Consider using kind 31. We put some thought into the tags, to meet academic citation standards.
{
"kind": 31,
"pubkey": "<citation-writer-pubkey>",
"tags": [
// mandatory tags
["u", "<URL where citation was accessed>"]
["accessed_on", "<date-time in ISO 8601 format>"],
["title", "<title to display for citation>"],
["author", "<author to display for citation>"],
// additional, optional tags
["published_on", "<date-time in ISO 8601 format>"],
["published_by", "<who published the citation>"],
["version", "<version or edition of the publication>"],
["location", "<where was it written or published>"],
["g", "<geohash of the precise location>"],
["open_timestamp", "<`e` tag of kind 1040 event>"],
["summary", "<short explanation of which topics the citation covers>"],
],
"content": "<text cited>"
}