Creation of High‐Performance Heterogeneous Photocatalysts by Controlling Ligand Desorption and Particle Size of Gold Nanocluster

Abstract Recently, the creation of new heterogeneous catalysts using the unique electronic/geometric structures of small metal nanoclusters (NCs) has received considerable attention. However, to achieve this, it is extremely important to establish methods to remove the ligands from ligand‐protected metal NCs while preventing the aggregation of metal NCs. In this study, the ligand‐desorption process during calcination was followed for metal‐oxide‐supported 2‐phenylethanethiolate‐protected gold (Au) 25‐atom metal NCs using five experimental techniques. The results clearly demonstrate that the ligand‐desorption process consists of ligand dissociation on the surface of the metal NCs, adsorption of the generated compounds on the support and desorption of the compounds from the support, and the temperatures at which these processes occurred were elucidated. Based on the obtained knowledge, we established a method to form a metal‐oxide layer on the surface of Au NCs while preventing their aggregation, thereby succeeding in creating a water‐splitting photocatalyst with high activity and stability.

When creating such heterogeneous catalysts,generally,1) metal NCs are first precisely synthesized using ligands.T hen, 2) the obtained atomically precise metal NCs are adsorbed onto the support. Ty pically,t he presence of ligands in ah eterogeneous catalyst leads to ad ecrease in catalytic activity because it inhibits the approach of the reactant to the surface of the metal NCs and induces am odification of the electronic structure of the metal NCs (Scheme 1(a)). [26,27] Therefore,i nm any cases,3 )s ome or all of the ligands are removed by calcination or other pretreatments to attain higher activity (Scheme 1(b)). [15][16][17][18][19][20][21][22][23][24]28,29] However,l igand removal also induces aggregation of metal NCs.W hen such aggregation occurs,the catalytic activity specific to metal NCs is diminished (Scheme 1(c)). [15-24, 28, 29] Therefore,i nl igand removal, it is extremely important to select conditions that remove only the ligands while maintaining the number of constituent atoms of the metal NCs.However,aclear understanding of the ligand-desorption mechanism during calcination has not yet been attained. To perform calcination under appropriate conditions and therefore create ah ighly functional heterogeneous catalyst, it is essential to attain ad eep understanding of this mechanism.
In this study,f or metal-oxide-adsorbed 2-phenylethanethiolate (PET;S cheme S1(a)) protected gold (Au) 25-atom NCs ([Au 25 (PET) 18 ] À ;S cheme S2(a)), which is ac ommonly used catalyst in heterogeneous catalytic applications, [18,[20][21][22][23][24][25][26][27][28][29][30][31][32][33] the ligand-desorption process during calcination was followed using five experimental techniques.T he results clearly demonstrate that the ligand-desorption process consists of ligand dissociation on the surface of the metal NCs,a dsorption of the generated compounds on the support and desorption of the compounds from the support, and elucidate the temperatures at which these processes occur.Based on the obtained knowledge,w eh ave established am ethod to load Au NCs while preventing their aggregation, thereby succeeding in creating aw ater-splitting photocatalyst with high activity and stability.  18 ] À (counter ion is tetraoctylammonium ion = TOA + ;h ereinafter described as Au 25 -(PET) 18 )w as used. Au 25 (PET) 18 was synthesized with atomic precision using areported [34] method with slight modification (Scheme 2(a), Scheme S3, and Figure S1A(a)). Form etal oxides,t oa pply the obtained heterogeneous catalysts as water-splitting photocatalysts (Scheme 3), [35][36][37][38] BaLa 4 Ti 4 O 15 (Scheme S2(b) and S4), [39][40][41][42] which is one of the most advanced photocatalysts,w as used. When metal oxides are placed in water, hydroxyl groups (-OH) are generally formed on their surfaces.M etal NCs protected by hydrophobic ligands,such as PET,are barely adsorbed on such hydrophilic surfaces. [43] However,t oe stimate the metal loading weight with high accuracy,itisnecessary to adsorb the metal NCs on the support with ahigh adsorption efficiency. Therefore,some of the PET in Au 25 (PET) 18 was replaced with hydrophilic pmercaptobenzoic acid (p-MBA; Scheme S1(b)) [44] (Scheme 2 (b) and Figure S1B(a)). [43] Theo btained Au 25 (PET) 18Àx (p-MBA) x (x = 5-12;h ereinafter described as Au 25 (PET, p-MBA) 18 )was stirred with BaLa 4 Ti 4 O 15 in acetone solution for 1h at aw eight ratio of 0.1 wt %A u, which gave the best water-splitting photocatalytic activity in our previous study. [40] Scheme 1. Schematic illustration of typical phenomena caused by increasing the calcination temperature in metal-oxide-supported ligand-protected metal NCs:a)small size is maintained but low activity, b) high activity emerges while maintaining small size, and c) decreased activity due to aggregation. Scheme 2. Schematic illustration of experimental procedure used in this work:a)synthesis of Au 25 (PET) 18 ,b )preparation of Au 25 (PET, p-MBA) 18 using ligand-exchange reaction, c) adsorptiono fAu 25 18 /Cr(OH) 3 /BaLa 4 Ti 4 O 15 was placed in an electric furnace and calcined under reduced pressure (Scheme S5). Forthe calcination temperature,itwas increased from room temperature to each final temperature at arate of ca. 7 8 8Cmin À1 and kept at the final temperature for 80 min. Thesample obtained before and after the calcination was examined by direct insertion probe-mass spectrometry (DIP-MS;Scheme S6 and S7), X-ray absorption fine structure (XAFS) analysis,F ourier-transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) analyses, and transmission electron microscopy (TEM).

Results and Discussion
Mechanism for Au 25 (PET,p-MBA) 18 To better understand the phenomena occurring on the metal oxide during calcination, we first examined the liganddesorption pattern of Au 25 (PET, p-MBA) 18 ,w hich was not loaded on the metal oxide.F igure 1A presents the DIP-MS spectrum of Au 25 (PET, p-MBA) 18 .This MS spectrum contains peaks derived from all the compounds desorbed from the sample from 80 8 8Ct o5 00 8 8C ( Table S1). Them ain peaks appeared at m/z = 91, 105, 137, 154, 254, 274, and 290. The peak at m/z = 254 is attributed to ac ompound derived from TOA + ( Figure S2), which is the counter cation of Au 25 (PET, p-MBA) 18 .C omparison of the DIP-MS spectra with Au 25 -(PET) 18 ,A u 25 (PET,3 -MPA) 18 ,a nd Au 25 (SC4, p-MBA) 18 (3-MPA = 3-mercaptopropionic acid;S C4 = 1-buthanethiolate; Scheme S1(c)(d) and Figure S1) with different ligand combinations revealed that the peaks at m/z = 91, 105, and 274 correspond to PET-derived compounds,t he peaks at m/z = 137 and 154 correspond to p-MBA-derived compounds,a nd the peak at m/z = 290 can be obtained only when both PET and p-MBAare present ( Figure S3-S5). It can be interpreted that the peak at m/z = 91 is caused by EI dissociation of PET, and the peak at m/z = 137 is caused by EI dissociation ( Figure S6 (Figure 2(a)). Although these results are overall consistent with previous reports, [45] the fact that p-MBAi sd esorbed from the surface of the Au NCs as at hiol rather than at hiolate was first demonstrated in this report. [46] Figure 1B  Figure S8). These results indicate that in the calcination of Au 25 (PET, p-MBA) 18 ,S À Ca nd Au À Sd issociations of AuÀPET begin to occur first, followed by AuÀS dissociation of AuÀp-MBA. For p-MBA, desorption was observed at several temperatures ( Figure 1B(b)). In Au 25 (PET, p-MBA) 18 ,t here are two types of Ss ites (Scheme S2(a)). In addition, the temperature required for desorption is likely to differ depending on the state of the p-MBAs,f or example,t he state in which the p-MBAs are gathered or the state in which p-MBAislocated next to PET. Thed esorption of p-MBAi sc onsidered to have occurred at multiple temperatures for these reasons.  Figure 3A presents the mass spectra of the compounds desorbed at temperatures ranging from 80 8 8Cto500 8 8C. Surprisingly,the peaks attributed to PE (m/z = 105), p-MBA( m/z = 154), (PET) 2 (m/z = 274), and PET À p-MBA( m/z = 290) were negligibly observed in the mass spectra. On the other hand, carbon dioxide (CO 2 ; m/z = 44), which is one of the final products of calcination, benzene (m/z = 78), and styrene (m/z = 104) were strongly observed in the mass spectra. Benzene is interpreted to form from the EI dissociation of styrene ( Figure S9). Figure 3B shows the correlation between the desorption temperature and ion intensity for CO 2 ( Figure 3B(a)) and styrene ( Figure 3B Figure 1B and S10). These results imply that the PE, p-MBA, (PET) 2 ,a nd PET À p-MBAt hermally dissociated from Au 25 (PET, p-MBA) 18 were once adsorbed on the BaLa 4 Ti 4 O 15 surface and then desorbed from the surface of BaLa 4 Ti 4 O 15 in the form of styrene or CO 2 (Figure 2(b)).
Au L 3 -edge FT-EXAFS analysis was performed on unsupported Au 25 (PET, p-MBA) 18 and calcined samples to attain ad eeper understanding of the temperature at which each step occurs (Figure 4a nd S11). Thep eaks at ca. 1.8 [43,47,48] attributed to the AuÀSb ond were clearly observed in the spectra of Au 25 (PET, p-MBA) 18 (Figure 4(a)) and the sample after calcination at 250 8 8C ( Figure 4(b)). On the other hand, for the sample calcined at 300 8 8C ( Figure 4 (c)), the intensity of this peak was significantly reduced. This finding indicates that almost all the AuÀSbonds dissociate in the temperature range of 250-300 8 8CinA u 25 (PET, p-MBA) 18 [49] of SO 3 2À or SO 4 2À even at 500 8 8C( Figure 5).    Au bond regions, respectively. [43,47] In (a), only weak peaks appear in the AuÀAu bond region because the Au 13 core (Scheme S2(a)) fluctuates at room temperature. [48]   3) At ca. 300 8 8C, most of the ligands are desorbed from the surface of the Au NCs;the Au 25 maintains its size even at this temperature (Figure 7(d)). However, based on our previous work, the geometric and electronic structures of Au 25 is considered to change,largely due to elimination of the ligands;for example,the gold core geometry changes from as pherical structure (Scheme S2(a)) to af lat structure. [43] 4) Af urther increase of the calcination temperature causes significant aggregation of Au 25 (Figure 7(e)). Some organic ( Figure 3B(a)) and Scompounds ( Figure 5(d)) continue to remain on BaLa 4 Ti 4 O 15 and cannot be completely eliminated even at 500 8 8C (Figure 7(e)).
In addition to the strength of the Au À Sa nd S À Cb onds, the interaction between the ligands on the surface of the Au NCs also significantly affects the temperature of the ligand desorption from the surface of the Au NCs ( Figure S12). It is also presumed that the temperature at which the compound is desorbed from the support is related to the magnitude of the compound-support interaction. [20] In addition, the ease of dissociation/desorption of the ligands and the resulting aggregation of Au NCs appears to slightly vary depending on the calcination atmosphere ( Figure S13). [20,21] However, the results suggesting 1)-4) have often been observed during previous calcinations performed with thiolate (SR) functional groups,s upports,a nd atmospheres different from this study: [20,30,40,[49][50][51][52] for example,A u 25 (SG) 18 /BaLa 4 Ti 4 O 15 (SG = glutathionate) and Au 38 (PET) 24 /CeO 2 (CeO 2 = cerium(IV) oxide). Therefore,although there are differences in the required temperatures,i ti si nferred that behavior similar to that described in 1)-4) occurs during the calcination of any SR-protected Au NCs (Au n (SR) m NCs; n = number of Au, m = number of SR ligands)/metal oxide.T odate,aunified view has not been presented for the behavior of these Au n (SR) m NCs/metal oxides during calcination. [49,53] In this study,wesucceeded in elucidating the details of the phenomena occurring during the calcination of Au 25 (PET, p-MBA) 18

Toward the Creation of High-Performance Water-Splitting Photocatalysts
As described above,the behavior of Au 25 (PET, p-MBA) 18 / BaLa 4 Ti 4 O 15 during calcination was elucidated, and most of the ligands were successfully removed from Au 25 with almost no aggregation (Figure 6(e)). However,AuNCs with exposed surfaces are prone to aggregation when left untended (Figure S14) and during catalytic reactions. [40] Therefore,tocreate  S cheme 2(e)), and the stability of Au 25 against aggregation was greatly improved. [43] Furthermore,t he formation of such aC r 2 O 3 film [54] suppressed the reverse reaction on the surface of the Au NCs, resulting in higher water-splitting activity. [43] In the precursor, Au 25 (PET, p-MBA) 18 ,t he ligand was strongly bound to the surface of the Au NCs.However, in Cr 2 O 3 /Au 25 /BaLa 4 Ti 4 O 15 , it is assumed that the amorphous structure of Cr 2 O 3 is weakly bound to Au NCs and forms an overlying structure on Au NCs because Au does not form bonds with Oe asily. [43,55] This appears to be the reason why Cr 2 O 3 /Au 25 /BaLa 4 Ti 4 O 15 showed high water-splitting activity without losing the high H 2generation activity of small Au NCs.I th as been reported by other groups that the formation of such metal/semiconductor oxide films on the surface of metal NCs improves the stability of metal NCs against not only photocatalytic water-splitting reactions but also thermocatalytic reactions. [17,56,57] Therefore, the establishment of amethod to form ametal oxide film on the Au 25 surface while suppressing the aggregation of Au 25 is expected to be extremely useful not only for the creation of high-performance water-splitting photocatalysts but also for the creation of high-performance heterogeneous catalysts.In our previous study,t he aggregation of Au 25 occurred in the Cr 2 O 3 layer. [43] In the current study,weattempted to elucidate the behavior of Au 25 (PET, p-MBA) 18 Figure 7. Indeed, the Au L 3 -edge FT-EXAFS ( Figure S15) and S2 pX PS ( Figure S16) results strongly support this interpretation.
However,t here are also some differences in the calcination mechanism between Au 25 (PET, p-MBA) 18 Figure 8B(b)) than that at which bond dissociation started on the surface of the Au NCs (195 8 8C; Figure 1B). Thes tudy using FT-IR spectroscopy ( Figure S17 and S18) revealed that some of the ligands in Au 25 (PET, p-MBA) 18 /Cr(OH) 3 /BaLa 4 Ti 4 O 15 migrated from Au 25 (PET, p-MBA) 18 to Cr(OH) 3 /BaLa 4 Ti 4 O 15 without heating. Such ligand migration is interpreted to be related to the start of the styrene desorption at 150 8 8Ci nA u 25 (PET, p-MBA) 18 / Cr(OH) 3 /BaLa 4 Ti 4 O 15 ( Figure S19 and S20).
TheA uL 3 -edge FT-EXAFS ( Figure S15) and diffuse reflectance spectra ( Figure S21) of aseries of samples indicate that the Au NCs change their geometric and electronic structures following ligand elimination, similar to the case of Au 25 (PET, p-MBA) 18 /BaLa 4 Ti 4 O 15 . [43] Figure 9A In order to clarify the reason for the observed 2.9 AE 0.9 nm particles,w ei nitiated UV-light irradiation within af ew minutes after calcination. Thea verage particle size of the Au NCs was suppressed to 1.5 AE 0.5 nm ( Figure 9A(b)). According to HAADF-STEM EDX elemental mapping, these Au NCs were embedded in the Cr 2 O 3 film ( Figure 9B and S22 and Table S2). These results indicate that the aggregation of Au 25 is relatively suppressed upon heating,   (Figure 10(f)). This phenomenon is assumed to be caused by the transfer of excited electrons generated in the photocatalyst to Au NCs and thereby the reduction of highly oxidized Cr (> 3 +)t o form ad eposit over the surface of the Au NCs (Figure S23).
Based on this understanding, it is extremely important to reduce the time between calcination and light irradiation as much as possible to create highly functional water-splitting photocatalysts with fine and stable Au NCs.I nf act, the sample in Figure 9A(b) exhibited ah igher water-splitting activity than the sample with more aggregation (Figure S24 and S25 and Scheme S8). In addition, further aggregation of Au NCs was suppressed even after long-term exposure to air for this sample,a nd this sample exhibited high durability during the water-splitting photocatalysis (Figure 11 and S26). Currently,C r 2 O 3 film formation by UV-light irradiation is performed in pure water ( Figure S23). However,the addition of asuitable sacrificial agent to the water would increase the consumption rate of the holes generated by the UV-light irradiation, [43,58,59] thereby allowing the reduction reaction on the surface of the Au NCs,i.e., Cr 2 O 3 film formation, to occur in ashorter time.Itisexpected that photocatalysts with even less aggregation of Au NCs can be created in the future by improving the Cr 2 O 3 film formation method.

Conclusion
In this study,the calcination mechanisms of Au 25 (PET, p-MBA) 18   Thef indings obtained in this study are expected to provide clear design guidelines for the creation of highly functional heterogeneous catalysts using metal NCs,w hich have been reported thus far. [12,60,61] Supporting Information: Experimental section, additional schemes,a dditional DIP-MS,X PS,F T-IR, FT-EXAFS spectra, TEM image,and photocatalytic activity. acknowledge also the instruments and scientific and technical assistance of Ashley Slattery and Microscopy Australia at Figure 11. Time course of water-splitting activity of Cr 2 O 3 /Au NCs/ BaLa 4 Ti 4 O 15 with Au NC particle-size of 1.5 AE 0.5 nm ( Figure 9A(b)). The red and blue circles represent H 2 and O 2 ,r espectively.