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Three PtCu-g-C3N4 materials are prepared following the procedures used to synthesise Pt-g-C3N4 (Chapter 2 and 4): simple impregnation, reduction under hydrogen gas and chemical reduction with NaBH4. The so prepared materials are tested for photocatalytic hydrogen evolution and the performances are compared in Figure 5.4. The results are compared with those of 1 wt.% Pt-g-C3N4 and 0.5 wt.% Pt-g-C3N4. When the co-catalyst is composed of only platinum, the hydrogen evolution rate at the steady state is found to be 1723 μmol·h-1_·g-1 _{for the 1 wt.% and 1531 μmol·h}-1_·g-1 _{for the 0.5 wt.%. For the simply} impregnated PtCu-g-C3N4 the hydrogen evolution rate at the steady state is 1443 μmol·h-1·g-1, 16 % lower than that of the 1 wt.% Pt, but comparable to the H2

Figure 5.4 H2 evolution rates of PtCu-g-C3N4. Comparison of the hydrogen evolution rates at

the steady state for PtCu-g-C3N4 prepared by simple impregnation, reduction by H2 and NaBH4.

For comparison, values for 1 wt.% Pt and 0.5 wt.%-g-C3N4 are also included.

0 500 1000 1500 2000 4750 5000 517 117 1443 1053 685 1531 1723 Pt-H2 Pt Pt05 PtCu-H2 PtC Cu u Pt-NaBH 4 PtCu-Na BH4 H 2 e voluti on r ate /  mol ·h -1 ·g -1 4789

production of the 0.5 wt.% Pt loaded g-C3N4. The results suggest that if a PtCu alloy is formed no improvement in activity is observed with the substitution of Pt with Cu atoms. On the other hand, if no alloy is formed then the performance is only due to the presence of platinum nanoparticles on their own, considering the negligible activity of Cu NPs (Figure 5.4).

When the catalyst is reduced in hydrogen a detrimental effect is produced on the performance and a H2 evolution rate of 1053 μmol·h-1·g-1 is obtained which is 27 % lower than the not pre-treated material. This behaviour is opposite to what has been seen for the 1 wt.% Pt loaded g-C3N4 where the reduction in hydrogen brought to an enhancement in performance by a factor 3 (Chapter 4). The H2 evolution of PtCu-g-C3N4 chemically reduced with NaBH4 is found to be 517 μmol·h-1·g-1. Compared to the PtCu impregnated material, the drop (64 %) in hydrogen evolution rate is of a similar magnitude to the one observed for the equivalent Pt system (60 %). This suggests that the introduction of copper is not favourable to enhance the performance of the catalyst.

To verify the presence of a PtCu alloy on the surface of g-C3N4, XRD analysis is performed on the reduced samples, the patterns are compared in Figure 5.5 with that of Pt-g-C3N4 reduced in H2 under the same conditions (Figure 5.5a). The impregnated sample is not illustrated in Figure 5.5 since, as demonstrated by Figure 4.6 (Chapter 4), in the case of 1 wt.% Pt no well-defined peaks in the XRD pattern could be identified.

Figure 5.5 XRD of PtCu-g-C3N4. XRD patterns of a) Pt-g-C3N4 reduced with H2 and

PtCu-g-C3N4 reduced with b) H2 and c) NaBH4. Bars: XRD patterns of Pt (PDF card #:04-0802,

black), Cu (PDF card #: 04-0836, red) and PtCu (PDF card #: 48-1549, blue). ♦: g-C3N4.

40 50 60 70 80 90 100 c a Nor ma li se d inte nsit y / a rb. unit 2 / degree b      

When the reduction of PtCu-g-C3N4 is carried out under hydrogen atmosphere (Figure 5.5b) the peaks are well defined and easy to identify. Only the main peaks characteristic of Pt0 can be observed: the (111) reflection at 39.8 ̊ and the (200) at 46.2 ̊. Compared to those of Pt-g-C3N4 (Figure 5.5a) the peak at 39.8 ° is smaller and broader. This could be due to the lower content of platinum (only 50 % of the original). The absence of the peaks characteristic of PtCu alloy can either indicate no alloyed particles are formed, they are too small or not crystalline enough, with the last possibility being the least probable.

When the reducing agent is NaBH4 (Figure 5.5c) the peaks are not well defined and therefore no clear identification is possible. Nonetheless, the broad peak at 2θ ~ 40.0 ̊ could be ascribed to the presence of both the (111) reflection of Pt0 and the (111) reflection of PtCu alloy (blue bars in figure 5.5, PDF # 48-1549). The peak at 2θ = 50.4 ̊ could be assigned to the (200) reflection of Cu0 _{suggesting the presence of copper metal.} However, this assumption cannot be confirmed since the (111) reflection (2θ = 43.3 ̊) partially overlaps with the peak of g-C3N4.

(HR)TEM images of all the PtCu-g-C3N4 samples are illustrated in Figure 5.6. The impregnated PtCu-g-C3N4 is analysed after photocatalytic test. As discussed in Chapter 4 this procedure can be seen as a photoreduction, hence coherent with the other loading procedures. After exposing to visible light the impregnated PtCu-g-C3N4, the average particle size is found to be 3.5 ± 2.8 nm (Figure 5.6a, inset). This value is higher than when platinum alone is impregnated on the surface (2.3 ± 1.2 nm, Chapter 4). The size distribution is also increased as seen from Figure 5.6a where big particles are accompanied by smaller particles. Figure 5.6b shows nanoparticles characterised by lattice fringes of values 0.219 nm and 0.188 nm which correspond to the (111) and (200) reflections of a cubic PtCu alloy (PDF card # 4-8731). In addition, some nanoparticles with lattice fringes of 0.226 nm characteristic of the (111) reflection of cubic Pt0 are also observed. This indicates that the degree of alloying is only partial. (HR)TEM should not be considered a quantitative technique since the analysed area is only a fraction of the overall sample. Nonetheless, in the limited number of TEM images acquired for this particular system the probability of finding PtCu nanoparticles compared to Pt is approximately 2:1.

When the sample is reduced under hydrogen the average particle size is found to be 3.9 ± 2.0 nm (Figure 5.6d, inset) showing no significant increase compared to the impregnated PtCu-g-C3N4. This behaviour is once again different from what was

observed in the case of Pt-g-C3N4 when the reduction in hydrogen atmosphere favoured the formation of big clusters of NPs and the average size increased by a factor two (4.9 ± 4.7 nm, Chapter 4). From the high resolution TEM images (Figure 5.6be-f) only platinum metal lattice fringes can be seen (0.226 nm). This result is found in agreement with the observation made in the XRD analysis. At this stage it is not possible to confirm the successful formation of the PtCu alloy. However, it is reasonable to believe copper remain present on the surface of g-C3N4, but the lack of evidence in both XRD and TEM could either indicate its presence as an amorphous phase or, less likely, monolayer-like configuration. For the reduction by NaBH4 (Figure 5.6g-i) PtCu alloy coexist with platinum metal NPs at a relative frequency in the TEM images of 1:1. This confirms what has been seen in the XRD, i. e. a broad peak suggesting the presence of both alloy and platinum metal. The overall average particle size is estimated to be 2.6 ± 1.5 nm which is lower than the other loading procedures, justifying the low resolution in the XRD pattern. The presence of the PtCu alloy is confirmed via HRTEM for both the simply impregnated and chemically reduced (NaBH4) samples. However, photocatalytic tests shows that no effect on the performance is produced. The absence of any significant change in hydrogen evolution can be explained if the amount of PtCu alloy is too low to have any quantifiable effect and only platinum is responsible for the production of hydrogen. The sample reduced under H2 does not show any evidence of PtCu alloy formation but still sees a decrease in performance. This indicates that despite not being visible a change is introduced by the Cu in the system. The results presented here are found in disagreement with the literature. Mu et al. showed that when PtCu nanoparticles were exposed to reduction treatments at 300 ̊C a platinum-rich surface was obtained.188 _{Furthermore, the} Cu@Pt core-shell systems is often reported in the literature with improvement in catalytic activity.185, 196 At this stage is not possible to provide a definitive explanation of why such a decrease in performance is observed. Nonetheless, some possible explanations can be speculated. Since only platinum metal is confirmed, it can be assumed that copper metal is present on its own on the surface of g-C3N4. Copper has a lower work function than platinum, therefore, it may receive electrons from the semiconductor more easily than Pt but at the same time facilitated their backflow and recombination. Alternatively, since when copper is introduced in the system no big clusters of NPs can be observed after reduction in H2, this suggests a higher dispersion of nanoparticles. As seen in the previous chapter a higher dispersion may be detrimental for the activity.