The reporting of the catalyst activity is an infamous aspect of the confusion that surrounds photocatalytic studies. (9) A study of the metal content/catalyst activity relationship could provide indirect proof that the activity is not likely related to the presence of the metal impurity. (8) However, we suggest that if any tiny doubt arises on the effective participation of unremovable metal traces, some efforts should be made to design some structural/activity experiments so that the activity can be robustly assigned to specific catalyst moieties. Moreover, in some cases, even the aforementioned base metals are not totally removed, although levels may be lowered to ppb. For some metal derivatives that are used in the synthesis, other side components may be incorporated that are not easily removed with simple acid washing. Attention should be paid to the use of high-purity acid to avoid possible secondary contamination. If a nanocarbon is used as one of the phases in catalyst formulation, thorough acid washing prior to its use is a standard preliminary step, as typical impurities include transition metals such as nickel, cobalt, iron, and copper that can be relatively easily dissolved in mineral acid. The design of the photocatalyst should be carried out with inclusion of a rigorous purification step to get rid of any presumably occurring impurity. Unfortunately, many articles discuss the activity on a single catalytic test, which has a large uncertainty. Reproducibility implies repetition (not less than three times) of the catalytic experiments with different batches of the same catalyst, with a report of the standard deviation. (7) The central message is therefore to carry out a rigorous characterization of the prepared photocatalysts by a combination of techniques and finally ensure that the catalyst’s activity is reproducible and is not dependent on serendipitous contaminations. (6) Similarly in photocatalysis, even by famous semiconductors such as TiO 2, it is a matter of fact that inclusion of foreign metals can create a positive synergy as well as a visible light region shift in the absorption spectra of TiO 2. Adsorbed foreign metal cations on electrocatalytic metallic surfaces has been recognized to bring positive or negative effects in many processes. On the other hand, this element of confusion regarding impurities is not exclusive to carbon nanomaterials. (3c) Just to mention one specific example, graphitic carbon nitride (g-C 3N 4) is a formidable photocatalyst that is enjoying great popularity due to its visible light activity and simplicity in preparation ( Figure 2), (4) and the boosting of its photocatalytic performance by metal doping has been well documented, (5) so that it has now become imperative to ascertain whether the apparent activity is tuned by any metal. A cross check among various characterization techniques, above all X-ray photoelectron spectroscopy (XPS), elemental analysis, and inductively coupled plasma mass spectroscopy, will confirm the total amount of surface and bulk metal impurities, and allow an understanding if such an amount is catalytically significant in relation to the overall activity. The presence of metallic contaminants should be evaluated and their possible contribution to the activity ruled out, as their adventitious incorporation in the catalyst structure may dramatically affect the activity, generating for example a reduction in charge recombination rate and thus inflating the claimed catalytic performance. However, a different but relevant question could be possibly answered: is it possible to establish a common protocol, set of rules, that makes all these publications useful for moving forward in a harmonized fashion the knowledge on photocatalysis? Figure 1 However, is this latter number truly justified or is the chain of events pining for sustainability and clean energy generating an overpopulation of contributions to the topic? Assuming that all such publications always reach a minimum standard of quality and novelty, the question for now must remain unanswered, because research is an unpredictable animal that can feed from any little fruit. Therefore, the fast-growing number of discoveries on photocatalysis comes as no surprise, and quite obviously they are accompanied by a ballistic number of publications ( Figure 1). Sunlight brings a gigantic energy pack, and modern scientists are investing a huge amount of time, resources, and intellectual exercise into best exploiting it. A never-tiring memento that, as long as the Sun shines in the sky, there will always be hope to heal a world suffering from an impressive increase in energetic demand. The line from the famous and comforting Beatles’ song sounds like a perfect slogan for today’s crusade toward a more sustainable world. “Here comes the sun, and I say it’s alright”.
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