Superscript Format

Perhaps not the best approach due to the inconsistent vertical positioning of ¹²³ vs. the other numbers… Alternatively, you could use axis post-selection to achieve the same effect.

<!DOCTYPE html>
<meta charset="utf-8">
<style>

.axis path,
.axis line {
  fill: none;
  stroke: #000;
  shape-rendering: crispEdges;
}

.axis text {
  font: 13px "helvetica neue";
}

</style>
<body>
<script src="//d3js.org/d3.v3.min.js"></script>
<script>

var margin = {top: 250, right: 20, bottom: 250, left: 20},
    width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom;

var superscript = "⁰¹²³⁴⁵⁶⁷⁸⁹",
    formatPower = function(d) { return (d + "").split("").map(function(c) { return superscript[c]; }).join(""); };

var x = d3.scale.log()
    .domain([1e0, 1e9])
    .range([0, width]);

var xAxis = d3.svg.axis()
    .scale(x)
    .orient("bottom")
    .ticks(10, function(d) { return 10 + formatPower(Math.round(Math.log(d) / Math.LN10)); });

var svg = d3.select("body").append("svg")
    .attr("width", width + margin.left + margin.right)
    .attr("height", height + margin.top + margin.bottom)
  .append("g")
    .attr("transform", "translate(" + margin.left + "," + margin.top + ")");

svg.append("g")
    .attr("class", "x axis")
    .attr("transform", "translate(0," + height + ")")
    .call(xAxis);

</script>

https://d3js.org/d3.v3.min.js