In Tandem Control of La-Doping and CuO-Heterojunction on SrTiO3 Perovskite by Double-Nozzle Flame Spray Pyrolysis: Selective H2 vs. CH4 Photocatalytic Production from H2O/CH3OH

ABO3 perovskites offer versatile photoactive nano-templates that can be optimized towards specific technologies, either by means of doping or via heterojunction engineering. SrTiO3 is a well-studied perovskite photocatalyst, with a highly reducing conduction-band edge. Herein we present a Double-Nozzle Flame Spray Pyrolysis (DN-FSP) technology for the synthesis of high crystallinity SrTiO3 nanoparticles with controlled La-doping in tandem with SrTiO3/CuO-heterojunction formation. So-produced La:SrTiO3/CuO nanocatalysts were optimized for photocatalysis of H2O/CH3OH mixtures by varying the La-doping level in the range from 0.25 to 0.9%. We find that, in absence of CuO, the 0.9La:SrTiO3 material achieved maximal efficient photocatalytic H2 production, i.e., 12 mmol g−1 h−1. Introduction of CuO on La:SrTiO3 enhanced selective production of methane CH4. The optimized 0.25La:SrTiO3/0.5%CuO catalyst achieved photocatalytic CH4 production of 1.5 mmol g−1 h−1. Based on XRD, XRF, XPS, BET, and UV-Vis/DRS data, we discuss the photophysical basis of these trends and attribute them to the effect of La atoms in the SrTiO3 lattice regarding the H2-production, plus the effect of interfacial CuO on the promotion of CH4 production. Technology-wise this work is among the first to exemplify the potential of DN-FSP for scalable production of complex nanomaterials such as La:SrTiO3/CuO with a diligent control of doping and heterojunction in a single-step synthesis.


Introduction
Photocatalytic storage of light photons in the form of fuels such as H 2 or CH 4 is globally envisaged as an environmentally benign technology [1,2]. To this front, in the last decades, photocatalytic semiconductors receive great consideration [3].
Strontium Titanate (SrTiO 3 ) has a classical ABO 3 perovskite structure, with the ideal cubic lattice [4]. SrTiO 3 has received enormous attention as a photocatalyst, oxidative catalyst, or catalyst-support, due to beneficial characteristics such as low price [5], chemical stability, and excellent thermal stability, e.g., melting point as high as 2080 • C, with carbon and sulfur tolerance, adaptability and modifiable oxidative properties [6]. SrTiO 3 has a highly reducing conduction-band-edge energy position (E CB ) of −1.2 eV vs. NHE [7,8], which makes it a highly efficient photocatalyst for hydrogen production from H 2 O [9]. SrTiO 3 has the disadvantage of a broad 3.2 eV band gap, permitting only the absorption of UV photons, thus to remediate this drawback, a common technique is to apply dopants or heterojunction with other materials [10]. heterojunction of CuO on La:SrTiO3; (iii) To study the H2 vs. CH4 photocatalytic process in comparison to CuO. For this reason, we have performed photocatalytic studies in an H2O/CH3OH mixture. CH3OH is a well-known hole-scavenger [49], however, most publications do not examine the CH3OH involvement in the reaction path and the final products, e.g., eventually CH4.

Flame Spray Pyrolysis (FSP) Synthesis of SrTiO3 Nanoparticles
The precursor solution contains Strontium acetate (STREM), Titanium(VI) isopropoxide (97%, Aldrich) for the synthesis of perovskite SrTiO3. For the deposition of cocatalytic metals, i.e., La atoms, with Lanthanum Acetylacetonate (97%, STREM). The Sr and Ti precursors were dispersed in a mixture of Acetic acid and Xylene (1:1 volume ratio), while for the La precursor a mixture of toluene and 2-EHA (1:1 volume ratio) that consisted of approximately 10% of the total volume of the final precursor solution. We underline that it is the control of FSP parameters that allows Sr-and Ti-to be engaged exclusively in the formation of the SrTiO3 crystals, while La-is introduced as a latticedopant.
Double-Nozzle FSP: In the DN-FSP as shown in Figure 1, two FSP-nozzles operate in tandem that are asymmetrically positioned, so that the system generates two different kinds of nanomaterials by controlling the properties of each nozzle independently, where the left FSP-nozzle contained the Sr/Ti/La precursors, while the right FSP-nozzle contained Cu(NO3)2·3H2O (Supelco) in Acetonitrile:Ethyleglygol (1:1 volume ratio). Screening experiments were conducted to find the preferent geometric parameters: α1 nozzle angle at 20 o , and α2 = 20 o . The internozzle distance was placed at x = 8 cm and the vertical intersection distance of the two flames, above the nozzle, was b = 10, see Figure 1C. The FSP parameters for the synthesis of the nanoparticles consisted of an oxygen dispersion flow rate of D = 5 L min −1 (Linde 99.999%) and a precursor flow rate of P = 5 mL min −1 . These D and P rates were also used for both nozzles in the DN-FSP setup. The pilot flame was ignited by premixed O2 and CH4 (4 L min −1 , 2 L min −1 ). In the Single-Nozzle as well as in the Double-Nozzle FSP, with the assistance of a vacuum pump (BUSCH), the The FSP parameters for the synthesis of the nanoparticles consisted of an oxygen dispersion flow rate of D = 5 L min −1 (Linde 99.999%) and a precursor flow rate of P = 5 mL min −1 . These D and P rates were also used for both nozzles in the DN-FSP setup. The pilot flame was ignited by premixed O 2 and CH 4 (4 L min −1 , 2 L min −1 ). In the Single-Nozzle as well as in the Double-Nozzle FSP, with the assistance of a vacuum pump (BUSCH), the produced particles were deposited on a glass microfiber filter with a binder (Albet Labscience GF_6_257) and collected by scrubbing the nanoparticles from the filter. The nanomaterials were collected in glass vials under an inert Argon atmosphere, until use.
For convenience, herein the produced materials, listed in Table 1, are codenamed as follows: X_La:STO/Y_Cu where STO = SrTiO 3 , X the nominal % La-content per weight of SrTiO 3 , Y = the nominal % Cu-content per weight of SrTiO 3 . In this way, we have prepared 0.9La:STO and 0.35La:STO, respectively, listed in Table 1. For the DN-FSP experiments, we have produced STO/2Cu, STO/1.2Cu, and STO/0.5Cu, respectively. Finally the 0.25La:STO/0.5Cu produced by DN-FSP is codenamed as La:STO/Cu.

Photocatalytic Evaluation
Photocatalytic experiments were performed in a double-walled photochemical reactor (Toption instrument co. Ltd.) with a total reaction volume of 340 mL, at a temperature of 25 ± 2 • C, controlled by a recirculation chiller cooling system. The UV source was a 250 W Mercury lamp, positioned at the geometrical center of the photoreactor inside the quartz-immersion well. The irradiation power at the experimental mean distance of 3 cm was 0.5 W cm −2 as measured with a power meter (Newport model, 1918-C).
A Gas-Chromatography System combined with a Thermal-Conductivity-Detector (TCD-Shimadzu GC-2014, Carboxen 1000 column, Ar carrier gas) was used to identify and quantify the produced H 2 and CH 4 gases. In each experiment, 50 mg of the catalyst was suspended in 200 mL Milli-Q water and 50 mL methanol (20% per volume) as a hole-scavenger. Photo-deposition of Pt-cocatalyst was implemented to increase the photocatalytic production, using hydrogen hexachloroplatinate (IV) hydrate, (H 2 Pt 4 Cl 6 ·H 2 O, 99.9%, Alfa Aesar). The error bars of approximately 8%, for the photocatalytic products H 2 and CH 4 , reflect the detection uncertainty and statistical deviation after three catalytic runs.

Characterization of Materials
XRD measurements were carried out to identify the crystal phase identification and structural properties of the materials, using a Bruker D8-Advance diffractometer with a Cu source (Kα, λ = 1,5418 Å), with operation parameters of 40 KV generator voltage and 40 mA current. The particle crystal size (d XRD ) as obtained from the XRD data was calculated with the Scherrer Equation (1) where K = 0.9, λ = 1,5418 Å, and FWHM is the full-width at half-maximum of the XRD peaks [50]. Transmission Electron Microscopy analysis was used to map the particle morphology. The amount of La-atoms and Cu-atoms of the nanomaterials was calculated by a home-built Energy-Dispersive X-Ray Fluorescence (EDXRF), and an annular 241 Am radioisotopic source was used for sample excitation. The source is fixed coaxially above a CANBERRA SL80175 Si(Li) detector (5 mm crystal thickness, 80 mm 2 area), with a 25 µm thick Be window and an energy resolution of 171 eV for the 5.9 keV Mn Kα line. Spectral analysis was carried out using the WinQxas software package (International Atomic Energy Agency, 1997-2002). Quantitative analysis was based on in-house standard samples and the construction of calibration curves.
The morphology and phase composition of the La:STO/Cu particle was studied using an FEI Talos F200i field-emission (scanning) transmission electron microscope (Thermo Fisher Scientific Inc., Waltham, MA, USA) operating at 200 kV, equipped with a windowless energy-dispersive spectroscopy microanalyzer (6T/100 Bruker, Hamburg, Germany).
The Specific Surface Area (SSA) and the pore size of the nanomaterials were measured by a Quantachrome NOVAtouch_LX2 to record the N 2 adsorption-desorption isotherms at 77 K. The SSA was calculated using the absorption data points in the range of 0.1−0.3 relative pressure P/Po. While the pore radius analysis was obtained by the BJH method [51] in the range of 0.35-0.99 P/Po. Diffuse-Reflectance UV-Vis absorption spectra were recorded with a Perkin Elmer Lambda-35 spectrometer with BaSO 4 powder used as a background standard, operating at room temperature for the wavelength range of 200-800 nm, while the band gap energy (E g ) was calculated using the Kubelka-Munk method [52].
The oxidation state of the Sr, Ti, O, and Cu atoms was monitored by X-ray photoelectron spectroscopy (XPS), using a SPECS spectrometer equipped with a twin Al-Mg anode X-ray source and a multi-channel hemispherical sector electron analyzer (HSA-Phoibos 100), a monochromatized Mg Kα line at 1253.6eV, analyzer pass-energy at 15 eV, and the base pressure at 2-5 × 10 −9 mbar. The binding energies were determined vs. the energy of C1s carbon peak at 284.5 eV. The peak deconvolution was performed employing mixed Gaussian-Lorentzian functions, using WinSpec software, developed at the Laboratoire Interdisciplinaire de Spectroscopie Electronique, University of Namur, Belgium.

SrTiO 3 -Based Perovskite Nanoparticle Synthesis by FSP
Highly crystalline SrTiO 3 has been successfully produced by Single-Nozzle FSP, see XRD data in Figure  0.1−0.3 relative pressure P/Po. While the pore radius analysis was obtained by the BJH method [51] in the range of 0.35-0.99 P/Po. Diffuse-Reflectance UV-Vis absorption spectra were recorded with a Perkin Elmer Lambda-35 spectrometer with BaSO4 powder used as a background standard, operating at room temperature for the wavelength range of 200-800 nm, while the band gap energy (Eg) was calculated using the Kubelka-Munk method [52].
The oxidation state of the Sr, Ti, O, and Cu atoms was monitored by X-ray photoelectron spectroscopy (XPS), using a SPECS spectrometer equipped with a twin Al-Mg anode X-ray source and a multi-channel hemispherical sector electron analyzer (HSA-Phoibos 100), a monochromatized Mg Kα line at 1253.6eV, analyzer pass-energy at 15 eV, and the base pressure at 2-5 × 10 −9 mbar. The binding energies were determined vs. the energy of C1s carbon peak at 284.5 eV. The peak deconvolution was performed employing mixed Gaussian-Lorentzian functions, using WinSpec software, developed at the Laboratoire Interdisciplinaire de Spectroscopie Electronique, University of Namur, Belgium.

SrTiO3-Based Perovskite Nanoparticle Synthesis by FSP
Highly crystalline SrTiO3 has been successfully produced by Single-Nozzle FSP, see XRD data in Figure   The dXRD values calculated by the Scherrer method [50], listed in Table 1, indicate dXRD sizes in the range of 45-55nm. The TEM images of the pristine SrTiO3, show the formation of quasi-spherical particles with a distribution of particle sizes, see Figure 2B, as commonly observed in FSP-made particles [19]. Specifically, according to TEM, the FSP-made SrTiO3 includes a few large particles with diameters of 40-50nm and many smaller particles with diameters below 20nm. The ensuing particle-size distribution calculated from the TEM images, Figure 2C, shows a mean size of dTEM = 17 ± 0.2nm as obtained from a Gaussian fitting. Comparison of dXRD and dTEM exemplifies the well-known effect of large The d XRD values calculated by the Scherrer method [50], listed in Table 1, indicate d XRD sizes in the range of 45-55nm. The TEM images of the pristine SrTiO 3 , show the formation of quasi-spherical particles with a distribution of particle sizes, see Figure 2B, as commonly observed in FSP-made particles [19]. Specifically, according to TEM, the FSPmade SrTiO 3 includes a few large particles with diameters of 40-50nm and many smaller particles with diameters below 20nm. The ensuing particle-size distribution calculated from the TEM images, Figure 2C, shows a mean size of d TEM = 17 ± 0.2nm as obtained from a Gaussian fitting. Comparison of d XRD and d TEM exemplifies the well-known effect of large particles to predominate the diffraction peaks in XRD, thus d XRD overestimates the average particle size. In Figure 3A, HRTEM images for La:STO/Cu are shown. Distinct Miller planes of CuO (110) are resolved with d = 2.75 Å [53], showing that CuO particles are deposited on the surface of the SrTiO 3 particle. These CuO particles are small, <2 nm, therefore they are not detected in the XRD. Figure 3C-F present the scanning-TEM images for the Ti, Sr, La, and Cu atoms mapping for material La:STO/Cu. Importantly, Figure 3E demonstrates that the La atoms are evenly dispersed throughout the whole volume of the SrTiO 3 particle. This verifies the atomic distribution of La, i.e., without La-clusters. The STEM for Cu atoms, Figure 3D, verifies the formation of dense/particle structures on SrTiO 3 . Caution is drawn to the fact that the faint blue hue in Figure 3D is the enhanced emissions from Cu-grid atoms employed for the TEM measurements that are in proximity to the Sr-atoms. These are secondary Cu-electrons enhanced via Strontium excitations. Thus, in Figure 3F The N2-adsorption isotherms for selected nanomaterials are presented in Figure 3A, while the data for nanomaterials with different CuO percentages are shown in Figure S1 in the Supporting Information. All nanomaterials have the characteristic of a type-IV isotherm. The SSA values and pore-volume analysis, see Figure 4B, reveal some interesting La-doped SrTiO 3 produced by Single-Nozzle FSP retain their high crystallinity and purity, i.e., no secondary phases are formed, such as La 2 O 3 . La-doping is evidenced by a pink-hue color developed in La:SrTiO 3 vs. the white SrTiO 3 . Upon increasing La-doping, the SrTiO 3 -particle size tends to increase, i.e., from 45 nm d XRD to 55 nm d XRD . Strikingly, the Specific Surface Area increases also, see Table 1. This counterintuitive observation is further analyzed in the BET-data analysis hereafter.
Double-Nozzle FSP used to form the SrTiO 3 /CuO heterojunctions, retains the high crystallinity of SrTiO 3 , see XRD data in Figure 2A, with a tendency towards larger particle sizes at increased Cu-content. In agreement with previous reports [28], this can be attributed to the increased contribution to the increased enthalpy that is added due to the second flame, thus increasing the overall synthesis temperature, and extending the temperature at the stage of agglomeration.
The N 2 -adsorption isotherms for selected nanomaterials are presented in Figure 3A, while the data for nanomaterials with different CuO percentages are shown in Figure S1 in the Supporting Information. All nanomaterials have the characteristic of a type-IV isotherm.
The SSA values and pore-volume analysis, see Figure 4B, reveal some interesting trends: in the Single-Nozzle FSP, the La-dopped particles show an increase in their SSA values, see Table 1. However, taking into account the XRD data, this increased SSA is not concurring with the particle size. More insightful information is obtained by examining the pore size and pore volume trends, see Figure 4B, which reveals an increase in pore volume upon La-doping. Typically, in literature, SrTiO 3 particles are reported to possess a total pore volume in the range of 15 cm 3 g −1 [18,54], which is in the same range observed for our pristine SrTiO 3 nanomaterial, Figure S2 in the Supporting Information. Upon La-doping, a sharp increase is observed for the SSA, but more importantly a 3-fold increase in the pore volume to 39 cm 3 g −1 . This trend can be attributed to a geometrical effect as depicted in the scheme in Figure 4. This is a result of the FSP process, where the La-doping decreases the packing/aggregation of the SrTiO 3 , even though some SrTiO 3 might grow bigger. It is worth mentioning the relatively high surface area and pore volume in our nanomaterials, i.e., compared vs. previous synthesis methods for SrTiO 3 that possessed low SSA, i.e., due to the application of high calcination temperatures [5,55].

Diffuse Reflectance UV-Vis Spectroscopy
SrTiO3, as a semiconductor, has been shown to possess an indirect bandgap of 3.25 eV and a direct bandgap of 3.75 eV [56]. It has been shown that La or Nb-dopings, or

Diffuse Reflectance UV-Vis Spectroscopy
SrTiO 3, as a semiconductor, has been shown to possess an indirect bandgap of 3.25 eV and a direct bandgap of 3.75 eV [56]. It has been shown that La or Nb-dopings, or oxygen vacancies can produce an n-type alteration in the Density of States (DOS) [56], i.e., extra DOS are formed within the band gap right below the Conduction Band bottom. In our data in Figure 5B, the indirect band gap has been calculated using the Kubelka-Munk method [52]. Notice that its absorption starts at 390 nm [57], resulting in all the nanomaterials having Eg values close to 3.2 eV, listed in Table 1. Lanthanum/Copper incorporation in the SrTiO 3 materials fundamentally changed the color of the nanoparticles, with a purple hue observed after the lanthanum doping, while a drastic brown tint was observed for the 2% CuO heterostructure [28], which is clearly visible from extra absorption in the 400-580 nm range, Figure 5A.

X-Ray Photoelectron Spectroscopy
XPS data for Sr-or O-atoms are presented in Figures 6A and 6C, respectively. Figure  6B presents Cu-XPS data, there is an emphasis on copper in order to confirm the oxidation state of the Cu-NPs. The material STO/2Cu, i.e., with the higher Cu-loading is exemplified, since it possessed strong Cu-XPS signals, to ascertain the Cu-oxidation state with precision. La-could not be detected by XPS in any of our materials. Overall, the present DRS-UV-Vis analysis verifies that the electronic structure of the SrTiO 3 semiconductors produced by SN-FSP and DN-FSP, as well as their trends upon La-doping and Cu-heterojunction, are in accordance with literature data.

X-ray Photoelectron Spectroscopy
XPS data for Sr-or O-atoms are presented in Figure 6A,C, respectively. Figure 6B presents Cu-XPS data, there is an emphasis on copper in order to confirm the oxidation state of the Cu-NPs. The material STO/2Cu, i.e., with the higher Cu-loading is exemplified, since it possessed strong Cu-XPS signals, to ascertain the Cu-oxidation state with precision. La-could not be detected by XPS in any of our materials.
In Figure 6B, the binding energies located broadly at 932.8eV and 952.7eV correspond to the Cu 2p 3/2 and Cu 2p 1/2 , respectively [58,59], while the presence of a strong satellite signal at 941eV and the broader peaks of Cu 2p 3/2 and Cu 2p 1/2 indicate that the Cu atoms are in the Cu(II) oxidation state [58,59]. Thus, XPS results confirm that the heterostructure is indeed SrTiO 3 /CuO. This is in agreement with the FSP settings, i.e., the P/D = 5/5 produces a typical oxidizing environment [19] that in our Cu-precursor promotes the formation of CuO. The binding energies at 132.2 eV and 134 eV were identified as the Sr 3d 5/2 and Sr 3d 3/2 , corresponding to the Sr 2+ state of the SrTiO 3 [60,61], Figure 6A. For the oxygen species, Figure 6C, three peaks were observed, with the binding energies at 528.9 eV that can be attributed to lattice oxygen species of the SrTiO 3 crystal structure [61,62]. The 531.1 eV is attributed to chemisorbed oxygen, and lastly, the peak at 532.4 eV is attributed to adsorbed oxygen on the particle surface or hydroxyl groups [61,62]. The oxygen species from the partially substituted materials, i.e., 0.25%La/0.5%CuO have distinctly different intensity ratios of the (lattice) vs. (chemisorbed) oxygen species. This can be possibly attributed to oxygen vacancies that are created from the bending of the lattice, impacting the adsorption and efficiency of the oxygen species at the catalyst surface [61,62]. For the Ti 2p, all materials have peaks located at 457.7 eV and 463.4 eV, which are the binding states of Ti 2p 3/2 and Ti 2p 1/2 , respectively, that correspond to typical Ti 4+ states, Figure S3 in the Supporting Information.

X-Ray Photoelectron Spectroscopy
XPS data for Sr-or O-atoms are presented in Figures 6A and 6C, respectively. Figure  6B presents Cu-XPS data, there is an emphasis on copper in order to confirm the oxidation state of the Cu-NPs. The material STO/2Cu, i.e., with the higher Cu-loading is exemplified, since it possessed strong Cu-XPS signals, to ascertain the Cu-oxidation state with precision. La-could not be detected by XPS in any of our materials.  Overall, the present XPS data show that the FSP-made nano SrTiO 3 consists of typical Ti 4+ states with some O vacancies in all materials. In STO/Cu, the CuO particle was confirmed by XPS in accordance with the oxidizing FSP process used herein.

Photocatalytic Evaluation
Photocatalytic results for photocatalytic hydrogen production of the materials were evaluated using a mixture of H 2 O/CH 3 OH as a catalytic substrate. The only products were H 2 or CH 4 , in all cases. The gas-production data are shown in Figure 7A,B for H 2 or CH 4 , respectively. The corresponding rates are presented in Figure 7C,D, respectively. The highest H 2-yield was 11980 umol g −1 h −1 , achieved by the 0.9%La:SrTiO 3 , which is~500% higher than for pristine SrTiO 3 with H 2-yield 2760 umol g −1 h −1 . A clear beneficial trend is observed, i.e., higher La-doping promotes H 2 production. This trend is in agreement with the literature [31,32]. Herein, however, we focus attention on the relative rates for CH 4 also. From Figure 7 we observe that in the absence of CuO, the production of CH 4 from the photoreduction of methanol was minimal. The SrTiO 3 /CuO heterostructure portrays very different results, with the 2%CuO having almost the same H 2 production as the pristine SrTiO 3 , however, there is a sharp increase in the CH 4 (76 to 136 umol g −1 h −1 ). This selectivity towards CH 4 is apparent for all CuO heterojunctions. Decreasing CuO content resulted in higher H 2 and CH 4 production, 2%CuO increasing the production to 704 umol g −1 h −1 CH 4 . Notice that with increasing Cu-loading the SrTiO 3 particle surface is fully covered with CuO, which in turn might act as an inhibitor of light-absorbance by the SrTiO 3 . This 'darkening' effect plays a key role in diminishing the photocatalytic activity, i.e., both H 2 and CH 4 . Most interestingly, the material 0.25%La/0.5%Cu showed an enhanced H 2 /CH 4 selectivity, with the CH 4 reaching 1469 umol g −1 h −1 and H 2 to 5907 umol g −1 h −1 .

Discussion
The present data show that FSP offers a versatile technology for the production of nano SrTiO3, La:SrTiO3, SrTiO3/CuO, and La:SrTiO3/CuO. These materials show significant photocatalytic activity that can be tailored towards either pure H2 production or CH4/H2 production from an H2O/CH3OH mixture. Table 2 allows a comparison of H2 photogeneration by the present FSP particles vs. literature, SrTiO3-based nanocatalysts. From Table  2, and the literature generally, the use of methanol or ethanol as a sacrificial agent to enhance CH4 production has not been reported so far. Most research publications utilize Xenon radiation, with the highest yields shown in Table 2, although there are several with UV irradiation.  Overall, the present data in Figure 7 show that: (i) in all instances, La-doping greatly increases the photocatalytic activity, (ii) CuO on SrTiO 3 drives the products toward the reduction of CH 3 OH to CH 4 . Importantly, after the photocatalytic application, the particles fully retained their structure as evidenced by XRD data, see Figure S4 in the Supporting Information.

Discussion
The present data show that FSP offers a versatile technology for the production of nano SrTiO 3 , La:SrTiO 3, SrTiO 3 /CuO, and La:SrTiO 3 /CuO. These materials show significant photocatalytic activity that can be tailored towards either pure H 2 production or CH 4 /H 2 production from an H 2 O/CH 3 OH mixture. Table 2 allows a comparison of H 2 photogeneration by the present FSP particles vs. literature, SrTiO 3 -based nanocatalysts. From Table 2, and the literature generally, the use of methanol or ethanol as a sacrificial agent to enhance CH 4 production has not been reported so far. Most research publications utilize Xenon radiation, with the highest yields shown in Table 2, although there are several with UV irradiation.
In the case of SrTiO 3 , full exploitation of its band gap > 3.2 eV dictates the use of UV light, in order to maximize the H 2 yield [63][64][65][66][67], in comparison with visible-light H 2 production [68,69]. In this context, Table 2, shows that the FSP-made 0.9%La:SrTiO 3 nanocatalyst has the highest yield vs. literature data on pertinent La:SrTiO 3 materials, although the irradiation method that was used herein (UV-Lamp) definitely boosts the photocatalytic efficiency. So far, stabilizing CuO or Cu 2 O NPs on metal oxides with lower conduction-band positions is a common strategy to enhance photocatalytic H 2 production compared with pristine CuO and Cu 2 O [70,71]. In this context, it was reported that coupling Cu 2 O with TiO 2 or ZnO leads to higher H 2 photoproduction [70,72]. Although there are no previous data on CH 4 [73]. Moreover, Chen et al. had shown that the co-existence of Cu 1+ /Cu 0 species on TiO 2 enhances the photocatalytic efficiency, by increasing the lifetime of electrons leading to an enhancement of CO 2 hydrogenation. In the same study, the reduction of CuO to Cu 0 was found to be more efficient for the photocatalytic production of CH 4 . A Cu-Cu 2 O/TiO 2 hybrid has shown an excellent selectivity for CH 4 which is attributed to the suppression of CO formation from Cu 0 species while Cu 1+ species act as the active sites for CH 4 production [74]. All these data pose the possibility that the promotion of CH 4 production by SrTiO 3 /CuO or La:SrTiO 3 /CuO heterojunctions might involve the formation of Cu 1+ or Cu 0 species at the SrTiO 3 /CuO interface.

Conclusions
In the present work, we introduce Double-Nozzle Flame Spray Pyrolysis technology as a synthesis method for production of nano SrTiO 3 perovskites, with in tandem control of La-Doping of SrTiO 3 crystal plus a CuO/SrTiO 3 -heterojunction. The proposed FSP technology allows controlled production of La:SrTiO 3 , SrTiO 3 /CuO, or La:SrTiO 3 /CuO with fundamental advantages of the one-step production, and the potential for scalable production of complex nanomaterials.
The resulting nanomaterials showed distinct structural-electronic properties, with the La-doping inducing a characteristic increase in SSA via the formation of larger pore voids. Diligent control of the La-doping and SrTiO 3 /CuO heterostructure allowed a selective control of photocatalytic production of H 2 or CH 4 from an H 2 O/CH 3 OH mixture. Ladoping in all cases increased the photocatalytic activity of SrTiO 3 nanocatalysts, with the 0.9La:STO showing a benchmark H 2 -production rate of 12 mmol g −1 h −1 . The incorporation of CuO drastically shifted the selectivity from H 2 toward CH 4 . The highest production was achieved with in tandem incorporation of La and CuO, i.e., La:SrTiO 3 /CuO catalyst, with a CH 4 production rate of 1.5 mmol g −1 h −1 .
Thus, the present work exemplifies FSP as a potent technology for the production of complex nanocatalysts, at the same time bringing new insights into photocatalysis for H 2 /CH 4 production.