Highly ordered mesoporous silica film nanocomposites containing gold nanoparticles for the catalytic reduction of 4-nitrophenol

We report that transparent mesostructured silica/gold nanocomposite materials with an interpore distance of 4.1 nm, as-synthesized from a templated sol–gel synthesis method using discotic trinuclear gold(I) pyrazolate complex, were successfully utilized for the fabrication of thin film mesoporous silica nanocomposites containing gold nanoparticles. The material exhibited a highly ordered hexagonal structure when subjected to a thermal hydrogen reduction treatment at 210 °C. In contrast, when the material was subjected to calcination as a heat treatment from 190 to 450 °C, the thin film nanocomposites showed an intense d100 X-ray diffraction peak. Moreover, gold nanoparticles inside the thin film nanocomposites were confirmed by the presence of the d111 diffraction peak at 2θ = 38.2°, a surface plasmon resonance peak between 500–580 nm, and the spherical shape observed in the transmission electron microscope images, as well as the visual change in color from pink to purple. Interestingly, by simply dipping the material into a reaction solution of 4-nitrophenol at room temperature, the highly ordered structure of the as-fabricated silica/gold nanoparticle thin film composite after thermal hydrogen reduction at 210 °C resulted in an improved catalytic activity for the reduction of 4-nitrophenol to 4-aminophenol compared to the material calcined at 250 °C. Such catalytic activity is due to the presence of gold nanoparticles of smaller size in the silicate channels of the highly ordered mesoporous film nanocomposites.


Introduction
Mesoporous silica nanomaterials with pore size between 2 to 50 nm [1] have been recently applied to the development of biomedical adsorbents [2][3][4], drug delivery systems [5][6][7], catalysts [8][9][10], as well as supports for metal nanoparticles [11][12][13] due to their large surface area, good thermal stability, high uniformity, and controllable pore size [1]. These nanomaterials with a hexagonal structure were independently discovered using a layered silicate kanemite as a template to form folded sheet materials (FSM)-16 [14] and cetyltrimethylammonium bromide (CTAB) as a cationic surfactant to form the material Mobil composition of matter (MCM)-41 [15]. Considering their low toxicity, their bio-degradable nature, and the fact that they are inexpensive and highly availability, non-ionic surfactants such as Brij [16] and Pluronic [17] block copolymers have also been employed as templates [18] to form as-synthesized mesoporous (mesostructured) silica with higher acidity and thicker pore walls. For the preparation of mesoporous silica, the templates need to be removed from the silicate nanochannels to allow the formation of porous structures [1]. Generally, this can be carried out by a calcination method at high temperature in the presence of air [1,[14][15][16][17]. However, this process can also lead to structural damage due to the removal of organic components from the template at high temperature [18]. Since the high quality of mesoporous silica nanomaterials can increase the performance for the above-mentioned applications, it is crucial to find a method for template removal.
As a host for metal nanoparticles, mesoporous matrixes have been extensively used for the controlled growth of gold nanoparticles (AuNPs) [11,19]. For mesoporous silica, gold sources derived from gold(III) chloride trihydrate (HAuCl 4 ) solution have been used as a precursor for post-synthetic grafting upon mixing with mesoporous silica [20]. Another method is to utilize the co-condensation reaction by the mixing solution of HAuCl 4 with surfactants and silica sources during the sol-gel synthesis [21]. However, after template removal at 450 °C or above, both approaches suffer from agglomeration, non-homogenous distribution, and low insertion of AuNPs in the silicate nanochannels. On the other hand, hexagonal mesostructured silica nanomaterials with dense filling of organic functional groups were successfully reported using a functional organic amphiphile surfactant as a template [22][23][24]. Lintang et al. [25] reported the fabrication of hexagonal mesoporous silica nanocomposites ([Au 3 Pz 3 ] C10TEG / silica hex ) using columnar assembly of an amphiphilic trinuclear gold(I) pyrazolate complex ([Au 3 Pz 3 ] C10TEG ) as a self-assembled template. Indeed, the resulting mesostructured silica nanocomposites not only exhibited perfect self-repairing properties, but also worked as a metal ion sensor [26] with excellent phosphorescent sensing and temperature imaging capabilities [27].
Hence, it is very interesting to utilize this nanocomposite for the growth of AuNPs in the silicate nanochannels.
Recently, mesoporous silica nanomaterials were not only used as a support to control the size of the AuNPs [28], but also to enhance the thermal stability after heat treatment at high temperature [29]. In our previous works, high purity [Au 3 Pz 3 ] C10TEG [30] was used to fabricate [Au 3 Pz 3 ] C10TEG / silica hex for synthesizing AuNPs in the nanocomposites via calcination ([AuNPs] cal /silica hex ) [31] and thermal hydrogen reduction methods ([AuNPs] red /silica hex ) [32]. However, the resulting quality of the mesoporous silica nanomaterials was quite low based on their characteristic diffraction peaks and microscopy images. Therefore, by varying the reduction temperature close to the thermal decomposition of its organic components during calcination and thermal hydrogen reduction methods, we report herein the successful fabrication of highly ordered mesoporous silica film nanocomposites consisting of AuNPs ([AuNPs] red /silica hex ) as shown in Figure 1. Since 4-nitrophenol (4-NP) has been reported as a chemical harmful to human beings due to their highly toxic nature and yet it can be easily found in industrial wastewaters due to its high solubility properties [33], it is very crucial to find an effective method for the degradation and transformation of 4-NP. Generally, the reduction of 4-NP by heterogeneous catalysts in powdered form usually involves a tedious and time-consuming recovery process such as filtration and centrifugation in order to retrieve the catalysts. Hence, we highlight the utilization of thin film nanocomposites [AuNPs] red /silica hex as a heterogeneous catalyst for the reduction of 4-NP to 4-aminophenol (4-AP), where a thin film was simply dipped into the reaction system containing an excess of sodium borohydride (NaBH 4 ).

Results and Discussion
Thermal decomposition of [Au 3 Pz 3 ] C10TEG Thermogravimetric analysis (TGA) was used to examine the process of weight loss as a function of temperature change [34]. The thermal behavior of the [Au 3 Pz 3 ] C10TEG was further supported by its TGA thermogram as shown in Figure 2. Based on the thermogram, the first weight loss step occurred around 50 to 190 °C at 1 wt %, indicating the removal of physically adsorbed water molecules. The second step took place from 190 to 260 °C due to the chain opening of organic complexes in [Au 3 Pz 3 ] C10TEG with weight loss of around 4 wt %. This was followed by the decomposition of long aliphatic alkyl chains and aromatics rings from 260 to 450 °C with 70 wt % loss. The total weight loss was calculated to be around 75 wt %. The remaining weight, at around 25 wt %, can be attributed to the presence of possible residual carbonaceous species [35] and inorganic components [36]. ] C10TEG /silica hex through a sol-gel synthesis with tetrabutyl orthosilicate (TBOS) as a silica source, followed by the synthesis of [AuNPs] cal /silica hex or [AuNPs] red /silica hex by calcination or thermal hydrogen reduction. The synthetic scheme is adapted from [25].

Structural analysis of the nanocomposites before thermal treatment
Generally, the study of the formation of mesostructured silica nanocomposites can be characterized by using X-ray diffraction (XRD) at the small-angle region [37]. Figure 3a [25], with orientation parallel to the substrate [38]. By using Bragg's law, the interpore distance of the hexagonal structure at 2θ = 2.2° was found to be 4.1 nm, which is close to the calculated molecular size of the complex with the assumption of a 1 nm wall thickness.  Figure 3b. These reveal the corresponding planes of mesostructured silica with a hexagonal structure [39]. Hence, the structural analysis from XRD and TEM measurements confirmed the   formation of a hexagonal structure in the silicate nanochannels of [Au 3 Pz 3 ] C10TEG /silica hex .

Structural analysis of the nanocomposites after heat treatment
The quality of the hexagonal mesostructured silica can be generally evaluated from its ordered or disordered structures by heating at high temperature. Figure 4a,b shows the XRD patterns of [Au 3 Pz 3 ] C10TEG /silica hex in the range of 190 to 450 °C (plots (a)-(e)). In both methods, the diffraction peaks for d 100 were still preserved due to good thermal stability and the well-ordered pore structure of [Au 3 Pz 3 ] C10TEG /silica hex . Interestingly, by using thermal hydrogen reduction, such preservation of d 100 in the intensity can also be observed even at a temperature as low as 190 °C (Figure 4b, plot (a)). Moreover, the position of the d 100 diffraction peak was shifted to a higher angle with increasing calcination and reduction temperature  (Table 1), indicating a small decrease of pore size and unit cell. For example, when the calcination temperature was changed from 190 to 250 °C in increments of 20 °C (Figure 4a, plots (a)-(d)), the position of the d 100 diffraction peaks for the resulting mesoporous silica ([AuNPs] cal /silica hex ) were shifted to a higher angle from 2θ = 2.2° (entry 1, Figure 4a, plot (a)) to 2θ =2.5° at 210 °C (entry 2, Figure 4a, plot (b)) and 230 °C (entry 3, Figure 4a, plot (c)) as well as 2θ = 2.8° at 250 °C (entry 4, Figure 4a, plot (d)). These shifts were obviously observed due to calcining at 450 °C to give the d 100 diffraction peaks at 2θ = 2.9° (entry 5, Figure 4a, plot (e)). Calculations of the interpore distance of the hexagonal structure showed a decrease from 3.9 nm to 3.6 nm at 210 °C (entry 2) and 230 °C (entry 3) as well as a decrease from 3.  Figure 4b(e)). The decrease in the interpore distance due to the thermal hydrogen reduction method at 450 °C was calculated to be 0.7 nm, which is less than for the calcination method (0.9 nm). Hence, the thermal hydrogen reduction was determined to be the best heat treatment method for the preservation of the hexagonal structure and to limit the disruption and shrinkage of triethylene glycol (TEG) interpenetration into the silica wall when the temperature was increased near the full decomposition of the [Au 3 Pz 3 ] C10TEG template.
The quality of the mesoporous silica nanocomposites can be indirectly identified using the intensity of the d 100 diffraction peaks. For samples treated with the calcination method, Figure 4a shows no significant change in the d 100 intensity of [AuNPs] cal /silica hex . In contrast, for samples treated with the thermal hydrogen reduction method, [AuNPs] red /silica hex presented a significantly improved d 100 intensity, even at 190 °C (Figure 4b). When the changes in intensity were compared as a function of temperature (insets in Figure 4a and Figure 4b), it can be observed that the thermal hydrogen reduction at 210 °C provided the best quality of mesoporous silica nanocomposites with a d 100 intensity four times higher than the highest d 100 peak after calcination at 250 °C. By holding and carrying out thermal hydrogen reduction at 210 °C, the composite will start to decompose its organic components in [Au 3 Pz 3 ] C10TEG and then form porous structures in mesoporous silica with high quality. It should be noted that [AuNPs] red /silica hex at 230 °C showed a significant decrease in the ordered structure, suggesting the competition of a reduction process and the decomposition of organic components. Of interest is the thermal hydrogen reduction at 250 °C which gave even better quality silica film composites compared to the best results using the calcination method.
In our previous reports [31,32], we highlighted the morphology of mesoporous silica composites after both types of heat treatments at 450 and 250 °C by calcination for 3 hours and thermal hydrogen reduction for 2 hours. In this current work, we have only selected the best two samples from each heat treatment process (calcination at 250 °C and thermal hydrogen reduction at 210 °C) for the TEM measurements. Figure 5a and correlation images (bottom inset figure) for the hexagonal honeycomb structure [39]. The high quality of hexagonal arrangement for [AuNPs] red /silica hex at 210 °C was supported by its intense diffraction peak of d 100 (Figure 4b(b)).
Crystallite size analysis of AuNPs Figure 6 shows the formation of AuNPs based on the X-ray diffraction peaks in the wide-angle area. In this case, diffraction peaks at 2θ = 38.2° were observed for all samples for both heat treatments ((a)-(e)). AuNPs with such particular diffraction peaks were generally confirmed to have a d 111 plane with a cubic phase [40]. In order to determine the crystallite size, Scherrer's equation was applied and the results are summarized as shown in Table 1. According to the TEM images (Figure 5b), samples treated by calcination or thermal hydrogen reduction at 450 °C (Figure 6a(e) and 6b(e)) resulted in larger AuNPs (around 27 and 20 nm) than expected based on calculations. When thermal hydrogen reduction was applied at 210 °C, the crystallite size of the AuNPs was 17 nm while the TEM image in Figure 5c showed particles with the size in that range. TEM images with magnification from 50 to 5 nm for calcination at 250 and 450 °C (Figure 7a and 7b) as well as thermal hydrogen reduction at 210 and 250 °C (Figure 7c and 7d) showed the presence of AuNPs. All film composites showed AuNPs with an indexed reflection in the FFT pattern corresponding to d 111 with a face centered cubic (fcc) structure at 2θ = 38.2° and a fringe spacing of 0.23 nm (ICDD 98-005-0876). Based on Figure 7, we have also observed from the TEM images that the AuNPs cover the external silicate nanochannels, suggesting a weak interaction between AuNPs with the porous silica structure and the possibility of Ostwald ripening during both heat treatments. Another reason for such an observation could be due to the use of TEM with higher electron energy during the measurement. Moreover, the presence of external AuNPs was suggested due to the sintering effect as reported by others [29]. As the source of the AuNPs, the organic functional groups are arranged inside the silica channels using a template sol-gel synthesis of mesoporous silica with a functional surfactant [22][23][24][25]. Due to the heat treatment, the organic functional groups produce AuNPs inside the silicate nanochannels, which may also be found outside the silica surface. Therefore, we strongly believe that the AuNPs in the film composites will be mostly arranged in the silica framework where they can be additionally supported by the TEM 3D tomography at low accelerating voltage with topography-based reconstruction to show the pore orientation at the various angles with the presence of AuNPs (see Supporting Information File 1 for the movie).

Optical properties of AuNPs
Surface plasmon resonance (SPR) peaks in the UV-vis spectrum in the range of 500 to 600 nm can be used to identify the presence of AuNPs [40]. Before calcination or thermal hydrogen reduction was conducted, [Au 3 Pz 3 ] C10TEG and [Au 3 Pz 3 ] C10TEG /silica hex showed absorption bands for π-π stacking of the benzene rings at less than 350 nm without SPR peaks at 500-600 nm [31,32]. After both heat treatments, Figure 8a and 8b shows the SPR peaks at various temperatures and the results are summarized in Table 1. Generally, a red shift of the SPR peaks was observed for both heat treatments from 450 to 190 °C due to the decrease in the average size of the AuNPs [40]. For example, at temperatures as high as 450 °C for both the calcination and thermal hydrogen reduction heat treatment methods (Figure 8a(e) and 8b(e)), the resulting silica film composites gave the lowest SPR bands centered at 538 and  Table 1 (calculated from their XRD diffractograms at wide-angle area as shown in Figure 6a(e) and 6b(e)). Nevertheless, after calcination at 450 °C, the images illuminated by daylight showed a change in color of thin silica film composites from colorless transparent for [Au 3 Pz 3 ] C10TEG / silica hex to a pinkish color for [AuNPs] cal /silica hex (Figure 8a, insert). By using thermal hydrogen reduction at the same temperature, colorless transparent of [Au 3 Pz 3 ] C10TEG /silica hex was changed to purplish colour of [AuNPs] red /silica hex (Figure 8b, insert). Both color changes indicate that different sizes of AuNPs were possibly formed in their thin film composites.

Catalytic activity
For the catalysis reaction, only the thin film composites [AuNPs] cal /silica hex treated at 250 °C for 3 hours and [AuNPs] red /silica hex materials treated at 210 °C for 2 hours were selected because these thin films displayed the best quality within the respective heating methods. In the preparation of the 4-NP solution, the addition of NaBH 4 in excess caused a red shift of the absorption spectrum from 315 nm to an intense peak at 400 nm due to the presence of 4-nitrophenolate ions [41]. By simply dipping the thin film sample into a 4-NP solution, the reaction was determined to be complete based on the time taken for the peak at 400 nm of 4-NP to completely disappear, while formation of a new peak at 300 nm, corresponding to the formation of 4-AP, was detected and then enhanced. In addition, a color change of the 4-AP solution from yellowish to colorless was also observed at the end of the catalytic reaction. After the calcination of the thin film at 250 °C, the catalytic reduction was completely observed within 160 minutes as shown in Figure 9a. The rate constant (k) for this reaction was calculated to be 1.77 × 10 −2 min −1 based on the slope in the graph of ln A/ A 0 versus time in Figure 9b. Interestingly, the thin film [AuNPs] red /silica hex at 210 °C showed an improved catalytic activity, in which the reaction was completed faster -within 140 minutes as shown in Figure 9c with k = 1.92 × 10 −2 min −1 (Figure 9d). Since AuNPs were the active site in this reaction, the increase in the catalytic activity of the film composite [AuNPs] red /silica hex at 210 °C is strongly suggested by the small particle size of the as-synthesized AuNPs in high-quality mesostructured silica. Since the NaBH 4 concentration largely exceeded that of 4-NP (and remains constant throughout the experiment), both films were found to follow pseudo-first order kinetics [41]. This reduction followed the Langmuir-Hinshelwood model, where both reactants (4-NP and BH 4 − ) were adsorbed on the AuNP surface at a fast rate. In the next step, electron transfer from the hydride ion to the AuNPs occurred to give the hydrogen atom that later will react with 4-NP. Finally, 4-AP was formed and dissociated from the AuNP surface [42]. The presence of an isobestic point in Figure 9a,c further proved that only one product, 4-AP, was formed [43,44]. Compared to other reports, the AuNP-film catalyst in this work showed higher catalytic activity for the reduction of 4-NP to 4-AP which was 10 times (0.199 × 10 −2 min −1 ) and 4 times (0.45 × 10 −2 min −1 ) higher than for AuNPs prepared from Gnida glauca leaf and stem extracts [45]. Another report by Shende et al. [46,47] demonstrated that AuNPs prepared from Litchi Chinensis peel extract and Platanus Orientalis leaf extract showed 1000 times lower catalytic activity with rate constants of 0.00136 × 10 −2 and 0.00191 × 10 −2 min −1 . Hence, the AuNP-film catalyst in the nanocomposite of this work shows the importance of mesoporous silica as a framework for growing AuNPs in the silicate nanochannels with higher ordered nanostructure. In particular, the amount of AuNPs in our AuNP-film catalyst was only 0.04 mg, indicating that it is a good catalyst even with such a small amount and with a simple method for the catalytic reduction of 4-NP to 4-AP. The turnover frequency (TOF) value of [AuNPs] red /silica hex at 210 °C as a thin-film catalyst was calculated to be 0.02 min −1 , which is higher than that of [AuNPs] cal /silica hex at 250 °C with a value of 0.01 min −1 . Such catalytic activity of the AuNP-film catalyst prepared by thermal hydrogen reduction at 210 °C is due to its higher TOF value [48]. Moreover, in the same catalytic reduction of 4-NP, this TOF value is comparable to Au-Fe 3 O 4 as a bifunctional catalyst [49]. The catalyst is active to reduce all 4-NP to 4-AP, giving a linear decomposition rate within the reaction time. Unfortunately, we found that the AuNP-film catalyst was pulled out of the glass substrate. Therefore, reusability tests were very difficult [50,51] since we only used a very small amount of the catalyst. We are now in the process of developing a new method to construct a strong coating of the AuNPs on the substrate.

Conclusion
By using calcination and thermal hydrogen reduction in the range of 190 to 450 °C, we have demonstrated that mesoporous silica/gold nanoparticle thin film composites with a hexagonal structure could be successfully fabricated from as-synthesized mesostructured template materials. The best quality of the silica/gold film nanocomposites was found by thermal hydrogen reduction at 210 °C as determined from its intense d 100 peak and clear TEM image with a hexagonal alignment of the nanopores. Both heat treatment methods could be successfully used to produce gold nanoparticles in the silica film nanocom- was firstly prepared as a pale-yellow sticky solid in 69% yield using a synthetic scheme as shown in Figure 1. The fabrication of mesostructured silica/gold thin film nanocomposites ([Au 3 Pz 3 ] C10TEG /silica hex ) was carried out using a templated sol-gel synthesis according to our previous synthetic protocol [25] as shown in Figure 1 with mole ratios of [Au 3 Pz 3 ] C10TEG /TBOS/EtOH/HCl/H 2 O 1:60:504:1.2:266. Subsequently, a medium comprised of dry ethanol (61.6 mg, 1.3 mmol), deionized water (11.9 mg, 0.7 mmol) and hydrochloric acid (0.3 mg, 2.9 × 10 −3 mmol) was prepared before being added to [Au 3 Pz 3 ] C10TEG (10.0 mg, 2.5 × 10 −3 mmol). The mixture was left to dissolve for 20 minutes before tetrabutoxysilane (TBOS, 48 mg, 1.49 × 10 −1 mmol) was added into the solution. The sol-gel solution was covered with aluminum foil and aged for 12 hours at room temperature. For fabrication as thin films, 70 µL of the final sol-gel solution was spin-coated on a glass substrate to produce a colorless transparent thin film (around 0.5 mg of composite was successfully coated). It was followed by airdrying for a day at ambient temperature.

Synthesis of [AuNPs] cal /silica hex and [AuNPs] red /silica hex
Thin film [Au 3 Pz 3 ] C10TEG /silica hex was placed in ceramic crucibles and calcined using a muffle furnace from 190 to 450 °C for 3 hours at a heating rate of 1 °C min −1 . Another heat treatment, thermal hydrogen reduction, was performed using a tube furnace by placing the thin film into the center of the tube with the same temperature treatment for 2 hours with flow rate of hydrogen gas in 25 mL min −1 .

Catalytic reduction of 4-NP
The catalytic reduction of 4-NP to 4-AP was studied using the resulting thin films [AuNPs] cal /silica hex and [AuNPs] red / silica hex . Typically, the catalytic study was conducted by firstly preparing 4-nitrophenolate ions as a yellow solution from the mixture of the 4-NP solution (5.0 mL, 0.3 µmol) and NaBH 4 powder (23.0 mg, 600.0 µmol) by stirring for 30 seconds. The thin film catalysts were dipped into 3 mL of 4-nitrophenolate ion solution containing a small magnetic stirrer at 1200 rpm until the absorption peak of 4-NP at 400 nm completely disappeared.

Supporting Information
Supporting Information File 1 TEM 3D tomography video of the AuNP film nanocomposites.