environmental remediation, photocatalysis can also drive organic transformation processes to selectively synthesize valuable chemicals [103–105]. Compared with thermochemical synthetic process, the photocatalytic selective organic transformation process often requires milder conditions and shorter reaction sequences and can exclude some undesirable side reactions. Based on the reported research, the photocatalytic selective organic transformation reactions can be divided into three categories, i.e., the oxidation reaction, reduction reaction and coupling reaction [103]. In Table 2, some of the most representative M/S nanocomposite photocatalysts applied in selective organic transformation are listed [106–120].


**Table 2.** M/S nanocomposite photocatalysts for selective organic transformation.

Towards the photocatalytic selective oxidation reactions, Yuzawa et al. investigated the effect of Pt loading on the properties of Pt/TiO2 photocatalysts for amination of benzene to aniline and unfolded the reaction mechanism governing the photocatalytic amination process [108]. As revealed in Figure 8, the conversion yield of Pt/TiO2 did not increase linearly with the Pt loading content and reached a peak at about 0.1 wt% Pt loading content, while the aniline selectivity of Pt/TiO2 photocatalysts was hardly influenced by the Pt loading content and remained about 97%. The mechanism governing the photocatalytic amination process was clarified through ESR spectroscopy. First, the holes on the TiO2 surface oxidized an ammonia molecule to form a neutral amide radical, which then reacted with the aromatic ring to form an intermediate, and afterwards the hydrogen of the intermediate was abstracted by the active sites on the Pt surface, leading to the formation of aniline. Thus, it could be deduced that the charge separation at the Pt-TiO2 interface could significantly enhance the properties of Pt/TiO2 photocatalysts for the amination of benzene. Zhang et al. investigated the properties of core-shell and yolk-shell Pt@CeO2 photocatalysts for oxidation of benzyl alcohol to benzaldehyde [111]. As revealed in Figure 9, core-shell Pt@CeO2 photocatalysts exhibited much higher benzaldehyde yield and selectivity than yolk-shell Pt@CeO2 photocatalysts. Zhang et al. attributed the unsatisfactory properties of yolk-shell Pt@CeO2 photocatalysts to the loose contact between the Pt core and CeO2 shell, which deteriorated the charge separation at the Pt-CeO2 interface.

**Figure 8.** (**a**) Conversion yield of Pt/TiO2 versus the Pt loading amount; (**b**) Aniline selectivity of Pt/TiO2 versus the Pt loading amount. Reproduced from [107], with copyright permission from American Chemical Society, 2013.

**Figure 9.** Results of photocatalytic selective oxidation of benzyl alcohol to benzyl aldehyde over the core-shell Pt@CeO2, yolk-shell Pt@CeO2, supported Pt/CeO2, CeO2 nanoparticles and blank CeO2. Reproduced from [110], with copyright permission from The Royal Society of Chemistry, 2011.

As listed in Table 2, the photocatalytic selective reduction reactions mainly involve hydrogenation of nitroaromatics and reduction of CO2. The investigation of Tada et al. on hydrogenation of nitroaromatics demonstrated that the life time of electrons could also be prolonged by the charge separation at the Ag-TiO2 interface, leading to the enhanced properties of Ag/TiO2 photocatalysts for hydrogenation of nitrobenzene to aniline [113]. For better properties of photocatalytic reduction of CO2 to CH4, Xie et al. combined P25 TiO2 with five noble metals, i.e., Pt, Pd, Rh, Ag and Au [114]. During the photocatalytic test, the properties of M/TiO2 photocatalysts increased in the sequence of Ag/TiO2 < Rh/TiO2 < Au/TiO2 < Pd/TiO2 < Pt/TiO2, which indicated the charge separation at the metal-TiO2 interface improved with the Schottky barrier height.

Towards the photocatalytic coupling reactions, Jiao et al. investigated the properties of Pd/SiC photocatalysts for Suzuki coupling of iodobenzene and phenylboronic acid [120]. Under visible light illumination, the conversion of iodobenzene and selectivity for the main product of Pd/SiC both reached nearly 100%. Jiao et al. also evaluated the effect of photogenerated electrons and holes in Pd/SiC on the Suzuki coupling process. After adding the electron-capturing agen<sup>t</sup> (or hole-capturing agent) into the reaction system, the photocatalytic properties of Pd/SiC decreased dramatically, which indicated both the photogenerated electrons and holes contributed to the Suzuki coupling process.

## *4.3. Hydrogen Evolution*

As a significant process to convert solar energy into chemical energy, photocatalytic hydrogen evolution has always been the hot spot in the research field of photocatalysis [47,121–123]. Table 3 lists some of the most representative M/S photocatalysts for hydrogen evolution [48,63,67,124–144].


**Table 3.** M/S nanocomposite photocatalysts for hydrogen evolution.

For the M/S nanocomposite photocatalysts listed in Table 3, their properties for photocatalytic hydrogen evolution achieved remarkable enhancement from the charge separation at the metal-semiconductor interface and the SPR of the metal component. In the work of Bi et al., they utilized Pt-Ni alloy nanoparticles to decorate g-C3N4 for saving the usage of Pt [124]. The results revealed the PtNix/g-C3N4 photocatalysts could exhibit comparable properties to that of Pt/g-C3N4 photocatalysts, which provided a possible approach for developing M/S nanocomposite photocatalysts with excellent properties as well as low cost. In the work of Ingram et al., the difference in the properties of Ag/TiO2 and Au/TiO2 (TiO2 was doped by nitrogen atoms, i.e., N-TiO2) photocatalysts for visible light-driven hydrogen evolution was investigated, which shed light on more effective utilization of SPR [125]. As revealed in Figure 10a, the SPR of Ag and Au was excited at the wavelengths of about 400 and 500 nm, respectively, while the absorption edge of N-TiO2 was around 400 nm. Therefore, the SPR of Ag was more efficient for exciting electron-hole pairs in N-TiO2 than Au, which was supported by the photocatalytic hydrogen evolution test results in Figure 10b.

Towards the core-shell and yolk-shell structure, Ma et al. investigated the effect of Au cores on the properties of core-shell Au@CdS photocatalysts for hydrogen evolution [136]. As revealed in Figure 11, under irradiation of visible light with wavelength ≥ 420 nm, core-shell Au@CdS exhibited apparently higher photocatalytic properties than pristine CdS, while under irradiation of visible light with wavelength ≥500 nm, the photocatalytic properties of core-shell Au@CdS were only slightly higher than pristine CdS. Thus, Ma et al. thought the radiative energy transfer from SPR-excited Au to CdS was the main contribution to the enhanced photocatalytic properties of core-shell Au@CdS, rather than the electron transfer from SPR-excited Au to CdS. In the work of Ngaw et al., the effect of Au content on the photocatalytic properties of yolk-shell Au@TiO2 for water splitting was investigated [137]. The

photocatalytic properties of yolk-shell Au@TiO2 also did not improve linearly with the Au content under the irradiation of both visible light and UV light, and reached a peak at 2 wt% Au content. In addition, Ngaw et al. ascribed the enhanced photocatalytic properties of yolk-shell Au@TiO2 to the void space and highly porous shell in yolk-shell Au@TiO2, which provided more active sites for H+ ions to be reduced and more channels for reactants to diffuse into and out of the photocatalytic particles.

**Figure 10.** (**a**) Ultraviolet-visible extinction spectra of TiO2, N-TiO2, Ag/N-TiO2 and Au/N-TiO2 samples. The inset shows difference spectra for Ag and Au (i.e., the Ag/N-TiO2 or Au/N-TiO2 spectrum minus the N-TiO2 spectrum); (**b**) Photocurrent responses (per macroscopic electrode area) under visible light illumination. Reproduced from [124], with copyright permission from American Chemical Society, 2011.

**Figure 11.** Photocatalytic water splitting of core-shell Au@CdS and CdS nanoparticles under irradiation of visible light with wavelength more than 420 nm and 500 nm, respectively. Reproduced from [133], with copyright permission from John Wiley & Sons, 2014.

In the work of Seh et al., the properties of Janus Au/TiO2 and core-shell Au@TiO2 photocatalysts for hydrogen evolution were compared and investigated [63]. As revealed in Figure 12a,b, Janus Au/TiO2 photocatalysts exhibited higher properties than core-shell Au@TiO2 photocatalysts and the properties of Janus Au/TiO2 photocatalysts increased with the size of Au nanoparticles. Moreover, Seh et al. attributed the enhanced charge separation in Janus Au/TiO2 photocatalysts to the strong plasmonic near-fields localized closely to the Au-TiO2 interface, which was supported by the discrete-dipole approximation simulation results.

**Figure 12.** (**a**) Hydrogen evolution through photocatalytic water splitting on Janus Au/TiO2, core-shell Au@TiO2, amorphous TiO2 and pristine Au nanoparticles; (**b**) Hydrogen evolution through photocatalytic water splitting on Janus Au/TiO2 with different Au nanoparticle sizes. Reproduced from [63], with copyright permission from John Wiley & Sons, 2012.

As for the array structure, Wang et al. demonstrated the excellent properties of multi-segmented Au/CdS nanorod arrays (NRAs) for photocatalytic hydrogen evolution [67]. As revealed in Figure 13a, the activities of Au/CdS NRAs for hydrogen evolution increased with the number of Au-CdS segments under irradiation of simulated sunlight. In addition, compared with pristine CdS NRAs, the activities of Au/CdS NRAs for hydrogen evolution gained significant enhancement under irradiation of simulated sunlight, which was presented in Figure 13b. Wang et al. attributed the excellent properties of Au/CdS NRAs for hydrogen evolution to the charge separation at the Au-CdS interface under irradiation of simulated sunlight.

**Figure 13.** (**a**) Hydrogen evolution on the multi-segmented CdS/Au NRAs in the photoelectrochemical cell under irradiation of simulated sun light. (I-V) SEM images of the side view of the CdS-Au NRAs. (**b**) Comparison of the activities for hydrogen evolution on the CdS/Au NRAs and pristine CdS NRAs under irradiation of simulated sunlight. Reproduced from [67], with copyright permission from John Wiley & Sons, 2014.

Toward the multi-junction structure, Yu et al. demonstrated the excellent properties of multi-junction CdS/Au/ZnO photocatalysts for hydrogen evolution [139]. To reveal the high charge separation efficiency bought by the Z-scheme electron transfer, Yu el al. also synthesized Au/CdS/ZnO photocatalysts by depositing Au on CdS/ZnO photocatalysts. As shown in Figure 14a, the properties of multi-junction CdS/Au/ZnO photocatalysts was 4.5 times higher than that of CdS/ZnO photocatalysts, while the properties of Au/CdS/ZnO photocatalysts achieved only a small enhancement compared with CdS/ZnO photocatalysts. The PL spectra in Figure 14b further confirmed the high charge separation efficiency due to the Z-scheme electron transfer in multi-junction CdS/Au/ZnO photocatalysts.

**Figure 14.** (**a**) Hydrogen evolution through photocatalytic water splitting on CdS/ZnO, Au/CdS/ZnO and CdS/Au/ZnO, respectively. (**b**) PL spectra of ZnO, CdS/ZnO and CdS/Au/ZnO at a 270 nm excitation wavelength. Reproduced from [136], with copyright permission from The Royal Society of Chemistry, 2013.

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