For a continuous and scalable synthesis of noble-metal-based nanocomposites, a novel photocatalytic reactor is developed and its structure, operation principles, and product quality optimization strategies are described.
In this work, a novel photocatalytic reactor for the pulsed and controlled excitation of the photocatalyst and the precise deposition of metallic nanoparticles is developed. Guidelines for the replication of the reactor and its operation are provided in detail. Three different composite systems (Pt/graphene, Pt/TiO2, and Au/TiO2) with monodisperse and uniformly distributed particles are produced by this reactor, and the photodeposition mechanism, as well as the synthesis optimization strategy, are discussed. The synthesis methods and their technical aspects are described comprehensively. The role of the ultraviolet (UV) dose (in each excitation pulse) on the photodeposition process is investigated and the optimum values for each composite system are provided.
Metallic nanoparticles, especially noble metals (e.g., Pt, Au, Pd) have vast applications in catalysis1. In general, decreasing the size of the nanoparticles (NPs) increases their catalytic activity while maintaining the cost (weight) constant, but it also makes their application more difficult. NPs (usually smaller than 10 nm) have great tendencies to aggregation, which degrades their catalytic activity; however, immobilization on suitable substrates can mostly resolve this problem. Furthermore, depending on the application type (e.g., electrocatalysis), it is sometimes necessary to immobilize the NPs on conductive substrates2,3. NPs can also be hybridized with semiconductors to form a Schottky barrier and avoid (delay) the electron-hole recombination (acting as electron traps)4,5. Hence, in most of the applications, the noble metal NPs (NNPs) are deposited either on a conductive (e.g., graphene) or a semiconductive (e.g., TiO2) substrate. In both cases, metal cations are usually reduced in the presence of the substrate, and the reduction technique differs from one method to another.
For the deposition of NNPs via a reduction of their cations, electrons (with proper electrical potential) should be provided. That can be done in two ways: by oxidation of other chemical species (a reducing agent)6,7 or from an external power source8. In any case, for the homogeneous deposition of monodispersed NPs, it is necessary to impose a strict control on the generation and transfer of the (reducing) electrons. This is very difficult when a reducing agent is used since there is virtually no control over the reduction process once the reactants (cations and reducing agent) are mixed. Furthermore, NPs can form anywhere and not necessarily on the target substrate. When using an external power source, the control over the number of the provided electrons is much better, but NPs can only be deposited on an electrode surface.
Photocatalytic deposition (PD) is an alternative approach, which offers more control over the number of the (photo)generated electrons since it is directly related to the dose of the illuminated photons (with a proper wavelength). In this method, the substrate material has a dual role; it provides the reducing electrons9 and stabilizes the formed NPs10. Moreover, NPs form only on the substrate since the electrons are generated by the substrate. A proper electrical connection between composite components (made by the photocatalytic reduction method) is also guaranteed11. Nevertheless, in conventional photocatalytic deposition methods in which the whole batch of reactants (photocatalyst and metal cation) is illuminated simultaneously, there is no control over the nucleation of the NNPs. Indeed, once a few particles (nuclei) are formed, they act as preferred transfer sites for the photogenerated electrons5 and act as a preferred growth site. This superior electron transfer promotes the growth of the existing particles and disfavors the formation of new nuclei, which results in the formation of large NNPs. This problem can be addressed by the pulsed illumination of UV light in a special continuous-flow reactor (Figure 1) that has recently been developed by our group12. The unique feature of this reactor is that it allows researchers to control both NP-size-determining factors, namely, nucleation and growth. In this reactor, a very small portion of reactants is illuminated for a very short period of time, promoting the formation of nuclei (more nuclei are formed) and restricting the growth (smaller particles are attained). In this method, by controlling the illumination dose (i.e., by adjusting the exposure duration [changing the length of uncovered parts of the reaction tube; Figure 1C] or intensity of the incident light [number of the lamps]), a very precise control over the number of photogenerated electrons and, consequently, on the reduction process (NNP deposition) can be exerted.
Figure 1: The fabricated photocatalytic deposition reactor. (A) The reactor. (B) Inside the illumination chamber. (C) A quartz tube with 5 cm x 1 cm illumination exposure length. Please click here to view a larger version of this figure.
Despite the great potential of the PD method for the controlled deposition of NNPs, its application is limited to semiconducting materials. Fortunately, it is possible to open a wide band gap in graphene (one of the best-conducting substrates13) by its simple chemical functionalization. Afterward, these functional groups (FGs) can be mostly removed and the resulting graphene will still be conductive enough for most of the applications. Among numerous functionalized derivatives of graphene, graphene oxide (GO), which exhibits considerable semiconducting properties14, is the most promising candidate for this purpose. This is mainly due to the fact that GO's production has the highest production yield among the others. Nevertheless, since GO consists of different types of FGs, its chemical composition varies continuously under UV illumination. We have recently shown that by a selective removal of weakly bonded FGs (partial reduction; PRGO), the chemical structure and electronic properties of GO can be stabilized, which is an essential requirement for homogeneous depositions of the NNPs12. In this report, we describe the structure of the reactor and provide detailed information for its replication and operation. The deposition mechanism (working mechanism of the reactor) and possible optimization strategies are also discussed in great detail. To validate the applicability of the developed PD reactor for both types of common substrates (conductor and semiconductor) and different NNPs, the deposition of platinum on PRGO and TiO2, as well as of gold on TiO2, is demonstrated. It is noteworthy that by a proper selection of the metal, photocatalyst and precursor materials (e.g., salt, hole scavenger), and the dispersion media, several other metallic particles (such as Ag and Pd15) can also be deposited. In principle-since, in the photodeposition of NNPs, the cations of the metal are reduced by the photoexcited electrons-the energy level of the semiconductor's conduction band minimum (CBM) should match with (be more negative than) the reduction potential of the aimed cations. Due to the extensive technical production aspects, the synthesis of PRGO is also described in detail. For further information regarding the chemical structure and electronic properties of PRGO, please refer to previous work12.
The detailed structure of the reactor is schematically depicted in Figure 2. The reactor has two main components: a UV illumination and a reservoir compartment. The illumination section consists of a quartz tube, which is exactly fixed along the central axis of a cylindrical tube with a polished aluminum liner. The reservoir consists of a 1 L sealed-cap glass bottle with gas and liquid (reactants) inlets and outlets. Use a silicon septum with an open-top screw cap for inserting the tubes. To take samples during the reaction without letting oxygen enter the reactor, an outlet with a valve is also installed. It should be mentioned here that the samplings on specific time intervals are not a part of the nanocomposite production process, and sampling only needs to be done once to obtain the concentration-time curves for each set of synthesis parameters (the application of these curves will be discussed in the Discussion section). The reservoir is placed inside an ice-water bath while being vigorously mixed on a magnetic stirrer. A magnetic pump circulates the reactant from the reservoir to the reaction chamber (illumination section) and back to the reservoir. A magnetic one is used since high flow rates are necessary (the flow rate in this work = 16 L·min-1) and peristaltic pumps (or other similar pumps) can hardly provide those flows. When using a magnetic pump, care should be taken to completely fill the impeller casing (pump housing) with the reactant liquid and evacuate any trapped air (oxygen source). The trapped air can also decrease the pump's real flow rate.
For a pulsed excitation of the photocatalyst material, specific lengths of the quartz tube are covered by a thick aluminum foil, leaving equal lengths between them uncovered (Figure 2). The duration of the pulsed excitation can be adjusted by changing the length of the uncovered parts (exposure length). The optimum exposure length is determined by various parameters, such as the quantum yield of the photocatalyst and the intended NP loading (concentration of the precursors; see Discussion).
1. Fabrication and operation of the photocatalytic deposition reactor
CAUTION: When UV lamps are turned on, use UV-C protective glasses.
Figure 2: Schematic illustration of the photocatalytic deposition reactor. T1 is the N2 gas outlet that goes into the bubbler. T2 is the sampling tube and Ps is the point for attachment of the syringe (for sampling). V0 is the initial feeding valve and V1 and V2 are the sampling and tube evacuation valves (after sampling), respectively. Please click here to view a larger version of this figure.
2. Synthesis of partially reduced GO
CAUTION: All the following experiments should be carried out inside a fume hood.
3. Photocatalytic deposition of NNPs
4. Sample preparation for characterizations
XPS is one of the most powerful techniques for confirming the formation of metallic NPs and study their chemical states. For this purpose, both survey spectra and high-resolution spectra (of Pt4f and Au4f) were recorded, which confirms the complete reduction of the metallic cations and successful deposition of the NNPs (Figure 3). For the deconvolution of both Pt4f and Au4f, initially, a Shirley background subtraction was performed. Afterward, the core-level spectra were decomposed into their components with mixed Gaussian/Lorentzian (70% Gaussian and 30% Lorentzian) lines by a nonlinear least-squares curve-fitting procedure, using the XPSPEAK 4.1 software. The reduced chi-square value for all the fittings was kept below 0.01. The binding energy separation of the Pt4f7/2 and Pt4f5/2 peaks was set to 3.33 eV and the intensity ratio between the Pt4f7/2 and Pt4f5/2 peaks was set to 0.75. Those values for Au4f7/2 and Au4f5/2 were 3.71 eV and 0.78, respectively.
Figure 3: XPS survey spectra. Spectra for the (A) Pt/graphene composite, (B) Pt/TiO2 composite, and (c) Au/TiO2 composite. Deconvoluted spectra for (D) Pt4f in the Pt/graphene composite, (E) Pt4f in the Pt/TiO2 composite, and (F) Au4f in the Au/TiO2 composite. Please click here to view a larger version of this figure.
The distribution of the nanoparticles and their morphologies, which have significant effect on the performance of nanocomposite catalysts, have been studied by TEM. These two characteristics are affected by numerous synthesis parameters, but in this work, through the optimization of the illumination dose per exposure (IDE), we have been able to obtain a fairly monodisperse and uniformly distributed NNPs on both of the substrates (RGO and TiO2) (Figure 4).
Figure 4: TEM images. (A) Pt/graphene composite produced by a 1 cm x 50 cm exposure tube (this figure has been modified from Abdolhosseinzadeh et al.12. (B) Au/TiO2. (C) Pt/TiO2. (D) Effect of a high IDE in the formation of large particles and the depletion of its surroundings in Pt/graphene composite. The mean particle sizes for panels A, B, and C are 1.75, 3.8, and 3.77, respectively. Please click here to view a larger version of this figure.
In the photocatalytic deposition of NNPs, since photoexcited electrons are responsible for the reduction of metallic cations, the reaction progress (NNP loading) can be studied by monitoring the concentration changes of the metallic cations. ICP-OES is one of the most accurate techniques for determining the cations concentration. The direct relation of the PD with IDE is clearly shown in these experiments, which provide significant insights and information for understanding the working mechanism of the developed reactor (Figure 5).
Figure 5: Concentration changes of the metallic cations during the PD in the developed reactor with various IDEs. Changes in (A) Pt/graphene, (B) Au/TiO2, and (C) Pt/TiO2. Please click here to view a larger version of this figure.
Interestingly, by comparing the photodeposition rate (obtained from the ICP-OES results) for different synthesis conditions (Table 1), a direct relation between the number of photoexcited electrons and the amount of the deposited NNPs can be demonstrated (see Discussion). The working (deposition) mechanism of the developed reactor is also explained based on this data set.
System | Pt/graphene | Pt/graphene | Pt/graphene | Au/TiO2 | Au/TiO2 | Pt/TiO2 | Pt/TiO2 |
Condition | 5 x 0.4 cm | 5 x 2 cm | 1 x 50 cm | 2 lamps | 4 lamps | 2 lamps | 4 lamps |
Slope in the linear region | ~0 | -0.00932 | -0.04412 | -0.066 | -0.12179 | -0.05112 | -0.08332 |
Table 1: Slope values of linear fits to cation concentration changes obtained from ICP-OES analysis.
Nanoparticles are the most widely used form of noble-metal-based catalysts. In almost all cases, NNPs are deposited either on a conductive or a semiconductive support material. This hybridization is mostly done by the reduction of the cations of the noble metal in the presence of the intended substrate (material). Hence, a successful synthesis method for the production of NNP-based nanocomposite should meet at least two main requirements: 1) the reduction of the cations should be efficient and complete; 2) the deposition rate, location, and amount should be controllable. In this work, we have demonstrated that the developed reactor (and synthesis method) can successfully address both of the aforementioned requirements.
The XPS survey spectra (Figure 3) clearly demonstrated the successful deposition of Pt and Au on RGO and TiO2 (a conductive and a semiconductive material). Deconvolution of the high-resolution Pt4f and Au4f peaks revealed no nonmetallic components, which verifies that the photocatalytic reduction method with both PRGO and TiO2 can completely reduce the Pt4+ and Au3+ to Pt0 and Au0, respectively. Considering the deposition yield and required time for it (Figure 5), the output of this method is comparable with (or even better than) most of the conventional NNP deposition methods (i.e., polyol method18).
As mentioned earlier, since photoexcited electrons (produced by the substrate itself) reduce the metal cations in a PD reaction, NNPs only form on the substrate, which allows researchers to control the deposition location (not everywhere in the suspension). Furthermore, thanks to the unique pulsed excitation of the substrate (photocatalyst), both nucleation and growth can be controlled. This is mainly due to the fact that, by limiting the exposure time (and/or UV light intensity), a very small and controlled amount of photoexcited electrons are produced in a specific location on the substrate. These electrons have a short lifetime and, due to the poor conductivity of the photocatalyst (semiconductor), cannot move too far from the location that was generated there. Hence, in order to form a stable nucleus which should possess a minimum radius, a specific number of electrons should be generated in one excitation pulse (exposure). This implies that a minimum IDE (exposure length and/or UV light intensity) was required to initiate the PD in the reactor, which can explain the unsuccessful Pt deposition on PRGO with the 5 cm x 0.4 cm exposure tube (Figure 5A, negligible concentration changes). Similar behavior was observed when using very low IDEs for TiO2-based composites (corresponding data are not provided here). This is mainly due to the fact that the critical IDE value for the formation of stable nuclei is directly related to the quantum yield (since it is usually hard to measure the absolute quantum yield, apparent quantum yield can also be used19) of the photocatalyst material (0.36%-0.41% for PRGO and 0.97%-1.1% for TiO2).
When the number of photoexcited electrons (per exposure) are larger than the value needed for a stable nucleus formation, as shown in Figure 5 and Table 1, the photodeposition rate (monitored by ICP-OES) has a linear relation with the exposure time and UV light intensity. Even when comparing the Au/TiO2 and Pt/TiO2 systems, since Pt4+ needs one more electron than Au3+, the photodeposition rate is higher (1.46) for Au/TiO2 (the expected ratio is 4/3 = 1.33). It should be mentioned here that in some cases, the obtained PD rate ratios have slight differences with the expected values (especially for Pt), which is probably due to the fact that the proposed simple model for the nucleation and growth in the PD systems presented here is not completely valid in multi-electron reductions, and other parameters should also be taken into account. At the final stages of the PD in all the systems (Figure 5), an abrupt change in the deposition rate has happened, which implies that a significant change in the deposition process has occurred. This is due to the fact that in the proposed model, it was assumed (but not specifically stated) that for each photogenerated electron, a metallic cation will be available immediately to consume it, which is apparently not the case when concentrations of the metal cations fall below a specific value (the diffusion of the cations should also be taken into account).
Since this deviation from the linear behavior occurs in low concentrations, where the major part of the cations is deposited, and the deposition of the remaining cations will require much more time, it is reasonable to conduct the PD in the linear region and recover the remaining cations from the solution by the well-developed hydrometallurgical extraction methods20. In this case, the developed reactor can operate in a continuous manner and the desired product can be collected (taken out of the reactor) after a known period of circulation. It is also possible that, by increasing the reactor length, by using either multiple parallel tubes or spring-like tubes, the precursors can enter the reactor from one side and the products can exit from the other side.
Considering the aforementioned discussions, it is clear that by minimizing the IDE (but keeping it higher than the critical value needed for the formation of the nuclei), researchers can get small monodisperse particles with a uniform distribution, but the production time will also increase significantly. For instance, as also shown in Figure 6, the time required for obtaining the same amount of Pt loading in the Pt/graphene system when using the 5 cm x 2 cm and 1 cm x 50 cm tubes is approximately five times longer (in the linear region). On the other hand, by increasing the IDE (either the exposure length or the UV light intensity), growth will dominate the nucleation (similar to the conventional PD methods) and large particles will form. In very high IDEs, as described earlier, a formed particle may suck the electrons from its vicinity (since it has a better conductivity) and disfavor the formation of other nuclei around it. In this case, the particle grows enormously, and the particle size distribution of the final product will be very wide (Figure 4D). This phenomenon is more problematic in high quantum-yield photocatalysts (comparing TiO2 with PRGO), and proper IDE adjustment is more challenging. As a result, shorter exposure lengths and a lower illumination dose (number of the UV lamps) are used for TiO2-based composites and still, as can be seen in TEM images, the graphene-based composite has a better monodispersity than the two other TiO2-based composites. Hence, the IDE should be carefully optimized to get the highest quality product with the highest yield. The results and discussions presented here clearly demonstrate the potential and abilities of the developed reactor for a precisely controlled synthesis of NNP-based catalysts (on both types of substrates) on a large scale and in a continuous manner.
The authors have nothing to disclose.
The authors would like to thank Sabanci University and Swiss Federal Laboratories for Materials Science and Technology (Empa) for all the support provided.
Chloroplatinic acid solution | Sigma Aldrich | 262587-50ML | |
Hydrogen tetrachloroaurate(III) hydrate | Alfa Aesar | 12325.03 | |
TiO2 Nanopowder (TiO2, anatase, 99.9%, 100nm) | US research nanomaterials | US3411 | |
Graphite powder | Alfa Aesar | 10129 | |
Sulfuric acid | Sigma Aldrich | 1120802500 | |
Hydrogen peroxide | Sigma Aldrich | H1009-100ML | |
L-Ascorbic acid | Sigma Aldrich | A92902-500G | |
Hydrochloric acid | Sigma Aldrich | 320331-2.5L | |
Sodium hydroxide | Sigma Aldrich | S5881-1KG | |
Potassium permanganate | Merck | 1050821000 | |
Corning® Silicone Septa for GL45 Screw Cap | Sigma Aldrich (Corning) | CLS139545SS | |
Polyvinyl chloride pipe | Koctas | UV-Reactor casing | |
Fuded silica (Quartz) tube | Technical Glass Products | ||
UV−C lamps | Philips | TUV PL-L 55W/4P HF 1CT/25 |