Peer review of: Quantifying and understanding the triboelectric series of inorganic non-metallic materials (2020)

Reviewers' Comments: 
 
Reviewer #1: 
Remarks to the Author: 
In this paper, the authors used a developed standard method to quantify the triboelectric series of 
inorganic nonmetallic materials. This work establishes a fundamental material’s property of 
quantitative triboelectrification for the first time. In addition, from the measured data, they further 
investigated the basic principles of the contact-electrification (CE), and verified the origin of CE. These 
are great breakthroughs for understanding the triboelectric series of solid materials. This paper not 
only provides an important database for manufacturing and engineering, but also deeply studied the 
fundamental science and reveal the secret of an old unresolved problems in physics. Therefore, I 
strongly recommend the manuscript to be published in Nature Comm. 
There are several questions to clear: 
1. How this setup avoids the initial surface charge since the materials may contact other materials or 
air, and they will carry different surface charges before CE. 
2. In the manuscript, the author seems not mentioned the size effect. Will different sizes affect the 
measuring results? 
3. Can the authors use air instead of ultra-pure nitrogen to monitor the ambient environment? 
4. The quantum transition model well explained the fundamental questions. It would be better for the 
authors to describe more details about the difference between the quantum transition model and 
contact potential theory. Why contact potential has a low voltage, but the contact electrification has 
such a large voltage? Although the authors have point out some, they may need more words to help 
the readers for better understanding. 
By reading through the paper, I am pleased to learn the demonstrations and theory model presented. 
I believe this work can have a significant impact on this area, and the model will be widely used to 
study the triboelectrification between different materials. 
 
 
 
Reviewer #2: 
Remarks to the Author: 
This work is an extended study of the author's previous work. In this work, they applied the previously 
developed method to understand CE of inorganic non-metallic materials. The results reported in this 
work can be very useful database for further advancement of triboelectric energy harvesting devices, 
sensors, or other applications utilizing turboelectric effect. Thus, I would like to suggest the publication 
of this work and further investigation on other triboelectric charging mechanisms in the future. 
 
One concern I have in this work is the following. The proposed method agrees pretty well with most of 
inorganic materials listed in Fig. 3. However, the charge density difference for the materials ranging 
from Fe2O3 to BaTiOs (materials in the blue line box) is very small to make a general conclusion. For 
instance, the difference between the same materials, which may be originated from different surface 
states density of the source materials, might be bigger than that between different materials. Thus, I 
wonder if it is reasonable to list the materials in the box in order. I also wonder how many samples 
were used for this and if the work function of the listed materials are directly measured or are the 
values that are reported previously. 
 
 
 
Reviewer #3: 
Remarks to the Author: 
Triboelectric nanogenerators (TENGs) are of wide interest, and one of the fundamental challenges is to 
quantify the triboelectric characteristics of emerging materials, and rank them in the so-called 
triboelectric series. The authors previously published a liquid mercury based method to quantify 
triboelectric materials [Nat Commun. 2019; 10: 1427], in which the liquid metal serves as one of the 
TENG electrodes. While the current work is useful, it largely builds on "exactly the same method" 
published by the authors, and is severely lacking novelty. As such, this reviewer does not find the 
present work suitable for publishing in Nature Communications. The present manuscript is also riddled 
with typos, and some of the sentences are borrowed from their previous publication [Nat Commun. 
2019; 10: 1427]. Few examples include the following: 
• Line 26 Abstract: The dual use of "inorganic" in “...inorganic nonmetallic inorganic materials...” . 
• Line 136: The units for the work function of mercury is missing. 
• Line 162: The notation EFA is not defined. 
• Figure 5b is not cited in the text.